# Switched Reluctance Motor



## PStechPaul (May 1, 2012)

I "rediscovered" this motor in my searches and it seems to be making a comeback because electronic controls are now ubiquitous and the design has many advantages and few downsides. As a point of reference this was touched upon here:
http://www.diyelectriccar.com/forums/showthread.php?t=75088&highlight=reluctance

Here are some links I posted elsewhere but I'll consolidate them here:

http://machinedesign.com/article/the-switch-to-switched-reluctance-1211
http://www.youtube.com/watch?feature=player_embedded&v=jt4Fa4H43Iw (simple VR motor)
http://www.youtube.com/watch?v=nQ8G5wnH5sc&feature=player_embedded (variable reluctance test motor 3p stator 4p rotor)
http://www.youtube.com/watch?v=b3hmkehrcUg&feature=player_embedded (SRM test)
http://www.youtube.com/watch?v=G2qS2TxU9KY&feature=player_embedded (first run) 

I made a drawing of a proposed design for a switched reluctance motor:










Here is a video of a simple SR motor and controller I just made:




 
From the description:

This is an experimental switched reluctance motor which uses only external electromagnets in the stator, and no windings or permanent magnets in the rotor. Thus it is a very simple, rugged, and low cost design. The expense and complexity may be in the controller. The principle is essentially using electromagnets to align a piece of steel and then switching the excitation to adjacent magnets to achieve motion. I think I need to work on the design of the rotor and the pole pieces so that the force aligns the rotor at an exact point of rotation, which means wider stator pole pieces or more narrow rotor tips.

I may make some changes and see if I can get significant improvement, especially for self-starting. I think I will need to use a full three-phase H-bridge so the pole pairs can be driven both positive an negative (N and S). I also found that it helps to drive two sets of poles together to get higher torque and smoother transitions.


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## bjfreeman (Dec 7, 2011)

how is this different from a squirrel cage reluctance motor using aluminum bars.


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## cpct (May 31, 2012)

bjfreeman said:


> how is this different from a squirrel cage reluctance motor using aluminum bars.


If I understand it correctly, an induction motor is based on slip of the rotor versus the stator to give power whereas a SR motor needs "exact" positioning, because the rotor aligns to minimize magnetic "resistance". Right?


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## PStechPaul (May 1, 2012)

The induction motor relies on the rotating magnetic field and the slip to cause current flow in the conductive squirrel cage and a back EMF.

The switched reluctance motor works on magnetic forces only so it is totally synchronous and can work with full torque down to zero RPM. The rotor has only magnetic losses so it runs much cooler.

I need to do more thinking and tinkering, but I think it is possible to achieve a lot more torque from the SR motor. 

Here is a diagram of how I plan to drive the stator to achieve rotation:


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## PStechPaul (May 1, 2012)

I found a good explanation and animation of a variable reluctance motor (just another name for the same):

http://www.wisc-online.com/objects/ViewObject.aspx?ID=IAU14208

I think these motors are commonly used for disc drives.

Yesterday I took apart a 1050 RPM 1/7 HP shaded pole blower motor. It has six pole pieces and a rotor with 33 slots and a skew of about 16 degrees or 1.5 slot pitch. I was able to cut the conductive ends off the rotor (with a hacksaw) but the bars of the squirrel cage could not be removed and I was unable to rearrange the rotor laminations to eliminate the skew in an attempt to make a SRM. I also tried, unsuccessfully, to mill or cut slots in the rotor, so I gave up on that attempt. However the stator may be useful. 

Now I plan to make a rotor from several steel plates. I have some transformer laminations that I might be able to press together tightly enough to be able to machine the outer edges to match the ID of the stator. I may make just a two pole stator rather than four pole as was the experimental motor in the video. I think it will work just as well, and perhaps better.

My first idea was to use a pattern of two coils and four coils as shown for the 4 pole rotor:










But I realized that I should be able to get much better magnetization and more torque by using all six coils for multiples of 60 degrees:










I found an interesting article about motor efficiency that indicated a reduced skew can increase efficiency:
http://www1.eere.energy.gov/buildings/appliance_standards/commercial/pdfs/tsd_sem_nopr_ch4.pdf

And here are other references to skew and other related topics:
http://www.rle.mit.edu/media/pr152/15_PR152.pdf
http://masters.donntu.edu.ua/2010/etf/allagulova/library/article_8.pdf
http://www.lmphotonics.com/InductionMotor/MCog.php (cogging and crawling)
http://www.scs-europe.net/conf/ecms2011/ecms2011%20accepted%20papers/eee_ECMS_0107.pdf (induction motor analysis)


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## PStechPaul (May 1, 2012)

I copied the following from a discussion on wheel motors:

I am thinking that the switched reluctance motor may be capable of multi-speed operation and a very high torque. Consider the following:










The operation shown requires six different states for a single revolution. But as the motor gets up to speed it may be possible to drive it by simply applying two states, one for 0 degrees, and the other for 180 degrees. But the speed is really only limited by how fast the drive electronics can switch the fields, so this may not really matter. And I think such a motor, with all of the fields fully magnetized, would have enormous torque compared to other motors where at any given moment only about 1/3 of the windings are being driven. This motor could be driven to "lock" in any of the positions shown, and the drive current could be varied using PWM techniques so that it would only pull whatever power is actually needed to maintain that position. And intermediate positions could be obtained by applying different PWM values to adjacent poles (much like microstepping of a stepper motor). That's what this is, actually, but without magnets.


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## EVfun (Mar 14, 2010)

Switched reluctance has been used before on an electric motorcycle. http://www.electricmotorbike.org/index.php?page=lectra


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## PStechPaul (May 1, 2012)

Thanks for the link. It has a good basic explanation of the principles of the variable reluctance motor (as they call it), and I definitely realize the challenge of the electronics which I have not (yet) implemented in my experimental design. Looks like a shaft encoder of some sort will be needed. I look forward to the challenge! 

I found more information from Microchip:










This comes from http://www.microchip.com/pagehandler/en-us/technology/motorcontrol/motortypes/sr.html

Inerestingly, they say that the polarity of the pole does not matter. But for the implementation I am going to try, it does matter, and I think it may also result in a more powerful (or torquey) motor.

Here is a section on their motor control module for the PIC24F series:
http://ww1.microchip.com/downloads/en/DeviceDoc/DS39735.pdf

Freescale has some similar information. http://www.freescale.com/webapp/sps/site/overview.jsp?code=WBT_MOTORSRTUT_WP I like their animated GIFs of the SR motor and ACIM (and stepper and BLDC):

SR motor:







ACIM:







Stepper:







BLDC:









And some more from Freescale: http://www.freescale.com/webapp/sps/site/application.jsp?code=APLSWRMOT

This seems to have the most detail on motor characteristics and control theory: http://cache.freescale.com/files/microcontrollers/doc/ref_manual/DRM032.pdf


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## shortbus (Sep 27, 2011)

Hi, I'm a long time lurker and this is my first post. 

I've been researching the SRM for about the last six months and now getting ready to build one. Your the first guy I've found thats also into these. I can give some links to some good information, if you would like. Both on drive circuits and types of SRM's that are being made.

As far as self starting they need to be of at least three phases. More than three gives less noise and cogging torque, but ups the complexity of the drive. There are two types of commutation for them, sensored and sensorless. The sensored is the easiest to make.

I'm modifying a single phase 36 slot stator for my prototype. It is going to be a three phase 9 pole, with a 6 tooth rotor. Also it will be a short flux path type. This type is a lower temperature due to the back iron basically on being magnetically active in the individual poles. The magnetic circuit also is more localized in both the stator and rotor.

Please let me know if you want some links to more information.


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## PStechPaul (May 1, 2012)

I would like any information you may have. You may post the links here or you can PM me, but let me know if you want any of these links kept "private". I believe in the free distribution of information but if you are seeking a patent or trade secret then I'd need to know that. Good luck with your build and I look forward to more discussion.


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## shortbus (Sep 27, 2011)

The links are all from the searching I've done. I too am all about sharing information and encouraging innovation. I'll dig up a few links and post them.

Maybe you can help with my latest problem on this. I have looked and can't find a chart giving acceptable amperage levels for stator windings. The windings in a motor or transformer are different than ratings for open air. Motor windings carry more amps than the open air amps, but can't find a guide for choosing what gage to use. Any information would be much appreciated.


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## shortbus (Sep 27, 2011)

OK, here are just a couple to start off with -

http://eprints.gla.ac.uk/2851/1/optimalmiller.pdf This is by Tim Miller, the guru of the modern SRM

http://www.qucosa.de/fileadmin/data/qucosa/documents/525/1235569858100-6251.pdf A large one that is pretty good.

http://vukosavic.etf.bg.ac.rs/srmi.pdf This is on some of the inverter/control topologies that are used.

I've got many more but these will keep you busy for a while.


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## PStechPaul (May 1, 2012)

Very interesting and valuable references!  I've only just skimmed them and the mathematical analyses are freankly beyond what I can comprehend. I'm a practical experimenter sort and I have some ideas that do not seem to have been explored in these papers or anywhere else. If you notice, I am proposing a wide pole face on both rotor and stator, which I think may produce more torque and/or smoother operation. Also, I propose to drive more than one pair of poles at a time in a sequence. And I want to use full H bridges for each pole pair to reverse the field. So I'm building the circuit and plan to try it on my simple prototype first. Then I have a fan motor that has six pole pieces and I want to make a rotor with either two or four poles. Lacking ability to cut thin laminations I plan to use a stack of steel washers a bit larger than I need and then turn them to size on a lathe and cut the notches for the poles.

As to your question of the wire size and heating of the stator windings, it depends largely on the current you need and the duty cycle and the maximum temperature of the wire insulation. You will need higher voltage as the speed and frequency increase because of the coil inductance, and that will vary depending on alignment. You want more current when out of alignment in order to create rotation torque, and that is when the inductance is low. At alignment, inductance is higher, but you don't need much torque except as holding torque for low speed and servo type use. 

You will need to determine the parameters from the physical construction of the motor, and probably just fill the winding space as efficiently as possible. The wire size will determine number of turns and in conjunction with speed will determine the voltage. You may need to take a first shot by an educated guess and then taking measurements when you apply power. Then you can determine exactly what you need. And just add a thermal sensor to detect temperature and increase power until it reaches a level you're comfortable with. That's what I would do!


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## shortbus (Sep 27, 2011)

Yeah, my math is very bad too. I haven't worried about the math part either, just the practical part of the design.

Hope you don't mind me sharing some thoughts on what your trying to do. Not trying to be negative or stop you from experimenting, just some things during my research I found about the beast -

The wide more than one pole turned on at a time doesn't seem to work. The SRM principle is as the unaligned rotor pole becomes aligned, it stops moving. So if you turn on more than one adjacent pole thats aligned it is then a brake. Because of the magnetic attraction.

The poles don't benefit from 'bipolar switching'. Because there is no magnetism in the rotor poles. But they need to have the opposing teeth of a pole pair wound to give a north and south pole. This is to complete the magnetic circuit. The mag circuit forms similar to two D's with the flat sides together. It goes through the rotor and around both sides of the stator back iron. If the poles aren't opposite polarity theres no circuit.

All of this is why I decided to go with a "short flux path" motor. Only the back iron and the immediate edge of the rotor under the stator tooth thats 'on' are involved in the magnetic circuit. The stator pole is made of two adjacent slots of the stator. One tooth wound as a north the other as a south. The rotor can then be made from either the original induction rotor or a solid piece of steel. Because there is no real eddy currents involved in it. And the shorted turns of the induction rotor aren't affected by the magnetism.

Sorry for the long post, but this is the first time I've had anyone to talk to about this stuff And again I hope you don't take offense to what I said about your design.


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## shortbus (Sep 27, 2011)

Forgot, thanks for the ideas about the wire gage. Thats pretty much what I came up with too.


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## aeroscott (Jan 5, 2008)

great information on sr motors . No wonder LTI uses them .


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## shortbus (Sep 27, 2011)

Yeah, they have a pretty flat torque level for the whole speed range, are capable of very high speeds and take no magnets. They operate at lower temperatures. They've been around since the late 1800s but have just now become practical due to mosfets and IGBTs for the inverters. With the right controller they can operate in all four quadrants of motor control.

The new Dyson motor is a SRM, and a lot of washing machines are now using them.
Mining equipment has them as wheel motors, and the actual digging power for the machines.


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## PStechPaul (May 1, 2012)

According to at least one of the articles cited, commercial acceptance and the need for special drives makes the SRM a specialized item that will likely not see extensive commercial and industrial use anytime soon. Perhaps if the drive electronics could be incorporated inside the motor so that it could be used as a simple line start/run motor, it might become more widely used. I'm excited about its possibilities for EV use but practically speaking, it's probably much more practical and less expensive to use off-the-shelf ACIMs and drives if they can be obtained used and surplus. 

I'm interested in the "short flux path" design. It makes sense to minimize the path which should reduce the amount of iron and it will probably have more torque with less weight by eliminating much of the iron in the center of the rotor. It is also probably possible to design a "partial stator" which can be placed around a rotor similar to a brake caliper on a disk brake. It would also work as a linear drive similar to MagLev vehicles. The track would be simply a steel rail with notches filled with a non-magnetic material to maintain a smooth surface. 

Another idea I had was to eliminate the air gap entirely and instead use roller bearings to carry the flux, rather than air. The motor would then not need bearings on the shaft as the surfaces of the rotot and stator would be a large roller bearing. For a linear motor it might require something like a recirculating ball bearing mechanism as used in linear actuators. More food for thought!


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## shortbus (Sep 27, 2011)

I'll see if I can find some of the sites again. I hate reading PDFs on the computer, so usually print hard copies to read and don't save the sites.

I did find one today that might interest you - http://ecad.hanyang.ac.kr/bbs/data/...metric_Converter_and_Full_Bridge_Inverter.pdf

They do have something called a "bearing less SRM" but have never looked into how their made.


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## PStechPaul (May 1, 2012)

Here is a video of a linear switched reluctance motor as I proposed. Not a very good video but shows what I'm talking about:





 
And this demo showing the problem of "cogging" and oscillation on each step:





 
More details of the concept:
http://wwwlea.uni-paderborn.de/file...ichungen/2008/08Speedam-schneider-boecker.pdf


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## shortbus (Sep 27, 2011)

That second video looks like they didn't follow accepted guide lines on tooth sizes and spacing. Again there are tons of designs on line that work. The LSRM are what is used in most "pick and place" machines for assembling printed circuit boards in factories. So they are capable of smoother more precise movements.

Found some of the links for short flux path motors -

https://uhra.herts.ac.uk/dspace/bitstream/2299/1593/1/901730.pdf Takes a short time to load

http://elth.ucv.ro/fisiere/anale/2008/3_6.pdf

http://www.ime.pw.edu.pl/zme/pub/05/004bienkowski.pdf

http://jimhendershot.com/Jim_Hender...sesrbrushlessdcwithlowlossmagneticcircuit.pdf
If you only read one this the one

https://uhra.herts.ac.uk/dspace/bitstream/2299/1594/1/901731.pdf


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## iti_uk (Oct 24, 2011)

Hello all,

Any further developments or thoughts about these motors?

I'm no expert when it comes to magnetism, but being a mechanical type of person I enjoy mocking ideas up in CAD; I'm just throwing together a basic design for a short flux path SRM with the possibility of manufacturing a prototype once I'm happy with everything.

I have a couple of questions;

shortbus, you suggested earlier that the rotor could either be made using either laminates or from solid -

- is there no concern whatsoever for eddy currents and resultant heating in this arrangement?

- if there is heating or a detrimental change in rotor magnetism due to any eddy currents, would this be offset satisfactorily (or even advantageously) by an increase in magnetisable material (the iron, less non-conductive laminations)?

In considering manufacturability for the prototype, I'm leaning towards doing laminations rather than a solid rotor, but if eddy currents are not a practical issue, then laminations of around 5mm thickness would cut down manufacturing time while allowing a simple water-jetting of the parts from sheet.

I've attached a couple of screencaps of my idea so far. It includes iron laminations (nominally 1mm thick for now) mounted on an aluminium boss (from which I haven't yet bothered to remove material to bring the rotor mass down). I haven't put much thought into the stator yet - it's just a mock-up to demonstrate the principal.

This is a 5-phase motor, based on one of the .pdf documents linked previously, but with pole numbers doubled. It currently resembles a pancake motor in dimensions (160mm OD of the stator, roughly 20mm thick).

Thoughts?

Chris


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## iti_uk (Oct 24, 2011)

Another small question as I walk to my car – I imagine windage to be a consideration with these motors, so I would be experimenting with filling the spaces between teeth with a polymer or ceramic of some sort, which got me to thinking about the notch between the paired teeth of the rotor. Is this notch actually necessary to clearly define a pole "face" (as I imagine it) or could I remove that notch and fully siamese the paired teeth?


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## aeroscott (Jan 5, 2008)

I think cutting out steel is less induction heating . If you filled the motor with hydrogen or helium , you would cut down on windage and have the best heat exchange gas .


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## cpct (May 31, 2012)

iti_uk said:


> - is there no concern whatsoever for eddy currents and resultant heating in this arrangement?


Both stator and rotor could still have iron losses, so lamination of both would be the best.



> - if there is heating or a detrimental change in rotor magnetism due to any eddy currents, would this be offset satisfactorily (or even advantageously) by an increase in magnetisable material (the iron, less non-conductive laminations)?


I have no "numerical" idea about that, but it would seem to me that the amount of iron you gain by not laminating is really small, while laminating is definitely known to improve motor efficiency. If I remember my textbooks right, the eddy current heating would decrease the magnetic permeability of iron as well.


I find the use of SRM quite interesting, but aren't they known for being larger than other types of motors of the same performance?


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## PStechPaul (May 1, 2012)

I think SRMs may be larger than PM BLDCs but smaller than ACIMs. The expense and fragility of BLDCs favor the SRM if size is not a major issue, and for EVs (particularly long range cars) the motor size/weight is much less than the batteries.

The laminated design seems to be best and I agree that filling in the voids with ceramic or plastic is a good idea to reduce windage. The short magnetic path design would allow most of the rotor mass to be on the periphery where it would contribute most to rotational inertia, and should also reduce cost. 

I have also heard of transformer designs (toroidal and C-core) which use parallel strands of magnetic wire rather than laminations. If the magnetic path is well defined the strands can be made to follow the lines of flux and it should further reduce eddy currents. But it might be more expensive to manufacture and may not be as strong as laminations.


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## shortbus (Sep 27, 2011)

iti_uk said:


> I have a couple of questions;
> 
> shortbus, you suggested earlier that the rotor could either be made using either laminates or from solid -
> 
> ...


 
Glad to hear someone else has caught the bug

I don't think the rotor is as effected by the eddy currents as the stator is. But may be wrong. A lot of the motors that college students make for their thesis projects just have solid rotors, because of the ease of using solid over laminations.

With the short flux path style, one of the ideas with it is the smaller amount of eddy current loss due to the short flux and no flux reversal in the stator, and I assume that goes to the rotor also.

As to your other question about the "notch" in the rotor face, it has to be there, maybe even deeper than you show. With out it, the pole pair that is un-aligned, would just be the same as an aligned rotor pole. And that wouldn't cause any movement or torque.

Wish I had access to a water jet like you. Did before retiring and closing of the company.


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## iti_uk (Oct 24, 2011)

Ahh interesting input, thanks all. I've been following this thread since it began. I went and did a load of googling on the subject and I'm sold on the idea. I was previously drawing up a BLDC motor, but was concerned about handling the sort of permanent magnets required. I'd far rather have something "neutral" when not energised to make assembly and servicing safer and more simple.

I suppose I'll be using laminations, then. At least I don't have to start over :-D

Tomorrow, when I have some free time I'll adjust that "notch" to deepen it.

I've been considering enlarging the aluminium boss and creating separated pole pair laminations, and mounting the pole pairs separately into the boss. I'll see what I can come up with...

As to SRM size, from what I've read it's true to say that they are smaller than ACIMs, but larger than BLDCs. The reason they are smaller than ACIMs is that the windings in the stator are far simpler - each pole is wound as an individual, instead of having to wind across poles in delta- or Y-patterns. Not having to have this extra copper "hanging off the ends of the stator" means a shorter motor.

I don't have direct access to a water jet cutter myself, but I've got a good contact who has done water-jetting for parts I used in a previous project. It's all about who you know ;-)


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## Salty9 (Jul 13, 2009)

To muddy the waters a little bit more, anyone considered an axial flux SR motor?


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## iti_uk (Oct 24, 2011)

Salty9 said:


> To muddy the waters a little bit more, anyone considered an axial flux SR motor?


I haven't read into axial flux SRMs, but here is my initial thought;

I guess an axial flux SRM would reduce the overall diameter of the motor chassis by moving the stator from the periphery to alongside and within the circumference of the rotor. Assuming that the axial flux path is similar to the short path design as discussed previously in this thread, I see this design as having no advantage over the periphery-stator, short path design other than it changes the packaging type from large diameter, small thickness to small diameter, large thickness. The rotor characteristics would remain the same.

It would seem to me that both the axial flux and periphery-stator (radial flux?) designs have exactly the same advantage over the diametric-path SRM (an example of which is shown in the first post of this thread).

You've certainly put the idea in my head now, though. I'll pay around with it to see if I can come up with anything interesting and I'll hit google at lunchtime to see what I can find...

edit:

After further thought and some googling, I am considering the following regarding the difference between what I'll call AFSRMs (Axial Flux SRM) and RFSRMs (Radial Flux SRM (short path));

The design consideration which has led us to the short-path design SRM is to have a short and complete a path of magnetic flux as possible. With an ACIM, BLDC or conventional SRM, the magnetic "path" is diametric, so the advantage of changing the design to that of an axial flux arrangement is effective because the length of the "path" reduces and simplifies significantly. However, with a short-path RFSRM, this consideration is already accounted-for - the "path" is short and well-defined within both the rotor and the stator.

The next thing to consider, then, is what I would call the "working radius" of the rotor - the average radius of the interface between rotor and stator. Assuming similar strength electro-motive force, it makes sense that the larger the working radius, the larger the moment acting on the motor shaft (more torque). For a RFSRM, this working radius is at the circumference of the rotor, whereas for an AFSRM, the working radius is measured to the centre of the iron face on the rotor. This means that for the two motors to have the same working radius (i.e. the same torque), the AFSRM will necessarily have to have a larger diameter rotor. This means more rotational mass, meaning a reduced maximum safe motor speed due to increased stresses in the rotor material. Lower motor speed means lower power for the same torque value.


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## Salty9 (Jul 13, 2009)

Good analysis, I think (I'm really not qualified to evaluate it). I was thinking of a sandwich with the rotor between two stators.


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## iti_uk (Oct 24, 2011)

Salty9 said:


> Good analysis, I think (I'm really not qualified to evaluate it). I was thinking of a sandwich with the rotor between two stators.


I guess that could be done. It could allow for stacking of multiple rotors, too which would be nice... This could be achieved in a similar manner with a radial flux motor by just increasing the number of laminations in the stator and rotor, although it wouldn't exactly be "modular" once manufacturing was complete...

I'm going to stick with a radial flux setup for the moment though; I've got a new and improved rotor in my head which I'll draw up in CAD tonight.

Honestly, I'm not qualified to speak with authority on this subject. My background is mechanical, not electrical (or anything regarding magnetism), I'm just applying what I know from a mechanical standpoint and making assumptions based on my limited understanding of magnetic motor theory. So I guess take what I say with a pinch of salt... lol. I'm ready and willing to be corrected on any of this.


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## shortbus (Sep 27, 2011)

Like most of you my background is mechanical and not electrical also. But motors have always been a on going interest.

@iti uk - I have been wrestling with the idea of commenting or not commenting on your drawing of your motor. And decided to make a comment, to maybe keep you from wasting some time and effort. This is not meant as criticism but just from the point of helping.

The stator would be great for a BLDC but not so great for a SRM. The SRM makes its torque/rotation, from moving the rotor from an unaligned to an aligned position. By that I mean, the rotor pole needs to be close to a stator pole but not under it. The SRM is a "doubly salient pole motor". For this to happen there needs to be "gaps" between the poles. The stator gap widths need to be close to the width of the rotor pole faces in size. The drawing you posted doesn't have the size gaps necessary to do this. Again this is meant to help not as criticism.

Here is a PDF that gives a lot of good information about pole sizing - http://eprints.gla.ac.uk/2851/1/optimalmiller.pdf


Here are good patent PDF's that explain a lot too - 
http://www.freepatentsonline.com/5111095.pdf this one is good for the use of five phase short flux

http://www.freepatentsonline.com/4883999.pdf

http://www.freepatentsonline.com/5015903.pdf


To all - the SRM is used in all different sizes, from the new Dyson vacuum sweeper all the way to 100 or more kW steel mill drives.


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## iti_uk (Oct 24, 2011)

shortbus said:


> @iti uk


Criticism is always welcome so please don't hold back if you've got something to say. I'd rather know that I'm wrong sooner than later. So am I to understand that the poles on the stator should have gaps of equal to/greater than a pole width? I'll include that in my next update, which I'm about to start messing with now.

Also, those links are good reading. I'll pour over them properly later when I get home.


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## iti_uk (Oct 24, 2011)

shortbus, I've tweaked the model of the stator (and re-matched the rotor). Is this more like it? (I'll be making the inter-pole cutout deeper in a minute...)


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## iti_uk (Oct 24, 2011)

Here's a quick idea of my rotor lamination idea (shown with only one endplate, the other being hidden to show the iron) - each pole pair is a seperate lamination stack, and the stacks are positioned using dowels in the stack/endplate holes. Assuming that the rotor doesn't get hot, the void could be filled with any choice of lightweight material for windage reduction purposes.
The endplates are aluminium.

edit: added an extra screencap with both endplates, for illustration (minus fasteners or dowels, for now).

Out of interest, the pictured assembly weighs 0.596kg, excluding the (arbitrarily chosen) 20mm diameter steel shaft (0.843kg including the shaft). The overall diameter of the rotor is 100mm.
The previous non-seperate pole-pair rotor weighed 0.678kg (0.925kg inc. shaft), so that means a saving of 82g (overall saving of 9%, inc. shaft)

edit 2: added a shot of the rotor sitting in the stator, with one endplate hidden.


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## shortbus (Sep 27, 2011)

Now your cooking! Glad you didn't take what I had to say as an offense. Some people do, and especially on line where you can't have a one on one talk. Don't want you or anyone else to get the idea that I'm an expert or anything close to that. Just a old guy with too much time on his hands that loves doing things and figuring stuff out.

What I'm using as a test bed for the stator is a GM car alternator. The stator has 36 slots just like the 5HP motor I'll be using for the full size one. 

Starting with an alternator or AC electric motor gives (in my opinion) a head start. The stator laminations are already made, even though they need modified. And the end bells are ready made. All thats needed to be fabricated is a rotor and shaft. To me thats a win-win.

I'm going with a three phase my self. That way I can use a MC3035 BLDC controller to do the commutation logic. http://www.onsemi.com/pub_link/Collateral/MC33035-D.PDF

http://www.onsemi.com/pub_link/Collateral/AN1046-D.PDF


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## shortbus (Sep 27, 2011)

@ PStechPaul, I want to apologize to you for hijacking your thread with my ideas and links. Didn't really mean to, it just sort of happened. I will understand if you ask a moderator to move the stuff to another post. Just thought it would be good to have it all in one post. Again I apologize.


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## iti_uk (Oct 24, 2011)

@PStechPaul, my apologies also. I've come into this thread treating it as an idea assembling zone...

@shortbus, no worries. I have had to learn to take criticism at work, as it is better to be seen to admit fault and then make a change than to deny and take it personally.

Sounds like you're well on your way to making a working motor! My thinking is that I can get all the laminations water-jetted fairly easily, so all that remains is a shaft, a housing and the endbells of the motor. I'm going to shop around for shafts before I specify a shaft bore diameter in my rotor design, and I'm trying to cook up something interesting for the housing which might allow machining from billet if I can find someone with a CNC mill. It's a lot of work, but I've got time and only vague vehicle plans in mind. 

In a final model (not this prototype), I'd like to take advantage of the fact that most of the heat in the motor will be produced in the stator by including some sort of liquid cooling in the housing. It's a complete ground-up project so I'm trying to include as much beneficial stuff as I can while still remaining plausibly manufacturable...

Who knows where it'll lead.

As for the 5-phase nature of my motor, I haven't even considered controllers yet. That may be a project for the future, or I'll find a 5-phase controller somewhere...


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## PStechPaul (May 1, 2012)

Don't worry about "hijacking" the thread. Your replies are very much on-topic and I am reading them with great interest. I've been sidetracked on other projects but I want to get back to doing some work on my ideas for a SRM design. I made a controller using a PIC18F2431 and two L298 drivers, so I have six independent drives which can provide either polarity to each of three pole pairs, or separate unidirectional drive to all six. I want to see how various switching algorithms work. 

I have a fan motor which has six pole pieces but they are shaded poles so I might need to remove the shorting bars, and the gap in each pole might not be ideal. I tried to rework the rotor and I was able to cut off the shorting caps to expose the aluminum winding ends, but it was not practical to remove the laminations and reuse them. I was able to drive the shaft out and I may make a rotor from a stack of steel washers, but I'd have to do a lot of cutting and milling, and the magnetic properties would be far from ideal. I might instead look at a short path design where the rotor would be mostly non-magnetic material and have sections of steel on the outer surface.

Thanks for all the ideas. I might be able to try some things and post results soon!


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## Salty9 (Jul 13, 2009)

Since SRMs are doubly salient, it should be possible to rewind a single phase ACIM stator for SRM use.


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## aeroscott (Jan 5, 2008)

I was thinking of cooling the stator . I like the idea of lower voltage motors with increasing voltage as speed is increased . So the Remy motor uses hairpin("U") square wire silver soldered together to make the winding . So now you can use ceramic insulators and if the copper is a tube, cooling gas or oil can be run through the conductor .Also the coils have less winding to winding voltage differential , standard wires can have large wire to wire voltage differential . that's if the first and last wires are next to each other .


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## iti_uk (Oct 24, 2011)

PStechPaul,

I liked the prototype you posted at the beginning of this thread. One thing occurred to me as I watched it, though; have you tried different diameter rotors? That rotor looks very large for the number and size of the stator poles. If it were me, I'd be tempted to halve the diameter of the rotor (and rearrange the stator poles to suit, of course) to see what happens...?

Is your controller scalable to 5 phases?

@aeroscott, thru-winding liquid cooling sounds interesting, although I'd be concerned about the drop in cross-section of the copper. Rather than replacing copper, my initial thought is to replace several of the laminations (1 in 10, maybe?) with a hollow "laminate-a-like" through which to pass coolant. I might mock this up this evening, I'll see how work goes...


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## aeroscott (Jan 5, 2008)

your concern for loss of cross section is a good point. This makes me think about the tube being thick wall with very small cooling hole .this may mean H2 or He as the need for low viscosity and high velocity . I just remembered where I saw tube conductor , supper conducting generator. It had very low number of turns .
The wound coils(as seen in some pm and most sr motors) have some room around them(space between coils) . I wounder if spaces could be made between layers of winding for coolant gas or oil . These type coils are more like transformers .


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## iti_uk (Oct 24, 2011)

Another difficulty regarding hollow copper wire is manufacturing it to our requirements. Further, once manufactured, it might be tricky to actually wind the wire effectively without crushing or kinking the tube at the corners of the laminate.

edit: Although I love the idea of cooling the coils directly either by having tubular wire or replacing laminates with laminate-oid cooling channels, after an afternoon of thinking, I'm coming to the conclusion that a (relatively more) simple solution is to use a housing which includes a water jacket or passageways to cool the outside circumference of the stator. Doing it this way doesn't compromise the performance of the electromagnets and is most likely far more simple to implement.

I really should get started on mocking up a housing...


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## shortbus (Sep 27, 2011)

Salty9 said:


> Since SRMs are doubly salient, it should be possible to rewind a single phase ACIM stator for SRM use.


Thats what I'll be doing. But you also need to remove the unused "teeth" of the stator. To keep stray reluctance to a minimum.


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## shortbus (Sep 27, 2011)

There's a lot of information already available for using micro controllers and DSP for the control of a SRM. Freescale and TI have been at it for a while - http://www.google.com/search?q=srm+controller+design&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7GWYE#hl=en&sugexp=les%3B&gs_nf=1&gs_mss=srm%20controller%20design%20&pq=srm%20controller%20design&cp=31&gs_id=v&xhr=t&q=srm+controller+design+freescale&pf=p&sclient=psy-ab&rls=com.microsoft:en-us%3AIE-SearchBox&rlz=1I7GWYE&oq=srm+controller+design+freescale&gs_l=&pbx=1&bav=on.2,or.r_gc.r_pw.&fp=811e24697edb2157&biw=1024&bih=526

But when it comes to that I'm pretty much a dinosaur, so going to try an analog approach first.


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## shortbus (Sep 27, 2011)

Since there are no magnets involved the heat issue isn't as big a deal, like in PMDC and BLDC. The heat that the high temp magnet wire can withstand is hard on rare earth magnets. Thats one reason most commercial DC motors use ceramic magnets, they can take the heat better without loosing their magnetic properties.


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## iti_uk (Oct 24, 2011)

I've only passingly thought about the controller electronics and I'll need to learn things pretty much from scratch if I take on a controller project myself.


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## PStechPaul (May 1, 2012)

Here is the schematic for my motor controller:









It has six independent outputs which can be driven to ground or V+ by the L298 bridge drivers. So it should be scalable to 5 phase if you are using a unipolar configuration. 

I realize that the narrow pole faces are less than ideal and thus do not really lock the rotor to their position. The fan motor I plan to use has six poles with wide pole faces so I think by matching the rotor to the stator it should achieve maximum reluctance when aligned. Another idea was to use a smaller radius on the tips of the rotor so that the gap would be smallest when exactly aligned.


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## shortbus (Sep 27, 2011)

Paul are you just using th L298 to drive the gate of a mosfet or IGBT? the L298 has a hard time switching stepper motors, from what I know. Or is this just going to be a proof of concept?


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## PStechPaul (May 1, 2012)

This is really just proof of concept. Rather than using half-bridge or hi-lo drivers and large MOSFETs or IGBTs, I decided to use the L298s because I had them already and they are simple to hook up in the prototype. I'm using 24 VDC relay coils which draw only about 100 mA so my motor will be very limited power. Once I have a better understanding of the concepts I may proceed to make a more comprehensive design (probably with a bigger PIC18 part or possibly a dsPIC). I already have the C18 compiler so I can code the PIC18 parts in C. The dsPIC compiler is a bit pricey so I'd rather not commit too much money to the project unless I can really justify it.

I'll probably make it so that it can drive a SR motor as well as a three phase ACIM.


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## iti_uk (Oct 24, 2011)

Any more thoughts or developments, anyone?

I haven't had a chance to progress my sketches in the past few days, hoping to do a bit tomorrow.

Chris


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## rochesterricer (Jan 5, 2011)

I just read an article about these motors in Charged EV Magazine, written by Jeff of Evnetics. Good stuff.


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## devoreaux (Jan 2, 2013)

Do you know any company apart nidec or maccon that sells SRMs (of course with controller)?


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## iti_uk (Oct 24, 2011)

I've been away for a while, but in between working I've had some thoughts regarding these SRMs. Specifically, I've been thinking about axial-flux setups (despite what I've said previously lol). I've got a question to run by people here; Is the torque of the motor proportional to the number of rotor-stator interfaces (i.e. a single stator, single rotor ARM has one "interface", a stator-rotor-stator sandwich has two interfaces, etc). If this is the case, I wonder what a split-rotor design might be like (See attached diagram).

My thinking is that the extra passive stator in the middle of the rotor sandwich will provide an extra two interfaces while adding little in the way of mass to the motor. I imagine that this would weaken the field (extra air-gaps, etc), but I wonder if overall an increase in torque might be felt...

The diagram is a bit abstract, but imagine that you are looking top-down on an axial flux motor assembly. The red sections are the stator(s). All poles on the stators (including the poles on the "middle" stator in the split rotor design) are aligned.

In the split rotor design, imagine a horseshoe magnet arrangement for the "outside" stators, with the EM coil wound on the horseshoe magnet. The "middle" stator is passive (has no coil).

Has this been tried before? Thoughts?

I'm still (slowly) going ahead with my plan of creating a prototype, but I'm currently having difficulty visualising ways of securing horseshoe EMs in a stator. Google's not been too helpful and I'm not totally happy with any of my ideas so far.


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## Salty9 (Jul 13, 2009)

Have you considered potting your stator EMs in an appropriate compound? I have been thinking about building an axial flux motor and it seems it would work in that configuration.


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## PStechPaul (May 1, 2012)

I think it is a good idea to have at least two interfaces as you picture, which is similar to a disc brake with a caliper that grips both sides of the surface. But the problem I see is that the components would need to be very accurately aligned and kept from touching. The magnetic field strength increases as the gap decreases, but if the disc is not perfectly aligned between the stator poles, it will pull harder to the side with less gap, and may bend the rotor enough to contact, at which point it becomes a brake.

I have also considered the possibility of using some means to hold the alignment and maintain the proper gap in the axial flux design. One possibility would be to use a Teflon spacer or coating on the surfaces, although I think it would still create too much friction and would be subject to contamination and degradation. Another idea is to use a ball bearing of some sort on the periphery of the rotor to keep it aligned. And yet another idea would be to have grooves in the rotor and stator for ball bearings which would also provide a magnetic path with essentially zero gap. That might be good for very low speeds and high torque.


The radial alignment is more robust because the rotor can be held on center with good bearings and the surface can be machined accurately to hold a very small gap. The problem may be at high speeds where the centrifugal force tends to expand the rotor (although expansion from temperature may be more problematic). However, the heat will be generated mostly in the stator and expansion would increase the gap.

I need to get back to work on this project. I'm trying to finish up another couple of jobs and then maybe I'll have time again.


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## iti_uk (Oct 24, 2011)

Interesting input there, Paul. I hadn't really considered that an axial flux SRM's rotor deflection (axially) is in a positive-feedback-style relationship with the stators, while an induction or PM motor would have a negative-feedback-style relationship. I will have a look into the maths at some point and see if I can determine how sensitive a system like this would be to deflecting to the point of contact.

After discussion with a colleague yesterday, I've come up with a solution for the stator mounting. It came after a long discussion starting with Salty9's suggestions (thanks for that) and it's ended up as a clamshell-style design. I'm fiddling about with it in CAD at the moment, so I might be able to post up some pics of it soon.

Chris


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## Salty9 (Jul 13, 2009)

http://endless-sphere.com/forums/viewtopic.php?f=30&t=46476

Triple stator axial flux motor build. Not SR but doubly (triply?) salient.


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## iti_uk (Oct 24, 2011)

Salty9 said:


> http://endless-sphere.com/forums/viewtopic.php?f=30&t=46476
> 
> Triple stator axial flux motor build. Not SR but doubly (triply?) salient.


I've seen his v1 build before, it'll be interesting to follow his v2 build.

His design is coreless, and I don't think this would work with an SRM armature. The way the coils are bonded into a sheet looks fine for his setup, but I would be concerned about structural integrity with a more powerful setup.

Nice to see another project being taken on, though. Always good.

Chris


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## kennybobby (Aug 10, 2012)

Have you guys ever dismantled a stepper motor?--if not then i would recommend it-- you might find some parts you can use to build your motors. When you do it will destroy the magnetic field in the rotor, but they are low cost. The SRM sure look like a stepper motor without the axially-aligned magnetic field of the laminated rotor. Steppers make their maximum torque when not moving--they are good for positioning and placement tasks such as hard drive heads and pick/place machines. They have a large cogging torque and the torque ripple can be quite high while moving and they have resonant frequencies in which the movement stalls. Many folks have viewed the simple digital aspect of controlling these sort of motors and tried to develop them into torque motors, but i have tested lots of electric motors on dynamometers in the past 20 years and have never found a SRM or stepper motor that would be a suitable candidate for a torque motor application such as an EV.


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## PStechPaul (May 1, 2012)

iti_uk said:


> I've seen his v1 build before, it'll be interesting to follow his v2 build.
> 
> His design is coreless, and I don't think this would work with an SRM armature. The way the coils are bonded into a sheet looks fine for his setup, but I would be concerned about structural integrity with a more powerful setup.
> 
> ...


Some nice pictures of the v1 build:
http://endless-sphere.com/forums/viewtopic.php?f=30&t=30061&p=434496#p434496

I wonder if an SRM would work better using repulsion? But I can't figure out just how to make it work. A piece of iron basically forms magnetic poles that attract the electromagnet that creates the field, so this would need to be somehow reversed. There is a repulsion motor but it uses brushes and commutator:
http://en.wikipedia.org/wiki/Repulsion_motor
http://www.brighthubengineering.com/diy-electronics-devices/48676-how-repulsion-motors-work/

I found a new article on a double rotor SRM that might be useful:
www.mdpi.com/1996-1073/5/10/4008/pdf

I had another idea for a type of motor using the principles of a railgun. There are several types, and what I had in mind was using the principle of the force generated in a loop of wire which tends to orce them apart. So a railgun would use two conductive rails across which a voltage would be applied on one end, and a conductive roller (like a bullet) would placed across them, closing the circuit and causing a force that moves the projectile toward the end of the rails. Now picture this with the rails arranged as two concentric loops, and the conductive projectile (roller) would be connected to an arm of a rotor. The loop would have to have a gap at which point the roller would transition from the end of the loop to the beginning, and the process should continue. Most railguns use a high capacitance discharge to achieve tremendous acceleration at the cost of much energy loss and damage to the rails and projectile, but if the position and velocity were known, the appropriate amount of current could be applied for a specific acceleration or rotational speed. Several of these could be assembled in parallel, or multiiple shorter loops, for smoother operation and higher torque. Here is a simple description of a railgun using a linear homopolar principle which requires magnets:
http://sci-toys.com/scitoys/scitoys/electro/railgun/railgun.html


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## iti_uk (Oct 24, 2011)

PStechPaul said:


> Some nice pictures of the v1 build:
> http://endless-sphere.com/forums/viewtopic.php?f=30&t=30061&p=434496#p434496
> 
> I wonder if an SRM would work better using repulsion? But I can't figure out just how to make it work. A piece of iron basically forms magnetic poles that attract the electromagnet that creates the field, so this would need to be somehow reversed. There is a repulsion motor but it uses brushes and commutator:
> ...


Isn't that just an induction motor or a series wound DC motor?

edit: just read through the wikipedia entry and I see what you mean. It's an interesting concept, but I'm trying to avoid the use of brushes.



PStechPaul said:


> I had another idea for a type of motor using the principles of a railgun. There are several types, and what I had in mind was using the principle of the force generated in a loop of wire which tends to orce them apart. So a railgun would use two conductive rails across which a voltage would be applied on one end, and a conductive roller (like a bullet) would placed across them, closing the circuit and causing a force that moves the projectile toward the end of the rails. Now picture this with the rails arranged as two concentric loops, and the conductive projectile (roller) would be connected to an arm of a rotor. The loop would have to have a gap at which point the roller would transition from the end of the loop to the beginning, and the process should continue. Most railguns use a high capacitance discharge to achieve tremendous acceleration at the cost of much energy loss and damage to the rails and projectile, but if the position and velocity were known, the appropriate amount of current could be applied for a specific acceleration or rotational speed. Several of these could be assembled in parallel, or multiiple shorter loops, for smoother operation and higher torque. Here is a simple description of a railgun using a linear homopolar principle which requires magnets:
> http://sci-toys.com/scitoys/scitoys/electro/railgun/railgun.html


Weirdly, I've thought of that before, too. It's a nice idea but even at lower power levels I'd worry about durability issues. Also, I'm trying to avoid the use of exotic materials and manufacturing to too high a tolerance, both of which are characteristics of railguns (low friction, high conductivity, high surface hardness/corrosion resistance).

One of my goals with this design is to eliminate the use of any special materials and for the possibility of near-100% recyclability, so no permanent magnets, just aluminium (structure), copper (coils - of course there's the coating but that can't be helped), iron (magnetic core laminations) and steel bearings.



kennybobby said:


> Have you guys ever dismantled a stepper motor?--if not then i would recommend it-- you might find some parts you can use to build your motors. When you do it will destroy the magnetic field in the rotor, but they are low cost. The SRM sure look like a stepper motor without the axially-aligned magnetic field of the laminated rotor. Steppers make their maximum torque when not moving--they are good for positioning and placement tasks such as hard drive heads and pick/place machines. They have a large cogging torque and the torque ripple can be quite high while moving and they have resonant frequencies in which the movement stalls. Many folks have viewed the simple digital aspect of controlling these sort of motors and tried to develop them into torque motors, but i have tested lots of electric motors on dynamometers in the past 20 years and have never found a SRM or stepper motor that would be a suitable candidate for a torque motor application such as an EV.


Thanks for the input, I've looked at steppers before and they're almost what I'm going for, but not quite.

I have read an article (I believe it's linked to earlier in this thread) which discusses multiple phase motors overcoming the cogging that you get with single or 3 phase - as far as I understand it, it's down to the way the phases overlap, and the maximum torque drop-off between phases.

I have been toying (in my head) with the idea of driving every pole pair separately, but instead of being driven by a sine wave generator they would be "fired" in a sort-of-CDI manner (see ICE ignition), where for each pole pair, a capacitor will be charged in between firing (let's say we have 8 pole pairs); 7/8 of the cycle, the capacitor will (have the opportunity to) charge, 1/7 of the cycle, the coil will fire using the energy stored in the capacitor. The motor can be "throttled" by changing the voltage to which the capacitor may charge. Of course, I might be mixing up my theory and maybe simple DC switching might be just as good at firing the coils, but I figure it's worth consideration. Allowing those 8 phases to overlap should, as I understand it, reduce cogging to an acceptable level.

Chris


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## PStechPaul (May 1, 2012)

The idea of charging a capacitor and discharging into the stator poles is an interesting concept. In a SRM, when the rotor is not aligned with the pole piece, the inductance is low, so the discharge will be at the highest current level due to the full voltage of the capacitor and the low inductance. As the rotor aligns to the pole, the inductance will increase while the voltage drops. The torque increases with a constant current in the pole as it comes into alignment, which would cause torque ripple, but the discharging capacitor and increasing inductance may reduce it. Worth looking into.


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## iti_uk (Oct 24, 2011)

Okay, I've had some free time recently so I've been able to mess about with my prototype design in CAD.

I've changed things up with motor poles. One of the issues I had with the design in this previously linked-to article was the fact that the stator pole pairings (and therefore the firing circuitry) were not (in my mind) "tidy", that is to say that poles were not paired, but instead the pairings would change between magnetisation events. This lead to uneven pauses between magnetisation (for example, coil "A" in that link's Fig.10 has the pattern: On, Off, On, Off, Off) which could complicate any sort of capacitor-firing system.

To remedy this, I swapped the stator and rotor poles. My new design (shown in the attached pic) has evenly spaced rotor poles (22) and unevenly spaced paired stator poles (20 poles, 10 pairs). This results in 5 distinct phases without the pole sharing of the previous design and even "firing" patterns.

In terms of geometry, this design with this number of poles results in a rotor rotation of 3.273 degrees between phases (22 cycles per rotor revolution, or 366 cycles per second (total triggering at 1833Hz, each phase firing at 366Hz) at 1000RPM). Although there will be a firing event ever 3.273 degrees, each firing event will last for up to 8.182 degrees, meaning a potential overlap of up to 2.5 phases. Along with the capacitor firing idea, this should help significantly with cogging.

I have also drawn up a quick stator housing which includes a water-jacket style cooling system with a fully assembled mass of around 5.5kg, excluding the copper windings and bearings (I haven't decided on those yet). The Rotor diameter is 100mm and the overall motor diameter (including housing) is 220mm. The length of the rotor is currently 22mm, although this is arbitrary and can be scaled without difficulty.

I feel that I'm getting close to manufacturing my first real prototype of this motor, I just need to sort out the bearings and a mount for a rotor position sensor. The rotor and stator laminations can all be water-jetted from sheet, and the boss and housings (the housing is in two identical halves) are all designed to be machined from one side using a 3-axis CNC mill.

Thoughts/comments?

Chris


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## shortbus (Sep 27, 2011)

The more usual way of doing the rotor/stator poles is, the stator has more poles than the rotor. They are usually listed with the stator number first, like 6/4, 6 stator poles 4 rotor poles. 

Since a SRM makes torque by moving the rotor from the unaligned position to an aligned position of the poles. Then repeating for a whole revolution. Cutting a stator and rotor out of paper and the rotating the rotor pole by pole is an easy way to get a better visual idea if a design does this. Or at least for me. and paper and scissors are cheaper than metal and machining. 

Using a higher number of poles, without changing the number of aligned poles, does not make more torque in a motor. Just makes it smoother rotating.

cary


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## shortbus (Sep 27, 2011)

PStechPaul said:


> http://endless-sphere.com/forums/viewtopic.php?f=30&t=30061&p=434496#p434496
> 
> I wonder if an SRM would work better using repulsion? But I can't figure out just how to make it work. A piece of iron basically forms magnetic poles that attract the electromagnet that creates the field, so this would need to be somehow reversed. There is a repulsion motor but it uses brushes and commutator


But a repulsion motor requires both rotor windings and brushes, which add to the cost of the motor. When you add the brushes and windings in, you would get more torque and efficiency from a brushed DC motor.

cary


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## iti_uk (Oct 24, 2011)

Cool, thanks for the reply.



shortbus said:


> The more usual way of doing the rotor/stator poles is, the stator has more poles than the rotor. They are usually listed with the stator number first, like 6/4, 6 stator poles 4 rotor poles.


Agreed. See my earlier posts, this was the direction my original design was going in. However, I believe by swapping the poles the result is simplified pole wiring with the same nature as if the poles were in the more usual arrangement. I think I've spotted a trick and I'm just trying to develop it...



shortbus said:


> Since a SRM makes torque by moving the rotor from the unaligned position to an aligned position of the poles. Then repeating for a whole revolution.


Correct. This is the nature of the pole combination I have posted.



shortbus said:


> Cutting a stator and rotor out of paper and the rotating the rotor pole by pole is an easy way to get a better visual idea if a design does this. Or at least for me. and paper and scissors are cheaper than metal and machining.


You can do that in CAD - I'm a CAE analyst by trade and I've been doing this on Catia (V5). I do sometimes print out a screenshot to do quick scribbles, but it's easier to rotate the CAD model on the screen to check the overlaps. I can confirm that the 20/22 design overlaps, pulling the rotor around as the phases progress.



shortbus said:


> Using a higher number of poles, without changing the number of aligned poles, does not make more torque in a motor. Just makes it smoother rotating.
> 
> cary


Yeah, my aim with this design is smoothness. Cogging is a major problem with SRMs being used as motors for vehicles (as far as I am aware). This is an attempt to address that problem, allowing SRMs to become a more realistic option. Torque is of course important, but this is just a proof-of-concept prototype.

I have doubled the number of poles from the minimum of 5 pole pairs (short flux path arrangement) so that the motor is symmetrical across the rotor. This is in an attempt to minimize vibration caused by pole attraction occurring on only one side of the rotor.

I've indicated the pole pairs in the screenshot to try and make what I'm suggesting a bit clearer. Pairs of pole pairs are coloured and labelled A to E.

Chris


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## PStechPaul (May 1, 2012)

So from the screenshot you would energize:

ABC
BC
BCD
CD
CDE
DE
DEA
EA
EAB
AB









Which is ten states for 1/2 revolution, 20 states per revolution. 3600 RPM is 60 RPS *20 = 1200 Hz. But I see that one more state may be needed for a full 1/2 turn, so that is 1320 Hz, or 367 Hz/1000 RPM. OK, that agrees with what you stated.

I wonder, though, if there would be more torque from a motor with more utilization of pole pieces and a larger percentage of the area being supplied with the magnetic field. It seems you have either four or six poles activated at any time, out of 20 total. That is a utilization of 20% and 30%.

In the design I was considering, with six pole stator and four pole rotor, either all six or four will be energized, which is either 100% or 67% utilization (ignoring the actual surfaces that align).

I think that is a critical factor in the design. If you think of it as holding torque, it should be proportional to the surface area of the mating pole pieces. But the force under that condition is radial, and does not contribute to angular rotational force. Perhaps the angular force can be determined by the angle of the misalignment and the area through which that angular magnetic field passes. It will be a vector force where the angular force increases with misalignment but the total force is reduced because of the subsequent reduction of surface area. It seems complicated, and it's making my head hurt!  

When I get back to work on this project, I will have to energize one or more pole pairs and then turn the rotor while measuring the torque at various angles. Then I might be able to determine a specific amount of current at each angle to produce a constant torque. I need to be able to see and feel this motor in operation.


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## iti_uk (Oct 24, 2011)

(By the way, apologies for dissecting quotes as I am doing here; it not meant in a rude way, I just find it easier to address points properly in this way. )



PStechPaul said:


> ...It seems you have either four or six poles activated at any time, out of 20 total. That is a utilization of 20% and 30%.


Almost. It would be 2 or 3 pairs of pole pairs, so 4 or 6 out of 10 pole pairs, or 8 or 12 poles out of 20. (40% and 60%).



PStechPaul said:


> In the design I was considering, with six pole stator and four pole rotor, either all six or four will be energized, which is either 100% or 67% utilization (ignoring the actual surfaces that align).


That sounds pretty interesting. Do you have a visual representation of the setup (or have you previously posted one?)?



PStechPaul said:


> I think that is a critical factor in the design. If you think of it as holding torque, it should be proportional to the surface area of the mating pole pieces. But the force under that condition is radial, and does not contribute to angular rotational force. Perhaps the angular force can be determined by the angle of the misalignment and the area through which that angular magnetic field passes. It will be a vector force where the angular force increases with misalignment but the total force is reduced because of the subsequent reduction of surface area. It seems complicated, and it's making my head hurt!


Absolutely, it's a complicated calculation. I would expect that it's the increase in the "resistance" to the flux across the misaligned poles which would drop the tangential force acting on the pole. As the poles come into alignment, the flux becomes less "resisted" and the tangential force increases, hence the cogging associated with SRMs.



PStechPaul said:


> When I get back to work on this project, I will have to energize one or more pole pairs and then turn the rotor while measuring the torque at various angles. Then I might be able to determine a specific amount of current at each angle to produce a constant torque. I need to be able to see and feel this motor in operation.


I'd be fascinated to see what you find.

I'm not confident that torque could be perfectly constant, but I would like us to keep in mind a comparison to internal combustion engines. Even a silky V12 will be producing only one "event" every 60 degrees of crank motion, with a non-constant moment acting on the crank caused by the non-linear burn. With that in mind, I don't think motors need perfectly smooth torque to obtain acceptable levels of NVH in vehicles, although something approaching "perfect smoothness" would be a very good selling point to those of us who are swayed more by comfort than environmental or financial concerns.

Chris


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## PStechPaul (May 1, 2012)

No apologies needed. I tend to bring up many points for discussion, and as you have explained, some of them may not be correct, or need further clarification and research.

I showed illustrations of my concepts on the first page:
http://www.diyelectriccar.com/forums/showthread.php/switched-reluctance-motor-77483.html

But I'll show them here for convenience:



























I think the design of the first image may be better, and could be scaled to add more rotor and stator poles for smoother operation.

[edit] One problem I see with these designs is that, in each position, there is a rather wide range of angular position where the aligned area is the same so the force will be the same, so the position will not be stable. I think the surfaces of the mating pole pieces need to be equal so any angular change will result in a change of tangential force.


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## shortbus (Sep 27, 2011)

Hate to be a downer, but the definition of a SRM is a "doubly salient, singularly excited motor". When a rotor pole is over half way aligned with a stator pole, the torque and rotation will be minimal. It's the completely unaligned to aligned pole movement that makes the most torque.

Here is a motor similar to the one you show Paul - http://www.google.com/url?sa=t&rct=..._M3lfhZ6S2u0NrTpg3QyA&bvm=bv.1357700187,d.aWM


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## PStechPaul (May 1, 2012)

That's very helpful, and clearly illustrates the principle of a definite change in torque for each degree of angular position. I'm not sure about my thought that the torque may be proportional to the aligned surface areas of the stator/rotor poles. It may produce more radial force, but that does not contribute to torque. 

I'll have to read that article more carefully and tweak my design accordingly. 

But right now I'm distracted by the Ravens/Broncos game. I'm not much of a football fan, but since Baltimore is in it, and it seems to be a good game, so I think I'll watch the second half and not try to do two things at once.


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## PStechPaul (May 1, 2012)

I just found a pretty good resource showing many motor topologies, mostly for a wheel motor but useful for any design:
http://robotics.ee.uwa.edu.au/theses/2012-REV-InWheelMotor-Hooper-ME.pdf


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## PStechPaul (May 1, 2012)

Still watching the game, in the second overtime period. But here is an idea I had for using slit toroids for a short flux design:










[edit] *Ravens WIN!!!*


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## Salty9 (Jul 13, 2009)

Good well written citation Paul. That vernier control is going to require some study. I liked the game outcome too.


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## iti_uk (Oct 24, 2011)

PStechPaul said:


> I showed illustrations of my concepts on the first page:
> http://www.diyelectriccar.com/forums/showthread.php/switched-reluctance-motor-77483.html
> 
> But I'll show them here for convenience:
> ...


The problem I see with the first image is the proximity of the N and S poles, when all 6 poles are energised, there's barely any gap at all. I'd be interested to see an FEA plot of the flux path I this setup - I suspect a large amount of flux would escape directly between the stator poles and avoid the rotor altogether.

In the diagram in your last post, it's interesting to see a 6/7 setup being suggested, which is the angle I'm trying to explore  

Since I'm starting to think seriously about making a real life prototype, does anyone have any suggestions as to where I can buy lamination iron (sheets for machining)? 

I'm still going to do some FEA before I cut my first lamination, but I'd like to plan for available materials. 

Also, I'm a structural/crash analyst, and have never done electromagnet simulation - what should I be looking for in the model? Do the relevant solvers prefer solid (tetra or hex?) mesh or shell mesh, or do they disregard depth and only analyse a single cross-section? And which solvers do people use? I suspect there's something basic built into the Catia environment, but I'd prefer a dedicated solver if they're easily available. 

Chris


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## Electric Forklift Guy (Dec 13, 2012)

Has anyone considered just how LOUD a 7 kw reluctance motor would be?


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## shortbus (Sep 27, 2011)

Electric Forklift Guy said:


> Has anyone considered just how LOUD a 7 kw reluctance motor would be?


That depends on the configuration. To get a quiet SRM, you have to have more than two opposite poles energized at a time. An 12/8 three phase motor is comparatively quiet. When compared to a 6/4 at least.

The noise of a SRM is mostly from the distortion and harmonics of the stator. Making the energized poles equally spaced, at 90 degrees apart is a big help in this.

cary


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## shortbus (Sep 27, 2011)

@iti uk, there are couple of SRM design/FEA software packages out there. 

The first was developed in part by TJE Miller, one of the SRM guru's. http://www.motor-design.com/downloads/speed_software.pdf

The other is supposed to be pretty good too. http://www.infolytica.com/en/products/motorsolve/srm/

Never checked into them price wise, computer stuff like this is to hard for me to figure out.


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## iti_uk (Oct 24, 2011)

Electric Forklift Guy said:


> Has anyone considered just how LOUD a 7 kw reluctance motor would be?


This is part of what I'm trying to address with the phase overlap (and possibly with the capacitor-driven controller).

Shortbus, thanks for those links, they look like interesting leads. Looks like I've got a lot of reading ahead of me 

Chris


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## shortbus (Sep 27, 2011)

Got lots of links, if it will help. I've been thinking about these motors for a while before joining here. You guy's are the only ones showing any interest in them too. 

http://eprints.gla.ac.uk/2842/1/vibration2miller.pdf

http://users.encs.concordia.ca/~pillay/25.pdf

http://people.clarkson.edu/~nanosci/jse/B/vol0236B/jse28.pdf

http://www.google.com/url?sa=t&rct=...gMOEz6wyyP3Bv3j1G80_A&bvm=bv.1357700187,d.aWM


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## iti_uk (Oct 24, 2011)

Excellent, thanks shortbus.  The more background on this topic the better.

Chris


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## Dex (Sep 9, 2008)

I have been working with switched reluctance motors for years. 

If you want a cheap motor to play with you could us an old maytag washing machine motor.
*Maytag Neptune*​*P/N 6 2702230 Model H55BMBJL-1820*​*Used only with controller P/N 6 2702240 Ref 29491*​ 
This is a three phase machine.

The only off the shelf switched reluctance motor you could purchase one from Rocky Mountain Technologies. The Densi motor they sell is four phase.

Dex


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## devoreaux (Jan 2, 2013)

Do you know how to use the controller? How to adjust the speed and how to change the rotation of the shaft?


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## cts_casemod (Aug 23, 2012)

devoreaux said:


> Do you know how to use the controller? How to adjust the speed and how to change the rotation of the shaft?


In open loop you split some frequency and the motor folows. Its a sincronous type, as such if it is overloaded it will stall. Closed loop, the controller waits for the motor to reach a certain position before energizing the next set of coils. It can be really fast, because there is no back EMF, however, the faster it goes, the less time the magnets have to actuate, hence the torque decreases.

To change direction, the rotating magnetic field is inverted. The coils are triggered in the oposite direction


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## devoreaux (Jan 2, 2013)

I was asking precisely for the application of Maytag SRM washing machine. The controller is there but they don't know how to use it, I was asking about how to change the speed and rotation direction of that SRM using their hardware.


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## cts_casemod (Aug 23, 2012)

devoreaux said:


> I was asking precisely for the application of Maytag SRM washing machine. The controller is there but they don't know how to use it, I was asking about how to change the speed and rotation direction of that SRM using their hardware.


Cant help you on that, but my advice is to start with your own controller. BLDC controllers work in a very similar way, you basically need 3 hall sensors to get the position of the rotor and energize a given set of poles at a time, so you *might* be able to get away by using an encoder from one, if not the whole setup with minor modificatios. But there are many options like using an absolute shaft encoder or just a normal encoder with a zero point, in which your CPU counts the pulses from the encoder with a longer pulse to define zero to find the rotor position (as used on Internal combustion engines).

Its all down to your personal taste.


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## devoreaux (Jan 2, 2013)

If I wanted to use a prototype I was thinking about absolute encoder. 

Indeed, a BLDC motor and SRM look quite similar in control, but magnet is linear, iron is nonlinear and thus much more difficult to simulate etc.


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## cts_casemod (Aug 23, 2012)

devoreaux said:


> If I wanted to use a prototype I was thinking about absolute encoder.
> 
> Indeed, a BLDC motor and SRM look quite similar in control, but magnet is linear, iron is nonlinear and thus much more difficult to simulate etc.


Not really, the only difference is a SRM rotor "pulls" the stator, while a PM usually "pushes" (Same magnetic polo).

I would go with an absolute encoder if the speed was low enought and there was a need for presision, like on industrial automation.
For EV application at 6000RPM+ a 1024PPR encoder would need a very fast CPU with high bandwith inputs. We are talking about roughly 100KHz.


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## shortbus (Sep 27, 2011)

While a BLDC controller "might" work, its output is not DC. A BLDC forgetting the name for a minute, uses trapezoidal AC. Each phase switches polarity to match the rotor magnets.

A SR motor can use this wave form but it's not a necessity. It may even be a liability, changing the direction of polarity in a SRM stator could cause extra heat and eddy currents in the core of the rotor and stator.

But if you want a cheap way to play with a BLDC controller, look into the MC33035 chip. Uses hall sensors or opto-interrupters for rotor positioning.
http://www.onsemi.com/PowerSolutions/product.do?id=MC33035


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## cts_casemod (Aug 23, 2012)

Okay, who gave me negative points and pointed out I was providing inaccurate statments?

Its a bit of a shameless attitude, if someone has anything to say please post to clarify, to my knewledge I dont believe I am providing wrong information.


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## cts_casemod (Aug 23, 2012)

shortbus said:


> While a BLDC controller "might" work, its output is not DC. A BLDC forgetting the name for a minute, uses trapezoidal AC. Each phase switches polarity to match the rotor magnets.
> 
> A SR motor can use this wave form but it's not a necessity. It may even be a liability, changing the direction of polarity in a SRM stator could cause extra heat and eddy currents in the core of the rotor and stator.
> 
> ...


 
I mentioned a modified BLDC controller, to clarify the actual hardware not the chipset. The idea is to use some of the inner bits like encoder/mosfet/mosfet drivers and design a control board to suit.

As with AC, I see no problem for the thing to run using AC, obviously the core losses would increase as would the complexity of the controller, but everything else being equal and electromagnet works both in AC and DC.


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## shortbus (Sep 27, 2011)

cts_casemod said:


> I mentioned a modified BLDC controller, to clarify the actual hardware not the chipset. The idea is to use some of the inner bits like encoder/mosfet/mosfet drivers and design a control board to suit.
> 
> As with AC, I see no problem for the thing to run using AC, obviously the core losses would increase as would the complexity of the controller, but everything else being equal and electromagnet works both in AC and DC.


The only thing I can see is that straight DC is easier to use for a SRM. Changing polarity on an inductor is an extra load on the mosfets/IGBTs, meaning extra heat and switch device size. With straight DC, diodes and capacitors can retain some of the voltage from the stator coils (inductors) when switching phases.

The other problem with a BLDC controller is most BLDCs are wired Delta, and SRMs are star.


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## sirwattsalot (Aug 27, 2012)

The common series wound motor will draw, or pass more current when under load. This is why a motor rated at 90% efficiency may be 1% efficient when over loaded. Any motor that works in the reverse principle and draws less current when loaded would be more efficient at any speed and load. How does the reluctance motor behave at different speeds and loads? The only motors that I know of that do not lose efficiency under load are hydraulic and piston driven, powered by compressed air since they restrict the flow from the supply when loaded.


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## EVfun (Mar 14, 2010)

sirwattsalot said:


> The common series wound motor will draw, or pass more current when under load. This is why a motor rated at 90% efficiency may be 1% efficient when over loaded. Any motor that works in the reverse principle and draws less current when loaded would be more efficient at any speed and load. How does the reluctance motor behave at different speeds and loads? The only motors that I know of that do not lose efficiency under load are hydraulic and piston driven, powered by compressed air since they restrict the flow from the supply when loaded.


What? A common series motor will draw more current when overloaded and therefore make more torque. RPM will drop some, but input watts and motor power will both increase. Efficiency will drop some, but even the drag racers rarely pull them below 50% efficiency, and then only briefly when the current is upwards of 2000 amps. Most street EV series motors stay between 70% and 90% efficiency at all but the lowest rpms (all motors are 0% efficient at 0 rpm.) Motor type isn't going to make a dramatic impact on range.


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## cts_casemod (Aug 23, 2012)

shortbus said:


> The only thing I can see is that straight DC is easier to use for a SRM. Changing polarity on an inductor is an extra load on the mosfets/IGBTs, meaning extra heat and switch device size. With straight DC, diodes and capacitors can retain some of the voltage from the stator coils (inductors) when switching phases.
> 
> The other problem with a BLDC controller is most BLDCs are wired Delta, and SRMs are star.


You are right, I just said it would work, not that it would be the best way to drive one. I should have clarified that.

I wouldnt drive a SMR in any. I would use a common for all 6 coils attached to VCC and 6 phases attached to a N Channel mosfet, driven by the controller. Not sure if there is a name for such connection, altought I understand that your point is probably use a 3 phase setup with the star center attached to VCC and each of the phases to the mosfet? If so, its quite similar.


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## shortbus (Sep 27, 2011)

cts_casemod said:


> You are right, I just said it would work, not that it would be the best way to drive one. I should have clarified that.
> 
> I wouldnt drive a SMR in any. I would use a common for all 6 coils attached to VCC and 6 phases attached to a N Channel mosfet, driven by the controller. Not sure if there is a name for such connection, altought I understand that your point is probably use a 3 phase setup with the star center attached to VCC and each of the phases to the mosfet? If so, its quite similar.


Yeah, In a 6 coil stator, the opposite coils are wired as pairs. Makes three phases. All of the coils have one leg tied to positive and each pair is turned on one pair at a time. The hall sensors and logic in the controller tell which pair to turn on, as the rotor poles come into position. The newer controllers are sensorless. Much harder to do though.

I think with a little work the MC33035 chip could be configured to work with a SRM though. Using the low side mosfets in stead of the H-bridge configuration. At around $3 - $4 for a chip, I'll be trying some day.


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## cts_casemod (Aug 23, 2012)

shortbus said:


> Yeah, In a 6 coil stator, the opposite coils are wired as pairs. Makes three phases. All of the coils have one leg tied to positive and each pair is turned on one pair at a time. The hall sensors and logic in the controller tell which pair to turn on, as the rotor poles come into position. The newer controllers are sensorless. Much harder to do though.
> 
> I think with a little work the MC33035 chip could be configured to work with a SRM though. Using the low side mosfets in stead of the H-bridge configuration. At around $3 - $4 for a chip, I'll be trying some day.


The other day I was thinking in the AC debate. For sensorless it might actually help to use an alternating current, in order to detect the current. The idea is that once the Pole in the rotor is aligned with the stator, the reluctance changes and the current decreases, so it would know when to skip next phase. Quite similar to a way an AC coil in a contactor works, if you are familiar with it. Of course the extra complexity on the controller side would probably neglect the benefit of not needing an encoder.


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## shortbus (Sep 27, 2011)

cts_casemod said:


> The other day I was thinking in the AC debate. For sensorless it might actually help to use an alternating current, in order to detect the current. The idea is that once the Pole in the rotor is aligned with the stator, the reluctance changes and the current decreases, so it would know when to skip next phase. Quite similar to a way an AC coil in a contactor works, if you are familiar with it. Of course the extra complexity on the controller side would probably neglect the benefit of not needing an encoder.


That's the way the sensorless SRM inverters work. Send a low voltage AC to all phases and detect rotor position. The complexity is way above my pay grade.

The opto-iterupter is what I'm thinking of using. Easier to make a chopper wheel than to make a magnet wheel. And the detection of position should be more precise.

An encoder could be used too, but seems to me some kind of memory would need to be put into the control/motor to always keep it 'timed' right. The shaft getting turned while the system was powered down would throw the whole thing off.


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## cts_casemod (Aug 23, 2012)

shortbus said:


> That's the way the sensorless SRM inverters work. Send a low voltage AC to all phases and detect rotor position. The complexity is way above my pay grade.
> 
> The opto-iterupter is what I'm thinking of using. Easier to make a chopper wheel than to make a magnet wheel. And the detection of position should be more precise.
> 
> An encoder could be used too, but seems to me some kind of memory would need to be put into the control/motor to always keep it 'timed' right. The shaft getting turned while the system was powered down would throw the whole thing off.



What will you be using as emitter? Ir diodes might be a bit too slow depending on what speed you pretend to use. I mostly use a geared wheel with a pickup hall/magnetic sensor. The hall is easier to use and can read locked rotor.


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## shortbus (Sep 27, 2011)

cts_casemod said:


> What will you be using as emitter? Ir diodes might be a bit too slow depending on what speed you pretend to use. I mostly use a geared wheel with a pickup hall/magnetic sensor. The hall is easier to use and can read locked rotor.


Haven't got any where near that point yet. There are quite a few built in college thesis though that use the IR opto-interupters though. With no shown problems. Why can't a opto read a locked rotor? The signal from both an opto or hall is still the same?


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## cts_casemod (Aug 23, 2012)

shortbus said:


> Haven't got any where near that point yet. There are quite a few built in college thesis though that use the IR opto-interupters though. With no shown problems. Why can't a opto read a locked rotor? The signal from both an opto or hall is still the same?


I didnt said an opto couldnt read a locked rotor. Of course it can!
I said a hall can, but a (normal) magnetic pickup cant.

Its hard to say what exactly what you can use because I dont know the resolution of your shaft signal (PPR). On my inverter I use a 124PPR signal, so thats a bit over 12KHz @ 6000RPM. I decided to use magnetic, because it has a good high frequency response, relatively inexpensive, water and dust proof and easy to find in any car parts store, should I have any failure. 

This is for induction, so even at 2Hz shaft speed I have a 250Hz signal that is not much of a problem to read (Encoder works in open loop up to 6Hz)

If I was using PMDC or VR I would choose hall. But beware that even a hall has a maximum frequency response and gets quite expensive for higher frequencies. I have no idea if a normal IR sensor could read such frequency without long term issues. The main reason I wouldnt use one is because they are open type, exposed to dust, moisture, light and sitting a few mm away from the receiver. A signal up to 1KHz would be alright, if the application was intended to work inside protected from contaminants.


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## shortbus (Sep 27, 2011)

The sensors(there are three for a SRM or BLDC) will be inside the end bell, so dirt shouldn't be a problem. Don't know how you would use any of the automotive ICE type sensors on a SRM? I guess a wheel with the right number and spacing of standing 'teeth' would work. Never gave it any thought.


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## cts_casemod (Aug 23, 2012)

shortbus said:


> The sensors(there are three for a SRM or BLDC) will be inside the end bell, so dirt shouldn't be a problem. Don't know how you would use any of the automotive ICE type sensors on a SRM? I guess a wheel with the right number and spacing of standing 'teeth' would work. Never gave it any thought.


Inside the bell housing should be fine, but how will you take them off in case of failure?

I use an automotive flywheel, so, pretty much the same way an ECU knows the crank position. Reads the missing teeth on start up to define the position and then adds up.


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## shortbus (Sep 27, 2011)

I thought we were talking SRM, switched reluctance motors? The sensors in them need to be "timed" to the rotor teeth, so the stator phase is turned on and off at the correct point for each rotor/stator position. Did I missed some thing along the way?


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## cts_casemod (Aug 23, 2012)

shortbus said:


> I thought we were talking SRM, switched reluctance motors? The sensors in them need to be "timed" to the rotor teeth, so the stator phase is turned on and off at the correct point for each rotor/stator position. Did I missed some thing along the way?


You asked me how I achieved the same results with automotive sensors.

If using one proximity:
Using a reference to know the position and adding a certain angle by each new teeth. 

If using 3 sensors:
Just like you do with IR. With the added advantage that an automotive hall gives you a clean 0 - 1 digital output. No filtering to be done.

PS: Only Automotive halls can be used for SR. Automotive magnetic pickups only read above a certain frequency.


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## shortbus (Sep 27, 2011)

How did you get your flywheel teeth in time with the rotor teeth? Any torsional distortion in the motor shaft from the flywheel?


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## cts_casemod (Aug 23, 2012)

shortbus said:


> How did you get your flywheel teeth in time with the rotor teeth? Any torsional distortion in the motor shaft from the flywheel?


Not sure I understand your question, torsional distortion? My shaft is 38mm, it is rock solid.

The motor shaft is key'ed. I only need the CPU to know that when it hits the zero (mark on the flywheel) the motor is at x angle.


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## shortbus (Sep 27, 2011)

Again I think there is something I missed.  This thread is about Switched Reluctance Motors. For them to work there needs to be more than one sensor and a "X" (don't know what that means) angle.


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## PStechPaul (May 1, 2012)

I finally ordered and received some materials to use for a version of the SRM. I took apart a small fan motor a while ago and I tried to machine the rotor with not so good results. The rotor is 2.75" diameter and I found some washers of that size. The ID is 1.25" so I also got some 1.25" steel rod, which fits pretty well. So I need to fasten several washers together to get the thickness I need, and then machine the edges to obtain four lobes. I'm not sure exactly how to do that, but probably a 0.5" to 0.75" end mill would do the job. If all else fails I can drill four holes and then use a saw to cut the slots. Then I would probably need to clean up the cuts with a mill.

Then I would need to cut the 1.25" rod to the same length as the thickness of the washers. I will have to drill a 0.500" hole in the center for the shaft. I think I can drill several holes around the periphery, drill and tap, and insert screws. For the shaft, it could be knurled like the original one. Actually I might even be able to use the old shaft. Here's what I have:


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## jlewis (Oct 13, 2013)

Just to test out the function and controller, you may consider just making a one-piece solid rotor. The losses will be very high, but you can play with the phase current and angles and get a better understanding of how the motor works. Then you could move to a laminated rotor. It will much easier to machine a solid block, assuming you can find the material for the right cost. I'm sure the washer method would be cheap but may ultimately be a big headache. If you can find the right place, you may get a reasonable cost for steel lamination cutting, laser or waterjet. The places that do dedicated motor lamination fab usually charge crazy prices because you'll get a near perfect stack, but some steel fab shops will have the equipment to at least cut some thick laminations. If you go to all that trouble, keep in mind you'll want to either oxidize or coat them to effectively insulate between each lamination.

I know I'm jumping around with all these suggestions, I've just seen these threads on SR motors from time to time and I work at a company that develops SR motors and always wanted to participate in discussions like these. I'm a technician, not an engineer, so I'm not the expert on simulation and design, but understand the practical aspects of assembling and running the motor. A lot of the information on SRs is available in textbooks, some good sources have already been linked, such as TJE Miller. Some of the books are very expensive because there's such a limited audience. Maybe I can help in ways, but I do have to be mindful of my employer as well. SR is very much a developing emerging technology still and it's cool to see people interested in it. As it's not very widespread, most developers are very protective of their tricks to make SR motors quieter and more efficient.

I wanted to offer up that introduction, as I found this thread but haven't looked around this site in general really, but don't want to take the focus off of the thread starter and maybe I can be more frequent around here to see what everyone else has going on as well. Looking forward to seeing your progress here.

- John


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## cts_casemod (Aug 23, 2012)

Paul, how are you getting along with this?



jlewis said:


> Just to test out the function and controller, you may consider just making a one-piece solid rotor. The losses will be very high, but you can play with the phase current and angles and get a better understanding of how the motor works.
> 
> - John


John,

There are many possible reasons to make a laminated rotor on a SRM, but losses wont be one of them. There is no EMF generated back on the rotor which is driven with DC. In fact even the stator could me made using a solid core if the polarity used is always the same.

If you remember those first DC motors (not universal DC motors) used to be built like that and many are still in operation today in older trains. An examples is Germany railway that still use 16⅔ Hz mains to avoid excessive losses and arcing.


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## jlewis (Oct 13, 2013)

I will have to talk to one of our engineers and I'll see if I can find some literature reference for you. I can assure you that the steel will become a very good heater at higher flux densities if eddy losses are not addressed. I will back that up as soon as I can. Every SR that I've seen in production today has a laminated stator and rotor. The flux still passes through the rotor and thus there are eddy currents.

- John

Edit: Eddy and hysteresis losses are what I should have said. Because of flux reversal in the rotor, i.e. you are pushing flux one way through the rotor and then as it rotates 180 deg, the flux will be pushed in the opposite direction, because the stator poles will always be energized with the same polarity. In some machines this is not the case, and polarity does matter in order to decrease flux reversal. Miller's paper talks about core losses only in a few places. The paper by Wichert is very in-depth and talks about core losses a bit, see Chapter 3. But really eddy losses are the thing being addressed by laminating. Without lamination, the rotor becomes an electromagnet in itself. Remember that though there is no EMF being generated inherently by the rotor because it's a lump of steel, once you create a field in it via the phase winding, it now behaves much like a magnet. This is why it's possible to generate electricity with an SR machine.


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## PStechPaul (May 1, 2012)

This project is on hold for now, as I want to finish my tractor project before the weather and daylight become major issue. The SRM may be a winter project. I recently got some mill holders and better lathe tools which should help with the machining. I think the washers will be a reasonable compromise between a solid block of iron and full laminations. If necessary I could cut slots in the washers with a bandsaw. Also the washers will allow me to experiment with rotor skew, and they are cheaper than a steel rod which would need much more machining.


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## jlewis (Oct 13, 2013)

This is a paper I really like on a minimalistic way of using an optointerrupter type sensor (or multiple in this case) for motor commutation. I've seen really good results in using optointerrupters, if you have the ability to make the interrupter wheel. It's a fairly easy way to do commutation with a digital signal. It may help you when you get around to it. http://www.jpe.or.kr/archives/view_articles.asp?seq=141

It is, as others have said, the same signal as a hall effect sensor, but of course you can't use a hall without some kind of magnet.


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## tylerwatts (Feb 9, 2012)

Paul. 
Winter's here. I hope you got your tractor together. What is it looking like for progressing this srm project? I'm keen on this design myself and have a few motors I might try building an srm rotor for one to compare performance. The control worries me as that is out of my comfort zone.


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## PStechPaul (May 1, 2012)

As usual, I bite off more than I can chew, and I keep getting sidetracked on other projects that never quite get past initial design and concepts. I got "stuck" on the tractor project trying to make an effective brake, which requires machining and materials which I finally ordered and received. But now it is deep into winter and the tractor will have to wait until the Spring.

I also got into the idea of making my own DC-DC converter and I got some ferrite cores and bobbins and have had some success with that, but I still need to make some PC boards, and I tried to add a BMS as well as other functions and once again got bogged down...

When I started into some of the machining aspects of the tractor project I got involved in a lively forum of home shop machinists, and it has proven fascinating as well as distracting. But as I learned more about machine shop practices and some cool projects I started thinking more about electric motors. One guy has made working models of some gas engines, and I thought I might try something similar with electric motors. I started with an idea to make a tiny 3 phase induction motor, and then I decided that perhaps the SRM would be even more unique and potentially practical. So, here are my threads "over there":
http://bbs.homeshopmachinist.net/threads/62003-Miniature-Switched-Reluctance-Motor-(SRM)
http://bbs.homeshopmachinist.net/threads/61983-Punch-and-die-for-thin-matal-(motor-laminations)
http://bbs.homeshopmachinist.net/th...-model-three-phase-AC-induction-motor-project

I had another look at my video of the SRM I was playing with and I am rather impressed that it worked at all without a position sensor. I think my miniature motor may be easier to work with and once I make some progress I can upsize with more confidence. Here are some images of my ideas and progress so far:



















Thanks for the kick in the butt!


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## tylerwatts (Feb 9, 2012)

Wow that tiny motor looks cool. What processor do you plan to use for control of the inverter? Is my understanding correct that you need an IGBT/Mosfet switch for each coil in the motor to properly time field position and polarity? That is to keep a field moving ahead of each rotor path for optimum continuous work? 

Just looking at your picture, will the shaft complete the magnetic path through the centre of your rotor? Can magnetic fields cross each other like this? Ie can the rotor have 2 magnetic paths running simultaneously in your motor? My mind returns to images of ABB SRM motor images showing strange lamination shapes and flux paths.


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## PStechPaul (May 1, 2012)

I'm still trying to wrap my (now frozen) brain around the SRM concept and the RSM (as introduced by ABB) which apparently can use existing VFD controllers. But as I understand it the SRM is basically a set of electromagnets and soft steel bars that are attracted sequentially to the magnets. Thus the polarity of the magnet is not an issue, and even AC could be used (although perhaps with a shading coil).

But with the little SRM I plan to build, I may need to control the polarity of the stator poles. Going back to the illustration of rotor position and stator energization:










Those images show the magnetic field passing through the center of the rotor to the opposite pole on the stator. But it may be simpler to think of each stator pole as an electromagnet exerting force on the nearest rotor pole. But there will always be what might be termed "salient" or "induced" poles and magnetic lines of force throughout the assembly, and it will tend to follow the path of least magnetic resistance to create a closed circuit. The force will increase as the gap decreases, but when two pole pieces are aligned, the force will be radial, rather than tangential which is needed for rotation. 

It is much like a stepper motor and it will have considerable holding torque when the pole pieces are aligned as at the 30 degree steps. I show two adjacent stator poles energized at the same polarity at 15 degrees, which should draw the rotor into alignment as shown, and the opposite end will also do the same.

But using the concept of short magnetic paths, adjacent stator poles could be energized with opposite polarity to create N and S poles, which would travel through two adjacent pole pieces of the rotor and not through the center. This may develop more torque, and it seems that the rotor design of the ABB RSM shows this as well, with the semicircular iron pieces that look like cut and inverted toroid sections. This makes it similar to my cut toroid design:










Here are some course lectures which include the SRM and RSM:
http://www.youtube.com/watch?v=AzjrpZtAaBo&list=PLuEu1smhK2DHJgb8sTiku3W8XmUSllbMn

As soon as my brain and fingers thaw out, maybe I'll do more work on this.


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## tylerwatts (Feb 9, 2012)

Cool. I like the short path design. Not sure how existing vfds could be used but I'll catch up on the video. 

Hope you are not caught in the north USA freeze! Good excuse at least to lock up in the workshop with the heater on and build a motor


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## PStechPaul (May 1, 2012)

Went down to about 4F last night. Warmed up to 18 today. Now back to about 12. Mid twenties tomorrow, and a chance for 40s to 50 by the weekend. 

I wrapped 33 turns of #24 magnet wire around one stator pole, and measured the inductance:

54 uH at 120 Hz, 43 uH at 1 kHz, and 26 uH at 10 kHz

With the stator in place the readings were:

84 uH, 51 uH, 27 uH

I'm not sure what that tells me, except perhaps that the relatively hard cold rolled steel used for washers does not have good magnetic properties at higher frequencies. When I put 5 turns or so on a steel washer the inductance at 10 kHz was virtually the same as with an air core with the same dimensions.

I put 4 amps (at 0.7 volts) through the stator coil and it exerts a pretty strong pull on the rotor, but it did not seem to develop much torque. I think the gaps will need to be larger, or possibly energizing two adjacent poles may align the rotor between them. The first SRM I built and showed in the video has rather small stator pole pieces and the rotor seemed to center itself on them. Well, at least with a tiny model like this it won't take much effort or material to try something different.


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## tylerwatts (Feb 9, 2012)

I expect you need opposite stator poles Paul, to generate a magnetic path to flow through the rotor and develop the torque we expect. Sounds good so far though. I'm looking at RSM for retrofit mods to my AC motors.


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## PStechPaul (May 1, 2012)

I just finished the concept drawings for this MiniSRM. I agree that there needs to be excitation of adjacent pole pieces with opposite polarity to create a N-S pole.









I plan to use 1/4" polycarbonate end bells with 6x10x3 mm ball bearings. Now I need to do some machining work.


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## tylerwatts (Feb 9, 2012)

That looks good Paul. I'm keen on the arrangement where one pole is coming into alignment as the other leaves. I wonder whether some motors use odd numbers. Of pairs for rotor a stator such that there's always a number of poles aligning to thus combat cogging issues.


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## PStechPaul (May 1, 2012)

I have been hearing the term "saliency" and "salient poles" in regards to SRM and RSM motors, and I found a pretty good simple explanation:

http://www.coilgun.info/theorymath/saliencyratio.htm

After watching more of the video I posted earlier, I understand a little more about the difference between the SRM and RSM designs. I think I will need to modify the stator and rotor for smaller pole faces to get more rotational force, which is probably equivalent to having a higher saliency ratio.


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## aeroscott (Jan 5, 2008)

PStechPaul said:


> I have been hearing the term "saliency" and "salient poles" in regards to SRM and RSM motors, and I found a pretty good simple explanation:
> 
> http://www.coilgun.info/theorymath/saliencyratio.htm
> 
> I just a looked at the vid, you can see the SRM VFD can't shoot through(both igbt's on at the same time) giving time to shut down without blowing things. near end of vid.


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## tylerwatts (Feb 9, 2012)

Scott I don't understand your post. What are you saying please? I. Didn't fully understand the video in that link tbh.


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## aeroscott (Jan 5, 2008)

tylerwatts said:


> Scott I don't understand your post. What are you saying please? I. Didn't fully understand the video in that link tbh.


 Almost at the end of the vid. they show the layout for SRM and standard VFD . If you turned on all the igbt's at the same time , the only path is through the motor coil.


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## aeroscott (Jan 5, 2008)

I had to run. In a standard vfd , 1 phase 1of the 2 igbt's stay on and the other comes on , it's a dead short between the igbt's . No protection is possible . In the srm vfd the motor winding are in between the igbt's . It's very hard to see the motor windings in the schematic . thanks for asking.


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## aeroscott (Jan 5, 2008)

The other advantage is srm's run on square waves meaning 1/10 the on /off cycles of sine wave vfd's . After talking to LTI about srm's, I Goggled srm and found a mill. contractor showing srm motor being done for a race car , 150 lbs. 20,000 rpm 750 continuous hp . next time(about 2010) I looked, link was gone . Doe
s this sound like the Nissan 750 hp ev that ran "the ring" added; may have been Toyota


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## Tesseract (Sep 27, 2008)

aeroscott said:


> Almost at the end of the vid. they show the layout for SRM and standard VFD . If you turned on all the igbt's at the same time , the only path is through the motor coil.


Huh? If you turn on *all six* of the IGBTs in a standard VFD you get three hard shorts across the DC link, regardless of how the motor is connected to the VFD.


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## aeroscott (Jan 5, 2008)

Yes, I should of made that clear . thanks
added ; In the srm model shown , dc bus connects threw motor with igbt's on .


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## aeroscott (Jan 5, 2008)

Tesseract said:


> Huh? If you turn on *all six* of the IGBTs in a standard VFD you get three hard shorts across the DC link, regardless of how the motor is connected to the VFD.


The srm motor did not use the standard vfd .


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## Tesseract (Sep 27, 2008)

Deleted because I really don't feel like arguing on the internet.


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## aeroscott (Jan 5, 2008)

aeroscott said:


> The srm motor did not use the standard vfd .


 The video in post 128 is what was referenced . Other vfd's can be used but this particular one has the no shoot through advantage.


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## dcb (Dec 5, 2009)

Of course, I start looking at srm and it is already being discussed  Yah yah, zombie thread, whatev.



PStechPaul said:


> 54 uH at 120 Hz, 43 uH at 1 kHz, and 26 uH at 10 kHz
> 
> With the stator in place the readings were:
> 
> 84 uH, 51 uH, 27 uH


As you have probably guessed, it means your Ld/Lq isn't going to be so good. A guy was trying discussing rotors (short path in the rotor) for induction motor stators (no rewinding, using standard 3 phase inverters) and concluded you need an Ld/Lq of at least .7 to get a power factor of .8 which puts you in the ballpark with induction motors. 

Anyway, I pretty much watched the whole series:
https://www.youtube.com/watch?v=Aa-QY5T0_8w
He calls the unmodified ACIM stator/inverter (except software) approach RSM, as opposed to SRM. With rather complicated rotors though (short path in the rotor).

But it leads to the question, what is the possibility of stamping out srm rotor plates for a brushed DC machine with the typical large 4 poles? using an existing 3 phase driver section, can you get useable startup with a 2 phase setup and rotor pole assymetry (do you need 4 phase?) Where does all the flux go if it is a 3 pole rotor, what about a 2 pole? 
basically, is it possible and remotely practical, i.e. for a small motor or even a warp9? As a starting point? With the full bridge 3 phase inverter (and losing the brushes/commutator of course)?

(note, this design allows shoot through, so don't shoot through, duh)

edit, I'm ok with unidirectional (one sided air teeth/snail curl/?), though it isn't for everyone of course (car w/transmission or direct drive motorcycle should be fine). Getting into the forward regen quadrant would be nice, even if bumpy.









Also do you guys recommend any open source tools for modelling magnetic flux with various shapes and gaps and etc? (edit: started looking at FEMM, let me know if there is a better option)


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## cts_casemod (Aug 23, 2012)

I would not be very worried with PF. Essentially the inverter is a buck converter so if one doesn't change the polarity (and rather energize the coils within a given order, with DC) this is not a problem at all, ie, CCM.

It also avoids the need for laminations. The rotor can be machined from a solid block of iron.


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## dcb (Dec 5, 2009)

Yah that doesn't sound quite right from the rotor perspective, especially when re-purposing an induction or dc motor stator, there will certainly be a pulsing flux (like ripple=ac losses) and lams don't bother me too much (though they may yet). Certainly there are many options for creating them with some precision and pressing them onto a shaft for the diy'er, in any configuration really, including salient vs flux carrier/flux barrier (lower aero drag at high rpm).

there are shops that laser cut lams, i.e.
http://www.polarislaserlaminations.com/laser-cut-laminations.html or http://www.lcscompany.com/electric-motor-laminations.html

i.e. for this (from the video) rotor which is the style abb uses which can repurpose (without rewinding) an induction stator apparently and inverter. Can't imagine how to make it diy without laminations really.

lower right first attachment are radial laminations, harder to make I'm sure, but no leakage in support webbing (though will see more eddy currents according to the lecture).


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## dcb (Dec 5, 2009)

But my question is on the repurposing of a DC 4 pole motor stator using 2 phases (and a 3 phase inverter topology) and a 2 or 3 pole rotor. i.e. what is wrong, if anything, with this picture? And do you need a custom inverter topology to maintain good efficiency? assymetrical rotor poles so they can self start. Are the extra wide dc stator poles a deal breaker for instance? Seems like it would be murder on the Ld/Lq if the next phase already has rotor iron under it when it fires.









Admittedly I've still a lot of head scratching to do, and there are plenty of perfectly useable as-is motors out there, so this is very much academic.


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## cts_casemod (Aug 23, 2012)

What I have on my mind is to re-purpose the motor with a suitable inverter and a new stator, rather than using the original as that will always tend to generate it's own "induction" field, which again may be convenient if one plans to make it start across the mains, but for us EVer's there's not much point in doing so. It will negatively impact efficiency. The point in going SR is so that the motor can operate synchronously.

I may try that using a car alternator with some hall sensors, which again is a more wise choice as the stator can remain unchanged if you wish so (only the rotor needs to be machined (for testing) or reconstructed (for final use at full torque).

Paul, did you finish your prototype?


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## cts_casemod (Aug 23, 2012)

dcb said:


> But my question is on the repurposing of a DC 4 pole motor stator using 2 phases (and a 3 phase inverter topology) and a 2 or 3 pole rotor. i.e. what is wrong, if anything, with this picture? And do you need a custom inverter topology to maintain good efficiency? assymetrical rotor poles so they can self start. Are the extra wide dc stator poles a deal breaker for instance? Seems like it would be murder on the Ld/Lq if the next phase already has rotor iron under it when it fires.
> 
> 
> 
> ...


The problem I see with that topology is torque ripple. Even if one were to assume 4 poles that's quite a large distance between them, since DC motors normally have 'flat' poles. What happens if the motor stops in between? One might not have enough torque to restart, due to the large distance.

Induction motors, on the other hand are composed of 36 slots. It's much easier to machine a rotor, also with 36 slots, and energyse/de-energise the stator coils at the right time to 'pull' the rotor (or push to regenerate). The torque will be pulsating, but at 360/36 = 10degree steps, the motor inertia tends to smooth the output very well (just like PWM in an electrical circuit).


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