# High rpm DC motor?



## PStechPaul (May 1, 2012)

I doubt that there is any new mechanical design that would greatly reduce brush wear and/or improve operation at higher speeds without becoming much more complex and expensive than time-proven designs.

But it might be possible to replace the brushes and commutator with a system to use high frequency electromagnetic coupling of current into the rotor (armature), rectify it, and change polarity to derive maximum rotational torque from the field of the stator, which could be permanent magnet or electromagnet.

There may be other ways to achieve more efficient commutation by using LEDs and photoelectric sensors (maybe even LASCRs), or perhaps inductive proximity sensors or reed switches. 

It might be fun to try some of these ideas on a small scale, but I don't think it will be possible to achieve better performance or lower cost or higher efficiency and longevity, compared to ACIMs or BLDCs or even synchronous motors which use slip rings to produce a magnetic field in the rotor while using AC on the stator. 

Remember that most DC motors are really AC where the alternating and rotating field is created mechanically. The only true DC motor is the homopolar design, but it is generally impractical for any significant power:
http://en.wikipedia.org/wiki/Homopolar_motor


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## Karter2 (Nov 17, 2011)

tylerwatts said:


> Hello all. I have been postulating the design of a high rpm DC motor. Brush arcing is the biggest limiter from what I've read so I was trying to think of a better 'brush' design.


 how high rpm ?
What size motor ?
I know of brushed motors that run at 250,000 rpm ! ...but they are small .


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## tylerwatts (Feb 9, 2012)

Paul,
forgive me but you're speaking a different language to what I understand. I want to replace the 'flat' friction surfaces of brushes with a continuously contacting roller that is in contact but not rubbing to create friction and hopefully not as susceptible to bounce and arcing.

I want to keep the mechanical timing of the rotor as a feature for simplicity but attain high rpm with sustained power. Asking for too much maybe but someone has to or we'd never make progress!

Karter
I am looking for around 12000rpm and 30-40kW continuous in this application but would hope it could be scalable in terms of power. No need for more rpm than this.


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## PStechPaul (May 1, 2012)

This might be something like your idea:
https://sites.google.com/site/alternativeworldenergy/roller-bearing-motor-commutators

The problems are that a roller makes only a small area of contact with the commutator, and it is difficult to conduct much current through the bearings of a rolling wheel brush. 

The usual carbon brushes wear so that they fit the commutator well and provide a lot of surface area for current. They also overlap two or more segments, but if the armature has a large number of poles, there will not be a large and abrupt change of current and thus less arcing than a motor with only a few poles.


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## tylerwatts (Feb 9, 2012)

Actually Paul the rollers would be where the brush is and the commutator would be unchanged. So you would have a smaller contact area but of the motor is designed right the can be minimised.

Do you see this not working? I'll have to get a small motor to try it on. I was this ok bring exotically of using solid self-lubricating ceramic bearings/bushings that have bear perfect contact with their 'axle'and the commutator for efficient conduction of current. I'll try with some small rc bearings first but scaling down is difficult as a large motor has more room for decent rollers. When I get time (if ever it would be soon) I'll let you all know how it works out. 

Night all...


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## Karter2 (Nov 17, 2011)

> using solid self-lubricating ceramic bearings/bushings that have bear perfect contact with their 'axle'and the commutator for efficient conduction of current.


 Are you not just moving the problem from the commutator to the "Brush /roller bearing area ??
..and you have also created a "point contact" area issue of roller to commutator.


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## mizlplix (May 1, 2011)

I vote for Paul's optical idea.

That sounds worthy.

Miz


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## Karter2 (Nov 17, 2011)

What is wrong with the existing "Brushless PM DC" motor designs ??


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## tylerwatts (Feb 9, 2012)

The point contact would be a current limiter but that can be designed for. I don't have a problem with PMAC motors. This design would be a simple version where the benefit of permanent magnets is available and the controller design and control would be simplified also, which should hopefully help both cost and DIY build/setup.

Those are my thoughts anyway. I'm proposing the idea and asking both is it possible and what do people think of it.


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## PStechPaul (May 1, 2012)

I think any major improvement over conventional brushes for a DC motor will require some electronics, and in that case it will become essentially a BLDC or PMAC motor. It may be possible to embed the electronics in the motor so that you can use a conventional PWM DC motor drive, but I doubt it will be any real advantage.

Here are some links I found, but really nothing new:
http://www2.mae.ufl.edu/designlab/Class%20Projects/Background%20Information/Electric%20DC%20motors.htm
http://www.electrical4u.com/constru...field-winding-commutator-brushes-of-dc-motor/
http://www.reliance.com/mtr/cmdcbr.htm (much detailed information on brushes and commutators)
http://www.google.com/patents/US5276372 (reciprocating electric motor)

If you want to pursue some novel mechanical brush designs, you may want to consider brushes with laminated copper or silver strips that favor conduction from the surface of the commutator to the brush holder connection, and reduce cross-conduction across the gap. 

If arcing is a problem, perhaps some sort of snubber or regenerative energy scavenging device could be employed.

For higher speed, perhaps a centrifugally operated brush timing advance mechanism could be designed (like a spark advance on an ICE) to get optimal performance.

Maybe you could design a mechanism that would engage and disengage contact between the brush and the commutator without sliding and causing friction and wear. 

It is an interesting challenge to dream up such possibilities, but since there are already many designs that are proven to be reliable, inexpensive, and efficient, I doubt there is any practical benefit. But perhaps you can come up with something and build it and show it on YouTube. There are some fascinating devices that people have come up with.


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## tylerwatts (Feb 9, 2012)

Thanks Paul. I'll have a go at some ideas and if I find anything I will post. 

In the mean time, what do we all agree to be the limiting features of a brushed DC motor for running at high rpm, between 6-12000rpm and at even power levels of over 20kW continuous? I'll start the list from feedback on this forum. 

Brush bounce or shudder.
Arcing on commutator. 
Excessive heat and friction on commutator and in brushes. 
Integrity of rotor components at high cf loads. 
Voltage requirement for high rpm, though winding for it can compensate.
Is cooling a concern?


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## Karter2 (Nov 17, 2011)

My thoughts..
BLDC are still relatively "new" technology and are still developing rapidly.
But already there are very powerful, lightweight, efficient motors and compact controllers that have good functionality. If they lack anything it is reliability, but that will come with refinement.
I fear all PM motors may be limited by the availability and cost of the best magnet materials.


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## PStechPaul (May 1, 2012)

Another technology to investigate for high speed operation is the ironless DC motor design (which has achieved 5 HP/lb):
http://www.designworldonline.com/Ironless-DC-Motors-Deserve-a-Second-Look/
http://itee.uq.edu.au/~walkerg/publications/2001/104_Greaves_AUPEC01.pdf
http://hal.archives-ouvertes.fr/docs/00/41/33/52/PDF/paper-ironless-machines.pdf
http://www.launchpnt.com/portfolio/aerospace/uav-electric-propulsion/

There are many more links I found but these should suffice. Unfortunately these all seem to require high strength permanent magnets, but perhaps they could be replaced with electromagnets. It should be theoretically possible to build a motor with nothing but air core inductors, or perhaps a material such as ferrite might replace the usual laminated iron and allow higher frequency operation.


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## epyon (Mar 20, 2008)

The one thing this motor "must" be able to do is be able to work with a DC controller from a 48v Curtis to a Shiva .


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## tylerwatts (Feb 9, 2012)

That is my idea for a brushed rotor and big PM stator, any DC controller would run the motor then! And it can be easily scaled and not cost the earth. Magnets would cost more than copper for the stator but perform better I feel.


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## vmrod (Jul 2, 2010)

Psytech,

thanks for posting those imbedded e-motor clips. Very cool!


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## epyon (Mar 20, 2008)

I almost got a dc motor an put some magnets on it too .


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## Siwastaja (Aug 1, 2012)

It's beyond me why anyone would want to design a "new" DC motor when you can design an AC motor and inverter.

What next, designing a "new" ICE, or how about a steam engine? They look so "cool" (hot really) and kind of steampunk!


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## tylerwatts (Feb 9, 2012)

I won't talk about steam engine's here. And no, I'm not reinventing anything. This motor exists, it is the Lynch and/or Agni design and offers better DC efficiency and I believe a more power dense motor. Simply being more efficient would mean more kW hit the road. Also I feel this motor configuration will offer more torque from the motor. But I want to try this out first. 

This thread was meant a discussion and proposal forum and nobody is necessarily correct or not. Just all contributing to a wider well-being.


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## PStechPaul (May 1, 2012)

OK, I found the www.agnimotors.com website and they claim that their motors provide high efficiency at a wide range of rpm, but the graphs do not show this:
http://agnimotors.com/95_Series_Performance_Graphs.pdf

Instead, it shows high efficiency (92%) only at about 3600 RPM and for a wide range of current from 100 to 400 amps. This is for 71 RPM/volt at 60V as well as 71 RPM/V at 48V. This is odd because the RPM is the same for those two voltages. And then at 36V and 24V the RPM drops to about 1800 but efficiency drops to 80-90%. Then at 12V peak efficiency is 85% at 850 RPM and 75A but drops to 65% at 660 RPM and 400A.

These curves are odd because they are usually plotted as a function of RPM. And if you want to run the motor at low speed (400 RPM) for start-up, using 60V, you get only 10% efficiency and 10 Nm torque at about 15 amps (900W) and maybe 90W output. If you run it at 12V and 400RPM you get 40% efficiency but only about 10 amps current and 20 Nm torque. In fact, it's really impossible to get good data from these graphs because they are really drawn as a function of current, and the results don't make any sense to me.


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## major (Apr 4, 2008)

PStechPaul said:


> OK, I found the www.agnimotors.com website and they claim that their motors provide high efficiency at a wide range of rpm, but the graphs do not show this:
> http://agnimotors.com/95_Series_Performance_Graphs.pdf
> 
> Instead, it shows high efficiency (92%) only at about 3600 RPM and for a wide range of current from 100 to 400 amps. This is for 71 RPM/volt at 60V as well as 71 RPM/V at 48V. This is odd because the RPM is the same for those two voltages. And then at 36V and 24V the RPM drops to about 1800 but efficiency drops to 80-90%. Then at 12V peak efficiency is 85% at 850 RPM and 75A but drops to 65% at 660 RPM and 400A.
> ...


Using the load as the independent variable (x-axis) while holding voltage constant is the normal method to present the performance characteristics of electric motors. Typically torque is used for the x-axis but because the torque-current relationship is singular (and linear for PM motors) having current as the independent variable (load) is often used. Current is typically easier for the end user to measure. 



> And if you want to run the motor at low speed (400 RPM) for start-up, using 60V, you get only 10% efficiency and 10 Nm torque at about 15 amps (900W) and maybe 90W output.


This makes no sense whatsoever. If you want to run this motor at 400 RPM, you apply 5.6V (at no-load, higher when loaded). You arrive at that voltage by dividing the desired speed (400RPM) by the machine constant (71RPM/V).

You have a knack for posting numerous citations on every imaginable topic. Try searching this forum concerning the proper way to interpret motor performance characteristic curves. I have covered the subject a number of times.


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## PStechPaul (May 1, 2012)

I realize that there is a linear V/RPM relationship for a DC motor but I still did not see how to interpret these curves. But now I see that there is a black line representing RPM which changes according to current. It varies from 550 to 850 RPM at 12V. But for vehicle traction applications with variable speed PWM drive it seems much better to express the relationships as a function of RPM (or voltage, if you prefer), but it is important to know the behavior of the motor under locked rotor conditions for start-up torque. I would expect the curves to be as they are shown here:
http://lancet.mit.edu/motors/motors3.html

Here they use torque for the horizontal axis and show the speed variations a, b, and c at the nominal and max/min limits of field strength:
http://www.globe-motors.com/dc_motor.pdf

The specs for DC servo motors show torque and current as a function of speed:
http://www.baldor.com/support/Literature/Load.ashx/BR1202-F?LitNumber=BR1202-F
http://www.glentek.com/images/pdfs/GM6000 PC.pdf

In this post for Warp9 motors, the horizontal axis is speed:
http://www.diyelectriccar.com/forum...-charts-ev-performance-spreadsheet-41565.html

I think the convention of using torque (or amps) as the variable comes from early days where most motors were used at a fixed voltage and a fixed nominal RPM, in which case it makes sense to be able to see where the best efficiency may be obtained, and a motor with good efficiency over a wide range of torque would be ideal for many industrial machines. But for EVs, and modern PWM VS drives, I think it is better to plot as a function of RPM. Of course, I'm mostly an AC guy, so DC motor curves are not something I normally deal with. 

But thanks for setting me straight. I had totally misread those curves.


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## major (Apr 4, 2008)

PStechPaul said:


> In this post for Warp9 motors, the horizontal axis is speed:
> http://www.diyelectriccar.com/forum...-charts-ev-performance-spreadsheet-41565.html


Because he redrew it.


maxvtol said:


> Chart 1 is just the torque curve from Netgain for a 9" motor formatted how I'm used to looking at torque curves.


 Go to the source for Warp9 and you'll find the proper speed torque curve.

If you read deeper into that thread, I link max here  http://www.diyelectriccar.com/forums/showpost.php?p=156130&postcount=2 to help him better understand motor curves.


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## epyon (Mar 20, 2008)

Siwastaja said:


> It's beyond me why anyone would want to design a "new" DC motor when you can design an AC motor and inverter.
> !


The lower amount of money and hi torque it can make , is it for me . $10,000 for a AC controller that can do half what a DC controller can do . Why hasn't any one put a magnetic stator on a Warp 11 motor ?


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