# LoBuck Controller for Hi Amps



## xtian999 (Apr 26, 2009)

This is awesome! I found this on a British site called: Battery Vehicle Society. I can imagine doing it with parts from an old motor and cutting away parts of the commutator to get the same effect.


*SimonR*



Joined: 03 Sep 2008
Posts: 16

Posted: Sat Mar 28, 2009 12:47 am Post subject: Mechanical Speed Controller  A few years ago (while we were doing Robot Wars in fact) I designed a mechanical speed controller. We needed a controller at the time which would handle 1200A at 24v - and such a thing just did not exist. 

As it turned out, I built an electronic one using big IGBT's - but another chap named Steve Davis built and used this. 

I thought it might be useful to anyone wanting to build an EV on a non existent budget. 










Take a cylinder of PTFE & cut a 'V' in one end (as shown on the left (the white bit) and then take a cylinder of Copper and cut such that the two fit neatly together. Drill an axial hole up the middle to mount the two components on a shaft. Glue the whole lot together. 

Put the spindle in a drill chuck to turn and use emery cloth to polish the surface & remove any burrs. 










Using the brush assembly from a DC motor (selected such that the brushes will handle the required current), mount two brushes diametrically opposite one another such that their mounting can slide along the length of the commutator (the Copper & PTFE bit), or arrange that the brushes remain stationary and the commutator slides. Connect a small, high speed motor such that the commutator assembly rotates on its axis. 

As the brushes move along the length of the commutator, the proportion of the time they are in contact with the copper changes from 100% to 0% changing the power delivered to your drive motors in the same proportion (PWM) 

Use an RC Servo to move the brushes with a spring return to the PTFE end such that it will fail safe, although I would recommend using a solenoid switch to engage & disengage the power to the whole system. Reversing is achieved with either a big relay or four solenoid switches acting as a change-over. 

A System such as this can be made to handle HUGE currents and the cost and complexity are roughly the same regardless. I would use the brush set from an automotive starter motor (removing the two brushes that connect to ground and separating the contacts which are normally live). The main loss in such a system is the motor which drives the commutator assembly although this should be fairly free running and the losses low. It does not require any special tooling to build (I've built one with nothing more than a hack saw and a drill (although I started with copper & PTFE tube into which the axle fitted neatly. Glue the components together with an epoxy otherwise the glue may melt!


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## aeroscott (Jan 5, 2008)

that is so simple, whats not to love . tell us how you built the igbt controller .


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## Madmac (Mar 14, 2008)

Would have thought you would still need a diode across the motor (or a set of shorting contacts) to keep the current flowing in the motor winding and prevent back EMF arcing.


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## lazzer408 (May 18, 2008)

Madmac said:


> Would have thought you would still need a diode across the motor (or a set of shorting contacts) to keep the current flowing in the motor winding and prevent back EMF arcing.


You thought correctly. The arking on that "rotory pwm" would quickly destroy it. Diodes will be needed.


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## Madmac (Mar 14, 2008)

You could make the system completely mechanical by shorting the motor coil. Would probably need to damp the coil while it goes from one set of contacts to the shorting set.


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## xtian999 (Apr 26, 2009)

I suppose that the smoother the surfaces, the better. So, close machining tolerances and good materials would help performance, but not budget. On the other hand, more brush material and slower rotating speed would help too. Who wants to build one? Raise your hand.


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## lazzer408 (May 18, 2008)

You want high rotational speeds. Over 1000rpm easily. Depends on the motor. If it rotates to slowly the fields will have time to collapse and you don't want that.


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## gerd1022 (Jun 9, 2008)

i really really want one... such a great idea.

I actually think im gonna make and test one... 

you would probably want it to be even faster than 1000 RPM, 20 kHz PWM is 20,000 pulses per second, which would be 60,000 RPM (each revolution is 2 pulses...)

obviously 60,000 RPM is way too much, but the faster the better...


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## Shanex-2 (Jun 7, 2008)

or just make more spike points say 4 instead of 2 and you will have 2 times the switching for the same rpm. Very good idea by the way. YES I WANT ONE lol.


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## ga2500ev (Apr 20, 2008)

Shanex-2 said:


> or just make more spike points say 4 instead of 2 and you will have 2 times the switching for the same rpm. Very good idea by the way. YES I WANT ONE lol.


The challenge there is crafting the rods so that you can get the correct effect as they widen out.

My answer to this is that's what gears are for. Change the gearing ratio between the motor and the rod to get the speed required for the application.

Some other resources. Here's an Ebay Buy it now on 6 1.19" x 4.75" copper rods:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270340994037

There's probably enough in one buy to make 12 controllers.

I was thinking that because of the heavy wiring that it would be prudent to keep the brushes stable and to make the rod/motor slide along a raceway. Anyone have any idea what type of travel distance to expect from a typical accelerator cable? 

Finally would this system still need capacitance integrated into it?

I cannot wait to put one together.

ga2500ev


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## Shanex-2 (Jun 7, 2008)

gerd1022 said:


> you would probably want it to be even faster than 1000 RPM, 20 kHz PWM is 20,000 pulses per second, which would be 60,000 RPM (each revolution is 2 pulses...)
> 
> obviously 60,000 RPM is way too much, but the faster the better...


do you realy need to have that high of switching speed ? other than noise is it realy necessary .The hertz on a standard ac outlet is 60 hertz which is 120 times a second. Granted A/C and D/C are different .


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## Batterypoweredtoad (Feb 5, 2008)

I very much doubt the posted design would take 1200 amps for any extended time because of the tiny brushes. I'm collecting the parts to build one using the brush set from a 6.7" motor I pulled from a forklift. They should handle a little more but I don't know if it will be enough. Maybe a full throttle contactor bypass? I agree that sliding the brushes isn't going to work with wires attached. Sliding the spinning portion is the first approach I will try. If that doesn't pan out maybe I will slide the brushes but use a live rod on each side to transmit the power? Who knows-updates will follow slowly.


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## mxmtech (Apr 21, 2009)

Wouldn't the brushes that power the motor be adequate to operate the controller? The exact same juice?
Was the discussion about diodes for ac applications?
Could anyone hazard a guess about the dimensions of this thing, based on the size of the blue stak-on connectors?
What gauge of wires are connected to the motor?


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## Batterypoweredtoad (Feb 5, 2008)

With this controller you use half the brushes to bring the power in and half to send it out so you effectively get twice the amps per brush you do even if you use the same brush set as the motor you are powering. To make things worse for me I plan on pushing two 6.7" motors with the controller I build using one brush set. Add to that the fact that the setup will be a direct drive setup and used for drag racing with no regard for battery health I think I will be testing the life of the controllers brushes. (and my poor little 6.7" motors) I have no idea about the diodes-can anybody explain why they would be needed and where they would go?


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## ga2500ev (Apr 20, 2008)

On the freewheeling diode the short answer is that a motor is a huge inductor. As you provide energy to it it creates a magnetic field. When you switch off the energy source, the magnetic field creates a huge voltage spike as it collapses. In this proposed system that arc would be created at the brush/copper connection, causing pitting and early brush failure.

The diode provides a path for the energy of the collapsing field to travel thus suppressing the high voltage spikes.

The diode(s) are wired directly across the motor terminals in the reverse direction of current flow.

Check out this wikidpedia page for more info:

http://en.wikipedia.org/wiki/Flyback_diode

hope this helps,
*
My question is are caps still necessary in this system? Will they help to protect the batteries?*

ga2500ev


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## ga2500ev (Apr 20, 2008)

Shanex-2 said:


> do you realy need to have that high of switching speed ? other than noise is it realy necessary .The hertz on a standard ac outlet is 60 hertz which is 120 times a second. Granted A/C and D/C are different .


As Lazzer already pointed out if you don't spin fast enough the motor magnetic field will completely collapse at each interruption, which will waste a ton of energy since it'll need to be rebuilt on each on transistion.

Toy motors can easily get up to 12K RPM. It shouldn't be too difficult to get the rod spinning at a reasonable speed. 

The other issue is that if you spin too slowly you can feel the motor pulses and you won't have smooth acceleration.

ga2500ev


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## mxmtech (Apr 21, 2009)

_Did anyone buy one of the copper rod ebay items suggested by ga2500ev? That seller will not ship to Canada. Has anyone a source for the teflon rod?_


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## xtian999 (Apr 26, 2009)

The original design looks good. However. could it be simplified by doing a single cut across the cylinders instead of a vee?


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## Batterypoweredtoad (Feb 5, 2008)

You need the "V" for that design to work. That design feeds the power in to one side of the V through a brush and the other side of the V picks it up.


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## ga2500ev (Apr 20, 2008)

Batterypoweredtoad said:


> You need the "V" for that design to work. That design feeds the power in to one side of the V through a brush and the other side of the V picks it up.


Let me add more detail to that answer. The "V" shape facilitates delivering a variable amount of power per unit time to the motor. It's a form of PWM where the further up the brushes goes towards the open part of the "V", the more time it spends in the copper section, so the more power gets delivered to the motor per revolution of the rod.

A straight gut would deliver constant power, which means that the car would not accelerate as you applied more pedal.

The "V" shape is a cut that a miter or jig saw can make pretty easily.

ga2500ev


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## hondo (Mar 30, 2009)

I had this idea years ago but was told by an electrical engineer that there would be too much arcing. But I think now that I know more about free wheeling diodes and capacitors, I think it should work if protected properly. I was going to use one angled cut on the copper with one moveable brush, and the other brush stationary on the solid copper end. This would work just as well as the v cut but the frequency would be cut in half.

Hondo


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## ga2500ev (Apr 20, 2008)

hondo said:


> I had this idea years ago but was told by an electrical engineer that there would be too much arcing. But I think now that I know more about free wheeling diodes and capacitors, I think it should work if protected properly. I was going to use one angled cut on the copper with one moveable brush, and the other brush stationary on the solid copper end. This would work just as well as the v cut but the frequency would be cut in half.
> 
> Hondo


But what about the mechanical setup? The V cut limits the amount of axial shift between the two rods because they are fitted together. With the single cut, the two rod parts can slide past one another.

ga2500ev


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## EV59RAG (Oct 6, 2008)

this is a very interesting project.... I assume that the horizontal movement of the brushes is caused by the gas pedal? No more 5k or 6k pot? less parts means simpler to work on.

That means that the dimension of how deep the V-cut is will all depend on how much play you have on your gas pedal?


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## hondo (Mar 30, 2009)

ga2500ev said:


> But what about the mechanical setup? The V cut limits the amount of axial shift between the two rods because they are fitted together. With the single cut, the two rod parts can slide past one another.
> 
> ga2500ev


I was planning on using a thick copper pipe and a nonconductive pipe with a nonconductive core in the middle to hold things together.

Hondo


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## ga2500ev (Apr 20, 2008)

EV59RAG said:


> this is a very interesting project.... I assume that the horizontal movement of the brushes is caused by the gas pedal? No more 5k or 6k pot? less parts means simpler to work on.


That's right.


> That means that the dimension of how deep the V-cut is will all depend on how much play you have on your gas pedal?


Right again.

ga2500ev


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## xtian999 (Apr 26, 2009)

hondo said:


> I had this idea years ago but was told by an electrical engineer that there would be too much arcing. But I think now that I know more about free wheeling diodes and capacitors, I think it should work if protected properly. I was going to use one angled cut on the copper with one moveable brush, and the other brush stationary on the solid copper end. This would work just as well as the v cut but the frequency would be cut in half.
> 
> Hondo


That's what I meant. One single oblique cut across both cylinders. Same angle as the vee, just all the way across. One brush at the base of the copper, one sliding up and down along the cylinder for the PWM. Should not have any other effect on the system than doubling the period of the pulses and making it easier to fabricate. Copper pipe is a great money saving idea too, Even for the "dead" side, as long as it was not connected to the "live" side. So really, you just need a ellipsoid spacer in the cut. You may even be able to wire up the other side for something like reverse or regen. 

I wonder if the outer mechanism can't be scrounged from a little DC squirrel fan or maybe an old printer. A printer would have almost everything you need already there, just modify the roller and put your moving brush where the cartridge went.


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## Shanex-2 (Jun 7, 2008)

the other thing needing to be considdered is balancing the roller. With the single cut design you have more copper on one side than the other and the copper is going to be heavier than the plastic or teflon side remember that this thing is spinning pretty fast (at least 1000rpm) . With the V-cut design it is natually balanced. However the v-cut does take more time to make . I am having a little bit of a problem making the v-cuts match even with a miter saw . I am using a 1 1/2" copper pipe and a 1 1/4" pvc pipe . The outside dia. are a close match.


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## ga2500ev (Apr 20, 2008)

Any suggestion for brushes?

ga2500ev


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## xtian999 (Apr 26, 2009)

Keep the off-cut piece on the barrel with a spacer. So, one cut for the copper, two for the spacer. All on the same jig. Should fit together and balance nicely.


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## SimonRafferty (Apr 13, 2009)

This is actually my design - and my post on the UK forum!

Si


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## mxmtech (Apr 21, 2009)

SimonRafferty said:


> This is actually my design - and my post on the UK forum!
> 
> Si


Welcome Si, your design is elegant. I would like to read more of the story of what you used it for and how it worked. Can I have your autograph, lol?
A few of us on this forum are determined to build your design and try it out. There are going to be a lot of tools used in this project (lathe and drill press) that I'm not too familiar with. 
If you have a video detailing its construction, and showing it in action, I would buy it.
Have you tried it on a full sized EV?


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## ga2500ev (Apr 20, 2008)

SimonRafferty said:


> This is actually my design - and my post on the UK forum!
> 
> Si


Care to share any building tips?

ga2500ev


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## mxmtech (Apr 21, 2009)

One thing that I can think of is to heat the copper pipe before measuring the diameter of the non-conductive plug, then heat it again for the assembly. The pipe will shrink when it cools and will be a snug fit.


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## SimonRafferty (Apr 13, 2009)

The controller was originally intended for this:
http://www.youtube.com/watch?v=htt-rIvU8V0 (I'm the tall one!)

The motor driving the drum was a 4.7Hp 12v running on 24v. Stalled, this drew nearly 1100A @ 24v. We used two strings of Odyssey PC680's in parallel which would deliver about 1000A into the motor. 

At the time, the biggest speed controllers were in the 500A region - but they were too expensive for us. I set about thinking of alternatives.

The original idea was to use the commutator from a larger motor. Connect each contact on the commutator to a slip-ring on the same shaft. Then use one brush touching the commutator and the other sliding side to side connecting to between 1 and all the slip rings. That gives a discrete number of steps in the duty cycle - but I don't think you often need particularly high resolution in the speed control unless you need differential steering.

I failed to find a big commutator - so the idea above was the alternative.

The intention was to use a freewheeling diode and spin the commutator pretty fast. Possibly to use a capacitor to 'average' out the voltage supplied to the motor if the switching speed is not high enough for the time constant of the motor.

In the end, we found a load of BIG IGBT's in a computer UPS at a junk yard. They were rated at 500A each - so we used three. We built a pretty low tech darlington low-side driver driven by a PIC controller. There was a freewheel diode in parallel with the motor - but not the IGBT as we wanted the motor to freewheel - not regen.

A chap named Steve Davis (who I have lost touch with) built the mechanical controller using the same brush pack as the motor he was going to drive. It appeared to work pretty well - but I remember him saying the rotation speed was quite important in keeping arcing to a minimum.

The reason I posted it on the UK forum was because, like here, not everyone has an unlimited budget and it might offer a low tech, low cost solution which I'm all in favour of. I've aquired a Siemens AC induction motor and I was wondering about a mechanical 3 phase controller - and I have a similarly simple plan!

Si


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## mxmtech (Apr 21, 2009)

Was this Robot Wars last years episode? I haven't seen it on our tv for a while. I didn't know they were still producing new shows.
Excellent & exciting YouTube video. I never saw anything like your robot when I was watching the show. Is it a unique design?
I thought I would insert a 1 1/2" dowel into a 1 1/2" copper pipe, cut out the v-shapes, then cut one side of the pipe lengthwise twice so that the brushes would have no electrical connection on that end. Fill the gaps with fiberglass resin?


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## xtian999 (Apr 26, 2009)

Totally awesome video! Effing genius bot! Thank you Simon for the cool design and inspiring application.


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## Wiredsim (Jul 4, 2008)

To reduce the required rotational speed couldn't you actually cut this into more of a crown shape? Use a copper pipe and make 4 smaller V cuts into it? This would potentially give you 4 or even 8 pulses per RPM. I hope my description is clear, I could post a picture later.


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## JimDanielson (Oct 19, 2008)

That sounds like it would work, but making the cuts for the v is very simple, while a crown gets much more complicated


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## mxmtech (Apr 21, 2009)

If you use a crown shaped cut you would probably have to choose the size of your brushes more carefully. Less gap between the points means smaller brushes and less current capability.
What are the contact dimensions of the brushes for a large DC motor?
I think I am going to choose a 40 hp motor 144 volt model.


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## SimonRafferty (Apr 13, 2009)

Another quickie for you.

For another project, I needed a 300A 24v controller, but only had a 100A 4QD 2 Quadrant controller. I used a contactor to connect the battery directly to the motor (bypassing the controller) then the throttle was above 90%.

This gave fine low speed control - but the absolute max acceleration and top speed if you floor it! Worked pretty well.

Not sure how useful this is to EV builders - but there must be some of you who would like to increase the power of your controller.

Si


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## Wiredsim (Jul 4, 2008)

I agree about the brush size, though you can negate that somewhat by increasing the O.D. of the tube. I would use a copper pipe instead of a copper rod as is to reduce the weight of the spinning element. T You can pick up type L hard pipe 2" or 3" for fairly cheap per foot. That should still allow for massive amperage, your brushes are going to be the limiting factor for that either way.


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## SimonRafferty (Apr 13, 2009)

Another little idea for you! You can do something similar for a single or three phase AC drive. 

You would probably need to have the parts laser cut - but that's not that difficult these days.

3 phase AC Drives tend to work by splitting the wave form in to the positive going and negative going half - which determines if the current is going to flow in or out of a particular phase wire.

You only use PWM on one half of the cycle - so when a phase is putting current in to the motor, it uses PWM but when it is receiving current from the other two phases, it just connects to the battery negative.

Mechanically, this is simplest to achieve using two commutators. One comprises 180 degrees split up in to a series of conductors and spaces to give the PWM profile for the top of the wave. The other just has 180 degrees of conductor and 180 degrees of space.

Each phase has two brushes, one on each commutator at the same rotational position.

The brushes for the three phases are just spaced at 120 degrees around the commutator.

Bit like this:








You then spin the commutator with a smaller motor - and the bigger AC motor should follow.

Alternatively, connect the commutator to the big motor (geared to allow for the number of poles in the motor) via a differential. The other side of the diff connects to a servo (RC type).

Now, the servo controls the degree of lead or lag in the phase of the commutator versus the motor - giving you vector phase control.

Although you might think this limits the resolution of the PWM too much, my Siemens controller, particularly at high speed only uses a few pulses per cycle. 

Si


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## xtian999 (Apr 26, 2009)

I have heard of some people getting three-phase power in their shops by using a small three phase motor as a sort of converter. Is this similar?

I am totally ignorant of AC motors and controllers. Please excuse if this is a silly Q.

Xtian


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## Bouche (Jul 9, 2008)

This controller is so simple and elegant even a newb like me gets it. I’m an Architect by training and about the only dealings I have with electrical design is fighting with my Electrical Engineers about the size of their electrical rooms, but I get this!!!

I was so inspired by the concept and subsequent suggestions I did a quick 3D CAD model of my own take on this design. I used two beefy sets of stationary brushes, one set for in and one set for out. The commutator is 3” diameter and moves a total of 7” laterally from a dead stop to full throttle. It came out to 3’-2” long but could probably be shortened with some tweaking.

Please feel free to offer constructive feedback for design enhancements.

I hope to do a conversion one day, until then I’ll have to be content to living vicariously though some of the members of this forum who have already done it.

Here’s some pictures:


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## Jason Lattimer (Dec 27, 2008)

That is wonderful. I love the design. Simple and elegant. Can I assume that the motor is designed to run at a fixed speed? And do you think the copper tube will need cleaned periodically? I may actually try to build this design. Thank you.


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## Amberwolf (May 29, 2009)

SimonRafferty said:


> Another quickie for you.
> 
> For another project, I needed a 300A 24v controller, but only had a 100A 4QD 2 Quadrant controller. I used a contactor to connect the battery directly to the motor (bypassing the controller) then the throttle was above 90%.
> 
> ...


With smaller controllers like the 4QD, you could also take the MOSFETs offboard, and parallel a whole bunch of them, with a larger (and if necessary actively-cooled) heatsink. Larger MOSFETs can also be used, but it's probably cheaper to buy a whole bunch of smaller ones, plus the RDSon is much lower by doing it that way. 

You might also have to upgrade the driver transistors, since the current required to charge the gates would go up.

I did this on a cheap little scooter controller, since I had to replace the destroyed drivers and MOSFETs it came with anyway, and it worked very well until further components on it failed (probably from some of the original damage to it, and/or the corrosion it had come with that I was never able to fully clean off). 

I am planning to do this on my 2QD right now, using NTY100N10 MOSFETs on a separately-mounted heatsink, along with an offboard capacitor (or set of them) wired together with busbars, rather than with a PCB. My current ebike doesn't need this kind of power, but I'd like to be able to use the controller for many experiments in the future that *will* need it, without any risk of destroying it from overheating or overvoltage. 
________
Lifestyle vaporizer heater cover


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## Woodsmith (Jun 5, 2008)

I have a video of my very, very low budget, parts out of the bin, working model of the mechanical speed controller in my thread here.


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## OHM (Jun 30, 2008)

Im all for spending the same amount of money for a reliable locally repairable 
mechanical controller that has a little less efficiency than
a modern efficient controller that can suffer all kinds of electronic issues.

Its a shame those that know how to make one wont share this info
to the world as poorer countries and individuals could be driving cheap
battery vehicles with 40-60 mile range on lead acid batteries and able
to repair their own issues rather than send a controller back to the
manufacturer.

Call me old fashioned but better progress can be made through the true
and tried antiquities of the past rather than modern delicate devices that 
need mass production for economical success.


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## camerondmm (Apr 28, 2009)

I think that if someone really wants to build a full size one of these, they should start with the core-shaft out of a similarly sized electric motor to the one they plan to control. Use similar sized brushes too.


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## Harold in CR (Sep 8, 2008)

There is a much simpler way to do this. Find an OLD Car GENERATOR, NOT alternator. Cut some of the copper "bars" out of the motor, after removing the windings. The bars are connected to the bars of the commutator. Cutting out some of the bars from the armature, will still allow the generator to become a "Chopper" or Pulse generating device. Don't see why a starter motor would not be as good, or even better. They handle more amps, right ???

I have searched for the original article, where David Arthurs, of Springdale Arkansas, did this, back in 1978 or so. He converted an Opel GT. My searching skills are not up to par with some of you guys. 

I'm thinking he used a large Rheostat to control the field voltage, of an old Aircraft Starter-Generator. He pulsed the feed into the motor, and that was his motor controller. 

He also used a 5HP lawn mower motor, to turn an alternator, and possibly, that "chopper" generator. He made a hybrid car, out of this design. He developed a very simple design, that could be reproduced today. 

Mother Earth News sold copies of those plans. Might still be available ???


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## Luka (Jul 31, 2009)

@ SimonRafferty
Hi and thanks for share the infomation you have supplied.









Re; the diagram you have supplied. 
This has been a great help for me understanding (mechanically) how and when a 3phase motor receivies its supply.
I can see what is happening when viewing it without the slotted portion in the rotating cylinder,but what i dont understand is the adding slotted arrangement as described.(ie. For simulataed PWM)

Could someone explain what is happening at the top 2 paired brushes when they come in contact with the slotted section and the non slotted section next to one and other?
My question relates to the top paired brushes as shown when they are presented with the slotted portion of the cylinder and the conducting portion. The paired brushes are connected with each other, would they not creat a short btw the batt pos. and neg. as the device is rotated?

I'm new to this site and this subject but find it very interesting learning from what you guys know and share. So any simpified explaination would be greatly appreciated.

Thanks again.
Luka


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## Amberwolf (May 29, 2009)

Luka said:


> My question relates to the top paired brushes as shown when they are presented with the slotted portion of the cylinder and the conducting portion. The paired brushes are connected with each other, would they not creat a short btw the batt pos. and neg. as the device is rotated?


The white areas are non-conductive, so that crossconduction *won't* happen.
________
Buy Bubblers


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## Luka (Jul 31, 2009)

Thanks Amberwolf,
What confused me was the white side surface area in contact with the battery terminals(also by brushed action i imagine).
I assumed the white was the conductive area.
Would the slots be equal spacing and the same width of a contact brush?
Luka


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## SimonRafferty (Apr 13, 2009)

Sorry - should have made that clearer!

Si


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## Luka (Jul 31, 2009)

Hi Si
Much of the mechanical rotation info you have provided has help clear up in my head, how/when electical theory is applied to motors.
Very useful instructional info for newbie like me.
Thank you once again.
Luka


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## Tesseract (Sep 27, 2008)

Hmmm... I don't see how this contraption can make the motor voltage proportional to frequency - the ratio of on time to off time is, quite literally, fixed. Yes, if you proportion each conductive strip just right you can induce a pretty good sine wave current to flow in each winding (you still need freewheeling diodes, btw), but the average voltage applied to the windings is fixed because there is no way to change the width of all of the strips (and any attempt I can think of to do this mechanically would have made Rube Goldberg proud).


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## SimonRafferty (Apr 13, 2009)

Why does the voltage have to be proportional to frequency? Why does the voltage have to vary at all?

I wonder if you are confusing the way a DC motor works with AC?

In a DC motor, the speed is proportional to voltage you are delivering to the motor - it will speed up until the generated back-emf equals the voltage applied. The torque generated is proportional to the current flowing through the coils. Therefore changing the duty cycle, varies the mean voltage and current and changes the speed and torque of the motor.

In an AC motor, the AC causes the magnetic field to rotate. It is the back emf generated in the coils in the rotor which generates a field for the stator field to react against. The back emf is generated by the field crossing the rotor coils.

If the rotor is spinning at the same speed as the AC field is rotating, the coils will never cross the field as they are moving together. Therefore the torque will be zero and the work done will only be in heating up the stator coils.

If the rotor is spinning at a different speed to the AC rotation, the number of field crossings per second is proportional to the speed difference. So the torque generated is proportional to the speed difference.

Therefore, to change the torque / speed of the motor you only need to change the frequency of the AC. The max achievable torque and speed are determined by the current and voltage - but the frequency (or more accurately the phase) is king inbetween.

Si


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## Tesseract (Sep 27, 2008)

SimonRafferty said:


> Why does the voltage have to be proportional to frequency? Why does the voltage have to vary at all?
> 
> I wonder if you are confusing the way a DC motor works with AC?


Well, see, AC motors rely on _inductive reactance_ to limit the current for a given voltage and frequency. For example, say the nameplate current of an AC motor is 100A at 240VAC/60Hz then the impedance of any one phase leg must be 2.4 ohms (comprised of both resistance and reactance, but it makes things a lot simpler to just assume there is no resistance for now).

This equation: 

Xl = 2 x Pi x f x L

says the inductance of the motor windings must be 6.37mH. With this in mind, it should now be apparent that if you cut the frequency in half the inductive reactance will be cut in half, too, or to 1.2 ohms in this case. Guess what happens if you persist in applying the same 240VAC but at 30Hz... or 3Hz...


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## SimonRafferty (Apr 13, 2009)

Tessaract - Sorry if my reply came across as insulting? Re reading it, it didn't sound how I'd hoped!

I had assumed (possibly incorrectly) that as the frequency was low, the reactance would be swamped by the DC resistance? I have to admit, I didn't run the maths on it - but it seemed reasonable.

I have built solid state 3 phase inverters - and I this is a bigger concern because the pwm switching frequency is in 20 to 40khz range. In general I have measured the current and slip angle and adjusted the PWM duty and phase to achieve the goal - so it is in effect doing what you suggest.

Another way to look at it I suppose would be that at low RPM, the resistance is all that is limiting current flow - so you get the highest torque. As the frequency increases, the reactance will have a progressively bigger effect, reducing torque as it goes. So long as the supply and motor are specified based on the DC resistance - it shouldn't catch fire!

Si


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## Drew (Jul 26, 2009)

SimonRafferty said:


> Another little idea for you! You can do something similar for a single or three phase AC drive.
> 
> You would probably need to have the parts laser cut - but that's not that difficult these days.
> 
> ...



Simon, 

I was looking at your design for a DC PWM device and thinking about an AC version myself. I thought you might be interested. My thought was actually to use something like a barrel cam attached to the back of the traction motor. The barrel cam would then drive 3 rods which would control the position of brushes on a modified version of your DC PWM controller, this allows much more accessable control of things like PWM amplitude and advance.

I can have a go at turning out a drawing if you're interested in a bit more detail, but I'm not sure how well I'll do with paint brush


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## few2many (Jun 23, 2009)

Looking over my golf cart motor gave me an idea.
The speed controller is the horrible resistive wire set up. If you know how it works, skip ahead! Basically, The accelerator switch uses several wires and connection points along a resistive heated wire. The accelerator switch receives power from either the low voltage/ high resistive section for low speed and then the low resistive/high voltage section for higher speeds, and what looks like 4 other spots in between for medium speeds. 

My idea is simple, my terminology may take some thought. It may or may not be feasible for a full on EV. Run youre PWM shaft off the traction motor itself, instead of a seperate motor! At the start of the brushed slider, use a resistive power section and relay to get the motor running, then , set it up so the pwm power level picks up a little lower than the resistive set up, as starting takes a bit more power. The frequency will change with motor speed, but you can set up the rotating pwm to have a good minimun frequency through multiple brushed contracts or gear reduction. Follow me so far, I'll try to get a pic in. It would also be possible to use a relay and resisive wire alone. this would just be fore starting, not cruising.


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## Darxus (May 10, 2010)

This stuff sounds great, why did the thread die a year ago? 

Did anybody every get further into implementation and testing than Woody's excellent video?


I was just pointed to this thread after posting about mechanical PWM.


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## OHM (Jun 30, 2008)

Darxus said:


> This stuff sounds great, why did the thread die a year ago?


Most people are interested in patenting their ideas for commercial reasons as
development costs money / time that most tinkerers cant afford. 

Im all for a cheap alternative to whats out there.


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## SiO2 (Mar 19, 2011)

Harold in CR said:


> There is a much simpler way to do this. Find an OLD Car GENERATOR, NOT alternator. Cut some of the copper "bars" out of the motor, after removing the windings. The bars are connected to the bars of the commutator. Cutting out some of the bars from the armature, will still allow the generator to become a "Chopper" or Pulse generating device. Don't see why a starter motor would not be as good, or even better. They handle more amps, right ???
> 
> I have searched for the original article, where David Arthurs, of Springdale Arkansas, did this, back in 1978 or so. He converted an Opel GT. My searching skills are not up to par with some of you guys.
> 
> ...


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## Guest (Mar 19, 2011)

Starter is not good because they usually use bushings and not bearings. The starter is designed for quick short bursts of high amperage low voltage use. The generators are good but usually don't have much voltage output and then it must be driven at high rpms to get that. The Opel was done with a high amp low voltage modified alternator and a low voltage Starter/Generator. The Starter/Generator is a good set up for small light weight vehicles. I have one. Modify an alternator for the gas driven extender. But it must be built for your particular setup. I'd love to have a gas driven generator to power my Starter/Generator. It would need to be a high amp output alternator and a decent voltage of at least 48 volts. Don't expect any speed records with this setup. Or you can just use a nice on off switch and 48 volts and high AH rating batteries in parallel. Use your clutch to smooth out the starting lurches from the on off switch. No amperage control. It will just dump in what that motor will suck out. I saw a nice buggy that performed quite well on 72 volts and one speed on off switch. Not a speed demon but it would do around town speeds quite well. Given a bit of time it would do freeway speeds in the 55 mph range. 

Pete


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