# Mechanical PWM



## major (Apr 4, 2008)

Darxus said:


> I've found evidence that people talked about using mechanical instead of electronic PWM for controlling electric motors in the past, but I'm not finding much information.
> 
> Why wouldn't mechanical PWM work great, and be very inexpensive?


Hi Dar,

A waste of time in my opinion  But WTH, try it and find out. It was discussed here http://www.diyelectriccar.com/forums/showthread.php/lobuck-controller-hi-amps-31134.html And I thought several members were building it. But then????

Go figure 

major


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## Darxus (May 10, 2010)

I might as well finish that brain dump here.

Please tell me why these wouldn't be very useful:


1, Non-parallel switching:

To my knowledge, all controllers have you wire all your batteries into a single pack before connecting them to the controller. They then rapidly (16,000 times a second = 16 kHz) switch the connection from the pack to the motor on and off via a parallel bank of switches (mosfets or IGBTs) (PWM). These need to be perfectly synchronized, because if one comes on or goes off before the rest, all the power goes through it, and it explodes. And then there are fewer switches to handle the same power, so the rest of them explode. 

So why not break the batteries up into a number of sets each isolated to a single switch? 

The ATMEGA168 controller used on both the open source EV controller and the Zeva controller has 6 PWM outputs, which could drive 6 mosfet controllers, which could drive 12 mosfets, which could control 12 isolated battery packs. Which should allow you to do:


2, Asynchronous switching:

As I mentioned, controllers switch full power from all batteries to the motor on and off at the same time (synchronously). This creates some problems, which are generally handled with freewheeling diodes (and / or synchronous rectification mosfets) and capacitors. 

Now that you've got the mosfets each controlling an isolated battery pack, why not stagger the firing of each of those mosfet controllers evenly? So instead of all firing at once, one sixth of them fire at once. One sixth the pulse, one sixth the work for capacitors, and, I think, a lot less back EMF.

Like firing the cylinders in a Chevy V8 one at a time, instead of all at once.


3, Capacitors on the motor:

Why doesn't anybody run capacitors across the motor terminals to further smooth its input, as a sort of electrical flywheel?


4, You could drive a mechanical PWM off the shaft of the motor it controls, without requiring an additional motor. As long as the variable speed doesn't cause too many problems.


I would expect all of these these to be at least as useful on a mechanical PWM. And you could do as many staggered PWMs with isolated battery packs as you like (up to the number of cells in your vehicle, of course).


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## Darxus (May 10, 2010)

major said:


> A waste of time in my opinion


That's not useful. Telling me why would be useful.



major said:


> It was discussed here http://www.diyelectriccar.com/forums/showthread.php/lobuck-controller-hi-amps-31134.html And I thought several members were building it. But then????


Excellent, thanks.


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## Batterypoweredtoad (Feb 5, 2008)

Ive got the 1/4" thick 3" diameter copper tubing, I've got the brush assemblies pirated from some old motors, Ive got the appropriate plastic stuff, I have many ideas on how to get the whole assembly spinning and all that wonderful stuff. The hitch I come to is how to control the movement of the brushes in relation to the rotating assembly or moving the rotating assembly in relation to the brushes. If you make the rotating assembly slide to vary the PWM you all of the sudden have something that can vary power based on chassis movement. Wouldn't be fun to have full power every time you hit the brakes or go over a bump. If you instead move the brushes, you now have to transfer 1000amps through either flexible wires or some type of sliding brush assembly. Braided wires aren't common or cheap at that power level, and the sliding brush setup is something you have to create all by yourself. I think it could be done and could be marketed. Imagine a controller that can deliver as much power as you can supply for minimal $. Battery and motor protection would be your own problem.


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## Darxus (May 10, 2010)

Batterypoweredtoad said:


> Ive got the 1/4" thick 3" diameter copper tubing, I've got the brush assemblies pirated from some old motors, Ive got the appropriate plastic stuff, I have many ideas on how to get the whole assembly spinning and all that wonderful stuff.


Nice. Pictures?



Batterypoweredtoad said:


> Wouldn't be fun to have full power every time you hit the brakes or go over a bump.


Oh relax and hook it up 

Seriously though, good call. Just hook it up to a linkage from a pedal with a heavy enough spring that its own inertia won't get it moving around too much. Actually, only allow movement along the axis of the vehicle, and have it move toward the _back_ of the vehicle to increase duty cycle (power throughput). That way:

A vertical or horizontal bump will do nothing.

Under hard braking inertia would carry the thing forward toward the low duty cycle / power throughput end.

Under hard acceleration inertia would tend to pull it toward more acceleration, which would be bad, but I think you can easily compensate for that with a spring on the accelerator pedal.

Or maybe run the linkage through some gearing to put it at significant mechanical disadvantage to shift around due to its own inertia? 

Or you could get carried away and include a failsafe disconnect separately activated by inertia. Heh.

You could use a step motor.

Want pictures?

In case you might think for a moment that I might not take this problem seriously: One time I was riding my Blackbird. I had my right wrist a little high. I was accelerating. I hit a bump. This caused my wrist to straighten out a little, which caused more throttle to get rolled on, which caused my wrist to straighten out more.... 

It turned out alright. But let there be no doubt: Inline four cylinder sport motorcycles with a liter or more displacement are just waiting for the moment you prove yourself unworthy to kill you. And this is why we love them 



Batterypoweredtoad said:


> Imagine a controller that can deliver as much power as you can supply for minimal $. Battery and motor protection would be your own problem.


Hehe. Amp / volt capacity: All of it.


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## major (Apr 4, 2008)

major said:


> A waste of time in my opinion





Darxus said:


> That's not useful. Telling me why would be useful.


Then it would be a waste of my time. You'll notice I did not bother to post in the referenced thread. And I should have known better this time around also.

There are numerous obstacles to be found with this method. For starters, you might want to look at current density in the contact patch upon start up.

This is the limit of the time I will waste on this topic. 

major


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## Batterypoweredtoad (Feb 5, 2008)

Major, you grump . O.K., say I actually have 1/2" thick copper tubing and somehow that tolerates the large currents on that tiny little point of copper, what are my other obstacles to overcome? (trying really hard to keep you sucked into this thread )


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## Tesseract (Sep 27, 2008)

Batterypoweredtoad said:


> Major, you grump . O.K., say I actually have 1/2" thick copper tubing and somehow that tolerates the large currents on that tiny little point of copper, what are my other obstacles to overcome? (trying really hard to keep you sucked into this thread )


Ok, since _you_ asked... 

There are all sorts of problems with this idea, but here are the four I see as most damning:

1. "Throttle" controls duty cycle, or the average voltage seen by the motor, but you want to control the average current. This makes for an awkward Human-Machine Interface (HMI in the vernacular) because at low RPMs you can only use a tiny fraction of the throttle travel while as RPM increases you have to apply more throttle to continue accelerating. That said, maintaining a constant cruising speed should be much easier 

2. At zero to low RPMs there is zero to little back EMF (Eg), so the motor pretty much looks like a dead short. As you apply throttle, moving the brushes to where they just start to contact the tip of the triangular copper section, the current density will be incredibly high in both the copper and the brushes. That is to say, a very small area has to carry a huge current. The metal will vaporize (forming pits) and the brushes will burn up.

3. There is no path for freewheeling current, so you will need some large diodes anyway unless you don't mind seeing this contraption vaporized in mere seconds.

4. The "scalloped" copper has to be mated with an insulator, likely made from an engineering plastic like Delrin. These two materials will wear down at vastly different rates which will soon lead to breaking brushes from the step change in height from plastic to copper.


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## Batterypoweredtoad (Feb 5, 2008)

Hi Jeff,
Didn't I call you the "Crusher of Dreams" when we talked? Guess I get to stand by that statement.  No real worries on my part, I am neck deep in 3 projects with short deadlines that have zero to do with the EV world, so no homebrew controller or any other controller for me at the moment. Maybe I will get back to chasing an expensive hobby with a paupers budget after October.


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## Darxus (May 10, 2010)

Tesseract said:


> 2. At zero to low RPMs there is zero to little back EMF (Eg), so the motor pretty much looks like a dead short. As you apply throttle, moving the brushes to where they just start to contact the tip of the triangular copper section, the current density will be incredibly high in both the copper and the brushes. That is to say, a very small area has to carry a huge current. The metal will vaporize (forming pits) and the brushes will burn up.


That's why I suggested using large flat blocks smacked into each other and then yanked apart, instead of spinning things.



Tesseract said:


> 4. The "scalloped" copper has to be mated with an insulator, likely made from an engineering plastic like Delrin. These two materials will wear down at vastly different rates which will soon lead to breaking brushes from the step change in height from plastic to copper.


The video Woody posted in the old thread is a copper pipe sliced diagonally with some kind of material between the two slices to separate current. One side is live, the other side isn't. There is still the seam to jump of course.


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## Tesseract (Sep 27, 2008)

Batterypoweredtoad said:


> Didn't I call you the "Crusher of Dreams" when we talked? Guess I get to stand by that statement.


 I am, aren't I? I blame physics, though- it's always such an obstacle to overcome... 




Darxus said:


> That's why I suggested using large flat blocks smacked into each other and then yanked apart, instead of spinning things.


In other words, use a contactor. This is, of course, a perfectly valid way to perform "mechanical pwm," but, you really ought to run some numbers first...

A well-built contactor should survive 10^6 to 10^7 cycles (mechanical life) but the number of electrical cycles (i.e. - switching under load) it can survive will be far less. Let's consider the venerable Albright SW200 to make this example more realistic. The mechanical life is 5 x 10^6 cycles but the electrical life - ie, when actually having to interrupt current (which it would have to do in this case) is not really specified. The Tyco/Kilovac EV200, though, has a rated life of 10^4 cycles if breaking 500A at 120VDC so we'll use that (even though the EV200 is hermetically sealed and, IMO, of superior construction). The minimum cycle time is the sum of the make, bounce and release times. Let's go with 40ms, 3ms and 30ms, respectively, for a total time of 73ms.

That results in a fastest PWM frequency of 13.7Hz. Assuming the torque ripples from such don't drive one mad, the real problem here is that the minimum on time is so long that the motor will essentially draw the short circuit current available from the battery pack. Hmmm... I can't imagine why this method isn't more popular?!?!

Next figure out the operating life by dividing 10^4 cycles by 13.7Hz and you get a breathtakingly awful 730 seconds! Even if the contactor could survive as long as its stated mechanical life that only comes out to 101 hours of operation.

So, mechanical PWM is just an incredibly bad idea.


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## Darxus (May 10, 2010)

Tesseract said:


> In other words, use a contactor. This is, of course, a perfectly valid way to perform "mechanical pwm," but, you really ought to run some numbers first...


Thanks, I really appreciate you taking the time to post that.

Can you give me an estimate of the minimum frequency?


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## TigerNut (Dec 18, 2009)

Darxus said:


> I might as well finish that brain dump here.
> 
> Please tell me why these wouldn't be very useful:
> 
> ...


A FLA battery string typically consists of a single series string, so there is no way to distribute the current load. If you distribute your switches over the various batteries in the series string, you achieve nothing because whatever switch closes last handles all the current, and whichever switch opens first gets the full impact of the inductive voltage spike.

If you're talking about paralleled strings of Li-Ion packs, then this could theoretically be done, but you'd have a tremendously complicated wiring arrangement since each string pack would need its own high-current wiring to the controller. And, the string/controller that had the highest impedance would get the hottest, which could be a thermal runaway in the making.



Darxus said:


> 2, Asynchronous switching:
> 
> As I mentioned, controllers switch full power from all batteries to the motor on and off at the same time (synchronously). This creates some problems, which are generally handled with freewheeling diodes (and / or synchronous rectification mosfets) and capacitors.
> 
> ...


Multi-cylinder car engines don't fire all their cylinders at once...


Darxus said:


> 3, Capacitors on the motor:
> 
> Why doesn't anybody run capacitors across the motor terminals to further smooth its input, as a sort of electrical flywheel?


A capacitor with a suitable voltage rating and enough capacitance to make a difference would be bigger than the motor, and probably three times as expensive.



Darxus said:


> 4, You could drive a mechanical PWM off the shaft of the motor it controls, without requiring an additional motor. As long as the variable speed doesn't cause too many problems.


I suspect there would be some problems at startup. And PWM has to be done at high speed, otherwise it's not "modulation" and you don't get any of the benefits of this method of voltage regulation.


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## Woodsmith (Jun 5, 2008)

There are an awful lot of reasons why the mock up I made and videoed stayed as a mock up. Many of those reasons have already been mentioned above.
The video is here in case anyone desperately needs to see it and don;t want to trawl through my thread to find it.


I love the idea of a whizzy spinny thing with lots of flashing sparky whorling bits going around, it would look great on the dash or in a clear blister sticking out though the hood.
But having played with it a bit I would only really use it on a non road going low budget toy. My tractor would have been a good candidate but I have a proper (conventional) controller for it.

Even running it as it was on the pillar drill there were a lot of sparks and the contact points, copper on copper, wore away really quickly.
It is low budget but to do a good job of making one you would need to have reasonably good making abilities and at least a lathe.


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## Harold in CR (Sep 8, 2008)

Check out this story. I believe the guy used a starter motor, and cut out most of the copper windings, and made a "Pulser". 

Google search for David Arthurs Electric car


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## Darxus (May 10, 2010)

Harold in CR said:


> Check out this story. I believe the guy used a starter motor, and cut out most of the copper windings, and made a "Pulser".
> 
> Google search for David Arthurs Electric car


Sounds great, but is there actually any information on the internet on how to build it? I'm not finding anything more than:



> "The motor will always have full voltage and full current, but the pulser makes it 'think' the voltage and amperage are cut down to about 1/4 of what's actually available. With this gadget-which is simply a combination of a reworked car generator and an old fan motor-I can keep the draw within limits and effectively control the car's acceleration . . . without sacrificing the maximum current or voltage that's necessary for high-speed driving. I could have achieved the same results with a commercially available FCR control ... but one of those units would have cut my power slightly, and cost in the neighborhood of $800! I can build my own device for about $25, and I can fix it myself if it breaks!"


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## Harold in CR (Sep 8, 2008)

Unfortunately, that article is over 30 years old. I remember seeing a photo of what he did. There are flat copper bars in the Generator. He cut out several of these. What he ended up with , was an armature that would send out the Pulse, through those copper contacts, that were still hooked onto the Commutator. Looked kinda like a skeleton of it's former self ??

Maybe someone else can figure what was run through the Generator, maybe the field windings ?? I'm just guessing here. 

He also had a humongous Potentiometer he used to control another circuit.

Maybe Google him, in Springdale Arkansas. He also converted a VW Bus, and, maybe other vehicles ?? Try to call him or maybe he has an email account ?? That's all I remember. That article was in Mother Earth Magazine, and, I believe plans were available ?? Might check there, also .


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## Darxus (May 10, 2010)

Thanks.

This got me thinking that the brushes and commutator used on a motor should be sufficient to also do PWM for that motor, and that the problem is the points at the end, where the energy density is highest?

So what if, instead of slicing the cylinder diagonally, you make it into one big commutator, with switches to adjust the number of contacts on the commutator that are live? 

So you could have, say, 12 contacts on the commutator / cylinder, each connected via slip ring to a switch, that would let you connect anywhere from 0 to 12 of those contacts, to give you 12 levels of power?

Or, you know, 256 contacts.


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## Harold in CR (Sep 8, 2008)

You got me. 

I did find an email address of someone that found the original ?? set of plans ?? He has offered to share ??

"i found the original blueprits to dave's idea.
if you are interested please email me.
[email protected]"

I just copied and pasted this info. Be careful about using it, because of Spyware-Hackers- whatever.


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## blueblizzard816 (Sep 19, 2010)

I was really hoping that someone would have tried this concept out for an EV application but I haven't seen anything posted (or am I wrong?)

Does anyone knowledgeable about this concept think it's worth designing (taking current density and wear of material out of the equation)? Or will it just be a complete waste of time? 

I feel like it would work just using the rotating copper/plastic shaft, brushes and diodes. But then again, I don't have any justification of this as I don't have an EE background.

Has anyone actually produced this concept for high voltage/current applications?

Also I was reading on the other thread about the same mechanical controller that "If it [the shaft] rotates to slowly the fields will have time to collapse and you don't want that." What does he mean by "the fields will have time to collapse"?


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## major (Apr 4, 2008)

blueblizzard816 said:


> Does anyone knowledgeable about this concept think it's worth designing (taking current density and wear of material out of the equation)?


Tessersact and I have given our opinions on this thread and other places.



> Or will it just be a complete waste of time?


I said it before and here I say it again. It is a waste of time.

Even a worse idea than the giant pot 

major


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## hondo (Mar 30, 2009)

Major, your "giant pot" made me laugh, but it also reminded me of my first controllor I built for my Fiero. For those of you who can get several GOOD contactors cheap, it actually works pretty well. You have 3-4 "banks" of resistors (I used 1/8" nichrome wire about 2 ft long) and you set up your throttle with a cam to turn on micro switches that activate the contactors. The secret to making it work is to have a series/parallel switch so you can start out at half voltage until the motor gets up to speed then switch to full voltage only if you need to accelerate quickly. I set up the contactors so that when I first stepped on the throttle, all the juice had to pass through all the resistors. A little farther and the second contactor would kick in bypassing the first resistor bank and so on until I had direct current from the batteries to the motor. You have to set up your micro switches so the next contactor makes contact before the previous one shuts off. This system worked fine but you really had to be on your toes. I melted the posts off of several batteries while in full voltage mode with too low of motor rpm. I have a 1000 amp meter and it would peg if you jumped on it too soon. I eventually bought a 800 amp Kelly controller and have had nothing but trouble with it and am thinking of switching back to my old setup. At least it never left me stranded, even when I melted a post off I could always limp home on half of the battery pack.

Hondo


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## major (Apr 4, 2008)

hondo said:


> I eventually bought a 800 amp Kelly controller and have had nothing but trouble with it


We should keep a record of each time somebody mentions trouble with Kelly


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