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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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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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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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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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Discussion Starter #6
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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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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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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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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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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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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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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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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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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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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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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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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Discussion Starter #18
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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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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