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Tuesday, September 22, 2009

4-Pole Motor Ideas

I thought of something a while back, but have not yet tested the idea beyond a simple quick check:

If I have a 4-pole PMDC motor, is it possible to run it using only two of the poles?

A few minutes ago, I decided to formally ask the question over at DIYElectricCar forums, where I've gotten useful answers before about motor stuff like this. I'll post the gist of the answers I get once a conclusion is reached.

Basically, this 4-pole motor is wired so that two brushes, 180° apart, are both connected to one of the input leads. To use just one pair, two consecutive brushes would be disconnected or removed.

So, if I apply power only to one set of brushes, should the motor operate "normally", except for consuming only part of the current, and producing only part of the speed and torque? Potentially with cogging issues at lower speeds since it is a PM motor?

I have tested with only one set of brushes installed and it does function, reducing (at 12V) the no-load current from about 4A to about 3.5A (using my gloved hand to grip the shaft as hard as I can, which slows it down a little, gives a load of about 6A with 4 brushes, and about 5A with 2 brushes).

The speed (loaded or unloaded) is significantly less with only one set of brushes, though I don't know by how much--I'd presume 1/2 speed; it sounds like it. I don't have a tachometer at the moment to test that.

I am thinking of doing this because most of the time I will not need all of the torque or speed that this motor can produce, and so do not want to "waste" the power the second set of windings would consume unless I am actively using them.

So if I setup a pair of relays (contactors) at the motor housing near the input contacts, and wire the brushes independently, so that I can cut off the power to one of the two brush pairs after I have gotten moving from a stop or finished going up a hill (the only times I should need the extra torque), is it likely to have any serious side effects?

I doubt it, but I'll wait for the experts to reply there before I perform vehicle-power-level experiments with it. :)

EDIT: I was typing up something to someone else, and thought of a more concise way to say some of the above test results:

All readings are approximate, and done on a harbor-freight DMM. ;) Single MKP 12V U1 used for power, no controller, just direct wiring.

no-load 6A
load of gloved hand 5A
current ratio around 1.2

no-load 3.5A
load of gloved hand 4A
RPM sounds about 1/2 of 4-poles.
current ratio around 1.14

Margin of error on the current ratios is too high to presume that the higher ratio on the 4-pole tests would be valid under higher voltages and higher loads without testing using a known load, such as a brake handle squeezed to a specific angle. But if it does continue upward like that, it would probably make a significant difference in current with only 2 poles vs 4 at the normal vehicle loads.

However, since the no-load draw at 12V with 2-poles is already almost as much as that of the 2-pole motor at 24V, I wonder if it will make much real difference.

Hmmpf. Any power savings is still a power savings, I guess. :)


  1. Well, it turns out my whole idea was based on a complete misunderstanding of what was happening inside the motor causing the current drop.

    For some reason, I had been thinking that reducing brush count reduced pole count, which is obviously absurd, but I do not know why I thought that before it was pointed out to me. :/

    What it does instead is to increase the brush and armature resistance, because now there is only one set at a time in use, instead of two, which passes less current thru it but increases the heat buildup in the motor for the same work done. (I think.)

    The increased resistance is why the current drops, and both of those are why the speed drops.

    It ends up being *less* efficient.

    Sometimes I feel really dumb when the obvious is pointed out to me, when I should have been able to see it myself. :(

    I guess if it would really have worked, there'd be switchable-brush-count motors on the market!

    The answer to my question was posted here.

  2. Wait there pardner! I don't see that as showing any dumbness. In fact, I can give a thought experiment which may support your idea.

    say we have two motors. One has a armature field resistance of 1 ohm while the other one is a ridiculously low resistance of say .001 ohm, for the sake of contrast.

    The only way to get the latter to be efficient is to be able to spin the motor ultrafast in order to get the inductive impedance high enough to be lower than that of the rest of the current loop.

  3. Oops, I think that last "lower" should be "higher".

  4. I'm afraid I don't know enough about motors to follow that, but this particular experiment definitely doesn't work the way I thought it did at first, nor how I had hoped it would.

    So while I do indeed get lower current into the motor, the difference in speed is even greater than *that*, in the wrong direction, making the total efficiency of the motor (as far as running my bike goes) *less* by using only two poles.

    This makes it unlikely I'll actually try to do this particular idea, as I can't see how it would be of enough (if any) benefit to be worth the time to do. :-(


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