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.

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

2-poles:
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. :)