Motor theory questions

I was looking at the differences between the two CIM motors and started asking some questions. What internally about a PM DC brush motor defines its characteristics. What factors determine the maximum speed, torque etc.

Motors like the CIMs are designed to be low speed high torque while motors like the old drill motors are high speed low torque. How do they do that?

James, this is a great question. I don’t know how much motor background you have, essentially knowing how a motor works, but I think there are a few websites out there that will give you a good description of how they work.

Here’s one that’s pretty good:
How Motors Work at

Once this is set, it really depends on the motor construction. Here are a few things off the top of my head:

Number of turns on the motor windings.
Thickness of windings (this affects the max current capacity).
Input Voltage (Higher voltage will yield a proportionally higher speed)
Magnet type & strength
Friction of brushes

I’m sure others can add to this discussion.


after reading that, what about the number of poles?
how important is that?

The number of poles is always even, since every north pole must return magnetic flux to a south pole.

Generally, a motor with a higher pole count will run slower and develop higher torque than another motor with a lower pole count, assuming that both motors are the same size and are rated for the same operating voltage and continuous-duty current.

Some of the factors involved are the power of the magnetic structure, the current through the windings and the number of windings. Now everyone knows that magnetic fields jump gaps but a gap is seen as a loss sort of like a resistor to magnetic field and different materials also act as losses to magnetic fields. For that reason, the choice of materials for both the magnetic structure and the armature come into play. A designer is looking to maximize a variety of factors like RPM, torque and input power for particular applications and does so by massaging all of these variables. A close look at a drill motor vs. a Fisher Price (both of which have openings at the brushes and are about the same size) will show only three commutator segments for the FP but many more for the drill. Although the free speed is close for both these motors the stall current and stall torque are nearly double for the drill.
The motor works on the principle of opposite magnetic poles attracting. Everyone who has played with permanent magnets knows that the greatest attraction occurs when the ends of the magnet are closest together, and the same is true inside the motors. In the case of the FP, there are only three coils of wire spaced 120 degrees apart so at any one time the energized coil is not very close to being inline with the magnets. The drill on the other hand has many more poles so a coil is much closer to the magnet field when current is passing through it. This is one of the reasons that the drill has superior low speed torque specs.
I know that there are other reasons, and a mentor out there (Mark McLeaod or Mike Betts, I don’t remember which) does work with motor design everyday can add much more to this discussion than I can.