# Design Project: DC Motor

#### DC Electric Circuits

The armature windings of your DC motor will have very low resistance. Too low, in fact, to accurately measure with an ohmmeter connected directly to the commutator terminals, because the contact resistance between the meter probes and terminals will be a large percentage of the resistance read by the meter.

One way to avoid this contact resistance problem and measure very low resistances using normal meters is to employ what is known as the *Kelvin resistance measurement technique*: measuring the voltage dropped by the low-resistance element when a substantial DC current is passed through it.

Draw a diagram showing how this measurement technique works, and explain *why* it works.

You will notice that a very important factor in your motor’s performance is the *brush angle*: the points of contact that the brushes make with the commutator bars, as compared to the axis of the field magnets. What brush angle results in maximum unloaded motor speed? Why can’t we just have the brushes contact the commutator bars at any arbitrary angle?

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