Question 1

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.


Question 2

If we apply Ohm’s Law to the calculation of armature current, based on the applied voltage and the known armature winding resistance, we obtain a value for current that far exceeds the value of current measured when the motor is running at speed. Explain why this is.


Question 3

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?


Published under the terms and conditions of the Creative Commons Attribution License