Discussion
A dc series motor is running at rated speed without any additional resistance in series. If an additional resistance is placed in series, the speed of motor __________.
Answer: A
The speed of the motor decreases.
For a dc series motor, \(N \propto \frac{V - I_{a}(R_{a} + R_{se} + R_{ext})}{\phi}\). An extra resistance in series increases the voltage drop, which reduces the back emf available at the same load, so the speed falls. This is the armature resistance (rheostatic) method of speed control, which gives speeds below the rated speed.
The speed does not stay the same or rise, because nothing in the change weakens the field or raises the voltage. The motor settles at a new lower speed rather than oscillating.
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