Discussion
Which of these is NOT a method of controlling the speed of a dc motor?
Answer: C
Correct answer: Changing the supply frequency
A dc machine has no supply frequency to change - its speed depends on armature voltage, flux and armature resistance, and nothing else. Frequency control belongs to ac machines, whose synchronous speed is \( 120f/P \). The other three are the three classical dc speed control methods: armature voltage below base speed, field weakening above it, and series resistance, which works but wastes the energy as heat.
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