Discussion
Following controls are considered for dc motors :
(a) Flux Control
(b) Armature resistance control
(c) Supply voltage control
Which of the above controls play significant role in the speed control of dc motors ?
Answer: C
All three controls, (a), (b) and (c), play a significant role.
The speed of a dc motor is
\[ N \propto \frac{V - I_{a}R_{a}}{\phi} \]
so it can be changed through each quantity on the right. Flux control (a field rheostat) weakens \(\phi\) and gives speeds above normal. Armature resistance control adds resistance in series with the armature and gives speeds below normal. Supply voltage control, as in the Ward-Leonard system or a controlled rectifier, varies \(V\) smoothly over a wide range.
Every option leaving one of them out is incomplete.
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