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GATE 2026 EE – Question 20

Electrical Machines · Synchronous machines: cylindrical and salient pole machines, performance and characteristics, regulation and parallel operation of generators, starting of synchronous motors · 1 mark · Multiple choice

A certain application requires power at a frequency of 16.67 Hz, while the available grid frequency is 50 Hz. A 3-phase synchronous motor connected to the 50 Hz, 3-phase grid, driving a synchronous generator, is used for this application.

Which one of the following combinations is a suitable choice for the number of poles in the motor and generator, respectively?

  1. 2, 6
  2. 6, 2
  3. 4, 6
  4. 6, 4

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Correct answer: (B) 6, 2

Explanation

Synchronous speed is $N=120f/P$. The motor and generator turn at the same speed, so $\dfrac{f_{gen}}{P_{gen}}=\dfrac{f_{mot}}{P_{mot}}$, i.e. $\dfrac{P_{gen}}{P_{mot}}=\dfrac{16.67}{50}=\dfrac13$. Only motor 6 poles and generator 2 poles satisfy this.