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GATE 2020 EE – Question 17

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 three-phase cylindrical rotor synchronous generator has a synchronous reactance $X_s$ and a negligible armature resistance. The magnitude of per phase terminal voltage is $V_A$ and the magnitude of per phase induced emf is $E_A$. Considering the following two statements, P and Q,

P: For any three-phase balanced leading load connected across the terminals of this synchronous generator, $V_A$ is always more than $E_A$.

Q: For any three-phase balanced lagging load connected across the terminals of this synchronous generator, $V_A$ is always less than $E_A$.

which of the following options is correct?

  1. P is false and Q is true.
  2. P is true and Q is false.
  3. P is false and Q is false.
  4. P is true and Q is true.

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Show answer and explanation

Correct answer: (A) P is false and Q is true.

Explanation

With a lagging load the armature reaction is demagnetizing, so $V_A<E_A$ always (Q is true). For a leading load $V_A$ exceeds $E_A$ only when the load is sufficiently leading; a load at unity power factor, or slightly leading, still gives $V_A<E_A$. So P is false.