If the speed of a 3-phase, 400 V, 50 Hz synchronous motor is tripled, the efficiency of the machine will _______.

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  1. become zero
  2. reduce to one-third
  3. remain constant
  4. become 3 times

Answer (Detailed Solution Below)

Option 3 : remain constant
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Power flow in the synchronous motor

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The efficiency of the synchronous motor is given by:

% η \(={P_{out}\over P_{in}}\times 100\)

% η = \(={P_{in}-3I_a^2R_a-P_c\over P_{in}}\times 100\)

Explanation

The synchronous speed is given by:

\(N_s={120f\over P}\)

where, Ns = Synchronous speed

f = Frequency

P = No. of poles

The efficiency of the motor is independent of the synchronous speed, hence the efficiency of the machine will remain constant.

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