Question

Synchronous Speed of an AC induction motor depends on:
(A) Frequency of the supply voltage
(B) Number of poles
(C) Current
(D) Voltage
Choose the correct answer from the options given below:

• (A) and (B) only.
• (B) and (C) only.
• (C) and (D) only.
• (A) and (D) only.

Hint:

Recall the formula Ns = 120f/P and see which quantities it actually contains.

Answer 1

The Correct Option is A

Step 1: The synchronous speed of an AC induction motor is the speed at which the rotating magnetic field of the stator winding rotates, given by \(N_s = \frac{120f}{P}\), where f is the supply frequency in Hz and P is the number of poles.
Step 2: From this formula, synchronous speed depends only on the supply frequency f (statement A) and the number of poles P (statement B), since both appear directly in the formula.
Step 3: Current (C) results from the load on the motor and affects torque and slip, but does not enter the synchronous speed formula. Voltage (D) affects torque and magnetising current but has no term in the synchronous speed formula either.
Step 4: Therefore only (A) and (B) determine synchronous speed, option 1.