Question

Synchronous Speed of an AC induction motor depends on
(A). Frequency of the supply voltage (f)
(B). Number of poles (n)
(C). Current (I)
(D). Voltage (V)
(E). Resistance (R)

Choose the correct answer from the options given below:

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

Hint:

Remember the classic speed formula \(N_s = 120f / P\).
Only speed-determining variables are frequency \(f\) and pole count \(P\).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Synchronous speed is the rotation speed of the rotating magnetic field created by the stator windings in an AC motor.

Step 2: Key Formula or Approach:
The synchronous speed \(N_s\) (in rpm) is defined as:
\[ N_s = \frac{120 f}{P} \] where:
\(f\) = supply frequency (Hz)
\(P\) = number of stator poles (denoted as \(n\) in the question alternatives)

Step 3: Detailed Explanation:
Looking at the formula, the synchronous speed is:
1. Directly proportional to the supply frequency (\(f\)).
2. Inversely proportional to the number of poles (\(P\) or \(n\)).
It is completely independent of load current (\(I\)), terminal voltage (\(V\)), and winding resistance (\(R\)).
Thus, only parameters (A) and (B) dictate the synchronous speed of the AC motor.

Step 4: Final Answer:
The correct option is 1, which corresponds to (A) and (B) only.