Question

The frequency of the emf induced in the rotor circuit of a three phase. 50 Hz, 4 pole induction motor running at 1425 RPM is

• A. 0.25 Hz
• B. 0.025 Hz
• C. 2.5 Hz
• D. 25 Hz

Hint:

Rotor frequency is directly proportional to slip. At standstill (start), slip is 1, so $f_r = f$. As the motor speeds up and approaches $N_s$, the slip and rotor frequency both decrease toward zero.

Answer 1

The Correct Option is C

Step 1: Find the slip speed directly, the RPM difference between synchronous speed and actual rotor speed. For a 4-pole, 50 Hz motor, synchronous speed is the standard value \( N_s = 1500 \) RPM, so the slip speed is \( 1500 - 1425 = 75 \) RPM.

Step 2: Use the direct shortcut relating slip speed to rotor frequency, \( f_r = \frac{P \times (N_s - N)}{120} \), which skips computing the slip fraction as a separate step.

Step 3: Substitute \( P = 4 \) and the slip speed of 75 RPM: \( f_r = \frac{4 \times 75}{120} = \frac{300}{120} \).
\[ \boxed{f_r = 2.5 \text{ Hz}} \]