Question

A 12-pole, 3-phase, 50 Hz induction motor runs at 475 rev/min. Determine the slip speed and frequency of the rotor currents.

• A. 20 rev/min, 2.5 Hz
• B. 25 rev/min, 3.5 Hz
• C. 25 rev/min, 2.5 Hz
• D. 20 rev/min, 3.5 Hz

Hint:

Rotor current frequency in an induction motor is directly proportional to slip. At synchronous speed, rotor frequency becomes zero.

Answer 1

The Correct Option is A

Step 1: Calculate the synchronous speed of the motor.
Synchronous speed is given by
\[ N_s = \frac{120 f}{P} \]
\[ N_s = \frac{120 \times 50}{12} = 500 \text{ rev/min} \]
Step 2: Calculate the slip speed.
\[ \text{Slip speed} = N_s - N \]
\[ \text{Slip speed} = 500 - 475 = 25 \text{ rev/min} \]
Step 3: Calculate the slip of the motor.
\[ s = \frac{N_s - N}{N_s} = \frac{25}{500} = 0.05 \]
Step 4: Calculate the rotor current frequency.
Rotor current frequency is given by
\[ f_r = s f \]
\[ f_r = 0.05 \times 50 = 2.5 \text{ Hz} \]
Step 5: Conclusion.
The slip speed and rotor current frequency are
\[ \boxed{25 \text{ rev/min and } 2.5 \text{ Hz}} \]