Question

A three-phase, 50 Hz, 6-pole induction motor runs at 960 rpm. The stator copper loss, core loss, and the rotational loss of the motor can be neglected. The percentage efficiency of the motor is:

• A. $92$
• B. $94$
• C. $96$
• D. $98$

Hint:

Efficiency of induction motors can be approximated using $\eta = 1 - \text{slip}$ when losses are negligible.

Answer 1

The Correct Option is C

Step 1: Calculating synchronous speed. The synchronous speed is: $$N_s = \frac{120 f}{P} = \frac{120 \times 50}{6} = 1000 \, \text{rpm}.$$ Step 2: Calculating slip. $$\text{Slip} (s) = \frac{N_s - N_r}{N_s} = \frac{1000 - 960}{1000} = 0.04.$$ Step 3: Calculating efficiency. Efficiency ($\eta$) is given by: $$\eta = 1 - s = 1 - 0.04 = 0.96 \, \text{or } 96\%.$$ Hence, the correct option is (3) $96$.