Question

The purpose of connecting resistance in the rotor circuit of slip ring induction motor is _______.

• A. to increase running torque
• B. to increase starting torque
• C. to achieve maximum running torque
• D. to increase motor efficiency

Hint:

Always add external rotor resistance to improve starting torque — especially in heavy load conditions.

Answer 1

The Correct Option is B

Step 1: Basic principle of slip ring motor.
Rotor resistance affects torque-slip characteristics.
Step 2: At start, slip $s = 1$.
Torque $T \propto \frac{sR_2}{(R_2)^2 + (sX_2)^2}$
Increasing $R_2$ increases numerator significantly at start, giving high torque.
Step 3: Why not other options?
- Running torque depends on rotor design, not resistance
- Maximum torque is fixed; we only shift the slip at which it occurs
- Higher $R_2$ reduces efficiency (waste as heat)
Conclusion: $\boxed{\text{To increase starting torque}}$