(Solved) Insertion of resistance in the rotor circuit of an induction motor to develop a given torque causes ______ in rotor current.

A) almost no change
B) increase
C) decrease
D) none of these

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✅ Answer:

➡️ A) almost no change

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🧠 Detailed Solution

◆ Step 1: Key Concept

➡️ Torque in induction motor:

$$T\propto I_2^2\cdot \frac{R_2}{s}$$

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◆ Step 2: Effect of Adding Rotor Resistance

→ Rotor resistance $R_2$ increases
→ To maintain same torque, motor adjusts slip (s)

➡️ Slip increases (motor slows slightly)

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◆ Step 3: Current Behavior

➡️ Increase in slip → increases induced rotor EMF
➡️ This compensates increased resistance

👉 Result:

→ Rotor current magnitude remains almost same

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◆ Step 4: Physical Interpretation

➡️ Operating point shifts:
→ Higher slip
→ Same torque
→ Nearly same current

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❌ Why Other Options Are Incorrect

◆ B) increase → Not required for same torque
◆ C) decrease → Not possible (torque must be maintained)
◆ D) none → Incorrect

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🔑 Key Points to Remember

◆ Adding rotor resistance shifts torque-speed curve
◆ Slip increases for same torque
◆ Rotor current remains approximately constant
◆ Used for high starting torque control

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🎯 Final Conclusion

➡️ For same torque, rotor current shows:
Almost no change ✔️