(Solved) For achieving high starting torque and high operating efficiency, an induction motor should have ______ rotor circuit resistance at starting and ______ rotor circuit resistance under operating condition.

A) high, low
B) low, high
C) high, high
D) low, low

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✅ Answer:

➡️ A) high, low

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🧠 Detailed Solution

◆ Step 1: Starting Condition

➡️ At starting:

$$s=1$$

➡️ Torque expression:

$$T\propto \frac{R_2}{R_2^2+X_2^2}$$

➡️ For maximum starting torque:

$$R_2\approx X_2$$

➡️ Hence, high rotor resistance is required

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◆ Step 2: Effect at Starting

→ High $R_2$:
✔ Improves starting torque
✔ Improves power factor
✔ Reduces starting current

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◆ Step 3: Running Condition

➡️ During normal operation:
→ Slip is very small

➡️ Copper loss:

$$\text{Loss}\propto I^{2}R$$

➡️ To reduce losses ⇒ low rotor resistance required

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◆ Step 4: Practical Implementation

➡️ In slip-ring motor:
→ External resistance added at start
→ Gradually removed during running

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❌ Why Other Options Are Incorrect

◆ B) low, high → Opposite requirement
◆ C) high, high → High losses in running
◆ D) low, low → Poor starting torque

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🔑 Key Points to Remember

◆ Starting → High resistance ⇒ high torque
◆ Running → Low resistance ⇒ high efficiency
◆ External resistance used only during starting
◆ Trade-off between torque and efficiency

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🎯 Final Conclusion

➡️ Required condition:
High resistance at start & Low resistance during running ✔️