(Solved) For a slip-ring induction motor, if the rotor resistance is increased:

A) starting torque and efficiency increase
B) starting torque decreases but efficiency increases
C) starting torque increases but efficiency decreases
D) starting torque and efficiency decrease

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✅ Answer:

➡️ C) starting torque increases but efficiency decreases

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🧠 Detailed Solution

◆ Step 1: Starting Torque Relation

➡️ At starting ($s=1$):

$$T_{st}\propto \frac{R_2}{R_2^2+X_2^2}$$

➡️ Since $R_2\ll X_2$:

$$T_{st}\propto R_2$$

➡️ Increasing $R_2$ ⇒ Starting torque increases

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◆ Step 2: Effect on Efficiency

➡️ Rotor copper loss:

$$\text{Loss}\propto I^2{R}_2$$

➡️ Increasing $R_2$ ⇒ losses increase

➡️ Hence efficiency decreases

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◆ Step 3: Physical Interpretation

→ High resistance improves starting performance
→ But causes more heat loss during running

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❌ Why Other Options Are Incorrect

◆ A) both increase → efficiency actually decreases
◆ B) torque decreases → incorrect
◆ D) both decrease → incorrect

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🔑 Key Points to Remember

◆ Starting torque ∝ rotor resistance
◆ Higher resistance ⇒ better starting torque
◆ Higher resistance ⇒ higher losses ⇒ lower efficiency
◆ Used only during starting, then removed

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🎯 Final Conclusion

➡️ Increasing rotor resistance results in:
Higher starting torque but lower efficiency ✔️