(Solved) In a 3-phase slip-ring induction motor, high starting torque is achieved by:

A) Adding external resistance to rotor
B) Adding external inductance to rotor
C) Adding external capacitance to rotor
D) Adding external RLC circuit to rotor

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✅ Answer:

➡️ A) Adding external resistance to rotor

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🧠 Detailed Solution

◆ Step 1: Key Concept

➡️ Starting torque depends on rotor resistance:

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

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◆ Step 2: Condition for Maximum Starting Torque

➡️ At starting:

$$s=1$$

➡️ Maximum torque condition:

$$R_2=X_2$$

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◆ Step 3: Practical Problem

→ Normally, rotor resistance is low
→ So starting torque is low

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◆ Step 4: Solution

➡️ Add external resistance in rotor circuit

→ Increases $R_2$
→ Improves power factor
→ Increases starting torque

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◆ Step 5: Practical Implementation

➡️ Slip-ring motor uses:
→ Slip rings + brushes
→ External resistors connected in rotor circuit
→ Resistance gradually removed during running

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❌ Why Other Options Are Incorrect

◆ B) inductance → increases reactance ⇒ reduces torque
◆ C) capacitance → not used in rotor circuit
◆ D) RLC → not practical for this purpose

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🔑 Key Points to Remember

◆ Starting torque ↑ with rotor resistance
◆ Maximum torque when $R_2=X_2$
◆ Only slip-ring motors allow external resistance
◆ Resistance is removed after starting

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🎯 Final Conclusion

➡️ High starting torque is achieved by:
Adding external resistance in rotor circuit ✔️