A 3-phase slip-ring induction motor is wound for 4 poles on stator and 6 poles on rotor. When a 50 Hz balanced supply is applied, the motor will run at:

A) 1500 rpm
B) 1000 rpm
C) 750 rpm
D) zero speed

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✅ Answer:

➡️ D) zero speed

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🧠 Detailed Solution

◆ Step 1: Synchronous Speed (Stator Field)

$N_s=\frac{120f}{P}$

$$N_s=\frac{120\times 50}{4}=1500\text{rpm}$$

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◆ Step 2: Key Concept (Very Important 🚨)

➡️ In induction motor:
Torque is produced only when stator and rotor poles are same

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◆ Step 3: Given Condition

→ Stator poles = 4
→ Rotor poles = 6

➡️ Poles are different

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◆ Step 4: Physical Meaning

→ Stator produces 4-pole rotating magnetic field
→ Rotor is wound for 6 poles

➡️ Magnetic fields do not synchronize
➡️ No proper electromagnetic coupling

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◆ Step 5: Final Result

→ No coupling ⇒ No torque
→ No torque ⇒ No rotation

➡️ Speed = 0 rpm

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❌ Why Other Options Are Incorrect

◆ A) 1500 rpm → Only if motor runs at synchronous speed (not possible here)
◆ B) 1000 rpm → No such condition
◆ C) 750 rpm → Incorrect assumption

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🔑 Key Points to Remember

◆ Torque requires same number of poles (stator = rotor)
◆ Pole mismatch ⇒ No rotating magnetic interaction
◆ No interaction ⇒ No torque ⇒ No motion
◆ Common concept-based trap question

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🎯 Final Conclusion

➡️ Due to pole mismatch (4 ≠ 6):
Motor will not run ⇒ Speed = 0 rpm ✔️