The rotor of a 4-pole, 50 Hz, 3-phase slip-ring induction motor runs clockwise when stator terminals 1,2,3 are connected to A,B,C. If connections are changed to A,C,B, the rotor runs anticlockwise at synchronous speed. The frequency of induced EMF across open-circuited rotor terminals is:

A) 50 Hz
B) zero
C) 25 Hz
D) 100 Hz

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✅ Answer:

➡️ B) zero

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🧠 Detailed Solution

◆ Step 1: Key Concept

➡️ Rotor frequency depends on slip

$$f_r=s\times f$$

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◆ Step 2: Given Condition

➡️ Rotor runs at synchronous speed

$$N=N_s$$

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◆ Step 3: Calculate Slip

$$s=\frac{N_s-N}{N_s}=0$$

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◆ Step 4: Rotor Frequency

$$f_r=0\times f=0$$

➡️ No relative motion between rotor and RMF

➡️ Hence no induced EMF frequency

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❌ Why Other Options Are Incorrect

◆ A) 50 Hz → Only at standstill
◆ C) 25 Hz → Requires slip = 0.5
◆ D) 100 Hz → Not possible

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🔑 Key Points to Remember

◆ Rotor frequency: $f_r=sf$
◆ At synchronous speed → $s=0\Rightarrow f_r=0$
◆ No relative motion ⇒ No induced EMF
◆ Phase reversal changes direction only, not frequency

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🎯 Final Conclusion

➡️ At synchronous speed:
Rotor EMF frequency = 0 Hz ✔️