In a 3-phase, 50 Hz induction motor, the voltage between slip rings at standstill is 50 V. At full load, the slip is 0.04. The voltage between slip rings at full load is:

A) 50 V
B) 2 V
C) 20 V
D) 5 V

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✅ Answer:

➡️ B) 2 V

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🧠 Detailed Solution

◆ Step 1: Key Concept

➡️ Rotor induced voltage is proportional to slip (s)

$$E_2=s\times E_{20}$$

Where:
→ $E_{20}$ = standstill rotor voltage
→ $E_2$ = rotor voltage at running condition

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◆ Step 2: Given Data

→ Standstill voltage $E_{20}=50V$
→ Slip $s=0.04$

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◆ Step 3: Calculate Rotor Voltage

$$E_2=0.04\times 50$$$$E_2=2V$$

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❌ Why Other Options Are Incorrect

◆ A) 50 V → Only at standstill (s = 1)
◆ C) 20 V → Incorrect multiplication
◆ D) 5 V → Wrong calculation

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🔑 Key Points to Remember

◆ Rotor voltage: $E_2=s{E}_{20}$
◆ Voltage reduces as slip decreases
◆ At full load → slip is small ⇒ voltage is low
◆ At synchronous speed → voltage = 0

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🎯 Final Conclusion

➡️ Voltage between slip rings at full load =
2 V ✔️