(Solved) In a 3-phase induction motor, the reactance under running condition is less than its standstill value. This is due to reduction in:

A) rotor inductance
B) stator magnetic flux
C) frequency of rotor emf
D) mutual flux linking the stator and rotor

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✅ Answer:

➡️ C) frequency of rotor emf

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🧠 Detailed Solution

◆ Step 1: Key Concept

➡️ Rotor reactance depends on frequency

$$X_2=2\pi f_{r}L$$

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◆ Step 2: At Standstill

→ Slip $s=1$
→ Rotor frequency:

$$f_r=f$$

➡️ Reactance is maximum

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◆ Step 3: Under Running Condition

→ Slip decreases

$$f_r=s\times f$$

➡️ Rotor frequency reduces

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◆ Step 4: Effect on Reactance

→ Since $X_2\propto f_r$

➡️ Decrease in $f_r$ ⇒ Decrease in $X_2$

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❌ Why Other Options Are Incorrect

◆ A) rotor inductance → remains constant
◆ B) stator flux → not primary reason
◆ D) mutual flux → not directly responsible

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🔑 Key Points to Remember

◆ Rotor reactance: $X_2=2\pi f_{r}L$
◆ Rotor frequency: $f_r=sf$
◆ Running condition ⇒ slip ↓ ⇒ frequency ↓ ⇒ reactance ↓
◆ Reactance is maximum at standstill

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🎯 Final Conclusion

➡️ Reactance reduces due to decrease in:

Frequency of rotor EMF ✔️