The frequency of rotor currents at standstill is:

A) zero
B) 2f
C) f
D) sf

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✅ Answer:

➡️ C) f

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🧠 Detailed Solution

◆ Step 1: Key Concept

➡️ Rotor current frequency depends on slip (s)

$$f_r=s\times f$$

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◆ Step 2: Condition at Standstill

➡️ At standstill:

$$s=1$$

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◆ Step 3: Rotor Frequency

$$f_r=1\times f=f$$

➡️ Rotor experiences full supply frequency

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❌ Why Other Options Are Incorrect

◆ A) zero → Happens only at synchronous speed (s = 0)
◆ B) 2f → No such condition in induction motor
◆ D) sf → General formula, not specific answer

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🔑 Key Points to Remember

◆ Rotor frequency: $f_r=sf$
◆ At standstill → $s=1\Rightarrow f_r=f$
◆ At synchronous speed → $s=0\Rightarrow f_r=0$
◆ Rotor frequency decreases as speed increases

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🎯 Final Conclusion

➡️ At standstill, rotor current frequency =
f (same as supply frequency) ✔️