A) zero
B) 50 Hz
C) 100 Hz
D) 200 Hz
✅ Answer:
➡️ C) 100 Hz
🧠Detailed Theory + Solution
◆ Step 1: Synchronous Speed
N_s = \frac{120 \times 50}{6} = 1000 \, \text{rpm}
◆ Step 2: Rotor Speed Condition
➡️ Rotor is driven at 1000 rpm in opposite direction
So:
N = -1000 \, \text{rpm}
◆ Step 3: Slip Calculation
s = \frac{N_s - N}{N_s}
= \frac{1000 - (-1000)}{1000}
= \frac{2000}{1000} = 2
◆ Step 4: Rotor Frequency
➡️ Rotor (slip-ring) frequency is:
f_r = s f
f_r = 2 \times 50 = 100 \, \text{Hz}
◆ Step 5: Key Insight 🚨
➡️ Rotor running opposite → relative speed doubles
➡️ Slip becomes greater than 1 (s = 2)
➡️ Rotor frequency increases accordingly
❌ Why Other Options Are Incorrect
◆ A) zero → Only at synchronous speed same direction
◆ B) 50 Hz → Only at standstill
◆ D) 200 Hz → Overestimation (double counting error)
🔑 Key Points to Remember
◆ Rotor frequency:
◆ Opposite rotation ⇒ slip can be > 1
◆ At standstill →
◆ At synchronous speed →
🎯 Final Conclusion
➡️ Slip = 2 due to opposite rotation
➡️ Rotor frequency = 100 Hz ✔️