A) 1.5 Hz
B) 48.5 Hz
C) 5.15 Hz
D) 75 Hz
✅ Answer:
A) 1.5 Hz
🧠Detailed Solution
🔹 1. Given Data:
Supply frequency → f = 50 Hz
Number of poles → P = 8
Rotor speed → N = 727.5 rpm
🔹 2. Calculate Synchronous Speed (Ns):
Ns = (120 × f) / P
Ns = (120 × 50) / 8 = 750 rpm
🔹 3. Calculate Slip (s):
s = (Ns - N) / Ns
s = (750 - 727.5) / 750 = 22.5 / 750 = 0.03
👉 Slip = 3%
🔹 4. Calculate Rotor Frequency (fr):
fr = s × f
fr = 0.03 × 50 = 1.5 Hz
❌ Why Other Options Are Wrong
B) 48.5 Hz ❌ → Rotor frequency is always less than supply frequency
C) 5.15 Hz ❌ → Incorrect slip calculation
D) 75 Hz ❌ → Not possible (greater than supply frequency)
📌 Key Points
• Synchronous speed depends on frequency and poles
• Slip indicates how much rotor lags behind rotating field
• Rotor frequency = Slip × Supply frequency
• Rotor frequency is very low in normal operation
🎯 Final Conclusion
✔ Rotor EMF frequency = 1.5 Hz
✔ Correct Option → A