A) 1,500 rpm
B) 750 rpm
C) 375 rpm
D) 720 rpm
✅ Answer:
D) 720 rpm
🧠Detailed Solution
🔹 1. Given Data:
Supply frequency → f = 50 Hz
Rotor frequency → fr = 2 Hz
Number of poles → P = 8
🔹 2. Calculate Synchronous Speed (Ns):
Ns = (120 × f) / P
Ns = (120 × 50) / 8 = 750 rpm
🔹 3. Calculate Slip (s):
fr = s × f
s = fr / f = 2 / 50 = 0.04
👉 Slip = 4%
🔹 4. Calculate Motor Speed (N):
N = Ns (1 − s)
N = 750 × (1 − 0.04) = 750 × 0.96 = 720 rpm
❌ Why Other Options Are Wrong
A) 1500 rpm ❌ → Corresponds to 4-pole motor, not 8-pole
B) 750 rpm ❌ → This is synchronous speed, not actual speed
C) 375 rpm ❌ → Incorrect calculation
📌 Key Points
• Rotor frequency = Slip × Supply frequency
• Slip = Rotor frequency / Supply frequency
• Actual speed is always less than synchronous speed
• Higher slip → Lower speed
🎯 Final Conclusion
✔ Motor speed = 720 rpm
✔ Correct Option → D