A) 1,500 rpm
B) 1,000 rpm
C) 750 rpm
D) Zero
✅ Answer:
C) 750 rpm
🧠Detailed Solution
🔹 1. Given Data:
Supply frequency → f = 50 Hz
Rotor frequency → fr = 25 Hz
Number of poles → P = 4
🔹 2. Calculate Synchronous Speed (Ns):
Ns = (120 × f) / P
Ns = (120 × 50) / 4 = 1500 rpm
🔹 3. Calculate Slip (s):
fr = s × f
s = fr / f = 25 / 50 = 0.5
👉 Slip = 50%
🔹 4. Calculate Motor Speed (N):
N = Ns (1 − s)
N = 1500 × (1 − 0.5) = 1500 × 0.5 = 750 rpm
❌ Why Other Options Are Wrong
A) 1500 rpm ❌ → This is synchronous speed (only at zero slip)
B) 1000 rpm ❌ → Incorrect slip assumption
D) Zero ❌ → Motor is running, not at standstill
📌 Key Points
• Rotor frequency = Slip × Supply frequency
• Slip = Rotor frequency / Supply frequency
• Higher rotor frequency → Higher slip → Lower speed
• At slip = 0.5 → Speed becomes half of synchronous speed
🎯 Final Conclusion
✔ Motor speed = 750 rpm
✔ Correct Option → C