A) Ns
B) sNs
C) N
D) sN
✅ Answer: B) sNs
🧠Detailed Solution:
◆ Step 1: Rotor Current Frequency
Rotor current frequency fr = s × f
where s = slip and f = supply frequency
◆ Step 2: Speed of Rotating Magnetic Field
N = (120 × f) / P
◆ Step 3: Rotor Field Speed
Nrel = (120 × fr) / P
Substitute fr = s × f:
Nrel = (120 × s × f) / P
Nrel = s × (120 × f / P)
Nrel = sNs
◆ Step 4: Interpretation
→ Rotor magnetic field rotates at slip speed relative to rotor surface
→ This is called slip speed
❌ Why Other Options Are Incorrect
A) Ns → Speed with respect to stator, not rotor
C) N → Rotor mechanical speed only
D) sN → Not a valid relation
🔑 Key Points
◆ Rotor frequency = s × f
◆ Rotor field speed w.r.t rotor = sNs
◆ Rotor field speed w.r.t stator = Ns
◆ Important concept for induction motor theory
🎯 Final Conclusion
Speed of rotating field due to rotor currents relative to rotor surface = sNs