The rotor of an induction motor never runs at synchronous speed, because then the relative speed between the rotating flux and rotor will be

February 13, 2026

0

1️⃣ Maximum and hence, torque will be maximum
2️⃣ Maximum and hence, torque will be zero
3️⃣ Zero and hence, torque will be maximum
4️⃣ Zero and hence, torque will be zero

✅ Correct Answer

✔️ Option 4: Zero and hence, torque will be zero

📘 Detailed Explanation

An induction motor works on the principle of electromagnetic induction.

For torque production, there must be relative speed between:

Rotating magnetic field (stator field)
Rotor conductors

1️⃣ Rotating Magnetic Field (RMF)

When AC voltage is applied to the stator:

✔️ A rotating magnetic field is produced.
✔️ This field rotates at synchronous speed (Ns).

N_{s} = \frac{120 f}{P}

Where:
f = Supply frequency
P = Number of poles

2️⃣ Induction of Current in Rotor

The rotating magnetic field cuts the rotor conductors.

This causes:

✔️ Induced EMF in rotor
✔️ Rotor current flows (since rotor is short-circuited)
✔️ Rotor current produces its own magnetic field
✔️ Interaction of stator field and rotor field produces torque

👉 This process occurs only when relative speed exists.

3️⃣ Condition at Synchronous Speed

If:

N_{r} = N_{s}

Then:

Relative Speed = N_{s} - N_{r} = 0

So:

❌ No cutting of flux
❌ No induced EMF
❌ No rotor current
❌ No rotor magnetic field
❌ No torque produced

4️⃣ Effect on Torque

When relative speed = 0:

➡ Induced current = 0
➡ Torque = 0

Hence:

The rotor cannot continue rotating at synchronous speed.

5️⃣ Slip Concept

Slip is defined as:

S = \frac{N_{s} - N_{r}}{N_{s}}

At synchronous speed:

S = 0

Torque in induction motor:

T \propto S

So:

If S = 0 → T = 0

❌ Why Other Options Are Incorrect

1️⃣ Maximum speed → Maximum torque ❌

At synchronous speed, speed difference is zero, not maximum.
Torque cannot be maximum.

2️⃣ Maximum speed → Zero torque ❌

Relative speed is zero, not maximum.

3️⃣ Zero speed → Maximum torque ❌

Zero relative speed gives zero torque, not maximum.

📌 Important Concept

Why Induction Motor Always Runs Below Ns?

Because:

✔ Torque is produced only when slip exists
✔ Slip exists only when Nr < Ns
✔ At Ns → Slip = 0 → Torque = 0

Therefore:

Motor must always run slightly below synchronous speed.

✍️ One-Line Exam Statement

An induction motor never runs at synchronous speed because at that speed slip becomes zero, resulting in zero induced current and zero torque.

📊 Summary Table

Condition	Relative Speed	Induced Current	Torque
Nr < Ns (Normal running)	Present	Yes	Yes
Nr = Ns	Zero	No	No

✅ Final Conclusion

If the rotor reaches synchronous speed, the relative speed becomes zero, no current is induced in the rotor, and torque becomes zero. Hence, an induction motor can never run at synchronous speed.

✔️ Correct Answer: Option 4 — Zero and hence, torque will be zero

Tags: Induction Machine MCQ