(a) dc only
(b) ac only
(c) ac and dc both
(d) none of these
Answer: (b) ac only
Explanation:
CONCEPT: Working Principle of Induction Motor
- An induction motor works on the principle of electromagnetic induction.
- It requires a rotating magnetic field (RMF) for operation.
- This rotating magnetic field is produced only when:
- A three-phase AC supply is applied to the stator winding.
ROLE OF AC SUPPLY
When AC flows in stator windings:
- An alternating current produces an alternating flux.
- Three-phase currents create a rotating magnetic field.
- This rotating field cuts the rotor conductors.
- According to Faraday’s Law:
- EMF is induced in rotor.
- Rotor current flows.
- Interaction between stator flux and rotor current produces torque.
Thus:
👉 AC supply is essential.
WHY DC SUPPLY CANNOT RUN INDUCTION MOTOR
- When DC is applied:
- Current is constant.
- Flux becomes constant.
- No rotating magnetic field is produced.
- No EMF is induced in rotor.
- No rotor current flows.
- No electromagnetic torque is produced.
Therefore:
❌ Induction motor cannot run on DC supply.
WHY OTHER OPTIONS ARE INCORRECT
(a) DC Only ❌
- DC produces stationary magnetic field.
- No induction occurs.
- Motor will not rotate.
(c) AC and DC Both ❌
- Only special motors (universal motors) can run on AC and DC.
- Induction motors are not designed for DC operation.
(d) None of These ❌
- Since AC only is correct, this option is wrong.
UNIVERSAL MOTOR (FOR COMPARISON)
On DC Supply
- Works like DC series motor.
- Has commutator and brushes.
- Produces unidirectional torque.
On AC Supply
- Field and armature current reverse together.
- Torque remains in same direction.
- Motor continues to rotate.
Hence:
👉 Universal motor works on both AC and DC.
But:
👉 Induction motor does NOT.
IMPORTANT EXAM POINTS ⭐
One-Line Facts
✔ Induction motor runs only on AC
✔ Requires rotating magnetic field
✔ DC cannot produce RMF
✔ Universal motor runs on AC and DC
MCQ Tip
Q: Which motor runs on DC only?
👉 DC motor
Q: Which motor runs on AC only?
👉 Induction motor
Q: Which motor runs on both?
👉 Universal motor
FINAL CONCLUSION ⭐
An induction motor requires a rotating magnetic field for operation, which is produced only by AC supply. A DC supply creates a constant magnetic field and cannot induce rotor current or torque. Therefore, an induction motor operates only on AC supply.
KEY ONE-LINE EXAM STATEMENT ⭐
An induction motor runs only on AC supply because DC cannot produce a rotating magnetic field.