1️⃣ Synchronous motor
2️⃣ Repulsion start induction motor
3️⃣ Transformer with a shorted secondary
4️⃣ Capacitor-start induction motor
✅ Answer (Detailed Solution Below)
✔️ Option 3: Transformer with a shorted secondary
📖 Detailed Solution
🔹 A 3-phase induction motor works on the principle of electromagnetic (mutual) induction.
🔹 This principle is the same as that of a transformer.
Similarity with Transformer
🔹 In a transformer:
✔️ Primary winding is connected to AC supply.
✔️ Alternating current produces alternating flux.
✔️ Flux induces EMF in secondary winding.
✔️ Secondary current flows if circuit is closed.
🔹 In a 3-phase induction motor:
✔️ Stator winding → Primary
✔️ Rotor winding → Secondary
✔️ AC supply produces rotating magnetic field.
✔️ Rotating flux cuts rotor conductors.
✔️ EMF is induced in rotor.
✔️ Rotor circuit is short-circuited.
✔️ Rotor current flows.
✔️ Interaction produces torque.
👉 Hence, induction motor behaves like a transformer with shorted secondary.
⚙️ Why Induction Motor is Called a Generalized Transformer
✔️ Stator = Primary winding
✔️ Rotor = Secondary winding
✔️ Power transferred by induction
✔️ No electrical connection between stator and rotor
👉 Therefore, induction motor is also called rotating transformer.
📊 Transformer vs Induction Motor
| Feature | Transformer | Induction Motor |
|---|---|---|
| Flux | Alternating | Rotating |
| Air Gap | No | Present |
| Secondary | Stationary | Rotating |
| Energy Output | Electrical | Mechanical |
| Secondary Frequency | Same as supply | Depends on slip |
❌ Why Other Options are Incorrect
(1) Synchronous Motor ❌
🔹 Works on magnetic locking.
🔹 Rotor is excited by DC.
🔹 No induced rotor current in steady state.
🔹 Not based on induction.
(2) Repulsion Start Induction Motor ❌
🔹 Starts by repulsion principle.
🔹 Later runs as induction motor.
🔹 Not purely based on induction from start.
(4) Capacitor-Start Induction Motor ❌
🔹 Single-phase motor.
🔹 Uses capacitor for starting.
🔹 Different construction.
🔹 Not direct analogy.
📈 Important Points
✔️ Induction motor → Rotating flux transformer
✔️ Transformer → Alternating flux transformer
✔️ Rotor frequency depends on slip
✔️ Mechanical power output in motor
⭐ Key Exam Notes
⭐ Induction motor = Generalized transformer
⭐ Rotor = Short-circuited secondary
⭐ Power transferred by induction
⭐ Converts electrical → mechanical energy
📌 Key One-Line Exam Statement ⭐
A 3-phase induction motor works on the same principle as a transformer with a short-circuited secondary.
📝 Final Conclusion
📍 A 3-phase induction motor operates on electromagnetic induction, where the stator acts as the primary and the rotor acts as a short-circuited secondary, similar to a transformer.
👉 Hence, Option 3 is Correct.