1️⃣ Always closed
2️⃣ Always open
3️⃣ May be open or closed depending upon the loading conditions
4️⃣ None of the above
✅ Answer (Detailed Solution Below)
✔️ Option 1: Always closed
📖 Detailed Solution
🔹 An induction motor works on the principle of electromagnetic induction.
🔹 For torque production, rotor current is essential.
🔹 Rotor current can flow only when the rotor circuit is closed.
⚙️ Why Rotor Circuit Must Be Closed
✔️ Rotating magnetic field cuts rotor conductors.
✔️ EMF is induced in rotor.
✔️ Current flows only if circuit is closed.
✔️ Rotor current interacts with stator flux.
✔️ Torque is produced.
👉 If rotor circuit is open → No current → No torque → Motor stops.
📘 Case 1: Squirrel-Cage Induction Motor
🔹 Rotor bars are short-circuited by end rings.
🔹 Circuit is permanently closed.
🔹 No external connection.
👉 Always closed.
📘 Case 2: Wound-Rotor (Slip-Ring) Induction Motor
🔹 Rotor windings connected to slip rings.
🔹 External resistance used during starting.
🔹 During normal running:
✔️ Slip rings are short-circuited.
✔️ External resistance is cut out.
✔️ Rotor becomes short-circuited.
👉 Circuit remains closed during operation.
⚙️ Transformer Analogy
🔹 Induction motor behaves like a transformer.
| Transformer | Induction Motor |
|---|---|
| Primary | Stator |
| Secondary | Rotor |
| Secondary | Short-circuited |
👉 Motor operates like transformer with shorted secondary.
❌ Why Other Options are Incorrect
(2) Always Open ❌
🔹 No rotor current.
🔹 No torque.
🔹 Motor cannot run.
(3) Open or Closed Depending on Load ❌
🔹 Load changes current magnitude.
🔹 Does not open rotor circuit.
🔹 Circuit remains closed.
(4) None of the Above ❌
🔹 Option (1) is correct.
📈 Effect of Load on Rotor Circuit
| Load | Slip | Rotor Current | Circuit |
|---|---|---|---|
| No Load | Small | Small | Closed |
| Full Load | High | High | Closed |
| Starting | Maximum | Maximum | Closed |
👉 Circuit always remains closed.
⭐ Important Exam Points
✔️ Rotor current is necessary
✔️ Closed circuit needed
✔️ Torque depends on rotor current
✔️ Circuit never opens during operation
📌 Key One-Line Exam Statement ⭐
The rotor circuit of an induction motor is always closed during operation to allow current flow and torque production.
📝 Final Conclusion
📍 For an induction motor to run and produce torque, rotor current must flow, which is possible only when the rotor circuit is closed. Hence, under operating conditions, the rotor circuit is always closed.
👉 Therefore, Option 1 is Correct.