1️⃣ Run normally
2️⃣ Get overheated
3️⃣ Not run
4️⃣ Make noise
✅ Answer (Detailed Solution Below)
✔️ Option 3: Not run
📖 Detailed Solution
🔹 A 3-phase induction motor works on the principle of electromagnetic induction.
🔹 Torque production in an induction motor requires:
✔️ Induced rotor EMF
✔️ Rotor current
✔️ Interaction between stator flux and rotor current
⚙️ Why Rotor Current is Necessary
🔹 When AC supply is given to stator:
✔️ Rotating magnetic field (RMF) is produced.
✔️ RMF cuts rotor conductors.
✔️ EMF is induced in rotor.
✔️ Rotor current flows (only if circuit is closed).
✔️ Torque is produced.
👉 Rotor current is essential for motor operation.
⚙️ Condition: Rotor Circuit Open
🔹 If rotor circuit is open:
❌ No current can flow in rotor
❌ Induced EMF cannot produce current
❌ No magnetic interaction
❌ No torque produced
👉 Therefore, motor cannot start or run.
📐 Relation with Slip and Rotor Frequency
Where:
fr = Rotor frequency
s = Slip
f = Supply frequency
🔹 At Starting
✔️ Rotor speed = 0
✔️ Slip = 1
✔️ Rotor frequency = f (maximum)
🔹 At Synchronous Speed
✔️ Rotor speed = Ns
✔️ Slip = 0
✔️ Rotor frequency = 0
✔️ No EMF induced
👉 Same situation as open rotor → No torque.
❌ Why Other Options are Incorrect
(1) Run Normally ❌
🔹 No rotor current.
🔹 No torque.
🔹 Motor cannot run.
(2) Get Overheated ❌
🔹 Overheating occurs in:
Locked rotor
Single phasing
Overloading
🔹 Open rotor mainly causes no operation.
(4) Make Noise ❌
🔹 Noise occurs in:
Single phasing
Unbalanced supply
Mechanical faults
🔹 Open rotor mainly causes stoppage.
📘 Comparison with Normal Operation
| Condition | Rotor Circuit | Rotor Current | Torque | Motor Runs |
|---|---|---|---|---|
| Normal | Closed | Present | Present | Yes |
| Open Rotor | Open | Absent | Zero | No |
⭐ Important Exam Points
✔️ Rotor current → Torque
✔️ Open rotor → No current
✔️ No current → No torque
✔️ Motor stops
📌 Key One-Line Exam Statement ⭐
If the rotor circuit of an induction motor is open, no current flows in the rotor and hence the motor will not run.
📝 Final Conclusion
📍 A 3-phase induction motor requires rotor current for torque production. When the rotor circuit is open, no current flows, no torque is produced, and the motor cannot run.
👉 Therefore, Option 3 is Correct.