1️⃣ Rotate in the same direction as before maintenance
2️⃣ Rotate in direction opposite to that before maintenance
3️⃣ Not start at all
4️⃣ Get heated and damaged
✅ Correct Answer
✔️ Option 2: Rotate in direction opposite to that before maintenance
📘 Explanation
The direction of rotation of a 3-phase induction motor depends on the phase sequence of the stator supply.
1️⃣ Rotating Magnetic Field (RMF)
When a 3-phase AC supply is applied to the stator windings, it produces a rotating magnetic field in the air gap.
This rotating field is responsible for:
✔️ Inducing current in the rotor
✔️ Producing torque
✔️ Determining direction of rotation
2️⃣ Phase Sequence and Direction
Let the original phase sequence be:
👉 R – Y – B
If any two supply terminals are interchanged (for example R and Y), the new sequence becomes:
👉 Y – R – B
(or)
👉 R – B – Y
This reverses the phase sequence.
3️⃣ Effect of Phase Reversal
Reversal of phase sequence ⇒ Reversal of rotating magnetic field direction
✔️ Rotor always follows the rotating magnetic field
✔️ RMF direction changes
✔️ Rotor direction also changes
So, the motor runs in the opposite direction.
❌ Why Other Options Are Incorrect
1️⃣ Rotate in the same direction ❌
This happens only if:
✔️ No phase is changed
OR
✔️ All three phases are shifted together (RYB → YBR)
Interchanging only two phases always reverses direction.
3️⃣ Not start at all ❌
A 3-phase induction motor is self-starting.
Even with reversed phase sequence:
✔️ Motor will start normally
✔️ Only direction changes
4️⃣ Get heated and damaged ❌
Simply reversing direction does not damage the motor.
Damage may occur only if:
❌ Motor is reversed while running (plugging)
❌ Load cannot run in reverse (pump, compressor, etc.)
Under normal switching, no damage occurs.
📌 Important Concept: Reversing Direction of 3-Phase Motor
✅ Method
To reverse direction:
➡ Interchange any two stator supply lines
Example:
Original → Reversed
R – Y – B → R – B – Y
A – B – C → A – C – B
⚠️ Note
✔️ Number of poles remains same
✔️ Speed remains same
✔️ Only direction changes
📊 Summary Table
| Change Made | Effect |
|---|---|
| Two stator lines interchanged | Direction reverses |
| Rotor/slip ring leads changed | No change |
| No interchange | Same direction |
✍️ One-Line Exam Note
The direction of a 3-phase induction motor is reversed by interchanging any two supply terminals.
📌 Final Conclusion
Interchanging any two supply terminals reverses the phase sequence, which reverses the rotating magnetic field. Therefore, the motor rotates in the opposite direction.
✔️ Correct Answer: Option 2 — Rotate in direction opposite to that before maintenance