1️⃣ Y–Δ starter
2️⃣ DOL starter
3️⃣ Auto-transformer starter
4️⃣ Interchanging any two of the supply lines
✅ Correct Answer
✔️ Option 4: Interchanging any two of the supply lines
📘 Explanation
The direction of rotation of a 3-phase induction motor depends on the phase sequence of the supply.
1️⃣ Rotating Magnetic Field (RMF)
When a 3-phase AC supply is applied to the stator, it produces a rotating magnetic field.
✔️ This field rotates at synchronous speed.
✔️ The rotor follows this rotating field.
✔️ Rotor starts rotating in the same direction as RMF.
2️⃣ Phase Sequence and Direction
Suppose the original phase sequence is:
👉 A – B – C
If any two supply lines are interchanged (for example A and B), the new sequence becomes:
👉 B – A – C
(or)
👉 A – C – B
This reverses the phase sequence.
3️⃣ Effect of Phase Reversal
Reversed phase sequence
⟹ Reversed rotating magnetic field
⟹ Reversed rotor direction
Therefore, the motor rotates in the opposite direction.
❌ Why Other Options Are Incorrect
1️⃣ Y–Δ Starter ❌
✔️ Used to reduce starting current
✔️ Provides reduced voltage starting
❌ Does NOT change direction
2️⃣ DOL Starter ❌
✔️ Used for direct starting
✔️ Provides full voltage
❌ Does NOT reverse rotation
3️⃣ Auto-transformer Starter ❌
✔️ Reduces starting voltage
✔️ Used for smooth starting
❌ Does NOT affect direction
📌 Important Concept: Direction Reversal Method
✅ Only One Practical Method
To reverse a 3-phase induction motor:
➡ 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 → Same
✔️ Speed → Same
✔️ Only direction changes
✍️ One-Line Exam Statement
The direction of a 3-phase induction motor is reversed by interchanging any two supply lines.
📊 Summary Table
| Method | Purpose | Reverses Direction? |
|---|---|---|
| DOL Starter | Direct starting | ❌ No |
| Y–Δ Starter | Reduce starting current | ❌ No |
| Auto-transformer Starter | Reduce voltage | ❌ No |
| Interchange two lines | Reverse phase sequence | ✅ Yes |
📌 Final Conclusion
Reversing a 3-phase induction motor is achieved by changing the phase sequence, which is done by interchanging any two supply lines.
✔️ Correct Answer: Option 4 — Interchanging any two of the supply lines