(a) delta
(b) star
(c) delta or star according to need
(d) none of the above
✅ Answer (Detailed Solution Below)
✔️ Option (b): star
📖 Detailed Solution
🔹 In a 3-phase slip-ring induction motor, the rotor has a three-phase distributed winding.
🔹 This winding is generally connected in star.
🔹 The three free ends are connected to three slip rings.
🔹 External resistance is added through these slip rings.
🔹 Star connection requires only three slip rings.
🔹 Delta connection would require six slip rings.
🔹 Hence, star connection reduces construction complexity.
🔹 It also simplifies starting and speed control.
👉 Therefore, the rotor winding is generally connected in star.
⚙️ Main Parts Related to Rotor Connection are:
✔️ Rotor Winding
✔️ Slip Rings
✔️ Carbon Brushes
✔️ External Resistance
✔️ Shaft
✔️ Bearings
🔄 Rotor Winding Connection:
🔹 Rotor winding is similar to stator winding.
🔹 Can be connected in star or delta.
🔹 Default connection is star in slip-ring motors.
🔹 Enables insertion of external resistance.
🔹 Provides balanced operation.
🔹 Offers a neutral point.
📌 Comparison of Rotor Connections:
| Connection | Slip Rings Required | External Resistance | Preference |
|---|---|---|---|
| Star (Y) | 3 | Easy | Preferred |
| Delta (Δ) | 6 | Difficult | Rare |
⚡ Stator and Rotor Windings:
🔹 Stator slots carry three-phase windings.
🔹 Rotor slots also carry three-phase windings.
🔹 Both windings are distributed.
🔹 Rotor winding resembles stator winding in construction.
🚀 Why Star Connection is Preferred
🔹 Easy insertion of external resistance.
🔹 Requires fewer slip rings.
🔹 Simplifies control circuit.
🔹 Improves starting performance.
🔹 Reduces cost and maintenance.
❌ Why Other Options are Incorrect
(1) Delta (Δ) ❌
🔹 Requires six slip rings.
🔹 Complicates external resistance connection.
🔹 Rarely used.
(3) Zig-zag ❌
🔹 Not used in induction motor rotors.
(4) Open Circuit ❌
🔹 Rotor must form a closed circuit to operate.
📌 Key Exam Notes
⭐ Rotor Type → Wound Rotor (Slip Ring)
⭐ Default Connection → Star (Y)
⭐ Slip Rings → 3
⭐ External Resistance → Possible
⭐ Starting Control → Good
📝 Final Summary
📍 In a 3-phase slip-ring induction motor:
✔️ Rotor has three-phase winding
✔️ Generally star-connected
✔️ Uses three slip rings
✔️ Allows external resistance
✔️ Improves starting and control
👉 Hence, Option b is Correct.