(a) single-phase winding
(b) three-phase winding
(c) heavy copper or aluminium bars placed in rotor slots
(d) heavy short-circuited end rings
Answer: (b) three-phase winding
Explanation:
📖 Detailed Solution
🔹 The rotor of a wound rotor induction motor contains a three-phase distributed winding.
🔹 This winding is similar to the stator winding.
🔹 The rotor winding is generally star-connected.
🔹 The three ends of the winding are connected to slip rings.
🔹 Brushes rest on the slip rings.
🔹 External resistances are connected through these brushes.
🔹 During starting, external resistance is added to control current and torque.
🔹 Hence, this motor is called a slip-ring induction motor.
👉 Therefore, wound rotor motors allow external resistance through slip rings.
⚙️ Main Parts of Wound Rotor Induction Motor are:
✔️ Stator
✔️ Wound Rotor
✔️ Slip Rings
✔️ Carbon Brushes
✔️ Shaft
✔️ Bearings
✔️ External Resistance
🔄 Rotor (Wound Type / Slip Ring Type):
🔹 Laminated steel core with slots.
🔹 Three-phase distributed winding.
🔹 Star-connected rotor winding.
🔹 Connected to slip rings.
🔹 Brushes provide external connection.
🔹 Allows insertion of external resistance.
🔹 Construction is comparatively complex.
🚀 Adding External Resistance to Rotor
🔹 External resistance is connected through brushes and slip rings.
🔹 It increases rotor resistance during starting.
🔹 This reduces starting current.
🔹 It improves starting torque.
🔹 As speed increases, resistance is gradually cut out.
🔢 Starting Torque Relation:
Where:
T = Starting Torque
R = Rotor Resistance
s = Slip
👉 Increasing rotor resistance increases starting torque.
❌ Why Other Options are Incorrect
(a) Single-phase winding ❌
🔹 Used in single-phase motors.
🔹 Not used in 3-phase wound rotor motors.
(c) Heavy copper or aluminium bars ❌
🔹 This is squirrel cage rotor construction.
🔹 Not applicable to wound rotor motors.
(d) Heavy short-circuited end rings ❌
🔹 Used in squirrel cage motors.
🔹 Wound rotors use slip rings and brushes.
📌 Additional Information
⭐ Only wound rotor induction motors allow external resistance.
⭐ This method is used for high starting torque.
⭐ Not applicable to squirrel cage motors.
⭐ Improves smooth starting under heavy load.
⚖️ Comparison: Slip Ring Motor vs Squirrel Cage Motor
| Feature | Slip Ring Motor | Squirrel Cage Motor |
|---|---|---|
| Rotor Type | Slip ring rotor | Squirrel cage rotor |
| Construction | Complicated | Simple |
| External Resistance | Can be added | Cannot be added |
| Starting Torque | High | Low |
| Brushes | Present | Absent |
| Maintenance | High | Low |
| Power Factor | Low | High |
| Speed Control | Possible | Not possible |
| Starter | Rotor resistance starter | Not used |
📝 Final Summary
📍 In a wound rotor (slip-ring) induction motor:
✔️ Rotor has three-phase winding
✔️ Connected to slip rings
✔️ External resistance can be added
✔️ High starting torque is obtained
✔️ Speed control is possible
👉 Hence, Option b is Correct.