(a) external star-connected resistors
(b) dc supply
(c) 3-phase ac supply
(d) equalizing coils
✅ Answer (Detailed Solution Below)
✔️ Option (a): External star-connected resistors
📖 Detailed Solution
🔹 In a slip-ring induction motor, the rotor has three-phase windings.
🔹 These windings are generally connected in star.
🔹 The three free ends are connected to slip rings.
🔹 Carbon brushes rest on the slip rings.
🔹 External resistors (rotor rheostat) are connected through brushes.
🔹 These resistors are connected in star.
🔹 During starting, resistance is added in series with rotor circuit.
🔹 This increases rotor resistance.
🔹 It improves starting torque.
🔹 It reduces high starting current.
🔹 As motor speed increases, resistance is gradually cut out.
🔹 At rated speed, the rotor circuit is short-circuited.
👉 Therefore, brushes are connected to external star-connected resistors.
⚙️ Main Parts Related to Starting Control are:
✔️ Slip Rings
✔️ Carbon Brushes
✔️ Rotor Winding (Star Connected)
✔️ External Resistor (Rheostat)
✔️ Shaft
✔️ Bearings
🔄 Rotor and Resistance Connection
🔹 Rotor winding → Star connected
🔹 Slip rings → 3 only
🔹 External resistance → Star connected
🔹 Brushes → Provide connection
📌 Comparison:
| Connection | Slip Rings Required |
|---|---|
| Star | 3 |
| Delta | 6 |
👉 Hence, star connection is preferred.
🚀 Purpose of External Resistance
🔹 Increase starting torque
🔹 Reduce starting current
🔹 Ensure smooth acceleration
🔹 Enable limited speed control
🔹 Improve efficiency at running
❌ Why Other Options are Incorrect
(2) Stator windings ❌
🔹 Brushes are connected only to rotor circuit.
(3) Rotor end rings ❌
🔹 End rings are in squirrel cage motors.
(4) Short-circuiting device ❌
🔹 Used only after starting.
📌 Key Exam Notes
⭐ Brush Connection → External resistance
⭐ Rotor Type → Wound rotor
⭐ Resistance → Star connected
⭐ Slip Rings → 3
⭐ Starting Torque → High
📝 Final Summary
📍 In a slip-ring induction motor:
✔️ Brushes connect external star-connected resistors
✔️ Starting current is reduced
✔️ Starting torque is increased
✔️ Resistance is cut out gradually
👉 Hence, Option (a) is Correct.