(a) connect extra resistance across them during starting
(b) connect them either in star or in delta as per need
(c) connect to 3-phase ac supply
(d) close the rotor circuit externally
✅ Answer (Detailed Solution Below)
✔️ Option (a): connect extra resistance across them during starting
📖 Detailed Solution
🔹 In a 3-phase slip-ring induction motor, the rotor has three-phase windings.
🔹 These windings are generally connected in star.
🔹 The three ends of the winding are brought out to slip rings.
🔹 Carbon brushes rest on the slip rings.
🔹 External resistances are connected through these brushes.
🔹 During starting, resistance is added to the rotor circuit.
🔹 This increases starting torque.
🔹 It reduces heavy starting current.
🔹 As speed increases, resistance is gradually cut out.
🔹 At normal speed, slip rings are short-circuited.
👉 Therefore, slip rings are mainly used to connect external resistance during starting.
⚙️ Main Parts Related to Starting and Control are:
✔️ Slip Rings
✔️ Carbon Brushes
✔️ Rotor Winding (Star Connected)
✔️ External Resistance Starter
✔️ Shaft
✔️ Bearings
🔄 Rotor Connection (Star Connection):
🔹 Rotor winding is connected in star.
🔹 Only three terminals are brought out.
🔹 Hence, only three slip rings are required.
🔹 Reduces construction complexity.
🔹 Improves reliability.
📌 Comparison with Delta Connection:
Connection Type Slip Rings Required Star 3 Delta 6
| Connection Type | Slip Rings Required |
|---|---|
| Star | 3 |
| Delta | 6 |
👉 Therefore, star connection is preferred.
🚀 Adding External Resistance to Rotor
🔹 External resistance is connected during starting.
🔹 It increases rotor resistance.
🔹 It limits starting current.
🔹 It improves starting torque.
🔹 Enables limited speed control.
🔹 Resistance is reduced step-by-step.
🔹 Improves running efficiency.
🔢 Starting Torque Relation:
Where:
T = Starting Torque
R = Rotor Resistance
s = Slip
👉 Higher rotor resistance gives higher starting torque.
⚡ Operation During Running
🔹 At starting → Maximum external resistance.
🔹 During acceleration → Resistance reduced gradually.
🔹 At rated speed → Slip rings shorted.
🔹 Motor runs efficiently.
❌ Why Other Options are Incorrect
(1) Improve power factor ❌
🔹 Not the primary purpose.
🔹 Main aim is starting control.
(3) Eliminate rotor losses ❌
🔹 Losses cannot be eliminated.
🔹 Resistance increases losses initially.
(4) Increase synchronous speed ❌
🔹 Speed depends on supply frequency and poles.
🔹 Not affected by slip rings.
📌 Key Exam Notes
⭐ Rotor Type → Wound Rotor (Slip Ring)
⭐ Connection → Star Connected
⭐ Slip Rings → 3 Only
⭐ External Resistance → Possible
⭐ Starting Torque → High
⭐ Speed Control → Limited
📝 Final Summary
📍 In a 3-phase slip-ring induction motor:
✔️ Rotor is star-connected
✔️ Three slip rings are used
✔️ External resistance is added
✔️ Starting current is reduced
✔️ Starting torque is increased
✔️ Better starting and control is achieved
👉 Hence, Option a is Correct.