(a) stator phases of the motor
(b) rotor at slip rings
(c) starting resistances in the starter
(d) none of the above
✅ Answer (Detailed Solution Below)
✔️ Option (b): Rotor at slip rings
📖 Detailed Solution
🔹 In a slip-ring induction motor, rotor windings are connected to slip rings.
🔹 External resistances are added through brushes during starting.
🔹 These resistances improve starting torque and limit current.
🔹 As motor speed increases, resistance must be removed.
🔹 This is done using short-circuit gear.
🔹 The short-circuit gear short-circuits the slip rings.
🔹 Hence, the rotor circuit becomes directly shorted.
🔹 After this, the motor operates like a squirrel cage motor.
👉 Therefore, short-circuit gear short-circuits the rotor at slip rings.
⚙️ Main Parts Related to Short-Circuiting are:
✔️ Slip Rings
✔️ Carbon Brushes
✔️ Short-Circuiting Gear
✔️ Rotor Winding
✔️ External Resistance
🔄 Operation of Short-Circuit Gear
🔹 At starting → External resistance connected.
🔹 During acceleration → Resistance reduced gradually.
🔹 At rated speed → Slip rings shorted.
🔹 Motor runs normally.
👉 Ensures efficient and smooth operation.
❌ Why Other Options are Incorrect
(1) Stator windings ❌
🔹 Short-circuit gear is connected only to rotor circuit.
(3) External resistance only ❌
🔹 Resistance is removed by shorting slip rings, not directly.
(4) Supply terminals ❌
🔹 Not related to rotor control.
📌 Additional Information: Rotor-Fed Induction Motor
🔹 If 3-phase supply is given to rotor.
🔹 And stator is short-circuited.
🔹 Rotating magnetic field is produced in rotor.
🔹 Stator cannot rotate.
🔹 Rotor rotates in opposite direction.
🔹 Motion reduces relative speed.
⚡ Special Case:
🔹 If stator is not short-circuited → No motion occurs.
🔹 Rotor remains stationary.
📐 Slip Frequency
🔹 Frequency of current in short-circuited stator = Slip frequency.
Where:
fs = Slip frequency
s = Slip
f = Supply frequency
📌 Key Exam Notes
⭐ Short-circuit gear → Shorts slip rings
⭐ Purpose → Remove external resistance
⭐ Running condition → Squirrel cage mode
⭐ Rotor-fed motor → Reverse rotation
📝 Final Summary
📍 In a slip-ring induction motor:
✔️ External resistance is used during starting
✔️ Short-circuit gear shorts slip rings
✔️ Rotor becomes short-circuited
✔️ Motor runs like squirrel cage
👉 Hence, Option b is Correct.