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Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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__________ motors have two separate rotor windings.
Repulsion motor
Capacitor start capacitor run motor
Repulsion start induction run motor
shaded pole motor
Repulsion start induction run motor
Quick Summary: A Repulsion-start induction-run motor is designed with both a stator winding and a rotor (armature) winding connected to a commutator. This dual-winding architecture allows it to provide high starting torque like a DC series motor and stable running performance like a standard induction motor.
A Repulsion-start induction-run motor is designed with both a stator winding and a rotor (armature) winding connected to a commutator. This dual-winding architecture allows it to provide high starting torque like a DC series motor and stable running performance like a standard induction motor.
Ts∝Is2 — Starting torque is proportional to the square of stator current
Nr≈0.75Ns — The switching speed threshold relative to synchronous speed
At starting, the motor operates on the repulsion principle where the stator field induces current in the rotor windings through the commutator brushes, creating high torque. Once the motor reaches approximately 75% of its rated speed, a centrifugal switch acts to short-circuit the commutator segments, converting the armature into an equivalent squirrel-cage rotor, allowing the motor to run as a normal induction motor.
Uses a commutator and brushes only during the starting period.
Eliminates the need for external starting capacitors.
Provides high starting torque (2.5 to 3.5 times full load torque).
Transition to induction mode is automatic via a mechanical centrifugal governor.
High starting torque
Constant speed characteristic at high loads
High maintenance due to commutator and brushes
Complex mechanical construction
Higher initial cost compared to capacitor-start motors
Compressors
Reciprocating pumps
Industrial floor polishers
The repulsion motor acts as a series motor at start and an induction motor when running.
Option B (Capacitor start capacitor run) uses a single squirrel-cage rotor, not a wound rotor with commutator.
C is correct — The repulsion start induction run motor features both a stator winding and a rotor winding connected to a commutator to combine high starting torque with induction motor running efficiency.
Always remember that in machines with brushes, the commutator's main purpose at high speed is often to short-circuit the rotor to simulate a squirrel-cage cage for efficiency.