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Examoogle • User • info@examoogle.com • EE-2024-8821
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The repulsion-start induction-run motor is used because of
Low cost
Good Power factor
High efficiency
High starting torque
High starting torque
A repulsion-start induction-run motor is a single-phase motor that starts as a repulsion motor and runs as an induction motor. It is specifically used in applications requiring high starting torque, typically around 350% to 450% of full-load torque, combined with a relatively low starting current.
A repulsion-start induction-run motor is a single-phase motor that starts as a repulsion motor and runs as an induction motor. It is specifically used in applications requiring high starting torque, typically around 350% to 450% of full-load torque, combined with a relatively low starting current.
Ts≈3.5 to 4.5×Tfl — starting torque relative to full-load torque
Is≈2.5 to 3×Ifl — starting current relative to full-load current
During starting, the short-circuited brushes positioned on the commutator produce a high electromagnetic torque through repulsion action between the main field and rotor field. As the motor accelerates to approximately 75% of full-load speed, a centrifugal mechanism short-circuits all the commutator segments together, converting the rotor winding into a cage-like structure so it operates as a standard induction motor with stable running characteristics.
Combines the extremely high starting torque of a repulsion motor with the constant-speed characteristics of an induction motor.
The centrifugal switch short-circuits the commutator segments when speed reaches about 75% of synchronous speed.
In some designs, the brushes are also lifted off the commutator when short-circuited to reduce wear and noise.
Extremely high starting torque (up to 450% of full-load torque)
Low starting current compared to high starting torque developed
Fairly constant speed under varying load conditions during running mode
High initial cost due to commutator and brush mechanism construction
Requires frequent maintenance of brushes and commutator
Higher mechanical noise during starting operation
Compressors and refrigeration units
Large air conditioners
High-inertia loads such as hoists, pumps, and machine tools
Option A is incorrect because repulsion-start motors are expensive due to complex commutator assemblies.
Option B is incorrect because its power factor is moderate, comparable to conventional single-phase induction motors.
Option C is incorrect because running efficiency is similar to standard induction motors, not significantly superior.
D is correct — The repulsion-start induction-run motor provides an exceptionally high starting torque (350%–450% of full load torque) with low starting current.
For competitive exams, remember that Repulsion-Start Induction-Run motors deliver the highest starting torque among single-phase induction motors (higher than Capacitor-Start motors).