SOLUTION

The repulsion-start induction-run motor is used because of high starting torque.

The repulsion-start induction-run motor starts as a repulsion motor but runs like a squirrel-cage motor. There are two types of repulsion-start induction-run motors:

1. The brush-riding type
2. The brush-lifting type

Brush Riding type Repulsion-start-induction Run Motor

The brush-riding type uses an axial commutator. The brushes ride against the commutator segments at all times when the motor is in operation. After the motor has accelerated to approximately 75% of its full-load speed, centrifugal force causes copper segments of a short-circuiting ring to overcome the force of a spring. The segments sling out and make contact with the segments of the commutator. This effectively short-circuits all the commutator segments together, and the motor operates in the same manner as a squirrel-cage motor.

Brush lifting type Repulsion-start-induction Run Motor

The brush-lifting-type motor uses a radial commutator. Weights are mounted at the front of the armature. When the motor reaches about 75% of full speed, these weights swing outward due to centrifugal force and cause two pushrods to act against a spring barrel and short-circuiting necklace.

The weights overcome the force of the spring and cause the entire spring barrel and brush holder assembly to move toward the back of the motor. The motor is so designed that the short-circuiting necklace will short-circuit the commutator bars before the brushes lift off the surface of the radial commutator. The motor will now operate as a squirrel-cage induction motor.

Operation of a Repulsion-Start, Induction-Run Motor

• The starting torque is good for either the brush-lifting type or the brush-riding type of repulsion-start, induction-run motor.
• The speed performance of both types of motors is very good because they operate as single-phase induction motors.
• The repulsion-start, the induction-run motor is of the single-phase type, ranging in size from about 1/6 hp to as high as 20 hp.
• The starting torque is 4 times higher than the running torque.
• The starting torque of the repulsion start motor is 350 percent with a moderate starting current.
• Because of the excellent starting and running characteristics for both types of repulsion- start, induction-run motors, they have been used for a variety of industrial applications, including commercial refrigerators, compressors, and pumps.