- It starts as a repulsion motor and runs as an induction motor after it reaches about 75-80% of its full speed, when a centrifugal short-circuiting device short-circuits its commutator.
- From that point on, it operates as an induction motor with a short-circuited squirrel-cage rotor.
- After the commutator is short-circuited, the brushes do not carry any current and can be lifted from the commutator to avoid unnecessary wear, tear, and friction losses.
- Repulsion-start motors come in two different designs:
- Brush-lifting type: In this type, the brushes are automatically lifted from the commutator when it is short-circuited. These motors generally use a radial form of commutator and are built in both small and large sizes.
- Brush-riding type: In this type, the brushes remain in contact with the commutator at all times. These motors use an axial form of commutator and are always built in small sizes.
- The starting torque of such motors exceeds 350% with moderate starting current.
- These motors are particularly useful when the starting period is comparatively long due to high-inertia loads. Applications include machine tools, commercial refrigerators, compressors, pumps, hoists, floor-polishing and grinding devices, etc.
Important MCQs on Repulsion Start Induction Motor:
1. At what speed does a repulsion-start motor transition to an induction motor?
A) 50-60% of its full speed
B) 75-80% of its full speed
C) 90-95% of its full speed
D) 100% of its full speed
Answer: B) 75-80% of its full speed
2. What device is responsible for short-circuiting the commutator in a repulsion-start motor?
A) Centrifugal switch
B) Brush-lifting mechanism
C) Centrifugal short-circuiting device
D) Rotor winding
Answer: C) Centrifugal short-circuiting device
3. What happens to the brushes in a repulsion-start motor after the commutator is short-circuited?
A) They continue to carry current
B) They are lifted to prevent wear
C) They increase the torque output
D) They remain active without any change
Answer: B) They are lifted to prevent wear
4. Which of the following is a feature of the brush-lifting type of repulsion-start motor?
A) Brushes remain in contact with the commutator at all times
B) Uses an axial commutator design
C) Brushes are automatically lifted after the commutator is short-circuited
D) Only built in small sizes
Answer: C) Brushes are automatically lifted after the commutator is short-circuited
5. What is the main advantage of lifting the brushes in a repulsion-start motor?
A) Enhances the starting torque
B) Reduces friction and wear
C) Increases current flow
D) Improves rotor speed
Answer: B) Reduces friction and wear
6. Which design of repulsion-start motor is built only in small sizes?
A) Brush-lifting type
B) Brush-riding type
C) Dual-type motor
D) High-torque motor
Answer: B) Brush-riding type
7. What form of commutator is used in the brush-riding type of repulsion-start motor?
A) Radial
B) Axial
C) Circular
D) Spiral
Answer: B) Axial
8. What percentage of starting torque can a repulsion-start motor achieve?
A) Up to 150%
B) 250%
C) Over 350%
D) 200%
Answer: C) Over 350%
9. Why are repulsion-start motors useful for applications with high-inertia loads?
A) They consume less power
B) They operate silently
C) They have high starting torque and can handle long starting periods
D) They are cheaper to produce
Answer: C) They have high starting torque and can handle long starting periods
10. Which of the following is an application of a repulsion-start motor?
A) Desktop computers
B) Ceiling fans
C) Commercial refrigerators
D) LED lighting
Answer: C) Commercial refrigerators