# Repulsion Motor MCQ (Multiple Choice Questions) :: Single phase Induction Motor

1. The type of rotor used in the repulsion motor is

1. Salient pole Type
2. Non-Salient pole type
3. Both 1 & 2
4. None of the Above

Answer.2. Non-salient pole type

Explanation: The rotor of repulsion motor  is of non-salient pole type, wound with single-phase winding for four, six, or eight poles. Non-Salient rotor have smaller diameter and longer axial length. They are used for High speed operation (typically speed will be 1500 and 3000 rpm).

2. The construction of stator of repulsion motor is similar to

1. Split Phase Motor
2. DC motor
3. Compound Motor
4. DC generator

Answer.1. Split Phase Motor

Explanation: The construction of stator of repulsion motor is similar to single phase or split phase motor. The stator of a repulsion motor contains only a run winding very similar to that used in the split-phase motor. Start windings are not necessary.

3. The construction of rotor of repulsion motor is similar to

1. Split Phase Motor
2. DC motor
3. Compound Motor
4. DC generator

Answer.2. DC Motor

Explanation: The rotor of the repulsion motor is same as the armature of a dc machine with a commutator and brushes assembly. The rotor also wound with distributed winding in the rotor core slots.

4. In repulsion motor which part is called as armature

1. Rotor
2. Stator
3. Can’t say
4. None of the above

Answer.1. Rotor

Explanation: The rotor is actually called an armature because it contains a slotted metal core with windings placed in the slots. The windings are connected to a commutator.

5. The repulsion motor work on the principle of

1. Electric repulsion
2. Force repulsion
3. Magnetic Repulsion
4. Repulsion & Attraction

Answer.3. Magnetic Repulsion

Explanation: The repulsion motor works on the principle of magnetic repulsion between two poles.  It means “similar poles repel each other hence two North poles will repel each other. Similarly, two South poles will repel each other. A repulsion motor will start with repulsion between two unlike poles and then runs as an induction motor.

6. The brush on the rotor of repulsion motor is short-circuited through ______

1. High resistance
2. Low Resistance
3. No need to short-circuit
4. None of the above

Answer.2. Low resistance

Explanation: In repulsion motor the brushes on the rotor are short-circuited with each other through a low resistance conductor called Jumper. There is no electrical connection to the rotor, unlike the rotor connection in a dc machine. A set of brushes makes contact with the surface of the commutator bars. The entire assembly looks very much like a DC armature and brush assembly. One difference, however, is that the brushes of the repulsion motor are shorted together. Their function is to provide a current path through certain parts of the armature, not to provide power to the armature from an external source.

7. Select the INCORRECT option with respect to repulsion type motor.

1. Commutator is used
2. One stator winding
3. Rotor similar to squirrel cage induction motor
4. A set of brushes used

Answer. 3. Rotor similar to squirrel cage induction motor

Explanation: The stator of the repulsion type motor is the same as the split-phase induction motor. The armature of the rotor is similar to the DC motor armature with drum-type winding connected to the commutator. Pair of brushes are used and it short-circuited to make a rotor squirrel cage. By changing the position of brushes starting torque can be varied.

8.  The direction of the induced current in the armature conductor of repulsion motor  depend on the position of

1. Commutator
2. Number of Poles
3. Stator
4. Brushes

Answer.4. Brushes

Explanation: When supply is given to the stator winding then the alternating field produced in the air gap. By means of transformer action, EMF induced in the armature conductor. the brushes of the repulsion motor are shorted together. Their function is to provide a current path through certain parts of the armature. However, the direction of the induced current in the armature conductor will depend on the position of the short-circuited brushes pair.

9. In repulsion motor when the brush axis is parallel or perpendicular to the magnetic axis then torque develop is

1. Maximum
2. Minimum
3. Zero
4. Constant

Answer.2. Minimum

Explanation: When the brush axis is parallel or perpendicular to the magnetic axis then no torque will be developed because the net voltage developed in the armature conductor is zero. When the brushes are at 90° electrical to the field axis or pole pieces then no torque is developed because the equal induced voltages in two halves of the armature winding will oppose each other hence there will be no induced current in the armature winding, therefore, no flux will be developed by the armature winding.

10. In repulsion motor the torque develop is maximum when the position of brushes is at angle of

1. 90°
2. 45°
3. 30°

Answer.3. 45°

Explanation:

For the production of torque in the rotor, we have to place the short-circuited brushes between 0° to 90° say α with respect to the magnetic axis. The torque in repulsion motor is given as,

Armatur torque (Ta) = k sin (2α)

When α is 45° than maximum torque will be produced.

11. In a single-phase repulsion motor, torque is developed on the rotor when the brush axis is fixed

1. At 90° electrical with the stator field axis
2. At an alignment with the stator field axis
3. At 90° mechanical with the stator field axis
4. At an angle with the stator field axis

Answer.4. At an angle with the stator field axis

In a single-phase repulsion motor, torque is developed on the rotor when the brush axis is fixed at an angle with the stator field axis. Since the brushes are movable on the commutator surface, they can be placed at right angles to the stator pole axis, along the stator pole axis, or at an angle with the stator pole axis.

12. The direction of rotation of repulsion motor can be changed by changing

1. Wiring Connection
2. Rotor Position
3. Brush Position
4. Direction can’t be changed

Answer.3. Brush Position

Explanation: The direction of rotation is determined by the position of the brushes with respect to the magnetic field of the stator. Hence we can change the direction of the motor by changing the brush positions. If the brushes are shifted clockwise from the main magnetic axis, the motor will rotate in a clockwise direction. If the brushes are shifted counterclockwise from the main magnetic axis, the motor will rotate in a counter-clockwise direction.

13. In repulsion motor direction of rotation of the motor

1. Same as that of brush shift
2. Independent of brush shift
3. Opposite to that of brush shift
4. None of the above

Answer.1. Same as that of brush shift

Explanation: The direction of rotation is determined by the position of the brushes with respect to the magnetic field of the stator. Hence we can change the direction of the motor by changing the brush positions. If the brushes are shifted clockwise from the main magnetic axis, the motor will rotate in a clockwise direction. If the brushes are shifted counterclockwise from the main magnetic axis, the motor will rotate in a counter-clockwise direction.

14. The application of repulsion motor is

1. Electrical traction
2. Hoist
3. Lift
4. All of the above

Answer.4. All of the above

Explanation: In repulsion motor the torque developed is proportional to the square of the stator current, the torque-speed characteristics of a repulsion motor is high. Magnitude of starting torque of a repulsion motor is high. Magnitude of starting torque depends on the position of the brush axis. Its speed regulation, like a series motor, is poor.  The found application in Air compressors, Lifts and hoists, Mining industries, Pumps and fans, Electric trains, etc.

15. The advantages of Repulsion motor is

1. High starting torque
2. Wide range of speed control
3. Low starting current
4. All of the above

Answer.4. All of the above

Explanation: The advantages of a repulsion motor are: excellent starting torque, low starting current and wide range of speed control with smooth variation of speed. The disadvantages are: noisy performance, poor speed regulation and periodic commutator maintenance requirement. Repulsion-type motors have the highest starting torque of any single-phase motor. The repulsion motor can race to very high speed if operated with no mechanical load connected to the shaft.

16. The main disadvantage of Repulsion motor is

1. Unstable speed characteristics
2. Noisy operation
3. Sparking Problem
4. All of the above

Answer.4. All of the above

Explanation: The repulsion motor has excellent starting and running torque but can exhibit unstable speed characteristics. The disadvantages of repulsion motor include the following. Sparks will occur at brushes, the power factor is very less at less speed, the speed at no-load condition is extremely high & unsafe, brushes & commutator exhaust quickly due to heat generation & arcing at the assembly of the brush.

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