SOLUTION

In the repulsion motor, maximum torque is developed when the brush axis is at 45° electrical to the field axis.

Repulsion motor

• 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.

Operating principle of repulsion Motor

This motor has a wound rotor that functions similarly to a squirrel-cage rotor. It also has a commutator/brush assembly. The brushes are shorted together to produce an effect similar to the shorted conductors of a squirrel-cage rotor. The position of the brush axis determines the amount of torque developed and the direction of rotation of the repulsion motor.

Brush Position of repulsion Motor

The position of the brushes is very important. Maximum torque is developed when the brushes are placed 15° on either side of the pole pieces. When the brushes are position are placed at 90°, a circuit is completed between the coils located at a right angle to the poles. In this position, there is no induced voltage in the armature windings and no torque is produced by the motor.

In Figure, the brushes have been moved to a position so that they are in line with the pole pieces. In this position, a large amount of current flows through the coils directly under the pole pieces. This current produces a magnetic field of the same polarity as the pole piece. Because the magnetic field produced in the armature is at a 0° angle to the magnetic field of the pole piece, no twisting or turning force is developed and the armature does not turn.

When the brushes have been shifted in a clockwise direction so that they are located 15° from the pole piece. The induced voltage in the armature winding produces a magnetic field of the same polarity as the pole piece. The magnetic field of the armature is repelled by the magnetic field of the pole piece, and the armature turns in the clockwise direction.

When the brushes have been shifted counterclockwise to a position 15° from the center of the pole piece. The magnetic field developed in the armature again repels the magnetic field of the pole piece, and the armature turns in the counterclockwise direction.

The direction of armature rotation is determined by the setting of the brushes. The direction of rotation for any type of repulsion motor is changed by setting the brushes 15° on either side of the pole pieces. Repulsion-type motors have the highest starting torque of any single-phase motor.

The torque in repulsion motor is given as,

${T_e} = \frac{K}{2}{\left( {{I_s}{N_s}} \right)^2}\sin 2\alpha$

From the torque equation of repulsion motor, maximum torque is achieved when stator and rotor magnetic axis are displaced from each other by 45°.

Note:-

• 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.
• Torque is developed when the brush position is 0° – 45° on either side of the pole.