Electrical Machines - Repulsion Motor

A single-phase repulsion motor consists of a stator carrying a single-phase exciting winding and a rotor which has a closed type armature winding with a commutator and brushes. The brushes on the commutator are short circuited. By adjusting the position of short-circuited brushes on the commutator, the starting torque can be developed in the motor.

Working Principle of Repulsion Motor

The figure shows the operating principle of a two-pole repulsion motor with its two short-circuited brushes.

Case 1

When the brush axis is parallel to the stator field and the stator winding is energised from a 1-phase supply, then an EMF is induced in the rotor conductors by the action of electromagnetic induction.

According to Lenz’s law, the direction of the EMF is such that the magnetic field produced by the resulting rotor currents will oppose the increase in the main field flux. The direction of the rotor currents is shown in the figure.

With the brush axis shown in the figure of Case-I, the current will flow from brush 2 to brush 1 where it enters the armature and flows back to the brush 2 through the two paths 1-3-2 and 1-4-2.

With the brushes set in this position, half of the rotor conductors under the N-pole carry current inward and half carry current outward. Similarly, half of the rotor conductors under the S-pole carry current inward and half carry current outward. Therefore, half of the rotor conductors experience a torque in one direction and the other half in other direction and both the torques are equal in magnitude. Hence, the net torque on the rotor is zero and the rotor remains stationary.

The rotor will also remain stationary if the brush axis is perpendicular to the stator field axis. It is because even then net torque on the rotor is zero.

Case 2

When the brush axis is at some angle other than 0° or 90° to the stator field axis, then a net torque is developed on the rotor and the rotor accelerates. The figure below represents the case-II, in which the brushes have been shifted clockwise through some angle from the stator field axis.

The EMF is induced in the rotor conductors and current flows through the two paths of the rotor winding from brush 1 to brush 2. Because of the new brush positions, the greater part of the conductors under the N-pole carry current in outward direction while the lesser part of the conductors carry current in the inward direction. Hence, with the brushes in this position, a net torque is developed in the clockwise direction (i.e., in the same direction in which brushes are shifted) and the rotor quickly attains the normal speed.

The direction of the rotation of the rotor depends upon the direction in which the brushes are shifted. If the brushes are shifted in clockwise direction from the stator field axis, then the net torque is produced in the clockwise direction and hence, the rotor accelerates in the clockwise direction.

If the brushes are shifted in the counter-clockwise direction, then the current in the rotor conductors under the pole faces is reversed and the net torque is developed in the anti-clockwise direction. Therefore, a repulsion motor may be made to rotate in the either direction depending upon the direction in which the brushes are shifted.

The armature torque developed in a repulsion motor is given by,

$$\mathrm{\tau_{a} \: \propto \: \sin 2 \theta}$$

Where, θ is the angle between the brush axis and the stator field axis.

To obtain the maximum armature torque,

2θ = 90°; or θ = 45°

Hence, by adjusting θ to 45°, the maximum torque can be obtained at starting.

Characteristics of Repulsion Motor

The characteristics of a repulsion motor are given as follows −

• Repulsion motor has a high starting torque and a high speed at no-load.
• Repulsion motor has relatively low starting current and high starting torque.
• The speed of the motor for any given load depends upon the position of the brushes.