Repulsion Motors

Repulsion Motors

Repulsion motors work on the principle of repulsion between two magnetic fields. These motors give excellent performance characteristics. Before going to actual discussion about motor let us consider the principle on which motor works.

1. Repulsion Principle

For understanding the torque production by motor using repulsion principle consider a two pole salient pole motor having magnetic axis horizontal. The armature of the machine consists of a d.c. windings having commutator and brushes. The brushes are short circuited by a low resistance jumper.

The stator winding is given excitation in such a way as to form the poles as shown in the Fig. 9.5.1.

The brushes are aligned in the same direction of the field axis. The stator winding will produce alternating flux which will induce emf in the armature conductors by transformer action. The direction of induced emf can be found by using Lenz's law. The direction of induced current will depend on position of brushes. These currents will lag behind the induced voltages by almost 90°. Because of the current flowing through the armature, it will produce its own magnetic field with the poles as shown in Fig. 9.5.1. Thus equal force of repulsion exists between like poles which will not produce any torque. Alternatively it can also be explained as the armature to be divided into four quadrants producing four torques T₁,T₂,3 and T4 which are equal and hence the net torque is zero.

If brushes are shifted by 90o, so the conductors undergoing short circuit are also changed. The induced emf are in the same direction as before. The arrangement is shown in the Fig. 9.5.2

Apart from the coils undergoing short circuit, the remaining armature winding gets divided into two parallel paths. It can be seen that the induced emfs are balanced and the resultant emf is zero. Thus no current flows through the brushes and the resultant torque is also zero.

Now if the brushes are in the position shown in the Fig. 9.5.3.

In this case, the brushes axis is not in the line of main field or at an angle of 90° to main field but it is at an angle of a with the main field.

Again the emf will be induced in the armature conductors and there will be net voltage across brush terminals which will produce current in the armature. Thus the armature will also produce its own magnetic field with the poles as shown in the Fig. 9.5.3. The north and south poles of stator and rotor will attract each other and there will be net torque available which will run the motor in the clockwise direction. Alternatively we can say that the north pole formed by armature winding will be repelled by the north pole formed by the main field winding and similarly the south pole will be repelled by south pole formed by the main field winding and the motor runs in clockwise direction. As the forces are of repulsion which contributes in the motion so the name of the motor is repulsion motor. If the brush is given shift in the opposite direction to that shown in the Fig. 9.5.4 then motor rims in anticlockwise direction which can also be explained on the similar lines. Hence the position of brushes decides the direction of rotation. The torque produced by the motor depends on the brush shift angle ɑ.

Key Point For a = 45 ° maximum torque is available.

Thus the control of speed and torque can be done with the help of brush shift. The variation of torque with brush shift is shown in the Fig. 9.5.4.

2. Repulsion Type Motors

The motors which use the principle as explained in the earlier section are called repulsion type motors which are categorised into four distinct groups which are as given below.

1) Repulsion motor       

2) Compensated repulsion motor

3) Repulsion start induction rim motor

4) Repulsion induction motor.

1) Repulsion motor      

The principle of operation of this motor is already explained in earlier section. This motor consists of a stator and rotor winding alongwith commutator and a set of brushes. The rotor winding is same as d.c. winding whereas brushes are short circuited having contact with commutator at all times. Normally there are four, six or eight poles on the stator.

The stator winding is of distributed non-salient pole type mounted in the slots. A rotor is also having slots for distributed winding. The winding on rotor is connected to the commutator which may be either axial or radial. The brushes are made up of carbon fitted in brush hold which press on the commutator and carries the current in the rotor winding.

With this type of motor, a very high starting torque (of about 300 to 350 % of full load) can be obtained with starting current of about 3 to 4 times the full load current. Thus it has got very good operating characteristics but the motors are expensive and rarely used in the applications. Also the speed of the motor changes with load. At no load its value is very high. There is possibility of sparking at brushes and the motor runs at low power factor. 

2. Compensated Repulsion Motor

It is the modified form of the repulsion motor. It is often required that the motor should run at constant speed and with higher power factor. These conditions can be met by the use of additional inner stator winding which is smaller than the outer commutator winding. The torques developed are thus additive. This additional winding is called compensating winding. It is connected in series with the rotor winding. It consists of additional set of brushes placed in the mid way between the normal short circuited brushes as shown in the Fig. 9.5.5.

The machine with this modification runs with improved power factor as the quadrature drop in the field winding is neutralised or compensated with this additional winding. Also the leakage between armature and field is reduced. It gives better speed regulation.

3. Repulsion Start Induction Run Motor

In order to obtain constant speed and high starting torque, repulsion start induction rim motor may be used. In this motor, when the speed of the motor reaches to half to three fourth of its speed, all the commutator segments are short circuited by means of centrifugal force-operated device. Due to this the motor runs like a squirrel cage induction motor.

With this arrangement, very high starting torque can be obtained. Whenever the commutator is short circuited, no current flows through brushes and hence they are lifted from the commutator which avoids unnecessary wear and tear and losses due to friction. Thus with the use of these motors high starting torques without excessive currents and constant speed operation is possible for wide range of torque. Hence these motors are used in machine tools, pumps, hoists, floor polishing, refrigerators, compressors and grinding tools.

4. Repulsion Induction Motor

With this type of repulsion motor, good overall operating characteristcs can be obtained. It is a combination of repulsion motor and induction motor. Thus with low starting current, high starting torque can be obtained. When the motor reaches its normal operating speed, the motor is converted to a single phase induction motor for getting constant speed operation. Thus it has got characteristics of both repulsion motor and of induction motor. 

In addition to the normal stator winding, it consists of inner squirrel cage winding mounted inside the slots below the commutating winding. Both the windings function during the operation of motor. The brushes are in continuous contact with commutator. The torques developed because of these two windings are additive.

During starting the major portion of torque is developed by the normal commutator winding as the squirrel cage winding has high reactance. During normal running condition the major portion of the torque is supplied by the squirrel cage winding as its reactance decreases. Unlike in repulsion start induction motor, this motor does not require short circuited centrifugal device. If such a motor is provided with compensating winding then it runs at improved power factor. Using brush shifting the reversal can also be achieved.

These motors are widely used in lifts, hoists, refrigerators, air pump compressors and machine tools.

Review Question

1. Discuss repulsion motor with diagram.