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.
- Repulsion Motor
- Compensated Repulsion Motor
- Repulsion Start Induction Motor
- Repulsion Induction Motor
2.1. 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 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 application. 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.
2.2 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.1.
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| Fig. 1 |
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.
2.3. Repulsion Start Induction Motor
In order to obtain constant speed and high starting torque, repulsion start induction run 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, refrigerator, compressor and grinding tools.
2.4 Repulsion Induction Motor
With this type of repulsion motor, good overall operating characteristics 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 insides the slots below the commutating winding. Both the winding 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 reactence 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, refrigerator, air pump compressor and machine tools.Sponsored Articles :