List of repulsion motors: 1. Compensated Repulsion Motor 2. Repulsion-Start Induction Motor 3. Repulsion-Induction Motor.
1. Compensated Repulsion Motor:
It is provided with an additional winding, called the compensating winding, to improve power factor and provide better speed regulation. This winding is much smaller than the stator winding and is usually wound in the inner slots of each main pole and is connected in series with rotor through an additional set of brushes placed midway between the usual short-circuited brushes, as illustrated in Fig. 1.47.
2. Repulsion-Start Induction Motor:
As the name implies the repulsion-start induction motor starts as a repulsion motor and runs as an induction motor.
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The general construction of a repulsion-start induction motor is quite similar to that of a repulsion motor (i.e., a laminated stator core complete with one winding similar to the main winding of a split-phase motor and a rotor, also called the armature, consisting of slots in which a winding is placed and connected to a commutator). The only difference is that in addition to the basic repulsion-motor construction it is equipped with a centrifugal device which operates at about 75-80 per cent of synchronous speed and short circuits all of the commutator segments.
The repulsion-start induction motor combines the desirable starting characteristics of the repulsion motor (i.e., high starting torque with moderate starting current) with operating characteristic (constant speed running) of the induction motor. A typical performance characteristic of a repulsion-start induction motor is given in Fig. 1.48. The starting torque is 3 to 6 times of full-load torque, and the starting current is approximately 3-4 times of full- load current.
The direction of rotation of such a motor may be reversed by shifting the brushes, but the brush rigging is not readily accessible, and the method is poorer than with any of the capacitor type motors.
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The efficiency and the maximum operating torque of the repulsion-start induction motor are usually less than those of a squirrel cage induction motor of comparative size i.e., a repulsion- start induction motor must be larger than a squirrel cage motor of the same rating to give the same performance.
As compared to capacitor motors, these motors:
(i) Need more maintenance because of commutator, brushes and centrifugal switch,
(ii) Are relatively more expensive,
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(iii) Cause radio interference during starting, and
(iv) Are noisier, especially when the brushes continue to ride on commutator.
However, despite all the drawbacks, these motors are still used in integral kilowatt ratings because they:
(i) Develop higher starting torque with low starting current and
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(ii) Can withstand longer starting periods than capacitor type motors.
Such motors are suitable for commercial refrigerators, compressors, pumps, hoists and other constant speed drives, particularly those which have a high inertia and prolonged starting period. The usual rating is from 1/3 kW to 12 kW but for special applications, ratings as high as 30 kW are available.
3. Repulsion-Induction Motor:
The repulsion-induction motor has a single phase stator winding, as a repulsion-start induction motor has, but it has two separate and independent windings on rotor in common slots. The inner winding is a squirrel cage winding with rotor bars permanently short- circuited while outer winding is a repulsion or commutator winding similar to a dc armature winding. When the motor starts, the squirrel cage winding, due to its high reactance, has no effect and the motor starts as a repulsion motor.
As the motor speeds up, the current shifts from the outer winding to the inner winding owing to decreasing reactance of the inner winding with the increasing speed and the squirrel cage winding comes into action. At normal speed both of the windings develop torque and the output of the motor is the combined output of both of the rotor windings.
The starting torque is about 2.5-3 times the full-load torque and its starting current is about 3-4 times the full-load current. The commutation of the motor is good at all speeds. No-load speed is above synchronous speed but normal speed is somewhat below synchronous one. No-load current is rather high, sometimes greater than the full-load current. The motor besides being costlier has brushes always on the commutator involving sparking and maintenance problems. This motor is particularly suitable where the load can be removed entirely by the declutching or by loose pulley.
Such a motor is used for applications requiring a high starting torque with essentially a constant running speed. The common ratings are 1/6 to 4 kW although in large ratings are also made. Common applications are household refrigerators, garage air pumps, gasoline pumps, compressors etc.