Single-phase motors:Repulsion Type Motors

Repulsion Type Motors

These can be divided into the following four distinct categories :

1. Repulsion Motor. It consists of (a) one stator winding (b) one rotor which is wound like a d.c. armature (c) commutator and (d) a set of brushes, which are short-circuited and remain in contact with the commutator at all times. It operates continuously on the ‘repulsion’ principle. No short-circuiting mechanism is required for this type.

2. Compensated Repulsion Motor. It is identical with repulsion motor in all re Repulsion induction motor spects, except that (a) it carries an additional stator winding, called compensating winding

(b) there is another set of two brushes which are placed midway between the usual short- circuited brush set. The compensating winding and this added set are connected in series.

3. Repulsion-start Induction-run Motor. This motor starts as a repulsion motor, but normally runs as an induction motor, with constant speed characteristics. It consists of (a) one stator winding (b) one rotor which is similar to the wire-wound d.c. armature (c) a commutator and (d) a centrifugal mechanism which short-circuits the commutator bars all the way round (with the help of a short-circuiting necklace) when the motor has reached nearly 75 per cent of full speed.

4. Repulsion Induction Motor. It works on the combined principle of repulsion and induction. It consists of (a) stator winding (b) two rotor windings : one squirrel cage and the other usual d.c. winding connected to the commutator and (c) a short-circuited set of two brushes.

It may be noted that repulsion motors have excellent characterstics, but are expensive and require more attention and maintenance than single-phase motors. Hence, they are being replaced by two- value capacitor motors for nearly all applications.

Repulsion Motor

Constructionally, it consists of the following :

1. Stator winding of the distributed non-salient pole type housed in the slots of a smooth-cored stator (just as in the case of split-phase motors). The stator is generally wound for four, six or eight poles.

2. A rotor (slotted core type) carrying a distributed winding (either lap or wave) which is connected to the commutator. The rotor is identical in construction to the d.c. armature.

3. A commutator, which may be one of the two types : an axial commutator with bars parallel to the shaft or a radial or vertical commutator having radial bars on which brushes press horizontally.

4. Carbon brushes (fitted in brush holders) which ride against the commutator and are used for conducting current through the armature (i.e. rotor) winding.

Repulsion Principle

To understand how torque is developed by the repulsion principle, consider Fig. 36.37 which shows a 2-pole salient pole motor with the magnetic axis vertical. For easy understanding, the stator winding has been shown with concentrated salient-pole construction (actually it is of distributed non- salient type). The basic functioning of the machine will be the same with either type of construction. As mentioned before, the armature is of standard d.c. construction with commutator and brushes (which are short-circuited with a low-resistance jumper).

Suppose that the direction of flow of the alternating current in the exciting or field (stator) winding is such that it creates a N-pole at the top and a S-pole at the bottom. The alternating flux produced by the stator winding will induce e.m.f. in the armature conductors by transformer action. The direction of the induced e.m.f. can be found by using Lenz’s law and is as shown in Fig. 36.37 (a). However, the direction of the induced currents in the armature conductors will depend on the positions of the short-circuited brushes. If brush axis is colinear with magnetic axis of the main poles, the directions of the induced currents (shown by dots and arrows) will be as indicated in Fig. 36.37 (a)*. As a result, the armature will become an electromagnet with a N-pole on its top, directly under the main N-pole and with a S-pole at the bottom, directly over the main S-pole. Because of this face-to- face positioning of the main and induced magnetic poles, no torque will be developed. The two forces of repulsion on top and bottom act along Y Y¢ in direct opposition to each other.**

the position shown in Fig. 36.37 (b) so that the brush axis is at right angles to the magnetic axis of the main poles, the directions of the induced voltages at any time in the respective armature conductors are exactly the same as they were for the brush position of Fig.

36.37 (a). However, with brush positions of Fig. 36.37 (b), the voltages induced in the armature conductors in each path between the brush terminals will neutralize each other, hence there will be no net voltage across brushes to produce armature current. If there is no armature current, obviously, no torque will be developed.

If the brushes are set in position shown in Fig. 36.38 (a) so that the brush axis is neither in line with nor 90º from the mag- netic axis Y Y¢ of the main poles, a net volt- age*** will be induced between the brush terminals which will produce armature cur-

rent. The armature will again act as an elec-

tromagnet and develop its own N-and S-poles which, in this case, will not directly face the respective main poles. As shown in Fig. 36.38 (a), the armature poles lie along A A¢ making an angle of a with Y Y¢.

Hence, rotor N-pole will be repelled by the main N-pole and the rotor S-pole will, similarly, be repelled by the main S-pole. Consequently, the rotor will rotate in clockwise direction [Fig.36.38 (b)]. Since the forces are those of repulsion, it is appropriate to call the motor as repulsion motor.

It should be noted that if the brushes are shifted counter-clockwise from YY ¢, rotation will also be counter-clockwise. Obviously, direction of rotation of the motor is determined by the position of brushes with respect to the main magnetic axis.

It is worth noting that the value of starting torque developed by such a motor will depends on the amount of brush-shift whereas direction of

rotation will depend on the direction of shift [Fig. 36.39 (a)]. Maximum starting torque is developed at some position where brush axis makes, an angle lying between 0º and 45º with the magnetic axis of main poles. Motor speed can also be controlled by means of brush shift. Variation of starting torque of a repulsion motor with brush-shift is shown in Fig. 36.39 (b).

A straight repulsion type motor has high starting torque (about 350 per cent) and moderate starting current (about 3 to 4 times full-load value).

Principal shortcomings of such a motor are :

1. speed varies with changing load, becoming dangerously high at no load.

2. low power factor, except at high speeds.

3. tendency to spark at brushes.