A repulsion motor in its simplest form consists of a field comprising a distributed winding, housed in slots, in a smooth-cored stator and an armature carrying a distributed winding connected to a commutator. The stator winding, which produces the main field, is connected to the main supply. The armature or rotor winding is not connected electrically to the main circuit, but the brushes, which are set at an angle to the direction of the main flux, are short-circuited as shown in Fig. 12.
Fig. 12. Repulsion motor.
- If the brush axis BB is set at right angles to the direction of the stator flux, the e.m.f induced in one-half of the rotor .winding is exactly balanced by the e.m.f. induced in the other half, so that resultant e.m.f. is zero; no current flows in the rotor winding and no-torque is developed.
- If the brush axis is placed in line with the direction of the stator flux, the e.m.f. in one half assists that in the other, so that a maximum current flows. Here again no torque is developed, since the torque of one half of the rotor conductors is exactly balanced by that due to the other half.
- If the brush axis makes an angle with the stator flux as shown, a resultant torque is produced. The torque is maximum theoretically, if this angle is 45°, but in practice the angle of inclination is about half this value.
It is clear from above that the speed of repulsion motor depends upon the brush position. Speed control of such a machine can be provided by mounting the brushes all a rocker which can be rotated by a lever handle mounted on the motor end-shield. If remote control is required, the lever handle may be manipulated by a simple system of rods and cranks.
Alternatively, if the motor is to be totally enclosed, or remote control from a
considerable distance is required, speed control may be obtained by the use of an external series resistance with fixed brush gear.
- The direction of rotation of a simple repulsion motor may be reversed by swinging the brushes into the position shown dotted in Fig. 12.
Atkinson repulsion motor. A modification of the simple repulsion motor is the Atkinson repulsion motor, in which the stator winding comprises two windings at right-angles to each other and connected in series, as shown in Fig. 13. One advantage obtained by this method is that the direction of rotation can be reversed by reversing the connections to one of the stator windings. Instead of moving the brush rocker, it is necessary only to throw the reversing switch, shown in Fig. 13.
It will be observed that, as the rotor is electrically connected to the stator, the compensated repulsion motor is not able to operate directly from a high-voltage supply, as was the case with the simple repulsion motor.
Fig. 15 shows the typical speed-torque characteristics of single-phase repulsion motors.
Compensated repulsion motor. The power factor of the machine may be improved by compensation, and this is done by providing an additional set of brushes as shown in Fig. 14. The
Fig. 15. Typical speed-torque characteristics of single-phase repulsion motors.
axis of one pair, BB, of brushes coincides with the stator winding, these brushes are short-circuited. The other pair, bb, of brushes is set at right-angles to the former and is connected in series with the stator winding.