Electrical Engineering ⇒ Topic : Repulsion motor
A repulsion motor in its simplest form consists of a field comprising a distributed winding,housed in slots, in smooth-cored stator and an armature carying 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.(a).
Fig.(a). Repulsion motor
If the brush axis BB is set at right angles to the direction of the stator flux, the e.m.f. inducted 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 a maximum theoretically, if this angle 45is but in practice the angle of inclination is about half this value.
It is clear above from that the speed of repulsion motor depends upon the brush position.Speed control of such a machine can be provided by mounting the brushes on a rocker which can be rotated by a lever handle mounted on the motor end-shield. If remote control is required, the lower 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 consider able 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.(a).
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