Repulsion Motor

Design of Repulsion Motor

The repulsion motor is the first single-phase AC motor in which the armature field builds up as a consequence of the voltage induced in the rotor. The rotor has the same design as the one in the universal motor – laminated sheet steel with an armature winding inserted into grooves, whose coil ends are connected to the collector segments. The carbon brushes no longer serve to feed current – they are attached on a moveable mount and short-circuited. The stator has no salient pole pieces – the exciter winding is inserted into the grooves of the dynamo steel core.

How the motor works

 

As was the case in the transformer the current carrying exciter winding induces a voltage in the coils of the armature winding. The position of the short-circuited brushes determines the current flow in the armature winding. If the brushes are aligned with the magnetic poles, the total current is equal to 0 – no magnetic field can build up in the rotor. The maximum current flows through the armature winding when the brushes are rotated by 90° from this alignment. A magnetic field arises whose direction coincides with the direction of the magnetic field in the stator. These brush positions are correspondingly defined as the "soft" and "hard" neutral positions.

Torque, speed and back emf

In order for torque to be exerted on the rotor, the brushes are turned by 15° out of the "hard" neutral position. Now the north pole of the rotor is repelled by the north pole of the stator. If the brushes are turned by 15° in the opposite direction, the rotation direction of the motor is reversed. At approx. 75% of the rated speed all of the collector segments are short-circuited with a ring and the brushes are lifted – their position no longer determines the amplitude of the armature current. Only the induced back emf is responsible for the armature current in the short-circuited rotor.