Do you know,What is Repulsion Motor ?

Do you know,What is Repulsion Motor ?

A lot of us may not have heard about repulsion motor . It is a type of electric motor for use on alternating current (AC) . It was formerly used as a traction motor for electric trains but has been superseded by other types of motors and is now only of historical interest.

 Repulsion motors are classified under single phase motors. In repulsion motors the stator windings are connected directly to the AC power supply and the rotor is connected to a commutator and brush assembly, similar to that of a direct current (DC) motor .

Construction

Repulsion motors consist of a stator, rotor, commutator and brush.

The stator is mostly of non-salient pole type provided with slots. The rotor is connected to the commutator which is identical to the construction of DC armature. The windings of rotor are of distributed type. They may be either lap winding or wave winding. Repulsion motors consists of a commutator which may be of axial type or vertical type. Carbon brushes are used to conduct current through the armature.

The principle difference between an AC series motor and repulsion motors is the way in which power is supplied to armature. In Ac series motor the armature receives voltage by conduction through the power supply. But In repulsion motors the armature is supplied by induction from the stator windings .

Its Operation

Repulsion motors are based on the principle of repulsion between two magnetic fields. Consider a 2-pole salient pole motor with vertical magnetic axis (basic functioning of salient pole and non-salient pole the construction is same) . The armature is connected to commutator and brush. The brushes are short circuited using a low-resistance jumper. when alternating current is supplied to field or stator winding, it induces an emf in the armature. The direction of alternating current is such that it creates north pole at the top and south pole at the bottom. The direction of induced emf is given by Lenz law, according to which the direction of induced emf is so as to oppose the cause producing it. the induced emf induces the current in the armature conductors. The direction of induced current in the armature conductors depends on the position of the brush.

If the brush axis is along the direction of the field (or) if the brush axis is collinear with the magnetic field, the armature behaves like an electromagnet and so an N-pole is formed directly below the N-pole of the stator and S-pole is formed directly above the S-pole of the stator. Now the net torque at this condition is zero. Both the N-poles repel each other and Both the S-poles repel each other. The two repulsion forces are in direct opposition to each other and hence no torque will be developed in this condition

As the same if we consider that the brushes are shifted through 90 degree, so that the magnetic axis is perpendicular to the brush axis ….. torque also at this condition is zero.

Every  time we change α and note the torque value to obtaine maximum torque as the value of starting torque of repulsion motors is determined by the amount of  brush shift α from the main magnetic axis. The maximum torque occurs when α=45 degrees. Brush shift also helps in speed control of repulsion motors.

The various types of motors which works under the repulsion principle are:

•        Compensated repulsion motor

•        Repulsion-start Induction-run motor

•        Repulsion Induction motor

Disadvantages :

•        Occurrence of sparks at brushes

•      Commutator and brushes wear out quickly. This is primarily due to arcing and    heat generated at brush assembly.

•        The power factor is poor at low speeds.

•        No load speed is very high and dangerous.

Applications :

•        Value Operators

•        Farm Motor Applications

•        Hoists

•        Floor Maintenance Machines

•        Air Compressors

•        Laundry Equipment

•        Mining Equipment