REPULSIONMOTOR SUBMITTED BY:AMRITANSHU KUMAR REG NO: - 20101008 EE 1 INTRODUCTION:An alternating-current motor having stator windings connected directly to the source of ac power and rotor windings connected to a commutator. brushes on the commutator are short-circuited and are positioned to produce the rotating magnetic field required for starting and running. TYPES:The various types of motors which works under the repulsion principle are: Compensated repulsion motor Repulsion-start Induction-run motor Repulsion Induction motor CONSTRUCTION:- . It was formerly used as a traction motor for electric trains but has been superseded by other types of motors and is now only of historical interest. They may be either lap winding or wave winding. The stator is mostly of non-salient pole type provided with slots. Carbon brushes are used to conduct current through the armature. The windings of rotor are of distributed type. The rotor winding is connected to a commutator which is in contact with a pair of short- . commutator and brush assembly. In Ac series motor the armature receives voltage by conduction through the power supply. rotor. The rotor is connected to the commutator which is identical to the construction of DC armature. Repulsion motors consists of a commutator which may be of axial type or vertical type.The repulsion-induction motor is a combination of a repulsion motor and a squirrel-cage induction motor Repulsion motors consist of a stator. But In repulsion motors the armature is supplied by induction from the stator windings. The principle difference between an AC series motor and repulsion motors is the way in which power is supplied to armature. The motor has a stator and a rotor but there is no electrical connection between the two and the rotor current is generated by induction. circuited brushes which can be moved to change their angular position relative to an imaginary line drawn through the axis of the stator. Consider a 2-pole salient pole motor with a vertical magnetic axis. it induces an electromotive force (emf) in the armature. PRINCIPLE:- Repulsion motors are based on the principle of repulsion between two magnetic fields. according to which the direction of induced emf opposes the cause producing it. The direction of alternating current is such that it creates a north pole at the top and a south pole at the bottom. The brushes are short circuited using a low-resistance jumper. The motor can be started. When alternating current is supplied to the field (stator) winding. The armature is connected to a commutator and brushes. The direction of induced emf is given by Lenz's law. the armature behaves like an electromagnet and a N-pole is formed directly below the N-pole of the stator and a S-pole is formed . stopped and reversed. and the speed can be varied. Stop positions Brush axis parallel to field If the brush axis is along the direction of the magnetic field. simply by changing the angular position of the brushes. The induced emf induces current in the armature conductors and the direction of the induced current depends on the position of the brushes. Both the N-poles repel each other and both the S-poles repel each other. Since there is no induced emf. Run positions If the brush axis is displaced at an angle to the magnetic axis. The two repulsion forces are in direct opposition to each other and hence no torque will be developed. zero. Apart from the coils undergoing short circuit. Brush axis at right angles to field If the brushes are shifted through 90 degrees. the coils undergoing short circuit change. a net voltage is induced at the brush terminals which will produce .directly above the S-pole of the stator. the voltage induced in the other coils between the brush terminals is neutralized and the net voltage is zero. there is no current in the circuit and the net torque developed is. so that the magnetic axis is perpendicular to the brush axis. The net torque in this condition is zero. again. If the brushes are shifted clockwise from the main magnetic axis. the motor will rotate in a counter-clockwise direction. so the rotor starts rotating. the North Pole is not directly under the North pole of the magnetic axis and the South Pole is not directly above the South Pole of the magnetic axis. If the brushes are shifted counter-clockwise from the main magnetic axis. Brush shift can also be used to control the speed of a repulsion motor. with North and South poles. Torque and speed control The starting torque of a repulsion motor is determined by the angle of brush shift from the main magnetic axis. The maximum torque is obtained from a brush shift of 45 degrees. the Npole of the stator field will repel the N-pole of the rotor field and the S-pole of stator field will repel the S-pole of the rotor field.current in the armature. Direction of rotation The direction of rotation is determined by the position of the brushes with respect to the magnetic field of the stator. but in this condition. The poles of the armature are slightly displaced from those of the stator. In this condition. the motor will rotate in a clockwise direction. The current in the armature circuit will produce its own magnetic field. . ADVANTAGES AND DISADVANTAGES:ADVANTAGES: Most commutator motors are limited to about 1.WHY NOT USED TODAY???...500 volts because higher voltages give rise to a risk of arcing across the commutator. Repulsion motors can be used at higher voltages because the rotor circuit is not electrically connected to the supply . The power factor is poor at low speeds. No load speed is very high and dangerous. repulsion-induction motors are ideal for: Value Operators Farm Motor Applications Hoists Floor Maintenance Machines Air Compressors Laundry Equipment Mining Equipment . This is primarily due to arcing and heat generated at brush assembly.Suitable where high torque and constant speed is required and also have high power factor DISADVANTAGES: Disadvantages of Repulsion Motor: Occurrence of sparks at brushes Commutator and brushes wear out quickly. APPLICATIONS:Application of Repulsion motors: Because of excellent starting and accelerating characteristics.