1. Niharika narayan

    Thread Starter New Member

    May 24, 2012
    An electric motor is a device which converts electrical energy into mechanical energy. In any electric motor, operation is based on simple electromagnetism. A current-carrying conductor generates a magnetic field; when this is then placed in an external magnetic field, it will experience a force proportional to the current in the conductor, and to the strength of the external magnetic field.The direction of the force is obtained with the help of Fleming's left hand rule.
    Ac motors
    Introduction: An ac motor is an electric motor driven by alternating current. It consists of two parts:
    1. Stator-(coil supplied with alternating current to produce rotating magnetic field).
    2. Rotor-(attached to shaft to which torque is applied by rotating field).
    It’s basically of two types:
    1. Squirrel cage motors.
    2. Slip ring motors.

    1. Squirrel cage motors: It is the most common type of ac motor used in any industry or for any domestic purpose. The motor takes its name from the shape of its rotor “windings"- a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. Usually only end rings are visible. During operation, motor may be viewed as a transformer with a rotating secondary. When the rotor is not rotating synchronous with the magnetic field, large rotor currents are induced; this magnetize the rotor and interact with the stator's magnetic fields to bring the rotor into synchronization with the stator's field.
    The bars provide a large surface area to provide cooling effect to the motor. In any design there are an optimum number of bars, if the number of bars increases beyond that it serves to increase the losses of the motor particularly when starting.
    Construction: squirrel cage motor consists of:
    Shaft, bearings, cooling fan, frame, rotor, stator.
    Construction difference between a single phase and three phase motor:
    A single phase motor will have a capacitor with it to help itself get started when power or electric current is supplied to it as it provides it the desirable torque. As a three phase motor get started itself when electric current is given it does not need any capacitor.

    D.C. motors
    A dc motor is one which is driven by direct current.
    Parts of a dc motors:
    • the armature coil ABCD mounted on an axle
    • the commutator that is a split ring divided in two parts S1 and S2
    • a pair of brushes B1 and B2
    • a horse - shoe electromagnet
    The coil ABCD is wound round a soft iron and is placed in between the pole pieces of a powerful horse - shoe magnet. The coil is free to rotate about its axis. The ends of the coil A and D are connected to split parts of the ring S1 and S2 respectively. Two brushes B1 and B2, made of carbon or copper, touch the split rings S1 and S2 respectively. A dc source is connected across the brushes B1 and B2. When the coil rotates, the split rings rotate but the brushes do not move.
    A wheel can be mounted on the axle placed along the axis of the coil so as to drive the desired parts of the machine such as electric fan, washing machine etc. where the motor is used.

    The plane of the coil is horizontal and the split ring S1 touches the brush B1 while the split ring S2 touches the brush B2. The brush B1 is connected to the anode of the D.C. Battery while the brush B2 is connected to the cathode. The current flows in the coil in the direction ABCD. The arms BC and DA being parallel to the magnetic field experience no force.
    According to Fleming's left hand rule force 'F' acting on the arm AB, is inward and perpendicular to the plane of the coil and the force on the arm CD is in just in the opposite direction. The forces on the arms AB and CD being equal and opposite form an anticlockwise couple, due to which the coil begins to rotate. It rotates in such a way that the arm AB goes in and the arm CD comes out.
    When the coil reaches the vertical position, the couple becomes zero since the forces on the arms now become collinear. But due to the inertia of motion, the coil does not stop in this position. As the coil passes from the vertical the split ring S1 comes in contact with the brush B2, while the split ring S2 comes in contact with the brush B1. Now the current flows through the coil in the direction DCBA and the forces acting on the arms DC and AB of the coil again form an anticlockwise, couple due to which the coil remains rotating in the same direction. Thus, whenever the coil comes in the vertical position, the direction of the current through the coil reverses and the coil continues to rotate in the same direction.
    The deflecting couple on the coil is maximum when the plane of the coil is parallel to the direction of the magnetic field and the deflecting couple is minimum when the plane of the coil is perpendicular to the magnet field.
    The speed of rotation of the coil depends on the deflecting couple acting on the coil. Hence the speed of rotation of the coil can be increased by,
    • increasing the number of turns of the coil
    • increasing the strength of the current
    • increasing the area of the coil
    • increasing the strength of the magnetic field

    Last edited: May 24, 2012
    PackratKing likes this.
  2. #12


    Nov 30, 2010
    So...why do you keep posting a quotation out of a book?