02-08-2011, 04:56 PM
In a single phase induction motor, it is necessary to provide a starting circuit to start rotation of the rotor. The single phase induction motor may rotate in either direction and it is only the starting circuit which determines rotational direction.
For small motors of a few watts, the start rotation is done by means of one or two single turn(s) of heavy copper wire around one corner of the pole. The current induced in the single turn is out of phase with the supply current and so causes an out-of-phase component in the magnetic field, which imparts to the field sufficient rotational character to start the motor. Starting torque is very low and efficiency is also reduced. Such shaded-pole motors are typically used in low-power applications with low or zero starting torque requirements.
Shaded pole induction motor
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Larger motors are provided with a second stator winding which is fed with an out-of-phase current to create a rotating magnetic field. The out-of-phase current may be derived by feeding the winding through a capacitor or it may derive from the winding having different values of inductance and resistance from the main winding.
Capacitor-start induction motor.
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In some designs, the second winding is disconnected once the motor is up to speed, usually by means of a switch operated by centrifugal force acting on weights on the motor shaft. Other designs keep the second winding continuously energized during running, which improves torque.
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DC MOTOR
PRINCIPLE OF OPERATION
A machine that converts DC power into mechanical power is known as DC motor. Its operation is based on the principle that when a carrying carrying conductor is placed in a magnetic field the conductor experiences a mechanical force. The direction of this force is given by Fleming’s left hand rule and magnitude is given by F=BIL newtons
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