Sunday, 26 January 2014

Motor Types- AC Motor

Motor Types- AC Motor

An AC motor is an electric motor driven by an alternating current (AC).
It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.
There are two main types of AC motors, depending on the type of rotor used.
For more related information please refer:
Motor Types-Introduction
Motor Types- DC Motor
Motor Types- AC Motor
Motor Types- Special Types of Motor
Servo Motors

Stepper Motor
The first type is the induction motor or asynchronous motor; this type relies on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current. The second type is the synchronous motor, which does not rely on induction and as a result, can rotate exactly at the supply frequency or a sub-multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet. Other types of motors include eddy current motors, and also AC/DC mechanically commutated machines in which speed is dependent on voltage and winding connection.


If the rotor of a squirrel cage motor runs at the true synchronous speed, the flux in the rotor at any given place on the rotor would not change, and no current would be created in the squirrel cage. For this reason, ordinary squirrel-cage motors run at some tens of rpm slower than synchronous speed. Because the rotating field (or equivalent pulsating field) effectively rotates faster than the rotor, it could be said to slip past the surface of the rotor. The difference between synchronous speed and actual speed is called slip, and loading the motor increases the amount of slip as the motor slows down slightly. Even with no load, internal mechanical losses prevent the slip from being zero.

The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:
 N_{s} = 120F/p
Ns = Synchronous speed, in revolutions per minute
F = AC power frequency
p = Number of poles per phase winding

1.Induction Motor: So called because voltage is induced in the rotor (thus no need for brushes), but for this to happen,the rotor must rotate at a lower speed than the magnetic

field to allow for the existence of an induced voltage.

2.Synchronous Motor: So called because rotor tries to line up with the rotating magnetic field in the stator. It has the
stator of an induction motor, and the rotor of a dc motor.

3.Three Phase Induction Motor:
An electric motor converts electrical energy into a mechanical energy which is then supplied to different types of loads. A.c. motors operate on an a.c. supply, and they are classified into synchronous, single phase and 3 phase induction, and special purpose motors. Out of all types, 3 phase induction motors are most widely used for industrial applications mainly because they do not require a starting device.
A 3 phase induction motor derives its name from the fact that the rotor current is induced by the magnetic field, instead of electrical connections.
The operating principle of a 3 phase induction motor is based on the production of r.m.f.

The stator of an induction motor consists of a number of overlapping windings offset by an electrical angle of 120°. When the primary winding or stator is connected to a three phase alternating current supply, it establishes a rotating magnetic field which rotates at a synchronous speed.

The direction of rotation of the motor depends on the phase sequence of supply lines, and the order in which these lines are connected to the stator. Thus interchanging the connection of any two primary terminals to the supply will reverse the direction of rotation.
The number of poles and the frequency of the applied voltage determine the synchronous speed of rotation in the motor’s stator. Motors are commonly configured to have 2, 4, 6 or 8 poles. The synchronous speed, a term given to the speed at which the field produced by primary currents will rotate, is determined by the following expression.
Synchronous speed of rotation = (120* supply frequency) / Number of poles on the stator

4.Single Phase Induction Motor:
Single phase induction motors require just one power phase for its operation. They are commonly used in low power rating applications, in domestic as well as industrial use. This article is aimed at giving you a conceptual overview of working of single phase motors.

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