Monday, October 3, 2011

Types Of Electric Motors (3)

*AC Motors:

Alternating current (AC) motors use an electrical current, which reverses its direction at regular intervals. An AC motor has two basic electrical parts: a "stator" and a "rotor" . The stator is in the stationary electrical component. The rotor is the rotating electrical component
, which in turn rotates the motor shaft. The main advantage of DC motors over AC motors is that speed is more difficult to control for AC motors. To compensate for this, AC motors can be equipped with variable frequency drives but the improved speed control comes together with a reduced power quality. Induction motors are the most popular motors in industry because of their ruggedness and
lower maintenance requirements. AC induction motors are inexpensive (half or less of the cost of a DC motor) and also provide a high power to weight ratio (about twice that of a DC motor).

* Synchronous motor:

A synchronous motor is an AC motor, which runs at constant speed fixed by frequency of the
system. It requires direct current (DC) for excitation and has low starting torque, and synchronous motors are therefore suited for applications that start with a low load, such as air compressors, frequency changes and motor generators. Synchronous motors are able to improve the power factor of a system, which is why they are often used in systems that use a lot of electricity.
A Synchronous Motor

The main components of a synchronous motor are :

􀂃 Rotor. 
The main difference between the synchronous motor and the induction motor is
that the rotor of the synchronous motor travels at the same speed as the rotating magnetic
field. This is possible because the magnetic field of the rotor is no longer induced. The
rotor either has permanent magnets or DC-excited currents, which are forced to lock into
a certain position when confronted with another magnetic field.

􀂃 Stator. 
The stator produces a rotating magnetic field that is proportional to the frequency
This motor rotates at a synchronous speed, which is given by the following equation:

Ns = 120 f / P
f = frequency of the supply frequency
P= number of poles


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