23-08-2017, 06:23 AM
23-08-2017, 10:25 AM
A 3-phase AC generator is called "Alternator". Like a DC generator, it is also composed of two windings, namely:
(1) Field winding
(2) Reinforcement of armature.
However unlike a DC generator, its poles are in the rotor and the armature winding is in the stator.
Airy field
Field winding on poles requires a direct current which is supplied using a pair of slip rings on the shaft and a pair of stationary carbon brushes. The number of poles is always uniform. All poles are identical and symmetrically fitted into the rotor with alternate polarities, ie a "North" pole is followed by a "South" pole, which in turn is followed by another "North" pole and so on.
Reinforcement winding
The stator is composed of thin alloy laminations. The laminate structure is used to reduce magnetic losses and also to provide comfort in construction. These laminations (also known as embossments) have been perforated around the inner periphery to produce grooves. Copper windings are placed in these slots. It is known as "armature winding" and "stator winding". It is a three phase winding generally in 'star'. All three terminals of the star connection, including the fourth terminal, the neutral, are removed in the terminal box of the machine.
![[Image: ac_generator_working.jpg]](http://marineengineeringonline.com/wordpress/wp-content/uploads/2014/05/ac_generator_working.jpg)
Working
The generator is driven by its motor in the right direction at its rated speed (RPM). DC The field current is then supplied to its field winding through the brush and slide ring arrangements. The poles produce a sufficiently strong magnetic flux. Since the poles are rotating, the magnetic field is also rotating. The armature windings in the stator windings cut off the flow of the rotating poles. Therefore, the electrical energy is produced in the stator winding. Note that the frequency of the electric power is kept constant at 50 Hz (or at 60 Hz) keeping the generator constant.