23-02-2013, 03:23 PM
SYNCRONOUS MACHINES
SYNCRONOUS MACHINES.ppt (Size: 919 KB / Downloads: 435)
Principle of Operation
The operation of a synchronous generator is based on Faraday's law of electromagnetic induction, and in an ac synchronous generator the generation of emf's is by relative motion of conductors and magnetic flux.
These machines can be used as either motors or generators but their predominant use is in generation.
Its characteristic feature is that the armature rotates through a stationary magnetic field, and the generated AC is brought to the load by means of slip rings and brushes.
The revolving-armature alternator is found only in alternators of small power rating and is not generally used. This is because a rotating armature requires slip rings and brushes to conduct the current from the armature to the load.
Single Phase Alternator
The stator is two pole. The winding is wound in two distinct pole groups, both poles being wound in the same direction around the stator frame.
As the rotor (field) turns, its poles will induce AC voltages in the stator (armature) windings. Since one rotor pole is in the same position relative to a stator pole as any other rotor pole, both the stator poles are cut by equal amounts of magnetic lines of force at any time. As a result, the voltages induced in the two poles of the stator winding have the same amplitude or value at any given instant.
Three Phase Alternator
When a rotor has rotated through an angle so that two adjacent rotor poles (a north and a south) have passed one winding, the voltage induced in that one winding will have varied through a complete cycle of 360 electrical degrees.
The magnitude of the voltage generated by an alternator can be varied by adjusting the current on the rotor which changes the strength of the magnetic field.