An induction motor is single excited machine whereas the three-phase synchronous machine is a doubly excited ac machine (two inputs are given) because its field winding is excited by the dc source and the armature winding is excited by the ac source. The aim of excitation is to covert stator and rotor into an electromagnet. The synchronous machine can be classified into two types.
- Synchronous generator
- Synchronous motor
It is an electromechanical energy conversion device when it operates as a generator it delivers AC power. When working as a motor it takes active power from the AC source. The synchronous machine is also called as an alternator.
The speed of the synchronous motor is independent of the load and voltage. It is depending on the frequency and number of poles.
Ns = 120*f / P
f = frequency
P= number of poles
In most of the generating station, synchronous generators are used.
The synchronous machine has stator and rotor. Armature winding is placed in the stator which is energized by AC source and field winding is placed in the rotor which is energized by DC source. Principle of operation of the synchronous generator is the same as electromagnetic induction type. Based on the construction rotor can be classified into two types
- Salient pole type(projecting)
- Non-salient pole (cylindrical type)
Salient pole type generator also called a hydro generator, It has small core length and large diameter whereas cylindrical pole type generator also called turbo generator, it has large core length and small diameter.