Synchronous generators or alternators are synchronous machine used to convert mechanical power to electrical power.
A DC current is applied to the rotor winding which produce a rotor magnetic field.
In AC machine the stator magnetic field and rotor magnetic field rotate at the same speed to produce an useful torque.
Although magnetic fields rotate at the same speed, rotor doesn’t have to rotate at this speed.
- If rotor rotate at the speed of stator magnetic field we call it synchronous machine
- If the rotor rotate at different speed then we call it asynchronous machine.
Two terms commonly are used to describe the windings on a machine
- Field windings -> produce main magnetic field
- Armature windings -> main voltage is induced.
Field windings are locate at rotor
Armature windings are locate at stator
The rotor of a synchronous generator is essentially a large electromagnetic. The magnetic poles on the rotor can be either salient or non salient construction.
Non salient pole rotors are normally used for two and four pole rotors ( high speed ) , white salient pole rotors are normally used for rotors with four or more poles ( low speed )
A DC current must be supplied to the field circuit on the rotor. There are two common approaches
- Supply the DC power from on external DC source to the rotor by means of slip rings and brushes
- Supply DC power from a special DC power source mounted directly on the shaft of the synchronous generator.
A brushless exciter is a small AC generator with its field circuit mounted on the stator shaft.
A brushless exciter circuit
Synchronous generators example