GENERATOR OPERATION

The stator, also called the armature, carries the three-phase AC winding. The rotor, also called the field, carries the DC excitation or field winding. The field winding therefore rotates at the shaft speed and sets up the main magnetic flux in the machine.

The fundamental magnetic action between the stator and rotor is one of tangential pulling. In a generator, the rotor pole pulls the corresponding stator pole flux around with it. In a motor, the stator pole pulls the rotor pole flux around with it. The action is analogous to stretching a spring, the greater the power developed, the greater the pull and greater the corresponding distance that is created between the rotor and stator flux axes.

When a machine is not connected to the three-phase supply but is running at rated speed and with rated terminal voltage at the stator, there exists rated flux in the iron circuit and across the air gap. This flux cuts the stator winding and induces rated emf in winding and hence rated voltage at the main terminals. Consider what happens in a generator. Let the generator be connected to a load, or the live switchboard bus bars. Stator current is caused to flow. The current in the stator winding causes a stator flux to be created which tends to counteract the air-gap flux that is produced by the excitation. This reduction of air-gap flux causes the terminal voltage to fall. The terminal voltage can be restored by increasing the rotor excitation current and hence the flux. So the demagnetizing effect of the stator current can be compensated by increasing the field excitation current. This demagnetizing effect of the stator current is called ‘armature reaction’ and gives rise to what is known as the synchronous reactance, which is also called a ‘derived’ reactance.