In a three phase circuit currents flow in each phase. If an inductive or capacitive load is connected, then the current waveforms will respectively ‘lag’ or ‘lead’ the voltage waveforms by 90°.

The current in the U phase will be zero, but current will be flowing in V and W phases. It can be seen that the lines of magnetic flux now enclose not only the rotor excitation current, but also the stator currents in the V and W phases.

The flux is the result of the mmf acting on a magnetic circuit, but it can now be seen that the mmf is a combination of the ampere-turns from the rotor and the stator winding. If Ir, Is, Nr, and Ns are the currents and turns in the stator and rotor windings respectively, then Φ = (IrNr + IsNs)/Rm.

It is seen that the stator currents oppose the field current in the rotor and their effect is to reduce the flux, with a corresponding reduction in the generated voltage. This demagnetizing effect is called ‘armature reaction’; it is the way in which Lenz’s Law operates in a generator.

The armature reaction effect therefore depends on the extent to which the stator currents lag or lead the voltages (often called the ‘phase’ or ‘phase angle’). If, for example, the generator load is capacitive, the currents will lead the voltages by 90°, and they will be opposite in direction to that for an inductive load.

The ampere-turns of stator and rotor windings will add in this case and the flux and the generated voltage will be higher. In the case of a resistive load, the ampereturns of the stator will act at 90° to the rotor poles, tending to concentrate the flux towards the trailing edge of the pole and producing magnetic saturation here when large stator currents flow; this reduces the flux and the output voltage, but not so much as in the inductive load case.

The output voltage is influenced not only by armature reaction, but also by voltage drop within the stator winding. This voltage drop is partly due to the internal resistance of the winding, and partly due to flux which links the stator winding but not the rotor winding; this flux is known as ‘leakage flux’ and it appears in the stator electrical circuit as a leakage inductance, which also creates a voltage drop.

The phase angle between stator currents and voltages will affect this voltage drop, producing a greater drop at lagging currents, and a negative drop (an increase) in voltage at leading currents. In order to maintain a constant output voltage it is therefore necessary to change the excitation current in the rotor to compensate for the load conditions.

To achieve constant output voltage, an Automatic Voltage Regulator (AVR) is used on the majority of generators, except for some small self-regulating units. The effects described so far can be conveniently summarized in a phasor diagram which combines stator and rotor mmfs as well as the emf drops due to resistance and leakage reactance of the stator winding. 

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