Similarities Between Alternator and Two Winding Transformers: An alternating current generator is often referred to as an alternator.
Many of the concepts used in transformer analysis will be found useful when dealing with alternator operating principles. The single-phase alternator is in some respects similar to a two-winding transformer, the primary of which rotates mechanically and is supplied with a direct voltage instead of an alternating voltage. Elements of an alternator crude form reveal this. The magnetic structure consists of an armature A and a rotor R. If the rotor is held in a vertical position, the application of an alternating voltage across H and X will cause an induced voltage E in the armature winding. This device would be similar to an ordinary transformer for this operating condition. Due to the air gap between A and R, the drive current would be higher than is normally encountered when using a closed magnetic circuit.
If now the element R is mounted on a shaft 0, find the leads H and X connected to slip rings on the shaft, the rotor winding could be supplied with DC current. This direct current would produce a flux in the rotor element. If the rotor rotates while direct current is flowing through its winding, a change in flux through A will occur and thus an alternating voltage will be generated in the armature winding. Under this operating condition, the structure becomes a single-phase alternator. If the structure of the rotor is such that the flux distribution in space is sinusoidal, then for constant speed the voltage E can be made sinusoidal. Therefore, the armature of this device will work in the same way as the secondary of a transformer. You will have a resistance drop and a leakage reactance drop in your winding under load conditions as in the transformer case.
The presence of secondary current in a transformer will cause the current in its primary to be such that the primary can maintain its constant flux. When this device is used as an alternator, the dc current in the rotor winding is determined by the relationship between the dc voltage and the resistance of the rotor winding. Flux constancy is not required in R as in the primary of the transformer. any resulting flux produced by the armature current will not be completely counteracted by a change in rotor current. If the armature current is supplied to a lagging load, it will tend to demagnetize the rotor. This will cause a reduction in core flux as lagging current Ia increases and a corresponding reduction in E. If the armature supplies a leading current, an increase in flux will occur. The changes in flux due to armature currents are large, and consequently the change in voltage E with change in magnitude or power factor of the load current will be correspondingly large. The voltage regulation of an alternator under some load conditions can be 20 to 30 times greater than that of a power transformer. For this reason, an alternator with fixed field current (rotor current) is a variable voltage device. It can be converted to a constant voltage device by using regulation devices that automatically change the field current when the load changes to maintain the armature voltage at a predetermined value.
