Similarities between the alternator and the two-winding transformers: An alternating, current generator is frequently referred to as an alternator.
Many of the concepts used in the analysis of the transformer will be found useful in dealing with the principles of operating of the alternator. The single-phase alternator is in some respects similar to a two-winding transformer, the primary of which is rotated mechanically and supplied with a direct voltage instead of an alternating voltage. The elements of a crude form of alternator reveal this. The magnetic structure consists of an armature A and a rotor R. If the rotor is held in the position vertically, the application of an alternating voltage at H and X will cause an induced voltage E in the armature winding. This device would be similar to an ordinary transformer for this condition of operation. Because of the air gap between A and R, the exciting current would be higher than is normally found when a closed magnetic circuit is used.
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 direct current. This direct current would produce a flux in the rotor element. If the rotor is rotated while direct current is flowing through its winding, it would cause a change in flux through A and thereby generate an alternating voltage in the armature winding. Under this condition of operation the structure becomes a single phase alternator. If the rotor structure is such that the distribution of flux in space is sinusoidal, then for constant speed the voltage E can be made sinusoidal. Thus the armature of this device will function in very much the same manner as the secondary of a transformer. It will have a resistance drop and a leakage-reactance drop in its winding under condition of load as in the case of the transformer.
The presence of secondary current in a transformer will cause current in its primary such that the primary can keep its flux constant. When this device is used as an alternator, the dc current in the rotor winding is determined by the ratio of the dc voltage to the resistance of the rotor winding. Constancy of the flux in R is not required as in the primary of the transformer. Consequently any fluxes produced by the armature current will not be completely counteracted by a change in the rotor current. If the armature current is supplied to a lagging load, it will tend to demagnetize the rotor. This will cause a reduction of the flux in the core as the lagging current Ia is increased and a corresponding reduction in E. If a leading current is supplied by the armature, an increase in the flux will take place. 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 conditions of load may be 20 or 30 times as large as that of a power transformer. For this reason, an alternator with fixed field current (current in rotor) is a variable voltage device. It can be converted to a Constant voltage device by the use of regulating devices that automatically change the field current when the load is changed so as to keep the armature voltage at a predetermined value.