Q15Electrical Machines and Drives

DC Series Generator

In a DC series generator, the field winding is connected in series with the armature and the external load circuit, so that the same current flows through the armature, field winding, and load - consequently, the field flux (and hence the generated EMF, before saturation) increases directly with load current, in sharp contrast to a shunt generator (where field current is essentially independent of load current, being determined by a fixed field-circuit resistance across the armature terminals) or a separately excited generator (where field current is set entirely independently by an external source).

The distinctive terminal characteristic (external characteristic, terminal voltage versus load current) of a series generator is highly non-linear and non-self-regulating: at very low load current, the field flux (and generated EMF) is negligible, so terminal voltage starts near zero; as load current rises, EMF rises correspondingly (nearly linearly at first, since the magnetic circuit is unsaturated), and terminal voltage rises with it despite the increasing armature-circuit voltage drop; at higher currents, magnetic saturation limits further flux (and EMF) increase while the armature and series-field resistance voltage drop continues to grow, causing terminal voltage to reach a maximum and then fall with further increase in load current, giving the characteristic 'rising-then-falling' shape distinct from the drooping (monotonically falling) characteristic of a shunt generator.

Because of this inherent inability to maintain constant terminal voltage as load varies, the series generator is entirely unsuitable for general-purpose power distribution applications (where a shunt or compound generator's much flatter voltage characteristic, made possible by field current being independent of, or in a compound machine only weakly reinforced by, load current, is required) - the series generator historically found use in specialized applications precisely exploiting its rising-voltage-with-current characteristic, such as series boosters (added in series with a feeder to compensate for the feeder's own voltage drop as load current increases, since the booster's own voltage naturally rises with the same current causing the drop) and, in DC arc-lighting and early constant-current series-distribution systems, as a constant-current source.

In summary, the series generator is distinguished from shunt, separately-excited, and compound generators primarily by this direct field-current-equals-load-current coupling, giving it a unique rising-then-falling external characteristic, and confining its practical use to specialized constant-current or voltage-boosting roles rather than general electrical power supply, where the far more stable voltage regulation of shunt, separately-excited, or (especially) compound generators is essential.