Transformers with D-C Flux

When there is a net d-c flux in the core, as in single-phase half-wave anode transformers, the choice of core depends on the same principles as in reactors with large a-c flux. The windings carry non-sinusoidal load current, the form of which depends on the circuit. Winding currents may be calculated with the aid of Table I. Generally the heating effects of these currents are large. Maximum flux density should be limited as described in Linear Reactor Design. This precaution is essential in limited power supplies like aircraft or portable generators, lest the generator voltage wave form be badly distorted. On large power systems the rectifier is a minor part of the total load and has no influence on voltage wave form. The chief limitation then is primary winding current, and maximum induction may exceed the usual limits.

In single-phase half-wave transformers, air gaps are sometimes provided in the cores to reduce the core flux asymmetry described in Wave Shapes. Transformers designed in this manner resemble reactors in that core induction is calculated as in Reactors and the following sections, depending on the operating conditions. Even in transformers with no air gap, there is a certain amount of incidental reluctance at the joints in both stacked laminations and type C cores. This small gap reduces the degree of core saturation that would exist in half-wave transformers with unbroken magnetic paths.