Polyphase Transformers

In large power rectifiers three-phase supplies are generally used. Accurate phase voltages must be maintained to avoid supply frequency ripple in the output. Delta-connected primaries are shown in Table VII for the various rectifiers; these are preferable to open-delta because phase balance is better, and to Y-connections because of possibly high third harmonics. Open-delta connections require only two single-phase transformers instead of three, but a similar saving may be had by using a single core-type three-phase unit which retains the phase-balance advantage. The main drawback to a three-phase core is its special dimensions. Often, to use standard parts, three single-phase units are employed in the smaller power ratings. But if the power is hundreds or thousands of kilowatts, the cores are built to order, and the weight saving in a three-phase core is significant.

Two- and three-phase filament transformers are used with output tubes for large broadcast stations to heat filaments uniformly and reduce hum in the r-f output.