Anode Transformers

Anode transformers differ from filament transformers in several respects.

(a)  Currents are non-sinusoidal. In a single-phase full-wave rectifier, for instance, current flows through one half of the secondary during each positive voltage excursion and through the other half during each negative excursion. For half of the time each half-secondary winding is idle.

(b)   Leakage inductance not only determines output voltage but also affects rectifier regulation in an entirely different manner than with a straight a-c load. This is discussed in Rectifier Performance.

(c)  Half-wave rectifiers carry unbalanced direct current; this may necessitate less a-c flux density, hence larger transformers, than full-wave rectifiers. Unbalance in the three-phase half-wave type can be avoided by the use of zigzag connections, but an increase in size over full-wave results because of the out-of-phase voltages. These connections are desirable in full-wave rectifiers when half voltage is obtained from a center tap. See Table VII.

(d)  Single-phase full-wave rectifiers with two anodes have higher secondary volt-amperes for a given primary v-a rating than a filament transformer. Bridge-type (four-anode) rectifiers have equal primary and secondary volt-amperes, as well as balanced direct current, and plate transformers for these rectifiers are smaller than for other types. Three-phase rectifier transformers are smaller in total size but require more coils. The three-phase full-wave type has equal primary and secondary v-a ratings.

(e)  Induced secondary voltage is much higher. Filament transformers are insulated for this voltage but have a few secondary turns of large wire, whereas anode transformers have many turns of small wire. For this reason the volts per layer are higher in anode transformers, and core windows having proportionately greater height and less width than those in Fig. 56 are often preferable. This trend runs counter to the conditions for low leakage inductance and makes it necessary to interleave the windings.

Fig. 57. Dimensions and coil section of anode transformer. Construction shown is for shell-type transformer with 2 Hipersil cores.

Figure 57 shows the windings of a single-phase full-wave rectifier transformer with the primary interleaved between halves of the secondary. This arrangement is especially adaptable to transformers with grounded center tap. The primary-secondary insulation can be reduced to the amount suitable for primary to ground. This is called graded insulation.

In large power rectifiers of the gas-filled or pool types, anode current under short-circuit conditions may be very great, and anode transformer windings must be braced to prevent damage. If the conductors are small, solventless varnish is useful for solidly embedding the conductors.