Filament Transformers

Low-voltage filament transformers are used for heating tube filaments at or near ground potential. Often the filament windings of several tubes are combined into one transformer. Sometimes this requires several secondary windings. In terms of a single secondary transformer a 5 or 6 secondary unit requires about 50 per cent greater size and weight. But these multiwinding transformers are smaller than five or six separate units; this warrants designing them specially in many instances.

Rectifier tube filaments often operate at high d-c voltages and require windings with high voltage insulation. It is usually not feasible to combine high-voltage windings with low-voltage windings when the high voltage is more than 3,000 volts direct current because of insulation difficulties, particularly in the leads. Large rectifier filaments are usually heated by separate transformers; in polyphase rectifiers, all tube filaments are at high voltage, and some secondary windings may be combined. See the three-phase full-wave rectifier in Table VII, where the + HV lead connects to a winding which heats the filaments of three tubes.

Low capacitance filament windings are sometimes required for high-frequency circuits. The problem is not particularly difficult in small v-a ratings and at moderate voltages. Here air occupies most of the space between windings. In larger ratings the problem is more difficult, because the capacitance increases directly as the coil mean turn length for a given spacing between windings. As voltage to ground increases, there comes a point beyond which creepage effects necessitate oil-insulated windings, whereupon the capacitance jumps 2 to 1 for a given size and spacing. There is a value of capacitance below which it is impossible to go because of space limitations in the transformer. What this value is in any given case may be estimated from the fact that the capacitance in μμf of a body in free space is roughly equal to one-half its largest dimension in centimeters.

Except for the differences just mentioned, the design of filament transformers does not differ much from that of small 60-cycle power transformers. The load is constant and of unity power factor. Leakage reactance plays practically no part, because of its quadrature relationship to the load. Output voltage may therefore be figured as in Fig. 3(c). It should be accurately calculated, however, to maintain the proper filament emission and life.

When a tube filament is cold, the filament resistance is a small fraction of its operating value. In large tubes it is often necessary to protect the tube filaments against the high initial current they would draw at rated filament voltage. This is done by automatically reducing the starting voltage through the use of a current-limiting transformer having magnetic shunts between primary and secondary windings. The design of these transformers is somewhat special, and is included in the section on current-limiting transformers.

Fig. 54. 15 kv filament transformer enclosed in insulating case.

High-voltage filament transformers are sometimes mounted in an insulating case, as in Fig. 54, with the tube socket on top. This arrangement eliminates the need for high-voltage wiring between the transformer and the tube, and provides the insulation for the socket. The problem of air pockets at the base of high-voltage bushings is also eliminated. It is still necessary to insulate well between windings and to fill the case fully with insulating compound in order to eliminate corona.