Current-Limiting Transformers

Filaments of large vacuum tubes sometimes must be protected against the high initial current they draw at rated filament voltage. This is done by reducing the starting voltage automatically through the use of a current-limiting transformer, with magnetic shunts between primary and secondary windings. The shunts carry very little flux at no load; as the load increases, the secondary ampere-turns force more of the flux into the shunts until at current I_sc, Fig. 194 (B), the output voltage is zero.

Fig. 194. (A) Current-limiting transformer; (B) output voltage versus current curve.

The same principle is used to limit current in transformers for oil-burner ignition, precipi-trons, and neon or other gas-filled tube signs.

Cross-sectional area through each shunt path is the same as that of the upper or lower leg of the shell laminations; then flux in the shunts does not exceed that in the core, shunt iron loss is not abnormal, and secondary voltage is sinusoidal. At short-circuit current I_sc, half the total flux flows through each set of shunts. The air-gap length in each shunt path can be found from equation 35:

where

N = secondary turns

B_m = allowable induction in the shunts (in gauss).

The constant 0.6 is generally too small because of the flux fringing around the gap. The increase of gap made necessary by fringing may be found from Fig. 72. If the shunts are too short, the transformer does not limit the current properly. It is best to have slightly less air gap than necessary, and find by trial the right length of shunt. Fringing flux heats the coils and core somewhat more than in an ordinary transformer. If the secondary current is heavy, coils are wound pancake fashion and connected in parallel; they may have to be cross-connected for the coils to divide the load equally.

If the ordinate for open-circuit voltage and abscissa for short-circuit current in Fig. 194(B) are equal, the curve is a quarter-circle for a perfect transformer because the secondary current at short-circuit is all reactive. With core, shunt, and winding losses the curve for an actual transformer falls some 10 to 15 per cent less than the quarter-circle at currents 0.5 to 0.75 times I_SC.