Peaking Transformers

Author: E.E. Kimberly

It is sometimes desirable to have the output voltage of a transformer sharply peaked instead of sinusoidal in form. One use for the peaking transformer is found in the grid-firing circuit of a thyratron, where the thyratron must fire at exactly the same instant in every cycle under varying load conditions or with changing characteristics of the thyratron caused by aging.

Fig. 28-13. Wave Forms in a Peaking Transformer

In the usual type of transformer, as the flux density rises into the curved part of the magnetizing curve, more magnetizing current is drawn from the line to maintain the sinusoidal form of the flux. As a result, the normal magnetizing current is of the form shown as ie in Fig. 28-13 (a).

If a resistance is placed in series with the transformer primary, it will tend to dominate the impedance and hence to subdue the peak of current which would flow if the resistance were not present. The failure of the current to reach its normal peak causes the flux wave to be "starved" where its peak would normally be, and so the flux wave tends to be flat-topped. As a result the voltage is practically zero while the flux is approximately constant at its maximum, and sharp peaks of voltage are produced near the instant when the flux is zero, as shown in Fig. 28-13 (b).