Fig. 4 describes in graphic form how zero-firing of SCRs can proportion the voltage to an AC load. For electric heating with nominally constant resistance heating elements, zero-firing, with its 100% power factor and harmonic-free waveform, is clearly the best way to proportion power in small or large amounts. As a result, this method of control is fast becoming the dominant method of power control for conventional electric resistance heating.
Unfortunately, zero-firing into a transformer primary is not generally suitable. If the transformer core flux is opposite in polarity to the initial voltage waveform applied, core saturation could occur, which would cause damaging high current to flow. There are various “soft-start” arrangements to deal with this situation but, for general usage, it is wise to employ phase-angle control into most transformer primary circuits.
For small instrument control transformers, which are connected to large zero-fired power controls to measure volts, amps or kW, one must also take care to limit these same “saturation currents”. They may be too small to affect the fusing of the power control, but they can easily damage the windings of the control transformer, which are not rated for continuous bursts of saturation current. Small, properly sized, series-connected resistors will eliminate the problem and allow one to safely use these control transformers with big zero-fired power controls.