Fig. 2 illustrates a typical installation of a 50 A single phase thyristor unit phase-angle controlling 480 VAC to the input of a transformer, with a low voltage resistive heater connected to the transformer secondary.
Note the resistance value RL. All properly designed thyristor controls have certain minimum resistance loads that they are able to control, if for no other reason than that this minimum resistance must be large enough to limit the current through each thyristor to its rating when it switches into the ON state. From Ohm’s law this value is, typically, the RMS mains voltage divided by the maximum nameplate current rating of the power control.
And for a direct-connected resistance heater, this would be a correct value.
However, some might also assume that this is the same value when connecting an SCR power control to a transformer. It is not. All transformer primaries have a DC resistance value much lower than the primary voltage divided by the primary current rating. As a result, the maximum current that can flow through the loop 1-4-3-2 is limited only by RL and whatever system impedance is available at terminals 1 and 2. If F1 is selected properly to protect the two SCRs and no consideration is given to the transformer RL, it is very likely that one will have nuisance fuse blowing during normal operation of the system, due to the transformer magnetizing current inrush.
When SCRs are either in the OFF state or in operation, one cannot assume that they will not misfire due to some line transient. After all, that is the reason all properly designed SCR controls always have an RC network connected across the SCRs, as shown in Fig. 1. The RC network and the dv/dt rating of the SCR combine to prevent over 90% of line noise from causing a misfire. Nonetheless, there is always a bigger transient on the mains, and every solid state power control must be designed to allow for that inevitability. By oversizing the SCR/fuse combination for transformer or other high-inrush loads, one can be assured that nuisance fuse blowing is unlikely to occur, nor will there be any fatigue damage to the fragile silicon SCR pellet.
Finally, one must also ensure that phase-angle control (Fig. 3) of the transformer primary always impresses a symmetrical AC voltage on the primary, with no significant DC component. The latter can cause saturation of the transformer core, which leads to excessive current and temperatures that cannot be tolerated for long-term operation.
Transformers and other inductive loads require full-cycle gating of the SCRs to ensure that the SCRs remain in the ON state, regardless of the inductive load characteristic (where the load current may lag the SCR voltage by 90 degrees or more).