Thyristor regulators are used to control the energy supplied to a wide variety of loads. There are a number of circuit configurations that can be used but all methods employ self-commutating semiconductors either as single switching elements (half-controlled) but more often each phase is controlled with inverse parallel connected pairs of semiconductors (fully-controlled). Two switching methods can be employed and these are referred to as "burst fire" or "phase-angle" types.
"Burst fire" involves switching on the power with the semiconductors on full conduction for a certain time (several full mains cycles) determined by a sensor such as a thermostat. Units that work on the phase angle control principle generate an electronic firing pulse that switches the thyristor on at any point on the sine wave, thus allowing current flow for part of the mains cycle. The thyristor automatically switches off when the current falls to zero and does not conduct again until it receives the next firing pulse.
In this way it is possible to vary the output voltage but in doing so the resulting wave form resembles a saw tooth, unless the thyristor is on full conduction.
It is possible to control currents up to thousands of amps in this way but as the power increases so does the cooling requirement.
REO manufacture stand-alone units of up to 400A per phase with water cooling. Above this current level the units tend to be purpose built and for this reason a regulating/firing module is used in conjunction with separate six-pulse thyristor stacks, using primary or secondary regulation.