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Thyristor Power Control Configurations
Single or Three Phase
Low power applications can be driven from a single phase AC supply. This will require one inverse-parallel
thyristor pair to switch the current and control the output to the heater. Three phase supplies for higher power
applications can be controlled by two or three inverse-parallel pairs, depending upon the configuration of the
system and firing control techniques being used.
Firing Control Techniques
Firing modes are usually determined by the type of load (resistive or inductive), and the accuracy of the
temperature control required. Inductive loads use phase angle firing which is very accurate, and can
accommodate a soft start power up, but RFI shielding may be needed to protect any sensitive equipment nearby.
The alternative to phase angle control is zero crossover firing which can be used for resistive loads where
appropriate, and generates no RFI. The control circuitry increases in complexity and cost as the power system
requirement increases from single phase to multi-phase, and from zero cross over to the more accurate phase