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Two phase control of three phase heating loads with thyristor (SCR) power controllers

07 September 2012

An economical means of switching three phase heating loads is to switch two phases only. This is possible on delta or floating star (no neutral) loads with constant resistance heaters.

This method of switching works due to the fact that if two phases are switched off then the third phase has no return conduction path. This type of two phase switching can only be used with burst cycle or on/off firing of the thyristors. Phase angle firing cannot be used with this method.

There are two main benefits of using two phase switching to control a three phase load. One is reduced cost because there is one less thyristor module in the circuit. The other is that the thyristor power controller dissipates less heat because there are only two thyristor modules in the circuit rather than three. The reduced heat dissipation minimises enclosure fan cooling requirements and allows the control panel to run cooler, maximising the life of the power electronics.

CD Automation provide a wide range of two phase switching thyristor power controller assemblies from 30 amps per phase with the REVO-S and REVO-M series right up to 2700 amps per phase with the MULTIDRIVE 2 Phase controller.

All these controllers have an RS485 communications port running Modbus RTU protocol. This allows the user not only to configure the thyristor remotely but also allows important electrical parameters such as current, voltage and power to be read. Fieldbus modules are available to convert Modbus RTU RS485 to Profibus, Ethernet, Devicenet and CANbus.

Semiconductor protection fuses are internal to the thyristor and are supplied as standard.
CD Automation can advise on all aspects of selection, application, configuration and commissioning.

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