Thyristor based Control Panel for 20KVA Salt Bath Furnace

The control panel featured here is designed to control the temperature of the salt bath furnace whose temperature and electrical power are controlled through the low voltage high current secondary windings of the transformer. The transformer is rated for 20KVA having 2 phase primary at 400VAC and secondary voltage of 10V at 2000 Amps. The secondary winding is connected to the 2 electrodes which are immersed in the salt bath of the furnace. When the secondary voltage is applied the current flows between the electrodes through the salt and thereby heating in the liquid salt takes place. The molten salt could be nitrates, nitrites, caustic soda, chlorides, carbonates, and cyanide.

The control panel incorporates 2 phase thyristor power regulator built using Libratherm 2 phase firing card model LTC-13-2 and a Semikron SCR module SKKT573/16E. The closed-loop control is achieved using the external PLC. The temperature signal sensed by the R-type thermocouple, which is immersed in the furnace is connected directly to the PLC or through a signal conditioner. The PLC generates a control signal of 0 to 10VDC proportional to the required temperature, which is fed to the firing card, which in turn controls the primary voltage. Instead of a PLC a PID temperature controller can also be used for temperature control.

Manual power control is achieved using the front panel potentiometer. The Auto/Manual control selection is made using the front panel rotary selector switch. Based on the control signal either in the manual control or in the auto control, transformer primary voltage is controlled from 0 to 400VAC which in turn will control the secondary voltage and current flowing through the salt bath of the furnace, and hence the temperature is controlled.

The panel front panel also has Analog meters to monitor the primary voltage, primary current, and 3 phase indicating lamps to monitor the incoming voltage. To turn ON and OFF the power to the panel a 600A MCCB is installed in the incomer power lines. The panel is constructed with CR sheet and duly powder coated with the Siemens gray color and provided with a suitable mounting stand.