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Enhanced thyristor power control

Product News Tuesday, April 2, 2013: CD Automation UK Ltd

CD Automation’s comprehensive range of thyristor power controllers has been enhanced with the addition of the REVO-Multidrive for current to 2600A. The product is also used with complex load types where there is a need for phase angle firing. These include transformer coupled loads, fast responding loads, loads with large resistance changes and loads requiring current limiting or soft-start.

Integral feedback control regulates the power to the heating element by monitoring the line supply voltage and compensating for any fluctuations. It is a way to linearise the output power by comparing it with the input for individual phases, providing true power control, load balance monitoring, and missing or incorrect phase rotation diagnostics.

“When you start up a silicon carbide heating element for example, the initial resistance is so low that it acts almost as a short circuit, causing a huge inrush current. Typically, this would be seven or eight times the nominal current, but could be as high as 15 times the nominal current,” clarifies Jez Watson of CD Automation.

“As the temperature rises to about 500 degrees centigrade for optimal operation, the resistance of the element also increases. We are only talking about 250 milliseconds, but in that time you can do a lot of damage with high inrush current into complex heating elements such as silicon carbide or Kanthal.

“With the feedback feature on REVO-Multidrive, you can monitor the temperature at start-up and then switch to voltage control once the required operating temperature has been reached. At this point the unit automatically increases the voltage to maintain the required power.

“Feedback is often used in conjunction with phase angle firing. When using silicon carbide for example, the resistance increases with age, which affects the power output. This is because, as the elements age, their resistance increases.

“This results in decreasing current, so after say a year, the transformer voltage would need to be adjusted by changing the taps so that the resistance of the heater is at the same level. This has traditionally been provided by two or three taps on the transformer. Feedback goes some way to replace multi-tap.”

Sitting at the top of the CD Automation range, there is no load that the REVO-Multidrive can’t handle. The keypad and display allow total flexibility for the user with full configuration of the process and digital inputs, firing and control modes, to suit the application.

Real time values such as the voltage, current and power can be read from the display. Internal quick-blow fuses as standard provide complete protection for the thyristor. Additional features such as current limit, profiling, heater break alarm and RS485 communications come as standard.