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Understanding Thyristor Control

15 March 2013

A thyristor is a semiconductor device which acts as a switch that can only pass current in one direction. Here, Jez Watson of CD Automation explains the application of thyristor control to simple and complex heating loads and temperature control

The concept of a thyristor was first described by William Shockley in 1950 when he referred to a bipolar transistor with a p-n hook- collector. It is in fact a switchable diode sometimes known as a silicon controller rectifier (SCR). machine. A manufacturer will typically choose a thyristor based on the type of heating element to be used. There are essentially six types of load; (1) normal resistive, (2) loads where resistance changes with time, (3) applications where resistance changes with temperature, (4) fast responding loads, and (5) and (6), transformer coupled loads of various types.

In the first case, a normal resistive load applies to any load element with a resistance change of less than 10%, with a typical element made of iron chromium or nickel chromium. Typical firing is zero-crossing, burst firing or single cycle.

In the second case, where resistance changes with time, resistance starts high when the element is new and decreases with age, then increases again as aging progresses. The typical element type is silicon carbide with phase angle firing.

The third case is where resistance changes with temperature, acting as a short circuit when cold, with resistance increasing as temperature increases. Here, the typical element type is molybdenum, tungsten or Super Kanthal and firing is phase angle plus current limit. The fourth case is fast responding loads with high surge currents and where high resolution is required, typically short wave infrared (SWIR) lamp using phase angle firing. Medium/long wave IR is treated as normal resistive.

CD Automation’s REVO range of thyristors handles current from 10A-2.5kA in three main ranges: S, M and E. The S signifies solid state relay which is a simple on-off type device using DC logic signals from a temperature controller or similar control mechanism. A normal resistance element that does not vary with temperature or time allows a basic type of firing. Because temperature response times are not critical, unlike pressure or flow measurement where quick reaction time is required, simple on-off firing is cheap and adequate.

REVO-S also has time- proportioned on-off or burst firing, which is used with DC linear type signals, such as 4-20mA current or 0-10V voltage. This will switch bursts off then on for better temperature control. Compact and low cost, the REVO-S is designed to replace contactors and is suitable for all resistive switching applications.

If the element type used is more complex, for example silicon carbide or Super Kanthal, which operates at high temperatures and varies with time and temperature, more sophistication is needed in terms of firing type, and this is provided by the REVO M, or the enhanced version REVO E. The REVO M includes RS485 Modbus interoperability so it can be configured for input type and firing type.

Suitable for resistive and inductive/transformer load types excluding two-phase, the REVO E series has current limiting, which is needed for complex load types, like certain heating elements where current limits need to be set manually, and power control modes. Designed specifically to drive inductive and transformer loads, REVO E provides optimum control of the process and ease of set-up and maintenance. 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.

REVO-TC is a temperature controller and thyristor power controller in one package, significantly reducing wiring and installation time. REVO-PC is a sophisticated load management system that provides basic thyristor controllers with enhanced control performance. Ideal when multiple zone control is required, REVO-PC can drive up to 24 single-phase or up to eight three-phase REVO-S units by synchronisation and power limiting, resulting in tight control of energy supply.

So, while Shockley’s original definition might be quite limited, the potential for thyristors in simple and complex heating loads and temperature control is far greater.

Key Points

  • A machine manufacturer will typically choose a thyristor based on the type of heating element to be used
  • CD Automation’s REVO range of thyristors handles current from 10A-2.5kA in three main ranges: S, M and E