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Thyristors: the smart way to control heater loads

11 October 2012

Jez Watson offers this short tutorial on the application of thyristor control to both simple and complex heating loads and other temperature control applications, illustrating the methods with examples from his company's Revo (‘Relay Evolution’) product portfolio.

A control cabinet at HVAC systems manufacturer, Collins Walker is opened to show installed CD Automation thyristor modules
A control cabinet at HVAC systems manufacturer, Collins Walker is opened to show installed CD Automation thyristor modules

A thyristor is a semiconductor device which acts as a switch that can only pass current in one direction. The concept was first described by William Shockley in 1950 as a ‘bipolar transistor with a p-n hook-collector’ – a device now frequently referred to as a silicon controlled rectifier.

A machine 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 percent, 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 ageing 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 a short wave infrared (SWIR) lamp using phase angle firing). Medium/long wave IR is treated as normal resistive. 

Finally, the last two cases apply to transformer coupled loads with high in-rush current on start-up. In the fifth case, the transformer is connected on the secondary winding and the firing method is typically phase angle or soft-start plus current limit. In the sixth case, the transformer is connected on the secondary winding and the firing method is typically phase angle plus current limit. 

Basic firing
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. The device is designed to replace contactors and is suitable for all resistive switching applications, medium and long wave IR load types. 

Complex firing
If the element type 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 former includes RS485 Modbus interoperability so it can be configured for input type and firing type. 

Suitable for normal resistive loads, medium and long wave infrared (IR) and silicon carbide load types plus short wave IR (SWIR) on single phase units, Revo M enables the selection of input, firing and feedback type. Users can change or view any parameter, either via the front panel display and keypad, or via the RS485 port, including reading and displaying process values such as voltage, current and power. 

Complex loads
Suitable for resistive and inductive/transformer load types (excluding two-phase), Revo E offers a current limiting function for complex loads such as heating elements that require current limits to be set manually. Designed specifically to drive inductive and transformer loads, this thyristor module is easy to set up via an integral keypad and display.

Revo CL, E and Multidrive are fundamentally aimed at the same application types and have a similar feature-set. Why one is chosen over another is down to the number of phases controlled and the size of unit. Multidrive can be considered as a combined Revo-CL and Revo-E, plus additional digital/analogue inputs/outputs. It handles current from 800A to 2,600A.

Revo SSR/Analog comes in two versions with dc logic input or dc linear 0-10V/4-20mA inputs and three different power outputs. Assembled without a heat sink (enabling it to be fitted to a customer’s own heat sink arrangement), multiple units can be mounted together, reducing unit depth and freeing up panel space.

Temperature control and PC power
Revo-TC is a temperature controller and thyristor power controller all in one, significantly reducing wiring and installation time, while Revo-PC is a sophisticated load management system that provides basic thyristor controllers with enhanced control performance.

Ideal when multiple zone control is required, the latter can drive up to 24 single-phase or up to eight three-phase Revo-S units via synchronisation and power limiting, to provide very tight control over energy consumption. 

CD Automation’s thyristor power controllers fire heating elements on ovens, furnaces, heat treatment systems and a wide spectrum of electrical loads up to 3,000kW. They are deployed on applications ranging from simple single-phase heaters to complex high temperature-coefficient three-phase loads.

Jez Watson is with CD Automation

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