Taming electrical energy with power electronics

Paul Boughton

A thyristor is a semiconductor device which acts as a switchable diode, also known as a silicon controller rectifier (SCR). Here, Jez Watson explains the application of thyristor control to simple and complex heating loads and to temperature control for applications like ovens, furnaces and heat treatment.

Thyristors can control electrical loads up to 3000kW from simple single-phase heaters up to complex high temperature-coefficient three phase loads. 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 per cent, with a typical element made of iron chromium or nickel chromium. Typical firing method 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.

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 connected on the secondary winding and firing method is typically phase angle plus current limit.

Load resistance

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.

Time-proportioned on-off or burst firing 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.

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, the firing method requires more sophistication.

In addition to various firing methods, proportional integral derivative (PID) feedback control may be employed. Supply voltage fluctuations change the power to the load so, to overcome this effect, the voltage supplied to the load is measured and compared with the power demand from the controller.

A PID controller examines signals from sensors placed in a process, called feedback or ‘error’ signals. The error signal is used to automatically hold the power at the value requested. When a feedback signal is received, it is compared with the desired value, or set value, and a calculation is made to match it to the set value. Process controllers generally work on the principle of a ‘closed-loop’.

Taking a typical application like an oven, the measured temperature, referred to as the process value (PV), is fed into the controller and compared to the set value (SV): the desired temperature. As the temperature rises, the power to the oven is reduced by the controller until the desired temperature is reached.

Most industrial processes such as plastic extrusion require stable ‘straight-line’ control of temperature. The example shown assumes that the variable is temperature, but the principles are equally applicable to all analogue variables.

The integral function prevents the initial overshoot on power-up, while the derivative function eliminates the temperature instability over time once the set value is achieved and the process is under control. Most heating applications can be handled by proportional control only.

PID terms

A proportional controller has the ability to vary its output between 0-100 percent. This enables it to continuously adjust the output so that the power input to the process is in balance with the process demand.

The range, or band, where the output power is adjusted is called the proportional band (PB). The difference between the stabilised PV and SV is called ‘offset’ and can be reduced by narrowing the proportional band. However the proportional band can only be reduced so far before instability occurs.

This is where the Integral term comes in. Integral, also called ‘reset’, has one primary function: to eliminate offset. This eliminates the temperature offset condition caused by proportional control on system start-up. To reduce or eliminate overshoot, we must use the D or derivative term of PID control.

In many thermal systems, overshoot (or undershoot) of the set value temperature is perfectly acceptable. However, in some systems this can produce poor quality products or perhaps even damage expensive equipment.

So derivative has one main job: to prevent or greatly reduce overshoot and undershoot. If the temperature rise is too fast, it will begin switching the heater off to prevent overshoot. If the temperature is falling too fast, it will begin switching the heater on longer to prevent undershoot.

Another way to think of derivative is that it opposes change. For example, if the temperature suddenly drops below the SV, derivative opposes the rapid drop by turning the heater full on. Luckily for us, most PID controllers come with automatic PID tuning.

Electrode boiler

Thyristor control can enhance pollution-free steam generation. Simple on-off switching was found to be too brute a force for electrode boilers over 100kW at Collins Walker Limited, a manufacturer of electric steam and hot water boilers, so CD Automation helped the company to switch from contactors to thyristor control.

An electrode boiler conducts an electric current through water between a pair of electrodes.  It is more robust than an immersion type. One of the reasons for this is that it does not use heating elements and associated switching needed to control them.

Collins Walker’s immersion boilers have traditionally used relay contactors to control power switching to the elements. Because of their smaller rating, this can amount to a bank of up to 60 contactors on larger boilers. With their associated wiring, contactors can be a source of faults, and require regular maintenance.

Although having multiple contactors gives some degree of control, by staging the switching, the boiler and the electrical supply can still be subject to power surges. With larger boilers rated at thousands of kilowatts, even Collins Walker’s power station customers are concerned about the huge draw when boilers are switched on! The heating elements themselves suffer thermal shock each time the power is switched, and this leads to element failures.

No arcing

Collins Walker replaced all 60 contactors with a single thyristor, which cut down wiring significantly. Contactors have a finite life and start burning out after prolonged use, giving rise to maintenance issues over time. A solid state thyristor is contactless so there is no arcing of the type that accelerates contactor wear.

Because CD Automation’s thyristor controllers are designed for ease of installation, Collins Walker was able to configure and set them up in house without the need for expensive engineering consultancy.

The current handled by the boiler is 850A at 415V. Collins Walker now uses thyristor controllers on all but the smallest boilers – those less than 100 kW, which only have one or two contactors. The controllers are programmed to bring in the power gradually, for example starting at 10 per cent for a few seconds before moving on to 20 per cent and so on.

The power is also reduced gradually instead of being abruptly switched off, smoothing the whole process of switching the power on and off. This gradual power process is similar to soft starting on electric motors and has been found to be particularly useful at one of Collin’s Walker customers, Jersey Power Station, where the need is to heat water by gradually switching in heater banks.

The thyristor is attached to a programmable logic controller (PLC) forming part of a proportional integral derivative (PID) loop. This helps to anticipate the lag in the system, so the on or off switching occurs just before it is needed.

This prevents thermal overshoot, where the elements are still supplying heat after the set temperature has been reached and the power switched off, making the boiler more energy efficient. This is only possible with thyristor control as a PID Loop cannot be implemented on a boiler using contactors. A PID temperature controller can take advantage of burst firing, which is a time-proportioned on-off method.

Vacuum furnace

Taking another example, in this instance a vacuum furnace, a thyristor can control furnace heating and be integrated into a closed loop PID control system, providing more accurate feedback control of the heat treatment process.

The 4-20mA signal from a controller adjusts the output from the thyristor, then the furnace temperature is fed back to the controller via thermocouples. A thyristor can use 0-10V voltage analogue signals as well, if needed, but some vacuum furnace users prefer to stick with 4-20mA current signals because this provides better control.

Furnace brazing is a semi-automated process through which metal components are joined using a dissimilar lower melting material. Furnace brazing allows design and manufacturing engineers to join simple or complex designs for one joint or several hundred joints.

Parts to be joined are normally cleaned and brazing alloy applied to the surfaces to be joined, and then placed into the furnace. The entire assembly is brought to brazing temperature after the furnace has been evacuated of air to eliminate any oxidation or contamination occurring - the braze alloy liquefies and flows throughout the brazed joints.

Firing methods

Zero crossing: Zero crossing firing mode is used with the logic output from a temperature controller and so the thyristor operates like a contactor. The cycle time is performed by the temperature controller. Zero crossing minimises interferences as the thyristor unit switches on-off at zero voltage.

Burst firing: Burst firing is performed digitally within the thyristor unit at zero volts, producing no EMC interference. Analogue input is used and the number of complete cycles must be specified for 50 per cent power demand. This value can be between 1 and 255 complete cycles, determining the speed of firing. When 1 is specified, the firing mode becomes single cycle.

Single cycle: Single cycle is the fastest zero crossing switching method. At 50% input signal, one cycle is on and one cycle is off. At 75 per cent, three cycles are on and one cycle is off. If power demand is 76 per cent the unit performs the same as for 75 per cent but every time the unit switches on the microprocessor divides 76/75 and memorises the ratio. When the sum is one the unit delivers one cycle more to the load. With this firing it is necessary to have analogue input.

Delayed triggering: Used to switch the primary coil of transformers when coupled with normal resistive loads (not cold resistance) on the secondary, delayed triggering prevents the inrush current when zero voltage (on-off) is used to switch the primary. The thyristor unit switches off when the load voltage is negative and switches on only when positive with a pre-set delay for the first half cycle.

Phase angle: Phase angle controls the power to the load by allowing the thyristor to conduct for part of the AC supply cycle only. The more the power required, the more the conduction angle is advanced until virtually the whole cycle is conducting for 100 per cent power. The load power can be adjusted from 0 to 100 per cent as a function of the analogue input signal, normally determined by a temperature controller or potentiometer, PA is normally used with inductive loads.

Soft start plus burst firing: This is an additional feature to Burst Firing. Starting in phase angle mode, the unit ramps from zero to full voltage at a preset time, finishing at full conduction for the remainder of the on period. Ideally used to switch small inductive loads, soft start plus burst firing avoids current surge and minimises electrical interference.

Jez Watson is with CD Automation UK Ltd, Eastbourne, East Sussex, UK. www.cdautomation.co.uk