This is a semiconductor device which acts as a switch. The semiconductor rectifier material either works as a conductor when current flows in one direction or as an insulator when it attempts to flow in the other. If the voltage being applied is the standard sinusoidal AC form only half of the waveform will be conducted, therefore current flows during half cycles only. To deliver maximum power to the load both halves of the AC waveform must be conducted and two silicon controlled rectifiers (SCRs) used, connected in an anti-parallel (back to back) configuration.
The term silicon controlled rectifier (SCR) is sometimes used interchangeably with thyristors however, the former is a brand name introduced by General Electric to describe a particular kind of thyristor that it manufactured. There are various other types including diacs and triacs which are designed to work with alternating current so the terms are not completely synonymous. Whether it’s called thyristor or SCR we're referring to the same semiconductor device.
In the case of back to back SCRs, these allow the full wave current to be conducted. The forward SCR conducts during the positive half of the cycle, the reverse SCR during the negative half. Each SCR is turned on at the appropriate time by a trigger pulse applied to the gate (a third leg) and the device will remain ON until the instantaneous load current through it drops to zero. The trigger pulses are generated by a drive circuit which times the pulse to ensure the thyristor unit output is a function of the input control signal and firing mode.
Thyristors are commonly used in AC circuits and for power control. The application or the load being driven or switched dictates which type of SCR and firing mode can be used. The main types of load power: resistive and inductive will each need a different type of firing of the SCR. Resistive elements are either variable or fixed and the choice of element is primarily chosen by the maximum temperature required and environment conditions. The most common inductive load is the transformer, normally used to provide galvanic isolation between the primary and secondary winding or to change the main supply voltage to the nominal load supply. Thyristors are used in motor speed controls, light dimmers, pressure-control systems and liquid-level regulators.
Using our automatic calculation tool is the easiest way but if you prefer the long way it is first necessary to recognise whether the application is single or three-phase. For single phase (1PH) you can use the calculation: total load (in watts) over the load voltage (L to N or L1 to L2) to give the current value (I = P/V). For example, load is 12kW and the voltage (L to N) is 240V giving a current of 50A. We normally add a safety margin of 15-20% to this nominal current in selecting a thyristor power controller to ensure we allow for any fluctuations in voltage supply or temperature, etc. This means we are not switching at the unit’s maximum which results in a longer thyristor life.
Three phase (3PH) systems are normally used for larger power consuming loads. The voltage between any two legs of a three system is a sinusoidal AC waveform but the voltage waveform between each successive pair of legs will be displaced in time by 120 electrical degrees from the other two. This corresponds to 6.67 milliseconds for a 50Hz system. Assuming the load is 12kW but connected to 3 phase we would have 12000 watts over the voltage, times by the square root of 3 or 1.73 for convenience. So the current would be 12000 / 415 X 1.73 = 16.8 amps.
The firing mode is determined by the electronics mounted on the thyristor (SCR) power controller (the electronics package is called the firing circuit).
Zero crossover is used with the logic output typically from a temperature controller and the thyristor operates like a contactor. The power controller is OFF if no input signal is present or fully ON if the input signal is present; there are no other conditions possible. The cycle time is determined by the control device ie a temperature controller. Zero crossover minimises EMC interference due to the fact that the thyristor unit switches ON/OFF at zero volts.
Burst firing is zero crossover but the ON/OFF periods are fixed for a specific cycle time (bursts of ON/OFF). The firing circuit determines how long the power controller should be ON for a given control signal. Analogue or proportional input is necessary for burst firing and the number of complete cycles must be specified for 50% 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. The main advantage over ON/OFF solid state control is that the unit will accept proportional inputs such as 4-20mA or 0-10V. Proportional inputs allow the process controller to vary the load more accurately as the process changes.
Single cycle is the fastest zero crossing switching method. At 50% input signal one cycle is ON and one is OFF. At 75% three cycles are ON and one is OFF. If power demand is 76% the unit performs the same as for 75% 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. Single cycle firing is ideal for simple resistance loads and where thermal mass of the load is small or as a low noise alternative to phase angle firing with some applications.
Phase angle controls the power to the load by allowing the thyristor to conduct for part of the AC supply cycle only. The more power that is required so the conduction angle is advanced until virtually the whole cycle is conducting for 100% power. The load power can be adjusted from 0 to 100% as a function of the analogue input signal, normally determined by a temperature controller or potentiometer; phase angle firing is normally used with inductive loads. Filters may need to be used to remove any resulting EMC interference generated and features including soft start and current limit may be required to control the load.
First consider the load and/or element type used; the most common is referred to as a fixed or normal resistance. This element doesn’t change more than 10% in resistance over time or with temperature variations. In our industry this accounts for more than 85% of elements used. Typical thyristor firing will be zero crossover either with a DC logic control signal (for zero crossover firing) or an analogue 4-20mA or 0-10V (for burst firing). No other element consideration or protection is really needed although having the longest cycle time that controls the load adequately is recommended to extend element life.
For additional load types including variable resistance elements, short-wave infrared lamps, inductive or transformer loads we recommend using our Product Selection Tool and/or contacting our technical team to help choose the right thyristor power controller for your application. Remember: we have the right product for all applications.
Reduced maintenance and operating costs as there are no moving mechanical parts to fail.
Electrically quieter as the device turns ON/OFF at zero volts so doesn’t create RFI interference.
Line distortion is eliminated.
Finer control and extended heater element life due to shorter cycle times that increase process stability and decrease heater thermal shock.
Heater shows less expansion and contraction caused by heating and cooling action.