Thyristors are controlled by sending the correct signal to the gate connection of the device. It will then continue to let current flow until the gate signal is removed and the voltage through it reaches zero. There are two main methods of firing the thyristors:
Zero Crossover Firing sends the gate signals to the thyristors only when the voltage through it is zero. Therefore, the thyristor will turn on and off only at the zero voltage crossover point of the sine wave which occurs every half cycle. The fully on and off periods are based on burst pulses of time which are set by the firing circuit and can be adjusted to suit the user’s load. For example if the total cycle time is set to 2 seconds and a 50% power output from the thyristor controller is required then the output will be fully on for 1 second and fully off for 1 second, this is shown in the diagram below:
This percentage output works linearly with reference to the amount of time on and off. For example if the total cycle time is set to 2 seconds and a 75% power output from the thyristor controller is required then the output will be fully on for 1.5 seconds and fully off for 0.5 seconds. To achieve this range of outputs a signal from the user is required which is normally 0-10Vdc or 4-20mA. This signal is proportional to the percentage power output required, for example when using a 0-10Vdc signal:
Signal (Vdc) Percentage Output (%)
Zero Crossover Firing (burst pulse) is the preferred method for standard resistive loads, in particular in the HVAC industry. By ensuring that the thyristors are always switched on at the zero voltage point of the mains half cycle, fast rising high voltage wave fronts are avoided which significantly reduces the level of generated electrical noise. This technique is cost effective, as the requirement for substantial filtering is now eliminated. Another benefit of this method is that it can be used on a three phase system with only two of the phases controlled by thyristors; this again saves cost and reduces heat losses produced by the thyristors.
Phase angle firing turns the thyristors on at a specific point in each half cycle of the mains frequency. Varying this switch-on point between the initial and final zero voltage points of the sine wave provides a variation from 100% down to 0% of the load voltage (and hence the output power). For example if a 50% power output from the thyristor controller is required then the waveform would be:
To achieve the range of output percentage a signal from the user is required which is commonly 0-10Vdc or 4-20mA. Much like burst pulse firing this signal is proportional to the percentage power output required; however instead of varying the amount of time the thyristors are on and off the signal is varying the amount of delay in the conduction angle of the waveform.
Phase angle control allows an extremely tight, accurate and smooth control of the load but will generate a significant amount of electrical noise unless substantial filter networks are employed. The level of electrical interference increases as the switch on point approaches the maximum peak voltage of the sine wave. Power factor is a calculation of available power vs. consumed power, at full output the phase angle method provides a power factor of 1 but this decreases as the output of the thyristors is reduced, i.e. the power factor is 0.5 at 50% output. The phase angle control method is used mainly for inductive loads, such as transformer coupled loads, and also some specific resistive loads which benefit from features that phase angle control can offer such as soft start to limit high inrush current.
There are a number of other variants of the firing methods detailed above; probably the most interesting one of these is the combination of zero crossover burst pulse and phase angle firing. If the user’s load will operate on zero crossover burst pulse the majority of the time but would benefit from a delayed or soft start function then this firing method is perfect. This then has the advantages of both firing methods. For example as phase angle control results in a poor power factor at power levels less than 100% the combination method can be used to avoid this and also reduce harmonics and conducted electromagnetic interference. The burst pulse cycle time would be set long enough to allow the phase angle soft start to complete, once this ramps the voltage up over a set time the burst pulse would take over the control until the cycle time has finished.
Power Products International offer a range of thyristor control options, including multiple phase configurable controllers. For assistance with your particular thyristor control requirements, and any other power semiconductor applications, please contact our technical sales team.
Power Products International is one of the largest UK manufacturers of thyristor controllers designed for the control of heater batteries in HVAC systems and resistive loads in electric furnaces. All of our controllers are self-powered and require a 0-10volt input signal. Available in single and three-phase versions, the units comply with current EMC and safety regulations, are CE compliant and built to BSEN 50178 standard. As with most Power Products thyristor assemblies, custom options are available.
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