SiC heating elements controlled by thyristor (Part 5)
Burst control switches through complete packets of AC sine waves in proportion to the control input signal. At 0% input the thyristor will be completely off, at 50% output the thyristor will be on half the time and off half the time, at 75% the thyristor will be on 75% of the time and off 25% of the time and at 100% the thyristor will be fully on. Traditionally thyristors used for the control of silicon carbide heaters have been phase angle control although recent innovations allow burst fire thyristors to be used as well.
Burst control thyristors used with silicon carbide heaters need to have a short time base. Even though the average power delivered to the heaters may well be within the heaters Watt density rating, the heater's watt density rating may well be exceeded during the “on time period”. If, for example, a slow cycle burst thyristor had a 30 second time base the heater would be on for 15s and exposed to the full mains voltage over that time period, causing it to fail prematurely. CD Automation thyristors can be operated in a single cycle burst mode which is optimum for silicon carbide heaters. At 50% output the thyristor is one cycle on, one cycle off i.e. 0.02s on, 0.02s off (at 50Hz) which minimises the risk of over stressing the silicon carbide heater during the on period.
Because burst control subjects the silicon carbide heaters to full mains voltage when bursting on the peak currents through the thyristors are higher than they would be if they were subjected to the reduced voltage obtained by phase control.
The thyristor must be sized to accommodate this higher peak current. As a result a burst firing thyristor used with silicon carbide heaters must have a considerably higher current rating than a phase control thyristor controlling the same load. Current rating is determined by dividing the RMS supply voltage by the minimum network resistance.
i.e. Current rating = supply voltage / minimum resistance
Minimum resistance needs to be taken at its worst case, with new heaters and at the temperature that gives the minimum resistance. Take the example of a single phase application with a silicon carbide heater with a minimum resistance of 2.5Ω on a 220V RMS supply voltage and a required output of 9kW. Using a thyristor with phase control we can deduce that the starting current is The phase control thyristor adjusts the voltage down to 150V to limit the maximum heater power. With the same heating load but using a burst control thyristor maximum peak current is
220V/2.5Ω = 88 amps
The burst controlling thyristor limits maximum power by limiting the “on” time of the heater.