Phase Fired SCRs can generate significant 3rd, 5th & 7th order harmonics above the line frequency due to the thyristor firing occurring between the AC zero crossings:
When Phase Fired SCRs must be used due to the load characteristics described in our previous blogs, EMI filtering can be added to the line to attenuate the harmonics. Several companies makes suitable high current filters for Phase Fires SCRs:
Commutation Problems: When an SCR or TRIAC is used to Phase angle control an inductive load, the load current lags the mains voltage. When the device turns off at zero current, the rate of rise of the reapplied voltage can retrigger the device and produce half cycling and blown fuses. To limit this rate of rise and obtain reliable commutation, an R-C (resistor–capacitor) snubber circuit should be connected in parallel with the SCR/TRIAC.
dv/dt Problems: When voltage transients occur on the mains supply or load of an SCR/TRIAC it can cause the device to turn on unexpectedly due to the fast rate of rise of voltage (dv/dt). This can result in false firing and half cycling of the load that can cause blown fuses when driving inductive loads. An R-C snubber circuit will help to limit the dv/dt seen by the device and will produce more reliable thyristor firing.
Snubber Suppliers: Check out our new RCSB Snubber: RCSB SNUBBER which is also available for SCRs and well as SSRs.
Snubber Sizing: Although most designers use an empirical approach to solving the aforementioned issues with snubbers, a number of great articles have been published on the mathematical basis for calculating snubber circuit values. When an SCR/TRIAC using an R-C snubber turns on, the capacitor is discharged through the resistor into the device resulting in high peak currents. It is critically important when sizing your snubber to make sure that the resistor value does not become so low that the ratings of the SCR/TRIAC are exceeded when the capacitor is discharged.
MOVs and TVSs: Metal Oxide Varistors and Transient Voltage Suppressors are both used on TRIACS/SCRs to “clamp” voltage spikes that can occur across the devices and damage them. Snubbers are not a substitute for MOVs/TVSs and vice versa. Snubbers and MOVs/TVs should be used together to get reliable performance and long life from the SCR/TRIAC application.
Solid State Relays (SSRs): AC output SSRs use either SCRs or TRIACS internally, so this information applies to them as well. Even though many SSRs have internal snubber networks, we have found these to be insufficient for use in many inductive load applications. The addition of external properly sized snubbers has improved performance in many applications we have encountered when using our Phase Angle Controllers and SSRs for driving inductive loads.
Soft Start is a method of limiting the rate of rise of the output of a phase angle controllers power output generally during start up only. Soft Change is a method of limiting both the rise and fall of of a phase angle control’s power output. Both of these methods are used mainly to limit the load’s inrush due to either the resistance temperature coefficient of a heater lamp or because the load is inductive. Soft start is useful on high inrush heaters such as Quartz, Molybdenum, Tungsten, or Graphite heaters. Soft Change can be a good choice for phase angle controlling inductive loads, where rapid changes in the phase angle can induce a DC imbalance in the in load and thus cause misfiring, heating in the inductor, or blown fuses.
Line Voltage Compensation in a Manually Controlled Heater Process: In a simple manually operated phase angle control arrangement the user sets the power level of the phase angle controller and this delivers a percentage of the full power to the heater. Ruling out any other process disturbances, the line voltage can change at any time affecting the power delivered to the heater dramatically (don’t forget P=E^2/R). Line voltage compensation effectively cancels out the change in line voltage keeping the power delivered to the load exactly where the user sets it.
Line Voltage Compensation in a Closed Loop Heater Process: In a closed loop heater process, a temperature controller such as a CAL 3200 sets the output level of the phase angle controller. The temperature controller (if it is a PID controller) will correct for changes in line voltage, but these changes have to ripple through the process lags before they arrive at the controller. This can affect the product quality, because often times the sensing point of the controller isnt perfect. Line voltage compensation cancels the affect of line voltage changes before they arrive at the temperature controller.