Heatrex’s Commercial Sales Manager, Rick Holmes, wrote this article on how to choose a SCR or VERNIER SCR Control.
SCR Proportional Control
SCR option is normally used when the total heater load is 96 amps or less. Master controller is used for heater loads of 48 amps or less with a Slave controller being added for loads up to 96 amps. The Master and Slave controllers proportion the entire heater output from 0 to 100% of the total heater kW. Working on a one second time base, the heater will be energized only for the number of AC cycles necessary to produce the exact amount of heat required. The SCR is switched on only as the voltage wave form crosses the zero voltage point, which virtually eliminates radio frequency interference (RFI). As an example with an input of 0-10 VDC the heater is full on at 10-volts and off at 0-volts. At 5-volts the heater will be full on for ½ second and off for the second half of. By modulating or proportioning the heater in this way excellent temperature can be achieved. This option is not recommended when the power is provided by a local backup generator or when the power supply has limited capacities. However, SCR option is the standard for the industry for small to medium loads. The use of phase angled fired SCRs is never recommended for use with electric duct heaters due to the harmonics produced when the wave is chopped at peak voltages. The following graph is typical of the temperature control output from the heater.
VERNIER Proportional Control
VERNIER SCR Control is recommended for large kW heaters. The VERNIER control system offers many of the advantages of full SCR control. One VERNIER stage is connected to a Slave SCR controller. Additional stages are sequenced on and off while the SCR-VERNIER stage automatically fills the gap between the step controlled stages, providing full proportional control over the entire heater kW output. Both the Slave SCR stage and the step-controlled stages are controlled by the step controller from a single temperature control signal. The VERNIER system is recommended for heaters drawing more than 96 amps. It is recommended the SCR be sized at 1/3 of the total heater kW for medium kW heaters and normally limited to 48 amps. Smaller kW VERNIER controlled heaters with multiple stages can result in poor temperature control as stages cycle or hunt to control the temperature. The optimum heater design is for each SCR and non-SCR stages be sized at or as close to 48 amps as possible. This also provides the most economical design. The following provides a graphic relationship of SCR and non-SCR staging.
Additional information on SCR can be obtained by contacting your local Heatrex representative at www.heatrex.com who will be glad to assist you with your heater selections.