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For almost 50 years, equipment manufacturers and maintenance engineers have used SCR (silicon controlled rectifier) power controllers to provide a reliable and flexible approach to controlling electric heaters.

An SCR is a solid-state switching device that can provide fast, infinitely variable proportional control of electric power. It can cycle as fast as 0.083 sec on a 60 Hz power line. If selected and used properly, an SCR power control can cycle on and off one billion times without any problem, said Paul Evalds of Avatar Instruments Inc., in Broomall, Pa.

An SCR power control has no mechanical parts to wear out. The SCR power control will not arc or be affected by dirty contacts.

Most SCR power controls are rated for ambient operating temperatures of 122oF (50oC) at full power ratings. Semiconductor 2ms I2T fuses, heat sinks, voltage spike protection and power lugs usually are included with SCR power controls.

SCR power controls can use several methods for applying electrical power to the heater loads.

  • On-off control provides power like a mechanical or mercury relay without the mechanical wear.

  • Zero-voltage switching proportionally turns on and off each full cycle of the power line cycle. By varying the number of AC power line cycles, the SCR provides power to the heaters. With a variable time base, the optimum number of cycles turned on/off is achieved. This switching is done at fractions of a second during the zero-voltage crossing of the power line cycles. Heater temperature can be held to tight tolerances.

  • Phase-angle fired control proportionally turns on a percentage of each power line half cycle. This gives smooth, infinitely variable application of power to the heaters. Phase-angle firing allows for options such as soft start, and current and voltage limit. Heater life can be increased up to seven times depending on heater type.

    Sizing Your Control

    When looking for an SCR power control, first determine what your maximum AC power line voltage is. Then, determine the maximum potential amperage or wattage of your heater load. This is a basic but critical piece of information, said Paul Evalds, Avatar Instruments Inc., Broomall, Pa. As with other power handling equipment, build some margin into your system design. Operating a 100 Amp load with an SCR rated for exactly 100 Amps will cause nuisance fuse blowing. Most manufacturers recommend sizing your SCR power controller by 110-125% of your maximum amperage load.

    Using the wrong controller could mean a blown fuse or a damaged controller after an excessive power surge or inrush current. Heating elements made of tungsten, molybdenum or graphite have large cold:hot resistance ratios. Transformer primaries also have inrush currents and transient voltages that can blow fuses on startup if not soft-started.

    For more information

    Contact Avatar Instruments. Call (610) 543-5155 or visit www.scrpower.com.