Whenever one wants to adjust voltage smoothly and proportionally to any given load, for power levels from 100 watts to 1 megawatt, the solid state power control (SSPC) is the best way to do it. SSPCs are small, lightweight, 99% efficient, and can be made to follow virtually any computer automation signal to drive any load arrangement, AC or DC.
For constant resistance heater loads, zero-firing, with 100% power factor at all times (Fig. 4), is clearly the best method to control large amounts of electric heating power. It can limit power by limiting the effective RMS voltage applied to the load, but it always applies the maximum voltage available from the mains. Therefore a given heater with its electrical insulation rated at 240 VAC cannot be operated with a zero-fired SSPC from 480 VAC mains. This is always the case, even if one were to restrict the proportioned output of a zero-fired SSPC to a nominal 240 VAC RMS value.
Phase-angle control (Fig. 3) has similar “insulation voltage” limitations. But due to its ability to provide an adjustable portion of every half sine wave, as opposed to the multiple-cycle bursts of zero-fired units, phase-angle control can be used over a wider range of voltage limit conditions.
For example, take a bottling plant cap seal: one needs 65 volts to provide the precise amount of heat necessary to form the seal. The plant engineer has 480 VAC three phase available, and wants to use a 480 VAC SSPC already on the shelf. Not really a good idea. If the heater is rated by its manufacturer at 120 VAC, the insulation is probably good for twice that. One could limit an SSPC to the 65 VAC RMS value, but coming from a 480 VAC forcing source, that heater would probably be smoke in a few months. Why not go to the plant’s lighting panel, pull out 120 VAC or even 240 VAC, and use that to drive the available 480 VAC SSPC? All SSPCs can run quite happily on all mains voltages below their nameplate rating.
As a general rule, most well-designed SSPCs can be used to limit their output voltage to a minimum of 40–50% of whatever their mains input voltage happens to be. In this way one can minimize nuisance fuse blowing and insulation damage to one’s heaters, and still draw the full-rated RMS current of the SSPC. Regardless, it is very important to use proper true RMS reading voltmeters and ammeters to determine the correct readings when using SSPCs operating at a higher mains voltage to control electric heaters rated at some lower voltage.
One other point should be stressed: long-term phase-angle control at retarded settings much below 60% of input voltage results in poor power factor usage of electricity. Remember, the mains voltage is still present when the thyristors are not conducting during the OFF period of each half-cycle. The power control meters interpret this as a lousy power factor.
References
- Blume, L., Transformer Engineering, New York, John Wiley & Sons, 1951.
- General Electric Company, SCR Manual, 5th ed., New York, 1972.
- Lowdon, E., Practical Transformer Design Handbook, 2nd ed., Blue Ridge Summit, PA, TAB Books, 1989.
- Payne, H., “Power Integrated Circuits: Know the Limitations”, Control Engineering, July 1986.
- Schaefer, J., Rectifier Circuits, New York, John Wiley & Sons, 1965.
Dr. Henry E. Payne III, P.E.
June 1994
AN11-18