To use a power control effectively, it is helpful to understand the three modes of transformer operation with thyristor controls.

Figure 1. Two thyristors arranged in an inversere-parallel connection can be used to adjust the primary voltage of a transformer.

Generally speaking, a transformer is an alternating current (AC) device. Because the ordinary thyristor (SCR) is basically a bistable direct current (DC) device - that is, when it turns on, it allows the source voltage to impress a DC current on the load and will not turn off until this current returns to zero for a few microseconds - it can be very destructive to transformers and itself when operated incorrectly into transformer loads.

By using two thyristors in an inverse-parallel connection (also known as an AC switch configuration), it is possible to effectively adjust the primary voltage of the transformer by using phase-angle control to vary the primary voltage from zero to the nominal mains voltage of the system (figure 1).

This article will address three basic modes of transformer operation with thyristor controls:

  • Mode 1: Transformer connected between the thyristor control and the load.
  • Mode 2: Small single-phase transformers connected to the load side of zero-fired thyristor controls.
  • Mode 3: Transformer connected before the thyristor control and the load.

The discovery of electromagnetic induction was made by an Englishman, Michael Faraday, in 1831. As the basic principle under which all transformers operate, Faraday found that a current of electricity flowing into a copper wire wrapped around an iron rod would create a magnet. If this magnet were inserted into another coil of wire, electricity would flow in that coil of wire as well. Unfortunately, since no one knew how to generate electricity at that time, the concept of AC electrical power transmission was unknown. In 1882, patents were granted in England for a system of distributing AC electricity with transformers, but this system was not successful. Shortly thereafter, George Westinghouse acquired the U.S. rights to these patents and managed to have the first successful AC transmission system installed in Great Barrington, MA, in 1886. The electric power industry as we know it today is a result of that humble beginning.



Figure 2. In a typical installation, a 50 A single-phase thyristor unit phase-angle controls a 480 AC voltage to the input of a transformer with a low-voltage resistive heater connected to the transformer secondary.

Phase-Angle Firing into Transformer Primary

Figure 2 illustrates a typical installation of a 50 A single-phase thyristor unit phase-angle controlling a 480 AC voltage to the input of a transformer with a low-voltage resistive heater connected to the transformer secondary.

Note the resistance value (RL). All properly designed thyristor controls will have certain minimum resistance loads that they are able to control, if, for no other reason, because this minimum resistance must be large enough to limit the current through each thyristor to its rating when it switches into the on state. From Ohm's Law, this value typically is the RMS mains voltage divided by the maximum nameplate current rating of the power control.

For a direct-connected resistance heater, this would be a correct value. However, some might also assume that this is the same value when connecting an SCR power control to a transformer. It is not. All transformer primaries will have a DC resistance value much lower than the primary voltage divided by the primary current rating. As a result, the maximum current that can flow through the loop 1-4-3-2 is limited only by RL and whatever system impedance is available at terminals 1 and 2. If F1 is selected properly to protect the two SCRs and there is no consideration given to the transformer RL, it is likely that one will have nuisance fuse blowing during normal operation of the system due to the transformer magnetizing current inrush.

When SCRs are either in the off state or in operation, one cannot assume that they will not misfire due to some line transient. After all, that is the reason that all properly designed SCR controls will always have an RC network connected across the SCRs. The RC network and the DV/DT rating of the SCR combine to prevent more than 90% of the line noise from causing a misfire. Nonetheless, there is always a bigger transient on the mains, and every solid-state power control must be designed to allow for that inevitability. By oversizing the SCR/fuse combination for transformer or other high inrush loads, one can assure oneself that nuisance fuse blowing is unlikely to occur, nor will there be any fatigue damage to the fragile, silicon SCR pellet.

Finally, one must also ensure that phase-angle control of the transformer primary always impresses a symmetrical AC voltage on the primary with no significant DC component. The latter can cause saturation of the transformer core, which will lead to excessive current and temperatures that cannot be tolerated for long-term operation. Transformers and other inductive loads require full-cycle gating of the SCRs to ensure that the SCRs will remain in the on state, regardless of the inductive load characteristic (where the load current may lag the SCR voltage by 90 degrees or more).



Figure 3. Zero-firing of SCR proportions the voltage to an AC load by varying the number of cycles on and off.

Zero-Firing into Transformer Primaries

Figure 3 shows how zero-firing of SCRs can proportion the voltage to an AC load. For electric heating with nominally constant-resistance heating elements, zero-firing - with its 100% power-factor and harmonic-free wave-form - is clearly the best way to proportion power in small or large amounts. As a result, this method of control is fast becoming the dominant method of power control for conventional, electric resistance heating.

Unfortunately, zero-firing into a transformer primary is not generally suitable to do. If the transformer-core flux is opposite in polarity to the initial voltage waveform applied, core saturation could occur, which would cause damaging high currents to flow. There are various soft-start arrangements to deal with this situation but, for general usage, it is wise to employ phase-angle control into most transformer primary circuits.

For small instrument control transformers - which are connected to large zero- fired power controls to measure volts, amps or kilowatts - one must also take care to limit these same saturation currents. They may be too small to affect the fusing of the power control, but they easily can damage the windings of the control transformer, which are not rated for continuous bursts of saturation current. Small, properly sized, series-connected resistors will eliminate the problem and allow one to safely use these control transformers with big zero-fired power controls.



Transformer Connected Before The Thyristor Control and Load

For economical general usage, this mode is the preferred method of using a transformer with any thyristor power-control unit. The SCR control can usually accept virtually any AC voltage input - simply wire the SCR control transformer to the conventional voltage on the transformer primary - so that whatever maximum load voltage is required can be easily provided by the transformer secondary in front of the SCR unit.

Whenever one wants to adjust voltage, smoothly and proportionally, to any given load - for power levels from 100 W to 1 MW - 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 its 100% power factor at all times, 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 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 - one can use phase-angle control over a wider range of voltage limit conditions.

As a general rule, most well-designed SSPCs can be used to limit their output voltages to a minimum of 40 to 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 important that one use proper true RMS reading voltmeters and ammeters to determine the correct readings when one wants to use SSPCs operating at a higher mains voltage to control electric heaters rated at some lower voltage.

One other point should be stressed: The mains voltage is still present when the thyristors are not conducting during the off period of each half-cycle. The power company meters interpret this as lousy power-factor.



References

  1. Blume, L., Transformer Engineering, New York, John Wiley & Sons, 1951.
  2. General Electric Company, SCR Manual, 5th ed., Auburn, NY, 1972.
  3. Lowdon, E., Practical Transformer Design Handbook, 2nd ed., Blue Ridge Summit, PA, TAB Books, 1989.
  4. Payne, H., "Power Integrated Circuits: Know the Limitations", Control Engineering, July 1986.
  5. Schaefer, J., Rectifier Circuits, New York, John Wiley & Sons, 1965.


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