Generally speaking, a transformer is an AC (alternating current) device.

Because the ordinary thyristor (SCR) is basically a bistable DC device,    ie., when it turns on, it allows the source voltage to impress a DC current on the load and will not  turn off until the 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, per Fig. 1, it is possible to effectively adjust the primary voltage of the transformer by using "phase-angle control" to vary the primary voltage from 0 to the nominal mains voltage of the system.

This application note will attempt to 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 electro-magnetic induction was made by an Englishman, Michael Faraday in 1831, As the basic principle under which all tansformers operate, Faraday found that a current of electricity flowing into a copper wire wrapped around an iron rod would create a magnet. And 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, Mass. In 1886. The electric power industry as we know it today is a result of that humble beginning.

Figure 1

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