AN11-18: Transformer considerations with thyristor controls, page 2

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

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

Figure 1. AC switch.

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 addresses three basic modes of transformer operation with thyristor controls:

A short history of the transformer

The discovery of electromagnetic induction, the basic principle under which all transformers operate, was made by an Englishman, Michael Faraday, in 1831. 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 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, Massachusetts, in 1886. The electric power industry as we know it today is a result of that humble beginning.