Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 635 of 707

Fig. 1. Primary thyristor. expense is relatively short. Most manufacturers will guarantee a well-designed transformer for 5 years; however, such well designed transformers will typically operate for a minimum of 15 years without problems. Rectification and Regulation The silicon diodes used in rectifiers are the simplest and most reliable rectifying devices available. Silicon, when properly treated with certain elements, allows cur- rent to flow in one direction only. When a silicon diode is hermetically sealed, it becomes completely impervious to external conditions, making it capable of with- standing the harsh environments commonly found in metal-finishing facilities. Another silicon device that is instrumental of today's rectifiers is the silicon- controlled rectifier, commonly known as a thyristor or silicon-controlled recti- fier (SCR). The thyristor is basically a silicon diode that will conduct only in one direction and only when a signal is applied to a terminal on the thyristor known as a "gate." In some instances, the thyristor functions as a regulating element, whereas in others, it acts as both a rectifying and a regulating device. In the primary thyristor configuration, illustrated in Fig. 1, thyristors are con- nected between the incoming voltage source and the transformer. In this design, a thyristor operates at a relatively high voltage and low current. Generally, all thyris- tors have a fixed forward voltage drop across them. This drop ranges from 1 to 1.5 V. When the highest quality thyristors are used as primary elements, with an input of 230 or 460 V, the efficiency of the thyristor network is greater than 99%. In the primary thyristor configuration, the thyristor is solely used to vary the AC supply voltage from zero through maximum. In order to make a fully regu- lated controller, each phase of the three-phase input must have two thyristors con- nected back to back, as shown, and their gates must be symmetrically triggered. The regulated voltage is then fed from the thyristors to the isolation trans- former, which converts the incoming high voltage/low current to a lower voltage and a higher current. From the transformer, you now have the desired output volt- age and current, but it is still in an AC form. It is here that the silicon diodes are 634

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