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Fig. 2. Secondary thyristor. 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 connected between the incoming voltage source and the transformer. In this design, a thyristor operates at a relatively high voltage and low current. Generally, all thyristors 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 regulated controller, each phase of the three-phase input must have two thyristors connected back to back, as shown, and their gates must be symmetrically triggered. The regulated voltage is then fed from the thyristors to the isolation transformer, 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 voltage and current, but it is still in an AC form. It is here that the silicon diodes are utilized. The function of the diodes, as stated earlier, is to allow conduction of current in only one direction. When the diodes are used, as shown in Fig. 1, they will rectify the transformer output and provide DC. Another method is to place the thyristors on the secondary side of the transformer, as shown in Fig. 2. This is known as a secondary thyristor design. In this configuration, the thyristors perform both the regulation and rectification operations, and no diodes are required. Either design can provide the desired DC output, and although each method has its advantages and disadvantages, the cost is usually the determining factor. The advantages of the primary method are as follows: Soft start���Because the controlling element is in the primary side of the transformer, it can control the inrush current to the transformer. 770

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