Fring of thyristors in a DC drive...






In a three phase dc drives, the three line voltages are applied to the thyristors. These are R-Y, Y-B, and B-R. Referring to the sketch below, thyristor 1 will conduct only when its anode is most positive compared to other two i.e. no. 3 and 5, provided, off course, the firing pulse is applied.

Looking at the three phase sinewave diagram, this possibility exists only after point "A", and continues to exist till point "B" which is 180 degrees away from "A". This means that the firing pulse generated by the electronics must be able to swing from A to B.




Pulse is generated using a dc level from current amplifier and a sinewave from the three phase supply. Both are first compared in a comparator. This comparator generates a square wave in which the rising and falling edges occur at the instant when amplitudes of sine wave and that of the dc level are equal. Therefore, one must have a sinewave signal which covers entire 180 degrees between "A" and "B" . This sine wave is normally derived by phase shifting a voltage which has 30 degrees phase shift with respect to Line voltage. Phase shifting capacitors are typically 0.1 mfd to 0.47 mfd. Resistor values are adjusted accordingly and are chosen to have low tolerance of 1 % . Output of these comparators ( 6 for three phase drive ) is passed through capacitors, to generate pulses. These are amplified and applied to gate of the thyristor through a pulse transformer.


It must be noted that the point A on the red sine wave above is already 60 degrees behind the zero crossing point of that sine wave. The firing pulse must be generated at point A if we want zero firing angle ( corresponding to maximum voltage ). Out of 60 degrees shift required, 30 degrees is achieved through R-C network and 30 degrees is achieved through Delta / Star configuration of Synchronising transformers.


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Firing Circuit


Power Circuit


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