Fring of thyristors in a DC drive...




In a three phase dc drives, the
three line voltages are applied to the thyristors. These are RY, YB, and
BR. 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 RC network and 30 degrees is
achieved through Delta / Star configuration of Synchronising transformers. Click
here to see more on firing pulse genration... 
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