What is the reason for imposing the limits of phase angle in a thyristor controlled phase shifter?

Answer 1

On the 0 degree side (full on), it's because a thyristor has to have a minimum amount of current flowing through it (called the 'latching' or 'holding' current) before it will stay on in the absence of gate drive. This current level (flowing through the load) usually doesn't occur until several degrees into the cycle.

On the 180 degree (full off) side it's for pretty much the same reason. You'd have to drive the control gate until the end of the cycle before you turned of the drive. And, if your timing circuit screwed up and drove the gate for too long, you might cause the thyristor to conduct for the entire cycle following.


Doug

Answer 2

Very good question :-P