Why SCR is fired after 60 degree

anvesh; are there 6 SCRs ? anyway once the SCRs is fired, firing later in the angle does nothing, they are problem firing 2 SCRs at the same time, 1 to start a new cycle & the 1 is already in a conduction cycle on a different SCR. perry

You have to understand the design of your rectifier and your transformer. I do believed that you are talking about your DC Furnace Rectifier. For DC furnace rectifier, the secondary transformer has two winding. One side is Star-Delta Winding and the other side is Delta-Delta winding.

The firing angle is still the same at 120 degree but what you see is a 6 pulse firing angle. 3 firing angle is from the Star-Delta configuration and the other 3 is from Delta-Delta configuration. Total, you will see 2 firing angle at 60 degree and not 120 degree.

Dear Anvesh,

your circuit is probably simpler this way (lower cost, higher reliability). Firing at 60 degree is at a "don't care" condition.

This is analog to 4 cylinder engines firing spark plugs twice during each 4-stroke cycle.

Dear Simon Wan you are right rectifier is for DC Arc furnace but the card where I measured is for individual rectifier and do not gives pulse to other rectifier. There are six port to take signal of pulse I mean one for each rectifier. And on CRO it is clearly visible that if an SCR is fired on 0 degree it will be fired again at 60 with another SCR and then it will be fired at 360 then 360+60 then 720 then 720+60 that way.