Goals and Symptoms

ATS48c79y:If starting torque is les than 50%no problems.But torque is ingreased ,thyristors nub. 2 and 5(from left) are blown.Do you have any idea,why. Is the problen in pover- or firing board?

Causes and Fixes

What is the control mode, torque or current limitation ?
Could you provide de datecode of this product and the trade mark of the thyristors ( components problems always possible ) ?

The main reasons to have a thyristor shorted are:
1) a to high dv/dt between anode and cathode. In this case, the thyristor can be fired at the bad time and a to high current could destroyed it.
2) a to high over voltage on the main power supply.
3) a to high current at the output of the ATS or in case of short-circuit and if the impedance of the main power supply is to low (main transformer to closed from the ATS).

For each of these case, a line choke and UR fuses has to be used for each ATS.
Coordination protection type 2 has to be chosen.