I have a spot welder has a problem with its thyristor, it has two back to back thyristors as indicated in the attached picture.

Thyristors are used for applying voltage for a bout 100 ms to the transformer. one of the two thyristors has been defected due to a mistake in the control circuit
the question is : can i remove that defected thyristor and apply only half wave to the transformer, has it any bad effect for transformer to add only positive part of the 380V wave??

 

Did you wonder why it was designed in that way in the first place ?

 

has it any bad effect for transformer to add only positive part of the 380V wave??

YES. The core will saturate and the magic smoke will escape.

 

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the question is : can i remove that defected thyristor and apply only half wave to the transformer, has it any bad effect for transformer to add only positive part of the 380V wave??

Not if the main's breaker opens before the transformer burns out.