why does 35KVA Transfomer for spot welding tripping?

[Mason Chong]

hi ,
Input primary 2 phase 415V with SCR control .
its was trips after trigger SCR .
if direct connection primary 415V without SCR , no trips happen .
I was wondering root cause of over current 100A MCCB keep on trip ?
Seeking advice

regards

 

[Pete999]

hi ,
Input primary 2 phase 415V with SCR control .
its was trips after trigger SCR .
if direct connection primary 415V without SCR , no trips happen .
I was wondering root cause of over current 100A MCCB keep on trip ?
Seeking advice

regards

You mention that when the SCR is out of circuit no trip occurs, whilst working on AGL installations, we found that O\on start up the SCR drew an inordinate amount of Amps when compared to normal run Amps, could this be a inrush problem? maybe the wrong type of OCPD is being used, just throwing this around, for comments, but the OCPD was changed and it cured the problem, please don't ask me what sort or size it was when I was a lot younger than I am now, hope this throws some light on the subject, fingers crossed.

 

[Lucien Nunes]

Yes it may be a result of where in the mains cycle the SCR fires. When continuously energised, the transformer magnetising current (and therefore the flux) lags the voltage by a quarter-cycle. If the SCR fires at or near the supply voltage zero, the flux is also at zero when it would normally be at peak value, i.e the phase angle between them is wrong. The following voltage half-cycle then drives the flux to twice the normal peak value. If this causes the core to saturate, the inductance falls drastically and with it the primary impedance, resulting in a heavy overcurrent that trips the breaker. The same thing would happen when switched on at the voltage zero by chance rather than by the firing circuit, so if it occasionally trips when switched on by hand without the SCR control that is a clue that this is the reason the trip. Also, snubber networks subjected to high dV/dt can pass high instantaneous currents. As Pete says the MCCB is probably unsuitable for this duty.