Re: [CAD_CAM_EDM_DRO] Re: Transformer Soft Start

>
>Most likely the inrush current is caused by the huge filter cap
>trying to instantly charge to full transformer voltage. It looks like
>an almost dead short to the AC side for a few cycles until the cap
>starts to charge. With no load the transformer should draw a small
>amount of magnetizing current and that's all.

Here's what's happening;

When a transformer is switched on, a transient current flows. The value of
this current over the first half cycle of mains voltage exceeds the normal
magnetizing current by a very large factor, and is often sufficient to blow
the primary fuse or lead to excessive tripping of the input circuit
breaker. This current decays exponentially over several cycles until normal
magnetizing current flows.

This phenom is common in machine tool and welding transformers, even with
no load on the secondaries. ie; Even though the panel on the machine says
50FLA, a 100A circuit breaker feeding the machine trips out when the
cabinet switch on the machine is thrown.

Standard breakers and fuses react too fast to the inrush current. What's
needed is a slower disconnect to allow the current to settle to normal levels.

We usually install a high inrush delay circuit breaker that can handle a 3X
overcurrent for a couple of cycles. No more problem trips. Dual element
time delay fuses can be used as well.

5Lgitchu10 the original poster can solve his problem with proper fuse
selection.

-Bob