Re: [CAD_CAM_EDM_DRO] Transformer Soft Start

Ted Walls wrote:

>tiffyted wrote:
>
>
>
>>Can anyone in this group advise me on a suitable circuit for soft
>>starting a 230v primary, 40v secondary 1000VA toriordal transformer.
>>
>>
>
>Jon Elson Replied:
>
> I think you have something serious wrong, here. I don't think soft start
>will help you.....
>
>Thanks for the reply Jon,
>
>The transformer is 230v only primary and 2 x 40v 12.5A secondary. As
>supplied by RS. It currently is not connected to any rectification circuit
>and the secondary windings are open circuit.
>
>I have checked the data sheet which indicates the normal running current
>would be 4.34A. However, there is a paragraph that states the start current
>can momentarily be as much as 67 times this, which would blow the fuse. This
>is what led me to assume I needed a soft start system.
>
>

The problem with transformers is that they are designed to use minimum
materials.
So, the core is just enough to handle the normal integration of voltage
* time, which
always balances out to zero over a full cycle. but, if the transformer
is turned on
at about zero volts, it is starting out with no magnetic field, and has
to integrate the
full half-cycle of one polarity without previously been fully charged
with the reverse
polarity from the previous half cycle. This causes the iron in the
transformer to
saturate, and the normal inductance of the transformer disappears,
leaving only the
wire resistance to limit current. This is why some power supplies produce a
loud hum when first turned on.

The surge limiter (Negative Temp Coefficient) devices are about one US
Dollar
each. You will need one of the big ones for this application. But, I'm
not sure
it will solve the problem. The saturated iron should resolve itself
after a half cycle
of the transformer. 67 x nominal load for one half cycle is not an extreme
event for motors and rectifier power supplies. You should really try
the light
bulb limiter first to make sure it is not a bad transformer (or the wrong
transformer, wrong wiring, etc.) Hmm, is there any possibility you have the
mains connected to the secondary?

Jon

>Les Newell has suggested that I use a surge arrester. I think if they are
>not too pricey I'll get one to try. If it works Ok I'll let you all know. If
>not I'll have to look into it a bit deeper.
>
>Thanks again to all for their replies.
>
>
>
>
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