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Re: 1350 KVA Current limiting

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Tesla list wrote:

Original poster: "David Thomson" <dwt@xxxxxxxxxxxx>
 > I strongly suspect that you actually have transformers rated
 > for 450 VA (0.450 kVA).
You are right.  I looked at the labels too quick.  The
transformers are rated to 450 VA, not KVA.
But my variac definitely says it is rated to 10 KVA.  So where is
my problem?  Why am I popping the fuses on the variac?

During power-up, larger variacs will often pop the mains circuit breaker due to current inrush. Inrush currents shouldn't pop fuses unless they are fast response type. If so, convert to a "slower" fuse or bypass fusing and use an external circuit breaker or the mains breaker.

The transformers you have aren't current limited. If you aren't using any external ballast resistance or inductance, than shorting the secondary side will probably blow the fuse or mains breaker.

The transformers, BTW, are not Staco, but RAYCO.  They are 120V
50/60 Hz input.  I guess I don't understand the output.  The
diagram for the secondary shows one lead in and one lead out, but
the winding diagram has a gap in it.  These are not center tap
secondaries.  Also, the output lead is intended to be fused, but
I have bypassed the fuse since they are $50 each plus $20
shipping.  According to the wiring diagram, the fuse is entirely
in the output lead and not between windings. The label says it is
a 5.5KV transformer, the wiring diagram has "2400 V, 450 VA"
between the output leads.

Your transformers appear to be small 20:1 potential transformers made to convert 2400 volts in to 120 volts out. The HV side fuse is likely rated for 5.5 kV, as well as the transformer insulation class. If so, a similar potential transformer by a different vendor can be found here (PT3-1-45-242F or PT3-2-45-242FF) on this page:

It's hard to tell what the model number is, because there are so
many numbers on the label.  It does have an military NSN number
on it, as I bought these as military surplus.  The part that
might be the model number is X80-7407.  There is also a CAGE
number, and two more unidentified numbers.
I have all the primaries in parallel and the secondaries in
 > You can limit current to ANY sized transformer by adding
 > inductance in series with the LV side primary. This works
 > irrespective of the actual KVA rating of the transformer(s),
 > and is a simpler, safer, more efficient, and usually much
 > more cost effective way than trying to limit current on the HV
Thanks, I have a suitable ballast I can use.

Excellent - adding a primary ballast should do the trick!

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