60A Variac on Ebay: inrush current problem

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Original poster: "Scott Hanson" <huil888@xxxxxxxxxxxx>

David -

This is a common problem when powering up big inductive loads (like
transformers or variacs) that have high magnetizing currents. Several
relatively easy "fixes" are available.

Possibly the simplest method is to use a low-value, large wattage wirewound
resistor in series with the load, but paralleled with a normally-open relay
or contactor. The coil of the contactor is connected across the line
downstream of the resistor. Initially, the entire load current passes
through the resistor, so the reduced voltage limits the current to a value
that will not trip the circuit breaker. Within a few hundred milliseconds
the initial inrush current drops, the line voltage downstream of the
resistor increases, and the contactor closes, shorting out the resistor and
applying full line voltage to the load. The contactor should be rated to
handle the full load current.

Because the resistor is in-circuit for a very short period of time, it does
not need to have a "huge" power rating, it just has to have a large enough
conductor to carry the initial current without self-destructing.

There are other similar arrangements that use an adjustable R-C timing
circuit to drive the relay coil so you can easily adjust the time that the
resistor is "in circuit".

Regards,
Scott Hanson
----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Saturday, November 12, 2005 7:49 AM
Subject: Re: 60A Variac on Ebay


> Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>
>
> Skip, all,
>
> I have a Staco triple variac assembly that looks exactly
> like that one.
(SNIP)
 However, I have had one problem when powering up
> these 3 paralleled variacs as a single phase, 84 amp
> triple stack - It has a TERRIBLE inrush current problem
> that will trip the 125 amp mains breaker about 8 out of 10
> times on initial power up. It's so bad that I initially thought
> that there was a dead short! I suppose the only way that I
> could use it would be to place the ballast unit between
> the wall plug and the variac instead of the standard setup
> with the ballast between the variac and the transformer.?
>
> David Rieben