Re: Soft transformer turn on without a variac

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Original poster: "Crow Leader" <tesla-at-lists.symmetric-dot-net> 

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, October 30, 2003 7:26 AM
Subject: Re: Soft transformer turn on without a variac


 > Original poster: davep <davep-at-quik-dot-com>
 >
 > Speaking as an EE, with some transformer test experience:
 > My understanding/observation:
 > Basic Physics:
 > It takes a (brief: milliseconds) period of time for
 > the mag field to 'penetrate' the core material.
 > Thus when initially powered up any transformer LOOKS
 > LIKE an air core coil: Very Low Impedance.  The effect
 > is not _noticed_ on small transformers as the
 > surge is small.  Rule of Thumb for large transformers
 > is: 'worst case' about 10x normal full load current.

As a note, distribution transformers, and even some large machine tool
transformers list their percent impedance on the nameplate. The lower the
value, the higher the surge current. 1/%Z x full load current will give you
the highest surge current you should be able to get from a transformer,
assuming no losses on the line that feeds it.

If you add up all your impedances you can find the peak inrush current you
have to deal with, and by adding more may be able to get past the switch on
load problem by adding just fractions of an ohm of resistance.

I live in a place several stories up with a lovely combination of cloth
covered rubber wire and more modern THHN copper wire. The impedance of my
wiring to the 120volt outlet I tested was 0.4 Ohms or something like that.
If I stage some short circuit accident, I'm limited to 120/0.4 or 300 amps
until a fuse blows or a breaker trips. The wiring to your load can make a
big difference with the surge current your toys can draw. The only way I can
power on a several kW industrial microwave oven transformer I have is to run
it at the the end of a 50 foot extension cord, to drop the inrush current to
something the circuit breakers don't mind. With no load, the transformer
draws almost no power at all once magnetized.

KEN