Re: Soft transformer turn on without a variac

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Original poster: "Robert Jones" <alwynj48-at-earthlink-dot-net> 

Hi all, not contributed anything for a long time marriage and other things
had side tracked me.

Anyway to the point. I believe you have to switch at the zero input current
i.e. switch on at a voltage when the steady state input current is zero.

For an efficient output open circuit transformer that's at the peak of the
input voltage because the input current lags the input voltage by 90degs.

I agree there is a transient caused by the penetration time and eddycurrent
loss but that's a second order effect.

The standard text book explanation for the turn on surge goes something like
this. At turn on the response consists of a steady state response and a
transient response. The steady state response is the current that flows
after the transient has died away. The transient response is an
exponentially decay with a time constant of L/R. At the instant of or just
before turn on the input current is zero hence the two responses must be
equal and opposite in sign.

If the turn on occurs at a point when the steady state input current is zero
(approximately at the peak of the input voltage) then the transient current
must also be zero i.e. no surge.

If the turn on occurs at a point when the steady state input current is at a
maximum the transient current must be equal and opposite in direction. If
the transformer is efficient (usually this mean kVA and up) the time
constant (L<<R) of the transient response decay time is larger than the
cycle time of the input. Hence the transient response is still large
compared to the steady state current when the steady state current has the
same sign as the transient. In the no loss limit the peak of the surge is
twice the steady state current (ignoring saturation) and repeats cycle by
cycle.

It may be that with a loaded transformer you again have to switch at a
voltage when the steady state input current would be zero. That may also
require that the output current is zero too.

I have experimented with phase control of MOTs. It can increase the output
voltage on the primary C presumable by dumping part of the energy in the
primary and secondary L. The problem is the second order effects heats up
the core.  If that could be eliminated some how (filter Cs reducing rise
time) it could be attactive for compact power control and even power arc
prevention with a triggered SG.

On a different thread. The steady/transient analysis can be used on the
primary circuit to determine peak voltages rms currents etc.

Bob

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, October 30, 2003 5:26 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.
 >   The surge is independent of the _actual_ load on the
 > transformer.  (Until mag field can build up in the
 > core the load is 'invisible' to/from the primary.)
 >