According to :
[ ... ]
Variacs are usually designed to run at pretty high flux
density because core temperature rise is not normally a problem.
This means that the magnetising (i.e.no-load) current can be a
lot higher than in comparable fixed ratio transformers.
The toroidal core used is a completely closed iron
circuit with no residual air gap. The the reluctance of this iron
circuit is so low that, when power is switched off, it remains
magnetised at almost the full value of the flux density that
existed at the instant of disconnection of the supply.
Interesting.
Steady state, this is no problem but it can result
in possible large current peaks for the first few cycles after
power is re-applied.
If power is re-applied at the same phase instant as
the previous disconnection phase instant there is no transient
disturbance and it simply resumes normal no-load current.
OlK.
If power is removed at the peak positive flux
density instant in the supply wave and power later re-applied at
the instant that requires peak negative flux density the core
saturates because it cannot support the doubled flux swing. Very
large peak currents then occur that decay in the first few cycles
of the supply waveform.
This means that the switch on current surge appears
to be random because the actual value depends on the precise
instants when power is removed and when it is re-applied.
O.K. Then this suggests to me that the best way to control a
large one like his (the rating of his breaker) is to replace the switch
with a solid-state relay, which maintains the "on" state until both the
control voltage is off, *and* the current through the load drops to
zero. This should minimize the spikes when switching on to half of
worst-case. (Even better would be if the turn-on control could be
zero-crossing as well.
Glad to see an explanation of the phenomenon which makes sense,
and this at least shows that the 30% setting of the wiper when it
tripped the breaker was just a coincidence.
Enjoy,
DoN.
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