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Review of Variac As Current Limit!!!
RE:
Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
Can anyone explain how a variac *really* works as a current limiter?
This post bothered me a little bit:
>>>>>Begin Snip <<<<<
If I now set the variac at itīs highest position (wiper is near the Ph
terminal), the density of the magnetic field is at itīs maximum, BUT
through a VERY small cross-sectional core area. There is NO current
flowing through the windings from W to N, so this core area see no
or very little (stray losses) magnetic flux (& current). The density of
the magnetic flux flowing through that minute core area has got to
be massive. So, in my eyes, the core MUST saturate and it will no
longer truly work as a limiting inductor. That poor (current limiting)
variac is going to start growling like crazy. Of course, I realize the
magnetic field is a result of amps and volts per turn (on the variac),
so if you run a controller containing a second variac
>>>>>End Snip <<<<<
When the variac is adjusted so that the greatest number of turns on the coil
are in series with the load, the variac is at it's highest inductance value.
This means it is at it's highest impedance. This translates into lowest
current for a given applied voltage. With the same applied voltage,
adjusting the variac knob such that fewer turns are in series with the load,
the variac presents a decreasing inductance, and thus a decreasing impedance
to the load. Decreasing impedance at a constant voltage translates into a
greater current. The "core area" is not changing, and is not becoming
minute. The inductance (impedance) is. The core area that is related to
the magnetic current toting capability of the core is that which is
__parallel__ to the windings. Changing the number of turns in the wire coil
does not change this cross sectional area. It should be a simple
overcurrent condition on the variac.
NOTE: All of the magnetic current flows through the whole core! Just like
the electric current in a coil.
Also note, as was mentioned by Gavin Hubbard, that the autotransformer (that
is what it was designed for in the first place, right?) action of the variac
can generate highly destructive voltages on the open end of the coil!
>>>>>Another Snip <<<<<
Why? Because of the auto-transformer configuration, the unused portion of
the winding was stepping up the voltage i.e. applying 240 volts to the first
quarter of the winding resulted in a voltage step-up of approx 1000 volts
(1:4) in the last portion of the winding. The variacs enamel insulation
broke down after only 10 seconds at this voltage.
>>>>>End Another Snip <<<<<
Pretty Picky for a Newbie, huh?
MPF
the_machin_shin-at-hotmail-dot-com
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