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Re: Variac controlled inductive ballast - WARNING
One must carefully recall transformer theory, especially with regard to
turns ratio and their effect on BOTH current and potential. If you decrease
potential you can radically increase current across a portion of the
windings. The optimum solution is to definitely NOT use a variac to control
current. They can work but without careful metering across sections of the
winding you may increase current or voltage to the point where damage will
quickly occur.
A separate reactor around 30-50 mH with a series 0.5 Ohm resistance always
works better. Sweet spots are eliminated and you can tune over the full
portion of the spectrum. Tuning is accomplished by "dipping the plate
current" much the same way a ham radio transmitter is tuned. At some point
power increases with the increasing variac setting while current
simultaneously decreases until it "dips" to a low point and then as the
variac increases the current and power once again increase in value. This
dip is the most efficient operation for the entire system. Slightly above
this point, however, is the setting at which you will get the longest sparks
for any system. This value is usually 10-15% above the dip point. Tuning
of the reactor can be continuous or accomplished with a series of taps every
5 turns.
This concept is known as "tuning the power supply" to match the load.
Proper metering is essential to see this effect. Be sure to meter the
current both before and after the reactor. You will note some very
interesting relationships that will provide solid clues to effective tuning
of your system.
Dr. Resonance
-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Monday, March 06, 2000 1:26 AM
Subject: Variac controlled inductive ballast - WARNING
>Original Poster: "Gavin Hubbard" <ghub005-at-xtra.co.nz>
>
>List members,
>
>I killed a variac type variable ballast this morning. In hindsight, it was
>pretty clear why it happened, but I haven't seen reference to this failure
>mode on the list before.
>
>Because of the recent list discusions on variable ballasting, I wanted to
>see for myself how well I could control the current in a mid sized (5kVA)
>coil system. I set the power controller so it started with the mains
>voltage across the full variac winding. I then energised the circuit and
>decreased the inductance by shortening the winding length.
>
>With about one quarter of the winding left, the variac started arcing to
>the core and let the magic smoke out.
>
>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.
>
>This failure mode may also (theoretically) occur in multi-tapped inductors
>through the auto-transformer action.
>
>Safe coiling,
>
>Gavin Hubbard
>
>
>