Variac controlled inductive ballast - WARNING

• To: tesla-at-pupman-dot-com
• Subject: Variac controlled inductive ballast - WARNING
• From: "Gavin Hubbard" <ghub005-at-xtra.co.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Sun, 05 Mar 2000 21:20:37 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

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

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