remote control inductive ballast

To: tesla-at-pupman-dot-com
Subject: remote control inductive ballast
From: "Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Wed, 27 Oct 1999 13:15:02 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Terry's comments on a remotely controlled primary cap reminded me of an
idea for a primary current control device (ballast) for pole pigs and the
like. 

What you want is a variable inductor, for which many have used a welder's
adjustable leakage inductance.

How about this.  Build an array of fixed inductors, switched by suitable
relays (SSR or not), in parallel.  The values would be chose in a
1,2,4,8,.. sequence to allow arbitrarily fine control of the current.

For example, say I wanted to be able to control anywhere from 50 Amps to
about 1 amp in about 1 amp steps.  This implies using 5 bits (32 steps). 
The smallest step would be 50/32, or about 1.6 Amps. So, my first inductor
would be sized so that its impedance (at line frequency) is such that it
would flow 1.6 amps at 240 V, or, about 154 ohms.  This is about 0.4 H. 
The next inductor would be half that (0.2H) which would allow 3.2 Amps, and
so forth, to the smallest inductor (25 mH) which allows 26 Amps to flow
(into a shorted load).

With all 5 relays off, no current flows.  With just the .4H relay on, my
current is limited to no more than 1.6 Amps. With all the relays on, my
current would be limited to 48.4 Amps (about the 50 A I started with). With
a binary switching sequence, I can limit the current in equal steps all the
way in between.  The relays would need to handle the appropriate amount of
current (2 Amps for the smallest, up to 30A for the largest).  If you have
a source of identical relays with limited power handling, you can split
some of the legs up (say, make the highest current "bit" a pair of 50 mH
inductors, each switched by a 15 Amp relay, with the coils in parallel) 
This might also have advantages for the inductor design, because you can
make tradeoffs of winding size, core size, etc.

You could use the same switching network to switch appropriate PFC
capacitors in, as well.