From: John De Armond
Newsgroups: alt.energy.homepower,alt.engineering.electrical,
	sci.electronics.repair
Subject: Re: uWave ovens, was: 280V motor on 230V circuit
Date: Fri, 30 May 2008 02:55:50 -0400
Message-ID: <5n7v34dlfks9k6ghjp6fu74lgseene7fpm@4ax.com>

On Thu, 29 May 2008 22:19:09 +0000 (UTC), David Lesher <wb8foz@panix.com>
wrote:

>All I've ever had to a u-w power supply was to replace the rectifier
>stack; or junk the oven because it was clearly smoked...
>
>Someone one mentioned they were F-R, and a casual look seemed to confirm
>that, so I never questioned it. A F-R is also current limited; short the
>output and it delivers rated current, period..

The transformer goes by several names, depending on where you are.  Variable
reluctance, leakage flux, stray flux, etc.  It is exactly the same
construction and operating principle as a neon transformer, some kinds of HID
light ballasts and some series streetlight constant current transformers.

The core is an almost standard "E" core (or "H" core if you prefer) with one
exception.  The center leg has an air gap.  The windings are on the end legs
of the "E" instead of the center leg.

There are two magnetic paths around the core for the field set up by the
primary to travel.  Around the periphery and across the secondary and around
the center leg and across the air gap.  Field that travels along the center
leg does not cross the secondary and induces no voltage.

With no load applied, the bulk of the field travels the peripheral, very much
lower reluctance solid iron path, inducing full secondary voltage proportional
to the turns ratio.  As current flows in the secondary, counter-MMF raises the
reluctance of the peripheral path so that some of the flux travels through the
center leg.  With less flux traveling around the periphery and cutting across
the secondary, the secondary voltage drops as the current remains about the
same.  At the limit, if the secondary is shorted, the peripheral path has so
much reluctance that most of the flux travels the center leg and across the
air gap.  The same current as before flows through the secondary but at zero
volts.

When the dimensions of the core and gap are set up correctly, the transformer
behaves as an almost perfect constant current device.  That is, the secondary
voltage varies as necessary to keep the same current flowing through a varying
load.  Just what the doctor ordered to keep the magnetron happy.

The secondary current can be increased by opening up the air gap.  This raises
the reluctance of that path and forces more field through the secondary leg.
Closing the gap has the opposite effect.

The center leg is often called the magnetic shunt and frequently it is a
separate piece of laminated iron stuck between the coils and TIG welded in
place.  It is a common trick for Tesla Coilers to open up a neon transformer
and either knock out the shunt entirely or grind it down to open the air gap.
This modification causes the transformer to output much more current than it
is designed for - for a little while, at least :-)  The same thing works with
microwave oven transformers (MOT).

This design in a microwave oven is a vital part of keeping the magnetron anode
current within spec.  The magnetron is electrically a diode.  A diode that
isn't emission-limited would draw destructive current if not externally
limited.  With this design, the filament can be heated good and hot for long
life and not have the tube run away.  The design also is vital for protecting
the magnetron from potentially damaging conditions such as operating the oven
empty, arcing, etc.

It's popular to use several MOTs to build an arc welder.  This works quite
well specifically because these transformers are constant-current devices -
exactly the characteristic stick welding needs.  If they were conventional
transformers, the first time the rod touched the work and shorted the
secondary, fault current would flow and the breaker would trip or blue smoke
would leak out.

Along similar lines, one can cut off the high voltage secondary and replace it
with a suitable number of turns of heavy wire, connect a bridge rectifier and
have a nice constant current battery charger.  Select the turns carefully and
it'll do the bulk/absorption stages of the smart 3 stage charging algorithm.

John