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 |