280V motor on 230V circuit



Mostly less! :-) My Panasonic inverter unit just released the magic smoke. First the magenetron died, and then after I replaced that, an IGBT in the switcher shorted and did a fair bit of collateral damage.
I finally did find a service manual...in spanish (which I read poorly,) but it did at least tell me what all the small resistors and diodes were supposed to be (before they melted.)
So, $100 in parts later, I now have a working microwave again. And, if I get tired of it, I can sell it on Craigslist for at least $35! :-/
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On Fri, 30 May 2008 01:50:29 GMT, snipped-for-privacy@hatespam.com (Fiat Sparks) wrote:

Don't try to operate that oven from a cheap generator with a less than perfect sine output. That's another excuse for the blue smoke to leak out. BTDT.
In my case I wasn't about to spend that kind of money to repair an oven that barely cost that much, especially since I used it in my restaurant always on high. Therefore I yanked out all those fancy electronics and installed the transformer/diode/cap assembly from another old oven. I drilled a hole through that nice touch pad and installed an Intermatic spring-wound timer from Home Depot.
Viola, good as new and bullet-proof against nasty power.
John
-- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN The secret to creativity is knowing how to hide your sources -Albert Einstein
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snipped-for-privacy@hatespam.com (Fiat Sparks) writes:

Sorry, I was referring mostly to the normal transformer/capacitor/diode circuit. :)

:)
--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html
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snipped-for-privacy@hatespam.com (Fiat Sparks) writes:

Ouch. We have one that's only a few months old; I hope it does better than that. I love it because all of the power settings from "3" to "10" actually vary the magnetron RF output while keeping it on continuously. At "3", I can cook a single egg in the oven without it exploding.
Conventional microwaves set to "3" run the magnetron about 4.5 seconds at full power followed by about 10.5 seconds off, and 4.5 seconds of full power into a single egg generates enough steam for an explosion.
(On the Panasonic, settings of "1" and "2" actually run the magnetron at 30% power, using 1/3 and 2/3 on time).

For that much, I'd be tempted to scrap it and buy a new oven.
    Dave
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snipped-for-privacy@cs.ubc.ca (Dave Martindale) wrote in

Saw something in a catalogue: a plastic 'Egg' shaped chamber that holds 4 eggs and some water. I am guessing that it is a 'plastic pressure cooker'. Anyway, they claim you can hard boil eggs in it in a microwave oven.
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snipped-for-privacy@cs.ubc.ca (Dave Martindale) wrote:

Mine lasted about eight years with daily use for a family of 5. So, hopefully you've got a ways to go. I agree, it does cook nicely, which is why I even bothered to repair it.

You know, that's what the wife said! :-)
Had I known it was going to go out again after the magenetron died, I probably would have. But, my pride was at stake!
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wrote:

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 -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN In theory, there is no difference between theory and practice. In practice, there is.
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Neon John wrote: <see originsl post>

Another use for defunct MOT's, even those with a bad HV winding, is using two as a line isolation transformer. Just remove the HV winding, and connect the two magnetron heater windings together. The normal line winding on the second transformer then supplies your workbench with power isolated from the line. (You might need to modify the "shunt" core.)
With a Variac on the input, this is a handy for working on line operated devices that might have a line-to-chassis defect.
--
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Do you actually use a setup like that? I'd think that for any sort of current, the filament windings would be melting down even though they are made of fat wire. For example, at 300 VA of output, you're looking at ~100 A in the filament winding.
--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html
Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.
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Connecting the HV windings together would probably work a lot better. Would want to knock out the magnetic shunts too, that can be tricky but I've done it on several without damaging the windings.
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I wanted some heavy copper wire to wind a coil with so I tore xxxx started to tear up an old microwave transformer. Found out that pretty copper wire was aluminum with a copper colored layer on top of it. :(
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bz 73 de N5BZ k

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bz wrote:

That's pretty common these days, the consumer microwave oven is a perfect example of something that has been engineered to perfection to be as cheap as humanly possible and yet still function adequately.
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They look similar. Both transformers have magnetic shunts, and there's a big capacitor near the transformer. But the circuits are different.
The FR transformer puts the capacitor across the secondary winding, or two secondary windings in series, and the windings plus capacitor resonate at the designed line frequency. This causes the voltage to rise above what you'd expect from the turns ratio alone, but the voltage is limited by the portion of the core that the secondary is wound on going into saturation. Sometimes the secondary voltage waveform looks pretty square because of this peak clipping, but sometimes there's a third winding that (somehow) reduces the second harmonic and gives something closer to a sine wave.
There's a magnetic shunt between the primary and secondary windings so that the primary current doesn't go through the roof when the iron in the secondary saturates. It limits the shorted output current to about *twice* the rated current, not equal to the rated current.
Still, the FR transformer runs hot with no load, dissipating about 20% of its full output rating as heat.
In comparison, the capacitor in the microwave is wired as part of a voltage doubler; it doesn't resonate with the transformer secondary. The transformer iron is not designed to saturate (though, as an intermittent-duty transformer that is fan-cooled in use, it is apparently designed to operate close to saturation to minimize the amount of iron).
    Dave
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snipped-for-privacy@world.std.spaamtrap.com (Michael Moroney) writes:

A motor designed for 50 Hz would run fast on 60 Hz, which might overload something like a fan motor. A clock would run fast. On the other hand, more iron is needed for 50 Hz motors (at a given voltage and number of turns), so the motor is actually overbuilt for 60 Hz operation. Same goes for ordinary transformers.
I've taken a number of microwave ovens apart over the years, and none of them used constant-voltage transformers. They do have a magnetic shunt to give them somewhat of a constant *current* characteristic, and that might reduce the magnetron current some when operated from 60 Hz. The magnetron itself operates from half-wave rectified DC, so it shouldn't care.
Some Panasonic microwave ovens use an inverter to generate the HV supply for the tube. Feeding it 60 Hz instead of 50 Hz should not be a problem; it will just reduce the ripple at the output of the initial rectifier.
    Dave
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I also asked about tap changers/regulators. Is this what these are, and if so, are they likely to be manual or automatic?
http://tinyurl.com/534ffq (Google Maps street view of a set of 3 on a pole in upstate NY. You may have to click on street view and rotate to see them)
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snipped-for-privacy@world.std.spaamtrap.com (Michael Moroney) writes:

Yes. Note the primaries are broken by insulators, and there is no secondary [240/120] service on the pole. [The lowest run is TV coax, then neutral above it.]
I've never heard of such as manual regulators so I assume they are automatic.
It's somewhat unusual to see all three on one pole; it must be a BIG one. Generally I see them staggered on 3 adjacent poles; or two poles hold a platform between them.
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Earlier in this thread it was mentioned the automatic ones are expensive and only used for big substations, not for long runs of distribution systems like that one. Before I read that I assumed they must be automatic.

At that location there used to be 2 adjacent poles with 3 transformers on a platform between them, when the downstream was an older delta- connected system. I bet they just reused one of the heavy duty poles. I wonder why they simply didn't reuse the platform setup.
I wonder what each of those puppies weigh, and how many MVA they can handle.
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snipped-for-privacy@world.std.spaamtrap.com (Michael Moroney) writes:

I know mine [which look to be very similar to the picture..] are automatic -- after my PSC complaint, the PEPCO phone droid read off the trouble report where the lineman went out, found higher-than-wanted voltage on my phase and set it to manual until the correct person could fix the regulator.
Alas, the first repair didn't work, as a week later, I was awoken again....
Later conversations with lineman in the area confirmed both that detail; and the fact their standard pole-pigs do NOT have a choice of taps. [They replaced the pig serving me with a larger one a few years back.]
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snipped-for-privacy@ipal.net writes:

Probably.
Interesting. Around here I see control boxes on the pole-mounted ones. Maybe they have something to read. (I know some read SIEMENS in large letters on some of them, apparently the manufacturer)
Long ago, during one of the past "oil crises" (1973 or 1978 or so) they proposed cutting back the voltage delievered to one's home by 5-10% from the normal voltage, a "brownout", "to save energy". I once wondered how they may have done that, assuming any automated regulating equipment was hardwired to provide 13.8kV or whatever, and that setting couldn't easily be changed. Also, any automated regulators downstream would try to compensate for the lower supply and raise their output voltage, so all regulators would have to be adjusted. But if was done manually, for equipment not tied to a particular voltage, they could do it on a substation-by-substation basis, and ones "downstream" could have been left alone.
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|
| wrote: |> |> | Yes -you are shorting a part of the winding but the switching is a bit |> more |> | complex than that so that short circuit currents are limited to |> reasonable |> | values. It is a multistep operation with reactor switching. On-load tap |> | changers are expensive and are generally limited to applications where |> this |> | is absolutely needed (I have seen one where the tap changer was nearly |> as |> | large as the transformer). |> |> What about multiple parallel transformers, or at least multiple parallel |> windings on the same core (on whichever side the tapping is to be done), |> where the taps are stepped incrementally on each winding? Instead of a |> shorted winding segment, you'd have windings of differing voltage in |> parallel as each of the windings change their taps one at a time. |> | | So when one is set for say 118V and the other is set for 120V, you have a | 118V source connected in parallel with a 120V source and the only impedance | is the transformer windings?? | | OUCH!!! I think the magic smoke will be spewing in no time
I was afraid of that.
That also means if you are going to parallel 2 transformers, they better have exactly the same winding ratio.
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