Microwave arcs after years of normal operation

Our 5 year old Kenmore microwave has run perfectly with no abuse, no metal used while cooking, etc. We have also used it to warm our plates, and typically we warm one, two, or three dinner plates for between 1 and 2 minutes total. They always emerge nicely warmed, not hot but just right. The plates are similar to what we'd call stoneware but not as dense, perhaps some sort of clay substance made by Pfaltzgraff. This shows the exact plate but not the material, at

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We have warmed plates in it for most of the 5 years since we discovered this seems to work quite well. Today, while warming two plates, we got a strong arc after only a second or two, which upon subsequent test seems to occur behind the flexible white plastic square on the right side of the cavity (assume this is the magnetron horn?)

We shut it off immediately and then experimented further; it appears to still function, warming a glass of water normally, warming a container of rice normally, etc. However, placing the plates back in causes an almost immediate arc.

So, what's changed? Have the plates somehow become more/less load now (unlikely; we tried two other plates as well with the same result.)

I'm guessing maybe a buildup of grease or residue in the magnetron area may have changed it's parameters somehow, to the extent that it's no longer 'happy' being asked to warm plates.

Ideas? (Other than "don't do that" please...)

It could just be a buildup of crud somewhere, once it starts to arc, you have to clean and patch it up good to prevent it from arcing again.

I've never heard of warming plates in a microwave oven, I would think that if the plates themselves get warm, they're probably not microwave safe. I would at least put a cup of water in there as a load.

Just remove the plastic bit, but make sure not to put food in there that can "burst" and splatter into the waveguide.

Good advice ... a cup of water is easy and prevents damage to the oven (which seems to have happened in this case).

Just on their own ? I wouldn't do that.

Maybe the plates now contain less moisture. Is it now sort of wintertime and the humidity will have fallen.

Can't explain why it didn't happen every winter though.

Graham

There is probably a build-up of carbonized food on the back side of the plastic plate. Do as the previous poster said. You may or may not be able to clean the plastic sufficiently to put it back. The potential for arcing has always been there, it is just that the food build-up finallly broke down under the high fields that occur when the oven cavity is lightly loaded. That is why you are always cautioned not to run the microwave under no-load conditions which is about what you are doing when you just pout loates in there. You can still warm plates, just put some water in a container in the microwave at the same time. The water will absorb much of the energy and it will take you longer to heat the plates. Why not just run them under hot water for a few seconds?

H. R. (Bob) Hofmann

ian field wrote:

does your operating manual suggest using a microwave oven to heat plates?

a microwave oven is designed to heat items containing water. a water molecule is composed of a single atom of oxygen with two atoms of hydrogen attached at a specific angle to the oxygen atom. the frequency of a microwave oven is designed to cause the hydrogen atoms to vibrate, causing friction with adjacent water molecules, thereby generating heat.

clays contain minerals rich in metal oxides. the metal in your plates possibly has been marginal in the past and due to usage, the addition of molecular metal particles from flatware may have caused the change.

snipped-for-privacy@pdxc> Our 5 year old Kenmore microwave has run perfectly with no abuse, no

I think you nailed it, Bob... there were a couple of blobs of stuff on the backside of the shield; easy enough to clean up with some steel wool and then I remounted on the reverse so the waveguide shoots through a different area of the material. Seems to clear up the problem. Running hot water might be an option but not nearly as convenient as just popping them in the micro.

Thanks.

If you checked your manual, you would find that shield is NOT to be cleaned with anything abrasive. It is made of a compound that is part of the distribution of the microwaves. My first unit manual said that (early 80's) and the one I bought a week ago also said that as well. My first unit was a Sharp, ($1,200) and included a 9 hour cooking course with it. The very first night, they had a Sharp tech there that said any dish that contained clay was not to be used as there is always trace amounts of metals in clay. He also said that if any container that got hot in the machine when there was no food/water, should not be used. Why don't you email the manufacturer of the plates and ask if they are microwave safe. I'll lay odds the answer is NO. Regards Lee in Toronto

dont do that, put a cup of water on the plates when you nuke them.

Keep the plastic bit clean to prevent this recurring.

Dont run the machine with the plastic waveguide cover removed, as food splatters will enter the waveguide, this will short out, and cleaning it out will then be difficult or impossible.

NT

They don't want you to destroy it but it is just a piece of plastic that is transparent to microwaves.

He's been doing it for years without incident. So, something else has changed. As long as the plates absorb some of the energy, it won't hurt the oven. That's the same thing a glass of water does.

I'd guess there is some burnt on crud which is causing the arcing.

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You should see the items Braniac (science abuse) TV show puts in the microwave - on average about half the items result in the steel casing being unfolded back to sheets of steel! Some of the less destructive items at least provide an entertaining light show.

Nukes are nothing like that fragile. You can heat plates in them, melt aluminium & iron in them, or put metal foil on food in them. The problem is just lack of load, the plates dont absorb much, plus burnt muck on the waveguide entry cover.

Contrary to myth, magnetrons are not killed by running them with no load, that only applied to the very first commercial microwave oven iirc. However it does tend to cause arcing, and that can do minor damage in the cooking cavity, the worst example of which is frying that plastic plate. Once theres carbon on that, it will deteriorate until it catches fire. So keep that lil cover clean.

NT

Nukes are nothing like that fragile. You can heat plates in them, melt aluminium & iron in them, or put metal foil on food in them. The problem is just lack of load, the plates dont absorb much, plus burnt muck on the waveguide entry cover.

Contrary to myth, magnetrons are not killed by running them with no load, that only applied to the very first commercial microwave oven iirc. However it does tend to cause arcing, and that can do minor damage in the cooking cavity, the worst example of which is frying that plastic plate. Once theres carbon on that, it will deteriorate until it catches fire. So keep that lil cover clean and you can cast iron in it in peace.

NT