Microwave ovens

[Capitol]

Have been looking for a new built in microwave with true controllable power for a couple of years. Found this article which may be of interest:-

What they don't say is that many of these microwave ovens are too small internally to accommodate tall items. Found this problem with a Panasonic last month.

Seems that these manufacturers have also not bothered to educate the sales force in either Currys, John Lewis of Wren, as enquiries there elicit a "what are you talking about" response. IME pulsed power units spatter the cooking contents all over the oven if the time is slightly too long. Our 40year old microwave (resurrected only once) has a true low power setting achieved by a capacitor switching system and she is not willing to have anything but a true continuous low power setting.

Surprised to find out that the US shops are now selling double oven built in cookers with an integrated microwave, which is what she wants but the pricing is cloud cuckoo land. (roughly £2K+) Currently these seem to be limited to 27" models which are not being sold in the UK. Separate units are too tall for our housing, without engaging in a major door resizing exercise.

Any other information available?

[Brian Gaff]

Dunno but ui was wondering why even new machines still use on/off cycles to simulate low power. all it really does is use high power less long after all.

Is there some inherent problem, perhaps the disapation of the controlling device maybe? Brian

[Andrew Gabriel]

Reducing the power by other means (such as reducing anode voltage, reducing cathode temperature, etc) doesn't necessarily result in a gradual reduction of output power. That's certainly the case with things like power LEDs, where the output frequency shifts, and may fail to properly excite the phosphors anymore, so you get a colour shift.

In a microwave magnetron, I'm not sure what would happen, but it might be that if you reduce some of the operating parameters, you have to change other things too such as magnetic field strength, cavity size and/or other things which are all simply impractical to change.

For heating food, it simply doesn't matter that it uses a long pulse width modulation, so it's not a problem that needs solving.

[Capitol]

So why was it solved 40+ years ago? It is being solved again now. The solution is to control the magnetron current. The voltage is fixed to the best of my knowledge of magnetrons at fixed frequency. The use of long cycle times is necessary to try to keep the magnetron filament alive for the guarantee period.

On/off cycling at full power is the cheapest way to control power, that's why it is used and it doesn't work properly if you are looking for a simmer condition on a small quantity of food.

[Andrew Gabriel]

How? (Not saying it can't, I don't know the electrical characteristics of a magnetron).

The filament cycles on and off too, because it is driven from the EHT transformer too, due to running at the -5kV cathode potential. So the cycle time has to be relatively large with respect to the filament warm-up time.

I looked up some circuit diagrams for inverter microwaves, and this apsect works exactly the same way. Only difference is the EHT transformer is much smaller due to running at high frequency. They can't be feeding continuous reduced power to the magnetron because the cathode heater then wouldn't operate properly.

That's what the article you pointed to was saying, but the way it was written, it just wasn't very credible.

[Graham.]

I am not yet convinced anything novel is being done. Panasonic have talked about "chaos defrost" for some time but I am unable to find out exactly what that means. Marketing speak?

I am wondering if this so-called true variable power is just more of the same duty-cycle control, but measured in milliseconds rather than seconds. The magnetron filament typically takes two seconds to heat so it will have to be supplied separately from the HT inverter.

I am intrigued by your old microwave and its capacitor switching, what make and model was this and how did it work?

[meow2222]

Well, thats semi-true. You get less splatter with continuous low power, less boiling over, and you can cook mousses.

IIRC - and I might not - it was somewhat randomised cooking pattern, a purely marketing concept enabling use of the briefly fashionable chaos concept.

In the 70s it was common to have 2 cooking power levels, the lower one being achieved by switching a capacitor into the HT feed. It does behave better with the food, but switching kilovolts is a recipe for switch failures.

NT

[Capitol]

The brand was Belling which was actually a Japanese produced product. The capacitors are mechanically switched at high voltage, in series with the cathode supply. It still works perfectly after a capacitor replace/redesign exercise.

Inverter power supplies now used are apparently pulse width modulated at the switching frequency. The anode current is measured and compared with a reference. The heater is continuously on, after a few seconds as I understand it. The cathode volts are maintained at just below the operating voltage during the off periods, I believe, to keep the heater supply alive. There seem to have been some reliability problems, with failures after 14 months for some customers. I haven't seen a full circuit diagram for these ovens yet.

[Graham.]

I worked on the first generation inverter Panasonic on behalf of an insurance company. They seemed to drive the magnetron particularly hard, and a common fault was a cracked magnet, something that I rarely encountered with a conventional transformer type. Possibly not helped by the customer running it empty, but still...

Received wisdom was that mag was "special" (and expensive) and you shouldn't use a cheap universal replacement.

We were set up to repair inverters to component level if we could source the parts, I never found a source for the IGBTs. IIRC an exchange PCB was about £80 which made them BER.

The experience has made me dislike inverter microwaves and I still discourage people from buying them.

[Edgar]

Our Panasonic microwave failed recently. The magnetron magnet is cracked and the air guide shows signs of overheating. The magnetron has a heat sink with tiny air passages which are all blocked up. I put it down to dust being blown through the heat sink. We live on clay and in hot summer weather everything gets very very dusty.

We've replaced the machine with another similar one and I propose to check the new one in a while to see if there's any dust build up. If so it'll need some kind of regular cleaning.

I'm considering repairing the old machine. A new magnetron and air guide would be reasonable (it has an grill/oven facility which still works) but if a new inverter board is needed it definitely becomes scrap. But how to test the inverter without first buying a new magnetron?

BTW my wife was very pleased with the operation of this cooker - much better than previous non-inverter ones - if not so long lasting. She says "chaos defrost" is good however it works.

Edgar