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Microwave oven PWM frequency
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tooki:

--- Quote from: coppice on June 23, 2020, 03:57:18 pm ---
--- Quote from: tooki on June 23, 2020, 03:52:05 pm ---The primary manufacturer of inverter microwaves is Panasonic, whose microwave ovens have a great reputation, and AFAIK no issues whatsoever with longevity. (They’re all I’ve used for 20 years, and I’ve never had one die. I’ve only gotten new ones because of moving to a country with different mains voltage, or to upgrade to one with new features.)

--- End quote ---
When I bought a Panasonic microwave oven 20 years ago they were not inverter types and they did last well. The early inverter ones had a poor reputation for reliability and longevity. I don't know if that has improved.

--- End quote ---
Weird. The Panasonic I bought in 2000 definitely was an inverter model, and I had it until I moved overseas in late 2008, at which point I gave it away.
Gyro:
By coincidence, we replaced our Panasonic Inverter Microwave today. From the IC datecodes the old one appears to have been manufactured in 2005. I remember it was still the only brand offering inverter models. The only hard failure over its life was a failed turntable motor (a cheap and easy replacement due to the snip-out panel in the bottom).

Anyway, it had recently begun to suffer dropping power as cooking time increased, with the odd cracking sound. I had it apart a couple of times but couldn't see any problem - other than the filament glow of the magnetron appearing to dim slowly over several minutes. I wasn't sure if this was due to the inverter or the magnetron (inverter seemed more likely) so effectively BER.

With the nice new one in place, it was time to 'scavenge' the old one. This time I removed the magnetron and found cracked magnets and loose cooling fins that I hadn't noticed previously - a sure sign of overheating. I'm kicking myself now, because I could have got a brand new Panasonic magnetron for £44 on ebay and probably got another 10 years of life out of it! Still, the new one is very shiny, no dents and we don't need the same volume these days (it's still 950W though).

I found the attached Panasonic training guide on the web today. Apparently the inverter controller senses the Anode current using a current transformer to control the power output. With the damaged magnetron, it was presumably dialling back the power as the (non-microwave-generating) anode dissipation increased, hence the slowly dimming filament. Ah, hindsight!

From the training guide, it appears that the PWM frequency varies between approx 20kHz and 45kHz depending on output power, the whole thing running on 100Hz un-smoothed bridge rectified mains. Surprisingly there was only a single Electrolytic in the whole thing, a small Panasonic one on the controller logic supply, rated at 105'C despite having good ambient airflow from the cooling fan (take note treez). :-+  I was expecting it to be chock full of the things!

Now, what to do with a 1kW rated IGBT driven ferrite cored transformer (once the nasty 2kV secondary is unwound)?  >:D
coppice:

--- Quote from: Gyro on July 12, 2020, 08:30:16 pm ---I could have got a brand new Panasonic magnetron for £44 on ebay and probably got another 10 years of life out of it!

--- End quote ---
Wow, that's a heck of a markup. You can get those magnetrons from TaoBao for about 20% of that price.
Gyro:
Yes, almost certainly - It was a Scrooge's remorse search for a 'genuine matching Panasonic part number label' made in China magnetron, versus a TaoBao made in China magnetron.  :D
Red Squirrel:
Real men just wind their own magnetrons. :P

Actually, what part normally fails in them?  Is it usually the windings, or more involved?  I know they use some rather special (and dangerous) materials so probably don't want to handle them more than you need, but I suppose in theory one could rewind one.  There's a pretty serious science to it though, to get the right resonance etc. So probably would not recommend.  :o
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