Microwave oven PWM frequency

[ogden]

I often eat things which are quicker to cook than that, such as fried eggs or some fish I already have cooked and just needs reheating.

Fish which is already cooked, can be cooked quicker than that. Good one 

I have steamed vegetables before and although I agree it's better, than microwaving, I don't think the difference is great enough to warrant the extra time and cleaning of kitchen utensils, for a single portion.

Agreed Cooking/eating is not "discrete" if I can say so. As long as your microwaved, steamed or caramelized in pan vegetables are what you like - enjoy!

[Ilima]

Actually, pulse width modulation.... is 100% on, then completely, 0% off, then on, then off, and repeat.

The trick is, they just turn on and off much faster than relays can manage by using more advanced switches (usually semiconductors).

So, no. PWM controlled microwaves do not output 495 watts for 100% of the time. Rather, over the course of, say, one second, they switch on and off rapidly enough that the sum energy output for that second is 495 Joules.

[David Hess]

Microwaves that control the power level by ~0.25 HZ pwm are pretty annoying.
I would recommend replacing it with an inverter microwave.

But they work pretty well and without the added complexity of an inverter, they are more reliable.

I can see there are more than a few who are not buying the microwave oven problem in the 802 IEEE standards for WIFI , XBEE and  blue tooth. I can understand why as you would think they would be smart enough to separate 1000 watt microwave oven frequency from .1 watt communication frequencies. Sadly they did not separate them. Even Sadder if it is your phone that rings when computer communications are down.

I have never had a microwave which did *not* completely trash 2.4 GHz WiFi.  But the frequency selection for WiFi was no accident.  The ISM bands were available *because* industrial and commercial devices like microwave ovens render them useless for reliable communications.  If microwave ovens were not polluting the 2.4 GHz band, then the 2.4 GHz band would not have been available for WiFi and general use.

[tooki]

Microwaves that control the power level by ~0.25 HZ pwm are pretty annoying.
I would recommend replacing it with an inverter microwave.

But they work pretty well and without the added complexity of an inverter, they are more reliable.

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.)

[coppice]

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.)

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.

[David Hess]

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.

Exactly, that is what I found when I looked into it last year.  And replacement inverters are either expensive or not available.

[NiHaoMike]

I wonder if it could be possible to hack an inverter microwave oven to be more Wifi friendly by preferring to operate when there is no Wifi activity and/or by broadcasting RTS/CTS packets (using an ESP8266 or similar) to tell Wifi networks to avoid sending data when the microwave wants to operate.

[tom66]

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.)

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.

Our is 8 years old and still going strong.   Really like the "turbo bake" feature where it can use the oven and grill simultaneously for 3000W cooking - makes for some really nice roast potatoes with a crispy top but cooked through. 

The only failure is the plastic that's glued to the front of the door has peeled off. Funnily enough the same failure has happened to the one in the work kitchen (same generation, different model.)  I have yet to find an adhesive that survives the high temperature of the oven's operation so I haven't been able to glue it back on permanently yet.

[Yansi]

A microwave oven is switching on and off every 8 m seconds as well as every second or so. The reason for this is it is against the law for a microwave oven to run off a filter DC power supply. It must be a unfiltered supply raw off the full bridge rectifier thus on and off every 8 m second. The reason for this law is to sweep a gigantically stupid law passed before under the rug. The gigantically stupid law was to have a 1000 watt microwave  and a tiny 100 m watt WIFI in the same frequency range. The solution , equally as dumb , was to out law filtered power supplies in microwave ovens so that the tiny 100 m watt WIFI can sneak a few bytes out in a 2 m second window between power cycles of a monster 1000 watt microwave. Is it just me or did someone drop the ball here? Thank god for 5 GHz WIFI.

Seems a pretty much nonsense my friend. 

[Yansi]

I wonder if it could be possible to hack an inverter microwave oven to be more Wifi friendly by preferring to operate when there is no Wifi activity and/or by broadcasting RTS/CTS packets (using an ESP8266 or similar) to tell Wifi networks to avoid sending data when the microwave wants to operate.

No, because inverter microwave just means that the bulky transformer supply was replaced by a high frequency inverter. The magnetron is still there, spewing all sorts of shit somewhere in the 2450 MHz or so. There is no control over the frequency possible. Neigher does the magnetron run on DC voltage, it still uses the pulsed rectified voltage from mains. There would be no benefit in running the inverter from DC, in fact that would be an disadvantage.

[NiHaoMike]

The only failure is the plastic that's glued to the front of the door has peeled off. Funnily enough the same failure has happened to the one in the work kitchen (same generation, different model.)  I have yet to find an adhesive that survives the high temperature of the oven's operation so I haven't been able to glue it back on permanently yet.

Have you tried JB Weld?

No, because inverter microwave just means that the bulky transformer supply was replaced by a high frequency inverter. The magnetron is still there, spewing all sorts of shit somewhere in the 2450 MHz or so. There is no control over the frequency possible. Neigher does the magnetron run on DC voltage, it still uses the pulsed rectified voltage from mains. There would be no benefit in running the inverter from DC, in fact that would be an disadvantage.

I don't see why you couldn't "emulate a busy network" using a separate transmitter and take advantage of the collision avoidance mechanism that Wifi uses. (Make the oven appear like another network that nearby networks should avoid colliding with.) Actually, come to think of it, you don't even need an inverter microwave to do it to some extent. The inverter just gives you more flexibility in being able to turn the output on/off at will. With a conventional microwave, you could add a separate filament transformer and then gate the HV supply on and off with a SSR.

[tom66]

The only failure is the plastic that's glued to the front of the door has peeled off. Funnily enough the same failure has happened to the one in the work kitchen (same generation, different model.)  I have yet to find an adhesive that survives the high temperature of the oven's operation so I haven't been able to glue it back on permanently yet.

Have you tried JB Weld?

I've not, actually.  Is there a specific one I should use?

[Yansi]

The only failure is the plastic that's glued to the front of the door has peeled off. Funnily enough the same failure has happened to the one in the work kitchen (same generation, different model.)  I have yet to find an adhesive that survives the high temperature of the oven's operation so I haven't been able to glue it back on permanently yet.

Have you tried JB Weld?

No, because inverter microwave just means that the bulky transformer supply was replaced by a high frequency inverter. The magnetron is still there, spewing all sorts of shit somewhere in the 2450 MHz or so. There is no control over the frequency possible. Neigher does the magnetron run on DC voltage, it still uses the pulsed rectified voltage from mains. There would be no benefit in running the inverter from DC, in fact that would be an disadvantage.

I don't see why you couldn't "emulate a busy network" using a separate transmitter and take advantage of the collision avoidance mechanism that Wifi uses. (Make the oven appear like another network that nearby networks should avoid colliding with.) Actually, come to think of it, you don't even need an inverter microwave to do it to some extent. The inverter just gives you more flexibility in being able to turn the output on/off at will. With a conventional microwave, you could add a separate filament transformer and then gate the HV supply on and off with a SSR.

It already appears as a busy network channel already. It doesn't matter how did a packet got corrupted.

Why would you add a SSR? What for? To further make more EMI/RFI and increase the apparent power input?

Microwave oven designs are already very optimal.  Adding unnecessary shit doesn't make it better.

[chris_leyson]

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.)

Modern microwave ovens fitted with inverters such as Panasonic, Bosch and Siemens will generate continuous microwave energy from 1kW down to 360W or so, below that they switch over to PWM. By clever and subtle design an inverter will control a magnetron over a 1:3 power range. If they come with a grill or convection fan they can be used with or without additional microwave heating and actually produce some very well cooked meals. I know because I help design them.

The older ovens with transformer driven magnetrons are really crude by comparison and there isn't a great deal of food stuff that you can cook in the traditional sense. My old school cheap popty ping, or is it popty beep these days, is just used for warming.

[tom66]

One thing I *don't* like about our Panasonic combi microwave-inverter is the fan.  It runs for about a minute to "cool down" even if you only used the oven for 10 seconds to briefly zap something, in which time almost no heat is produced.  There seems to be no temperature monitoring, or estimated power dissipation, in the control logic. The fan is noisy and while it's not a massive problem, it strikes as something that could have easily been addressed in the software.

[NiHaoMike]

I've not, actually.  Is there a specific one I should use?

Just plain JB Weld is rated to quite high temperatures.

It already appears as a busy network channel already. It doesn't matter how did a packet got corrupted.

Wifi has a mechanism for APs to tell each other that they're going to transmit in order to avoid collisions. If the microwave could act like an AP and tell nearby networks to avoid transmitting for some time, they'll wait until the microwave has finished outputting its pulse before trying to send data, thereby avoiding retries.

[tom66]

I recall an industrial (i.e. chef's) microwave that used 915MHz RF.  I don't know if that's common in industrial ovens or not, or if it's any better at heating food, but it was interesting.  Probably only legal in the USA as 915MHz is protected in Europe.

[David Hess]

900 MHz works fine for heating food and probably even better than 2.4 GHz.  Early microwave ovens in the US were 900 MHz.

The problem with 900 MHz is that there is no room for a choke ring in the door so metal finger stock is used instead to seal the door from RF leakage.  But the metal finger stock proved to be too easy to damage, especially in a kitchen, leading to problems with leakage.

[tom66]

I've never heard of a choke ring.  I'm assuming that's a tuned antenna of some kind inside the door that helps reduce the emissions by shorting leakage?

[David Hess]

I've never heard of a choke ring.  I'm assuming that's a tuned antenna of some kind inside the door that helps reduce the emissions by shorting leakage?

It is something like that.  There is an open quarter-wave or shorted half-wave channel along the door seam which causes an out of phase reflection of the 2.4 GHz energy canceling out leakage.  Since it only works at 2.4 GHz and harmonics, this is why microwave ovens are not cheap substitutes for Faraday cages.

[tooki]

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.)

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.

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)? 

[coppice]

I could have got a brand new Panasonic magnetron for £44 on ebay and probably got another 10 years of life out of it!

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. 

[Red Squirrel]

Real men just wind their own magnetrons.

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.