Microwave oven PWM frequency

[SiliconWizard]

Also, there are often two things at work:
1. the duty cycle of the microwave — usually it's 100% at maximum power. At lower power, most microwaves have a PWM frequency in the seconds. Like 10 seconds on, 10 seconds off for 50%. Inverter microwaves (like my Panasonic) seem to both vary the microwave intensity and the duty cycle.

Yeah. Also depends on if it has some kind of "grill" function (usually a quartz lamp or something like that). I have an LG oven that has that. If you use a cooking mode with the Grill function, it will alternate between microwave and grill, making the cycles longer, and you can definitely hear when the magnetron restarts.

[Red Squirrel]

Why is it that they can't just vary the power level though?  It would basically be like a radio transmitter where you can dial down the watts.  I imagine most modern microwaves just use a high voltage inverter to power the magnetron now days, so it should be easy to simply dial down the voltage.  Or is it more complicated than that?

[ogden]

Microwave is good only at one job: flash heating. It is awfully bad for cooking or defrost. It means all you need - max power. Well... unless you are victim of toxic marketing and truly believe that you can *actually* cook something with microwave. In such case you are lost anyway and regulation of microwave power is least of your worries

[bob91343]

I just bought a microwave oven and on reduced power it's a few seconds on and a few seconds off.  Same with the one I just took apart.  That one yielded a nice power transformer to build a hipot tester, which I have done.

It's enough of a cost increment to do reduced power cleverly that I understand why they don't do that.  We are dealing with a kilowatt or so, not trivial enough to use a little FET to PWM it.  Further, there is the filament in the magnetron that needs steady heat, as shown by the fact that the oven doesn't do, say, 12 seconds on and 12 seconds off for 50%, but lengthens the on time to allow for heating filament.

[soldar]

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.

Rather than get into the technical aspects I will ask for a cite that supports "it is against the law for a microwave oven to run off a filter DC power supply." I am quite sure I know the technical explanation of why it is done this way but let's see the law first. Where can I find it?

[Zero999]

Microwave is good only at one job: flash heating. It is awfully bad for cooking or defrost. It means all you need - max power. Well... unless you are victim of toxic marketing and truly believe that you can *actually* cook something with microwave. In such case you are lost anyway and regulation of microwave power is least of your worries

Well I wouldn't agree with that. Microwaving is the most healthy way to cook certain foods, such as vegetables, because it doesn't excessively heat them or add fat like frying does or require them to be submerged in water which washes out some of the nutrients.

The one thing which microwaving doesn't do is brown food. I've just had a microwaved jacket potato and I crisped it up by placing it on top of the toaster for a couple of minutes.

[soldar]

My German-made induction stovetop has ten settings and at the five upper ones it does seem to regulate the power but at the lower ones it will cycle on and off, like a microwave oven, and it is annoying as hell and makes frying an egg impossible.

[thm_w]

My German-made induction stovetop has ten settings and at the five upper ones it does seem to regulate the power but at the lower ones it will cycle on and off, like a microwave oven, and it is annoying as hell and makes frying an egg impossible.

What kind of frequency?
I have a bosch and it does pulse at the lower power settings but its about 0.5-1Hz or so, perfectly fine for cooking. All pots/pans that I have that work with induction are reasonably thick so buffer the heat, and heat distribution is incredibly even. Maybe something else is wrong here.

A typical resistive element heated stove top is much worse, maybe 0.1Hz or lower.

[soldar]

What kind of frequency?

I did some measurements. Max power consumed is 1600 W. Then it goes down in 200 W steps down to 1000W. Then from there down it cycles on/off at 1200 W and in the one setting I tried at about T=14 seconds which is unusable for low heat. You can't blast 1200 w for 4 seconds and then switch off for 8 seconds to obtain an average of 400 W. It makes it unusable.  Furthermore, even when switched off  the thing is consuming 0.7 Amps of reactive power so I have to disconnect it entirely. It's crap. German crap but crap nonetheless.

Regarding microwave ovens they are good for reheating stuff and that's about it. I have never used one for real cooking.

A typical resistive element heated stove top is much worse, maybe 0.1Hz or lower.

The resistive plates I have do not switch but even if they did it would not matter much because they have great thermal mass and you can turn them off and the pot will continue to boil for 5 or 10 minutes. I have one that combines three resistor elements in different configurations so I can set from 150 W to 1700 W in six steps: https://www.eevblog.com/forum/beginners/calculating-value-of-resistors-in-parallel/msg2086750/#msg2086750

With resistor elements you can also use a triac power control.

[langwadt]

What kind of frequency?

I did some measurements. Max power consumed is 1600 W. Then it goes down in 200 W steps down to 1000W. Then from there down it cycles on/off at 1200 W and in the one setting I tried at about T=14 seconds which is unusable for low heat. You can't blast 1200 w for 4 seconds and then switch off for 8 seconds to obtain an average of 400 W. It makes it unusable.  Furthermore, even when switched off  the thing is consuming 0.7 Amps of reactive power so I have to disconnect it entirely. It's crap. German crap but crap nonetheless.

get pans with more thermal mass?

how much does the temperature change in those few seconds?

[thm_w]

I did some measurements. Max power consumed is 1600 W. Then it goes down in 200 W steps down to 1000W. Then from there down it cycles on/off at 1200 W and in the one setting I tried at about T=14 seconds which is unusable for low heat. You can't blast 1200 w for 4 seconds and then switch off for 8 seconds to obtain an average of 400 W. It makes it unusable.  Furthermore, even when switched off  the thing is consuming 0.7 Amps of reactive power so I have to disconnect it entirely. It's crap. German crap but crap nonetheless.

ok that does seem like a poor design. Unless they are switching internally with a relay there is no reason to have it that slow and generally a relay wouldn't be used.
Do you pay for reactive power?

The resistive plates I have do not switch but even if they did it would not matter much because they have great thermal mass and you can turn them off and the pot will continue to boil for 5 or 10 minutes. I have one that combines three resistor elements in different configurations so I can set from 150 W to 1700 W in six steps: https://www.eevblog.com/forum/beginners/calculating-value-of-resistors-in-parallel/msg2086750/#msg2086750

With resistor elements you can also use a triac power control.

Well thats a huge problem to me, they heat up, temperature spikes too high, the element shuts off then it slowly goes down again. If I turn the temperature from 10 to 1, I want the temperature to go down as fast as reasonably possible to avoid burning something, and never overshoot.

Triac would be nice, I don't understand why its not used or why it took so long (if modern cooktops are actually using it now).

[andy2000]

...

Perhaps keeping the tube warm using a separate supply would be one solution, but magnetron heaters are quite power hungry.  One example I saw was 3.5Vac at 25 amps.  So the additional supply for this is probably not worth it for such a mild improvement.  It is just derived off the main transformer....

There were a few microwaves that had a separate filament transformer so they could keep the filament hot while pulsing the HV rapidly.  My Amana "Radarange" from the 1970s cycles at about 1-2 Hz, which is fast enough to be useful.  The filament transformer is quite large, so I'm sure it adds significant cost. 

[ogden]

Microwaving is the most healthy way to cook certain foods, such as vegetables, because it doesn't excessively heat them or add fat like frying does or require them to be submerged in water which washes out some of the nutrients.

Try steaming. You will never use microwave for vegetables again. All you need - steaming insert for pot you already have. It is hard to manage heat your food receive in microwave because it is proportional to mass/water_content/power ratio.  I want to eat, not math class. Pot, electric oven and multipot are much better at actual cooking. Not to mention pan which as you already mentioned, is uncontested by microwave. Disclaimer: I do love microwave and use it often, but not for cooking.

ok that does seem like a poor design.

It looks like poor design or even failure indeed. My German (LOL) induction stovetop have around 2sec period and it is completely fine. Thou you want heavy pan anyway. For induction stove tops heavy pan is *very* important - because such most likely will have heavy iron inlay which will result in much better heating performance. Well... honestly heavy pan will perform better on any kind of stovetop. Thin pan is good only for high temperature searing but for such you need gas burner.

[soldar]

Do you pay for reactive power?

Good question. As a home consumer I do not pay for reactive power as such ... but (1) there will be some losses in the wiring, maybe a watt or a few watts, that I will pay for and (2) we pay for "capacity" which means the maximum power you can draw at any one time. But this is not measured in terms of real power, it is just a circuit breaker that trips by current. So suppose I have 10 A allowance, if I have some parasitic reactive power connected then I am decreasing the max power I can pull at any one time. And they charge dearly for this power allowance capacity part because it means you pay even if you don't consume at all. You have a second home where you only go a few weekends a year? You will still be paying through the nose.  The crooks in government have us well screwed.

I do not think there is any excuse for a device that is turned off to be using current like that. I have not determined if it is capacitive or reactive. I just see the current in a meter.

So I keep that induction plate disconnected most of the time and hardly use it. I also have a theory that it is not good to apply too much power to a pot because it will deform. The induction plate has a max setting of 1600 W but I would not like to use it above say, half that but I would want it continuous, not pulsing.

Regarding temperature response time:

Well thats a huge problem to me, they heat up, temperature spikes too high, the element shuts off then it slowly goes down again. If I turn the temperature from 10 to 1, I want the temperature to go down as fast as reasonably possible to avoid burning something, and never overshoot.

Gas I think has the fastest response time. OTOH, I don't have a problem with electric plates that have a large mass and longer response time. I just work around it. If I need to lower heat I just take the pot or pan off the plate. Not a problem at all.  In fact, I much prefer the longer response time of a hot plate than the bursts of the inductive plate because I can work around the first but there's nothing I can do about the bursts.

Well, now that I think about it, I do have a steel plate, about 30 cm square and 1 cm thick, that I can put on the induction stove and it just heats up and maintains temperature. But I have hardly used it because I just use the resistive plates and I am not sure if having a very hot plate right on the induction heater might be bad for it. I just avoid using it altogether.

Triac would be nice, I don't understand why its not used or why it took so long (if modern cooktops are actually using it now).

Well, yes and no. Having a single high power resistive element switching on and off seems like a bad design and you can easily set it to max and then control with a triac. But switching high loads with a triac for power control has bad power factor and introduces lots of noise in the line unless you filter really well. The best solution is what I have always seen here which is a plate with three resistive elements which can be switched in different configurations. I think pretty much all cooktops I have seen around here are like that.

[soldar]

get pans with more thermal mass?

Why? Better get a heat source that doesn't cycle.

how much does the temperature change in those few seconds?

So much that it makes frying an egg impossible. Most cooking that requires a certain dosage of heat needs it applied constantly and you can't cycle like that.

You can add thermal mass but I'd rather just cook on devices that don't have the problem in the first place.

That is one reason I only use the microwave oven for reheating things.

[Zero999]

Microwaving is the most healthy way to cook certain foods, such as vegetables, because it doesn't excessively heat them or add fat like frying does or require them to be submerged in water which washes out some of the nutrients.

Try steaming. You will never use microwave for vegetables again. All you need - steaming insert for pot you already have. It is hard to manage heat your food receive in microwave because it is proportional to mass/water_content/power ratio.  I want to eat, not math class. Pot, electric oven and multipot are much better at actual cooking. Not to mention pan which as you already mentioned, is uncontested by microwave. Disclaimer: I do love microwave and use it often, but not for cooking.

Steaming is still not as healthy as microwaving because it's above boiling point, but it does cook more evenly.

What do you use your microwave for if you don't cook with it?

[coppice]

Do you pay for reactive power?

No domestic consumers pay for reactive power, although introducing charges has been discussed in some countries. In some places some large industrial consumers either pay for reactive power, or pay a punishment factor if their power factor is too bad too much of the time.

In most countries there are strict controls over how electrical energy may be charged. For example, most European countries follow the Welmec practices for most forms of legal metrology, including electrical energy.

[ogden]

Steaming is still not as healthy as microwaving because it's above boiling point, but it does cook more evenly.

Steaming in regular pot is precisely at the boiling pot point, at max. I did not advise to use pressure pot to steam vegetables BTW do you know about microwave hotspots - reason why plate or special reflector shall rotate?

What do you use your microwave for if you don't cook with it?

Heating food which for some reason is below temperature of tasty consumption.

[thm_w]

Yeah I'll agree with ogden here, steaming is going to be ~100C max no matter what.
Put a piece of cheese in the microwave for 10 minutes and let me know how black it gets.

Not that the microwave is unhealthy in any way. But generally you need to get up to 150+C? to start running into issues. ie frying, deep frying, BBQ, using trash oil like soybean, etc.

Microwaving did not raise dAGE content to the same extent as other dry heat cooking methods for the relatively short cooking times (6 minutes or less) that were tested.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3704564/
https://suppversity.blogspot.com/2014/12/the-quest-for-optimal-cooking-oil-heat.html

[Red Squirrel]

Microwaves arn't really good at cooking nor are they really meant for it.  They are for reheating.  For real cooking you want a source of radiating heat like from a fire or a hot stove element, or even just hot air/steam.   The way microwaves cook is just different and you'll never get the same kind of result from it.   It also depends on the type of food though, some foods it won't matter while others it will. 

Putting eggs in the microwave for example is a really bad idea (but it's fun as long as it's not your microwave.  )

[Zero999]

Steam is above 100°C.

There are plenty of foods which microwave well. Of course putting a whole egg in the microwave will end badly, but scrambled egg is fine and the result will be indistinguishable from the stove. I also microwave porridge, which is more efficient than the stove for a single portion.

There is one food I only will cook in the microwave: onions. I stab the onion with a knife a few times, put it on a plate and zap it for a couple of minutes at full power. I leave it to cool for a bit, squeeze the onion and the cooked bit in the middle pops out. If I want to brown it, I'll put it in a frying pan or under the grill for a couple of minutes. I do this because I react badly to chopping raw onions and blasting them in the microwave neutralises them, making them safe to cut up.

[ogden]

Steam is above 100°C.

Compare water boiling point (99.97 °C at 101.325 kPa) with steam equilibrium data and you will see that you are mistaken: https://en.wikipedia.org/wiki/Water_(data_page)#Water/steam_equilibrium_properties

[Zero999]

Steam is above 100°C.

Compare water boiling point (99.97 °C at 101.325 kPa) with steam equilibrium data and you will see that you are mistaken: https://en.wikipedia.org/wiki/Water_(data_page)#Water/steam_equilibrium_properties

Yes, I was mistaken. I knew that chart and that water never exceeds 100°C. What I missed and I've now realised is: as long as there's boiling water, underneath the steam, then the temperature of the steam will never exceed 100°C, because the boiling water protects it from further temperature rise. It isn't the same as steam inside a conventional oven, which can easily exceed the boiling point.

I remember having this sort of discussion with a colleague who would always hold the switch down on the electric kettle after it had already boiled, in the hope to heat the water more. I explained to him that all it does is waste energy and produce more steam. At the time he didn't seem to accept it, but I did notice he didn't bother the next time he boiled the kettle, it might have sunk in.

Anyway, I can't say that I'll bother with steaming my vegetables, as I don't think it's worth the bother for one person, but if I'm cooking for more than two, I certainly will.

[ogden]

I remember having this sort of discussion with a colleague who would always hold the switch down on the electric kettle after it had already boiled, in the hope to heat the water more.

LOL BTW you can tell him that holding switch down may work when water is completely evaporated and steam is all that's left.

Anyway, I can't say that I'll bother with steaming my vegetables, as I don't think it's worth the bother for one person, but if I'm cooking for more than two, I certainly will.

Remember please - microwaving is not cooking [kidding] What you eat? Even - chicken needs more time than vegetable steaming. Small pot + glass of water = vegetables for two in < 30 minutes. Just saying. It's like thread about 60fps versus 120fps monitors where people who never experienced 120fps, have strong opinion that it sux

[Zero999]

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.

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. I like the convenience of being able to put the vegetables in a bowl or mug, microwave them at full power for a minute, stop, stir and microwave them for another minute.