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Microwave oven PWM frequency

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thm_w:

--- Quote from: soldar on August 12, 2019, 10:33:10 pm ---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.

--- End quote ---

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?


--- Quote ---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.

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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:

--- Quote from: tom66 on February 12, 2018, 06:51:22 pm ---...

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....

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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:

--- Quote from: Zero999 on August 12, 2019, 08:42:35 pm ---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.

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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.


--- Quote from: thm_w on August 13, 2019, 01:27:36 am ---ok that does seem like a poor design.

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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:

--- Quote from: thm_w on August 13, 2019, 01:27:36 am --- Do you pay for reactive power?
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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:

--- Quote from: thm_w on August 13, 2019, 01:27:36 am ---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.
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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.


--- Quote from: thm_w on August 13, 2019, 01:27:36 am ---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).
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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:

--- Quote from: langwadt on August 12, 2019, 10:42:31 pm --- get pans with more thermal mass?
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Why? Better get a heat source that doesn't cycle.


--- Quote from: langwadt on August 12, 2019, 10:42:31 pm ---how much does the temperature change in those few seconds?
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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.

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