Dual magnetron microwave oven, are they efficient?

[Bendba]

Hi,

Most is in the title, are dual magnetron microwave oven efficient, compared to a normal single magnetron.
Are there any interference between the two magnetron other than the same constructive/destructive effect you get from the reflected beam in a normal oven?
I know that there is some effort put in researches aiming at building semiconductor microwave emitters to replace the cavity magnetron and they appear to be based on having multiple emitters for one oven cavity. So I'm assuming the principles are the same.

Also, cavity magnetron do not work as antennas, is that right? So the two devices wouldn't influence each other, would they?

Thanks

[N2IXK]

The antenna is an integral part of the magnetron. The little stub that sticks into the waveguide is the antenna.

I think the dual magnetron ovens are just a way to get more cooking power from the standard mass-produced cheap magnetrons, rather than designing special higher power tubes for the smaller market.

[stj]

probably right, the big magnetrons for catering ovens cost a fortune.

[Zero999]

It shouldn't be a problem. Usually the magnetron is operated with a half wave voltage doubler, so the duty cycle is under 50%.


http://www.microtechfactoryservice.com/doubler.html

I imagine a dual magnetron microwave will fire the magnetrons alternately, giving near 100% duty cycle. The circuit will look similar to the above, but the transformer will have another anti-phase winding, to power the other magnetron.

[Bendba]

Hi,

Sorry, I didn't phrase that properly, I meant receiving antenna, as in having the ability to turn electromagnetic wave into electric current as a dipole does. If you aim a powered magnetron (1) to another magnetron(2), you won't measure any current at magnetron2's connector, even if the heater is powered. It that correct?

I'm just wondering if two 750W magnetrons would be the equivalent of one 1500W one. If not, where are the losses?

Also wondering how significant the RF efficiency difference is between two magnetron from different factory. Would it be so bad that a "750W" brand microwave oven could actually be more powerful than a "1000W" cheap one?

I was about to post when I saw the other messages.

It shouldn't be a problem. Usually the magnetron is operated with a half wave voltage doubler, so the duty cycle is under 50%.


http://www.microtechfactoryservice.com/doubler.html

I imagine a dual magnetron microwave will fire the magnetrons alternately, giving near 100% duty cycle. The circuit will look similar to the above, but the transformer will have another anti-phase winding, to power the other magnetron.

That'd make sense. And that would explain why I never saw a triple magnetron microwave oven.

[SeanB]

There is no difference between a 650, 700, 750 and 1000W microwave magnetron wise, they all use the same size magnetron, the same transformer and rectifier and the only difference is the high voltage capacitor, which controls the current in the magnetron. All are roughly 700W input power, and around 500W of RF power applied to the cavity. The dual magnetron units just get higher power, and as you have some form of stirrer in there ( either the food rotating on a turntable or an antenna reflector in the roof turned by either airflow or a small motor) they move the RF energy around inside the cavity, giving a general overall roughly even field in the cavity.

The magnetron’s are pretty robust, and so long as they are cooled enough they handle a very large load imbalance, even surviving the worst case test of no load in the oven, which results in almost all the RF energy being reflected back into the magnetron. Multiple magnetrons, so long as they do not directly feed into the slot antenna cavity ( the thing the magnetron sticks the purple end into) of each other, will survive so long as there is some load in there, or at least some spreading out of the RF energy.

There are some industrial units that use multiple commercial magnetrons, and properly rated power transformers ( around 3 times the mass of iron and real copper in the windings, so they actually are run at a surface temperature under 130C), to provide RF curing of things like veneers and laminates, and these are all firing into a common cavity with stub filters at the material inlet and outlet ( the cavity at the door, that is a quarter wave stub at 2.4GHZ and thus acts like a reflector) so you just run the stuff through and it gets heated and cured. They have magnetron cooling fans that make servers sound quiet, and move a lot of air through those fins to keep them cool.

[Bendba]

So,

I always thought that it was strange that they always appear to be the same size and weight. I just thought that the power capability of the magnetron itself was due to some imperfections in production process and then they would match the capacitor they need to limit the current.

So, if you were able to provide sufficient cooling and had a proper transformer outputting double the voltage, could you use full rectification and near double the RF power output?

Well, more like double the duty cycle, not the instantaneous power.

[Zero999]

I don't understand the question. You can have multiple heating elements in a conventional oven, so why would it be any different for a microwave?

Oh and by the way, at least here in the UK and possibly the rest of the EU, the power rating specification of a microwave is always the output power, not the input power.

[Bendba]

I don't understand the question. You can have multiple heating elements in a conventional oven, so why would it be any different for a microwave?

Oh and by the way, at least here in the UK and possibly the rest of the EU, the power rating specification of a microwave is always the output power, not the input power.

Because conventional oven use thermal energy to heat the air, not radio frequencies to be absorbed by the food, so the heat of one element doesn't interfere with the heat of another one. Where with radio frequencies, there are all sorts of feedback, reflections, interferences to account for.
Try to simultaneously transmit 2x500W for an extended amount of time  with two UHF having their aerials a couple of meters apart, they are not going to be happy.

If there was a sufficient about of food to absorb the energy and the two magnetrons don't aim at each other, that probably wouldn't matter but with a near empty oven muggy be a very different story, thus my question.

[Zero999]

I don't understand the question. You can have multiple heating elements in a conventional oven, so why would it be any different for a microwave?

Oh and by the way, at least here in the UK and possibly the rest of the EU, the power rating specification of a microwave is always the output power, not the input power.

Because conventional oven use thermal energy to heat the air, not radio frequencies to be absorbed by the food, so the heat of one element doesn't interfere with the heat of another one. Where with radio frequencies, there are all sorts of feedback, reflections, interferences to account for.
Try to simultaneously transmit 2x500W for an extended amount of time  with two UHF having their aerials a couple of meters apart, they are not going to be happy.

If there was a sufficient about of food to absorb the energy and the two magnetrons don't aim at each other, that probably wouldn't matter but with a near empty oven muggy be a very different story, thus my question.

Well that makes sense but as far as I'm aware the two magnetrons are not powered simultaneously but alternately.

[Cerebus]

( the thing the magnetron sticks the purple end into)

"Ooh, Matron!"

[Bendba]

Well that makes sense but as far as I'm aware the two magnetrons are not powered simultaneously but alternately.

Thus my answer after you first message:

That'd make sense. And that would explain why I never saw a triple magnetron microwave oven.

[PCChazter]

That'd make sense. And that would explain why I never saw a triple magnetron microwave oven.

Actually, 3 magnetron microwaves exist. Some of the commercial microwaves used by fast food restaurants have 3 in them. I worked on one that was rated for 2200W, and it had the magnetrons all in their own channels, kind of staggered so that the RF would reach the antennas at different rates. I don't know what each magnetron was rated at, so I can't answer your question as to whether or not they were adding their power proportionally.

Edit: they are not staggered, one is on the bottom. The model was DQ22HS if you're interested in looking it up

[Zero999]

That'd make sense. And that would explain why I never saw a triple magnetron microwave oven.

Actually, 3 magnetron microwaves exist. Some of the commercial microwaves used by fast food restaurants have 3 in them. I worked on one that was rated for 2200W, and it had the magnetrons all in their own channels, kind of staggered so that the RF would reach the antennas at different rates. I don't know what each magnetron was rated at, so I can't answer your question as to whether or not they were adding their power proportionally.

Edit: they are not staggered, one is on the bottom. The model was DQ22HS if you're interested in looking it up

I was going to say three phase but just Googled and found it's single phase.
http://shared.whirlpoolcorp.com/assets/pdfs/literature/10492915.PDF

It has three separate magnetrons, transformers, HV capacitors and diodes.
https://www.manualslib.com/manual/898268/Amana-Dq22hs.html?page=24#manual

Perhaps the magnetrons will all lock to the most powerful one's frequency?