Inverter Microwaves (not a product recommendation thread)

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sdav

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#269895 13-Apr-2020 12:48

Hey all,

I've been thinking about my inverter microwave because at home we had a discussion about it... It must be day 19 of level 4.

Anyway, my question is around the electronics side of inverter microwaves and clarifying the meaning of inverter in a microwave. This is my basic understanding so far.

The inverter microwave converts AC wall power to DC inside the inverter circuit. This provides the magnetron with a constant power at various levels and enables even cooking. I get that bit (I think!). What stumps me is why are they called inverter microwaves? Doesn't inverting only apply when converting AC > DC and not the other way round? Is it a different use of the word inverter that I'm unaware of?

I tried to look for a simple explanation of this but wasn't easily able to figure it out.

Thanks all.

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#2460486 13-Apr-2020 12:55

because a "Rectifying Microwave" sounds funny? ;-)

EDIT: Facetiousness aside, this actually looks like it has a pretty good explaination: https://media.datatail.com/docs/manual/371449_en.pdf

Looks like the inverter bit of the name refers to what they do after rectifying the AC to DC.

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JeremyNzl

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#2460490 13-Apr-2020 12:59

Happy to be wrong,

I understood the non inverter models would do reduced wattage at full power with a shorter on cycle. The inverter offers a more consistent result, that is tuned to the power setting required.

boosacnoodle

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#2460528 13-Apr-2020 13:59

I find the performance to be much better with even cooking. The model I have also has no physical moving parts inside and so is much easier to clean. It's a commercial model.

Delphinus

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#2460532 13-Apr-2020 14:03

JeremyNzl:

Happy to be wrong,

I understood the non inverter models would do reduced wattage at full power with a shorter on cycle. The inverter offers a more consistent result, that is tuned to the power setting required.

Based on my porridge observations I would back this up. Cooking porridge in an old one a while ago I would put it on 50% power and watch it going full blast with the porridge boiling/rising in my bowl, then the tone of the microwave would change for 5-10 seconds and the porridge lava would subside, until the tone changed again back to full power.

richms

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#2460536 13-Apr-2020 14:07

Delphinus:

Based on my porridge observations I would back this up. Cooking porridge in an old one a while ago I would put it on 50% power and watch it going full blast with the porridge boiling/rising in my bowl, then the tone of the microwave would change for 5-10 seconds and the porridge lava would subside, until the tone changed again back to full power.

Yeah, they constantly are switching with high startup loads so really hard on generators etc, with that old fashioned magnetic transformer they are really hard for inverters to drive too. Im guessing they picked up on the name inverter from other things that use it like fridges and aircons where it means much better efficiency and control vs old induction motor type things.

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TwoSeven

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#2460561 13-Apr-2020 15:17

I understood the inverter part refers to dc input inverter rather than a transformer, the output is fed into a cavity magnetron.

I am not an electronics person but I believe them to be pwm inverters.

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#2460605 13-Apr-2020 15:40

An inverter in common parlance converts DC to AC, not vice versa. Apparently these microwaves have an inverter instead of a transformer. I suspect this has something to do with oscillating at microwave frequencies. And they use substantially less electricity. (For us old timers, the original microwaves were amazing because they used so little electricity).

From Panasonic's website, it appears that an inverter microwave can reduce its output power somehow, whereas non-inverter microwaves are just on/off, and they approximate the desired power level by turning on for a few seconds, then off (Pulse Width Modulation in techspeak).

sdav

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#2460616 13-Apr-2020 15:50

sidefx:
because a "Rectifying Microwave" sounds funny? ;-)

EDIT: Facetiousness aside, this actually looks like it has a pretty good explaination: https://media.datatail.com/docs/manual/371449_en.pdf

Looks like the inverter bit of the name refers to what they do after rectifying the AC to DC.

Thank you, I did actually see that article and agree, it was super helpful. It was obviously aimed at someone that knew what all those terms meant so I thought I would double check my understanding. Essentially, there is converting and rectifying happening to the AC signal but no inverting? Why would a DC signal need inversion after rectifying? My assumption is the other components require a signal that the inverter can supply that rectifying cannot.

Agree “rectifying microwave” or “converting microwave” might sound a bit strange in the marketing department!

Thanks as always for the replies folks.

sidefx

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#2460869 14-Apr-2020 08:10

sdav:

Thank you, I did actually see that article and agree, it was super helpful. It was obviously aimed at someone that knew what all those terms meant so I thought I would double check my understanding. Essentially, there is converting and rectifying happening to the AC signal but no inverting? Why would a DC signal need inversion after rectifying? My assumption is the other components require a signal that the inverter can supply that rectifying cannot.

Based on the diagram on the beginning of page 3 (figure 1) and some of the wording, I think they rectify to DC, then re-invert but to the frequency required by the magnetron (instead of using the standard 50 or 60 Hz you get from the grid combined with a transformer?) - this allows them to drive the magnetron continuously at different power levels instead of the transformer + start\stop approach used by conventional microwave?

Disclaimer: No idea if this is right; I studied electrical engineering over 20 years ago but have not done a thing in it since ;-)

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SomeoneSomewhere

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#2460984 14-Apr-2020 12:27

My understanding is that to vary the power output of the magnetron, you need variable frequency AC. So the power flow is mains DC > rectifier > DC bus > inverter > variable frequency AC > transformer > magnetron.

Designing for an output frequency in the kHz allows the transformer to be much smaller for the same power frequency, as long as the magnetron is also designed for the same frequency range.

This is the same as other electrical equipment described as 'inverter', particularly aircon units. They output a variable-voltage variable-frequency (VVVF) supply to the motors allowing efficient slower-speed operation. Again, mains > rectifier > DC bus > inverter > load. They also have the advantage of generating 3P AC from a single phase source.

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#2461004 14-Apr-2020 12:40

I understand cavity magnetrons use pulsed dc (or sometimes rectified ac).

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#2461006 14-Apr-2020 12:42

Actually, that's a fair point. I think there is a rectifier on the transformer output of a normal microwave.

Perhaps it's effectively an isolated DC boost converter then.

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#2461080 14-Apr-2020 13:42

SomeoneSomewhere:

My understanding is that to vary the power output of the magnetron, you need variable frequency AC. So the power flow is mains DC > rectifier > DC bus > inverter > variable frequency AC > transformer > magnetron.

Designing for an output frequency in the kHz allows the transformer to be much smaller

(and lighter and cheaper). That's right. Aircraft use 400Hz power for the same reason.

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sidefx

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#2461082 14-Apr-2020 13:43

SomeoneSomewhere:

My understanding is that to vary the power output of the magnetron, you need variable frequency AC. So the power flow is mains DC > rectifier > DC bus > inverter > variable frequency AC > transformer > magnetron.

that was sort of my understanding from that document, but more like:

mains AC > rectifier > DC > INVERTER > variable frequency AC > magnetron

So I was under the perhaps mistaken impression from that document that the transformer wasn't required with the inverter circuit design and that's how they get the higher efficiency (because transformers are pretty inefficient at the high frequencies required to generate microwaves?)

(It truely must be Day 20 of lockdown :D )

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elpenguino

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#2461084 14-Apr-2020 13:45

SomeoneSomewhere:

Actually, that's a fair point. I think there is a rectifier on the transformer output of a normal microwave.

Not that I recall, for powering the magnetron anyway. I have an old PSU in the garage and there's just a whopping transformer followed by a voltage doubler/tripler type unit as seen in the old CRT TVs.

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