this post was submitted on 30 Nov 2023
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69
Why do many microwave ovens hum in an interval of a minor 7th? (lemmy.wtf)
submitted 11 months ago* (last edited 11 months ago) by [email protected] to c/[email protected]
43 comments fedilink hide all child comments
 

Something that I've noticed across most of the microwave ovens that I've used is that when they hum while cooking food, I can pick out 2 distinct tones. One of them is pretty clearly ~~60~~ 120 hz, the 2nd harmonic of the AC power frequency. The other is consistently a minor 7th above that (which would be somewhere around ~~106-108~~ 212-214 hz depending on the exact ratio). What causes this 2nd frequency to be produced?

Edit: after checking against a tone generator, the low frequency is actually 120 hz, double the grid frequency. The question is still the same, just an octave higher.

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[–] [email protected] 60 points 11 months ago (3 children)

There are two things making noise in the microwave when it's running. There's the transformer that's making the high voltage for the magnetron, which is you 120 Hz noise you're hearing.

The other noisy thing in the box is the fan. The fan is most likely a three blade metal fan running on a little shaded pole induction motor. That motor is very simple, just one moving part, and two poles. Two poles at 60 Hz gives us 3,600 RPM, which the motor can't quite reach because as the RPM gets closer to that magic number of 3,600 rpm, the motor draws lesa current and makes less power. This difference between the speed the motor is trying to run ("syncronous speed") and the actual speed is called the "slip" and is probably around 3%-5%. This gives us a fan speed between 3400 and 3500 rpm.

I don't know if that is giving you the second noise you're hearing, or if it's mixing with the transformer noise to make it. But now you've caused a situation where I'm going to be caught holding a guitar tuner to my microwave.

[–] [email protected] 10 points 11 months ago

He did the math!

[–] [email protected] 9 points 11 months ago

Can I give a second upvote to just the last sentence?

[–] [email protected] 2 points 10 months ago* (last edited 10 months ago) (1 children)

Wow. This question is a good but very specific observation, and I did not expect an actual answer.

[–] [email protected] 2 points 10 months ago

I don't know much about music or why some sounds are more pleasant than others... but I do know about electric motors.

[–] [email protected] 25 points 11 months ago (1 children)

There is a fan running and then the actual microwave itself. If you lower the power setting, you can hear the fan stay on consistently but hear the microwave shut off and on. If the power is at 100%, the microwave runs the entire time.

[–] [email protected] 6 points 11 months ago

A few microwaves don't do that because they're able to continuously run the magnetron at less than full power by using an inverter instead of a transformer.

[–] [email protected] 16 points 11 months ago

Why do microwaves hum

Because they don't know the words, obviously

[–] CameronDev 15 points 11 months ago* (last edited 11 months ago) (2 children)

Could it be the motor?

The second harmonic from 60hz is 120, which is close to where your hearing it? For what its worth, i get a peak at 100hz, which for the australian grid would make sense?

I used this app: https://play.google.com/store/apps/details?id=org.intoorbit.spectrum

I dont see a 50hz peak, but 100hz was clearly represented.

[–] [email protected] 9 points 11 months ago* (last edited 11 months ago)

I checked against a tone generator, and realized that the lower frequency was actually 120, not 60, so I'd guess you're probably hearing that. That puts the higher frequency at 212-214, which I also checked and that range matches what I'm hearing (for 100hz, it should be at around 177-180).

[–] [email protected] 1 points 11 months ago (1 children)

Interesting. I saw mostly 38Hz, 117, and 240. 100W LG microwave on US 60Hz with an audible fan.

[–] CameronDev 2 points 11 months ago

Mine is also LG, not sure what its power output is, but i think its either 1000W or 1200W, yours must take forever to heat anything 😉.

[–] [email protected] 13 points 11 months ago

you're hearing the magnatron's power supply

[–] [email protected] 6 points 11 months ago

50 / 60 hz from the transformer under load?

[–] [email protected] 4 points 11 months ago

Do you also observe this when running the microwave on max power? Usually, when a microwave states its 800 watts but you can adjust it down, it actually only changes the duration that power is supplied. So 400 watts would be 50% of the time the power is supplied at max.

[–] [email protected] 3 points 11 months ago

I honestly have no idea, and never before knew that I do not.