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.
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.
Wow. This question is a good but very specific observation, and I did not expect an actual answer.
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.
Why do microwaves hum
Because they don’t know the words, obviously
you’re hearing the magnatron’s power supply
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.
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).
Interesting. I saw mostly 38Hz, 117, and 240. 100W LG microwave on US 60Hz with an audible fan.
