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 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.
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.