yea, they have 13 'loops' in each phase, so that is 39 poles? maybe... lets try some math
i think electric motors usually run at 1750 or 3500 which is double that, and then with a a pulley you could change that but lets say 3500 since i don't know the ratios
39*3500/60 = 2275 Hz, what happens when you rectify that? doubles? so your looking at 5k Hz ... in the range... didn't count the pulley ratio, maybe i did all this math wrong but it kind of makes sense.
Does anyone have an frequency measurements off their alternator, for curiosity sake?
I know sometimes if you don't use carbon resistor spark plugs in your car you can hear sound coming from the alternator picked up in the audio system in your car, it produces a frequency in the audio range that interferes with the electronics, the spark plug wires act like antenna.