Measuring 6V AC Voltage with Arduino to determine wheel speed (of dynamo)

I'm trying to measure AC voltage from a dynamo hub (6V 3W AC), I really just want some method of tracking how many peaks there are and extrapolating the wheel speed. For now I'm going to power the arduino separately, I have read that negative voltages may damage the arduino, so I'm not sure how hooking it up to the AC hub would work. Any help would be great.

gime: I really just want some method of tracking how many peaks there are

Then you want to measure the frequency. It's easy with a zero-cross detection circuit connected to Arduino. There are many examples on the web.

Do you have any information on the hub ? (a link or datasheet ?)

I'd be inclined to rectify the AC, then use that through a hall switch to get a pulse each rev of the wheel. Much easier and much more accurate.

Optocoupler with series resistor and reverse diode across its input, output to Arduino using internal pull-up.

Fully isolated, output at dynamo frequency with not far from 50% duty cycle.

Do not be tempted to use interrupts for such a trivial function. Just poll it.