It depends where you measure. If you measure across the inductor, it absolutely goes negative.

The frequency is generally fixed, the duty cycle will vary.

A variable speed drive can be fed with DC. Is the output AC or DC? I know you need a three phase AC motor to wire up to it.

Is audio DC? It doesn’t have a fixed frequency. Amplifiers pulse DC and then remove or ‘block’ the DC offset so speakers see AC.

It seems like people in this thread have a very strict definition of AC being a 60Hz sine wave, and everything else must be DC.