AC generator supplies AC current. To determine how many times the direction of an Alternating Current changes we have to find out about its frequency. Frequency of an AC current implies number of complete waves present in a second. If the coil completes one revolution in one second then the frequency of the AC generator is said to be 1 hertz. The given picture will clear the concept,

In the picture there is only one complete wave in a second, hence its frequency is 1 hertz. Which means that the coil has completed one revolution. Hence we can that in one revolution the current change its direction twice first positive than negative.