why ac current has variable frequency and voltage? I din't understand about hertz. please make me clear about it?

The amount of voltage generated is varied by the process of moving wire in a magnetic field, as a coil of wire is rotated. the faster it rotates, the higher the frequency (hertz). Current then follows what the voltage does when applied to a load. Current is dependent on the electrical load placed on the voltage available. If there is no load, current is zero, regardless of the voltage.