now in AC current its drawn like a sin wave signal so half of wave in positive phase and the other half starts to be in negative and so on ... so it means the change of current direction right ? so why that doesnt change the rotation direction in AC motors
Because AC motors are designed to know about this and operate accordingly.
In an AC induction motor the chancing phase of the supply current is used to create changing polarity in the motor magnetic field. If the rotor is started to spin, it synchronizes itself with the changing field of the stator and will continue to spin. This is an over simplification of the process but holds true in this case.
Your question is why does the motor spin instead of vibrate back-and-forth. The answer is that when the voltage is changing to negative, the motor rotor has spun half-way around so that the rotor is presenting the opposite magnetic polarity to the stator coil, just in time as the coil reverses its magnetic polarity. If it's a single-phase motor, the motor could run in either direction except that either the stator has one pole shaded, or a capacitor is added, so that there is a slight tendency for the motor to always start rotating in the same direction.
