Why is 3 phase motor neutral not used?
A 3 phase motor does not require a neutral conductor because the impedances that make up the stator coils are of equal magnitude and it is what is known as a balanced 3 phase load. If the loads of a wye connected 3 phase system were unbalanced, i.e. the load currents in each of the 3 phases are different, then the unbalanced currents would require a return path to the service transformer via a neutral conductor. The neutral conductor is the transformer centre tap and is usually earthed locally.
The impedance of each phase winding of a three phase motor is identical (we call it a 'balanced load'). For a star (wye) connected motor, the phasor sum of three identical phase currents is zero, so no current will flow through a neutral, so a neutral isn't required. For a delta-connected motor, there is no provision for a neutral.