Two-axis theory

A theory based on DC separate-field motor, to deal with the behavior and control of an AC motor. The direction of the field flux (also known as main flux) is the direct axis, and the direction of the magnetic flux due to armature winding is the quadrature axis. It is regarded that in an AC motor the magnetic field is rotating, and the 3-phase currents are converted to the direct and quadrature component according to the following equation.