The purpose of vector control is to drive an AC motor with an inverter, which is a variable voltage and variable frequency power source, to achieve variable speed drive. The essence of the method consists of three stages. In the first stage, the three-phase AC currents supplied to the motor are detected and converted to a pair of two-phase DC currents by three-phase to two-phase transformation in the controller. One of these two currents corresponds to the field current and the other to the armature current, respectively. The respective reference values for the both currents are given to the controller. In the second stage, some kinds of controls are applied so that the actual values calculated in the first stage match these reference values. The resultant outputs of the control are reference values for the field voltage and the armature voltage. These are also DC quantities. In the third stage, these two DC voltage reference values are given to the inverter as three-phase AC voltage reference values by a two-phase to three-phase transformation. All the currents and voltages in the controller are recognized as vectors. The origin of the name vector control lies here. The above is the essence of vector control.