Dc motor has a rapid response, large starting torque, rotating speed of from zero to rated speed can provide the rated torque performance, but the advantages of dc motor is also its drawback, because the direct current machine to produce a constant torque under the rated load performance, the armature magnetic field and rotor magnetic field must be constant to maintain 90 °, this is about to borrow from the carbon brush and commutator. As the brush and commutator produce sparks and powdered carbon when the motor is turned, the use of the brush is restricted in addition to damage to the components. The ac motor has no carbon brush or commutator and is maintenance-free, robust and widely used, but it requires complex control techniques to achieve the performance of the dc motor. Nowadays, the semiconductor develops rapidly, the switching frequency of power component is accelerated, and the performance of driving motor is improved. The speed of microprocessor is also getting faster and faster, which can realize the control of ac motor in a rotating two-axis linear ac coordinate system, and properly control the current component of ac motor in the two-axis, so as to achieve the similar control of dc motor and have the same performance as dc motor.