Fig. 3 shows, that in case of a disorientation of the esti- mated coordinate system F ˆ in comparison to the „real“ dq coordinate system F the anisotropy of the AC machine re- sults in an “imaginary” (orthogonal) component of the high frequency carrier current ic , which does not occur as long as the estimated coordinate system F ˆ is in agreement with the „real“ dq coordinate system F. The orthogonal current com- ponent ic can be used to adjust and correct the orientation of the estimated coordinate system F ˆ to the „real“ dq coordi- nate system F. The imaginary (orthogonal) component of the current ic is directly proportional to the estimation error a Δδ and can consequently be used to adjust the orientation of the injected high frequency voltage signal uc by a closed tracking control loop (see Fig. 4 – [15]).
There is no need for any additional hardware being not available in an industrial drive anyway, and there is no im- pact of any machine or drive parameter on the rotor position control loop.
To realize an encoderless drive control in an industrial drive the field oriented control of a standard industrial drive must be completed by a PLL structure for the estimation of the rotor position. The respective software effort can be com- pared with the rotor model for the field orientation of an induction machine. Standard microcontrollers being able to realize a complete field oriented control are therefore suffi- cient for the encoderless control described here.