Countermeasure for instability in voltage source inverter fed field oriented synchronous machines without using a position sensor

A particular instability phenomenon as well as the countermeasure in a voltage source inverter-fed synchronous machine with a field orientation control circuit without using a position sensor. A superposed control circuit generates the reference values of the rotor current and the field-oriented stator voltage vector. The necessary signals for the generation of these reference values are calculated using a current model which is fed by the reference values of the currents. Due to the utilization of the load angle signal calculated in this way, the torque component of the stator current is strongly coupled with the flux, which results in an operation range where, under normal excitation, the machine is not able to work stably at the desired operation point. This instability is independent of the parameter tuning and the orientation correctness. The required decoupling is realized by feeding back of the actual flux signal to the control unit, and the unstable operation range will be eliminated