Speed estimation and compensation for harmonics in sensorless wound rotor synchronous machines

The conventional approach to control synchronous machines without a motion sensor in medium and high speed range is model-based and evaluates the electromotive force (EMF). In this case the information about the rotor position is included in the fundamental wave of the EMF and therefore the presence of harmonics will distort the estimation result. In order to compensate for harmonics information about their frequency is a prerequisite. In the case of wound rotor synchronous machines it is possible to obtain an accurate estimate of the speed by tracking the effect of the stator slots in the rotor current. For the rotor winding the stator slots represent a position dependent reluctance variation and therefore the rotor current will contain a harmonic component which is related to the rotor speed. The frequency of this harmonic component is tracked using an adaptive tracking filter and is then used to compensate for harmonics in the stator currents. The compensation is done by using adaptive resonant controllers. The suggested approach is verified experimentally by a highly utilized machine which is designed for automotive traction applications and features an extra-low voltage DC-link.