A new direct rotor field oriented scheme, using the third harmonic component of the air gap flux

The paper presents a new direct rotor field oriented scheme for induction motor, based on determination of the position and amplitude of the air gap flux, from the third harmonic component of the stator voltage. This field control utilizes spatial saturation harmonic components rotating at synchronous frequency that are generated in the air gap flux when the induction motor is in a saturated condition. This method is used when the induction machine is wye connected because the sum of the three phase voltages results in a signal dominated by the third harmonic component. The scheme uses a minimum number of sensors. The controller eliminates the flux sensing coils placed into the air gap on the wedges of the stator slots (or Hall elements in the air gap). This direct method of the flux estimation, in comparison with other direct or indirect methods, has the quality to be insensible to changes in the stator or rotor parameter´s values. Simulated and experimental results showing the nature of the third harmonic voltage and the responses of the oriented scheme are presented