Direct flux control – sensorless control method of PMSM for all speeds – basics and constraints

The limitations of sensorless control of permanent magnet synchronous machines (PMSMs) are discussed and a viable solution is proposed. The main concept of sensorless control of drives relies on additional information given by the machine during its normal operation. This information provided by the machine is essentially the back-electro motive force and the variance of the stator inductivity, which are dependent on the rotor position. Several approaches and methods have discussed these problems, and in most cases they are not avoidable and that some methods work better on certain speeds of the drives. The direct flux control (DFC) method to combat the above problems at all speeds is presented. The flux linkage signal which contains the necessary information about the rotor position can be measured between the neutral point of a PMSM and an artificial one. The mathematical derivation and the observations from the experiments show that this signal contains a second and a fourth harmonic, which can be used to calculate the rotor position. Furthermore, the limitations of implementing DFC are also addressed