Start-up of sensorless electronically commutated motor

Abstract

The paper presents the EKM take-off method without a rotor position sensor. The method is adapted for integration into already existing methods of sensorless work of EKM, which are characterized by simplicity and efficiency, but do not have a well-solved start-up. The developed method consists of several steps. By exciting the two phases of the stator, the rotor is brought to a known position. Then follows the determination of the moment of the first commutation by means of a special test that is performed before the actual take-off. The run starts from a known position, and the first switching is performed at the moment determined by the previous test. After the first switching, the EKM is controlled by a sensorless algorithm based on the detection of the passage of the EMS inactive phase through zero. As part of the theoretical research, a mathematical model of the motor was set up, which is correct regardless of the choice of commutation moments, and as such represents an improvement with respect to the existing EKM models. During the initial research, it was determined that the models used so far are correct only in the case when the commutation takes place at ideal moments. Since the first commutation in the proposed run-up is atypical, it was necessary to set up a model that is correct for every switching state regardless of the mutual position of the MMS stator and rotor. Based on this model, a simulation model was created in the MATLAB - SIMULINK software package, using which the proposed method sensorless start-up was investigated by simulation in different electromechanical conditions. In particular, the influence of the load torque on the start-up was investigated. Based on the simulation results, it was concluded that the proposed sensorless start-up is usable for drives with different torque characteristics. In order to theoretically investigate the dynamics of the ECM during the first switching state a linearized mathematical model of the system consisting of a rotor and two phases of the stator winding was used. Based on this model, an analytical expression for the moment of the first commutation was derived, which, as it turned out, is in accordance with the experimental and simulation results. The method was experimentally tested on a model built on digital signal processor (DSP) and intelligent power module (IPM) platforms. In order to avoid current stresses of the IPM, the start was made at a reduced voltage of the DC intermediate circuit. The experimental results of the take-off are shown in the engine unloaded mode and at 25% of the nominal load torque. The obtained results approximately correspond to the simulation results, and confirm the applicability of the method for different applications. Certain limitations and shortcomings of the method during experimental research occur due to the fact that in the laboratory plant, the run is integrated with the sensorless method, which does not have a built-in correction mechanism. All measurements important for start-up and for sensorless control are performed on inactive phases, which indicates that all conclusions derived from the conducted research could also be applied to drives that would be controlled by pulse-width modulation. With the expansion of the research in the proposed direction, the integration of the proposed start-up and some of the quality sensorless guidance methods that do not intrinsically solve the start-up problem, represents a good basis for the development of simple and efficient sensorless drives with EKM.

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