Research On The Electromagnetic Field Theory And Sensorless Technology Of High-speed BLDC Motors

High speed motors become a key technology in many applications, and is the hot issue in thefield of high speed motors because of its high power density, high efficiency, simple control and so on.The high-speed BLDC motor drive system has been widely used in the spindle drive system and theturbine mechanical system, such as aerospace, automotive, mold, semiconductor processing,electronic manufacturing, turbocharger, turbo air compressor and gas turbine generator etc. The keyproblems of the high speed BLDC motor are reflected in that the centrifugal force and the temperatureraise impact the mechanical properties and magnetic properties, position sensor can’t meet therequirement of high speed running and the contact bearing limit the highest speed. In view of theabove problems, many scholars have launched the aspects of the research that are the rotor eddycurrent loss analysis, rotor dynamics analysis, rotor stress analysis, sensorless technology and bearingsupport technology.This paper does research about the rotor eddy current loss analysis and the sensorless technology.Firstly, this paper establishes the BLDC motor electromagnetic field mathematical modelsincludes no-load magnetic field model, the armature electromagnetic field mathematical model andthe load electromagnetic field mathematical model, on the base of the existing literature. Thereference of this electromagnetic field mathematical model is the static coordinate system. It’s aharmonic analysis method, based on double Fourier series in the complex domain and get theelectromagnetic field distribution by solve the diffusion equation derived from Maxwell equations.The proposed models are general that means they are applicable to different motor configurations.Analytically calculated field distributions from both models are compared with finite elementpredictions for a slotted motor having a radial magnetized rotor and overlapping stator windings.Secondly, this paper establishes the rotor eddy current loss analysis model based on theelectromagnetic field mathematical model and Poynting’s theorem. According to this model, theinfluence factors of the rotor eddy current loss, such as the material electromagnetic characteristics,the width of slot opening and the copper shielding layer thickness, are researched. The research resultslaid theoretical foundation for the optimal design in order to reduce the rotor eddy current loss. Theanalytically calculated results are compared with the finite element simulation results. The comparedresults verify the correctness of the mathematical model.At last, this paper presents a novel sensorless technology in high-speed BLDC motor control demand. Based on the characteristics of the back electromotive force (EMF), the rotor position signalswould be constructed. It is intended to construct these signals, which makes the phase differencebetween the constructed signals and the back EMFs be controllable. Then, the rotor position error canbe compensated by controlling the phase difference angle in real time, and the opening angle can becontrol according to the control demand. Furthermore, a new method was proposed in this paper forthe permanent magnet brushless dc (BLDC) motor speed estimation. Based on the orthogonalcoordinate transformation, two orthogonal sine and cosine waves can be constructed from thetrapezoidal back electromotive force (EMF). It brings the opportunity for the rotation speedestimation. The proposed speed estimation method does not need the timer to calculate the speed,which is necessary for the traditional speed calculation, and not to be based on the assumed conditionthat the rotation speed does not change significantly over a certain period of time.Based on the theoretical analysis, this paper design the high speed BLDC motor whose ratedspeed is60krpm and rated power is2.5kW. The prototype is tested by no-load experiment and loadexperiment. The mechanical properties and working properties of this prototype is obtained based onthe test data, and the proposed sensorless technology is verified.