Research On Flux-weakening Level Of Permanent Magnet AC Servo Motor

Poor flux-weakening level of permanent magnet AC servo motor is the main reason to limit its development in high-speed field. In this paper, permanent magnet AC servo motor is researched and the method is proposed to improve its flux-weakening level. The main contents are as follows:Firstly, the reasons that permanent magnet AC servo motor has poor flux-weakening level are analyzed, and the main reason that d-axis inductance is small due to the presence of permanent magnet in d-axis magnetic circuit is found. And then, the flux-weakening levels of different rotor magnetic circuit structures are analyzed. Based on the analysis, a series of novel rotor magnetic circuit structures are proposed and their flux-weakening levels are researched. By analyzing, the rotor magnetic circuit structure which can improve flux-weakening level and has better air-gap flux density waveform is obtained. D-axis inductance of the structure is increasing by 30% than that of traditional rotor structure, and the highest speed is increasing by 24% when normalized value of current is 1.1.Secondly, the effects of q-axis and d-axis inductance and no-load PM flux linkage on maximum electromagnetic torque and basic speed and power factor are analyzed. The conclusions are obtained. Namely, maximum electromagnetic torque and basic speed and power factor are increasing with d-axis inductance being reduced. When q-axis inductance is increased, maximum electromagnetic torque is increasing and power factor is almost unchanged, but basic speed is reducing. No-load PM flux linkage is linear direct proportion to maximum electromagnetic torque and negative linear correlation with basic speed, and it is V-shaped curve relationship to power factor. Therefore, theoretic reference for designing permanent magnet AC servo motors with flux-weakening control is provided.Finally, two permanent magnet AC servo motors with different rotor structures are designed. One rotor structure is interior radial, the other is interior segmented which is proposed in this paper. The two prototypes are tested. The results are that temperature rise of the interior segmented rotor structure motor is 96K, and its efficiency is 88.56%. Its d-axis inductance is 5.7mH, which is 1.76 times than that of the interior radial rotor structure motor. The conclusion that the motor with interior segmented rotor structure can effectively improve flux-weakening level of permanent magnet AC servo motor and still meet basic performance requirements is verified.