Research on the Reduction of Synchronous Parasitic Torque by the Rotor Axial Piecewise Staggered Structure of Cage Induction Machine
Xu Wei1, Ren Xiaoming1, Ning Yinhang1, Bao Xiaohua2
1. School of Electrical Engineering Shanghai Dianji University Shanghai 201306 China; 2. School of Electrical Engineering and Automation Hefei University of Technology Hefei 230009 China
Abstract：The smoothness and stability requirements of motor output torque have gradually increased in recent years, especially in emerging applications such as electric vehicles. The induction machine (IM), due to its low price, high reliability, and strong overload capacity, still has certain competitive advantages and application needs. In classical motor design theory, the range of slot combinations is limited to ignore the large synchronous parasitic torque, such as the equivalent slot combination. However, the stray loss of IM is small when the number of stator slots and rotor slots is similar. For some of these special slot combinations, the output torque of the induction machine is related to the starting position of the motor. In order to suppress the torque ripple caused by the periodic torque variation with the starting position, a rotor axial piecewise staggered structure is proposed to weaken the synchronous parasitic torque. The selection range of slot combinations can be expanded by solving the starting difficulty of the IM with an equivalent slot combination. The electromagnetic torque calculation model is established, the torque amplitude expression about the initial rotor position is deduced, and the magnetic field order and the motor speed conditions for generating constant torque are determined. Based on the assumption of the linearly distributed magnetic potential, the mechanism of weakening the harmonic electromotive force is discussed using a piecewise staggered rotor structure. The relationship between the weakening degree of synchronous parasitic torque and the number of staggered rotor segments is quantified. Taking four kinds of special slot combinations as examples, the effects of the single skewed rotor, the piecewise staggered rotor, and the rotor with the combined structure on the fundamental torque and synchronous parasitic torque are simulated and analyzed. Finally, the dual skewed rotor prototype machines are trial-produced with the equivalent slot combination scheme, and the motor locked-rotor torque tests are carried out. Simulation results on the torque components show that the single skewed rotor weakens the synchronous torque of the motor with different slot combinations while slightly reducing the fundamental torque. However, the single skewed rotor still cannot change the spatial periodicity of the output torque. For the piecewise staggered rotor, the synchronous torque is weakened only for certain slot combinations, and the amplitude of the fundamental torque remains approximately unchanged. Regarding the combined rotor structure, taking the two-stage staggered skewed rotor as an example, the attenuation degree of the harmonic magnetic field is greater than that of any single rotor structure. Experimental results on the locked-rotor torque show that the offset effect of the staggered bar structure on the synchronous torque generated by the odd harmonics deviates from the simulation results by a degree. An optimal skewed rotor design is necessary to minimize the synchronous parasitic torque. The following conclusions can be drawn from the simulation analysis and prototype test. (1) The amplitude of synchronous torque changes periodically with the starting position of the motor, and the spatial period is inversely proportional to the order of the rotor harmonic magnetic field that generates torque. The rotor pitch is the least common multiple of the period. (2) The rotor axial staggered structure can reduce the rotor harmonic electromotive force, change the spatial periodicity of the output torque of the motor with certain slot combinations, and improve the minimum output torque. (3) Compared with the skewed rotor and the piecewise staggered rotor, the combined structure of the two has a better effect of weakening the synchronous parasitic torque. When the skewed distance of the dual skewed rotor is one tooth pitch, the minimum value of the starting torque ratio of the prototype is about 1.5 times, thus helping the normal starting of the motor with equal stator and rotor slot number. However, the partial magnetic flux leakage of the rotor may result in certain disadvantages. For example, it may increase no-load loss.
徐威, 任晓明, 宁银行, 鲍晓华. 笼型感应电机转子轴向分段错开结构削弱同步附加转矩的研究[J]. 电工技术学报, 2023, 38(20): 5411-5420.
Xu Wei, Ren Xiaoming, Ning Yinhang, Bao Xiaohua. Research on the Reduction of Synchronous Parasitic Torque by the Rotor Axial Piecewise Staggered Structure of Cage Induction Machine. Transactions of China Electrotechnical Society, 2023, 38(20): 5411-5420.
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