How Eddy current losse of permanent magnet will be effect permanent magnet rotor 's temperature
0 Introduction: rare earth permanent magnet synchronous motor (REPMSM) has the features of small volume, light weight, high efficiency, theoretically no fundamental loss of rotor, rotor temperature rise should be low, but the reality is not. The author developed an increased safety type rare earth permanent magnet synchronous motor, for example, occur when testing the rotor up to 1 25 ° c temperature rise. The rotor temperature is too high, causing risk of demagnetization of neodymium-Iron-Boron permanent magnet, affecting the normal work. This paper analyzes the possible causes of excessive temperature rise causes, put forward measures to reduce temperature rise.
1 rotor structure: REPMSM take the stator induction motor stator, it generally refers to the rotor structure. Asynchronous starting of REPMSM by the squirrel-cage rotor, shaft, rotor and magnet component, the rotor core formed by the lamination pressure and fill in the rotor core with neodymium-Iron-Boron permanent magnet and cast aluminum form a cage, as shown in Figure 1. Its with the asynchronous motor starting process, when people in the armature winding of the stator three phase AC symmetrical, circular rotating magnetic field, rotor stationary at this time, the rotor squirrel cage cut magnetic lines, and induces an alternating form an alternating magnetic field, and the magnetic field of the stator, rotor began to turn. When the rotor speed near synchronous speed, not induced in the squirrel cage, but the constant magnetic field of permanent magnets to form the stator magnetic field rotates synchronously, in normal running.
2 rotor temperature rise causes: Runtime fever comes from motor loss. REPMSMsynchronization runs, includes permanent magnet rotor loss loss and harmonic losses.
2. 1 permanent magnet depletion: the resistivity of neodymium-Iron-Boron (1. 44xl0 ˉt) ω ? m, has some conductivity in an alternating magnetic field of Eddy current losses. Thermal conductivity of ND-Fe-b 7.7cal/m. h. ° C, heat and poor. Neodymium-Iron-Boron magnets rust, oxidation, making it difficult for heat outward transfer,exacerbating the rotor temperature.
2. 2 harmonic losses: the slot effect, the stator magnetic field and other factors, motors harmonic magnetic field in the air gap is complex. Harmonic magnetic field in the air gap at different speeds relative to the rotor, the induced current in the rotor and squirrel cage, resulting in harmonic losses, making the rotor temperature.
3 measures to reduce temperature rise: from the analysis of proposed solutions are as follows.
3.1) segmented, tiered force of permanent magnet: magnet is no longer the placethroughout the material, instead of a permanent magnet is divided into multiple small segments or multiple layers, as shown in Figure 2. And magnet (layer) surfaceelectrophoresis treatment to reduce Eddy current losses, reducing the rotor temperature rise.
3.2) increased air gap: for asynchronous motor, increasing the air gap will increase the leakage flux, the excitation current increases, the less efficient. As for rare earth permanent magnet synchronous motor, increasing the gap, you can increase the air-gap magnetic field higher harmonic magnetic resistance and harmonic leakage and reduce the degree of cross-linking of the flux, weakening harmonic currentand reduce the loss of stator and rotor surfaces and harmonic losses and so on, so as to reduce the role of temperature.
3.3) rotor is half closed El Groove or closed slot: This reduces pulsating within the rotor core surface loss and tooth loss, and reduce the effective air gap length, improving power factor, while reducing the air gap permeance harmonics of pulse amplitude, reduce the magnetic harmonic losses caused by harmonics.
3.4) select the appropriate slot fits: Harmonic number, the lower the number of rotor slots the more greater loss; stator and rotor slot ratio close to 1 o'clock, loss, soselected slot combination.
3.5) double short-distance distribution of stator winding winding: double layer winding short distribution you need to select a different span can reduce the high harmonics and harmonic EMF reduction is unlikely, so as to improve the air-gap magnetic field waveforms, reduced harmonic losses, lower temperature rise.
3.6) high quality neodymium-Iron-Boron permanent magnet: found in practical applications, from different manufacturers with grades of ND-Fe-b magnets propertiesare different. Neodymium-Iron-Boron grades are different, have different size of Eddy current losses, and also different thermal conductivity. Select a relatively largethermal conductivity high performance ND-Fe-b permanent magnetic materials and magnets on the conduction of heat, thereby reducing the rotor temperature rise.
4 prototype improving measures of rotor temperature rise and its effect: from theabove analysis, replace the prototype used in the grades of ND-Fe-b permanentmagnets, composed of former 40SH 33UH, temperature-rise test again, the resultis the stator temperature 80 ℃, temperature was 51 degrees, the rotor core temperature is 140 ° c temperature is 110 degrees. After changing the permanent magnet rotor temperature dropped by 10 degrees, visible effects of permanent magnet Eddy-current loss on rotor temperature rise is very large.
5 conclusion: this article discusses the causes of rare earth permanent magnet synchronous motor rotor temperature rise too high, proposed to reduce the rotor temperature rise and analysis methods. Magnet designation was replaced on the original prototype testing indicate that permanent magnet Eddy current loss is a great impact on the rotor temperature. Therefore, if motor manufacturing process can take permanent magnets segment or tiered measures such as rotor temperaturerise will decline.