Mechanical stress and deformation in the rotors of a high-speed PMSM and IM
Martin Gerlach, Maximilian Zajonc, Bernd Ponick
Abstract
Abstract High-speed electric machines are gaining importance in the field of traction drives and aviation due to their high power density. The evaluation of the mechanical stress in the rotor is one crucial part in the design process for this type of machines. The mechanical stress cannot be measured directly. Accordingly, a validation of the calculated mechanical stress is difficult and normally not performed. Instead of the mechanical stress, the deformation at the rotor surface can be measured using a spin test machine with distance sensors. The deformation can then be used to validate the calculation results. In this paper, the mechanical load exerted on an IM rotor for a $60\,\text{kW}/20000\,\frac{1}{\text{min}}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>60</mml:mn><mml:mspace/><mml:mtext>kW</mml:mtext><mml:mo>/</mml:mo><mml:mn>20000</mml:mn><mml:mspace/><mml:mfrac><mml:mn>1</mml:mn><mml:mtext>min</mml:mtext></mml:mfrac></mml:math> high-speed electric machine and an PMSM rotor for a $75~\text{kW}/25000\,\frac{1}{\text{min}}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>75</mml:mn><mml:mspace/><mml:mtext>kW</mml:mtext><mml:mo>/</mml:mo><mml:mn>25000</mml:mn><mml:mspace/><mml:mfrac><mml:mn>1</mml:mn><mml:mtext>min</mml:mtext></mml:mfrac></mml:math> high-speed electric machine is analysed in detail. The mechanical stress and the deformation are calculated and analysed using a FEM simulation model. Then, a spin test is performed on the two rotors. First, the burst speed is determined by operating two rotor samples above their defined test speed. Then, the deformation is measured at the rotor surface for different operating speeds and the defined test speed. The measurement and the simulation results are compared and discussed. It can be shown that the two designs do not exceed the maximum mechanical stress for the defined operating range. In the deformation measurement of the IM rotor, a plastic deformation up to $\varepsilon _{\text{IM, pl}} = 8$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ε</mml:mi><mml:mtext>IM, pl</mml:mtext></mml:msub><mml:mo>=</mml:mo><mml:mn>8</mml:mn></mml:math> μm and elastic deformation up to $\varepsilon _{\text{IM, el}}=22$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ε</mml:mi><mml:mtext>IM, el</mml:mtext></mml:msub><mml:mo>=</mml:mo><mml:mn>22</mml:mn></mml:math> μm can be seen. In regards to plastics, PMSM rotor expands up to $\varepsilon _{\text{PMSM, pl}}= 5$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ε</mml:mi><mml:mtext>PMSM, pl</mml:mtext></mml:msub><mml:mo>=</mml:mo><mml:mn>5</mml:mn></mml:math> μm. The maximum elastic deformation of the PMSM rotor is $\varepsilon _{\text{PMSM, el}}=40$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ε</mml:mi><mml:mtext>PMSM, el</mml:mtext></mml:msub><mml:mo>=</mml:mo><mml:mn>40</mml:mn></mml:math> μm. The comparison of the calculated and the measured elastic deformation shows good accordance for the two rotor types. Both models are capable of describing the deformation and the state of stress in the rotors. In burst tests, both rotors withstand rotational speeds far above the defined test speed.