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Integrated Platform for Motor
Stator Sound Power Simulation
W. Huang, Z. Wang and J. Cherng University of Michigan – Dearborn
P. Ding, ANSYS, Inc.
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Background
• The pulse width modulation (PWM) technology is widely used in inverter fed motor control including electrified vehicles.
• PWM will cause high frequency harmonics due to its amplitude switching.
• Harmonics of electromagnetically induced forces will excite the motor structure and cause mechanical resonances and noise radiation.
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Objectives
• Develop an electromagnetical model of a complete motor-inverter system including inverter control circuit.
• Develop a FEA dynamic model of a motor stator. • Interface the electromagnetical model with the
mechanical model. • Determine the resonant vibration modes of the motor
stator structure and surface velocity. • Calculate the sound power level and sound pressure level
generated by the stator frame due to the electromagnetical excitation.
• Identify the optimum PWM frequency for best power output and lower noise.
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Energy flow of electric motor
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Acoustic generation from Motor
For an electrical motor below 10 Kw, although the radiated acoustic power is very small, approximately sound power level 60 to 80 dB could be generated.
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Motor Stator Work Flow
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Input frequency calculation
• 𝑓 = 𝑁𝑆 ∗𝑝
120
• 𝑓 = 3000 ∗8
120= 200Hz
• where,
𝑓 is the frequency of the AC supply current in Hz.
𝑝 is the number of poles per phase.
𝑁𝑆is the RPM.
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SPWM Inverter
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Main circuit
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Integrated circuit
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Motor Model: TOYOTA Prius 2004
Machine Type Internal Permanent Magnet
Adjustable Speed Motor
Rated Output Power 75kW
Number of Poles 8
Stator Teeth 48
Rated Speed 3000rpm
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Motor torque performance
Torque output with SPWM AC input
Torque output with ideal AC input
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Stator system vibration analysis Use harmonic analysis for forced vibration to get surface velocity 𝑥 𝑡 :
𝑀 𝑥 𝑡 + 𝐶 𝑥 𝑡 + 𝐾 𝑥 𝑡 = 𝐹
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Interval set up
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Surface velocity at 5000 PWM
frequency
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PWM harmonics analysis
• The harmonics frequencies which are mainly contained in the voltage are 𝑓𝑐 ± 2𝑓𝑖, 𝑓𝑐 ±4𝑓𝑖,2𝑓𝑐 ± 1𝑓𝑖, 2𝑓𝑐 ± 5𝑓𝑖, 3𝑓𝑐 ± 2𝑓𝑖, 3𝑓𝑐 ± 4𝑓𝑖, 4𝑓𝑐 ± 1𝑓𝑖 and 4𝑓𝑐 ± 5𝑓𝑖,
• 𝑓𝑖:fundamental frequency (inverter output frequency)
• 𝑓𝑐:the frequency of the carrier wave.
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PWM harmonics analysis
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Stator system acoustic analysis Stator considered as cylindrical radiation, sound power radiated from structure is:
𝜋𝑟𝑎𝑑 = 𝑢2𝜌0𝑐0𝐴𝛿𝑟𝑎𝑑
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Acoustic Body
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Acoustic Body mesh
Mesh Size: 7 mm Nodes: 374877 Elements:74124
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Acoustic Calculation
SPL== 20*log10[(Pmax/( 2)/Pref]
Where Pmax= 𝑃𝑟𝑒𝑎𝑙2 + 𝑃𝑖𝑚𝑎𝑔
2
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PWM frequency at 5000 Hz
0
20
40
60
80
100
120
5000PWM SPL
5000PWM SPL
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PWM frequency at 5000 Hz
Motor sound pressure level at 4800Hz
Peak SPL is 92dB
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SPL at difference path
SPL at five different distances at 4800Hz
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Resonance at 7200Hz
Stator mode Eigen frequencies
Stator mode shape at 7200.4 Hz
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Theoretical Motor performance
• Motor Performance Vs. Switching Frequency
Frequency
Energy loss
Smoothness ofRotation
Acoustic noise
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SPL and Efficiency Tradeoff based on
ANSYS simulation result
94.00%
94.10%
94.20%
94.30%
94.40%
94.50%
94.60%
94.70%
50
60
70
80
90
100
5000 6000 8000 10000
SPL(dB)
PWM Switching Frequency(Hz)
SPL peak
Efficiency
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Filter Design
• Trap Filter
0
1
2
3
4
5
4600 4800
Harmonics Amplitude
(A)
Frequency(Hz)
Without Filter
With Filter
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Filter Performance
Peak SPL is 89dB after add filter, 3 dB drop
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Experimental Test
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SPL at 7000 PWM frequency
Red line is SPL at long axial, Green line is SPL at short axial
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Conclusion
• A complete integrated CAE model of motor-inverter-stator was established to calculate and predict the motor magnetic noise.
• The motor magnetic noise is decreasing when PWM switching frequency is increasing.
• A noise and efficiency tradeoff is necessary before increasing the switching frequency to reduce the noise.
• An optimum 8000 Hz switching frequency was identified.
• Trap filter was found to be effective in reducing the magnetic noise.
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Acknowledgements
• The authors would like to express their gratitude to Mr. Mike Hebbes, Dr. Zed Tang, and Dr. Paul Larsen of ANSYS for their support on this project.
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References
• [1].Derrick E. Cameron, Origin and reduction of acoustic noise in variable-reluctance motors, Massachusetts Institute of Technology January 1990
• [2]. Jacek F. Gieras, Noise of polyphase electric motors, 2006 Taylor & Francis Group, LLC.
• [3]. S.D. Garvey, J.E. Penny, M.I. Firswell, C.N. Glew, Modeling the vibration behavior of stator cores of electrical machines with a view to successfully predicting machine noise.
• [4] Professor John Cherng’s lecture ME570 Chapter 4 Noise and vibration of Motors, Generators, and Convertors
• [5] Mounir Zeraoulia, Mohamed El Hachemi Benbouzid, Demba Diallo, Electric Motor Drive Selection Issues for HEV Propulsion Systems: A Comparative Study, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 55, NO. 6, NOVEMBER 2006
• [6] Li Junwu, Chen Shukang, Research on the Vibration and Noise of the Mini Type Permanent and Magnet Synchronous Motor
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Questions?
THANK You