Integrated Platform for Motor

Stator Sound Power Simulation

W. Huang, Z. Wang and J. Cherng University of Michigan – Dearborn

P. Ding, ANSYS, Inc.

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.

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.

Energy flow of electric motor

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.

Motor Stator Work Flow

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.

SPWM Inverter

Main circuit

Integrated circuit

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

Motor torque performance

Torque output with SPWM AC input

Torque output with ideal AC input

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

Interval set up

Surface velocity at 5000 PWM

frequency

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.

PWM harmonics analysis

Stator system acoustic analysis Stator considered as cylindrical radiation, sound power radiated from structure is:

𝜋𝑟𝑎𝑑 = 𝑢2𝜌0𝑐0𝐴𝛿𝑟𝑎𝑑

Acoustic Body

Acoustic Body mesh

Mesh Size: 7 mm Nodes: 374877 Elements:74124

Acoustic Calculation

SPL== 20*log10[(Pmax/( 2)/Pref]

Where Pmax= 𝑃𝑟𝑒𝑎𝑙2 + 𝑃𝑖𝑚𝑎𝑔

2

PWM frequency at 5000 Hz

0

20

40

60

80

100

120

5000PWM SPL

5000PWM SPL

PWM frequency at 5000 Hz

Motor sound pressure level at 4800Hz

Peak SPL is 92dB

SPL at difference path

SPL at five different distances at 4800Hz

Resonance at 7200Hz

Stator mode Eigen frequencies

Stator mode shape at 7200.4 Hz

Theoretical Motor performance

• Motor Performance Vs. Switching Frequency

Frequency

Energy loss

Smoothness ofRotation

Acoustic noise

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

Filter Design

• Trap Filter

0

1

2

3

4

5

4600 4800

Harmonics Amplitude

(A)

Frequency(Hz)

Without Filter

With Filter

Filter Performance

Peak SPL is 89dB after add filter, 3 dB drop

Experimental Test

SPL at 7000 PWM frequency

Red line is SPL at long axial, Green line is SPL at short axial

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.

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.

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

Questions?

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