coupled electromagnetic and thermal solution for electric machine design

© 2009 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

Coupled Electromagnetic and Thermal Solution for Electric Machine Design

Coupled Electromagnetic and Thermal Solution for Electric Machine Design

Xiao HUZed (Zhangjun) TANG

ANSYS, INC.

Xiao HUZed (Zhangjun) TANG

ANSYS, INC.

© 2009 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary

Introduction

• Electric machine design is a multi-physics problem– Electromagnetic– Fluid and thermal– Mechanical (Stress, Vibration)– Power electronics/control

• Electromagnetic, thermal and mechanical designs are interrelated

– Losses from electromagnetic design affect temperature– Temperature rise will change material properties– Thermal induced mechanical stress

• A design environment that accommodates all physics and their interaction is highly desired

– ANSYS Workbench environment


Simulation Driven Product Development - Electric Machine Design Methodology

Much better solution withANSYS CFD/Mechanical


Maxwell2D for Electromagnetic

• Majority of Electromagnetic Designs are Done in 2D for Electric Machine

– >80%– Faster– Enough accuracy

• Maxwell2D Transient Solver– Transient excitation– Transient motion– Motion induced transient effects

• Coupling between Maxwell3D and ANSYS is possible and follows the same design flow


Design Flow

Geometry

Losses

Centroids

Mapped Losses

Temperature

Workbench DMMaxwell UDP

Maxwell

Workbench Mesher

ANSYS Mechanical (automated)ANSYS CFD (Scripted)


Accurate Loss Coupling

• Most Losses are Distributed– Eddy loss (PMs) – Core loss (Stator & Rotor)

• Time Averaged Spatial Losses– Time constants are very different for electrical

and thermal


Export Thermal Data toANSYS Mechanical


Import Maxwell Loads toANSYS Mechanical


Maxwell 2D – ANSYS Thermal


Mechanical Eigenmode analysis of thermal pre-stressed model with Maxwell 3D transient losses


1.75 KHz mode results ofpre-stressed structural model

Thermal deformation


Need for Computational Fluid Dynamics (CFD)

• CFD is the science of predicting fluid flow and heat transfer by solving mathematical equations

• Electric machine cooling involves fluid flow and heat transfer and thus can benefit from CFD simulation


CFD Models for Electric Machine

• Conjugate heat transfer with mapped losses from Maxwell– Solids with different properties– Liquid or air for cooling– Air trapped inside electric machine

• Multiple Reference Frame (MRF) used to account for rotor rotation – Steady state solution with the impact of

rotating rotor


Cooling Methods for Electric Machines

• Forced convection liquid cooling– Most effective cooling– Expensive

• Forced convection air cooling– Effective cooling– Somewhat expensive

• Natural convection air cooling– Not as effective– Cheap


Test Cases and Purposes

Cooling Method Mesh Size (K)Case 1 Forced Water 916

Case 2 Forced Air 1007

Case 3 Natural Air 899

• Three test cases are conducted to see the effectiveness of cooling and different temperature and its gradient distribution


Geometry/Mesh

• A sector of geometry is used– Periodic boundary

• Hex is used in most of the regions– Except for the winding and the fluid region

surrounding it, etc.• Forced air cooling has an air domain outside


Loss Distribution for All Cases

• Spatial eddy loss distribution for the magnets• Spatial core loss distribution for the rotor, stator yoke, and stator teeth• Stranded winding copper loss• All losses, which are highly non-uniform, are from Maxwell2D


Temperature Distribution

• Max temperature are 398K, 517K, and 550k respectively• Forced water cooling is the most effective and natural air cooling

is the least.• Forced water cooling gives similar max temperature gradient

• Temperature gradient is responsible for thermal stress. • To keep both temperature and its gradient low is the best

Forced water cooling Forced air cooling Natural air cooling


Summary for Forced Cooling

• Forced water cooling is the most effective.• Natural air cooling is the least effective.• Forced water cooling, however, does not

necessarily give the least temperature gradient.

• Natural air cooling may face challenge of high temperature.

• Forced water cooling may face challenge of high temperature gradient.


Observations about Natural Convection Cooling

• Natural convection Heat Transfer Coefficient (HTC) is relativelyuniform compared with forced convection

• Natural convection cooling can be simulated by using a constant HTC instead of a full CFD calculation.

• Well accepted industry practice.

• Air trapped inside electric machines is not effective in heat transfer and thus can be removed from the calculation.

• Air gap kept but modeled by STILL air (details next)

• If air domains both inside and outside of the electric machine are removed, the problem becomes purely conductive

• No full CFD


Ineffectiveness of Trapped Air

• Relatively low velocity and uniform temperature of the trapped air explains its ineffectiveness for heat transfer

Velocity vector of trapped air (note the max velocity is only 2.5 m/s)

Temperature distribution of trapped air (note the temperature scale goes from 500K to 530K)


Test Cases Using Natural Convection Air Cooling

Cooling Method

Trapped Air

Full CFD

Air Gap Mesh Size (K)

Case 3 Natural Air

Yes Yes Yes 899

Case 4 Natural Air

No No No 394

Case 5 Natural Air

No No Yes 397

• Case 3 is from previous study and is used as a based line case here.• Case 4 contains only solids• Case 5 also contains the air gap between the rotor and stator to

improve the accuracy.• The air gap is treated as if it is solid


Temperature Distribution

• Max temperatures are 550K, 566K, and 563k respectively

• Trapped air has minimum impact on max temperature as expected

• Air gap has an impact on rotor temperature distribution

Full CFD (case3) Solid only, no CFD (case4) Solid and air gap, no CFD (case5)

Air gap


Comparison

Max Winding Temperature (K)

Error Max Rotor Temperature (K)

Error Performance on 4 CPUs

Case 3 550 0% 528 0% 4 ~ 12 hrsCase 4 566 2.9% 508 3.8% <10 minutesCase 5 563 2.4% 523 0.95% <10 minutes

The full CFD case is assumed to be correct.


Summary for Natural Cooling

• Trapped air in general does not have significant impact on temperature distribution except for the air gap between the rotor and stator

• Adding a layer of STILL air in the gap can improve accuracy– This could be the best comprise considering its much quicker

solution than a full CFD calculation.

• Note that forced cooling still needs CFD due to highly localizedheat transfer coefficient


Conclusion

• Forced convection cooling is effective and its thermal analysis needs CFD due to highly localized heat transfer coefficient

• Natural convection cooling can be effectively simulated without full CFD and thus making the simulation much easier and faster– Trapped air has impact on the solution only in the gap region,

which can be modeled using a layer of STILL air.

• ANSYS CFD can be used to perform either the full CFD calculation or the simplified conduction calculation

• ANSYS Mechanical can be used to perform the pure conduction, thermal stress, free modal, and pre-stress modal analysis

coupled electromagnetic and thermal solution for electric machine design

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