Electric Motor Thermal Management for Electric Traction Drives ...

ELECTRIC MOTOR THERMAL MANAGEMENT FOR ELECTRIC TRACTION DRIVES Kevin Bennion, Justin Cousineau, Gilbert Moreno National Renewable Energy Laboratory

SAE 2014 Thermal Management Systems Symposium September 22–24, 2014 Denver, CO

14TMSS-0086

NREL/PR-5400-62919 NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC

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Relevance – Why Motor Cooling?

• Current Density

• Magnet Cost

o Price variability

o Rare-earth materials

• Material Costs

• Reliability

• Efficiency

2

Stator Cooling Jacket

Stator End Winding

Rotor Stator

Sample electric traction drive motor.

Photo Credit: Kevin Bennion, NREL

Photo Credit: Kevin Bennion, NREL

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Motor Thermal Management – Passive and Active Cooling

3

Passive Thermal Design

Active Convective

Cooling

Motor Thermal

Management

Passive Thermal Design •Motor geometry •Material thermal properties •Thermal interfaces

Active Convective Cooling •Cooling location •Heat transfer coefficients •Available coolant •Parasitic power

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Background – Motor Thermal Management Challenges

Problem Extracting heat from within the motor to protect motor and enable high power density

Example 4 to 9 kW of heat could be produced with an 80-kW motor operating with an efficiency between 90% and 95% [1].

4

ATF: Automatic Transmission Fluid

Stator Laminations

Slot Winding

Rotor Laminations Rotor

Hub

Case

Air Gap

Stator-Case Contact

Nozzle/Orifice

ATF Impingement

Shaft ATF Flow

ATF Flow


End Winding

Motor Axis

Motor Cooling Section View

[1] S. Oki, S. Ishikawa, and T. Ikemi, “Development of High-Power and High-Efficiency Motor for a Newly Developed Electric Vehicle,” SAE International, 2012-01-0342, Apr. 2012.

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Stator Laminations

Slot Winding


Hub

Case

Air Gap

Stator-Case Contact

Nozzle/Orifice

ATF Impingement

Shaft ATF Flow

ATF Flow


End Winding

Motor Axis




Challenges 1. Orthotropic (direction

dependent) thermal conductivity of lamination stacks

1

1

5

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Stator Laminations

Slot Winding


Hub

Case

Air Gap

Stator-Case Contact

Nozzle/Orifice

ATF Impingement

Shaft ATF Flow

ATF Flow


End Winding

Motor Axis






2. Orthotropic thermal conductivity of slot windings

1

1

2

6

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Stator Laminations

Slot Winding


Hub

Case

Air Gap

Stator-Case Contact

Nozzle/Orifice

ATF Impingement

Shaft ATF Flow

ATF Flow


End Winding

Motor Axis







3. Orthotropic thermal conductivity of end windings

1

1

2 3

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Stator Laminations

Slot Winding


Hub

Case

Air Gap

Stator-Case Contact

Nozzle/Orifice

ATF Impingement

Shaft ATF Flow

ATF Flow


End Winding

Motor Axis








4. Convective heat transfer coefficients for ATF cooling

4

1

1

2 3

8

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Stator Laminations

Slot Winding


Hub

Case

Air Gap

Stator-Case Contact

Nozzle/Orifice

ATF Impingement

Shaft ATF Flow

ATF Flow


End Winding

Motor Axis









5. Thermal contact resistance of stator-case contact

4

1

1

2 3

5

9

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Stator Laminations

Slot Winding


Hub

Case

Air Gap

Stator-Case Contact

Nozzle/Orifice

ATF Impingement

Shaft ATF Flow

ATF Flow


End Winding

Motor Axis









5. Thermal contact resistance of stator-case contact

6. Cooling jacket performance

4

1

1

2 3

5

6

10

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Research Objective

Problem

Research Tasks

Objective

Support broad industry demand for data, analytical methods, and experimental techniques to improve and better understand motor thermal management

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Stator Laminations

Slot Winding


Hub

Case

Air Gap

Stator-Case Contact

Nozzle/Orifice

ATF Impingement

Shaft ATF Flow

ATF Flow


End Winding

Motor Axis


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Research Focus

•Measure convective heat transfer coefficients for ATF cooling of end windings •Measure interface thermal resistances

and orthotropic thermal conductivity of materials

Support broad industry demand for data to improve and better understand motor thermal management

Objective

Research Tasks

Automatic Transmission Fluid Heat Transfer

Material and Thermal Interface Testing

Photo Credit: Jana Jeffers, NREL

Photo Credit: Justin Cousineau, NREL

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Active Convective Cooling – ATF Heat Transfer Coefficients

• Measure convective heat transfer coefficients for ATF cooling of end windings

Photo Credit: Kevin Bennion, NREL Photo Credit: Jana Jeffers, NREL

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Direct impingement on target surfaces

Impingement on motor end windings

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ATF Impingement Test Section

D (mm) d (mm) S (mm) S /d D /d12.7 2.06 10 5 6.2

Test Sample

Nozzle Plate: Orifice Diameter

(d) = 2 mm

Sample Holder/Insulation

Resistance Heater Assembly

Fluid Inlet

Aluminum Vessel

ThermocouplesS = 10 mm

D = 12.7 mm

Oil Impingement Test Section Schematic Photo During Operation


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ATF Impingement Target Surfaces

Baseline 18 AWG 22 AWG 26 AWG

Radius (wire and insulation), mm

N/A 0.547 0.351 0.226

Total wetted surface area, mm2 126.7 148.2 143.3 139.2

AWG = American Wire Gauge

18 AWG 22 AWG 26 AWG

Credit: Gilbert Moreno, NREL

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Note: Heat transfer coefficient calculated from the base projected area (not wetted area)

50°C Inlet Temperature

0

2,000

4,000

6,000

8,000

10,000

12,000

0 2 4 6 8 10 12

Hea

t Tra

nsfe

r Coe

ffici

ent [

W/m

2 K]

Velocity [m/s]

18 AWG22 AWG26 AWGBaseline

ATF Heat Transfer Coefficients

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ATF Heat Transfer Coefficients

ATF flowing over surface

ATF deflecting off surface

Note: ATF viscosity decreases as temperature increases



0

2,000

4,000

6,000

8,000

10,000

12,000

0 2 4 6 8 10 12

Heat

Tra

nsfe

r Coe

ffici

ent [

W/m

2 K]

Velocity [m/s]

50°C70°C90°C

18 AWG sample data for all inlet temperatures

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Passive Thermal Design – Material and Interface Thermal Measurements

• Measure interface thermal resistances and orthotropic thermal conductivity of materials


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• Stacked lamination thermal conductivity

• Slot windings

Effective thermal properties for motor

design and simulation

Photo Credit: Kevin Bennion, NREL Photo Credit: Kevin Bennion, NREL

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Passive Thermal Design – Effective Through-Stack Thermal Conductivity

Measured Stack Thermal Resistance

Lamination-to-Lamination Thermal Contact Resistance

Effective Through-Stack Thermal Conductivity

Cold Plate

Hot Plate

Copper Spreader

Copper Spreader

Copper Metering Block


Lamination Stack

Thermal Grease

Thermal Grease

Force Applied

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0

500

1000

1500

2000

2500

0 2 4 6 8 10

Mea

sure

d La

min

atio

n St

ack

Ther

mal

Res

istan

ce (m

m²-

K/W

]

Number of Contacts

M19 29 Gauge, 138 kPa Data

Data with Systematic Error

Weighted Curve Fit

Cold Plate

Hot Plate

Copper Spreader

Copper Spreader



Lamination Stack

Thermal Grease

Thermal Grease

Force Applied

Error bars represent 95% confidence level

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Passive Thermal Design - Effective Through-Stack Thermal Conductivity

0

50

100

150

200

250

300

350

138 276 414 552

Lam

inat

ion-

to-L

amin

atio

n Th

erm

al

Cont

act R

esist

ance

[mm

²-K/

W]

Pressure [kPa]

M19 26 Gauge

M19 29 Gauge

HF10

Arnon7


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1.51.71.92.12.32.52.72.93.13.3

0 10 20 30 40 50Thro

ugh-

Stac

k Th

erm

al C

ondu

ctiv

ity

[W/m

-K]

Number of Laminations (NL)

M19 29 Gauge, 138 kPa Example


Asymptote(𝑁𝑁𝐿𝐿 → ∞)

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0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 200 400 600

Asym

ptot

ic T

hrou

gh-S

tack

The

rmal

Co

nduc

tivity

[W/m

-K]

Pressure [kPa]

M19 26 Gauge

M19 29 Gauge

HF10

Arnon7


23




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Passive Thermal Design – Effective In-Plane Lamination Stack Thermal Conductivity

• Measured in-plane thermal conductivity of lamination stacks provided by Oak Ridge National Laboratory (ORNL)

Cold Plate

Hot Plate

Copper Spreader

Copper Spreader



Lamination Stack

Thermal Grease

Thermal Grease

Force Applied


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Passive Thermal Design – Effective In-Plane Lamination Stack Thermal Conductivity

• Confirmed in-plane thermal conductivity is close to bulk material thermal conductivity

1. Based on measured thermal conductivity of similar material 2. Calculated assuming 99% stacking factor 3. Average of measured orthotropic property in setup shown in figure

21.9 21.7 21.8

0.0

5.0

10.0

15.0

20.0

25.0

BulkMaterial

CalculatedValue

Measured

In-P

lane

The

rmal

Con

duct

ivity

[W/m

-K]

M19 29 Gauge Laminations

1 2

3

25


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Passive Thermal Design – Effective Wire Bundle Cross-Slot Thermal Conductivity

• Measured cross-slot thermal conductivity of wire bundles prepared and provided by ORNL

Cold Plate

Hot Plate

Copper Spreader

Copper Spreader



Wire Bundle

Thermal Grease

Thermal Grease

Force Applied

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Photo Credits: Justin Cousineau, NREL

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Passive Thermal Design – Effective Wire Bundle Cross-Slot Thermal Conductivity

Note: Wire fill factor includes copper and insulation

0.48

1.06

0.66

0.54

0.98

0.60

0

0.2

0.4

0.6

0.8

1

1.2

Test Results Model: PerfectFill Factor

Model: 50%Voiding

Cro

ss-S

lot T

herm

al C

ondu

ctiv

ity (W

/m-K

)

Wire Fill Factor: 0.67 Wire Fill Factor: 0.63

• The agreement between model and experimental results depends on assumptions for fill factor, voiding, and thermal contact resistance

• Modeling approach appears to match, but additional testing is needed

Photo Credits: Justin Cousineau, NREL

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Conclusion

Relevance • Supports transition to more electric-drive vehicles with higher continuous power

requirements • Enables improved performance of non-rare earth motors and supports lower cost through

reduction of rare earth materials used to meet temperature requirements (dysprosium)

Technical Accomplishments • Received sample motor materials from ORNL and measured orthotropic thermal

conductivity • Completed expanded lamination thermal tests • Measured ATF heat transfer convection coefficients on target surfaces • Received ATF fluid property data from Ford Motor Company to support future work to

develop correlations and computational fluid dynamics models

Collaborations • Motor industry representatives: manufacturers, researchers, and end users (light-duty and

medium/heavy-duty applications) • Oak Ridge National Laboratory

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For more information, contact:

Principal Investigator Kevin Bennion [email protected] Phone: (303) 275-4447

APEEM Task Leader:

Sreekant Narumanchi [email protected] Phone: (303) 275-4062

Acknowledgments:

Susan Rogers and Steven Boyd, U.S. Department of Energy Team Members:

Justin Cousineau (NREL) Jana Jeffers (NREL) Charlie King (NREL) Gilbert Moreno (NREL) Tim Burress (ORNL) Andy Wereszczak (ORNL)

Electric Motor Thermal Management for Electric Traction Drives ...

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