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
SAE INTERNATIONAL

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

Stator Cooling Jacket

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

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End Winding

Motor Axis

Motor Cooling Section View

Background –

Motor Thermal Management Challenges

Problem

Extracting heat from within the 

motor to protect motor and 

enable high power density

Challenges

1. Orthotropic (direction 

dependent) thermal conductivity 

of lamination stacks

1

1

5
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Stator 

Laminations

Slot Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End Winding

Motor Axis

Motor Cooling Section View

Background –

Motor Thermal Management Challenges

Problem

Extracting heat from within the 

motor to protect motor and 

enable high power density

Challenges

1. Orthotropic (direction 

dependent) thermal conductivity 

of lamination stacks

2. Orthotropic thermal conductivity 

of slot windings

1

1

2

6
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Stator 

Laminations

Slot Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End Winding

Motor Axis

Motor Cooling Section View

Background –

Motor Thermal Management Challenges

Problem

Extracting heat from within the 

motor to protect motor and 

enable high power density

Challenges

1. Orthotropic (direction 

dependent) thermal conductivity 

of lamination stacks

2. Orthotropic thermal conductivity 

of slot windings

3. Orthotropic thermal conductivity 

of end windings

1

1

2 3

7
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Stator 

Laminations

Slot Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End Winding

Motor Axis

Motor Cooling Section View

Background –

Motor Thermal Management Challenges

Problem

Extracting heat from within the 

motor to protect motor and 

enable high power density

Challenges

1. Orthotropic (direction 

dependent) thermal conductivity 

of lamination stacks

2. Orthotropic thermal conductivity 

of slot windings

3. Orthotropic thermal conductivity 

of end windings

4. Convective heat transfer 

coefficients for ATF cooling 

4

1

1

2 3

8
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Stator 

Laminations

Slot Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End Winding

Motor Axis

Motor Cooling Section View

Background –

Motor Thermal Management Challenges

Problem

Extracting heat from within the 

motor to protect motor and 

enable high power density

Challenges

1. Orthotropic (direction 

dependent) thermal conductivity 

of lamination stacks

2. Orthotropic thermal conductivity 

of slot windings

3. Orthotropic thermal conductivity 

of end windings

4. Convective heat transfer 

coefficients for ATF cooling 

5. Thermal contact resistance of 

stator-case contact

4

1

1

2 3

5

9
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Stator 

Laminations

Slot Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End Winding

Motor Axis

Motor Cooling Section View

Background –

Motor Thermal Management Challenges

Problem

Extracting heat from within the 

motor to protect motor and 

enable high power density

Challenges

1. Orthotropic (direction 

dependent) thermal conductivity 

of lamination stacks

2. Orthotropic thermal conductivity 

of slot windings

3. Orthotropic thermal conductivity 

of end windings

4. Convective heat transfer 

coefficients for ATF cooling 

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

11

Stator 

Laminations

Slot Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End Winding

Motor Axis

Motor Cooling Section View
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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

12
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Active Convective Cooling –

ATF Heat Transfer Coefficients

• Measure convective heat transfer coefficients for ATF cooling of end 

windings

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

13

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/d

12.7 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

Thermocouples

S=10 mm

D=12.7 mm

Oil Impingement Test Section Schematic Photo During Operation

Photo Credit: Jana Jeffers, NREL

14
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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

15
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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

Heat Transfer Coefficient [W/m

2K]

Velocity [m/s]

18 AWG

22 AWG

26 AWG

Baseline

ATF Heat Transfer Coefficients

16
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ATF Heat Transfer Coefficients

ATF flowing over surface

ATF deflecting off surface

Note: ATF viscosity decreases as temperature increases

Photo Credit: Jana Jeffers, NREL

Photo Credit: Jana Jeffers, NREL

0

2,000

4,000

6,000

8,000

10,000

12,000

0 2 4 6 8 10 12

Heat Transfer Coefficient [W/m

2K]

Velocity [m/s]

50°C

70°C

90°C

18 AWG sample data for all inlet temperatures

17
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Passive Thermal Design –

Material and Interface Thermal Measurements

• Measure interface thermal resistances and orthotropic thermal 

conductivity of materials

Photo Credit: Justin Cousineau, NREL

18

• 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

Copper Metering Block

Lamination Stack

Thermal Grease

Thermal Grease

Force Applied

19

Photo Credit: Justin Cousineau, 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

0

500

1000

1500

2000

2500

0 2 4 6 8 10

Measured Lamination Stack 

Thermal Resistance (mm2-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

Copper Metering Block

Copper Metering Block

Lamination Stack

Thermal Grease

Thermal Grease

Force Applied

Error bars represent 95% confidence level

20
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Passive Thermal Design -

Effective Through-Stack Thermal Conductivity

0

50

100

150

200

250

300

350

138 276 414 552

Lamination-to-Lamination Thermal 

Contact Resistance [mm2-K/W]

Pressure [kPa]

M19 26 Gauge

M19 29 Gauge

HF10

Arnon7

Error bars represent 95% confidence level

21

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity
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Passive Thermal Design –

Effective Through-Stack Thermal Conductivity

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

3.1

3.3

Through

0 10 20 30 40 50

-Stack Thermal Conductivity 

[W/m-K]

Number of Laminations (NL)

M19 29 Gauge, 138 kPa Example

Effective Through-Stack Thermal Conductivity

Asymptote

(NNLL → ∞)

22

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity
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Passive Thermal Design –

Effective Through-Stack Thermal Conductivity

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 200 400 600

Asymptotic Through-Stack Thermal 

Conductivity [W/m-K]

Pressure [kPa]

M19 26 Gauge

M19 29 Gauge

HF10

Arnon7

Error bars represent 95% confidence level

23

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity
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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

Copper Metering Block

Copper Metering Block

Lamination Stack

Thermal Grease

Thermal Grease

Force Applied

Photo Credit: Justin Cousineau, NREL

24
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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

Bulk

Material

Calculated

Value

Measured

In-Plane Thermal Conductivity [W/m-K]

M19 29 Gauge Laminations

1 2

3

25

Photo Credit: Justin Cousineau, NREL
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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

Copper Metering Block

Copper Metering Block

Wire Bundle

Thermal Grease

Thermal Grease

Force Applied

26

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: Perfect

Fill Factor

Model: 50%

Voiding

Cross-Slot Thermal Conductivity (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

27
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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

28
For more information, contact:

Principal Investigator

Kevin Bennion

Kevin.Bennion@nrel.gov

Phone: (303) 275-4447 

APEEM Task Leader:

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov 

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)