Electric Motor Thermal Management 

PI: Kevin Bennion

National Renewable Energy Laboratory

June 19, 2018

DOE Vehicle Technologies Office

2018 Annual Merit Review and Peer Evaluation Meeting

This presentation does not contain any proprietary, confidential, or otherwise restricted information.

NREL/PR-5400-71208

Project ID: ELT075 
NREL | 2

Overview

• Project start date: FY2017

• Project end date: FY2019

• Percent complete: 50%

Timeline

• Motor Industry – R&D Input and 

Application of Research Results

– Suppliers, end users, and researchers

• Oak Ridge National Laboratory 

(ORNL)

– Motor Research Lead

• Ames Laboratory

– Magnet Materials Lead

• National Renewable Energy 

Laboratory (NREL)

– Thermal and Reliability Lead

Partners

• Total project funding

– DOE share: $1,100K

• Funding for FY 2017: $600K

• Funding for FY 2018: $500K

Budget

• Cost, Power Density, Life

Barriers
NREL | 3

Relevance

• Research enabling compact, reliable, and efficient 

electric machines

– Motor 10x power density increase (2025 versus 2015 targets) [1]

– Motor 2x increase in lifetime [1]

– Motor 53% cost reduction (2025 versus 2015 targets) [1]

Stator Cooling Jacket

Stator End Winding

Rotor

Photo Credit: Kevin Bennion, NREL Stator

[1] U.S. DRIVE Electrical and Electronics Technical Team Roadmap, 2017.
NREL | 4

Relevance

Advanced 

Electric Motor

Material 

Physics-Based 

Models

Base Materials

Non-Rare Earth 

Magnets

Conductive 

Material

High Voltage 

Insulators

Thermally 

Conductive 

Epoxy and Fillers

[1] U.S. DRIVE Electrical and Electronics Technical Team Roadmap, 2017.

• Material conductivity thermally 

drives the amount of material 

necessary to create the required 

magnetic field to create 

mechanical power [1]

• Material performance 

characterization techniques are 

not well known or identified in the 

literature [1]

• It is important to reduce the 

thermal resistance of the motor 

packaging stack-up to help 

increase the power density [1]

Electric Drive Motor R&D Areas [1]
NREL | 5

Relevance

• Increased accuracy of key parameters with less data 

can enable fewer design iterations and reduce time 

and cost

Key Parameter 

Characterization Data and 

Models

Component 

Design

Hardware 

Build

Design Iterations

Photo Credits: 

a,b: Emily Cousineau, NREL

c: Jana Jeffers, NREL

d: Kevin Bennion, NREL

a

b

c d
NREL | 6

Milestones

Date Description

December 2017

(Complete)

Milestone

• Measure in-situ convective cooling resistance to quantify impacts 

of reducing thermal stack-up resistance through material, interface, 

and cooling improvements.

March 2018

(Complete)

Milestone

• Research motor thermal management technologies and improved 

heat transfer designs with input from industry and ORNL for non 

rare-earth and reduced rare-earth motors.

June 2018

(In Progress)

Go/No-Go

• Select heat transfer technologies for improved active convective 

and passive cooling research.

September 2018

(In Progress)

Milestone

• Prepare report on research results.
NREL | 7

Overall Approach

Material and Interface Thermal 

Characterization

Motor System Impacts of Active 

Cooling

• Research emphasis on heat transfer of new 

materials and active cooling interactions

Photo Credits: 

a: Doug DeVoto, NREL

b: Kevin Bennion, NREL

a

b
NREL | 8

• Material and interface thermal resistance impact heat 

transfer

Slot windings

Slot liner or ground insulation

Stator laminations

Cross-slot winding thermal 

conductivity

Winding-to-liner thermal 

contact resistance

Liner-to-stator thermal 

contact resistance

Stator-to-case thermal 

contact resistance

Approach: Material Thermal Characterization
NREL | 9

Technical Accomplishments and Progress

Windings

• Cross-slot winding thermal conductivity 

measurement methods

• Collaboration with ORNL

• Journal publication [1]

Laminations

• Lamination thermal contact resistance 

measurements and through-stack lamination 

thermal conductivity measurements

• NREL technical report [2]

• Journal publication describing thermal 

modeling results in progress

[1] A. A. Wereszczak et al., “Anisotropic Thermal Response of Packed Copper Wire,” J. Thermal Sci. Eng. Appl, vol. 9, 

no. 4, pp. 041006, Apr. 2017 (9 pages).

[2] E. Cousineau et al., Characterization of Contact and Bulk Thermal Resistance of Laminations for Electric Machines.

NREL Technical Report NREL/TP-5400-63887, June 2015.

Photo Credit: Emily Cousineau, NREL

Photo Credit: Kevin Bennion, NREL
NREL | 10

Technical Accomplishments and Progress

Stator-to-Case Thermal Contact 

Resistance

• Demonstrated measurement methods

• Developed and validated model with 

experimental data

• Publication in press [1]

Magnet Materials

• Measured thermal properties and 

mechanical properties of magnets

• Collaboration with Ames

[1] J. E. Cousineau, et al., “Experimental Characterization and Modeling of Thermal Contact Resistance of Electric 

Machine Stator-to-Cooling Jacket Interface Under Interference Fit Loading,” J. Thermal Sci. Eng. Appl., Accepted 

for publication. 

Photo Credit: Kevin Bennion, NREL

Photo Credit: Doug DeVoto, NREL
NREL | 11

Technical Accomplishments

Wire size, type, insulation, 

Impregnation material

Winding

Slot liner to winding interface

Slot Winding Bonded to Slot Liner

Materials for Wide-Bandgap 

(WBG) inverters

Slot Liner/Ground Insulation

Thermal Measurements and Reliability

Temperature Cycles

Slot liner to stator interface, 

System validation

Motorette

Vibration Cycles

Photo Credit: Doug DeVoto, NREL

Winding

Stator teeth 

and back iron

Slot Liner
NREL | 12

Approach: Active Cooling Impacts

• Active cooling technologies influence the impact of new 

motor materials

Direct Cooling of Motor Windings Indirect Cooling of Motor through Case

Photo Credits: 

Kevin Bennion, NREL
NREL | 13

Technical Accomplishments and Progress

Orifice jet impingement

• Impingement on fixed target 

surfaces

• Quantify impact of wire gauge, 

velocity, and fluid temperature [1]

Fan jet impingement

• Quantified heat transfer over 

equivalent target surface area

• Not all of the fluid impinges on 

measured target surface

[1] K. Bennion and G. Moreno, “Convective Heat Transfer Coefficients of Automatic Transmission Fluid Jets 

with Implications for Electric Machine Thermal Management,” in ASME 2015 International Technical 

Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems and 

ASME 2015 12th International Conference on Nanochannels, Microchannels, and Minichannels, San 

Francisco, CA, United States, 2015.

Photo Credit: Jana Jeffers, NREL

Photo Credit: Xuhui Feng, NREL
NREL | 14

Technical Accomplishments and Progress

• Direct Automatic Transmission Fluid (ATF) 

Cooling

– Developing experimental methods to measure heat 

transfer

– Quantifying impact of new or alternative cooling 

approaches for ATF cooling of motors to take 

advantage of new motor materials

Orifice Jet Center Impingement Orifice Jet Edge Impingement

Photo Credit: Kevin Bennion, NREL

Photo Credits: Bidzina Kekelia, NREL
NREL | 15

Technical Accomplishments and Progress

Front View h =

qqss

AAss TTss − TTll

Experimental Factor Values

Jet incidence angle [degrees] 90, 60, 45

Jet distance from target [mm] 5, 10, 15

Jet position on target [mm] 0 (center), 6.3, 11.4

Fluid inlet temperature [°C] 50, 70, 90

Surface temperature [°C] 90, 100, 110, 120

h = heeeeee tt tttttt tt cccc cccccc

qqss = heeeeee rr rr rr tttt ss

AAss = AA tttt ss

TTss = TT ss tt tt tt

TTll = FFFF tt tt tt

Heater 

Assembly

Nozzle

Distance from target Jet incidence angle

Jet position
NREL | 16

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

0 1 2 3 4 5 6 7 8 9 10

Heat Transfer Coefficient [W/m

2·K]

Nozzle Jet Velocity [m/s]

Center Impingement, Tl=50°C

Ts=120°C

Ts=110°C

Ts=100°C

Ts=90°C

Technical Accomplishments and Progress

Temperature of the cooled 

surface impacts the heat 

transfer coefficient
NREL | 17

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

0 1 2 3 4 5 6 7 8 9 10

Heat Transfer Coefficient [W/m

2·K]

Nozzle Jet Velocity [m/s]

Tl=50°C

Ts=120°C

Ts=110°C

Ts=100°C

Ts=90°C

Ts=120°C

Ts=110°C

Ts=100°C

Ts=90°C

Center

Edge

Technical Accomplishments and Progress

As the orifice jet impingement 

moves away from center, the heat 

transfer coefficient decreases.
NREL | 18

Responses to Previous Year Reviewers’ 

Comments

• Multiple comments highlighted that the experimental results were very useful; 

once the results feed into an electric motor design process, the work will become 

more valuable.

– We are working on plans to perform tests on sample stator assemblies (motorettes) to 

validate models and quantify performance of material improvements combined with 

cooling strategies.

• Reviewer comments mentioned that the project needs to focus more on 

quantifying the impact of different insulation materials on the motor’s thermal 

performance, and the long-term/life impact of jet cooling on the insulation system 

needs to be quantified.

– The comment matches the updated focus in the revised U.S. DRIVE Electrical and 

Electronics Technical Team Roadmap. For this reason we are working to build material 

samples that could be thermally cycled, vibration tested, and exposed to driveline fluids.

• Reviewers mentioned that active cooling of the electric machine will allow size 

reduction and will improve peak load capability. However, cost and complexity due 

to active cooling system in terms of pumping power or performance should be 

justifiable for a given application.

– For this reason potential future work will include integrated system tests including sample 

stator assemblies (motorettes) with active cooling.
NREL | 19

Collaboration and Coordination with 

Other Institutions

• Industry

– Motor industry suppliers, driveline fluid suppliers, end users, and 

researchers

• NREL providing

– Experimental data

– Modeling results

– Analysis methods

• Industry providing

– Driveline fluid information and properties

– Information on insulation materials

– Boundary conditions for experimental and analytical work

– Application information
NREL | 20

Collaboration and Coordination with 

Other Institutions

• Other Government Laboratories

– ORNL

• Collaboration on motor designs to reduce or eliminate rare-earth 

materials

• NREL supported computational fluid dynamics for fluid flow and heat 

transfer analysis

– Ames

• NREL supporting magnet material physical property measurements
NREL | 21

Remaining Challenges and Barriers

• Research enabling compact, reliable, and efficient 

electric machines

– Motor 10x power density increase (2025 versus 2015 targets) [1]

– Motor 2x increase in lifetime [1]

– Motor 53% cost reduction (2025 versus 2015 targets) [1]

[1] U.S. DRIVE Electrical and Electronics Technical Team Roadmap, 2017.

Material and Interface Thermal 

Characterization

• Material conductivity improvements

• Methods to quantify thermal 

interfaces

• Reliability to support increased 

lifetime targets

Motor System Impacts with Active 

Cooling

• Methods to improve heat transfer 

through convective cooling (flow 

geometry and placement)

• Opportunities for improved driveline 

fluids targeted for motor cooling

• Impacts of aged fluids

• Alternative winding configurations 

(bar windings) 
NREL | 22

Proposed Future Research

“Any proposed future work is subject to change based on funding levels.”

Material and Interface Thermal 

Characterization

Active Cooling

System Impacts 

(Materials and Active Cooling)

Wire size, type, insulation, 

impregnation material slot liner, 

interface resistances

Motorette with alternative winding geometries, materials, 

and convective cooling approaches

Fluid characterization

Photo Credits: Bidzina Kekelia, NREL

Winding

Stator teeth 

and back iron

Slot Liner
NREL | 23

Summary

Relevance

• Supports research enabling compact, reliable, and efficient electric machines aligned with Roadmap 

research areas

Approach/Strategy

• Engage in collaborations with motor design experts and component suppliers within industry

• Collaborate with ORNL to provide motor thermal analysis support on related motor research at ORNL

• Collaborate with Ames to provide material properties to support Ames-led magnet development

• Develop and document characterization methods of materials, interface thermal properties, fluids, 

and cooling techniques

Technical Accomplishments

• Published data and analysis methods related to material and interface characterization methods

• Developed thermal interface models to predict thermal resistances of new motor designs

• Quantifying factors impacting the cooling performance of driveline fluids such automatic transmission 

fluid for cooling electric motors

Collaborations

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

medium/heavy-duty applications)

• Oak Ridge National Laboratory

• Ames Laboratory
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.

www.nrel.gov

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.

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.

Thank You

Acknowledgments

Susan Rogers, U.S. Department of 

Energy

EDT Task Leader

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov 

Phone: (303)-275-4062

Team Members

Emily Cousineau (NREL)

Doug DeVoto (NREL)

Xuhui Feng (NREL)

Bidzina Kekelia (NREL)

Josh Major (NREL)

Gilbert Moreno (NREL)

Jeff Tomerlin (NREL)

Tim Burress (ORNL)

Iver Anderson (Ames)

Liangfa Hu (Ames)

Emma White (Ames)

For more information, contact

Principal Investigator:

Kevin Bennion

Kevin.Bennion@nrel.gov

Phone: (303)-275-4447 
Technical Back-Up Slides
NREL | 26

Technical Back-Up Slides

• Stator-to-case thermal contact resistance test apparatus

Photo Credit: Emily Cousineau, NREL
NREL | 27

Technical Back-Up Slides

• Magnet Transverse Rupture Testing

o 3-mm x 3-mm x 32-mm beam samples

o Follow ASTM B528-12 test standard

o Samples each tested at 25°C

o Calculate transverse rupture strength:

TRS = (3 × P × L)/(2 × t2 × w)

where:

TRS = transverse rupture strength (MPa)

P = force required to rupture specimen (N)

L = distance between supporting rods (25.4 mm)

t = specimen thickness (mm)

w = specimen width (mm)

Measurements performed with Instron test system 

with samples inside environmental chamber

Photo Credits: Doug DeVoto, NREL 
NREL | 28

Technical Back-Up Slides

• Data show improved 

transverse rupture 

strength as compared to 

previously tested 

materials

• Baseline material shows 

consistent results 

between tests