1
Electric Motor Thermal
Management R&D
Kevin Bennion, Emily Cousineau, Xuhui Feng, Charlie King, Gilbert Moreno
National Renewable Energy Laboratory
IEEE Power & Energy Society General Meeting
Denver, CO
July 26-30, 2015
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.
2
Relevance – Why Motor Cooling
• Current Density
Stator Cooling Jacket
• Magnet Cost
•
Price variability
•
Rare-earth materials
Stator End-Winding
• Material Costs
• Reliability
Photo Credit: Kevin Bennion, NREL
Stator
• Efficiency
• Temperature Distribution
Photo Credit: Kevin Bennion, NREL
Sample electric traction drive motor
Rotor
3
Passive and Active Cooling
Passive Thermal Design
• Motor geometry
• Material thermal properties
• Thermal interfaces
Passive
Thermal
Design
Motor
Thermal
Management
Active
Convective
Cooling
Active Convective Cooling
• Available coolant
• Cooling location
• Parasitic power
• Heat transfer coefficients
3
4
Passive Thermal Design
Stator Laminations
Thermal Contact
Resistance
Measure
• Material thermal properties
• Thermal interfaces
Force Applied
Hot Plate
Copper Spreader
Thermal Grease
Copper Metering Block
Test Sample
Copper Metering Block
Thermal Grease
Copper Spreader
Cold Plate
Slot Paper
Slot Windings
Test apparatus built in accordance with ASTM
D5470-12 steady state technique
5
300
250
M19 29 Gauge
200
M19 26 Gauge
150
HF10
Ridged
Arnon 7
100
Ridged Average
50
Photo Credit: Emily Cousineau
Interlamination Thermal Contact
Resistance (RC) [mm²-K/W]
Thermal Contact Resistance
Smooth Average
0
138
276
414
552
Clamping Pressure [kPa]
Full technical report available at:
www.nrel.gov/publications/
Smooth
6
• Thinner laminations
have more contacts per
unit length
• Higher pressure lowers
contact resistance
• Error bars represent
95% confidence interval
Effective Through-Stack Thermal
Conductivity [W/m-K]
Lamination Thermal Conductivity
2.0
1.5
M19 26 Gauge
1.0
M19 29 Gauge
HF10
0.5
Arnon7
0.0
138
276
414
Pressure [kPa]
552
𝑘𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 =
Full technical report available at:
www.nrel.gov/publications/
𝑡
𝑅𝐶 + 𝑅𝐿
keffective = effective thermal conductivity
t = thickness of one lamination
RC = thermal contact resistance
RL = thermal resistance of one lamination
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Slot Materials and Interfaces
Slot Winding
(paper removed)
Slot Paper
Photo Credit: Emily Cousineau, NREL
Slot Winding
(paper bonded)
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Slot Materials and Interfaces
• Preliminary results indicate substantial thermal resistance
present between slot winding and paper
• Preliminary results measured to be 1,800 mm²-K/W
• As large as or larger than the slot liner paper alone
(1,676 mm²-K/W )
• More tests are necessary to confirm results and reduce
measurement uncertainty
• Contact resistance between paper and stator may also be
significant
Paper
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Active Convective Cooling
Direct impingement on
target surfaces
Impingement on motor endwindings
Measure heat
transfer coefficients
for ATF cooling of
end windings
Photo Credit: Jana Jeffers, NREL
Photo Credit: Kevin Bennion, NREL
Average Heat Transfer Coefficients
• Establish credibility of experiment and data with comparison of flat target
surface results to existing correlations in literature
• Produce new data for textured surfaces representative of end-winding wires
Spatial Mapping of Convective Heat Transfer Coefficient
• Jet local convective heat transfer
• Large-scale end-winding convective heat transfer mapping
ATF: automatic transmission fluid
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Average Heat Transfer Coefficients
D (mm)
12.7
Fluid Inlet
d (mm)
2.06
S (mm)
10
S /d
5
D /d
6.2
Aluminum
Vessel
Nozzle Plate:
Orifice Diameter
(d) = 2 mm
Thermocouples
S = 10 mm
D = 12.7 mm
Test Sample
Resistance
Heater Assembly
Photo Credit: Jana Jeffers, NREL
Sample Holder/Insulation
Oil impingement test section schematic (left). Photo during operation (right).
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11
ATF Impingement Target Surfaces
•
•
ATF impingement baseline target is flat polished copper with 600-grit sandpaper
Additional targets mimic wire bundles with insulation (18, 22, and 26 AWG)
Credit: Gilbert Moreno, NREL
Baseline
18 AWG
22 AWG
26 AWG
Radius (wire and insulation),
mm
N/A
0.547
0.351
0.226
Total wetted surface area, mm 2
126.7
148.2
143.3
139.2
18 AWG
22 AWG
26 AWG
AWG = American wire gauge
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12
Comparison to Plain Surface
Correlations
ATF fluid properties provided by Ford.
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ATF Heat Transfer Coefficients
50°C Inlet Temperature
Heat Transfer Coefficient [W/m2K]
12,000
10,000
Heat transfer coefficients of all target
surfaces at 50°C inlet temperature
8,000
• At lower impingement
velocities, all samples achieve
similar heat transfer
6,000
4,000
• Average heat transfer
coefficients increase with
larger wetted areas based on
wire size
18 AWG
22 AWG
26 AWG
Baseline
2,000
0
0
2
4
6
8
Velocity [m/s]
10
12
Note: Heat transfer coefficient calculated from the base projected area (not wetted area).
13
14
ATF Heat Transfer Coefficients
Fluid splatter observed at higher velocities for 70°C and 90°C inlet liquid temperatures
Splatter at 90°C occurred at lower velocity than at 70°C
As temperature increases, it is expected that the fluid splatter will occur at lower velocities
ATF flowing over surface
18 AWG sample data for all inlet temperatures
12,000
10,000
8,000
6,000
ATF deflecting off surface
4,000
50°C
70°C
90°C
2,000
0
0
2
4
6
8
Velocity [m/s]
10
12
Note: ATF viscosity decreases as temperature increases.
Photo Credits: Jana Jeffers, NREL
Heat Transfer Coefficient [W/m2K]
•
•
•
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Spatial Mapping of Heat Transfer
ATF jet
more cooling
less cooling
Photo Credit: Kevin Bennion, NREL
• Not all motor end-winding
surfaces are directly impacted
by ATF jets
• It is important to map the
spatial distribution of the heat
transfer coefficients to know
the overall cooling effect
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Local Convective Heat Transfer
Spatial mapping of local heat transfer with
ATF jet impingement:
•
•
Jet impingement heat transfer coefficients are
not uniform over the entire cooled surface
d, nozzle
diameter
Liquid jet
Air
Stagnation zone
Wall jet zone
The rate of decrease in the heat transfer
coefficient is unknown
Tw
Heated surface
Heat transfer coefficient
Nozzle
Experimental velocity profile of jet
impingement showing variation in
velocity at target surface. Measured
using particle image velocimetry
(PIV) at NREL.
Jet
Wall or Target
Boundary
r/d
Radial Distance
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Local Convective Heat Transfer
Test article cross-sectional view
Foil: heated via joule
heating
Built test apparatus to
spatially map local heat
transfer coefficients with ATF
jet impingement.
Lexan
insulation/support
Teflon insulation/
support
Bus bars
Assembled test article
View of the TLC-coated foil
Photo Credits: Gilbert Moreno, NREL
TLC: thermochromic liquid crystals
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Large-Scale Heat Transfer
Spatial mapping of large-scale end-winding convective heat
transfer with direct ATF cooling
Stator winding removed for
sensor package
Photo Credit: Kevin Bennion, NREL
Photo Credits: Kevin Bennion, NREL
3D drawing of stator with
sensor packages installed
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Sensor Design
Sensor
Package
Targets
Flat
18 AWG
20 AWG
Exploded View
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Spatial Mapping of Heat Transfer
ATF
jets
1
2
1. Fluid Jet Geometry
• Location and
orientation of ATF
fluid jets
• Nozzle type/
geometry
• System flow rate
• Jet velocity
• Parasitic power
2. Relative position
between measured heat
transfer and jet location
• Impact of gravity and
free fluid flow
• Fluid interactions
between jets
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• Cast aluminum cooling
jacket pressed around
the stator
• Water-ethylene glycol
(WEG) circulated
through three cooling
channels within the
cooling jacket
Coolant
Channels
(Credit: Kevin Bennion, NREL)
Motor Thermal Management
Outlet
Inlet
View of the cooling channels showing the WEG flow path
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Motor Thermal Management
Use computational fluid dynamics and finite element
analysis
• Validate the models using experimental results
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Electric Motor Thermal Management
End winding – side 1
End winding – side 2
Stator – middle
solid lines – experiment
symbols – model
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Electric Motor Thermal Management
Lpm: liters per minute
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Conclusion
Relevance
•
Supports transition to more electric-drive vehicles with higher continuous power
requirements.
Approach/Strategy
•
•
Engage in collaborations with motor design experts within industry, national laboratories, and
universities.
Perform in-house thermal characterization of materials, interface thermal properties, and
cooling techniques.
Summary
•
•
•
•
Published results for lamination stack thermal tests.
Published results for ATF convective heat transfer measurements.
Built experimental apparatus to measure variation in local convective heat transfer
coefficients.
Developed design and initiated construction of test equipment and sensors to map largescale convective heat transfer coefficients on motor end-windings with ATF direct cooling.
Acknowledgments:
For more information, contact:
Susan Rogers and Steven Boyd,
U.S. Department of Energy
Kevin Bennion
Kevin.Bennion@nrel.gov
Phone: (303) 275-4447
Tim Burress and Andy Wereszczak
of Oak Ridge National Laboratory
Jana R. Jeffers, previously of NREL
Section Supervisor
Sreekant Narumanchi
Sreekant.Narumanchi@nrel.gov
Phone: (303) 275-4062
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