IEEE Electrification Magazine - June 2014 - 46

exploring nonconventional Cooling Strategies
nonconventional power electronics cooling strategies
have also been explored. two-phase cooling offers high
heat-transfer rates, significantly higher than is possible
with traditional Weg-cooled systems. tests at nrel have
demonstrated that using a passive, immersion (twophase) cooling strategy can reduce the junction-to-liquid
thermal resistance by more than 60% as compared to the
ls 600h double-sided automotive cooling system. the
advancement of high-temperature, wide-bandgap (Wbg)
semiconductor devices (e.g., silicon carbide and gallium
nitride) have enabled the possibility of air-cooled power
electronics. nrel is currently collaborating with oak ridge
national laboratory to demonstrate a 55-kW air-cooled
silicon carbide-based inverter system. analysis indicates
that the air-cooled system can meet the 2015 doe aPeem
power density and specific power targets. although the
high temperature capability of Wbg devices has its advantages, the packaging of these devices requires advances to
auxiliary electronics (e.g., capacitors), bonded interface
materials, and electrical interconnects.

Stator Cooling Jacket
Nozzle/Orifice

Case
Stator-Case
Contact

Oil Flow

Stator
Slot
Winding

Oil
Impingement

Air Gap
Rotor
Laminations
End
Winding

Rotor
Hub
Shaft
Oil Flow
Motor Axis

Figure 6. A cross-sectional view illustration of an electric motor and
some typical thermal management approaches incorporating a stator
cooling jacket and oil cooling with ATF within the rotor and end
windings.

Tackling the Complexities of electric
Motor Thermal Management

high-temperature (>100 °c) coolant. the delphi project
developed low-thermal resistance packaging to decrease
silicon and reduce cost and also developed hightemperature capacitors. the high-temperature inverter,
shown in figure 5, meets the 2015 doe advanced Power
electronics and electric motors (aPeem) Program targets
and is currently being commercialized.

Air Gap

High Speed, Low Torque
Low Speed, High Torque

End Winding Radial

motor cooling presents a unique challenge because of the
complex geometries of motors and moving components
(e.g., rotors). the fact that the heat-generating and temperature-sensitive components (i.e., magnets and windings) are not easily accessible for direct cooling and are
embedded within nonthermally conductive materials
(e.g., steel laminations) adds to the challenge. motor thermal
management is further complicated by 1) variation in the

End Winding Angular

End Winding Axial

Rotor In-Plane

Rotor Axial

Slot Insulation

Slot Winding In-Plane

Slot Winding Axial

Stator-Case Contact

Stator In-Plane

Stator Axial

Stator Axial
0

2
4
6
8
Increase in Power Output (%)
(a)

High Speed, Low Torque
Low Speed, High Torque

End Winding Radial

2
4
6
8
Increase in Power Output (%)
(b)

Figure 7. An IPM model parameter sensitivity analysis for a 20% increase in component thermal conductivity. This impact is measured in terms
of increased motor output power. The results compare two heat distributions (high speed, low torque and low speed, high torque) for two different
cooling conditions: (a) a baseline cooling condition and (b) an aggressive low thermal resistance cooling condition.

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2014

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2014