IEEE Electrification Magazine - June 2014 - 47

potential motor configurations or designs, 2) the heat generation distribution and variation based on operating conditions, and 3) uncertainty in material thermal properties
or critical thermal contact interfaces.
the ability to remove heat from the motor depends on
the passive stack thermal resistances within the motor
and the convective cooling performance of the selected
cooling technology (figure 6). the passive thermal design
refers to the geometrical layout, material selection, and
thermal interfaces that affect the heat spreading
capabilities within the motor. the ability for heat to spread
through the motor dictates the thermal temperature gradients within the motor. the convective cooling technology is the cooling mechanism that ultimately removes the
heat from the motor and rejects it to the ambient environment. Without the ability to remove heat, the motor cannot operate without sacrificing performance, efficiency,
and reliability.

Stator Yoke
Stator Teeth
Air Gap

Rotor
Laminatio
Laminations

Rotor Hub
Magnet
Slot Winding

Case
(a)

C
Case
Cooling

Seeking the Optimal electric
Motor Cooling Strategies
the type of motor topology [e.g., permanent magnet (Pm),
induction, and switched-reluctance] selected depends on
the application and its requirements. for today's edvs, Pm
motors are widely used because of their superior performance. to allow for higher-temperature operation of the Pm
motor, expensive rare-earth materials (i.e., dysprosium) are
required. in an effort to reduce cost, there is interest in
either reducing or eliminating the expensive rare-earth
materials by either finding cost-effective material replacements or by improving thermal management to allow for
lower-temperature operation. to evaluate thermal
management strategies, a material property sensitivity
study was conducted for two types of Pm motors-a surface Pm motor and an interior Pm (iPm) motor. the finite
element-based analysis evaluated the effect of increasing a
component's thermal conductivity by 20% on the total thermal resistance (maximum winding temperature to coolant
temperature). figure 7 summarizes the results of the material sensitivity study for the iPm motor with cooling applied
to the case and end windings.
two operating points were evaluated-a high-speed,
low-torque operating point and a low-speed, high-torque
operating point. in the high-speed, low-torque operating
point, higher losses are expected within the rotor. in the
low-speed, high-torque operating point, higher losses are
expected within the stator. additionally, two cooling conditions were evaluated-a baseline cooling condition
[figure 7(a)] and a more aggressive cooling condition
[figure 7(b)]. in both cooling conditions, cooling in the form
of an effective thermal resistance was uniformly applied
to the stator end-windings, the stator case, and the rotor
end surface to simulate convective cooling (e.g., Weg
coolant or impinging atf jets) (figure 8). the baseline cooling condition is representative of the convective cooling
performance typical of automotive applications, while the

End-Winding
En
Cooling
Co

Rotor Cooling
(b)

Figure 8. (a) The IPM thermal model geometry and (b) convective
heat transfer boundary conditions.

aggressive cooling condition represents a significantly
higher convective cooling performance.
for the high-speed, low-torque condition, the thermal
properties (e.g., conductivity) within the rotor have the
most impact, while at the low-speed, high-torque operating conditions, the thermal properties associated with the
stator in general have a greater effect. the difference
between figure 7(a) and (b) emphasizes the interactions

Figure 9. Experiments have been conducted to measure heat transfer coefficients for impinging ATF jets. The photo shows an ATF jet
impinging on a heated surface.

IEEE Electrific ation Magazine / j une 2 0 1 4

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