IV. COOLING SYSTEM
Low-speed high-torque wheel motors must meet tight
requirements in terms of overload capability, and this severely
constrains the motor design. Furthermore, mounting the motor
within the wheel is desirable but imposes restrictions on the
machine diameter and demands totally enclosed construction
to provide protection against the environment. Hence, very
effective cooling of the machine is needed to achieve the all
required performance. In this respect, PM machines have the
great advantage over induction machines that significant
power loss occurs only in the machine stator and this
somewhat simplifies heat removal. For the wheel motor
development herewith discussed an original cooling system
arrangement has been conceived and patented. A liquid cooled
aluminum ring is used together with a high thermal
conductivity epoxy in order to remove heat directly from the
end windings at the outer diameter of the machine stator.
Further to that, the particular cooling ring-epoxy assembly
handles the reaction torque in the motor by transmitting it to
the motor mounting structure.
Figures 6 (a) and (b) are simplified plan views of the stator
winding and the cooling arrangement. Figure 6(a) shows the
cooling ring when the aluminum cooling fins are separate from
the stator core and the winding, while Figure 6(b) shows when
they are assembled. Stator has annular core with poles (teeth)
and with coils interspersed between poles. Coils are wound
from a flat ribbon and therefore possess significant lateral
strength, that is, the ability to withstand significant
circumferentially directed forces. Annular shaped stator
support has hollow interior through which coolant circulates.
Inwardly extending from stator support are tooth-like
protrusions shaped and spaced so as to fit intimately between
coils and in close proximity to core of the stator, as can be seen
in Figure 6(b). This arrangement provides for low thermal
impedance so that heat may be readily extracted from coils of
the stator as well as from the poles (stator teeth) as shown by
arrows in Figure 6(b). Stator cooling ring is attached to stator
by a thermally conductive epoxy, insertion molded thermally
conductive plastic, or equivalent material placed between
protrusions, winding coils and stator teeth. This insures
intimate thermal contact between cooling ring and stator.
Cooling ring also reacts the circumferentially directed forces
created by motor by means of, for example, circumferential
attachment plate shown in Figures 6(a) and (b), by which
motor is coupled to the vehicle frame.