Magnetics Business & Technology - January/February 2023 - 22

TECH TIPS
Luvata Pursues EV Market to Apply Hollow Magnet Wire in Direct Cooled Stators
To obtain extremely high magnetic fields, the magnetic winding of a motor can be made from internally cooled hollow sections of copper wire.
Square hollow conductors with a round or rectangular cooling channel are the most common profile, but the demand for different outer dimensions
and hole sizes is growing, says Luvata of Finland.
Direct-cooled motors with hollow conductor stators offer several
advantages over the indirect-cooled motors currently used in EVs,
says Luvata of Finland, which has been making copper hollow
conductors for more than sixty years. Adoption of the technology,
typically used in more exotic applications such as superconduction,
high-energy physics and magnetic resonance imaging, into massmarket
electric vehicles faces considerable technical obstacles,
however, and much higher cost than conventional magnet wire.
Recently, Luvata has teamed up with a German motor engineering
firm to help drive adoption of the technology. The partner, hyperdrives
GmbH, is a Munich-based start-up that designs and tests
electric drivetrains using oxygen-free copper hollow conductors
from Luvata. Based in Pori, Finland, Luvata is a group company of
Mitsubishi Materials Corp.
The attraction is that more effective cooling of the motor windings
enables a higher power output, increased efficiency especially at
partial loads, and the use of lower temperature grade permanent
magnets, meaning smaller, more efficient motors can be built using
cost-effective off-the-shelf materials. In electric motors, the current
flows in the windings, which are made of copper because of
its low resistance. Electrical losses are an unavoidable obstacle
and increase with the amount of current; for example, doubling the
current quadruples the losses, which increases heat in the motor. In
addition, magnetic losses heat both the stator and the rotor, further
increasing the heat load. This heat can be extracted by direct or
indirect cooling. Luvata explains the case in a white paper available
22 Magnetics Business & Technology * January/February 2023
on its website, highlighted in this article.
Most smaller electric motors today use indirect cooling, as they
have for more than a century. A cooling fluid, water glycol, in the
motor housing transports heat to a heat exchanger where it is recooled
by air before being returned to the housing. The drawback
of this method is its limited heat flow, especially for the wire enamel
and slot liner, which have low thermal conductivity. This can be
seen in Figure 1, taken from a thermal simulation of conventional
indirect liquid cooling.
Direct stator cooling using hollow conductors enables more efficient
cooling as a cooling fluid or gas can be circulated in direct contact
with the inside of the conductor. This allows for significantly higher
stator currents and therefore smaller, more compact motors. The
big benefit of this method is that it treats the heat where it is generated,
in the winding itself instead of in the housing. Effective cooling
of the stator also ensures a lower rotor temperature, enabling the
use of less expensive lower temperature graded permanent magnets.
Hollow conductors in electric motors are not a fundamentally new
invention. They have been used for decades in large electric motors
and generators where the distance between the heat-generating
winding and the coolant in the housing is too long for indirect cooling
to be effective. However, with smaller motors the challenge up
to now has been the dimensions of the conductor, as little as 1 mm,
which increases manufacturing complexity and negatively affects
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