MOTORS
gNaeinw t rEaVct mionotors
Axial ux traction motors are commencing early-stage production for use
in electric vehicles, but how do they work and what is their potential?
ABy Steed Webzell
xial- ux traction motors
are said to hold enormous
promise for use in electric
vehicles (EVs). Although
these motor types are
already common in stationary applications
such as agricultural machinery and lifts,
a number of companies are now evolving
the technology for automotive use.
In basic terms, an axial- ux motor is a
geometry of electric motor construction
where the gap between the rotor and
stator, and therefore the direction of
magnetic ux between the two, aligns in
parallel with the axis of rotation, rather
than radially, as with the concentric
cylindrical geometry of conventional
radial- ux motors. Among the advantages
is more torque for a given volume of
motor, as the active magnetic surface area
is the face of the motor’s rotor rather than
the outside diameter (see diagram, right).
Further bene ts include the potential to
provide more compact solutions, while
the shorter, one-dimensional ux path can
also deliver high e ciencies.
Leonardo Ferretti, motor designer at
Belgian rm Magnax, sets out the axial-
and radial- ux motor concepts and their
primary di erences, which essentially
focus on magnetic eld orientation.
“Compared with traditional radial- ux
electric motors, axial- ux machines,
especially single-stator, double-rotor
topologies, are more e ective in an
electromagnetic sense,” he says. Electrical
machines generate torque on a rotor shaft
thanks to magnetic forces created by
magnetic elds. To explain the impact of
magnetic eld orientation, a straight coil
of wire fed by a DC current can be used to
generate a nearly uniform magnetic eld,
similar to a bar magnet. When feeding the
same coil by a sinusoidal current source
(AC), the magnetic eld produced will be
oriented as a function of the current.
“In permanent-magnet PM motors
radial and axial, the set of AC windings
generates a series of north and south
magnetic poles along the air-gap of
the machine,” explains Ferretti. “The
interaction between these poles and the
permanent magnetic poles on the rotor
surface is responsible for the torque
production. The ux direction in the
air gap therefore designates electrical
machines as either radial or axial ux. In
a radial- ux machine, the air-gap ux
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