Magnetics Business & Technology - May/June 2020 - 23

RESEARCH & DEVELOPMENT

Tech Tips: Slotless Versus Slotted Permanent Magnet Motors
Slotted motors are good for high torque density and high acceleration, while slotless motors are optimum for smooth operation and good Kt (torque constant) linearity when operating in
a servo control system. The reasons behind this are explained
in an excellent white paper, "Slotless Vs Slotted Motors", from
Celera Motion that illustrates how their designs yield different
attributes.
Permanent magnet brushless motors come in all shapes and
sizes providing both rotary and linear motion. They are known
for high torque density and high efficiency. They are also routinely referred to as Brushless DC motors, Brushless AC motors, Synchronous Permanent Magnet motors, or Servo Motors.
The fundamental output of these machines is torque (rotary) or
force (linear).
Smooth and predictable torque/force while moving is the main
challenge for many applications in imaging, scanning, metrology, photonics, and tracking. Traditional permanent magnet
brushless motors exhibit cogging torque, a torque disturbance
from the interaction between the permanent magnet and the
slots between stator teeth. Cogging is cyclical torque with angle
that creates torque ripple, (and corresponding velocity ripple),
adding a non-linear element to the control of the motor.
Various methods to minimize cogging have been used for
years. They include skewing laminations, skewing magnets,
special mechanical modifications, and electrical compensation in the motor controller. In most applications that require

smooth motion with light loads the success of these methods is
marginal.
Slotless motors are
designed to optimize
smoothness and create predictable torque
output with minimal
non-linear effects.
Commonly referred
to as slotless motors
when rotary and air
Slotless stator
core motors when linear,
slotless motor designs place
only copper phase coils in the air gap of the motor. These coils,
when placed properly, interact with the permanent magnetic
flux to create force or torque. Cogging torque is eliminated
because the discontinuous iron teeth are removed from the
motor air gap. Slotless technology is especially effective with
direct drive precision systems because all torque is a function
of phase current and there are no unwanted or uncontrolled
torque disturbances from the motor. For more details, diagrams
and illustration, see the White Paper.
Celera Motion, based in Bedford, MA, is a motion control business of Novanta. Celera makes encoders, motors, servo drives
and mechatronic solutions.
For more info, see www.celeramotion.com.

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May/June 2020 * Magnetics Business & Technology

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Magnetics Business & Technology - May/June 2020

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