Page 4 - Industrial Technology EXTRA 22nd February 2021

ELECTRIC MOTORS...

Slotless versus slotted

brushless DC motor designs

Brushless DC motors are the technology of choice for applications operating at higher

speeds, but two different motor architectures can present design engineers with a
selection conundrum. Addressing the design considerations, Madhavan Ramanujam,
R&D Technologist at Portescap, and Cyril Baud, R&D Manager at Portescap, compare
and contrast slotted and slotless brushless DC motors.
he last decade has seen a growing compromise in one area or another. But there is a
acceptance of brushless DC motors as an new kid on the block – the slotless brushless DC
alternative to brushed DC motors for motor – and the latest developments are giving
Tapplications requiring combinations of engineers pause to rethink their design decisions.
high speed (above 12,000 rpm) and long operating The conventional brushless DC (BLDC) motor is
life. There are trade-offs, of course: Coreless brush a slotted design, by which we mean that the coils
commutated motors offer simple control and free are wound within slots around the stator. In the
from cogging, while the complexity of construction slotless design on the other hand, the coil is wound
of brushless DC motors mean the cost is higher. in a separate external operation and is then inserted
For many applications, the choice between the directly into the air gap during motor assembly. Let’s
two is clear, but for others where the requirements look at how these different approaches impact on
sit uncomfortably between the performance performance.
limitations of each technology, the decision is more The first thing of note is that slotless BLDC
difficult and frequently involves a design motors can be made smaller in size. In slotted
BLDC motors, the presence of stator teeth prevents
the overall size of the motor from being minimised.
Moreover, the winding process becomes
progressively more difficult as the motor size is
reduced. In contrast the slotless BLDC motor has
either skewed or axial type windings fixed on the
cylindrical stator iron core, enabling size to be more
readily reduced.
The slotless design also has a cost advantage
through reduced complexity and a stator core that is
easier to manufacture. Instead of having to insert

February 2021 • INDUSTRIAL TECHNOLOGY EXTRA • p4