Page 5 - Industrial Technology EXTRA 22nd February 2021

Slotted vs slotless technology diagram

the windings into slots, the
windings in the slotless motor
can be self-bonded with skewed
or cup shaped windings placed
on the air gap.

Performance considerations
In the early days of slotless BLDC motor
design, the power density was significantly lower operation (from 40,000 to 60,000 rpm), usually
than for an equivalent slotted motor. Now, the the slotless rotor has a two-pole permanent magnet
emergence of high energy permanent magnets design. In addition, the stator core losses are
(such as NdFeB and SmCo) and their alternative restricted to an acceptable range while the motor
magnetisation arrangements (including radial, operates at high speed thanks to the large air gap.
parallel and halbach) have closed the performance Core losses are relatively small due to its slotless
gap and caused a renewed interest in slotless stator structure and therefore offers high power
motors. density.
One might think that the same performance
advantage could be gained in slotted BLDC motors, Motor losses
but they are less able to employ high energy Slotless motors offer low inductance, which
magnets because the thicker teeth required to introduces a motion controls challenge. Low
increase the magnetic loading of the motor has the inductance results in higher motor losses if pulse
effect of reducing the slot area and thus the width modulation (PWM) control is applied.
decreased electric loading of the motor. Controls with higher switching frequency (80 –
That said, the slotted BLDC motor can still offer 100kHz) or series compensation inductors can be
higher torque than the slotless design. Part of this used to mitigate the low inductance issue.
comes from the fact that the slotted design can With these characteristics, the different
handle higher temperatures, thus allowing more brushless DC motor technologies lend themselves to
torque generation. However, due to the saturation of different applications. Slotted BLDC motors, for
the magnetic circuit during overloading operation, example, are ideal in applications such as electric
the motor torque is reduced. On the other hand, the vehicles or home appliances where a higher number
absence of teeth in the slotless design has no of poles is required and where ultimate size is less
magnetic saturation and therefore offers better of an issue. They are also preferred in harsh
overloading. environment since in slotted design the coil is easier
Another performance consideration is operation to protect and is mechanically held by stator teeth.
at high speed, and while both designs are capable On the other hand, when high speed and a small
of operating at far higher speeds than brushed DC size are required, for example in medical devices or
motors, slotted and slotless designs have different portable industrial tools, slotless BLDC motors can
characteristics at these elevated speeds. In order to offer the best solution.
obtain mechanical stabilisation under high speed www.portescap.com

February 2021 • INDUSTRIAL TECHNOLOGY EXTRA • p5