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Slotless vs slotted brushless DC motor designs

04 May 2021

BRUSHLESS 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 and Cyril Baud compare and contrast slotted and slotless brushless DC motors

The last decade has seen a growing acceptance of brushless DC motors as an alternative to brushed DC motors for applications requiring combinations of high speed (above 12,000 rpm) and long operating life. There are trade-offs, of course: Coreless brush commutated motors offer simple control and free from cogging, while the complexity of construction of brushless DC motors mean the cost is higher.

For many applications, the choice between the two is clear, but for others where the requirements sit uncomfortably between the performance limitations of each technology, the decision is more difficult and frequently involves a design compromise in one area or another. But there is a new kid on the block – the slotless brushless DC motor – and the latest developments are giving engineers pause to rethink their design decisions.

The conventional brushless DC (BLDC) motor is a slotted design, by which we mean that the coils are wound within slots around the stator. In the slotless design on the other hand, the coil is wound in a separate external operation and is then inserted directly into the air gap during motor assembly. Let’s look at how these different approaches impact on performance.

The winding process becomes progressively more difficult as the motor size is reduced

The first thing of note is that slotless BLDC 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 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 than for an equivalent slotted motor. Now, the emergence of high energy permanent magnets (such as NdFeB and SmCo) and their alternative magnetisation arrangements (including radial, parallel and halbach) have closed the performance gap and caused a renewed interest in slotless motors.

One might think that the same performance advantage could be gained in slotted BLDC motors, but they are less able to employ high energy magnets because the thicker teeth required to increase the magnetic loading of the motor has the effect of reducing the slot area and thus the decreased electric loading of the motor.

That said, the slotted BLDC motor can still offer higher torque than the slotless design. Part of this comes from the fact that the slotted design can handle higher temperatures, thus allowing more torque generation. However, due to the saturation of the magnetic circuit during overloading operation, the motor torque is reduced. On the other hand, the absence of teeth in the slotless design has no magnetic saturation and therefore offers better overloading.

Another performance consideration is operation at high speed, and while both designs are capable of operating at far higher speeds than brushed DC motors, slotted and slotless designs have different characteristics at these elevated speeds. In order to obtain mechanical stabilisation under high speed operation (from 40,000 to 60,000 rpm), usually the slotless rotor has a two-pole permanent magnet design. In addition, the stator core losses are restricted to an acceptable range while the motor operates at high speed thanks to the large air gap. Core losses are relatively small due to its slotless stator structure and therefore offers high power density.

Slotless offer low inductance, which introduces a motion controls challenge. Low inductance results in higher motor losses if pulse width modulation (PWM) control is applied. Controls with higher switching frequency (80 – 100kHz) or series compensation inductors can be used to mitigate the low inductance issue.

With these characteristics, the different brushless DC motor technologies lend themselves to different applications. Slotted BLDC motors, for example, are ideal in applications such as electric vehicles or home appliances where a higher number of poles is required and where ultimate size is less of an issue. They are also preferred in harsh environment since in slotted design the coil is easier to protect and is mechanically held by stator teeth. On the other hand, when high speed and a small size are required, for example in medical devices or portable industrial tools, slotless BLDC motors can offer the best solution.

Madhavan Ramanujam is R&D technologist at Portescap, and Cyril Baud R&D manager

Key Points

  • In slotless motors the coil is wound in a separate external operation and then inserted directly into the air gap during motor assembly
  • High energy permanent magnets have closed the performance gap between slotless and slotted motors
  • To obtain mechanical stabilisation under high speed operation, usually the slotless rotor has a two-pole permanent magnet design