Slotless versus slotted 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.

The conventional brushless dc (BLDC) motor is a slotted design – the coils are wound within slots around the stator. With a slotless design the coil is wound in a separate external operation and is then inserted directly into the air gap during motor assembly.

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 minimized. 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.

Performance considerations

In the early days of slotless BLDC motor design, the power density was lower than for an equivalent slotted motor. However, the emergence of high energy permanent magnets and their alternative magnetization arrangements has closed the performance gap.

Slotted BLDC motors 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.

However, the slotted BLDC motor can still offer higher torque than the slotless design as the slotted design can handle higher temperatures, allowing more torque generation. But, due to the saturation of the magnetic circuit during overloading operation, the motor torque is reduced whereas an absence of teeth in the slotless design has no magnetic saturation and therefore offers better overloading.

Another performance consideration is operation at high speed. While both designs can operate at far higher speeds than brushed dc motors, slotted and slotless designs have different characteristics at elevated speeds. To obtain mechanical stabilization 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 offers 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 to 100 kHz) 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 are suitable for 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 environments as the coil in a slotted design 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.

This article originally appeared on Control Engineering Europe’s website.

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