May 2021

n MOTORS To slot or not to slot T he past decade has seen a growing acceptance of brushless DC motors as an alternative to brushed DC motors for applications requiring a combination of high speeds (above 12,000 rpm) and long operating lives. There are trade-offs, of course: coreless brush-commutated motors offer simple control and freedom from cogging, while the complex construction of brushless DC motors means they cost more. 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 making some engineers rethink their design decisions. The conventional brushless DC (BLDC) motor is a slotted design – the coils are wound in slots around the stator. In slotless designs, the coil is wound externally in a separate operation and is then inserted into the air gap during motor assembly. Let’s look at how these different approaches affect performance. The first thing to note is that slotless BLDC motors can be made smaller. In slotted BLDC motors, the stator teeth necessitate a larger motor, and the winding process becomes more difficult as the motor size is reduced. By contrast, the slotless BLDC motor has either skewed or axial windings fixed on the cylindrical stator iron core, making it easier to reduce their size. The slotless design also has a cost advantage because of reduced complexity and a stator core that is easier to manufacture. Instead of having to be inserted into slots, a slotless motor’s windings can be self-bonded with skewed or cup-shaped windings placed on the air gap. Performance In the early days of slotless BLDC motors, the power density was much lower than that of an equivalent slotted motor. Now, with the emergence of high-energy permanent magnets (such as those made of NdFeB or SmCo) and their alternative magnetisation arrangements (including radial, parallel and halbach), the performance gap has closed and interest in Brushless DC motors are ideal for high-speed applications, but there is a choice of architectures, causing a selection conundrum. Madhavan Ramanujam, r&d technologist at Portescap, and Cyril Baud, the company’s r&d manager, compare slotted and slotless motor designs. Conventional slotted brushless DC motors offer more torque than slotless designs