May 2021

MOTOR n slotless motors has revived. One might think that the same performance advantage could be gained in slotted BLDC motors, but they are less able to use high-energy magnets because the thicker teeth needed to increase the magnetic loading of the motor reduces the slot area, and thus decreases electric loading of the motor. That said, slotted BLDC motors can still offer higher torque than slotless designs. Part of this comes from the fact that the slotted machines can handle higher temperatures, leading to higher torque generation. However, due to the saturation of the magnetic circuit during overloading, the motor torque is reduced. On the other hand, the absence of teeth means that slotless motors avoid magnetic saturation and therefore offer better overloading. While both designs are capable of operating at far higher speeds than brushed DC motors, slotted and slotless designs have different characteristics at elevated speeds. To obtain mechanical stabilisation under high-speed (40,000– 60,000 rpm) operation, the slotless rotor usually 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 the slotless stator structure and they therefore offer high power densities. Slotless motors offer low inductance, introducing a motion control challenge. Low inductance results in higher motor losses if PWM (pulse width modulation) control is applied. To mitigate the low- inductance issue, controls with higher switching frequencies (80–100kHz) or series compensation inductors can be used. The different brushless DC motor technologies lend themselves to different applications. Slotted BLDC motors, for example, are ideal for applications such as electric vehicles or home appliances where a higher number of poles is needed and size is less of an issue. They are also preferred in harsh environments because in slotted designs the coil is easier to protect and is held mechanically by the 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 be the best option. n In slotted motors (left), the coils are wound in slots around the stator. In slotless designs (right), the coils are inserted into the air gap during assembly. GOING DON’T ROUND KEEP ON ea 365 days a y s a day 24 hour & Mechanical M vice, Ser , Sales motor and au o Contact us f AND RO r! eek, 7 days a w , ing Engineer aintenance ation related needs tom ic our electr r all y UND .on line s ic-motor .electr www .on line s ic-motor sales@electr 02476 980833 Slotless brushless DC motors can be made smaller than slotted machines.