Over the past decade, brushless DC motors (bldcs) have increasingly replaced brushed DC motors, especially in applications where high speeds (above 12,000 rpm) and long life are required.
But BLDC motors don't have all the advantages: BLDC motors offer simple control and no cogging, while BLDC motors' complex structure means higher costs—conventional BLDC motors are slotted designs , that is, the coils are wound in the slots around the stator.
As a result, a BLDC motor with a slotless design was developed, which has 4 major advantages over conventional slotted BLDC motors.
Slotless BLDC motors use a slotless design. The coils are wound in a separate external operation and then inserted directly into the air gap during motor assembly.
In slotted BLDC motors, the presence of stator teeth prevents the overall size of the motor from being minimized. In addition, as the size of the motor shrinks, the winding process becomes increasingly difficult. In contrast, slotless brushless DC motors have windings that are slanted or axially fixed on a cylindrical stator core, making size reduction easier.
The slotless design also has cost advantages as it reduces complexity and the stator core is easier to manufacture.
While both designs can operate at speeds much higher than brushed DC motors, slotted and slotless designs have different characteristics at high speeds. To obtain mechanical stability at high speeds (from 40,000 to 60,000 rpm), slotless rotors usually have a two-pole permanent magnet design. In addition, due to the existence of the large air gap, when the motor runs at high speed, the loss of the stator core is limited to an acceptable range. This means that a slotless BLDC motor benefits from a slotless stator structure with relatively low core losses and therefore high power density.
In fact, in the early days of slotless BLDC motor design, its power density was lower than that of the equivalent slotted motor. However, the advent of high-energy permanent magnets and their alternative magnetization devices has narrowed the performance gap. Slotted BLDC motors are less able to use high-energy magnets because of the thicker teeth required to increase the magnetic load on the motor, which has the effect of reducing the area of the slot and thus the electrical load on the motor.
Slotted BLDC motors can provide higher torque than slotless designs because slotted designs can handle higher temperatures, allowing more torque to be produced. However, due to the saturation of the magnetic circuit during overload operation, the torque of the motor is reduced, and the toothless in the slotless design has no magnetic saturation, thus providing a better overload.
Although slotless BLDC motors have many advantages over standard bldcs, in practical applications, slotless BLDC motors are not always the best choice. For example, slotless BLDC motors offer low inductance, which poses a challenge for motion control. If pulse width modulation (pwm) control is used, lower inductance results in higher motor losses. Controls with higher switching frequencies (80 to 100 khz) or series compensated inductance can be used to alleviate the low inductance problem.
In fact, different BLDC motor technologies are suitable for different applications. Slotted BLDC motors are suitable for applications such as electric vehicles or home appliances that require a high number of poles, and final size is not an issue. They are also preferred in harsh environments, as the slotted design coils are easier to protect and mechanically held by the stator teeth. And for applications that require high speed and small size, such as in medical equipment or portable industrial tools, slotless BLDC motors are a better choice, offering the best solution.
