WHITE PAPER CHALLENGES AND OPPORTUNITIES IN MINIATURE SLOTTED MOTOR DESIGN The high torque density and enhanced cooling capability of miniature brushless DC (BLDC) slotted motors are instrumental in numerous applications ranging from robotics to consumer electronics. However, to optimize the efficiency, performance, and reliability of the application, their specification requires careful consideration. Engineering integration advice in the selection of off-the-shelf slotted BLDC motors is advantageous, and considerations on bespoke design will benefit the application’s performance in the long term. With the absence of brushes for the process of commutation, miniature brushless DC motors have advantages over their brushed counterparts, including improved efficiency, high speed operation, longer lifespan, and opportunities for improved control. Within BLDC motors, there is a design choice for either slotted or slotless configurations. The stator, which is the outer stationary part surrounding the rotor, contains the windings that channel the flow of electric current. This generates a magnetic field that interacts with the rotor’s magnets, causing it to rotate and create motion. In a slotless BLDC motor, the windings are typically secured in the stator by resin or are held in place by a tape or mechanical retention device. Alternatively, in a slotted design, windings are retained within slots or grooves. Slotless motors are generally more compact, and these designs are also beneficial for applications that require the smoothest operation and high speed. However, slotted BLDC motors enable an increase in the amount of magnetic flux density in the airgap, which results in higher torque generation; this helps improve the motor’s dynamics, making this design more reliable. The slots also help to dissipate heat, enabling improved cooling. WHITE PAPER | Page 1 of 4 ©2023 Portescap. All rights reser

SIZING SLOTTED BLDC MOTORS Slotted BLDC motors present unique challenges during the design phase, meaning that careful specification is required. This includes accurate core loss calculation, as well as the accountability of material saturation and magnet demagnetization at higher operating temperatures, which can impact performance, efficiency, and thermal management. Sizing the motor to meet the torque requirement is a critical step. This calculation depends mainly on the electrical and magnetic loadings, alongside rotor and stator dimensions and the selection of the materials. As a broad principle,...

There are different slot/pole combinations possible with miniature slotted motors. A high winding factor should be considered to increase the torque constant of the motor. A high LCM, or Least Common Multiple, between the number of slots and poles (S, P), reduces the cogging torque magnitude, while a high GCD, or Great Common Divisor (S, P) helps to reduce the unbalanced magnetic radial forces and increase the radial symmetry. Traditional Intergral Slot Concentrated Windings (ICSW) two-pole slotted design shown in Fig.1 is possible, but the torque constant is lower than the four-pole design;...

In addition to the slot-pole combination, another configuration known as fractional slot concentrated windings (FSCW) is also possible. FSCW includes multipole ring magnet configurations with short magnetic circuits, and this design is suitable for low-speed, high-torque applications. Table 1 identifies the various slot-pole combinations and their benefits. SPECIFYING MINIATURE SLOTTED BLDC MOTORS The motor’s generated torque is proportional to its volume, the magnetic and electrical loading, and its slot-pole combination. Specifying these characteristics is dependent not only on the application’s...