PERMANENT MAGNET MOTORS
Permanent magnet motors are increasingly utilized across various industries, including automotive, aerospace, medical, and home appliances. Their high efficiency, high power density, and capability for variable speed operation have led to their widespread adoption. Brushless DC (BLDC) motors and permanent magnet synchronous motors (PMSMs) are two common types of permanent magnet motors. They share a similar structure but differ in their power supply waveforms: BLDC motors are powered by a DC supply with a trapezoidal back EMF waveform, whereas PMSMs operate with a three-phase sinusoidal waveform. Despite these differences, the design process for both motor types remains largely the same.
Figure 1: Permanent magnet motor
THE MOTORWIZARD
The design process for a permanent magnet motor encompasses multiple critical steps, such as material selection, structural determination, and the geometrical sizing of components. One key geometrical parameter, the airgap length, is influenced by mechanical, thermal, and electromagnetic factors. This blog examines how the airgap length affects the performance of a surface-mounted BLDC motor, using the MotorWizard software for simulation. MotorWizard is a template-based finite element analysis tool designed for the electromagnetic simulation of electric motors. It offers various topologies, enabling users to easily modify them. Its straightforward workflow, automatic winding editor, diverse results, and extensive, customizable material library greatly facilitate the motor design and simulation process.
Figure 2: Flux distribution of permanent magnet motor at no load condition
Torque and Torque Ripple
In this study, a BLDC motor with three different airgap lengths (g = 0.5, 0.7, and 0.9 mm) was analyzed. According to the finite element analysis (FEA) results generated by MotorWizard, reducing the airgap length leads to an increase in airgap flux density. This increase enhances the flux linkage and the back EMF generated in the motor's phases, offering a distinct advantage. The back EMF of the machine is directly linked to its electromagnetic torque, indicating that the torque varies with changes in the airgap length, as depicted in Figure 3.
Figure 3: Electromagnetic Torque of the machines
Figure 3 reveals that with an applied current of 8 A, the machine featuring an airgap length of 0.5 mm generates an average torque of 7.58 Nm. In comparison, machines with airgap lengths of 0.7 mm and 0.9 mm produce average torques of 7.19 Nm and 6.84 Nm, respectively. Alongside the increase in average torque, there's also a rise in torque ripple, which is partially attributable to cogging torque. Figure 4 illustrates the cogging torque for the three different machines, clearly indicating that the machine with the smallest airgap length experiences the highest cogging torque.
Figure 4: Cogging Torque of the machines
SUMMARY AND TAKEAWAYS
The airgap length in permanent magnet motors is a critical design parameter that significantly influences their performance. Smaller airgaps can enhance flux linkage and back EMF, leading to higher torque, an advantage for many applications. However, this comes with increased cogging torque and torque ripple, presenting a design trade-off. Optimizing airgap length requires balancing these effects to meet specific operational goals, highlighting the complexity of motor design.