Motors and Generators
By Mohamed Watouti | 26/08/2021


Permanent magnet motors are increasingly utilized in the different parts of industry such as automotive, aerospace, medical and home appliances. High efficiency, high power density and variable speed operation are some of advantages that resulted in widespread applications. Brushless DC (BLDC) motor and permanent magnet synchronous motor (PMSM) are two common types of permanent magnet motor which have almost same structure but are supplied with two different supply waveforms. While DC supply and trapezoidal back EMF waveform are used for BLDC motor, PMSM operates with three-phase sinusoidal waveform. Despite the differences in their structure and operations, their design process is almost same. 


Figure 1: Permanent magnet motor


Designing a permanent magnet consists of several steps such as choosing the materials and selecting the proper structure for the machine and geometrical sizing of the various components. The airgap length is one of the important geometrical parameters and it is determined based on mechanical, thermal, and electromagnetic factors. In this blog, the effects of airgap length on performance of a surface mounted BLDC motor are studied. For this purpose, MotorWizard software is used to simulate the machine performance. MotorWizard is a template-based finite element software which provides electromagnetic simulation of electric motors for SOLIDWORKS users. Providing different topologies, ability to easily edit the topology, and simple and straightforward workflow makes motor simulation easy. Furthermore, auto winding editor, diversity of results and large customizable material library are just other advantages of this software. 


Figure 2: Flux distribution of permanent magnet motor at no load condition

Torque and Torque Ripple

A BLDC motor with three different airgap lengths is studied (g = 0.5, 0.7 and 0.9 mm). Based on the FEA results produced by MotorWizard, reducing the airgap increases the airgap flux density which, in turn, increases the flux linkage and back EMF generated in the phases, thus, an advantage. The back EMF of the machine directly contributes to the electromagnetic torque.  Hence, the torque also changes versus the airgap, as shown in Figure 3.


Figure3: Electromagnetic Torque of the machines

Figure 3 shows that applying a current of 8 A, the machine with the airgap length of 0.5 mm produces an average torque of 7.58 Nm. The machines with the airgap length of 0.7 and 0.9 mm produce average torque of 7.19 and 6.84 Nm, respectively. In addition to the increase in the average torque, the torque ripple also increases. A part of the torque ripple arises from cogging torque.  Figure 4 shows the cogging torque of three machines.  Clearly, the machine with the lowest airgap length has the highest cogging torque. 


Figure 4: Cogging Torque of the machines


As a conclusion, it is proven that the airgap length affects the machine performance. According to the results, the smaller airgap length increases the flux linkage and the generated back EMF of the machine which, consequently, increases the developed torque. These are considered as advantages of having a smaller airgap length. However, at the same time it increases the cogging torque and torque ripple, which is a disadvantage.

Therefore, in addition to mechanical and thermal concerns, choosing the optimum airgap length depends on the objectives of the design and type of applications and requires a trade-off between several outputs of the machine.