Test and Simulation of Variable Air Gap Concept on Axial Flux Electric Motor

One of the most promising methods to reduce vehicle fuel consumption and CO2 emissions is by using an electric motor in the vehicle powertrain system to assist the internal combustion engine, or propel vehicle by itself. This paper discusses a new potential method to improve axial flux motor performance and efficiency, by dynamically changing the air gap between the rotor and the stator. A series of experiments have provided insight into how certain key characteristics of the variable air gap (VAG) across a wide range of air gap settings. The results show that, on increasing the air gap from the normal 1.2mm to as much as 18mm, the peak torque reduces from 72Nm to 16Nm while the maximum speed of the motor increases from 5500rev/min to over 7000rev/min. It was seen that the high efficiency region moves towards the higher speed region as the air gap increases. Also, on increasing the air gap, the motor had a higher torque output at high speed. This behaviour is of limited benefit in a fixed geometry design, but the implementation of a software controlled air gap design allows the motor characteristics to be varied to suit the prevailing operating conditions. To demonstrate this benefit, the experimental data were used to build a model of the motor with a dynamically variable air gap concept incorporated into it. This model was then used with a fixed ratio powertrain, combined with a simple vehicle model and exercised over the NEDC drive cycle to predict the savings it would achieve when compared to a standard electric motor of similar technical specifications. The model predicts the overall battery energy usage reduced by 0.72% when using a VAG design. In addition, the VAG concept has the potential to reduce gearbox complexity and provide better drivability at higher speeds over the standard motor.