Electric machines

Introduction

Electrical machines, devices which convert mechanical power into electrical power and vice versa, are completely ubiquitous in the modern world. They generate electricity in power stations, wipe car windscreens, run computer fans and perform all manner of other tasks. Applications range from tiny motors for computer hard drives to large-scale industrial motors and machines for power generation. Moreover, electrical machines are constantly evolving and expanding into new applications. The developments in Axial Flux Permanent Magnet (AFPM) machines, enabling higher torque densities and higher efficiencies, promotes their application into new markets such as electric vehicles (EV) and wind turbines, hence encouraging also the evolution towards a more sustainable future.

Electric machines for electric vehicles

The ability of an electric machine to operate at rated torque over a wide speed range lends it self extremely well to vehicle applications. This factor alone allows the bulky and expensive gearbox to be replaced with a fix gear ratio or even direct drive. Advanced motor control schemes such as Space Vector Pulsed Width Modulation (SVPWM) allow the speed range to be extended. Two advantages of electric machines over Internal Combustion Engines (ICEs) are: they have a response time of a few milliseconds, over several hundred milliseconds of an ICE, and their torque can be measured accurately via current sensors. These allow improvements in vehicle control such as ABS, traction control and torque vectoring.

Surrounded by bits of motors

Electric machines for wind generation

AFPM machines have short axial length and require less active materials (permanent magnet, iron cores, copper and back iron) as compared to Radial Flux Permanent Magnet (RFPM) machines. Replacing RFPM machines, which are widely used in the current direct-drive megawatt scale wind turbines markets, with AFPM machines, increases efficiency and reduces cost. However, the disadvantages of AFPM machines are the large attractive forces due to the permanent magnets and their large outer diameter. In order to maintain the airgap clearance under such large forces, conventional AFPM machines result in a bulky and heavy back iron design. The weight and large outer diameter of conventional AFPM machine is one of the main problems on installation. Therefore, the challenge for engineers is to design a light and smaller diameter AFPM machines for wind generation.

Thermal considerations in electric machines

In electrical machines temperature has an effect on lifetime/ degradation and performance. Current density, which is usually proportional to torque density, is thermally limited. Temperatures which are too high will lead to premature failure, for example of stator winding insulation. The insulation lifetime of a machine varies inversely with working temperature. In axial flux electrical machines, the stator surface convective heat transfer is a key factor affecting cooling. In order to measure this we developed a novel electrical method to measure heat flux, shown in the photographs below, and also compared the results obtained with computational fluid dynamics simulations. We are currently researching a variety of aspects of the thermal performance of electrical machines, such as novel cooling approaches, prediction of temperatures using models, and improved thermal designs.

Heater array

Heater array