Assessment of Dielectric Fluids in Aerospace Electrical Machines
With the rise of more-electric and hybrid aircraft there is a drive to increase the power density of electrical machines from around 8kW/kg to >20kW/kg. Better magnetic materials or electrical conductors are unlikely to deliver this step change, so to achieve this increase in power density the existing materials must be pushed harder and the associated heat generation (losses) must be managed. In many cases this will make it necessary to liquid cool electrical machines, the coolant will also need to be a dielectric (electrical insulator) and may also be needed to act as a lubricant.
The selection of an appropriate dielectric fluid for an aerospace electrical system should be made with supporting data. Some of the data will be available from the supplier but may need independent verification. Other data will depend on the operational environment and duties of the fluid. This ARP lists the key physical properties of a dielectric fluid which would be necessary to make an informed fluid selection and a managed introduction of a fluid into an aerospace application. The appropriate test methods for characterising the physical properties is referenced and briefly described. This guide does not replace the full standard, but seeks to highlight the appropriate test standard & methods.
Rationale: This recommended practice guide was developed to provide the electrical systems designer with sufficient data to design the electrical and thermal aspects of a liquid dielectric cooled electrical system.