Jonathan Newell talks to Professor Simon Hodgson of Teesside University about pumping up the power for electric aviation without burning out the motors.
Hybrid and electric power could be the future for cleaner, greener aviation and all the major engine manufacturers are developing the technology that will keep them competitive in the future era of electric aircraft. However, such development isn’t without its challenges.
One significant challenge is ensuring that electric motor windings can endure the rigours of extreme environments. To find out more about this, I spoke to Professor Simon Hodgson, Pro Vice-Chancellor (Research and Innovation) at Teesside University, whose team is working on high-temperature wire insulation technology for the aviation industry.
The work undertaken at Teesside has so far been of benefit to two aviation giants, Rolls-Royce and Safran Electrical & Power UK, each of which has interests in very different applications.
Limitations on insulation
A constraint that has always existed with motor windings has been the heat resistance of the insulation and until now, the industry hasn’t made any massive leaps and bounds in development.
The windings of a century ago were insulated with Shellac and could withstand temperatures of no more than 150-170C before failing. Since that time, motors have typically seen and increase of 5-10C every decade. The move to man-made polymer insulation led to improvements resulting in the current resistance levels of 220-240C.
According to Hodgson, this is a key design limitation on producing reliable, high power motors. “As the power of the motor increases, it reaches the limit of the insulation causing failure over time. A rule of thumb is that the motor life can halve for every 10C of over-heating,” he says.
Teesside is approaching the challenge based on the fact that there are two significant sources of heat that the motor is exposed to. There is internally generated heat, which can be considerable and hard to model and there is environmental heat.
Internal heat – Temperature patterns within the windings can be very variable and can be a simple function of the amount of electrical energy or it can be more complex based on localised winding densities, which are harder to model and can vary considerably with manufacturing methods. If such localised heat sources are located near to a heat sink, they present less of a problem.
Traditionally, to overcome the possibility of hot-spots, motors tend to be over-specified. This itself causes a problem in weight sensitive aerospace applications.
Environmental heat – The main challenge associated with environmental heat is the need to use embedded motors in hybrid powertrains, where the motors will be exposed to the extreme heat generated by the engine in which they’re located.
Motors in hybrid aero engines
Rolls-Royce’s Future Technologies Group is engage in a major push to make step changes around engine development with moves towards hybrid propulsion combing gas turbine, generator and electrical propulsion motors.
According to Alexis Lambourne, a Novel Materials Specialist at Rolls-Royce, a key enabling technology for an aerospace hybrid propulsion package comes from the ability to integrate electrical systems into the core of a gas turbine. “Our need was to demonstrate high-temperature electrical wire insulation technology capable of operating at 450°C, thereby enabling this technology to go into electrical motors that would be used in a gas turbine engine,” Lambourne explains.
Teesside University provided the high temperature wire, and wire encapsulation through their coatings, that could survive such extreme temperatures.
For windings that are to be used in such hostile environments, Hodgson explains that the use of organic materials are not an option. The university team therefore looked at the use of ceramics. However, although such materials have the required thermal properties, they are mechanically poor. For this reason, Hodgson’s team developed a ceramic composite.
Ceramic composite insulation
Teesside university overcame the problem of ceramic rigidity by creating a composite from ceramic and polymer at the nano-scale. Mixing the materials at such a tiny scale creates a ceramic matrix with finely distributed organic material, which provides flexibility for winding.
Once it is wound, curing the winding “burns off” the organic material leaving just the matrix. The voids are so tiny that the ceramic settles into them, leaving a fully insulating coating.
This hard ceramic insulation has sufficient remaining flexibility for thermal expansion and contraction without being compromised.
More motors, more power
The second challenge faced by Professor Hodgson and his team was to enable higher power density motors that could be distributed throughout the highly electrified aircraft of the future.
The replacement of many hydraulic and pneumatic systems with electrical systems is more energy efficient and saves weight but places a requirement on more motors, which are smaller and more powerful.
According to Hodgson, in this scenario, insulation is the most usual point of failure and so one solution was to use more exotic polymers but this has a significant cost penalty..Instead, his team examined the ways in which the polymer fails.
“When a polymer is heated, the chains become more mobile, opening oxygen diffusing gaps and eventually disintegrating,” he tells me.
The answer that Teesside University came up with was to add molecular scale ceramic to link the polymer chains together. This successfully stopped the oxidisation of the polymer and took the temperature resistance up by 40C, a significant step in terms of operational life expectancy.
An advantage of creating this polymer / ceramic mix is that it lends itself to additive manufacturing using the precursor chemicals.
The use of additive manufacturing in this application as it overcomes some of the repeatability issues of existing techniques. “The winding becomes less of a birds-nest of wires and more of a repeatably manufactured structure,” Hodgson concludes.
More Electric Aircraft
The Advanced Electric Machine Technologies for Aircraft (AEMTA) Research & Technology programme, part-funded by Innovate UK, was created to re-establish and further develop electric machine design capability at Safran Electrical & Power UK. The programme included a large number of different technology strands, including high-temperature wire insulation.
Naveed Sheikh, Safran Electrical & Power UK Research & Technology Programme Manager, explained the rationale for the AEMTA programme: “The increasing electrification of functions on board aircraft is a formative and irreversible change that will move faster and intensify with future programmes. Hydraulic and pneumatic power is gradually being replaced by electricity. This has many advantages, both in terms of safety and the environment. We wanted to develop technologies to facilitate (More Electric Aircraft) MEA and establish a future UK supply chain for their commercial exploitation.
“Teesside University’s role was to create novel technologies required to produce the high performance, robust and lightweight electrical machines used in the MEA – particularly the electrical insulation materials which are one of the key performance limiting factors in these systems. We had problems where wire is used in high temperature, insulation would break down, and we needed enhanced capability in this technology. Professor Hodgson and his team successfully developed coatings to protect the wire and enabled it to perform at temperatures which met our needs.”