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Thermal Expansion In Aircraft
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Department of Mechanical Engineering, University of Maryland, 8228 Paint Branch Dr., Rm 3131, College Park, MD 20740, USA
Date received: 2019 April 16 / revision date: 2019 August 9 / Date of admission: 2019 August 24 / Publication date: 2019 September 20
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Improved cooling, combined with new semiconductor designs and packaging, revolutionized communications, computing, lighting, and energy conversion. The time has come for a similar revolution that will unlock the potential of electrified propulsion technologies to increase fuel efficiency, reduce emissions, and increase power and torque density in aviation and beyond. Electric motors with high efficiency and high specific power (kW/kg) are key factors in the electrification of future aviation. To improve the cooling of new synchronous motors and achieve performance and cost metrics for next-generation electric motors, electromagnetic and thermomechanical co-design can be achieved using innovative design topologies, materials and manufacturing methods. This article focuses on recent advances in thermal management of electric motors, with special emphasis on electric motors that are important for aeronautical propulsion.
The global aircraft fleet is expected to more than double to 48,000 in the next 20 years. Growth in air travel is driven by population, economic growth and a growing global middle class [1]. One of the biggest expenses for airlines is jet fuel, which accounts for about 20% of annual costs [2]. As shown in Figure 1 [3, 4, 5, 6], electric motor propulsion can reduce costs by improving aerodynamic efficiency through the wider use of distributed propulsion rather than single-aircraft propulsion in production jet aircraft. . Other advantages of electric motor propulsion include less noise and vibration.
For electric drive to be widespread, electric machines must have a high power density. High gravimetric power density improves aircraft efficiency by reducing takeoff weight, while increased bulk power density reduces frontal area and drag [7]. However, current commercial high-power motors can only operate continuously at about 5 kW/kg. Peak power above 10 kW/kg indicates thermal limitation of the motor, but the duration is limited to about 20 s before the temperature rise becomes uncontrollable [8, 9, 10]. A high conversion efficiency is also desirable because, for example, a motor producing 60 kW of rotor power and operating at 95% efficiency produces 3 kW of heat. In aircraft, heat production is the aircraft's energy (storage mass) that is not used for propulsion, but larger capacity cooling systems are generally larger and heavier. Therefore, to increase the viability of electric aircraft by integrating high-performance cooling systems, it is necessary to increase the power density of the DC motor while maintaining efficiency. The development of high power density engines can also increase the possibilities of electric personal transporters, multi-passenger vehicles and trains, and compact generators.
The compactness inherent in permanent magnets, created without bulky electromagnets inside the rotor, combined with more efficient operation, makes this motor architecture useful for aircraft propulsion. Most permanent magnet motors have an electromagnetic stator that synchronously drives a permanent magnet rotor that then transmits power to the drive blades. Figure 2a shows an engineered radial flux permanent magnet motor capable of driving an aircraft propeller. Figure 2b shows a computer-aided design (CAD) radial cross-section of a similar simplified engine, illustrating the main heat generation locations of such an engine.
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Figure 3a shows an exploded view of an axial flux permanent magnet machine, and Figure 3b shows an axial cross-section of the same machine. Axial flux motor architectures generate magnetic flux along the motor shaft, so the rotor and stator must be of the same diameter and adjacent to the motor shaft. Axial flow engines driving individual propellers have been demonstrated in high-performance electric aircraft competitions [11].
As shown in Figure 4, four common types of motor architectures for electric propulsion have been demonstrated. Figure 4a is probably the most "traditional" motor architecture, where the stator surrounds a rotor that drives the shaft. The magnetic flux flows radially from the stator windings to the permanent magnets. Figure 4b shows a somewhat similar configuration to Figure 4a, except that the rotor is hollow, allowing the drive vanes to be inside the rotor. It is said to be an "inner rotor" rotor design. Figure 4c shows an "outer rotor" rotor design where the stator is inside the rotor and the rotor may have blades on the outside. "Dual-rail" rotors combine an inner and outer rail configuration, which can increase efficiency at the expense of increased manufacturing complexity [12]. Figure 4d shows a similar cross-section of an axial flux permanent magnet motor, which is commonly used in a shaft drive configuration. Propulsion configurations include fixed-pitch propellers, variable-pitch propellers, counter-rotating (counter-rotating) fans or propellers, ducted fans, and impeller-driven fans [13]. Each of these configurations is compatible with a distributed engine.
Like any electrical device, permanent magnet motors generate heat. For example, a motor producing 60 kW of rotor power and operating at 95% efficiency produces 3 kW of heat. Without proper cooling, heat generation can cause temperatures to rise, which can have many negative effects. First, excessive temperatures can cause catastrophic failure due to the loss of integrity of the polymer electrical insulation and bonding materials. Second, temperature cycling induces mechanical stress due to thermal expansion, leading to fatigue, which is further increased by the extent of thermal cycling. In electric motors, the coefficient of thermal expansion of permanent magnet materials is about twice that of silicon steel, which can lead to mechanical failure of the magnets under excessive temperature cycling [14]. High temperature demagnetization can also lead to loss of permanent magnet function [14].
Increased temperature is also associated with reduced engine efficiency. The resistivity of copper increases with temperature, and the thermal load due to Joule heating increases with temperature. Efficiency can also decrease due to permanent degradation of magnetic properties (decrease in remanence and coercive force) at high temperature [14].
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A heat generation (loss) reduction strategy that has long been integrated into electric motors is the use of laminated rotor and stator cores. Electrical insulation between sheets of high-permeability silicon steel increases the resistance of the electrical current path in the material, reducing eddy current and iron losses. The same strategy can be applied to permanent magnets to reduce magnet losses [14]. However, it should be noted that electrical insulation is often also thermal insulation, which reduces cooling efficiency under residual heat loads. The bonding material holding the permanent magnet is also thermally resistant, but in most cases the heat generated in a permanent magnet is less than that of copper or iron [14].
In modern electric motors, the biggest factor in heat production is the copper stator windings. For example, in an air-cooled motor optimized for NASA's SCEPTOR program, copper (resistive) losses accounted for 43% of the total thermal load, and iron losses (mainly due to Joule heating due to eddy currents) accounted for 37%. and the permanent magnet rotor load accounted for 20% of the balance [12]. Depending on the engine configuration and operating conditions, copper losses can be the dominant heat source, accounting for more than 64% of the total heat [15].
The main way to increase efficiency is to model and optimize engine parameters. The NASA SCEPTOR engine was modeled using 2D finite element analysis [12]. Optimization variables include rotor configuration (inner rotor, outer rotor or dual rotor), number of stator slots and rotor poles, stator slot to rotor pole ratio, motor mass, operating speed, torque, voltage, lamination size,
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