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FAST MOTOR -A new motor for EV-PartⅡ

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1001775651285196800


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3.Material and Thermal Management Choices


The FAST team have leading expertise in both magnetic material characterisation as well as in advanced motor thermal management. In this section these two aspects and downselections are discussed.


3.1.Material Choices


As the flux-sources and key magnetic circuit flux paths, hard and soft magnetic materials play a crucial role in the design and performance of electrical machines. Fig. 3.1 shows commercially-available soft magnetic materials, namely CoFe-alloys and SiFe-alloys mapped against their two most important properties for high performance machines – the losses at high frequency (W/kg, 1T 1kHz) and the yield strength (MPa). Mass-produced machines used for transport applications currently predominantly use 0.35mm-thick silicon-iron (SiFe) grades. Higher performance machines in the automotive industry are increasingly moving towards the use of 0.2mm-SiFe, such as the grade NO20 which reduces the core losses by 40% at the reference frequency, down to 40 W/kg. Further outstanding iron loss reduction in SiFe can be achieved by going down to 0.1mm thickness, and doubling the amount of Silicon to 6%, as in the grade 10JNEX900 which halves the losses to 20 W/kg [3]. The saturation flux density of SiFe grades is around 1.8T. If further weight/volume savings are required by the application, CoFe grades may be used which have a saturation flux density of around 2.3T.


The best-in-class CoFe grade currently goes down to 0.055 mm in thickness and is made up of 50% cobalt [4]. The improved performance of CoFe comes at a significant cost premium, with the cost per kg being roughly around ten times that of SiFe in the same thickness. Electrical steels have typical yield strengths of ~300-400 MPa. This limits the choice of topology and the rotational speed for high-speed applications. Recent developments, using techniques such as dislocation strengthening, enable electrical steels to achieve yield strengths of over 800 MPa
[5], such as the grade 35HXT780T, which enables the increased uptake of topologies such as Induction Machines and interior PM machines for high speed applications [6].

Considering the high frequency operation of the intended motor application, NO20 is initially selected for stator lamination material due its good balance between technical and costperformance metrics. Similarly, for the high speed traction motor requirement, 35HXT780T is found suitable to be used as rotor lamination due to its high strength, good ductility and existing use in automotive applications.


For the permanent magnets, considering the manufacturing of the FAST motor in China, the main material of interest is Neodymium Iron Boron (NdFeB) in order to achieve high power density, however ferrite magnets will also be considered at the initial design stage for a complete picture including possible benefits of a rare-earth free topology.


Within the early stages of the FAST project, Induction Machines will also be investigated. The choice of material for the rotor cage of high speed induction machines necessitates careful considerations. Pure copper is often not used due to its lower yield strength and the tendency to soften at higher temperatures. Different types of copper alloys can be considered as shown in Fig. 3.2 [6], however, in general the stronger the alloy is, the less is its electrical conductivity which increases the losses. With induction machine losses in the rotor are more difficult to dissipate. With many commercial high speed induction machines, operating to the limit line of Fig. 2.1, often alloys of CuAl2O3, CuCr, and CuZr, with yield strengths of around 400 - 450 MPa are used. For higher strength, above 700 MPa, the material of choice has traditionally been CuBe, however it is well known that using this alloy is difficult due to Beryllium being a carcinogen, and its use and machining is only allowed in special circumstances with high levels of protection for those in close proximity. A recent interesting development are the so-called Colson alloys (CuNiSiCr) which achieve similar performance to CuBe, without the use of


Beryllium, thus facilitating their implementation in a mass-production environment [7]. Current research is looking at pushing the aforesaid alloys to ever higher performance, with strengths in excess of 900 MPa and conductivities above 70% IACS (International Annealed Copper Standard).

For this report for the IM topology CuCr is chosen for squirrel cage bars due to its good conductivity and yield strength, while for the end-rings, where the stresses are higher, CuNiSiCr is considered.


3.2.Thermal Management Choices


As the power density requirement of electrical motors becomes higher, as in the case of the application in hand, one of the main bottlenecks becomes the heat loss dissipation from the motor windings. Various types of cooling techniques can be used for electrical motors, including air, water and oil [8].

 

Traditional industrial motors are naturally-air cooled or air-forced cooled. While an air cooling system is simple to implement, from Fig. 3.3 the heat transfer coefficient (htc) is typically low, up to around 150 W/m2K. Current densities of around 3-5 A/mm2 can be used with forced air cooling, as shown in Fig. 3.4, which limit the power density. Mainstream automotive motors use water cooling. The main advantage is that the heat transfer coefficient of the water cooling system is very high, in the order of several thousands of W/m2K. On the other hand the drawback with water is that since it is electrically conductive, it is not possible to have the windings (where the main heat sources are) in direct contact with the water [9]. Thus an indirect cooling system with a water jacket is typically used, which increases the thermal resistance path between the copper losses and the coolant, due to the various materials in between, such as enamel, resin, air-bubbles, and the contact resistances. Typical continuous current densities which can be sustained with water-jacket cooling are around 8-12 A/mm2, with the higher end values corresponding to the use of special resin materials with a higher thermal conductivity.


Oil-cooling has been used at UoN for the last decade for aerospace electrical machines with world-record power densities [10, 11]. The advantage with oil-cooling is that the oil is an electrical insulator which means it can be directly in contact with the winding (direct-cooling), enabling continuous current densities of up to 30 A/mm2, and correspondingly very high power densities [12]. For the aforesaid aerospace machines, a semi-flooded oil cooling arrangement was developed by UoN, with the stator and rotor separated by the composite sleeve, resulting in a wet-stator, and the dry-rotor, as shown in Fig. 3.5.

 

While such a thermal technique is proven effective, the introduction of the stator sleeve increases the magnetic airgap. While for aerospace this is not often a problem, with automotive machines where the magnet mass (and cost) minimization are critical design points, such an arrangement is unfavourable. On the other hand, flooding the entire machine with oil at high speed is unreasonable as the windage losses would be huge [13].


A recent technique is to use oil-spray cooling for automotive motors. The oil-spray provides similar heat transfer coefficient as the oil-jet, and since it consists of tiny spray particles, the resulting windage losses can be kept low. The drawback with spray is that cooling can be uneven due to the placement of nozzles, gravity effects, as well as oil-film formation effects. UoN is leading research into this to minimize such effects and accurately model the spraying effects [12].


With the aforesaid intentions of maximising the power density and efficiency, whilst reducing the material cost, oil-spray cooling is chosen for this development.

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