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Next generation motors for Electric Vehicles - Part I

2023-07-17


1.Background 

This article describes the key considerations related to high performance electric passenger car traction motors, and is organised as follows. Section 2 describes typical requirements, including the torque-speed characteristic, dc link voltage, available coolants as well as the target performance metrics. In Section 3, the choice of magnetic materials (hard and soft) is discussed, together with the cooling technologies. In Section 4, using a unique sizing environment the Interior Permanent Magnet (IPM), Permanent Magnet Assisted Synchronous Reluctance (PMaSynRel), and Induction Machines (IM) are compared in details on various considerations including power density, efficiency and VA requirement for a range of pole numbers. Section 5 discusses the main outcomes from this article.

2.Machine requirement

The typical machine requirement is firstly described in this section. Consequently two selected commercial EV machines are reviewed for putting the performance metrics in perspective.

The typical target requirement is summarised in Table 2.1, corresponding to a next generation electric motor. The rated power is 80 kW, with the said power which needs to be achieved between 4000 and 16000 rpm,corresponding to a torque of 190 Nm at the base speed. The overall target power density is 20 kW/L in line with the APC next generation passenger car motor roadmaps [1], while the DC link is 800 V in line with the next generation trends [2]. Within such systems both water and oil are typically available as coolants for the electrical machine thermal management.

Table2.1 Typical motor requirements for EV

From Fig.2.1,when the drive cycle is further examined, it is evident that over 50% of time is spent between 2000 rpm and 10000 rpm at intermediate loads.

Fig.2.1 Torque speed characteristic and key motor operation points 

3.Material and Thermal Management Choices 

Improved materials and targeted thermal approaches plan an important role in improving the technical and cost-performance of EV motors. 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 as much as ten times that of SiFe. 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].

Fig. 3.1 Electrical steel grades map

Considering the high frequency operation of the next generation EV motors, NO20 can be considered suitable for stator lamination material due its good balance between technical and costperformance metrics. Similarly, 35HXT780T can be considered suitable to be used as rotor lamination due to its high strength, good ductility and existing use in automotive applications.

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

Induction Machines can also be considered for EV applications. 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, 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 to use in a high volume manufacturing environment. 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).

Fig. 3.2 Copper alloys map

For this article 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 future EV motors, 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]. 

 

Fig. 3.3 HTCs for various cooling media and strategies

Fig. 3.4 Comparison of continuous current densities for different cooling methods

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 current densities which can be sustained with water-jacket cooling are around 8-12 A/mm2.

Oil-cooling has been used at UoN for the last decade for 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 current densities of up to 30 A/mm2, and correspondingly very high power densities [12]. For some of the aforesaid 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. 

Fig. 3.5 Illustration of the oil semi-flooded cooling concept, and oil-spray cooling

While such a thermal technique is proven effective, the introduction of the stator sleeve introduces manufacturing complexities in a high volume automotive production environment. 

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 can be considered as one of the most suitable thermal management techniques for EV motors.