The dominant trend within the automotive industry for battery electric vehicle (BEV) traction electric motors is the use of permanent magnet synchronous machines (PMSM), and within that category, the internal permanent magnet (IPM) radial flux (RF) machine seems to be the preferred topology of choice. 

Within the Hypercar and S-Segment vehicles the surface permanent magnet (SPM) RF machine is more prevalent due to the higher performance capability and the reduced package size and mass. 

The magnet-free switched reluctance or induction machines are relatively common from manufacturers that have a desire to be magnet-free. Still, also this motor topology is good for overall vehicle efficiency if more than one motor is installed on a vehicle because it has very low no-load losses and can be free-spinning without significantly reducing the vehicle range. These machines are also typically slightly lower in cost, making them desirable for cost-sensitive segments. Still, they have low power density meaning they are rarely used in higher performance vehicles. 

The use of IPM RF machines provides relatively good efficiency in the low speed and low load region which means that on a WLTC or WLTP drive cycle test they tend to have an advantage over other topologies. 

The axial flux (AF) PMSM topology is gradually gaining publicity and is likely to be introduced to the market soon by Mercedes-Benz and other manufacturers. The AF motor design can offer performance and/or packaging benefits but has been held back from wider adoption due to concerns over manufacturability and reliability. 

Performance comparison between main EV/HEV topologies

System Level Trade-Offs 

The definition of a BEV traction motor system involves several considerations, which result in trade-offs and compromises: 

1. The battery is rated for a certain power drain (kW) which is referred to as the C-rating.  
2. The inverter(s) to control the vehicles traction motor(s) has a maximum phase current, maximum continuous power rating and a maximum modulation index, which is the space vector modulation used to apply positive or negative voltage across the motor phase connections. 
3. The electric motor has torque, power and maximum speed trade-offs which are a characteristic of the electromagnetic design but also have a relationship with the inverter due to the electromagnetic circuit, i.e. required phase currents, electrical resistance, inductance, required switching frequency, maximum power factor, back EMF and short circuit current. 
4. The gearbox to provide increased torque and tractive effort at the wheels is a mechanical element which needs to be matched to the target maximum vehicle speed (the equilibrium of vehicle drag and motor power) and the maximum motor speed, but should also consider vehicle performance and efficiency targets, and how it will be integrated with the motor, lubricated and cooled. 

This means that the design of the battery, inverter, electric motor and gearbox should be performed holistically, knowing that each component has parameters and characteristics which should be considered and tuned to suit the overall system and vehicle performance objectives.

To understand the system level trade-off better it is important to note the relationship between torque and power. 

P_kW = T_Nm * N_rad/s ➪ Power equals Torque multiplied by angular velocity 

In the example of a car with a conventional internal combustion engine (ICE) that produces about 200kW (268bhp) with a typical maximum vehicle wheel speed between 1800rpm to 2000rpm, a multi-ratio gearbox is used to provide the required low-speed tractive effort, as shown in Figure 2 below, and achieve vehicle acceleration targets

To replace the whole ICE powertrain and driveline with an electric motor the optimum system is still to use a gearbox, but now instead of needing multiple gear ratios to achieve the desired vehicle acceleration and maximum speed, a single ratio gearbox typically provides sufficient mechanical advantage (low speed and tractive effort for towing, kerb crawling etc) and can still achieve the desired vehicle acceleration and maximum wheel speed.

Generally, designing a system without a gearbox, so motor speed = wheel speed, the required torque from the motor will mean that the size and mass of the motor becomes uneconomical from a packaging and efficiency perspective.

The gearbox has losses, and the losses (measured in Watts) tend to be fairly constant no matter what gearbox topology or torque input is applied, but the losses increase as input speed increases.

This means that with low input torque the gearbox efficiency is very poor. Therefore when a high-speed motor is used, with a high gear ratio, the input speed is higher and the input torque is lower meaning the efficiency of the gearbox is worse. With a lower ratio gearbox, the motor needs to produce more input torque to achieve the same vehicle target, so the input speed is reduced, and this means that gearbox losses decrease.

The efficiency map below (Figure3) shows the general trend where efficiency improvement with torque is significant, but with speed there is no change. So increasing speed and decreasing input torque result in lower efficiency.

Therefore we should always consider at a system level the motor and gearbox efficiencies together. It is highly probable that using an electric motor which can produce higher torque will be beneficial.

Electric motor design characteristics have a torque and power curve defined by the constant power speed ratio (CPSR). The CPSR is the ratio between the maximum speed of the motor and the speed at which peak power can be achieved. The process is a little more complex than this, but from a high level, it is not worth going into the details at this time.

If we consider that the target power is still 200kW for the vehicle, and we have reduced the gear ratio and increased the motor torque to improve system efficiency, then we have not changed the CPSR, but we have proportionally reduced the Nmax of the motor.

If we were to make the assumption, from an electric motor and gearbox perspective, that it is better to have higher motor torque and a lower gear ratio. We need to understand that higher torque from the electric motor involves trade-offs. For instance the motor could be designed with lots of iron and copper to generate a strong magnetic flux. But the motor will then be compromised at lower loads and at high switching frequencies.

Typically with an RF motor for increased torque the axial length of the machine can be increased. However this is not a straight-forward gain because the EM circuit has changed meaning that there is a trade-off elsewhere, typically in terms of losses at higher speeds.

The AF PMSM technology has a known advantage for the potential torque it can generate. This is because of the larger radius of the magnets compared to a RF machine. This can be shown by the formula*1 below where the Torque is a factor of the diameter.

Therefore for the same inverter phase current, and the same diameter of motor, the torque will be increased.

But we could just increase the inverter phase current! However this has some drawbacks on the inverter and motor design.  

• The inverter cost will increase

• The busbar size will increase

• The required phase cable CSA and mass will increase

• The motor size and mass will increase (due to increased copper and iron)

So from a system level matching the inverter specification, in terms of peak phase current, peak and continuous power ratings and switching frequency, with the motor can mean a lower cost and lighter inverter, but also a lower cost and lighter electric motor.

N.B.: It is also worth pointing out that there is a small change to the overall motor length component with an AF machine. This is because the rotor back-iron will get thicker with increased diameter.

Low Motor Size and Mass 

 In the previous sections we have explored that matching the inverter, motor a gearbox to reduce phase current, increase motor torque and reduce gear ratio will provide system level improvements and a lighter overall EDU.

If we consider the vehicle now within that system, a lighter EDU will allow the vehicle manufacturer to either increase range for the same battery capacity or increase battery capacity for the same vehicle mass, which overall gives the same benefit to the end-user.

But if we now consider that reducing EDU mass is also highly likely to result in a reduced EDU size, and that for some high-performance vehicles a dual motor (torque vectoring) EDU is required, the yokeless dual rotor AF motor design allows further benefits. This is because the torque for a fixed outer diameter can be maximized and the total length (axial length ð aligned with the drive-axis) can be smaller compared to a RF motor design, giving packaging benefits within the vehicle.

Any vehicle with a smaller and lighter powertrain will have certain advantages which can be exploited by the vehicle designer.

For instance lower mass can have a compounding affect because loads and stresses are reduced, and ancillary and peripheral components therefore reduce in mass for the same stress.

Smaller packaging size/volume means that there is more freedom to place components in the available axial space between the wheels, and this means inverters or battery control modules or other ancillaries can be relocated and this is likely to result in more space for batteries or passenger volume, or improved vehicle design for other dynamic or application specific benefits.

Lower mass also improves vehicle handling and dynamics, and there are various studies which prove the compounding effects, but no better example than motorsport where the weight and space savings are exploited fully to make a more optimum vehicle design.

It would be very rare and unusual for a smaller and lighter EV powertrain to not show benefits at vehicle system level. However, the lighter and smaller powertrain is unlikely to pose any challenges. For instance, even in an application where the mass of the vehicle is high, such as a heavy goods vehicles, the overall unladen mass of the vehicle is still critical to the application because they may be limited by the gross laden weight, and also the cost of usage is a critical aspect being considered for the lifetime cost analysis, so weight savings will be a factor in the efficiency of the vehicle calculated over the full lifetime of ownership/use and not just as a metric for marketing or sales.

The lightest and therefore most torque-dense or power-dense electric motor designs tend to be based on the SPM design, in either RF or AF topologies.

The yokeless dual rotor AF machine provides benefits due to the double rotor architecture and a stator design which due to the straight flux path can take advantage of higher flux density silicon steels, however typically the cooling of this architecture is inferior to RF designs resulting in lower continuous performance.

Traxial have developed an oil-cooled yokeless AF motor which has improved cooling compared to other AF motors and has very high torque and power density.

Another factor, not yet considered deeply by many manufacturers when comparing and selecting the PM motor topology, is the mass of raw materials used in the motor, i.e. iron, copper, magnets etc, which all have an environmental impact. In the EU this is considered as the CO2 created during manufacturing but is also considered as the environmental load units* (ELU) which translates CO2, or environmental damage, into a monetary value.

Likewise, the USA has an equivalent called life cycle assessment (LCA) and Japan has material flow cost accounting (MFCA).

In all assessments, the lower raw material usage of an AF motor results in a reduction compared to an equivalent power RF motor, with one study showing 47% reduction.

Relationship between Max Speed and Max Continuous Power 

The relationship between the target vehicle maximum speed (wheel and motor speed) and the continuous output power of the battery, inverter, motor and gearbox is an interesting dynamic which is often not fully explored at the concept stage of the vehicle design. 

However, this is one of the most important factors which affect the vehicle efficiency. 

The battery, which so far has not been considered in too much detail, has a certain current capacity or derate capacity which is the C-factor. A higher C-factor means the battery can sustain a higher draw. 

The efficiencies of all the downstream devices, inverter, motor and gearbox therefore affect how much power can be delivered (continuously) to the vehicle wheels. 

Improving the efficiency of this system is essential to delivering maximum continuous power.

But for the powertrain the peak efficiency is typically optimized to be across a certain vehicle drive cycle in order to improve the operating range under ‘typical’ conditions.

This means that the motor’s continuous high-speed power and efficiency is compromised slightly, and this may compromise a vehicle’s maximum speed due to some form of thermal derate or voltage derate under high continuous loads.

When considering this factor, it is also important that the motor design has very good thermal performance at high speeds to reduce the affects of thermal derate under continuous conditions.

In summary, a machine that has low mass, small package size, high torque per phase current, good balance between high peak (and continuous) power and maximum speed, good drive cycle efficiency and good high speed heat rejection will provide system level benefits and overall be a good match for many vehicle applications and sectors.

This is where the Traxial yokeless dual rotor AF motor is believed to be superior to all other motor topologies and will start to penetrate the automotive and e-mobility markets. 

References: 

Ghazali, A., Hassan, M. K., Mohd Radzi, M. A., & As’arry, A. (2023). Optimizing energy harvesting: A gain-scheduled braking system for electric vehicles with enhanced state of charge and efficiency. Energies, 16(4561). https://doi.org/10.3390/en16124561

Verbelen, F., Defreyne, P., Sergeant, P., & Stockman, K. (2019). Efficiency measurement strategy for a planetary gearbox with 2 degrees of freedom. EEMODS 2019, Energy Efficiency in Motor Driven Systems, Proceedings. Presented at the EEMODS 2019, Tokyo. 

Zhang, R., Li, K., Yu, F., He, Z., & Yu, Z. (2017). Novel electronic braking system design for EVs based on constrained nonlinear hierarchical control. International Journal of Automotive Technology, 18(5), 707-718. https://doi.org/10.1007/s12239-017-0070-0

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