Ajedium™ PEEK Slot liners reduce eMotor length(-13% for stack length), decrease stator/rotor mass (-6.3%), & improve eMotor efficiency (2.1 %) vs NKN

By DeeDee Smith, Luigi Marino, Benoit Devaux, Victoria Lee, Jason Rich, Brian Baleno

Solvay Materials

One of the greatest challenges for eMotor 800 Volt designers is to identify solutions that can enable smaller and more compact eMotors while also finding ways to reduce the mass of both the eMotor and battery pack, by unlocking higher eMotor efficiencies. A new slot liner material, Ajedium™  PEEK slot liners, provides the potential to achieve all of these design breakthroughs. Conventional eMotor slot liner materials include paper and paper laminates. However, there are design limitations with these types of slot liner materials.  In fact, in the case of increased voltage,  the thickness of these incumbent materials also needs to be increased, thereby limiting the benefits provided by an 800 V system.

Therefore, in order to balance the copper fill factor, heat rejection capability, proper electrical insulation and expected life span, eMotor designers should first select the right insulation properties and thickness.

In this study, we utilized a virtual engineering software (ALTAIR FluxMotor 2021) to determine better ways to  incorporate  slot liners in the effort to reduce  eMotor length, light-weight the motor and battery pack, and improve the efficiency of the eMotor. Figure 1 introduces the eMotor design envelope used in the simulation.

This evaluation incorporates a comparative study and a sensitivity analysis on how slot liner thickness and thermal conductivity impact eMotor performance, efficiency and temperature distribution. The scope is not to determine the absolute values and behavior for each of the simulated designs. If that was the case, physical demonstrators would be needed to complete the study.

Figure 1: Design inputs for virtual engineering simulation

In the study, we consider three different thicknesses of Ajedium™  PEEK: 100 micron, 150 micron, and 250 micron and compare these three PEEK thickness to 250 micron NKN. 

As a first step, virtual engineering was used to determine the number of conductors that can be allocated in the slot by keeping the same number of turns and winding layout. Figure 2 below shows the improved thermal behavior enabled by the introduction of PEEK instead of NKN where PEEK allows for increased slot fill factor (at 150 and 100 microns), reduced magnet temperature and lower stator yoke temperatures.  

This design breakthrough is enabled by using thinner slot liners to give improved heat transfer in conjunction with the higher thermal conductivity value of PEEK (0.17 W/mK).

Figure 2: Virtual engineering results comparing NKN to PEEK

The second step consisted in converting the thermal benefit into more compact eMotor designs. While holding the same initial steady state winding temperature, the advantage of the thinner and more thermally conductive slot liners  allowed for an increase in fill factor and better heat dissipation capability. 

In the new study, we took into account a completed drive cycle rather than a single working point that was repeated until  a steady-state temperature in the windings was achieved (Figure 3). This drive cycle was designed specifically to take into account all  typical BEV driving conditions  such as urban traffic, highways, regenerative braking, etc.

Figure 3: Drive cycle conditions

An NKN baseline was established using 250 micron thick slot liners. Then the same was done with 150 micron in a series of two steps. The same motor length (84 mm) was initially taken into account, which showed a temperature drop. The next step was to factor in the reduction of the maximal winding temperature in an effort to make the eMotor more compact. A reduction in the motor length results in an increased motor temperature because there is less heat dissipation, due to the lower heat exchange surface, and a higher eMotor load (phase current) to recover the torque lost due the length reduction. The overall target was to reduce the length until the original maximal winding temperature was observed. However, the fully optimized design with a thinner and more thermally conductive slot liner would probably have a slightly different L/D (length over diameter) ratio. Hence, this optimization method would lead to conservative results on the benefits that an improved slot liner would provide.  

As highlighted below in Figure 4, using 150 micron PEEK slot liners provided  a 1.3 kg reduction in stator mass and a 0.9 kg reduction in rotor mass. The length of the eMotor using 150 micron PEEK slot liner decreased 6 mm from 84 mm to 78 mm, allowing for an overall weight reduction of 2.2 kg (-3.5%) on active parts only. This figure can be estimated to be as high as 2.64 kg if non active parts are taken into account.  

Figure 4: Stator and Rotor mass reduction with thinner PEEK (150 micron) slot liners

A further reduction in PEEK slot liner thickness (100 micron) was performed to determine if the eMotor  could be made more compact and if additional weight savings were possible. Figure 5 reveals that the length can be further decreased to 73 mm which is a savings of 11 mm (-13% on the stack length) from the original motor using NKN. Additionally, the  stator and rotor reductions from 250 micron NKN are 2.3 and 1.6 kg respectively, allowing for an overall weight reduction of 3.9 kg (-6.3%) on active parts only. This figure can be estimated to be as high as 4.68 kg if non active parts are taken into account.

For longer stack eMotors, the weight reduction benefit can potentially be even higher, getting closer to the stack length percentage of reduction (-13%), since end-windings, which basically remain the same with an improved slot liner, would account less in the overall weight computation.

Figure 5: Stator and Rotor mass reduction with thinner PEEK (100 micron) slot liners

Figure 6 summarizes the overall aspect of optimizing  eMotor slot liner thickness. Not only do thinner PEEK slot liners provide  both motor compactness and light-weighting, there is also a 2.1% improvement in motor efficiency. This efficiency figure is calculated as the average obtained for the simulated drive cycle. The drive cycle was purposely designed to span all the typical operating points, with reasonable cumulative time spent on each of them. Therefore, this result has a direct impact on the expected range of the BEV as well as the potential  battery downsizing for that same range.

Figure 6: Summary of Results with varying slot liner thickness

A more compact eMotor contributes to saving vehicle weight and makes packaging and vehicle integration simpler. Lower weight and easier vehicle integration allow for more design freedom for vehicle designers and potentially improve vehicle dynamics as well as weight on other vehicle components.

Finally, Ajedium™  PEEK slot liners provide many design advantages over paper and paper laminates. Some benefits of using PEEK slot liners include improved eMotor efficiency, potential reduction in the length of the motor, and also weight reduction of both the stator and rotor.