Monday, 27 May 2019

High Performance Polymers in Electrification: A Must-Have Or A Nice-To-Have (Part 2: Traction Motors)




High Performance Polymers for E-Mobility (Part 2: Traction Motor); Source: findoutaboutplastics.com


Welcome back to the second part of the high performance plastics for electrification series. In the first part, we have discussed the polymeric materials used in battery systems. Now, we investigate the high performance plastics used in traction motors and transmission components.

Traction motor with transmission – turning voltage into movement:

In the next years the power density of electric cars will increase and this is followed by an increase in voltage. We will see cars with 800 V systems, which doubles today’s system voltage. As a consequence, system temperatures (120-140°C) and torque (up to 300.000 RPM) will increase and insulation properties and safe operation will have highest priority.

-Magnet wire coating:

To ensure safe operations at increased torque and voltage system levels, traction motors and power modules require the usage of polymers such as polyphthalamides (PPA), polyphenylene sulfide (PPS), and polyether ether ketone (PEEK). Magnet wire insulation can be achieved by using an extruded layer of PEEK on the rectangular shaped copper cable. The PEEK magnet wire coating allows for higher cable packing density and thus increased power. Furthermore, by using PEEK, high dielectric strength, chemical resistance, and thermal stability can be achieved. Abrasion resistance of magnet wires is a key property too and is fulfilled by PEEK.
In the picture below the cross section of a traction motor can be seen. In light brown, the PEEK extruded magnet wires can be seen. This example was shown at the Fakuma 2018 (Friedrichshafen, Germany) at the Solvay booth. The traction motor is from Essex Furukawa.


Magnet wire coating solution using PEEK, Solvay booth, Fakuma 2018  

Slot liners which guide the copper cables can make use of different high performance polymers such as PEEK or polyimide (PI) film solutions. Liquid crystal polymers (LCP) can be used for slot liners as well. LCP allows achieving long flow length and wall thickness down to 0.4 mm. Furthermore, LCP’s have a high relative temperature index (RTI) of 220°C combined with inherent V0 properties. Other alternatives are PI and aromatic polyamide (=aramide) sheets, which are folded and inserted in the stator as insulation.
Electronic components of the traction motor include terminals, bobbins, resolvers, and inverters. For such kind of components PPS and PPA can be used. PPA can achieve a Comparative Tracking Index of over 600 V and is available with organic-based heat stabilizers to prevent galvanic corrosion. PPS is inherently V0. However, it does not reach as high CTI values as PPA. Most PPS grades show CTI values of 280. There are PPS grades with 500 V, but the mechanical properties drop significantly, which makes them not suitable for such applications.
Traction motors can be directly cooled by using dielectric fluids such as PolyFluoroPolyEthers (PFPE). The latter, can also be used as lubricant components to prevent friction and wear. Proper lubrication ensures that traction motors can be operated up to 1.2 million kilometers with little maintenance efforts.

Power Distribution Units (PDUs)

For PDUs, the materials requirements are similar to traction motors: high dielectric strength, easy flow, weldline strength, thermal shock resistance, dimensional stability and flame retardant (V0) properties. PPA and PPS, together with PEEK fulfil this set of criteria.

– Insulated-gate bipolar transistor (IGBT):

In components like the insulated-gate bipolar transistor (IGBT), plastic components can be in direct contact with the printed circuit board (PCB). As a result, requirements for selected polymers are dimensional stability around 120°C - 150°C, CTI of 600 V, electric volume resistivity and most importantly, halogens are not allowed to gas out of the part. PPA and PPS can fulfill such stringent requirements since they have a high dielectric strength and are easy to process. Weldlines represent always a design challenge, however PPA and PPS have a good weldline strength (approx. 100 MPa). Increased current leads to temperature increase, which can also be easily handled by PPA and PPS type of polymers.


With this I will close the second part of this high performance polymers series for EV. In the third part, we will look into autonomous driving systems.

Thank you for reading!
Till next time!
Herwig Juster

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