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

If you liked this post, share and like!

Interested in my monthly blog posts – then subscribe here.
New to my Find Out About Plastics Blog – check out the start here section.
Check out also my personal webpage.

What is the thermal conductivity of plastics? Thermal conductivity of 96 plastics for EV application design support

Thermal conductivity of 96 plastics

In emerging electric vehicle applications which includes traction motors, battery cages, and power electronics the focus is kept more and more on the material parameter “thermal conductivity”. Removal of entrapped heat is important due to heat’s high impact on overall system performance. Thermal conductivity is the transfer of heat from one body to another body which is in contact with first. It is measured as W/mK.

The rule of thumb for thermal conductivity in plastics is as follows:

1. For amorphous thermoplastics at 0-200°C, the thermal conductivity lies between 0.125-0.2 W/mK.

2. For semi-crystalline thermoplastics at 0-200°C, values can exceed 0.2 W/mk. Inherently ordered crystalline regions lead to higher thermal conductivity

“What is the thermal conductivity of xyz-plastic?”

I created a table based on published literature, which can support you in quickly accessing thermal conductivity data of different polymers. There are some compounders which have specialized in offering thermal conductive, electrical insulated polymers, reaching over 1 W/mK.

Thermal conductivity of plastics overview

Thank you for reading and till next time!
Herwig Juster

If you liked this post, share and like!

Interested in my monthly blog posts – then subscribe here.
New to my Find Out About Plastics Blog – check out the start here section
Check out also my personal webpage.


Literature:
[1] https://omnexus.specialchem.com/polymer-properties/
[2] Saechtling Plastics Handbook

High Performance Polymers in Electrification: A Must-Have Or A Nice-To-Have (Part 1: Introduction and Battery Systems)

High Performance Polymers in Electrification: A Must-Have Or A Nice-To-Have (www.findoutaboutplastics.com)



In automotive industry, electrification gains traction and design engineers all over are working to increase the energy density of batteries, create higher voltage traction motors and increase allover system voltage. In this three part series, we will discuss the requirements of different system components, i.e. batteries, traction motor, power electronics and autonomous driving in terms of polymers. Let’s get started with batteries.

Introduction
- Plastic parts in electric vehicles (EV’s)

In 2019, we see all major OEMs and Tier-1 supplier continue their efforts toward electrification and new mobility solutions. Currently, development progress of internal combustion engines is at 1% per year [1], whereas electrification developments progress at a rate of 7 %. Industry experts predict that the connected mobility can grow according metcalfe's law with a rate of 70% (taking Facebook user rates as a baseline). The overall plastic weight per car will not change significantly with EV’s. However, there will be a slight increase in weight in total. There are currently 10 000 parts made out of plastic in an average car and these use ca. 39 different polymers. Out of the 39, 6 are used the most, i.e. polypropylene, polyurethane, polyamides, polyethylenes, acryle-butadien-sytrenes, and polyvinylchloride [2].

Check out here my infographic on “Plastics in Automotive”

Also in electrification, light weighting together with fuel economy will continue to be a megatrend. The rule of thumb says that for every 10% of weight reduction, fuel economy improves by ca. 6-8%. This additionally drives the consumption of plastics in automotive. The Chinese car market shows a lot of potential for light weighting since most cars run on old technology platforms. Furthermore, this is the largest car market today.

- What changes with EV’s?

EV’s have no longer the need for fuel tanks, AdBlue tanks, pumps and fuel connections. Here, we will see a drop in engineering polymers, together with elastomers since fuel lines and gaskets are not in demand. On the commodity side, high density polyethylene (HDPE) will also lose out since it is used for making tanks. Under the hood, combustion engine covers will not be needed either, however new engine covers for traction motor will rise.

- Which challenges are there concerning material selection?

In the past, material selection for internal combustion cars was easier since lots of experience as well as know-how were already built-up. This included the handling of specifications. For under-the-bonnet applications for example, aliphatic Nylons such as PA 6, PA 6.6., and aromatic Nylons such as polyphtalamide (PPA) are widely used. Furthermore, polyphenylene sulfide (PPS) is used where the highest heat and chemical requirements over the lifetime of the car are needed. PBT, PP, PU and POM/Acetals round up the polymers used in several exterior and interior applications.

Today, in hybrid and full EV’s, material selection tends to be much more differentiated, since applications need to fulfill specific requirements. The one-fits-all approach is no longer working in a similar sense. Standards from other industries such as electronics influence now material selection in automotive. As a result, a “wedding” between e.g. consumer electronics and automotive standards may take place.

Looking into the high performance material portfolio, semi-crystalline polymers fulfill stringent electronic requirements such as high CTI (>600), intrinsic flame retardancy, dielectric strength, creep, tracking resistance, EMI shielding, and high (140°C) Relative Temperature Index (RTI).

Lithium ion batteries – the heart of the EV

For coating the separator, fluoropolymers such as PVDF in aqueous dispersions can be used. Commodity polymers such as ultra-high-molecular-weight polyethylene (UHMWPE) can be used as separator substrate material. PVDF plays a role in making electrode binders (anode and cathode) too. In cell manufacturing, high adhesion between electrodes and separator is needed to obtain good laminates. Most common cell systems are pouch and jelly roll systems.

Battery pack and module – holding all together

Going outside the cell to the finished battery pack, several challenges need to be overcome. In general, a battery pack has between 10 to 16 modules and each module can have between 10 to 12 pouch cells.

For battery packs, three things need to be balanced:

• energy density (range),

• power (rapid charging),

• safety (non-flammable).

Every battery is connected over the pack’s housing with an electrical control unit, which protects the cells from overloading. The housing must ensure that all batteries stay in place and impact or vibration do not compromise performance. Figure 1 summarizes the potential plastic materials for cells, module, and pack.

Figure 1:  Overview of polymeric materials for battery technologies in the electrical powertrain.

.

Cells are connected to one another over busbars. Stability must be ensured. Busbars are usually long copper parts (up to 30 cm) overmoulded with PPS to obtain low dimensional changes. Flame-retardant plastics become increasingly important for packaging of the cells into modules. Modules have endplates and separator plates. Polymers such as PPS are a perfect candidate to be used as endplate and separator material in such environments since it ensures a best-in class thermal and chemical stability (RTI up to 220°C), inherently flame retardancy, and high dimensional stability. Selecting a linear PPS grade is beneficial since it has lower flashing during moulding. This minimizes post-processing steps after moulding and the investment of a de-flashing station. In addition, polycarbonate and their blends offer long-term capability and low-temperature impact strength which are needed for the battery modules as well.


Generally, all different assembles can benefit from thermal conductive materials which support the removal of the heat generated by the cells. Different polymer grades can be improved in this context by means of additives.


Thermal management – keep it cool
Next to the classic thermal management systems which use water-glycol, new ways of cooling are on the rise. This is driven by water-glycol reaching its performance borders with higher voltage system (>500 V). A new way is the so called “direct liquid cooling” which uses a dielectric cooling fluid and the cooling component is directly immersed in this fluid. This can be realized with fluorinated fluids such as perfluoropolyether (PFPE). These kinds of fluids have good thermal conductivity and combine a low electrical conductivity with low viscosity. Furthermore, excellent chemical resistance is given through the fluoro properties.


Heating systems for interior and battery
In contrast to traditional combustion cars, EV’S dissipate less heat to enable sufficient heating of the passenger cabin. Therefore, additional heaters need to be installed. Most of such heaters are based on the Positive Temperature Coefficient (PTC) effect. The ceramic based PTC elements can be lined up next to each other holding together via a support frame.


Here, a stringent requirement is not having ions in your polymer formulation. This is important to prevent corrosion of overmoulded metal parts and connectors. Galvanic corrosion can reduce the electric performance down to failure. Therefore, electrical friendly, halogen free stabilized semi-aromatic Nylons or PPS might be suitable here.


During operation, the battery pack temperature should be kept at 60°C. Heaters are used to keep temperatures on a certain temperature to ensure efficient charging. Heaters can operate in the high-voltage range (500V) with an output of 7 kW. High performance polymers will ensure their proper function over lifetime.


Charging Systems – high voltage with high safety
Charging the batteries of your electric vehicle in a reasonable time requires high voltage charging systems. Temperatures should be kept between 0°C to 45°C. Additionally, interconnection systems provide the power for the electric engine. Electrical properties such as dielectric strength, volume resistivity, creep, tracking resistance need to be carefully considered by design engineers.


Color coding helps safely handle parts in the event of an accident. Above 60 V, a system is considered to be high voltage and then orange color coding of connectors and cables is required.


For covering all these needs, polymeric materials need to fulfill Comparative Tracking Index (CTI) values of over 600, dielectric strength in the range of 30 kV to 35 kV, and Relative Temperature Index (RTI) of 140°C.


Aliphatic Nylons such as PA6, PA66, PA 46 fulfill this stringent criteria. Exception are certain temperature levels. Semi-aromatic Nylons such as PPA (6T/6I; 6T/6.6; 6T/6I/6.6) fulfill the temperature and electrical requirements. High mechanical strength combined with reflow soldering durability and laser welding properties make them a good choice. Important is to select types which are halogen free and free of red phosphorous which in turn allows Nylons to reach high CTI values. Electric corrosion of assembly bins is prevented by not using ionic heat stabilizers. Most Nylons have an HB flame rating and V0 rating is possible, however it might involve inorganic heat stabilizers.



With this I will close the first part of this high performance polymers series for EV.

In the second part we will look into the traction motor.


Thank you for reading!

Till next time!

Herwig Juster


If you liked this post, share and like!

Interested in my monthly blog posts – then subscribe here.
New to my Find Out About Plastics Blog – check out the start here section
Check out also my personal webpage.

Literature:
[1] http://www.professoren.tum.de/lienkamp-markus/ and https://www.youtube.com/watch?v=5moHQFbEsDU&t=2238s
[2] A. Patil: An overview of Polymeric Materials for Automotive Applications, Materials Today: Proceedings 4 (2017) 3807-3815

Material Selection Guide Based On Mechanical Properties [Infographic]

Thanks for reading & till next time!

Greetings,

Herwig Juster

If you liked this post, share and like!
Interested in my monthly blog posts – then subscribe here.
New to my Find Out About Plastics Blog – check out the start here section
Check out also my personal webpage.