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A new frontier for high performance stators
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This article presents PFA coated wire as a potential universal base for the stator winding of high power electric motors. Tre Tau Engineering's DryCycle® process enables an environmentally friendly and economical application of PFA. The article presents the main properties of the wire, including adhesion, elongation, springback, BDV, PDIV and RPDIV, and highlights the advantages of using this polymer in electric motor stators. | PFA coating Wire Insulation Electric motor Stator PDIV, BDV |
Magnetic wire is a key component in various electronic products such as transformers, inductor coils, electromagnets, electric motors and generators. The latter are particularly important to the automotive industry, and the proper functioning of an electric motor for an electric car depends on the stator. All electric motors consist of a rotating part – the rotor –, a stationary part – the stator –, bearings, and a shaft [1]. A review of several studies about electric motor failures resulted from the experience of manufacturers, repair centers and academic studies shows that the faults distribution percentage in electric motors is approximately 45% in the bearings, 35% in the stator, 10% in the rotor, and the remaining 10% in other components [2]. This implies that one-third of all motor failures are to be attributed to stator failures, the most frequent cause being the breakage of its winding. The winding, comprised of a copper conductor with polymer insulation, is highly susceptible to electrical stresses, and mechanical and thermal loads; therefore, it is the most vulnerable component of the stator. Factors such as cracks, pores, wire oxidation, insulation damage, and permeability to gases and liquids can all diminish the dielectric properties of the polymer coating. [3].
Undoubtedly, the thermal and chemical resistance of the wire is directly contingent on the selection of its insulation material. The most used insulation materials for wires include PI, PEI, PAI, and PPS. Additionally, new high-performance polymers such as PEEK and PFA have been recently introduced. Polyimides, which are applied using classic enameling technology, remain the most popular insulation material on the market due to their ability to provide a uniform coating. However, the primary issue with this technology is the use of highly toxic solvents, such as cresol, which can have adverse effects on both worker health and the environment. Consequently, finding an alternative technology for electric wire production is an urgent task. Nowadays, extrusion represents a promising alternative, as this method offers a wider range of applications. For example, it allows to apply polymers that are not suitable for the conventional enameling technology, such as PEEK, PPS, and PFA.
PFA (perfluoroalkoxy) has great potential compared to other materials due to its excellent chemical resistance, which makes it suitable for use in aggressive environments. Moreover, PFA exhibits a higher resistance to partial discharges compared to common materials such as PEEK or PAI. Partial discharge is the main cause of wire insulation failure, which leads to the failure of the entire electric motor. Resistance to partial discharges is a crucial parameter to consider when designing high-voltage motors. Thus, PFA-coated wires have the potential to become a universal stator winding base for high-performance electric motors.
However, the widespread use of PFA in magnet wire manufacture is limited by the difficulty of applying it to the wire due to its extremely poor adhesion to the copper surface. In order to overcome this challenge, Tre Tau Engineering developed the patented "DryCycle® technology" (patent EP 3518255B1), which allows most extrusion polymer grades to be applied to wires without the use of solvents, primers, or other copper processing. The DryCycle® process requires minimal polymer application and offers fast application times, making it both economical and environmentally friendly.
This article presents the main properties of the resulting PFA wire, thus showing the wide range of possibilities and advantages of using this polymer in electric motor stators. The PFA coating was applied using the commercially available polymer Daikin AP-201. The tests were conducted in accordance with international standards IEC 60851-3, IEC 60851-5, and IEC 61934.
In order to fully characterize all the properties of the wire, standard tests for magnetic wire were selected: adhesion, elongation, springback, BDV and PDIV tests. One of the non-standard test methods we used was to make hairpins and compare their electrical properties with those of the unformed samples. These characteristics were chosen to provide a comprehensive evaluation of the properties of the wire.
+ Adhesion: the ability of the wire to bend and stretch is a function of this factor. This is the major weakness of most fluoropolymer coatings. Tre Tau Engineering has been able to achieve strong adhesion without the need for additional primers or other fixing compounds.
+ Elongation: this is a measure of the tensile strength and yield strength of the wire and the resistance of the insulation to this kind of stretching.
+ Springback: an important parameter to create hairpins, it indicates the deflection of the wire when it is bent.
+ BDV and PDIV: crucial parameters to determine the electrical properties of a wire. BDV indicates the maximum discharge that the wire can withstand, PDIV indicates the inception voltage of partial discharge. Another important parameter in this study is RPDIV (initial voltage of repeated partial discharge). PDIV and RPDIV can be used to simulate the actual operation of the electric motor by predicting the possible moment of stator failure.
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About Tau
Tau is a technology company at the core of energy transition, decarbonization of energy and electrification of transport. Its patented technologies reconcile the necessity of sustainable, resource-minimizing manufacturing with the power, reliability and performance that electrification requires to enable smaller, greener, longer-lasting and more powerful motors, transformers and generators. Tau focuses on carbon-conscious protective coatings for high-performance and standard copper, aluminium and steel wires.
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