Powering Motor Performance with PEEK Extrusions

Polyether ether ketone, also known as PEEK, is a well-known thermoplastic polymer. This organic compound belongs to the family of polyaryl ether ketone (PAEK) polymers which include other proverbial names such as polyether ketone ketone (PEKK) and polyether ketone (PEK). Similar to many PAEK family members, PEEK exists as a semi-crystalline polymer at room temperature and contains ketone and ether linkages on both sides of single aryl moieties, as shown in Figure 1. Since PEEK is a thermoplastic, it does not decompose at its melt temperature.

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This makes it highly amenable to melt processing, where it can be developed into a highly different collection of other forms. While new and ever more specialized plastics have been developed by the thermoplastic industry, PEEK continues to be a significant member of this highly influential group of polymer plastics.

Figure 1. Polyether ether ketone polymer, or [4-(4-methoxyphenoxy)phenyl]-(4-methyl-phenyl)methane ^[1]. A) Bond-line drawing of the PEEK repeating monomer unit; B) ball-and-stick model and C) van der Waals spheres. White = hydrogen; gray = carbo; red = oxygen. (shown with terminal hydroxyls).

Popular Properties

PEEK, despite being a relative newcomer to the world of thermoplastics (ca. 1978), has quickly become popular due to its many useful properties. In addition to its processability, this organic material is a very hard material which translates into long life and high strength for finished products. Further, PEEK’s hardness imparts superior resistance to galling and abrasion (wear) with a low coefficient of friction. Similar to many members of the PAEK family, PEEK is not susceptible to hydrolysis and has relatively low water absorption ^{[2, 3]}. These properties contribute to PEEK’s prolonged usage life prior to embrittlement. Also, PEEK is resistant to nearly all commonly encountered detergents, solvents and other chemicals, except for a few substances such as sulfuric acids and certain halogenated acids. However, one of PEEK’s most cited properties is its high temperature tolerance which is not exhibited by many other similar polymers. PEEK melts at about 343 °C (649 °F) and has a practical service temperature upper limit of 260 °C (500 °F) with certain properties retained over this temperature, as indicated in Table 1. (PEEK’s lower temperature limit extends to less than -70 °C / -94 °F) ^{[4, 5]}. Therefore, PEEK is considered as one of the most thermally stable polymers. These properties collectively make PEEK suitable for many potential applications across many different environments.

Table 1. Representative PEEK properties

Versatility of PEEK

As a polymer, PEEK has found its way into a range of general and highly specialized application fields. PEEK is a thermoplastic with excellent hardness, and hence it can be injection molded into different parts such as engine components, pipe flanges, structural parts, dental implants, and even load-bearing parts such as bearings or bushings. PEEK is machinable for finer tolerances such as those essential for threaded parts such as screws. It can be extruded to create a broad spectrum of radial parts such as conventional tubing, joint seats, gears, and even a heat shrinkable form (Zeus PEEKshrink^®).

The exceptional properties of PEEK also make it well-suited for products such as components used in biomedical applications, food packaging and pharmaceutical. It can be extruded as a monofilament, or can be produced as a drawn fiber. PEEK is capable of fine extrusions and can be coated over wires as a protective and insulating layer or over fiber optics as a strengthening layer (Figure 2). With PEEK’s ability to be made into a host of products, it is no wonder that it is being used in many industries such as aerospace, automotive, medical, chemical, and oil and gas exploration.

Figure 2. Close-up view of PEEK insulated wire examples.

Electrical Relevance

New applications for PEEK are always on the horizon, thanks to its many excellent attributes as a thermoplastic polymer. By the late 1970s, polymer engineers started investigating whether PEEK can be used as a commercially viable coating to electrical wires due to its dielectric properties ^{[6, 7]}. PEEK’s behavior as an insulator is of major interest for electric performance. PEEK has no mobile electrons as it is completely covalently bonded, resulting in exceptional insulating properties. However, as a dielectric, PEEK is easily polarizable and this enables it to store electrical energy. PEEK’s dielectric strength is the limit of a dielectric to store this energy. A material will break down or fail as an insulator when it is placed in a voltage that is beyond the material’s dielectric strength. This failure can manifest as an electrical or thermal failure at which the dielectric is no longer viable. (Therefore, dielectric strength is also called breakdown voltage). In practical terms, if the dielectric strength of a material is higher, its insulating properties will be better. Dielectric strength is dependent on thickness and is often given in volts per unit thickness, kV/mil or V/mil. PEEK’s dielectric strength may range from 0.5 to 20 kV/mil (ASTM D149) based on its purity and other physical factors, thus allowing it to be used for a wide range of insulating requirements.

In addition, PEEK’s morphology plays a role in its insulating abilities. While PEEK polymeric chains can be seen as linear overall, their conformation in space is of a bent molecule that involves multiple twists as shown in Figure 3. Therefore, based on the packing nature of the polymer chains, PEEK’s morphology is variable from almost completely amorphous to more crystalline. In thermoplastics, more crystalline forms show better insulating properties, and this holds true for PEEK as well. In addition, steps in polymer processing make it possible to control morphology within certain limits. This feature is important for PEEK-coated magnet wire for use in electric motors. Hence, during the coating process, PEEK’s insulating properties can be guided to match with the physical requirements to create optimal dielectric strength based on the proposed application. Dielectric strength is partly a function of morphological properties and material thickness, and hence it is logical that the choice of material is weighed carefully under conditions where there are likely to be high voltages or when the preferred thickness of the dielectric – such as a coating over wire – is quite small.

Figure 3. PEEK 2n polymer showing bent and twisted conformation. A) Ball-and-stick model; B) van der Waals spheres model. White = hydrogen; gray = carbon; red = oxygen. (shown with terminal hydroxyls).

A material’s insulating properties, aside from dielectric strength, can be described by its dielectric constant. This parameter (also called permittivity) shows how a material polarizes at the atomic level when compared to a vacuum’s polarizability and thus the degree to which it can store electrical energy. Dielectrics are insulators because of their polarizability; the more easily they are polarized, the more electrical energy they to store, thus making them better insulators. Dielectric constant shows the amount of an electric field that is reduced or dampened in the presence of the dielectric compared to a vacuum. As an instance, the dielectric constant for air is given as 1.00059, for a vacuum 1, and for PEEK about 3.0. When the dielectric constant is lower, the material’s insulating properties is better due to its ability to be polarized yet it stores electrical energy in the presence of an electric field and does not conduct electricity. An insulating wire coating with such properties thus provides clear benefits: As a case in point, PEEK coating on a magnet wire, acting as an insulator or dielectric, reduces the effective electric field between the wire and its surrounding or external environment such as ground or a metal motor frame.. Thus, well-insulated wire reduces electrical loss during operation and boosts the motor’s efficiency.

A material’s insulating capacity is not limited to planar dimensions, but volume resistivity also accounts for the material’s resistance (inverse conductance) in 3D space, usually given for a volume of unity. Generally, volume resistivity is given as ohms • cm (Ω • cm). In practical terms, this characteristic measures how strongly a material resists the flow of electric current via a cubic volume of the material; in other words, the lower the resistivity, the greater the material’s conductivity and vice versa. When it comes to volume resistivity, it is generally believed that materials with > 10⁹ resistivity are considered insulators while those with < 10⁵ Ω • cm resistivity are considered conductive ^[8]. Usually, the volume resistivity for PEEK lies in the range of 1.6-7 × 10¹⁶ Ω • cm (ASTM D257) identifying PEEK as a good insulator ^[8].

PEEK Insulated Wire for Electric Motors

Although the mechanical characteristics of PEEK impart considerable strength and protection to wire as a coating, its ultimate benefit as an extruded coating for motor applications and magnet wire is its electrical performance characteristics. PEEK is capable of resisting electrical leakage and can be applied in even thickness for predictable performance. The insulating benefits of this polymer can be obtained with extremely thin extruded coatings while still maintaining considerable crystallinity. However, the thin extruded PEEK layer over the wire means that the wire can still be manipulated such as bending and wrapping for coil windings. In particular, the chemical resistance of PEEK is preferred because it may be exposed to electrical coolants, solvents, and varnishes when used over magnet wire. The strong exterior of the PEEK-coated wire also protects it against abrasion and damage during the installation of normal motor build and wire. No one type of polymer can offer all the optimal traits for enhanced motor performance, but PEEK’s across-the-board characteristics make it highly attractive for motor and magnet wire insulation.

Comparison of OEM Motor and Motor Rebuilt with ZEUS PEEK Insulated Wire and Other Peek Products

Considering the various potential benefits that PEEK insulation could provide to electric motor performance, performance attributes for an OEM motor and the same motor redeveloped with PEEK insulation products was compared. Here, the motor used was a small 4-pole, 0.75 horsepower, 460 volt, AC induction motor that is readily obtainable (Figure 4). Class F (Class 155C) OEM-installed wire and insulation system was used that contained 24 AWG standard enameled magnet wire 35, Dacron^®-Mylar^®-Dacron^® (DMD) insulation, Class F lead wire, Nomex^® and Nomex^® laminate. The rebuilt motor contained PEEK Lay-Flat^® tubing for slot liners and phase insulation and Zeus crystalline PEEK insulated magnet wire 24 AWG, heavy build (1.1 mils), as shown in Figure 5. This rebuilt motor was Class H (Class 180C), and a trickle varnish resin insulation was used by both motors. Shown below are abbreviated test results.

Figure 4. Cut-away of representative test motor showing basic motor components describes in this work.

Figure 5. Cross-sectional view of rebuilt motor with rotor and shaft assembly removed. The rebuilt motor contained PEEK insulated magnet wire (stator windings), PEEK Lay-Flat^® tubing used as phase insulation, and PEEK Lay-Flat^® tubing for slot liners.

Electrical testing was performed using an ALL-TEST PRO 5™, an Amprobe^® AMB55 high voltage insulation resistance tester, an Electrom iTIG 12D, and an ALL-TEST PRO OL™. It was observed that for basic operating criteria, the motor that was rebuilt with Zeus products exhibited almost identical outputs to the OEM motor: insulation to ground, capacitance to ground, impedance, low ohm resistance, current/frequency (I/F) response, and inductance (Table 2). Next, polarization index was tested (according to IEEE Std 43-2013 Annex D; AMPROBE^® AMB55). The OEM motor polarized to 10 GigaOhm during the 10-minute test (data not shown). Before the varnish was applied, the motor rebuilt with Zeus products polarized to 1 TeraOhm in 12 seconds; after the trickle varnish was applied polarization was reduced to 5 seconds.

Table 2. Comparison of basic motor operation criteria of OEM motor and motor rebuilt with Zeus PEEK insulation products.

Next, the condition of the motor insulation system was tested using an Electrom iTIG D12 for high and low voltage assessment. Partial discharge (PD) was measured for the rebuilt and OEM motors in order to spot voids inside the insulation which can cause failure in variable frequency drive (VFD) applications.

The windings of the motor rebuilt with Zeus PEEK products and the OEM motor seemed to be similar based on earlier tests, but PD tests showed that the OEM motor was prone to failure with a PD of 668,070 pC, whereas the rebuilt motor exhibited a PD of just 703 pC (data not shown). With regard to other performance factors, the rebuilt motor demonstrated improvements over the OEM motor for D factor, Q factor, and capacitance (Table 3). Such improvements were found to be consistent for each of three different lead configurations tested. In addition, the rebuilt motor performed at somewhat higher efficiency than the OEM motor, 1746 rpm vs 1715 rpm, respectively, in spite of the slightly higher voltage harmonics of the Zeus PEEK motor (data not shown) [Oak Ridge Motor Efficiency and Load software (ORMEL '96)].

Table 3. Comparison of performance factors of OEM motor and motor rebuilt with Zeus PEEK insulation products

During the entire testing, the motor that was rebuilt with Zeus PEEK insulated magnet wire (and other PEEK products) showed equal to or better performance attributes when compared to the OEM motor in multiple areas. Shorter absorption times and lower leakage were also showed by the motor rebuilt with Zeus PEEK products. Further, this motor showed greater efficiency with minimum insulation system losses. These results indicate that there is less aptitude for failure in inverter applications after motor rewind with Zeus PEEK insulated magnet wire (and other PEEK products). Together, the tests demonstrated the excellent insulating properties of PEEK and how these properties can be used in a realistic yet highly beneficial way.

Conclusion

PEEK is a highly versatile thermoplastic polymer. This material has many beneficial properties, including its ability to be melt processed into many different forms. Components made with this polymer have high wear resistance and hardness, thanks to its unique properties. These components also have excellent chemical resistance. PEEK has high temperature tolerance which sets it apart from many polyaryl ether ketone (PAEK) family members and is capable of withstanding working temperatures to 260 °C (500 °F). Therefore, PEEK has become the material of choice for a wide range of applications.

With growing knowledge and interest in polymer plastics, these materials were specifically preferred for use as insulators. The materials’ processability provides some amount of control over many of their properties such as shape, thickness, and crystallinity relevant for use in electrical settings. PEEK, as a representative dielectric from this group, has ability to be polarized and has many properties that are particularly preferred for use in electric motors. Its mechanical attributes allow it to be extruded over wire for motor coil windings, and its heat resistance easily survives the high temperatures that could be reached under nonstop motor operation. PEEK and other polymer plastics have indeed become commonplace in a wide range of electrical applications.

To find out whether PEEK can positively affect the motor performance, a small readily obtainable 0.75 horsepower, 4-pole, 460 volt, AC induction motor with magnet wire and original equipment insulating materials was compared with the same motor rebuilt with Zeus PEEK insulation products such as PEEK Lay-Flat^® tubing and PEEK insulated magnet wire for phase insulation and as slot liners. It was observed that for basic motor operation, the rebuilt and OEM showed almost identical outputs for insulation to ground, capacitance to ground, impedance, inductance, low ohm resistance, capacitive phase angle, and current/frequency (I/F) response. For polarization index, the motor that was rebuilt with Zeus PEEK products polarized to 1 TeraOhm when compared to the 10 GigaOhm of the OEM motor. For other test outcomes, Zeus PEEK-based motor demonstrated improvements over the OEM motor in capacitance, Q factor, and D factor. Compared to the OEM motor, the rebuilt motor demonstrated slightly higher rpm under the same load indicating a gain of efficiency for the rebuilt motor. On the whole, the motor rebuilt that was with Zeus PEEK insulated wire (and other PEEK products) demonstrated performance equal or better than the OEM motor. This shows the excellent insulating properties of the PEEK insulated wire and other PEEK products and the potential to enhance the performance and efficiency of motors.

References

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This information has been sourced, reviewed and adapted from materials provided by Zeus Industrial Products, Inc.

For more information on this source, please visit Zeus Industrial Products, Inc.