Nanotechnology based wire enamels by ELANTAS

The new product Deatherm E 641 GL combines the positive effects of nanotechnology with the established and well-known properties of highly temperature resistant wire enamels. Intensive endurance tests have proven that this new wire enamel shows significantly improved the resistance against partial discharge and voltage spikes.

The application of nanotechnology to the electrical insulation industry represents a very attractive way to upgrade and increase properties of conventional insulating coatings. ELANTAS Electrical Insulation has been particularly active in the investigation of such novel technology applied to wire enamels over the last decade. Among possible ways to combine nanotechnology with wire enamels manufacturing technology, ELANTAS strategically decided to leave the conventional application of resulting nano-modified products untouched in order to protect customer investments in their enamelling machines.

Deatherm E 641 GL is a nano-modified wire enamel based on polyester-imide (PEI). Developed by ELANTAS, this product is a clear example of the successful application of the 21st century most innovative technology to electrical insulation.

Thanks to its intrinsic nature, such novel nano-modified enamel is characterized by exceptional performance in high voltage environments and by excellent electrical, thermal and mechanical properties. Tests of electrical life run in medium-high voltage fields (3-5 KV) showed excellent resistance compared to conventional PEI. Tests done by accelerated degradation under high frequency AC voltages showed much higher resistance to partial discharge (PD) and voltage overshoots compared to conventional enamelled wires. With the increased thermal resistance typical of Deatherm E 641 GL, the thermal damage caused by heat from high frequency pulses is prevented.

Deatherm E 641 GL has a synergistic effect between the organic and inorganic components. The organic resin was developed to be highly heat resistant and to maintain its flexible structure even after high temperature treatments, while the inorganic material has high affinity for the resin and its excellent dispersion in the binder which provides the necessary homogeneity before (liquid enamel) and after application (cured coating).

By incorporating nano-sized inorganic matter, the hardness and brittleness that micron sized equivalent materials bring to resulting coating is significantly reduced. A well-designed organic resin, in which thermal resistance is combined with excellent mechanical properties, along with crucial cross-linkers and incorporated nanofillers, guarantees the homogeneous distribution of the later ones in the final, cured coating.

By keeping the distance between the inorganic fillers in Deatherm E 641 GL quite small, a very compact texture is formed in the insulator minimizing defects. This results in positive effects for dielectric breakdown strength, which highly depends on internal defects of the cured enamel. Inorganic fillers have the capacity to enhance the resistance against insulation degradation originated by partial discharge (PD), dielectric heating and space charge.

When Deatherm E 641 GL is exposed to partial discharge, the surface erosion is much smaller than for conventional enamels when evaluated by Scanning Electron Microscopy (SEM). This surface improvement is explained by a "ceramization" phenomenon that occurs on the surface of the enamel in the presence of partial discharge activity, creating a barrier against degradation.

Deatherm E 641 GL shows increased service life under Surface Partial Discharges (SPD) aging conditions related to both high frequency sinusoidal and pulsed waveforms.

This behavior makes Deatherm E 641 GL particularly suitable for inverter-fed motors which are exposed to transient surge voltages made of bipolar pulses with the rise time in the micro or nano second order.

The windings of such motors can be exposed to SPD activity by way of over voltages caused by resonance and reflection phenomena.

Tests were run on twisted pairs of Deatherm E 641 GL related enamelled wires manufactured according to IEC 60851-5 at the Department of Electrical Engineering of Genoa University, Italy.

The aging tests were carried out applying to the twisted pair specimens voltage waveforms aimed at reproducing the electrical stress induced by the output of a switching VSD (Variable Speed Drive).

In the testing protocol the specimens (three for each wire) were preheated at 150 °C for 24 hours and kept at such temperature during the measurement. The total breakdown (i.e. the enamel failure) was chosen as end criterion.

The power supply circuit consisted of an arbitrary waveform generator and a linear power amplifier having a bandwidth from 10 Hz to 100 kHz at the maximum output voltage equal to 6 kV peak-to-peak. The aging tests were carried out applying high frequency (HF) PWM-like waveform at 5000 V peak-to-peak with the following characteristics: fundamental frequency 3000 Hz, switching frequency 24000 Hz, rise time 0,67 KV/μs (measured at 5000 V voltage peak-peak), duty cycle 50%. The pulsed voltage (Figure 1) had peaks every rise and fall fronts having amplitude equal to the constant part of each pulse in order to simulate a PWM power supply in presence of reflection and resonant phenomena.

The outcome of the tests carried out at 5000 V applying the HF-PWM-like voltage waveform showed a 7-fold increase of Tbd (time to total breakdown) versus conventional PEI.

Further electrical aging tests were carried out using the HF-PWM-like waveform. The aim of the tests was to trace lifetime diagrams of the tested enamels and to investigate their medium long-term behavior. Three voltage amplitude levels were considered; the relevant times to breakdown (Tbd) were related to the voltage amplitude by the inverse power law (graphic).

where, Vpp is the peak-to-peak voltage amplitude, K is a constant that depends on the material and n is the so-called voltage endurance coefficient (VEC). When plotted in a log-log V versus t plane, (1) results in a straight line, whose angular coefficient is -1/n. The coefficients K and n can then be determined by linear regression techniques.

Deatherm E 641 GL exhibited much better duration than conventional PEI in the same voltage stress range. In an extrapolation of the obtained life curves, the voltage amplitude can be estimated at a corresponding theoretical duration of 20 years (V20). Such an extrapolation results in V20 = 125 V for PEI and V20 = 2050 V for Deatherm E 641 GL. Besides the obvious consideration that the stress corresponds to an expected lifetime of 20 years of Deatherm E 641 GL, exceeding it by more than an order of magnitude to conventional material, it is interesting to compare the esteemed lifetime with the Partial Discharge Inception Voltage (PDIV), measured before the tests. The PDIV of the enamels, considering the actual test voltage waveform, is about 1750 V peak-to-peak. This means that in theory ELANTAS' new, high-tech Deatherm E 641 GL could withstand the exposure of PDs for more than 20 years, resulting in a significant reliability enhancement of electric motors.

It has to be pointed out that an enamel able to work in the presence of PDs for several years at such working voltage (Bipolar square pulses, overshoot factor =2, V >PDIV) has never been recorded so far.