# A Method for Endurance Testing of Enameled Round and Rectangular Wires for Motors Controlled by SiC-based Inverters

Veronika Gavrilenko, Andrey Leonov, Andrey Bukharkin, S Hlioui, Stéphane Lefebvre

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Veronika Gavrilenko, Andrey Leonov, Andrey Bukharkin, S Hlioui, Stéphane Lefebvre. A Method for Endurance Testing of Enameled Round and Rectangular Wires for Motors Controlled by SiC-based Inverters. IEEE Transactions on Dielectrics and Electrical Insulation, 2021, 28 (6), pp.2091-2098. 10.1109/TDEI.2021.009587 . hal-03470575

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# A Method for Endurance Testing of Enameled Round and Rectangular Wires for Motors Controlled by SiC-based Inverters

Veronika Gavrilenko¹,²

¹Laboratoire des Sciences de l'Ingénieur Appliquées à la Mécanique et au génie Electrique – Fédération IPRA, Université de Pau et des Pays de l'Adour/E2S UPPA, Pau, France

Andrey Leonov² and Andrey Bukharkin²

²National Research Tomsk Polytechnic University
Tomsk, Russia

Sami Hlioui³

³SATIE, UMR 8029, CNRS, CYU, Univ. Paris-Saclay
Neuville-sur-Oise, France

Stephane Lefebvre⁴

⁴SATIE, UMR 8029, CNRS, CNAM
Paris, France

## ABSTRACT

The purpose of this article is to propose a method for investigating the influence of the impulse voltage waveform generated by inverters based on wide bandgap semiconductors on the insulation endurance of magnet wires. In this regard, endurance tests of enamel insulation are performed on a designed test bench, which generates repetitive voltage pulses with ns-level rising and falling times corresponding to modern silicon carbide-based inverters. Furthermore, we suggest a novel specimen's shape for testing enameled rectangular section magnet wires which find their application in electric traction motors. Considerations related to the deterioration of dielectrics subjected to impulse voltage with high slew rates and high switching frequency are given.

**Index Terms** — enamel insulated wires, machine windings, magnet wire, partial discharges, pulse width modulated inverters, rotating machine insulation testing, silicon carbide

## 1 INTRODUCTION

ELECTRICAL insulation quality plays a vital role in providing electrical machines' reliability. This essential component of electrical machines operates under combined thermal, electrical, mechanical, and environmental stresses [1]. This results in aging and gradual deterioration of insulation properties that can reduce the motor's service lifetime. The impact of these factors on dielectrics' aging and deterioration will depend on the electrical machine's environmental conditions (temperature, humidity, and pressure), its power supply, and its control system. In case of standard 50-Hz AC supply voltage, thermal stresses are the main deterioration

cause for low-voltage electrical machines insulation [1]. This is due to the fact, that voltage magnitude and surges are not high enough to cause an insulation breakdown or an ionization.

Previous studies [2–4] highlighted that the use of voltage source pulse width modulated inverters increases electrical stresses in powered low-voltage electrical motors. The winding insulation of inverter-fed motor is influenced by a nonlinear voltage distribution along its winding resulting from the impulse voltage waveform. Indeed, an overvoltage can occur at motor terminals due to reflected waves' phenomena in cable line caused by the impedances mismatching between inverter, supply cable, and motor winding [2–4]. Moreover, in a random wound motor winding, the first and the last turns of one or several coils may be adjacent and could consequently be exposed to the entire input voltage [3, 4]. The resulting
overvoltage can therefore be higher than the partial discharge inception voltage value. Partial discharges (PD) activity induces complex physicochemical processes leading to dielectrics' accelerated degradation and decreasing the insulation service life of inverter-fed motors [5, 6].

The detrimental effect of the repetitive impulse voltage becomes more important with the use of wide bandgap semiconductors such as silicon carbide (SiC) in power converters. The modern advances in semiconductor technology allow us to create powerful inverters with higher switching speed and frequency range in comparison with silicon technology, providing losses reduction, compactness, and lower weight [7, 8]. Nevertheless, resulting electrical stresses are severe for the controlled motors: $dv/dt$ with very short rise/fall times (between 10 and 50 ns), a rise of common-mode currents and electromagnetic radiation [9, 10]. Since the faster rise time leads to the higher transient overvoltage, PD may occur with a higher probability accelerating the motor insulation aging and failure [11, 12].

The use of wide bandgap power electronics in motor drives requires new approaches to study PD effects on motor winding insulation. In [13] the experimental setup is performed with SiC components reproducing operational conditions. However, the fact that the tests are conducted on a complex motor drive system makes them relatively expensive and difficult to use when studying physical processes of dielectric aging. The author of [12] proposes a PD decoupler to study PD extinction voltage and magnitude on tested twisted pairs for rise times between 15 and 800 ns and commutation frequency up to 600 kHz. This work highlights the effect of the ultra-short rise time resulting in fewer PD but of higher magnitude. The influence of the impulse voltage repetition frequency from 5 to 200 kHz on repetitive partial discharge inception voltage (RPDIV) in a partial vacuum is studied in [14]. The reduction in RPDIV has been observed in correlation with the switching frequency rise that could limit the use of SiC-based power modules. However, aging and deterioration processes in insulation subjected to impulse voltage with high $dv/dt$ and high switching frequency are not thoroughly investigated. In addition, taking into account the influence of thermal stresses affecting the insulation during motor operation, the process of electro-thermal aging needs to be considered as well. Thus, this work aims to investigate the influence of the voltage waveform on enamel insulation lifetime, highlighting the effect of high $dv/dt$ high switching frequency impulses. To achieve these goals, we have designed a test bench allowing us to perform insulation endurance tests using SiC transistors. This test bench makes it possible to reproduce the stresses affecting the winding insulation of motors fed by inverters based on high-speed SiC switches.

This paper is organized into four sections. After this introduction, the testing methodology and the main goals are discussed in section II. This section provides the specifications of studied magnet wires and constructions details of specimens. A novel specimen's shape for testing enameled rectangular wires is introduced. The original test bench intended to study the effect of electrical stresses on motor winding insulation

generated by modern SiC high-speed switches is proposed. Section III gathers the experimental results of magnet wires endurance when subjected to electrical and thermal stresses provoking PD activity. The last section of this paper draws some conclusions resuming the main observations.

## 2 TESTING METHODOLOGY

The proposed methodology is based on insulation endurance tests, in which the insulation of a test object is exposed to one or more aging factors related to service conditions. This paper focuses on enamel insulation aging investigation incorporating both voltage and temperature effects as they are significant aging factors in inverter-fed motors [1, 2].

To induce the enamel insulation electrical aging, the impulse voltage is applied to wires' specimens representing turn-to-turn winding insulation. The voltage amplitude is set at a higher level than the RPDIV determined by preliminary tests when partial discharges have been detected. The electrical aging of insulation is done at an ambient temperature equal to 25 °C.

Furthermore, combined electrical and thermal stresses are applied simultaneously in order to enlarge the study considering the interaction of temperature and impulse voltage effects. In this case, the temperature in a thermal chamber is set at 180 °C. This value corresponds to the peak temperature into a stator coil which magnet wire having a thermal class H can support.

All tests have been performed under PD activity until enamel insulation breakdown occurred. Thus, the times to breakdown are recorded conforming to the International Electrotechnical Commission (IEC) standard 62539:2007 [15].

The tests are carried out with the following goals:

* to study the effect of partial discharges on enamel insulation aging and destruction when applying high $dv/dt$ high frequency impulse voltage;

* to evaluate the influence of voltage waveform parameters on insulation endurance by comparing the times to breakdown;

* to study the effect of high temperature accelerating insulation aging and breakdown when combined with electrical stresses; and

* to estimate and compare the ability of different wires to withstand PD activity depending on the applied stresses.

Test objects and procedures will be detailed in the following sections.

### 2.1 TEST OBJECTS AND SPECIMENS

The experimental study is conducted on enameled wires with Type I insulation conforming to IEC 60034-18-41 [16]. These wires are generally used in machines with rated voltage $\leq$ 700 V RMS. This insulation type, contrary to Type II, is not designed to withstand partial discharges during the operation. Since repetitive PD may occur at rising and falling edges of the impulse voltage in inverter-fed low-voltage motors, it may lead to a premature insulation failure.

Specimens in form of twisted pairs as shown in Figure 1 are made of enameled copper round section wires having 1 mm of
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Twisted pair specimen for testing round section wires: (a) general view; (b) schematic view, and (c) cross-section at the contact point.</figcaption></figure>

external diameter. This type of wire is usually used in random wound windings. The twisted pair specimens have been manufactured in compliance with IEC 60851-5 [17]. Two conventional wires and one corona resistant wire have been tested. These commercially available wires have a grade 2 double insulating layer. The enamel insulation compositions are as following:

* conventional wire type 1: polyesterimide (PEI) and polyamide-imide (PAI), 30 µm coating thickness;

* conventional wire type 2: modified polyurethane and polyamide, 30 µm coating thickness;

* corona resistant wire: tris(hydroxyethyl)cyanuric acid modified PEI comprising nanoparticles of silicon dioxide, and PAI, 40 µm coating thickness.

The specific composition of corona resistant nano-filled wires allows a better resistance to degradation caused by PD, providing improvement in terms of mechanical and chemical erosion, electrical breakdown behavior, and space charge mitigation compared to conventional enameled wires [18].

In this work, a rectangular enameled wire having 1 mm thickness (54 µm coating thick) and 3 mm width (68 µm coating thick) for use in motors with concentrated or coil stator windings has also been tested. This wire has a grade 2 double insulating layer composed of tris(2-hydroxyethyl)isocyanurate modified PEI and PAI.

The twisted pair is the well-known specimen for testing round enameled wires. As for rectangular enameled wires, the standard IEC 60851-5 [17] recommends to perform breakdown voltage tests on a bent piece of wire placed in the container and surrounded by metal shot. This method is not suitable for our tests. A turn-to-turn specimen intended to test rectangular wires for form-wound winding is given by IEC 60034-18-42 (Ed.1) [19] describing qualification and acceptance tests for PD resistant electrical insulation systems of Type II. The specimen is a pair of insulated conductors held together with the terminals splayed apart. The insulated conductors should be in contact along their length that may require to process the specimens using pressing operations and/or impregnation. Another specimen proposed in [20] consists of two pieces of wire where one wire is bent, and a plastic clip provides contact between the wires. This specimen allows conducting PD measurement into the wires' contact point. However, it is less suitable for insulation endurance testing considering the eventual non-uniformity of the enamel coating, uneven enamel thicknesses, and possible defects within the insulation.

The proposed specimen's shape for turn-to-turn insulation in concentrated or coil windings is depicted in Figure 2. It

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Coil specimens for testing rectangular section wires: (a) general view and (b) cross-section at the contact area.</figcaption></figure>

represents a small coil made of two rectangular wire's pieces bent in parallel around a metal tube in a repeatable manner. The metal tube used to wound manually the coils has 130 mm diameter that is greater than a mandrel's diameter specified for rectangular wires tests in [17]. Thus, the bending radius is large that allows to avoid coating deformations and cracks. The tested area (wound part) of 125 mm length and the number of turns in coils (4 turns for the studied rectangular wire) are kept similar for all specimens. The both wires' terminals are splayed apart to prevent flashovers and false failures there. The tested wire's dimensions provide sufficient rigidity, while wires with smaller sections might be placed on a dielectric tube to keep the coil's shape during testing.

## 2.2 EXPERIMENTAL SETUP AND PROTOCOL

With the purpose to study the electrical stress effect on turn-to-turn insulation induced by SiC high-speed switches, a test bench has been designed. The electrical test bench acts like a pulse generator (H-bridge inverter topology) assembled with a measurement unit and a thermal chamber where tested specimens are installed. The main components of this setup are shown in Figure 3 and detailed in Table 1. The photographs showing the complete test bench and the SiC inverter are given in Figure 4.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Scheme of experimental setup for high $dv/dt$ tests.</figcaption></figure>

The designed inverter plate made from a double-sided printed circuit board (PCB) has a symmetric realization. It allows us to use it as a full-bridge inverter applying bipolar
<table><caption>**Table 1.** Main components of high *dv/dt* test bench.</caption><thead><tr><th>Component</th><th>Characteristics</th></tr></thead><tbody><tr><td colspan="2" style="text-align: center; font-weight: bold;">Pulse generator</td></tr><tr><td>Switches, S<sub>1</sub> - S<sub>4</sub></td><td>MOSFET SiC Wolfspeed transistors with built-in SiC body diode, ref. C2M1000170D, V<sub>DS</sub> = 1700 V, R<sub>DS(on)</sub> = 1 Ω at 25°C</td></tr><tr><td>Decoupling capacitance, C<sub>1</sub> and C<sub>2</sub></td><td>TDK DC Link Film Capacitors, ref. B32776G, 8 μF, and Multilayer Ceramic Capacitors SMD, 100nF</td></tr><tr><td>Gate resistance, R<sub>G</sub></td><td>3.5 Ω</td></tr><tr><td>Drivers</td><td>Dual 1700V SiC MOSFET Driver, ref. PT62SCMD17</td></tr><tr><td colspan="2" style="text-align: center; font-weight: bold;">Measuring tools</td></tr><tr><td>Oscilloscope</td><td>Tektronix MDO3024 200 MHz</td></tr><tr><td>Kilovoltmeter</td><td>AC/DC, 0-120 kV, precision 0.25 %</td></tr><tr><td> Divider</td><td>RC voltage divider, 3000:1, 35 MHz, input resistance = 100 MΩ, input capacitance ≈ 3pF</td></tr><tr><td> Antenna</td><td>150 Ω coaxial radio frequency cable with matching resistors R<sub>1</sub> = R<sub>2</sub> = 150 Ω</td></tr><tr><td> Differential probe</td><td>Tektronix THDP0100 100 MHz, up to 6000 V differential</td></tr><tr><td colspan="2" style="text-align: center; font-weight: bold;">Test chamber</td></tr><tr><td>Thermal chamber</td><td>Maximum temperature 200 °C ± 5 °C</td></tr></tbody></table>

impulse voltage on tested specimens or as a half-bridge inverter generating unipolar pulses. In this study, only the full-bridge inverter topology is used to conduct the insulation endurance tests. Performed with modern SiC MOSFET 1700 V transistors, the test bench can generate adjustable impulse voltage waveforms with short rising and falling times. Selected transistors allow reaching high-voltage levels with high *dv/dt* due to low output capacitance with a maximum switching frequency up to 125 kHz. The implemented dual SiC MOSFET driver boards enable to set the dead time to a value varying from 250 to 1000 ns and to adjust the blanking time due to an on-board dead-time generator (the dead time is by default set at 500 ns). The gate drivers support *dv/dt* max up to 100 kV/μs and provide galvanic isolation up to 1700 V. Equipped with under voltage lock out and over current protection, gate drivers ensure inverter protection when the insulation breakdown of tested specimens occurs. The required voltage waveform is settled by means of a function generator connected to drivers' inputs. This test bench offers a possibility to adjust various voltage waveform parameters, such as the amplitude, the switching frequency, and the duty cycle. It makes then possible to examine their effects on PD and insulation aging.

The tested specimen's connection points are shown in Figure 3: both wires of tested twisted pairs or coils are connected to midpoints of the inverter. In the present setup, tested specimens are installed in the thermal chamber that allows a temperature control and also serves as a grounded metal screen protecting the inverter from electromagnetic interferences issued from partial discharges. The electrical connection between the inverter output and the tested specimen placed in the thermal chamber is made using a 1 m cable length. The used cable is insulated by polytetrafluoroethylene enabling operation at high temperatures.

<figure><img src="image_5.png" /><figcaption>**Figure 4.** General view of the experimental setup for high *dv/dt* tests (a), SiC-based inverter (b), and tested specimen installation (c).</figcaption></figure>

Figure 5 provides the bipolar voltage waveforms and the enlarged rising edges of two voltage levels obtained by the designed SiC-inverter. They have been measured at the inverter output (0.15 m from the inverter's PCB) as well as at connection points in the thermal chamber (1 m from the inverter's PCB) with and without tested specimen. The rising and falling edges are symmetric. The reported waveforms are measured using the Tektronix THDP0100 differential voltage probe connected to the Tektronix MDO3024 oscilloscope. These results demonstrate the effect of fast switching (rise time of 20 ns estimated for 10-90 % of peak-to-peak amplitude) on transient overvoltage and voltage oscillations when adding the power cable and the twisted pair (17 pF capacitive load). The voltage waveform measured on the tested twisted pair above the RPDIV which shown in Figure 5 curve (c) has a slower rate. It is probably deformed by partial discharges: caused by the influence of ionized air conductivity or by the overlapping of PD pulses on the rising edge. The PD detection methods will be discussed in section 3.1.

The experimental protocol consists of nine endurance tests applied to each of the studied magnet wires. Different combinations of electrical and thermal stresses are tested. These combinations are obtained by varying the following parameters:
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Voltage measured below the RPDIV $V_{DC}$ = 600 V (left) and above the RPDIV $V_{DC}$ = 1000 V (right). Legend: the inverter output (a), connection points in the thermal chamber (b), and the tested specimen installed in the thermal chamber (c).</figcaption></figure>

* DC bus voltage $V_{DC}$ = [1.0; 1.2; 1.4] kV,

* 2 levels of switching frequency $f$ = [20; 50] kHz,

* 2 levels of temperature $T$ = [25; 180] °C.

The tests are conducted at normal atmospheric pressure; the humidity $\phi$ = [15÷30] % slightly varies during tests. The duty cycle is set at 50% being the most appropriate when performing insulation tests as reported by [21].

The endurance testing has been performed until enamel insulation breakdown occurred, and the time to breakdown has been measured for each stresses combination.

## 3 EXPERIMENTAL RESULTS AND ANALYSIS

The endurance tests of magnet wires' enamel insulation have been carried out on the described test bench. The main aging factor is the repetitive high frequency impulse voltage with ns-level of rising and falling times related to SiC-based inverters. Furthermore, the electrical stresses are combined with the thermal ones that accelerates the insulation aging. The applied impulse voltage activates the ionization processes in the insulation engendering partial discharges in air gaps between turns of enameled wires. In the present study, partial discharges signal measurements have been performed with the aim to demonstrate the presence of PD during the endurance tests.

### 3.1 PARTIAL DISCHARGES DETECTION

An optical method is used to detect partial discharges. For this purpose, an ultraviolet (UV) imaging corona monitoring camera is used. This device allows to record the electromagnetic radiation in the UV spectrum band in the case where surface partial discharges and corona discharges

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Images of PD at impulse voltage $V_{DC}$ = 1.4 kV, $f$ = 50 kHz: specimen's installation (a); photograph of PD on the specimen (b), and PD optical image obtained with the UV corona monitoring camera (c).</figcaption></figure>

appear [22]. This monitoring camera is often used for detecting defects and failures in high-voltage electrical equipment (power transmission lines, insulators). In our experiments, it allows us to detect the PD inception when discharges are weakly luminous by obtaining their UV image. The brightness of optical emission increases proportionally to PD intensity and can be visible without using special equipment. The twisted pair installation photograph, as well as the photograph and the optical image of PD observed on this specimen, are given in Figure 6.

Considering that the partial discharges activity impacts the electromagnetic field, the detection of PD is also performed using an antenna similar to the coaxial cable sensor reported in [23]. The antenna is made with a coaxial cable for radio frequency signal transmission having a characteristic impedance of 150 Ω. The cable terminations are connected to the impedance matching resistors: $R_1 = R_2 = 150$ Ω. One of the cable ends is fixed at a distance of 15 cm from the specimen, as shown in Figure 4, and the other is connected to the oscilloscope. When a PD occurs near the antenna, the displacement current created by the PD is detected by the antenna and observed on the oscilloscope. The antenna is equipped with a high-pass filter, however, the PD signals still could be superimposed with the fast transistors switching. The RC-divider 3000:1, used to obtain the phase synchronization signal, measures the voltage at the one end of the tested specimen and is connected to the ground, as shown in Figure 3.

Waveforms in Figures 7a and 7b measured below and above the RPDIV (detected optically) depict high-frequency perturbations on the DC bus (DC bus kilovoltmeter), at the inverter output (RC-divider) and near the tested specimen (antenna). The observed perturbations are caused by transistors fast switching and, at the same time, induced by partial discharges. In fact, when the impulse voltage rise time is estimated to be 20 ns, and the PD pulses' duration is coming up to 10-30 ns, these two signals overlap in the time domain. This phenomenon complicates the analysis of PD when the impulse voltage has very high $dv/dt$ rates. Nevertheless, the reported scopes demonstrate that the PD on the tested specimens can be detected using the described antenna.
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Voltage waveforms measured below the RPDIV $V_{DC}$ = 600 V (a) and above the RPDIV $V_{DC}$ = 1200 V (b).</figcaption></figure>

## 3.2 ENDURANCE TESTS

According to the experimental protocol given in section 2.2, the endurance tests are conducted on four magnet wires using the designed test bench to apply constant stress combinations until the insulation breakdown appears. Figures 8, 9, 10, and 11 report the lifetime recorded for different combinations of electrical and thermal stresses repeated on five specimens.

Among the different tested magnet wires, the corona resistant one (Figure 8) shows the best endurance since it has the highest times to breakdown, especially at low voltage amplitude. In case of corona resistant wire, the complete data set corresponding to $V_{DC}$ = 1.2 kV has not been obtained; this is due to the fact that the specimens were not broken down after more than 7 hours of test duration. However, the performance of this wire is also limited and decreases remarkably when the

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Time to breakdown of twisted pairs made of the corona resistant wire.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 9.** Time to breakdown of twisted pairs made of the conventional wire type 1.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 10.** Time to breakdown of twisted pairs made of the conventional wire type 2.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 11.** Time to breakdown of coils made of the rectangular wire.</figcaption></figure>

voltage amplitude and the frequency both increase. The rectangular magnet (Figure 11) wire demonstrates a good endurance compared to other conventional wires, especially when the stresses are increased. It can be related to the enamel composition, to its thicker insulation, and to its higher thermal class, up to 200 °C.
As for the conventional wire type 2 (Figure 10), its lifetime is very low under high *dv/dt* impulse voltage, which complicates the analysis of factors effect, but it also means that this type of wire (insulation composition) should not be used for electrical machines operating in such conditions.

In general, the reported plots highlight the significance of voltage parameters variation, which mostly dominates the thermal stresses obtained by heating in the thermal chamber. According to the data probability distribution analysis, Weibull and lognormal distributions may be appropriate for fitting the time to breakdown distribution. Considering the lognormal distribution, an analysis of variance has been performed on experimental data logarithms. It reveals the significance of all factors. More tests with higher repetition number should be done in order to determine which of these factors have the most significant effect on insulation aging.

Microscope images of broken specimens presented in Figures 12 and 13 show the impact of thermal and electrical stresses on enamel insulation degradation depending on test conditions. The “frost effect” (decomposition products of solid dielectric on its surface) can be observed on the enamel surface at low voltage amplitude (Figures 12a, 12b, and 13). However, when the electrical stress reaches critical values (Figure 12c), the enamel insulation is severely damaged, effects of carbonization and sticking of turns are observed that indicate a thermal failure mode. This destruction process in most cases is very fast, e.g., only 30-40 s for the corona resistant wire before the breakdown occurred.

<figure><img src="image_13.png" /><figcaption>**Figure 12.** Twisted pair's photograph and microscope images after testing under high *dv/dt* impulse voltage *T* = 25 °C, *f* = 50 kHz: (a) *V*<sub>DC</sub> = 1 kV; (b) *V*<sub>DC</sub> = 1.2 kV and (c) *V*<sub>DC</sub> = 1.4 kV.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 13.** Coil's photograph and microscope image after testing under high *dv/dt* impulse voltage *T* = 180 °C, *f* = 20 kHz, *V*<sub>DC</sub> = 1.2 kV.</figcaption></figure>

The present study shows that the applied ns-level rise time impulse voltage leads to an accelerated insulation deterioration, and damages tend to be more significant in the central part of the specimens as demonstrated in Figure 12. This effect is observed at the switching frequency of 50 kHz which leads to dielectric heating. The increase in temperature is related to the higher repetition rate and the higher partial discharges energy; the observed increase of optical emission indicates the rise of the partial discharges current. The temperature increases the enamel insulation permittivity which leads to the higher electric field in the air gaps, facilitating the PD inception [24]. The fast dielectric destruction is a result of the ionization in air gaps between wires' turns, high ozone concentration, and local temperature rise. The interaction and mutual intensification of both electrical and thermal stresses accelerate the insulation aging and destruction, leading to a thermal runaway and a thermal insulation breakdown.

## 4 CONCLUSIONS

This paper presents a testing method intended to evaluate winding insulation endurance in motors fed by SiC-based inverters. The designed test bench allows to reproduce critical constraints arising in motors' windings caused by the use of SiC high-speed switches. These constraints are characterized by impulse voltage having steep rising/falling rates and high switching frequency.

Furthermore, the proposed test bench makes it possible to perform an insulation aging investigation incorporating both electrical and thermal stresses as they are significant aging factors in inverter fed motors. Thus, voltage parameters in the developed pulse generator and temperature levels in the thermal chamber can be varied.

In this paper, we proposed to conduct tests on unimpregnated magnet wires' specimens to evaluate their endurance to partial discharges in motor windings. It is based on the consideration that partial discharges can occur in voids and cracks of winding impregnation. Testing magnet wires with different enamel compositions allowed us to compare their withstand capability to PD. The standard specimens in form of twisted pairs for round wires have been tested. For rectangular magnet wires, a novel specimen's shape is proposed. Made of two rectangular wire's pieces wound in parallel, this specimen represents a coil for testing insulation between turns like in a coil type winding.

In order to be able to compare different wires' endurance, tests have been conducted until the enamel insulation breakdown. The specimens subjected to high *dv/dt* impulse voltage have “frost-effect” traces on their enamel film surface. This effect is caused by mechanical and chemical erosion due to partial discharges. The increase of the environmental temperature in the thermal chamber decreases the insulation lifespan. A similar effect is observed when the switching frequency of impulse voltage is increased. High energy partial discharges and local temperature rise induce fast dielectric destruction. Therefore, it may be assumed that electro-thermal aging and the related deterioration of the dielectric's physicochemical properties due to PD will eventually lead to a
thermal failure of dielectrics, lowering the threshold breakdown voltage.

Among our studied wires, the tested corona resistant round section wire and the rectangular section wire demonstrate the best endurance having the highest time to breakdown. However, the time to breakdown decreases significantly with the rise of both voltage amplitude and switching frequency. Thus, the fast-rising impulse voltage combined with high switching frequency can be a major limiting factor that determines the applicability of insulating materials in inverter-fed motors. Conducting insulation endurance tests on magnet wires' specimens gives an opportunity, at the machine design stage, to estimate and compare the insulation materials' ability to withstand applied stresses and thus to minimize the risk of unexpected machine's failure.

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[14] D. R. Meyer et al., "Influence of impulse voltage repetition frequency on RPDIV in partial vacuum," IEEE Trans. Dielectr. Electr. Insul., vol. 25, no. 3, pp. 873-882, June 2018.

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[17] Winding wires - Test methods - Part 5: Electrical properties, IEC 60851-5:2008, Ed. 4.0, 2008.

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[19] IEC 60034-18-42:2008. Rotating electrical machines - Part 18-42: Qualification and acceptance tests for partial discharge resistant electrical insulation systems (Type II) used in rotating electrical machines fed from voltage converters, TS Ed. 1.0, 2008.

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[24] M. Kaufhold, G. Borner, M. Eberhardt, and J. Speck, "Failure mechanism of the interturn insulation of low voltage electric machines fed by pulse-controlled inverters," IEEE Electr. Insul. Mag., vol. 12, no. 5, pp. 9-16, Sep. 1996.

<figure><img src="image_15.png" /></figure>

**Veronika Gavrilenko** got both M.Sc. degrees in Electrical Engineering from the Tomsk Polytechnic University (TPU), Russia, and from the CentraleSupelec, France, in 2014. In 2020 she completed jointly supervised Ph.D. in Electrical Engineering at the ENS Paris-Saclay, France, and at the TPU. She is currently an Associate Professor at the Université de Pau et des Pays de l'Adour. Her research interests include electrical insulation systems, partial discharges in inverter-fed motors, and wide bandgap semiconductor electronics.

<figure><img src="image_16.png" /></figure>

**Andrey Leonov** (member IGIP) got his M.Sc. and Ph.D. in Electrical Engineering in 1996 and 2003 from the Tomsk Polytechnic University (TPU), Russia. He is currently an Associate Professor at the TPU and Head of the Master's program "Electrical insulation, high voltage and cable engineering". His research activities deal with reliability of cables and electrical insulation systems.

<figure><img src="image_17.png" /></figure>

**Andrey Bukharkin** was born in Tomsk, Russia on 27 January 1989. He received the Master's degree in electrophysical technique from the Tomsk Polytechnic University (TPU) in 2012. He is currently working on his Ph.D. degree at TPU, on the study of electrophysical processes in sedimentary rocks.

<figure><img src="image_18.png" /></figure>

**Stéphane Lefebvre** received the Ph.D. degree in electrical engineering from the École Normale Supérieure de Cachan, Cachan, France, in 1994. He is currently a Full Professor at the Conservatoire National des Arts et Métiers (Cnam), France, where he teaches electrical engineering. His current research interests with SATIE laboratory include robustness of power semiconductor devices and integration in power electronics.

<figure><img src="image_19.png" /></figure>

**Sami Hlioui** obtained the electromechanical engineer diploma in 2004, his M.Sc. degree in electrical engineering from the Ecole Normale Superieure de Cachan, France and his Ph.D. degree in electrical power engineering from the University of Technology of Belfort-Montbeliard (UTBM), France, in 2008. He is currently a Full Professor at the Cergy Paris Université (CYU), France. His main research interests are the multidisciplinary modeling of electromagnetic

actuators and the optimal design of these actuators for embedded applications.