Massinissa Hadjloum
email: massinissa.hadjloum@univ-nantes.fr

Mohammed El Gibari

Hongwu Li

Afshin S Daryoush

Leaky waveguide deflector for 40 GS/s and 6 bits all-optical analog-to-digital converters

Keywords: All-optical analog-to-digital converter, Electro-optical polymer, Leaky waveguide, GS/s optical sampling, Bits of resolution

Analog-to-digital converters (ADC) are an important part of realizing direct digital receivers in future communication systems, where broad bandwidth, high effective number of bits, and low DC power consumption are an important requirement of achieving it at microwave frequencies. Due to a number of physical limitations of electrical ADC, design of an all-optical analog-todigital converter (AOADC) is pursued and presented here based on highly stable mode-locked laser based clock signal pulses for optical sampling, and a combination of leaky waveguide optical deflector using electro-optical (EO) polymers and stationary optical windows followed with high-speed photodetectors as optical quantizer. The reported principle of the AOADC is to convert a broadband RF signal into a spatially sampled light using optical deflection angle variation before quantizing it using either binary or Gray coded optical windows. The design and modeling of an AOADC working up to 20 GHz RF frequencies (with Nyquist sampling rate of over 40 GS/s) and for providing a resolution of better than 6 bits with under 4W of power consumption. Performance is currently limited by both optical and microwave attenuation in the available EO polymers.

Introduction

Telecommunications and microwave imaging systems performance is enhanced by digital signal processing functions [START_REF] Daryoush | Digitally Beamformed Multibeam phased array antennas for future communication satellites[END_REF] and in particular direct digital receivers are target of broadband signal detection using an array of signal processed detection such as FFT (Fast Fourier Transform). Fast ADCs are required to cope with the increase of the transmission rate and/or the carrier frequency in the range of several tens of GHz.

Electronic ADCs are running today at constantly higher sampling rate. In recent years, Fujitsu Inc. introduced their second generation ultra-fast ADC in CMOS technology operating with sampling rate up to 65 GS/s. However, the ADC is constructed from many interleaved elementary ADCs [START_REF]Fujitsu Launches Second Generation Ultra-fast 65GSa/s 8-Bit ADC Technology for 100G Optical Transport[END_REF], resulting in very high complexity and effective number of bits (ENoB) limitation of the ADC due to matching problems between interleaved elementary ADCs. As fundamentally, electronic ADCs ENoB is limited by the aperture jitter in electronic clocks and ambiguity in comparators, for elementary electronic ADCs to the state of the art with sampling rate of 12 GS/s, the resolution is limited to 5 bits [START_REF] El-Chammas | A 12-GS/s 81-mW 5-bit time-interleaved flash ADC with background timing skew calibration[END_REF]. Photonic ADCs can achieve much higher sampling rate and ENoB thanks to mode-lockedlaser with an aperture jitter 4 to 5 times smaller than electronic clocks [START_REF] Khilo | Photonic ADC: overcoming the bottleneck of electronic jitter[END_REF] and repetition rate over 160 GHz [START_REF] Hou | 160 GHz Passively Mode-Locked AlGaInAs 1.55 µm Strained Quantum-Well Lasers With Deeply Etched Intracavity Mirrors[END_REF] for optical sampling. Compared with electronic ADCs, AOADCs performs both sampling and quantization in optical domain, as opposed to hybrid designs where sampling is done in optical domain while quantization is performed in electrical domain [START_REF] Han | Photonic time-stretched analog-to-digital converter: fundamental concepts and practical considerations[END_REF]. Our design approach relies on using spatial sampling by transferring the amplitude of the sampled RF signal to deflection angle using a leaky waveguide optical deflector [START_REF] Hou | All-optical analog-to-digital converter up to 40 GSPS on spatial techniques[END_REF]. The optical deflector operates based on principle of an electro-optical (EO) polymers as waveguiding core of leaky waveguide structure. The EO polymers are more attractive over LiNbO3 and other ceramics because of a closer velocity matching [START_REF] Zhang | Polymer-based hybrid integrated photonic devices for silicon on-chip modulation and board-level optical interconnects[END_REF] between optical and RF waves with a stronger EO coefficient [START_REF] Huang | Highly efficient electrooptic polymers through improved poling using a thin TiO2-modified transparent electrode[END_REF]. Moreover, the selected EO material has reported bandwidth of over 100 GHz [START_REF] Chen | Demonstration of 110 GHz electro-optic polymer modulators[END_REF]. Recent progress in chromophore synthesis allows obtaining polymers with EO coefficient up to 350 pm/V [START_REF] Huang | Highly efficient electrooptic polymers through improved poling using a thin TiO2-modified transparent electrode[END_REF] that also meets board bandwidth due to the excellent velocity matching between microwave and lightwave signals. In addition, by combining the strong EO effect of the polymer with the very low timing jitters of actively mode-locked laser pulses [START_REF] Baili | Timing jitter reduction of a mode-locked VECSL using an optically triggered SESAM[END_REF], better conversion resolution, larger bandwidth and lower power consumption can be obtained than with electronic ADCs. These improvements are specific to optical solutions and makes AOADCs a very promising solution over electrical counterparts for 40 GS/s and effective number of bits (ENoB) of larger than 6 with low power consumption of under 4W. In this paper, we present the practical architecture and design of both optical waveguide and microwave driving electrodes of a leaky waveguide optical deflector using EO polymers. Design challenges and performance optimization are introduced by taking into account realization constraints of a 40 GS/s AOADC [START_REF] Hou | All-optical analog-to-digital converter up to 40 GSPS on spatial techniques[END_REF][START_REF] Huang | Highly efficient electrooptic polymers through improved poling using a thin TiO2-modified transparent electrode[END_REF] meeting 6 ENoB of binary code and under 4W of power consumption. The paper presents the design topology of a leaky waveguide deflector and principles of achieving high angular deflection with a small angular divergence. Bandwidth constraints are addressed by analyzing both velocity mismatching and RF electrode attenuation by incorporating special design modifications.

Operation principle of AOADC

The conceptual realization of AOADC is presented in Fig. 1, where it is composed of three main blocks [START_REF] Hou | All-optical analog-to-digital converter up to 40 GSPS on spatial techniques[END_REF]:

-A mode-locked laser provides short optical pulses of the order of picoseconds and pulse repetition interval of about 25 picoseconds (as minimum sampling time for Nyquist bandwidth of 20 GHz) a low timing jitter (of the order of 100 femtoseconds) for optical sampling of the applied RF signal to leaky waveguide based optical deflector.

-An optical deflector realized using an EO polymer in the region between RF electrodes to adjust deflection angle out of a leaky waveguide by converting each analog amplitude voltage variation of dV into an angular variation of dθ followed by a cylindrical lens to illuminate on the spatial coding mask.

-A stationary coding mask (in either binary or Gray codes) as optical windows for illuminating at a constant optical power level on an array of ultra high-speed photodetectors (with bandwidth of at least 40 GHz) to directly quantize the sampled RF signals, as 1 when illuminated and 0 when blocked off.

Since both mode-locked laser pulses with low jitters and quantization using array of photodiodes with optical masks have been already reported [START_REF] Hou | All-optical analog-to-digital converter up to 40 GSPS on spatial techniques[END_REF][START_REF] Baili | Timing jitter reduction of a mode-locked VECSL using an optically triggered SESAM[END_REF], the leaky waveguide deflector remains the central element of the AOADC. The detailed cross sectional view of the optical deflector is depicted in Fig. 2, which is composed of an embedded coplanar strip line (CPS) that applies a broadband RF signal (designed for bandwidth of 20 GHz) to the EO polymer. The EO polymer forms an optical waveguide with a small leaky coefficient to the superstrate. The leaky coefficient is selected optimally to maximize propagation length along the leaky waveguide, while satisfying velocity matched bandwidth of at least 20 GHz between optical and electrical travelling waves. When pulses from the mode-locked laser propagate through the waveguide, part of the optical power leaks to the superstrate. A thin buffer layer in between EO waveguide and superstrate controls the amount of light leakage over the optical deflector length as the leaky coefficient. The effective deflection angle is being adjusted by EO changes in index of refraction of optical waveguide. The typical values of RF permittivity and optical index of refraction for various materials are highlighted in Fig. 2. The physical parameters of W, G, h t and h g are optimized for the performance compromise of bandwidth, power consumption, and effective number of bits (ENoB).

The leaky waveguide deflector of the AOADC has a structure similar to that of a conventional EO phase modulator (cf. Fig. 2), except that the top cladding (buffer layer) is thin enough, so a tail of the optical field extends into the superstrate with refractive index n s . The index of refraction of n s is selected to be greater than the effective core index of refraction of the optical waveguide to result in transfer of energy in the normal direction of substrate and superstrate interface. The leaked power will propagate in a direction normal to this interface and the power distribution along the length of leaky optical waveguide controls in far-field diffraction angle. Both the bandwidth and resolution of the optical ADC depend on the thickness of the buffer layer h t , optical and microwave losses in the EO polymer since the optical distribution along the leaky waveguide are to be controlled.

The short mode-locked laser pulses with the low timing jitters are used for digital sampling of the applied RF signal. The sampling of RF signal is achieved by transforming applied RF signal to the leaky optical waveguide deflection angle. The travelling RF amplitude applied to the CPS results in changes in the effective refractive index of the poled EO polymer that is used as the waveguide core in the optical deflector. In particular, the applied RF signal induces this change due to a linear electro-optical (i.e., Pockels) effect, which effectively results in a change in deflection angle of θ, while satisfying Snell's law of refraction. The leaky wave is diffracted after propagation through superstrate-prism and is transmitted into the air at output angle of θ o . The diffracted beam is focused by the cylindrical lens across an image line of the optical coding mask as optical windows. The opening of optical windows provides a constant optical energy on the array of high-speed photodiodes that are located behind each optical window. For an ENoB of b=log 2 N, where N is the number possible states, b columns with N rows of optical windows are required with Nxb photodetectors arrays. Alternatively, Nxb fiber bundles could combine deflected light on each column of the optical window to a N:1 optical combiners that are remoted to a single ultra high-speed photodiode using parallel combination of optical fibers mounted behind the optical masks. The image line moves up and down, resulting in N distinct image lines, each one corresponding to an angle, according to an applied voltage as illustrated in Fig. 1.

The AOADC is designed for sampling rates f s up to 40 GS/s and at least b=6 bits of resolution, with a very high resolution-sampling rate product of 2.56×10 12 LSBs-Hz (where LSB means Least Significant Bits). In addition to the general advantages of all-optical ADCs, such as high-rate sampling, transportation of the binary output signal and the sampling signal by optical fibers, and the resulting increased EMI immunity, the proposed architecture also offers [START_REF] Hou | All-optical analog-to-digital converter up to 40 GSPS on spatial techniques[END_REF] better immunity to optical noise because of a lesser sensitivity to the transmitted optical power fluctuations. Moreover, since all bit states are inputted by the same deflected optical power due to its spatial sampling operation and is not close to the shot noise limit in the photodetectors, a higher linearity for this ADC is expected. Finally by taking advantage of traveling wave electrodes of CPS, this AOADC exhibits an inherently high sampling rate and broadband modulation capability.

AOADC Effective Number of Bits (ENoB)

The bit resolution of the AOADC depends on the beam divergence angle GT, which is being controlled by optical distribution along length of the optical deflector and its leaky wave attenuation coefficient, and the total angular swing 'T of the deflected beam. The 'T depends on maximum voltage swing provided by the RF signal and the EO coefficient of the polymer. The number of resolved lines on the coding mask N = 'T⁄δθ is related to the number of bits of resolution, b=log 2 N. For a given total angular swing of the deflected beam, one should reduce the leaky beam divergence to increase the resolution, so two conditions must be met:

-A single-mode optical waveguide. Indeed, multimode waveguide causes an enlargement of the deflected beam because of modal dispersion. The core width and height will be chosen according to the refractive index contrast between the core, the lower cladding and the buffer to obtain a single-mode waveguide.

-A large effective leaky waveguide length L eff ensuring a small leaky beam divergence limited by the diffraction angle limit. The effective leaky waveguide length L eff is controlled by the leakage coefficient D f , in turn by the buffer layer h t . A long effective length of the leaky waveguide requires a weak leakage meaning that the optical wave is still well confined in the waveguide core region and a small portion of it extends to the superstrate-prism.

The buffer layer thickness h t is the key parameter to obtain the right leakage coefficient D f . Fig. 3a shows its influence on D f . The thicker is the buffer layer, the smaller is the leakage coefficient and larger is the effective leaky waveguide length, the smaller is the beam divergence, so the higher is the resolution. However, a long deflector length reduces the modulation bandwidth due to velocity mismatch of electrical and optical waves along the CPS and leaky wave optical waveguide. So, a design compromise is made between the resolution and the bandwidth of ADC. The initial optical leakage coefficient is considered between 0.1 and 0.3 Np/cm, corresponding to a buffer layer thickness 3.75 and 3.35 µm respectively, and an optimum effective waveguide length between 2 and 3 cm.

The optical waveguide structure has been designed and optimized for short optical pulses from a mode-locked laser at O o =1.55 µm using the software OptiBPM. The design assumes employing lower cladding and buffer layer (cf. Fig. 2) in polymer NOA63 (n g = n t = 1.56), core in EO polymer PMMI/CPO-1 (n c =1.63), superstrate in a glass (n s =1.7), resulting in an effective index n eff =1.582. Optical attenuations over the all length of the leaky waveguide deflector is considered equal to 0.2 Np/cm (or 1.74 dB/cm). The single mode operation requirement leads to a waveguide core (cf. Fig. 3b) of 3-µm width and a 1-µm thickness, which meets the practical limits of low-cost micro-printing technique in the IETR laboratory.

By summing the contribution at each incremental segment of dz along the length of the waveguide to the optical leak wave and taking into account optical losses, the total leaky wave at the deflection angle θ is expressed as:

(1) Where A: magnitude of the input lightwave in the waveguide at the origin z=0; : leakage coefficient; : total optical losses due to material absorption and waveguide imperfection particularly the surface roughness;

: phase propagation constant of the lightwave in the leaky optical waveguide; : wave number in the superstrate; r: position vector at the observation point.

The effective refractive index of the waveguide is changed due to the Pockels effect of the core in electro-optical polymer as:

Where : effective refractive index in absence of applied RF electrical field; r eo : electro-optical coefficient of EO polymer;

: overlap integral between the RF and optical fields; : microwave losses in the CPS line; E(z): RF electrical field amplitude at z and expressed as due to exponential decrease of RF field for a travelling wave, where E o is the magnitude of the RF field at the input of the CPS line.

Similarly, the phase constant as function of the applied electrical RF field is:

(2)

where is the wave number in free space. By incorporating the Eq. ( 2) into Eq. ( 1), the total leaky optical field at the deflection angle of θ is then expressed as:

(3)

The influence of various parameters on the resolution has been studied using numerical calculation of Eq. (3) in Matlab. These results are rendered in Fig. 4.

Fig. 4a shows the normalized optical intensity as a function of the output beam angle θ for different values of the leakage coefficient D f by assuming the EO polymer optical losses of α opt = 0.2 Np/cm (i.e., 1.74 dB/cm). The leaky light intensity increases with the leakage coefficient D f . Indeed, for D f = 0 Np/cm, the intensity of the leaky wave is zero because the lightwave is totally confined in the optical waveguide. But a high leakage coefficient leads to a larger beam divergence δθ by reducing effective waveguide length L eff due to diffraction angle, as depicted in Fig. 4b. In fact, since the output beam divergence δθ is inversely proportional to L eff with a higher leakage coefficient a larger beam divergence is to be experienced. It could be understood that increase in optical losses results also in an increase of the beam divergence via reduction of the effective waveguide length.

The results in Fig. 5 have been obtained with waveguide length L = 3 cm, optical losses α opt = 0.2 Np/cm, RF losses α RF = 0.01 Np/cm thanks to a pre-emphasis technique based on an appropriate modification of the gap between the strips of CPS electrode (to develop in detail in a coming paper), leaky coefficient α f = 0.1 Np/cm, EO coefficient r eo = 100 pm/V, overlap integral between the RF and optical fields * = 100%, applied voltage V = ± 20 V and gap of CPS line = 6 µm. The two peaks in Fig. 5.a correspond respectively to applied voltages V RF = -20 V and +20 V, the spacing between them corresponds to the total angular sweeping 'T. According to the curve in the Fig. 5.b, the number of resolvable lines is almost proportional to the EO coefficient of the core material. It has been easily demonstrated that the input voltage is linearly converted into spatial angular deflection.

Bandwidth

As mentioned above, the AOADC's resolution is proportional to the effective length of the EO deflector, while the modulation bandwidth is inversely proportional to the length L eff due to the microwave and optical fields transit time mismatch limitations. To preserve the bandwidth, the deflector is designed to obtain the best velocity match possible between the optical and electrical waves. Indeed, if the velocities are matched, an optical pulse experiences the EO effect due to the input electrical RF field applied at the same instant throughout its propagation, and the leaky optical wave would be focused on even finer line. The maximum EO modulation frequency taking in account only the velocity mismatching is described by [START_REF] Rahman | Design optimization of polymer electrooptic modulators[END_REF]:

(4)
Where n eff is the effective refractive index of the optical waveguide, H eff is the effective dielectric constant of the CPS line, L is the interaction length between optical and microwave fields, and c is the light velocity in free space.

According to equation ( 4), one should optimize the design of both optical and microwave structures of the EO deflector to make the effective refractive index n eff and the square root of the effective dielectric constant H eff as close as possible to achieve broader bandwidth. When manufacturing, the deflector will be deposited on a silicon wafer, which serves as a popular substrate for Si Photonics combined with electronics (cf. Fig. 2), its high dielectric constant (H r =11.9) influences significantly the effective dielectric constant H eff and increase the difference between the effective refractive index and the square root of the effective dielectric constant, and hence, decrease the modulation bandwidth. To avoid this limitation, the electrical field must be isolated from the high dielectric constant of silicon by increasing the thickness of the lower cladding (h g ), with a lower dielectric constant.

All microwave features of the leaky waveguide have been calculated and optimized using the 3D full-wave electromagnetic field analysis software HFSS V. 15, such as characteristic impedance Z o of the CPS line, effective dielectric constant ε eff and microwave losses. Fig. 6 depicts the influence of the lower cladding thickness of NOA63 polymer (H r = 3.2, deposited on the silicon wafer of H r =11.9) on H eff and Z o . As we can see, H eff varies from 5.6 to 4.1 when the lower cladding thickness varies from 10 to 100 µm. A thick lower cladding keeps the microwave signal away from the silicon substrate and allows H eff to get matched to the effective refractive index n eff of 1.582. So the bandwidth can be improved.

Using travelling wave configuration, the EO deflector must be loaded with a resistor that matches the characteristic impedance of the electrodes. The CPS line is designed for a Z o = Z L = 50 Ω, where Z L is the load impedance, and for velocity-matched bandwidth of over 20 GHz. Z o depends not only on the geometric parameters of the CPS line but also all the surrounding materials. The gap G between CPS electrodes is fixed by the constraints of the optical waveguide manufacturing using micro-printing, core width of 3 µm and lateral claddings width of 3 µm, so G = 9 µm. Optimal electrode width is W = 350 µm, and the copper thickness t = 1 µm.

Compromises are made for achieving high bandwidth with required bit resolution in the design of the leaky waveguide deflector. The optimum length of L = 2.5 cm allows reaching the bandwidth of 20 GHz, for a thickness of the lower cladding of h g = 100 µm (this provides an eff = 4.1). An EO polymer with r eo = 300 pm/V, which is below the state of the art of EO polymers [START_REF] Huang | Highly efficient electrooptic polymers through improved poling using a thin TiO2-modified transparent electrode[END_REF], with an applied RF signal of magnitude V = ± 20 V (for a CPS gap = 6 µm at the input face of the waveguide and optimized over its length for electric field pre-emphasis) allows 6 bits of resolution for an optimum leaky coefficient of 0.1 Np/cm, corresponding to the buffet layer thickness of h t = 3.75 µm. The leaky coefficient can be chosen higher than 0.1 Np/cm if we use higher EO coefficient. This allows increasing the bandwidth of the leaky waveguide deflector by reducing its length, and keeps the resolution to 6 bits.

Conclusion

A practical architecture of an all-optical spatial sampling ADC based on leaky waveguide deflector is presented. Optimum design of the central element of the AOADC, EO polymer leaky waveguide deflector, is reported here. The resulting sampling rate is 40 GS/s with 6 bits of resolution using available EO polymers, corresponding to a very high resolution-sampling rate product of 2.56×10 12 LSBs-Hz. The current design takes into account the state of art of microprinting manufacturing limits in our laboratories. Manufacturing and realization of this device are being addressed for experimental evaluations.

Figure captions : Fig. 1 .

 :1 Figure captions:Fig.1. Conceptual diagram of the AOADC based on short-pulse laser for sampling, a leaky waveguide optical deflector and optical coding mask with array of photodiodes for quantization of applied RF signal. Fig.2. Cross section view of the optical leaky waveguide deflector on silicon substrate using EO polymer. Fig. 3. (a) Predicted influence of the buffer layer thickness on the leaky wave coefficient with core refractive index n c =1.63 and lower cladding and buffer layer refractive index n g = n t = 1.56 at the wavelength O o =1.55 µm. (b) Divergence angle of the leaky lightwave as function of the leakage coefficient. Fig. 4. (a) Normalized optical intensity as a function of the output beam angle θ for different values of the leakage coefficient α f . (b) Divergence angle of δθ for the leaky wave as a function of α f . Fig. 5. Simulations results obtained with L= 3 cm, α opt = 0.2 Np/cm, α RF = 0.01 Np/cm, α f = 0.1 Np/cm, *=100%, V RF = ± 20 V and CPS gap = 6 µm. (a) Leaky intensity peaks for applied voltages V RF = -20 and +20 V with r eo = 100 pm/V. (b) the number of resolvable lines as a function of EO coefficient. Fig. 6. Influence of the lower cladding thickness on dispersion of CPS waveguide. (a) Effective relative permittivity. (b) Characteristic impedance.

Figure

  Figure(s)

Acknowledgement

The authors would like to thank the Region of Pays de la Loire for its support through the project ADC PolyNano. The authors appreciated helpful discussion and technical input of Prof. H. Gundel, Prof. B. Guiffard and Dr. S. Ginestar from IETR on micro-printing manufacturing limits.


Thibaut Billard

Cedric Abadie

Bouazza Taghia

Advanced Partial Discharge Testing of 540V Aeronautic Motors Fed by SiC Inverter under Altitude Conditions

The present paper reports non-electrically intrusive partial discharge investigations on aeronautic and electric vehicle motors fed by SiC inverter drive under variable environmental conditions. A representative test procedure and experimental set-up based on operating aeronautic conditions are essential to ensure the accuracy and reliability of partial discharge test on aircraft systems to make informed decisions on insulation system design choice. The aim of this paper is to demonstrate the feasibility of partial discharge test of conditions, both electrically and environmentally.

To do so, the paper will start by detailing the innovative experimental set-up to be used in the study. It mainly consists in a high-voltage (1000V) inverter drive using SiC components to provide fast rise time surges. A vacuum chamber is used to simulate altitude while the based signal processing provided partial discharge detection. voltage magnitude trigger partial discharge events. The study helps to limits of the insulation system under various pressure and electrical conditions. It is shown that a representative insulation system performance picture could be drawn experimentally and used to enhance insulation design and manufacturing choices. This paper will also review the ability of the non-intrusive test method and the associated numerical signal processing to detect partial discharge in a motor fed by fast-rise time surge and under and thoughts about future work regarding advanced test procedure.

Introduction

Background

Optimization of energy sources aboard aircraft and permanent improvements in more electric technologies are pushing the aeronautic industry to aim for the more electrical aircraft (MEA). power generation, distribution and conversion are easier because achieved than with traditional hydraulic systems needing large, heavy and maintenance intensive distribution system. But, this shift towards MEA is not without consequences on the electrical stress the insulation system has to withstand. Thus, with primary voltage increasing, power distribution architecture evolution and high power density power electronics, partial discharge (PD) is now a serious cause of concern for aircraft integrator, system designer and component manufacturer. Once PDs are occurring regularly, degradation of the insulation system until premature failure of the aircraft system is irremediable.

Scope and Goal of the Work

This topic of interest has already been discussed by several key actors of the aerospace industry, research center such as US Air force laboratory or international academic actors. All acknowledge that PD risk in a harsh aeronautic environment where pressure, temperature and humidity variation, electrical stress created by inverter and harness length and weight increase. [1,2,[START_REF] Cella | Partial discharges measurements at the constituents' level of aerospace power electronics converters[END_REF]4] Among key issues of aeronautic companies that has to be investigated electric motors in an electrically non-intrusive way and under representative aeronautic environment when fed by a silicon carbide (SiC) inverter drive will be addressed in this paper and will be the main contribution. Indeed, it has been recently demonstrated that on-line PD detection on electric motors fed by inverter drive proves to be possible using non-intrusive sensors. A broad range of operating electric motors have been tested on industrial test benches under nominal conditions detection method [START_REF] Billard | Recent advances in on-line Partial Discharge detection in electric power conversion chains used in aeronautics[END_REF]. Moreover, a previous analysis illustrated the gains of carrying out a detailed and progressive analysis of the insulation system both under AC and impulse voltage from twisted pair sample to a complete stator [START_REF] Billard | Non-Intrusive Partial Discharges Investigations on Aeronautic Motors[END_REF]. But, as a whole, a key link in the testing chain was still missing. So, in order to make one step further, a unique and new testing mean has been designed at IRT Saint-Exupéry and will be used in this paper. It is a highly customizable three phases SiC inverter drive with a high voltage DC bus of 1kV.

The aim of this experimental set-up is thus to help electric motor designer evaluate the quality of the insulation system under harsh operating conditions. The three kind of PD tests conditions (simple thus complementary and give the full picture of the insulation system.

Outline

system will be recalled. In a second step, key features of more electrical aircraft will be recalled and how they relate to the innovative experimental set-up used here. Then, its main characteristics will be presented and an analysis is made regarding the relation between time, dV/dt and overvoltage.

The impact of a SiC inverter drive on the non-intrusive detection and wavelet numerical processing will be presented as a mean to double-check partial discharge inception (PDIV) values.

Then, details of the 3 stator will be presented and will be followed by tests results and analysis. Stator n°1 illustrates the potential consequences of undetected partial discharge on the insulation system and its consecutive failure. Stator n°2 illustrates the performance of the whole experimental set-up both at atmospheric and low pressure. discussion is followed by details of future work.

Partial Discharge Basics

What is a Partial Discharge? IEEE Definition

7] a partial discharge is:

An electrical discharge that only partially bridges the insulation between conductors. A transient gaseous ionization occurs in an insulation system when the electric stress exceeds a critical value, and this ionization produces partial discharges.

opening time 30s [START_REF] Billard | Non-Intrusive Partial Discharges Investigations on Aeronautic Motors[END_REF]

Consequences of Partial Discharge

Partial discharge events have been known and studied for more than a century, starting with Paschen, Peek and Townsend among others. Partial discharges are feared because they contribute to the degradation of the insulation system and lead to its premature failure [START_REF] Bartnikas | Engineering dielectrics -Volume 1 -Corona measurements and interpretation[END_REF] as will be shown in this paper.

Partial Discharge in More Electrical Aircraft

With the recent increase in voltage from 115 V AC to 230 V AC or Moreover, it is known from Paschen curve that partial discharge risk is highly linked to pressure level. Since this variation is non-linear with pressure, it is assumed that no simple scale factor could be applied to predict partial discharge inception voltage from results at atmospheric pressure. Indeed, it is impossible to state a priori that the weakest insulation point test at atmospheric pressure is also the weakest insulation point at low pressure so that a scale factor is a sound hypothesis. It is thus mandatory to carry out test across the whole range of pressure.

Partial Discharge Inception Voltage

According to IEEE Standard [START_REF]IEEE Guide for the Measurement of Partial Discharges in AC Electric Machinery[END_REF], PDIV is

The lowest voltage at which continuous partial discharges (PDs) permissible background noise) occur as the applied voltage is including the rate at which the voltage is increased as well as the previous history of the voltage applied to the winding or component thereof PDIV could be expressed either at RMS or peak voltage depending on the applied voltage during test.

During partial discharge test of equipment, PDIV is usually a representation of the insulation system quality. In other words, it partial discharge occurs.

It is thus of a critical importance to that tests are robust, easy to reproduce and relevant for the foreseen application. It is also important to have an accurate PDIV measurement method that could simple material characterization test at atmospheric pressure, to an operating system in a low pressure environment.

have dramatic consequence on its reliability, leading to short-circuits creating electrical arcs. Safe level of target PDIV should be set in order to ensure equipment are PD free during operational life of the system.

More Electrical Aircraft Paradigm

The MEA approach underlines the use of electrical system for non-propulsive application such as environmental control systems Other applications such as electrical taxiing or anti-icing are also aircraft has no on-engine hydraulic power generation and bleed air

Key Role of Highly Integrated Power Electronics

A key enabler of such development is the breakthrough in high power density, high voltage and reliable silicon-based power semiconductor switching device such as insulated gate bipolar transistor (IGBT).

It is expected that state-of-the-art switching device such as Silicon Carbide (SiC) or Gallium Nitride (GaN) will dramatically improve inverter drive, converters, motor controller and other semiconductor The evolution of power electronics towards fast switching in the primary voltage is putting even more stress on motor insulation system [START_REF] Roboam | New trends and challenges of electrical networks embedded in "more electrical aircraft[END_REF][START_REF] Rosero | Moving towards a more electric aircraft[END_REF]11,[START_REF] Moir | Aircraft Systems: Mechanical, Electrical and Avionics Subsystems Integration, 3rd Edition[END_REF] The combination of harness length, fast rise time is creation overvoltage at motor terminals and an uneven voltage distribution is found on coils increasing electrical stress on the turn-to-turn insulation system. The following experimental set-up aims to recreate such electrical stresses

Experimental Set-Up

Power Supplies

High Speed High Voltage Power Amplifier 10kV peak and a maximum of 40mA RMS current with a bandwidth tested such as AC at various frequency, square bipolar with adaptable duty cycle, frequency and other point-per-point signals.

High Voltage SiC Inverter Drive

An inverter drive has been designed at IRT Saint Exupéry to recreate voltage, two 15kW 500V ElektroAutomatic DC power supplies are in series with the mid-point grounded, the total DC bus voltage could remotely be varied from ± 10V to ± 500V.

The inverter drive consists in three 1700V SiC Cree power modules delivering fast rise times on each phase with driver boards accepting Matlab-Simulink model could be used to control the inverter drive thanks to the use of an OPAL-RT real time calculator. Switching frequency, carrier frequency and modulation factor could easily be per phase (enable and command) allowing the use of the inverter as a

High Voltage Measurements

Voltage level are monitored using a Testec TT-SI 9010A (1/1000

Signal Acquisition

Analog to Digital Conversion

Data are displayed and recorded using a Keysight DSOS204A oscilloscope with a sampling rate of 20GSa/s and numerical bandwidth numerically, the high resolution mode (12 bits) is used instead of the peak detect mode at the higher sampling frequency available.

Analog High Pass-Filtering

When performing measurement using non-intrusive sensor, it is usual remove noise coming from inverter drive switches or power have been used in the following experiments

Partial Discharge Sensors

Technological research at IRT Saint-Exupéry are focused on non-intrusive sensors. These sensors and associated method have both electric motor in automotive [2] and aeronautic test benches [START_REF] Billard | Recent advances in on-line Partial Discharge detection in electric power conversion chains used in aeronautics[END_REF].

Capacitive Sensor

The non-intrusive sensor used to detect partial discharge is taking sensor behavior in the next part. This sensor has already been used in numerous study, particularly [2] and [START_REF] Billard | Non-Intrusive Partial Discharges Investigations on Aeronautic Motors[END_REF].

Environmental Conditions

All the tests have been carried out within a grounded vacuum chamber, acting as a faraday chamber, connected to a vacuum pump. The system is currently able to regulate pressure from atmospheric to 20mbars with an accuracy of +/-5 mbars.

Rise Time and Overvoltage Measurement

time dependent rise time and overvoltage analysis has been carried out while feeding a three phase electric. Measurements were probe between the DC-and the output of the power module. Table 1 details the used operating point.

In IEC 60034-18-41 standard [13], stress magnitude categories are nominal DC bus voltage. It is thus interesting to track the time dependent evolution of this parameters to check when the electrical stress is the most severe.

Typical SiC inverter behavior could be observed on both rise time and overvoltage which is both are current dependent. Although current was not monitored during measurement, the time dependency and periodicity could be observed on rise time which is maximum at current polarity reversal while minimum when current is at his highest value. On the contrary, overvoltage is maximum when current is at its maximum while overvoltage is minimum at current polarity reversal. As a result, the electrical stress created by the combination of dV/dt and overvoltage is time dependant. These phenomena could be explained by the very nature of SiC switch, current magnitude, grid resistor and other power electronics characteristics. In essence, dV/dt is maximum for low current turn-on 14]. This is shown on for falling edge in red.

phase could be observed by small perturbations on overvoltage.

Non-Intrusive Sensor Performance in Motor Supplied by SiC PWM Inverter

Capacitive Sensor Study

impulses are transduced into voltage variations through the capacitance between the copper wire and oscilloscope impedance.

The Jack-SMA sensor is a robust, inexpensive, and standardized way of detection PD in a non-intrusive way. But, for consistency to be achieved, the positioning of the sensor has to be ensured. Due to its small geometry, achieving good contact and adequate holding on the 3D printed pliers to ensure good contact regardless of the cable gauge and vibrations

The sensor is based on a capacitive coupling. Sensor sensitivity can therefore be increased simply by improving the coupling capacitance value. This is achieved by increasing the contact surface by putting copper tape on the surface of the power cable to control the interacting area and thus the value of the induced capacity ( ).

It is important to note that this sensor is sensitive to the dynamic of the partial discharge compared to the "standards" sensors, which are only sensitive to the amplitude of the discharge.

Switching Noise Magnitude Evolution: Relation with Common Mode Current

When performing non-intrusive PD detection, periodic pulses numeric, it is important to track how maximum switching noise is evolving with DC bus voltage. It is expected that a higher DC bus will lead to a higher switching noise during the rise and will thus hide potential partial discharge signals.

, a typical sequence of switching could be observed with;

another phase and a slow turn-on. Each of these switching is intrusive sensor.

This source of noise collected by the sensor is dependent of the common mode capacitance of the whole set-up value. Indeed, since partial discharge towards the input impedance of the oscilloscope and its low voltage potential; it should also collect a part of the common mode current created by switching.

The whole set-up common mode capacitance is greatly increased impedance path for high frequencies common mode current. Whereas the common mode capacitance is decreased when stator frame is will be low when the stator frame is grounded, as it is usually the case in aeronautic applications.

The sensitivity of the measurement of PD pulse high frequency current will thus be function of the sample common mode capacitance, detection capacitance and the spread of frequency content between switching common mode current and partial discharge current.

The following measurements (Table 2) were made while feeding a three phases stator with the inverter drive with the sensor without an When an the detection capacitance is increased, the common mode impedance. (Table 3)

Analogical Filters Performance

Since the sensor is sensitive to quick changes in current, any phenomenon that induces this type of variation can cause a sensor response. When seeking to detect PDs in equipment supplied by inverter using PWM., a key challenge is to distinguish the raw signal with respect to electromagnetic noise induced by fast switching inverters. The magnitude of the signal associated with partial discharges generally has an amplitude of several tens of mV, while the magnitude of the noise signal is on the order of several hundred mV and depends on the voltage magnitude as has been demonstrated previously.

greater than the frequency for noise suppression of commutations and must be adapted to the voltage rise time.

In our work, a simple technique for observing the presence or the only the high-frequency components of the signal using analog parameters can alter the discharge spectrum like the pressure.

It is therefore important to determine the amplitude of the noise induced by the switching, detected by the sensor. A study was carried increased capacitance.

It can be seen in that the noise increases with the amplitude of This is explained by the fact that the dV / dt is not only dependent on the amplitude of the voltage but also the switched current. It can be noise seems to be suppressed since the noise does not exceed 10 mV.

white noise induced by the oscilloscope. It is thus recommended, for amplitude

Noise Suppression Improvement

In aeronautical environments, some equipment operates under pressure and temperature conditions that can vary greatly. These environmental condition variations can alter the signals associated with PD. The denoising technique must therefore work regardless of during testing, of the pressure as observed above as well as the fully automated digital method to replace the expertise of an operator. We therefore decided to use the wavelet transform method.

Invented by Jean Morlet, the wavelet transform (Wavelet Transform mathematical signal processing tool that decomposes a signal into and cosine, the result of applying this method provides information only on the frequency content of the signal of interest. The disadvantage lies in not knowing the moment at which each frequency component appears. Besides that, the basic functions of the WTs called "wavelets" allow two-dimensional resolution in the frequency and time domains. [START_REF] Altay | Wavelet base selection for denoising and extraction of partial discharge pulses in noisy environment[END_REF]. It is therefore necessary to check whether this method is functional for denoising PDs occurring under PWM. We used this technique to remove noise superimposed on PDs.

We therefore compared the results given through an analog high-pass ignition of PDs was observed on a stator supplied by a SiC inverter

The stator (n°2) was positioned in a vacuum chamber in order to carry out tests at atmospheric pressure and at low pressure.

pressure.

signal reconstructed by CWT (red) at atmospheric pressure It can be observed in that the noise amplitude is greater than 500 mV and corresponds temporally to the switching. In this case, there are two close switching. The oscillations associated with the which can lead to the appearance of PDs.

It can be seen in signal can then be seen which corresponds temporally to the greatest seen that the signal reconstructed by the wavelet transform has an The method is functional and allows to recover a signal with a much higher Signal to Noise Ratio (SNR).

It is now important to check the method at low pressure (100 mbar). It should be noted that although PDIV could be determined with this method, no calibration or PD pulse shape analysis could be carried out at the moment with the proposed algorithm. Indeed, due to the dependency of the sensor to the derivative of the current pulse rather than being a simple proportional impedance, the charge of the discharge current would have to be doubly integrated which is further complicated by the fact the parasitic oscillations of the circuit in response to the PD pulse.

Electric Motors Description

In order to validate our detection method and to make the best use of our test bench based on the SiC technology, three low voltage stators chosen of designed. The table below summarizes some of their properties.

These are stators of three-phases Permanent Magnet Synchronous Motor (PMSM), they are designed to be powered with Pulse Width Modulation (PWM) inverters. (Table 4)

Stator n°1: Laboratory Dummy Sample

The design of this stator has been extensively detailed in [START_REF] Billard | Non-Intrusive Partial Discharges Investigations on Aeronautic Motors[END_REF] are tables regarding insulation and electric design are recalled below (Tables 5,6,7). The two terminals of each of his three phases are accessible. This allows to test each phase separately. It is very important to highlight that the stator is not impregnated and that it has been rewound before starting the test campaign to ensure a PD free previous history. ( )

Stator n°2: High Mechanical Integration Constraint Stator

pressure (VPI process), this allows the varnish to penetrate as much as possible within the windings. Its end-windings are mechanically compressed to optimize the axial length of the machine ( ). The neutral point is not accessible so the evaluation of the PDIV of each phase alone (turn to turn PDIV) is not possible. Therefore, only the star-connected case will be tested.

Due to non-disclosure agreement with our industrial partners, insulation and electrical design details could not be provided for stator n°2 except that this is a three phases motor with four parallel coils per phase. The winding diagram of stator n°2 is similar to stator n°1.

Stator n°3: Low Cost Stator

Unlike the other two stators, this stator is cooled by air. The insertion of its coil is automated, this allows a more reproducible winding but end-windings are not compressed and are insulated by relatively thick tapes. It is also varnished using dipped impregnation instead of vacuum pressure impregnation as shown in Each of its three phases (U, V and W) is composed of four coils (U1, U2, U3, U4, V1, V2, V3, V4 and W1, W2, W3, W4). These are each phases. This stator is designed for electric traction application under atmospheric conditions. Therefore, the proposed tests don't

Advanced Partial Discharge Testing Results

General Purpose

both 100mbars and atmospheric pressure except for stator n°3. The purpose of these tests is to investigate the PDIV under SiC inverter drive electrical stress compared to typical AC tests usually carried out. On one hand, AC tests are used to determine the PDIV between phases (if neutral point is disconnected), and between phases and stator frame. On the other hand, SiC inverter tests will be used to determine PDIV in a more realistic approach with fast rime time impulse compared to aeronautic standard (>5V/µs).

PDIV of Experimental Set-Up

Before starting any investigation, PDIV of the experimental set-up Table 8)

Stator n°1

Stator n°1 has been designed at IRT Saint-Exupery to represent a typical aeronautic electric motor insulation system.

Phase to ground

grounded to test the slot insulation only. Then each phase is tested separately in respect to the stator frame. Tests are performed with 50 Table 9) Results are similar with our previous technical report [START_REF] Billard | Non-Intrusive Partial Discharges Investigations on Aeronautic Motors[END_REF], showing that even a complete rewound of the electric motor lead to stable between phases could be noted suggesting that slot insulation is balanced in all slots.

Phase to Phase

The phase to phase insulation is now tested, with two phases Table 10)

Again, results are similar with our previous technical report [START_REF] Billard | Non-Intrusive Partial Discharges Investigations on Aeronautic Motors[END_REF], with very few variations between what seems to be a balanced phase to phase electrical insulation. On average, the phase to phase insulation is also weaker than the phase to ground insulation system.

Stator n°2

Stator n°2 is also an aeronautic electric motor with the neutral point star connected meaning only few tests are possible both in AC and possible.

Phase to Ground Tests

A typical phase to ground insulation test is made on all phases at the Phase to ground insulation performance is better than for stator n°1 which presents similar insulation characteristics except for test for the VPI varnishing process. It is believed that the VPI process helps most of air gaps.

Stator Supplied by Three-Phase SiC Inverter at Atmospheric Pressure

The electric stator is fed with the inverter drive at atmospheric pressure with a short harness with a typical operating point. The non-intrusive sensor is positioned on one of the power cable (harness) feeding the electric stator with an increased capacitive used to perform PD detection. Although not reported in details here, a is a typical case with a PD signal appearing for the maximum peak to peak voltage (1410V peak-peak) for a short impulse case. It should be noted that this PD location suggests a turn-to turn or a phase to phase default. Indeed, a phase to ground PD is highly unlikely because the peak voltage does not increase above 1000V, which is lower than phase to ground PDIV under AC voltage reported in Table 11. On the contrary, a turn default could be tested with SiC impulse around 10kV/µs.

Stator Supplied by Three-Phase SiC Inverter at Atmospheric Pressure

The electric stator is tested at low pressure (100mbars). PDIV is also 100 mbar Table 13. Phase to ground AC Partial Discharge tests for stator n°2

It can be seen on that the discharge appears at 641 Vpeak at 100 mbar for the maximum voltage. Any insulation could trigger a This PD location is not characteristics of any insulation on the contrary to the previous case. Other parasitic signals of lower amplitude at 100 mbar could be observed and are labelled as residual noise. As a summary (Table 13), these tests prove that non-intrusive PD detection is possible at both atmospheric and low pressure on electric stator fed by a SiC inverter drive. Detection with high-pass test and numerical signal processing.

Stator n°3

Stator n°3 is only tested at atmospheric pressure In table 14, PDIV values for each phase are more widely distributed regarding stator n°3 than for both previous stators which is phase-to-ground insulation performance. Lowest phase to ground PDIV for stator n°3 is very similar to stator n°1 average phase to ground PDIV value.

Phase to Ground

Phase to Phase

The phase to phase insulation is made with the same material as the slot insulation (insulating paper). Results (Table 15) are consistent with this observation and should lead to better phase-to-phase insulation performance, although PDIV value are also more widely distributed. Another possible cause for this better performance is the concentric windings.

Coil-to-Coil Tests

Since 12 coils are available, each one is tested relatively to each other within each phase. This is not a conventional test since these coils are supposed to be connected in parallel within each phase and should coils of the same phase are in close vicinity to each other and the magnet wire and varnish is the only insulation and it could lead to a fairly low PDIV if the start of one coil is close to the end of the other. Or coils are wide away meaning that PDIV should be very high compared to other insulation. but a standard deviation of 3% is representative of the accuracy of PDIV evaluation in table 16.

Phase U presents the lowest coil to coil insulation of all phase at This weakness obviously does not appears with AC test because all coils are at the same voltage. This may be explain by the fact that this motor has already be tested numerous time regarding partial discharge level with an inverter.

On the contrary, phase V presents the higher PDIV between coils (above 2kV peak) whereas phase W only shows one weakness between W1 and W4 which will be very useful later on. nominal current of the second one is twice smaller and that of the third one is four times smaller ( ). The maximum torque is often the most dimensioning parameter. In the three proposed winding ).

SiC Inverter Drive Tests

In this case, the four coils of each phase were connected in parallel and the neutral point with a SiC PWM waveform (see table 17 for inverter drive is feeding the three phases at the same time with a PWM waveform. table 18, it seems that each phase has the same PDIV under SiC impulse stress, namely around 1300V peak. In the nominal case when all phases are star-connected, the PDIV could not be determined which is consistent with the fact that the most stressing case is when only one phase is tested. Indeed, when all phases are star-connected, each phases is facing a 2/3 of the voltage at maximum under a PWM waveform.

When comparing results with coils results for phases V and W one could observe that it is likely that phase V presents a turn-to turn PD whereas phase W presents a coil-to coil PD. Impulse tests have PD location. The results in table 19 actually tested when applying SiC impulse on phase W. In fact, the phase test presents a slightly higher PIDV than the W1/W4 coil PDIV. This is an interesting result because it shows that depending on could also appear between coils of the same phase.

Results (Table 20 assumption that phase V is presenting turn-to-turn PD on coil V2. It should be noted that the rise time during impulse coil test are shorter than in the case with 4 coils in parallel. This is due to the fact that the current supplied is lower and the tendency of SiC switch to produce faster dV/dt under low current, the applied electrical stress regarding turn insulation is thus higher. An example of PD acquisition can be seen in .

two test is only 20Vpeak, within the error margin, and no large observed on the turn to turn PDIV. This may be due to the fact that there are actually few turns per coil and a lot magnet wires (strands) in parallel per coil.

Reduction of Electrical Stress

Preventing PD in Turn-to-Turn Insulation

The critical area of insulation for phase V is the turn-to-turn insulation of coil 2 (named blue/yellow). It thus seems interesting to change the coil connection to try to reduce the electrical stress applied to it. The weakest coil V2 was connected in series to coil V4 which does not present turn-to-turn PD. ( ) It can be seen in that the electrical stress applied to the turn-to-turn critical area has been decreased because PD are no longer observed at coil V2 PDIV. This was expected since the DC bus voltage is now divided between the 2 coils in series instead of being increase of the DC bus up to 970VDC ( ), no PD could be ringing or very uneven voltage distribution could be observed along the 2 coils, explaining why no PD could be observed at coil V2 PDIV.

Preventing PD in Coil to Coil Insulation

coil insulation between W1 (black yellow) and W4 (black).To with 2 coils series in parallel. ( ) We can see in that the electrical stress decreased because there is not any PD at the voltage previously observed. As before, voltage is now divided between coils in series which, in turn, reduce the coil to coil stress. DC Bus voltage has been increased up to 970VDC without observing any PD signals.

Discussion about Winding Configuration Influence

Using results from coil-to-coil and impulse tests, the advantage of the same insulation system. What could have been expected from a simple voltage distribution analysis on coils have been tested experimentally on a typical electric stator. This holds true for this segregating coils and coils of few turns which does not prevent Although these conclusions could not be directly extended on any enhanced PDIV performance on other electric motor even without knowing a priori of all insulation in the motor. Obviously, this better PDIV performance come at a cost regarding electromechanical torque and speed.

Summary and Conclusions

This paper illustrates with stator n°2 the robustness of non-intrusive detection method at low pressure under SiC impulse voltage. Both PDIV with a better signal to noise ratio for the latter.

The experimental set-up consisting in a three phases SiC inverter drive and a vacuum chamber is able to reproduce any PWM waveform on a wide range of electric stator both at atmospheric pressure and at low altitude. The 1000V DC bus is able to test electric stator with a large design margin compared with 540VDC aeronautic electric network. The failure of stator n°1 illustrates the destructive potential of non-detected partial discharge This study showed how, without any knowledge of PDIV of each similar insulation system and identical electromechanical performance to evaluate insulation performance

Future Work and Improvements

As a conclusion of this technical report, further investigations will be performed.

  signal reconstructed by CWT (red) at 100 mbar One can see in that the method is able to detect PDs 100 mbar, however in this case the amplitude of the signal is only two timesThe digital denoising method based on continuous wavelet transform is functional. Moreover this method sets all the wavelet transform 21]. This method has real no longer required to remove noise in signals. It is still important to necessary for the signal to be discretized with a large sampling frequency in order to represent fast signals.Another important point is that the amplitude of the signal frequency band overlap corresponding to each level of decomposition removed or attenuated. It has also been reported that PD pulse shape processing based on wavelet transform allows the detection of PDIV under SiC pulse at both atmospheric and reduced pressure.

  Unfortunately, an insulation failure during preliminary tests prevents us from reporting the complete set of results under impulse test. The a 700V DC bus with all phases star connected. Initially, PDIV with these conditions was around 460-500V DC (650-700 Vpeak) bus at 100 mbars. Voltage was increased above PDIV to observe partial discharge evolution electrically and optically to rule out possible false positive signals. The weak point was failure occurred quite quickly (about half-an-hour of equivalent the failure is reported. ( ) PDs eventually led to an electrical arc between phases (short circuit) believed that the electrical arc even hang to the stator frame in aeronautic electromechanical chain which could have cause another failure mechanism or more damage.

  take advantage of having access to its 12 coils and connect them to functional performance, the number of turns per phase ), therefore the base b b represents the speed from which the consequently the current consumed by the motor. configuration1, E2 for configuration2 and E3 for configuration3 configuration1, I2 for configuration2 and I3 for configuration3 for configuration1, T2 for configuration2 and T3 for configuration3 (Analysis valid for the same DC voltage)

••

  Impact of variable dV/dt on each phase PDIV • Impact of temperature and low pressure on compressed endwinding • Detailed study of electromagnetic noise function of dV/dt • Development of an advanced three-phase wavelet numerical processing taking into account dV/dt value

Table 1 .

 1 Inverter drive operating point

Table 2 .

 2 Measurement sensitivity

Table 3 .

 3 Measurement sensitivity with increased capacitance

  The result of these two transforms time-invariant and stationary techniques, while WT screens all components produced by transients, of variable time and nonstationary [15], [17].

	Therefore, with the PD signal being of a non-periodic nature and
	exhibiting very fast transient characteristics, the WT approach seems
	more suitable in this context. The method has proved its ability to
	denoise PD signals, but mainly under AC voltage and in the presence
	of white noise [16], [18], [19],

Table 4 .

 4 Stators main characteristics

Table 5 .

 5 Stator n°1 winding characteristics

Table 6 .

 6 Enamel wire properties (Norm IEC 60317-13)

Table 7 .

 7 Insulation system reference and thickness for stator n°1

Table 8 .

 8 Experimental set-up PDIV

Table 9 .

 9 Phase to ground AC Partial Discharge tests for stator n°1

Table 10 .

 10 Phase to phase AC Partial Discharge tests for stator n°1

SiC Inverter Drive Test

Table 11 .

 11 Phase to ground AC Partial Discharge tests for stator n°2

Table 12 .

 12 SiC inverter drive operating point atmospheric pressure

Table 14 .

 14 Phase to ground AC Partial Discharge tests for stator n°3

Table 15 .

 15 Phase to phase AC Partial Discharge tests for stator n°3

Table 16 .

 16 Phase to phase AC Partial Discharge tests for stator n°3

Coil Configurations and Electric Stator Characteristics

Table 17 .

 17 Phase to ground AC Partial Discharge tests for stator n°3

Table 18 .

 18 Phase to ground AC Partial Discharge tests for stator n°3 atmospheric pressure on stator n°3

Table 19 .

 19 Phase to ground AC Partial Discharge tests for stator n°3

Table 20 .

 20 Phase to ground AC Partial Discharge tests for stator n°3

Acknowledgements

The information presented in this paper is the result of contributions from many team members of the more electrical aircraft -reliability individuals for thoughtful discussions and input, Dr. Bernardo Cougo and Dr. Laurent Albert.

Contact Information

Please direct contact to Dr. Thibaut Billard, research engineer and electromechanical designer at IRT Saint-Exupéry on temporary Thibaut.billard@irt-saintexupery.com


•Proportion of initiation time spent gazing at Region 3 was negatively correlated with processing speed only in correlated weakly (positive) in English only.

Individual Differences in Planning Strategies Among French, German, and English Speakers • Previous studies have found associations between WM (working memory) and planning scope in language production [START_REF] Swets | Individual differences in the scope of speech planning: Evidence from eye movements[END_REF][START_REF] Petrone | Consequences of working memory differences and phrasal length on pause duration and fundamental frequency[END_REF] such that high-span speakers plan in larger increments than low-span speakers. Although these studies each found these effects in different languages, no studies have assessed such individual differences cross-linguistically in the same study.

-In addition, previous research did not distinguish WM effects from processing speed. On one hand, it is MEASURES: Speech initiation time, gaze time (percent initiation time looking at Region 3 of image), number of pauses per utterance (defined as 70 ms or more between vocalizations).

• HYPOTHESES AND PREDICTIONS -Hypothesis 1: Individual differences in WM and speed account for overlapping (shared) variance in speech planning. If so, they will show similar patterns of association with planning, i.e., that higher levels effects from processing speed. On one hand, it is possible that WM and processing speed account for similar variability in planning scope. On the other, they might each account for unique variance in planning tendencies.

BACKGROUND MEASURES AND PREDICTIONS RESULTS CONTINUED

English

-RESEARCH QUESTIONS: Across languages, do planning strategies vary? Do individual differences in planning scope vary across languages?

patterns of association with planning, i.e., that higher levels show larger planning scope.

-Hypothesis 2: Individual differences in WM and processing speed will show effects that are robust to crosslinguistic differences in planning strategies and grammatical structures.

METHOD

French (n = 32), German (n = 31) and English (n = 30) speakers described 3-object arrays with similar-looking (contrast) or different (control) objects in Positions 1 and 3.

Contrast Condition Control Condition

RESULTS

SUMMARY OF RESULTS

• French speakers began their speech more quickly, but paused more often than German and English speakers, suggesting a narrower planning scope for French speakers, and a longer scope of planning for English and German speakers.

-French speakers only modified the initial noun phrase postnominally, unlike English and German speakers, who used both prenominal and postnominal modifiers. • Processing speed significantly accounted for unique variance in planning tendencies, over and above variance due to WM.

• Processing speed accounted for variance in initiation time, pauses and gaze patterns only in French, and did so in a Mean speech onset time varied across languages and condition, but not as a function of WM (SE in parentheses): French speakers initiated speech and paused more often than Speakers of English and German, indicating a more incremental strategy. Speed of processing predicted initiation time and pause frequency only in French, whereas WM showed similar non-significant patterns across all 3 languages for pauses. • Working memory and processing speech may serve different functions under different languages. • Processing speed may be more useful in more incremental languages in which speakers begin speech more quickly and create smaller prosodic chunks, e.g. French, while WM might be more useful in languages that tend to be planned in larger increments.

French German English

Target utterances

CONTRAST: "The four-legged cat moves below the train and the three-legged cat moves above the train." CONTROL: "The cat moves below the train and the wheel moves above the train."

Experimenter served as addressee: Moved objects around in Powerpoint to match descriptions.

VARIABLES

•Sentence type (contrast vs. control)

• Language spoken: German, English, French • Individual differences measures (left as continuous in analyses using linear mixed effects models): WM assessed by reading span variant [START_REF] Swets | The role of working memory in syntactic ambiguity resolution: A psychometric approach[END_REF] Processing speed assessed by letter comparison task [START_REF] Salthouse | The processing-speed theory of adult age differences in cognition[END_REF]. Task: To accurately complete as many "same" or "different" judgments as possible in 30 s. Task executed twice, and average scores were pauses and gaze patterns only in French, and did so in a direction that was opposite that of previously found WM effects: Speakers with high processing speed tended to adopt more efficient incremental strategies in that they required less time to look ahead and initiate speech, while also pausing less frequently.

CONCLUSIONS

Benjamin Swets (

 Swets Grand Valley State University), Caterina Petrone (Laboratoire Parole et Langage), Susanne Fuchs (Leibniz-Zentrum Allgemeine Sprachwissenschaft), & Jelena Krivokapić (University of Michigan & Haskins Laboratories)

  MEAN NUMBER OF PAUSES AS A FUNCTION OF LANGUAGE, WM SPAN, AND SENTENCE TYPEEnglish patterns across all 3 languages for pauses.