Final Report Summary - HT° MOTOR WINDINGS (Reliability assessment of key technologies for high temperature electrical machines)
Executive Summary:
Reliability assessment of key technologies for high temperature electrical machines
HT° Motor windings (JTI-CS-2011-1-SAGE-02-008)
The project objective is to evaluate the reliability of motor winding technologies able to work at higher temperatures than those currently used in the field of aeronautics or other applications.
To achieve this goal, it is necessary to rely on existing European technologies, which offer opportunities for rapid implementations on conventional machines, while opening new opportunities towards higher temperatures with innovative technologies.
Three sets of technologies will be studied:
- The technologies based on enamelled wires associated to polymer impregnation varnishes. These technologies are conventional for small and medium electrical machines that operate at temperatures defined by current standards.
- Technologies based on wires wrapped with glass fibber associated to impregnations made with epoxy resins. These technologies are currently used for large machines.
- The insulation technologies based on ceramics, which offer potential prospects for very high temperatures. However, these technologies will be more difficult to implement in the short term because they require exploratory studies.
The three sets of technologies have been tested for increasing temperatures on representative test vehicles (twisted pairs, and motorettes) involving measurement systems able to get the main parameters of the insulation system (DC and AC leakage currents, partial discharge inception voltage, breakdown voltage,…). The analysis of changes in recorded parameters during the test campaigns were estimate the maximum permissible temperature in the heart of the winding for each technology. These data, combined with constraints of implementation of each technology are defining recommendations for making electrical machines able to operate at high temperatures for each selected technology.
The part relating to the use of ceramic insulations opens the way to a major technological leap. Indeed, with such materials, the maximum operating temperature of electrical machines will no longer be imposed by the winding insulation system but by other parts of the motor (magnetic or mechanical parts), consequently new design methods will have to be developed. It is therefore important to put this new approach against the best we might obtain from other technologies by proposing solutions applicable in the short term.
The three components of the project are estimated of equal importance; the research effort devoted to insulation technologies based on ceramics, which correspond to long-term applications with a major technological leap, is equal to one third of the overall effort.
Finally, the project gives special attention to propose one or more key technologies for high temperature electrical machines for future.
Project Context and Objectives:
Summary description of the project context and the main objectives.
The more-electric aircraft will require higher performance and more compact electric actuators (motors) which will run hotter putting severe limitations on their operation. Today's objective of the aircraft industry is to push up the temperature limit.
The context of this project is Propellers electrical de-icing system: reliability assessment of key technologies for high temperature electrical machines .It is a part of the SAGE2 Demonstration Project aims at designing, manufacturing & testing a Counter-Rotating Open-Rotor Demonstrator. It involves most of the best European Engine & Engine Modules & Subsystems Manufacturers.
The SAGE2 Demonstrator incorporates two counter-rotating propellers, which should be deiced. An electrical deicing system is studied to supply and transfer the power necessary to the deicing. For this system several type of electrical machines are considered. High reliability of such machines in a harsh environment (high temperature) is a key target for the project.
The activities of this topic concern the assessment of reliability of materials and windings for high temperature electrical machines through tests campaigns.
The project objective is to evaluate the reliability of motor winding technologies able to work at high temperatures. To achieve this goal within the time define in the Clean Sky call sheet, it is necessary to rely on existing technologies, which offer opportunities for rapid implementations on conventional machines, while opening new opportunities towards higher temperatures with innovative technologies.
This project analyses existing insulation options and advances the state of the art of high temperature electrical motor windings. All this work is new because the objective is to push up the temperature limit. There are some initial studies on ceramic wire coatings which are very promising technologies.
However, the base material of the windings must be specified and the general mechanical characteristics of the insulated wires, required for various coil geometries, needs to be investigated.
Three sets of technologies have been studied:
- The technologies based on enameled wires associated to polymer impregnation varnishes.
- Technologies based on wires wrapped with glass fibber associated to impregnations made with epoxy resins.
- The insulation technologies based on ceramics, which offer potential prospects for very high temperatures.
The three sets of technologies have been tested for increasing temperatures on representative test vehicles (twisted pairs and motorette) involving measurement systems able to get the main parameters of the insulation system (DC and AC leakage currents, partial discharge inception voltage, breakdown voltage,…). The analysis of changes in recorded parameters during the test campaigns estimated the maximum permissible temperature in the heart of the winding for each technology. These data, combined with constraints of implementation of each technology defined recommendations for the design of electrical machines able to operate at high temperatures for each selected technology.
The part relating to the use of ceramic insulations opens the way to a major technological leap. With such materials, the maximum operating temperature of electrical machines are no longer imposed by the winding insulation system but by other parts (magnetic or mechanical), consequently new design methods will have to be developed. It is therefore important to put this new approach against the best we might obtain from other technologies by proposing solutions applicable in the short term.
This project reinforces European competitiveness in the field windings of electric motors. It also provide some information about the applicability of ceramic windings. Also, the project provide material and process data useful for motor health monitoring.
In the short term, this study will help the design electrical machinery of conventional construction, able of overcoming the current thermal limits. The recommendations for machine design give a range of technological solutions based on the desired maximum temperature. For a temperature range higher than another, some additional constraints appear. This study helps to make sound choices leading to the construction of a reliable machine in the context of severe application for example in the core of an open rotor where the temperature and mechanical stress are important. In the longer term, ceramics insulated windings will produce insulation systems able of withstanding temperatures until the Curie point of magnetic steel. When these coils will be developed, it will be possible to design radically different electrical machines able of operating in hot atmospheres. At room temperature, this solution will increase the temperature difference between hot spots and cold ones that can greatly increase the current densities, therefore the specific power. However this study had showed that this wires aren’t yet optimized for winding. Magnet wire with ceramic insulation needs to be improved particularly on their breakdown voltage and reducing the radius of curvature too restrictive for winding applications
Project Results:
Description of the main Scientific and Technical results
Project CleanSky: JTI-CS-2011-1-SAGE-02-008 [Propellers electrical de-icing system: reliability assessment of key technologies for high temperature electrical machines]
Project acronym: HT° Motor windings
Project full title: Reliability assessment of key technologies for high temperature electrical machines
Project number: 296503
1 Introduction
The project objective is to evaluate the reliability of motor winding technologies able to work at higher temperatures than those currently used in the field of aeronautics or other applications.
To achieve this goal within the time define, it is necessary to rely on existing European technologies, which offer opportunities for rapid implementations on conventional machines, while opening new opportunities towards higher temperatures with innovative technologies.
Three sets of technologies are studied:
- The technologies based on enamelled wires associated to polymer impregnation varnishes. These technologies are conventional for small and medium electrical machines that operate at temperatures defined by current standards.
- Technologies based on wires wrapped with glass fibber associated to impregnations made with epoxy resins. These technologies are currently used for large machines.
- The insulation technologies based on ceramics, which offer potential prospects for very high temperatures. However, these technologies will be more difficult to implement in the short term because they require exploratory studies.
The three sets of technologies are tested for increasing temperatures on representative test vehicles (twisted pairs, transformer coils and motorettes) involving measurement systems able to get the main parameters of the insulation system (DC and AC leakage currents, partial discharge inception voltage, breakdown voltage,…). The analysis of changes in recorded parameters during the test campaigns are estimate the maximum permissible temperature in the heart of the winding for each technology. These data, combined with constraints of implementation of each technology will define recommendations for making electrical machines able to operate at high temperatures for each selected technology.
The part relating to the use of ceramic insulations opens the way to a major technological leap. Indeed, with such materials, the maximum operating temperature of electrical machines will no longer be imposed by the winding insulation system but by other parts of the motor (magnetic or mechanical parts), consequently new design methods will have to be developed. It is therefore important to put this new approach against the best we might obtain from other technologies by proposing solutions applicable in the short term.
The three components of the project are estimated of equal importance; the research effort devoted to insulation technologies based on ceramics, which correspond to long-term applications with a major technological leap, is equal to one third of the overall effort.
Finally, the project gives special attention to propose one or more key technologies for high temperature electrical machines for future.
2 Measurements and tests on material sample and twisted pairs made with enameled wires
2.1 SAMPLES
In order to test the ability of some European Commercial Polyimide (PI) enameled wires, different dielectric measurements have been performed.
Three kinds of wires existing in the market have been selected and tested (Table 1). For investigations, these wires have been prepared in the form of twisted pairs (Figure 1), according to NEMA MW 1000.
Table 1: Main characteristics of enameled wires tested
Manufacturer Diameter (mm) Grade Insulation composition
Vonroll 0,5 2 PI
Essex 0,5 2 PI
Ederfil 0,5 2 PI
Figure 1 : Twisted pair and manufacturing device
2.2 RESULTS ON POLYIMIDE WIRES
The results of PDIV measurements, dielectric strength and lifetime under pulse voltage have been reported respectively in Figures 2, 3, 4 and 5.
Figure 2: Experimental values of PIDV (T=25°C – 50% RH)
Figure 3: Experimental values of dielectric strength (T=25°C)
Figure 4: Experimental values of lifetime under pulse voltage versus temperature (V = +/-1kV, f=10 kHz)
Figure 5: Experimental values of lifetime under pulse versus temperature (V = +/- 0.5kV f=10 kHz)
In comparison with others Polyimide wires, the vonRoll’ wire has shown the best dielectric behavior. In particular, the lifetime of this wire at high temperature (60°C above its thermal class) submitted to partial discharges has been found to be the higher. Note that all the wires have the same insulation composition: i.e.: Polyimide enamel. The differences observed in the dielectric performance are therefore related not only to quality of enamel, but also to its implementation on the copper (i.e.: the wire manufacturing).
2.3 REPORTS ON LIFETIME OF COMMERCIAL NANO-FILLED WIRES
2.3.1 Results on dielectric strength measurements
The results are shown in Figure 6. These results show that there is a lot of dispersion in the different breakdown voltage values. These differences may be attributed to the enameling process which can be different from one manufacturer to another (even if they use the same enamel) and on the dispersion in the enamel thickness. There is no strong difference between standard and nano-filled enameled wires.
Figure 6: Dielectric breakdown voltage (kV rms) of standard (wire 1) and nano-filled enameled wires (wires 2 to 5). (Thermal class: 180°C, diameter: 0,5 mm, grade 2, temperature: 25°C, frequency: 50Hz, dV/dt=1kV/s)
2.3.2 Results on lifetime measurements
Figure 7: Lifetime of standard (wire 1) and nano-filled enameled wires (wires 2 to 5) versus temperature: from the left to the right: -55°C, 25°C and 180°C (frequency: 5 kHz, voltage: +/- 1kV)
These results showed on figure 7 strongly demonstrate the impact of the nano-fillers on the lifetime of these wires when they are submitted to partial discharges (PD). An increase in lifetime between one and two orders of magnitude has been found. The use of these particular wires is consequently highly recommended when motors are fed by inverters, as this kind of feeding leads to strong over-voltages at the motor terminals which can initiate PD.
2.4 TESTS MADE FOR CERAMIC MATERIALS
2.4.1 Introduction
First, the description of the wires used and the method of manufacturing the specimens are presented. Then the measurement system is described. The report ends with a paragraph presenting results and discussions. The results show the variation of various parameters of the insulation of ceramic-coated wires as a function of temperature.
2.4.2 Presentation of ceramic samples
The measurements presented in this report were carried out on specimens made with various ceramic insulated wires of different diameters. The wires used are named in this report W1, W2 and W3. The properties, given by the manufacturer, of these wires are shown in table 2.
Table 2: Properties of used ceramic insulated wires.
Designation Insulation Conductor Temperature Diameter Insulation thickness Voltage rating/Test voltage
W1, W2 Ceramic Nickel Clad Copper +500°C
(peak +1000°C) 0.5 mm
1 mm Not available (U0/U) 100/170 V
W3 PbO, TiO2, SiO2, MgO 27% Nickel Clad Copper +537°C
(peak +815°C) 0.8 mm 7.6 – 15 µm 200 VDC
Usually the dielectric properties of the insulation of the magnet wires are measured on twisted pairs according to [IEC 60851-5] standard. This standard details the different testing procedures and the method which must be used for manufacturing enameled twisted pairs. The ceramic insulation layer of the ceramic wires remains quite fragile. It implies that the ceramic layer cannot be strained beyond a quite low limit. Consequently, the wire bending can be an issue. For W1 and W2, technical data specifies that the ceramic coating does not crack when wound on a form 7x diameter of wire. The manufacturing of standard twisted pairs with ceramic wires may introduce premature failures in insulating coating, thus this type of specimen is not suitable for ceramic wires. For wires with a small radius, the same standard proposes the use of another type of specimen. This method is used to manufacture specimens for testing insulation properties of ceramic wires presented in this report. This method for manufacturing specimens is exempt of mechanical stresses in terms of small bending radius. The method consists in measuring the properties of the insulation of a wire wound around a metallic tube whose diameter may be different.
According to the wire diameter of ceramic wires the measurements were made on specimens wound on cylinders of different diameters:
• 12 mm for W1 0.5 mm,
• 50 mm for W2 1 mm and W3, 0.8 mm.
The specimens are placed in a metallic frame as shown in figure 8. This frame is made with steel bars to ensure that the ends of the specimens surpass it, to make the necessary connections for the dielectric tests. To insulate the specimens from the metallic frame, two electrical insulators are used: Neffalite11 (this insulator resists a temperature up to 1100°C) and fiber glass cloth. Due to small dimensions of oven, ten specimens have been made and placed in the metal frame. The load shown in figures 8 and 9, applies the required force on the cylinder to ensure an intimate contact between the specimen and the cylinder; the fiberglass cloth on the supporting bar gives the necessary softness. These allow measuring the dielectric parameters of the insulating layer of the full length of the wire wound around the cylinder.
Figure 8: Specimens wound on cylinder and placed on a metallic frame
2.4.3 Description of the measurement system
The capacitances, resistances and losses (tan δ) measurement system includes the impedance analyzer Agilent E4980A and a connecting cables system based on a 4 points method in order to eliminate their influences on the impedance measurement. This cable system transfers the measurement point near to the specimen which will allow us to measure the dielectric parameters of the specimens only. The measurements must be made at a given temperature, and therefore, when the temperature become too high, it is impossible to manipulate the measuring cables and connect it them to each specimens. Two high temperature (up to 600°C) cables (each cable has 12 conductors) were placed and fixed in the oven and were connected to each specimen (Figure9).
The accuracy of the capacitances measurement is given by the abacus taken in the technical data of the measuring instrument. We note that to measure the capacitance between 10 pF and 200 pF, the range of frequencies providing a measurement accuracy of 0.3% is between about 1 kHz and 100 kHz. The frequency of 10 kHz has been chosen for these measurements.
Concerning the parallel resistance of the insulation this impedance analyzer is able to measure values up to 100 MΩ with an accuracy of 10%. When an accuracy of 1% is required values are limited to 10 MΩ. For values higher than 100 MΩ measurement accuracy of the impedance analyzer goes outside the area of 10% and therefore it is only capable to give an order of magnitude of this parameter.
Figure 9: Photograph of oven and connection cables to specimens.
3 Measurements and tests made on motorettes
3.1 MEASUREMENT AND TEST PROTOCOL
3.1.1 Aim of the study
In order to evaluate the ability of different primary and secondary insulations resins to be used in windings of HT° (high temperature) rotating machines, different tests have been performed. For that purpose, 12 motorettes have been produced with the same shape and size (see Fig. 10).
Figure 10: View of one motorette under study
3.1.1 Tested solutions
Solution number Wire Ground insulation Impregnation varnish
0 Diameter: 0.5mm insulation polyimide Fiberglass textile impregnated with polyimide, thickness : 200µm polyimide
1 Diameter 0.8mm
nanocharged, class 180°C Mica fiberglass composite; thickness: 90μm for tooth + mica sheets 250μm on the lateral plates Silicon
2 Diameter 0.5mm ceramic coated Silicon impregnated fiber glass, thickness: 200µm Silicon
3 Diameter 0.5mm ceramic coated Mica fiberglass composite; thickness: 90μm for tooth + mica sheets 250μm on the lateral plates Ceramic cement
Solutions 0 correspond to a standard organic insulation solution, which is used as a reference.
Solution 1 is performed to test a nanocharged wire associated to a silicon impregnation.
Solution 2 associates a ceramic-coated wire and a silicon impregnation.
Solutions 0, 1, and 2 are limited in temperature because of the impregnation, they are tested up to 300°C for increasing and decreasing temperatures. With this procedure, it is possible to observe irreversible changes in insulation layers. The graphs are presented between 100°C and 300°C in order to eliminate variations due to difference in the ambient humidity.
Solution 3 is fully inorganic; it has been designed for very high temperatures, beyond the capabilities of polymers and silicones. Resistance measurements are made between 300°C and 500°C.
For each combination, three motorettes have been prepared in order to take into account the dispersion to find the characteristic values of both PDIV and dielectric strength.
For each motorette, the PDIV (Partial Discharge Inception Voltage) has been estimated between coils and between coils and ground from 20°C to 300°C, during both increasing and decreasing temperatures. After that, the dielectric strength has been measured at 300°C.
3.1.2 Experimental results
The experimental results of PDIV are presented in Fig. 11 to 14.
Figure 11: Evolution of PIDV between phases versus temperature from 20°C to 300°C.
Figure 12: Evolution of PIDV between phases versus temperature from 300°C to 20°C.
Whatever the combination of insulations, the PDIV decreases versus temperature, which is a classical result. There is no strong difference between the measurements made with an increasing and a decreasing temperature. We can conclude that the insulation between phases has not been deteriorated or has not moved during these heat cycles.
The solutions “no ceramic” (0 and 1) show higher PDIV values than the solutions “ceramic” (2 and 3). This can be explained by the fact that:
- Ceramic materials exhibit higher permittivity than polymer ones. Consequently, if we consider that the thickness of secondary insulation is the same in both cases, the electric field is reinforced in both embedded and vented voids, leading to lower PDIV’s.
- The primary insulation of ceramic wires contains much more voids than polymer wires ones, leading to lower PDIV’s.
The PDIV observed for ceramic solutions is relatively low, typically lower than the minimum of Paschen’s curves at high temperature. We can conclude that in ceramic wires, partial discharges may also occur at the surface of imbedded voids (surface discharges), leading to PDIV lower than those obtained with partial discharges in gaz.
Figure 13: Evolution of PIDV between phases and ground versus temperature from 20°C to 300°C. Weibull’s values with 90% confidence intervals.
Figure 14: Evolution of PIDV between phases and ground versus temperature from 300°C to 20°C. Weibull’s values with 90% confidence intervals
The solution (3) ‘all ceramic’ exhibits the lower PDIV. The assumptions proposed for inter-phases PDIV may also be used here (higher permittivity).
Solutions (1) and (2), impregnated with the same secondary insulation (silicon), have approximately the same PDIV. This can be attributed to the good penetration of silicon between wires and ground, leading to a lower impact of the higher permittivity of the primary insulation of solution (2).
The PDIV at room temperature of solution (0) has been found to be lower when measured for the second time. In this case, the ground wall insulation (polyimide reinforced with glass fiber) may have been affected by the thermal cycle (either deterioration or movement).
The experimental results of dielectric strength are presented in Fig. 15.
Measurement have been performed at 300°C.
Figure 15: Dielectric strength of motorettes at 300°C
The solutions (0) and (1) have approximately the same breakdown values. Moreover, they exhibit the higher breakdown strength values (for both inter-phases and ground wall values). This can be attributed to the fact that the dielectric strength of polymer wires is higher than those of ceramic wires.
The solution (3) has not been broken in order to keep it intact for experiments to be done at higher temperatures (i.e. 500°C).
These experiments need to be completed by lifetime measurements. Indeed, there is no strong correlation between the dielectric strength value and the lifetime. In other words, there is no evidence that both solutions (0) and (1) will present a higher lifetime at high temperature and under nominal voltage than ceramic solutions.
3.2 TEST MADE FOR CERAMIC MATERIALS
3.2.1 Introduction
This section is divided in 2 parts. The first concern the results of the measurements made until 300°C in the range of the study. The second one concern measurement made up to 500°C in order to give a first idea of the real capabilities of ceramic insulation.
3.2.2 Results up to 300°C
Figure 16 presents the variations of the parallel capacitance with the temperature. The results are plotted relative to the capacitance value at 20°C.
Figure 16: Parallel Capacitance up to 300°C on full ceramic solution
The measurements of the parallel capacitance Cp show few variations with the temperature. For the 4 testing frequency the capacitance decrease at most 20%.
The variation of resistance Rp is given in figure 17. The plotted results are always given in relation with the value at 20°C. The resistance is very difficult to measure at 50 and 100Hz due to its very high value, therefore, only the results to 1kHz and 10kHz are presented.
Figure 17: Resistance Parallel Rp up to 300°C on full ceramic solution
We can see on this figure a clear increase of the resistance until 100°C due to the evaporation of the water contained in the ceramic porosity. Then the resistance returned to values closed to initial one.
The figure 18 presents the PDIV values during a cycle during a thermal cycle of 20°C thus amount to 300°C and back to 20°C for the turn-to-turn insulation and the turn to ground insulation. The measurement is made at 50Hz.
Figure 18: PDIV up to 300°C on full ceramic solution
First the PDIV decrease slowly with the temperature increasing, as is typically seen. In addition we can see clearly that the variations are the same during the warming and the cooling.
3.2.3 Results up to 500°C
Given the capability of the ceramic material, we have made a series of measurement until 500°C to see the behaviour of the insulation at this kind of temperature. The measurements are made at 1 kHz with a thermal cycle similar to that used for PDIV measurements: from 300°C to 500°C and back. The parallel capacitance variations give in figure 19 still very stable during warming and cooling.
Figure 19: Parallel Capacitance Cp
The parallel resistance have the same dynamics as the lowest temperature and continues to fall sharply as we can see in figure 20.
Figure 20: Parallel resistance up to 500°C on full ceramic solution
Finally the PDIV tends to stabilize beyond 400°C (Figure 21). The ceramic material seems to be slightly better with the increase of the temperature. It is possible that the materials need heat treatment after the mechanical process to have its maximum potential.
Figure 21: PDIV up to 500°C on full ceramic solution
4 Conclusion
Different solutions (solutions (0) to (3)) different vehicles test as twisted pairs and motorettes have been tested to measure their PDIV and dielectric strength.
The fully organic insulation is a mature technology, its thermal class defined by standards is 240°C. The definition of the thermal class corresponds to a life span of 20,000h with a single thermal constraint. In other words, after an aging test at a constant temperature of 240°C during 20,000 hours (2 years and 4 months), half the standard samples are destroyed by the final test which imposes 1kV/50Hz during 1 minute. The definition of the thermal class is rather theoretical and corresponds to a single thermal constraints, it is far from the real life of an operating electrical machine. The tests made on this mature technology show that, over the thermal class, at 300°C (25% over the thermal class) irreversible degradations are noted for short times. Consequently it is not possible to work over the thermal class of a fully organic insulation system except for short live spans corresponding to very specific applications.
Compared to the fully organic mature winding technology (solution 0), the high temperature tested solutions (solutions 1, 2 and 3) have a much lower insulation resistance. However, the equivalent resistance at 10 kHz is over 1 M, which is an acceptable value for a low voltage electrical machine.
For solution 1, achieved tests corresponds to the ground insulation that means a mica / fiberglass with a silicon impregnation. The measurements made with increasing and decreasing temperatures up to 300°C show that there is no significant irreversible degradation of the ground insulation. Consequently, the silicon varnish is a good candidate for further explorations toward temperature slightly higher than the classical organic solutions. For the solution 1 the thermal limit will be the turn-to-turn organic layer, which is defined by the thermal class of the wire.
Solution 2 avoids this problem using a ceramic-coated wire combined to a silicon impregnation. This fully inorganic insulation has mechanical performances close to the organic one but it shows higher resistances for decreasing temperatures. More extensive investigation should be made on these phenomena; the ceramic layer is porous and interface phenomena between ceramics and silicon require deeper studies beyond the scope of the exploratory approach of the project. However the silicon varnishes have thermal limits and it will be difficult to work over 300°C for long life span.
Solution 3 is a fully ceramic solution that can resist to extreme temperatures but the coil has mechanical characteristics very different that organic ones, it hard and brittle and the insertion of such coils in a motor will be more difficult. These preliminary tests shows that it will be possible to work up to 500°C with this technology when mechanical problems will be solved.
As expected, in solutions using ceramic materials, the PDIV may be reduced due to their permittivity. Moreover, due to the presence of micro-voids in the primary insulation of ceramic wires, surface partial discharges occurring in these voids lead to PDIV down to 150V at high temperatures (i.e.: 300°C).
Solutions using ceramic wires exhibit lower values of breakdown strength at high temperature (i.e.: 300°C). Nevertheless, lifetime measurements have to be performed to really have an idea of the gain achieved when using ceramic materials instead of polymer ones.
The measurements made on the ceramic material have shown that the insulation performance are slightly poorer that for organic materials. Nevertheless, the material presents a good stability with the temperature. The capacitance is stable, the PDIV decrease slowly. A special attention is needed with the parallel resistance, which seems to decrease strongly.
That kind of insulation material cannot replace the polymer in a range of temperature inferior to 300°C. Indeed, the difficulty to process the wire and the limited capabilities make it impossible the used of the ceramic wire in industrial applications. Therefore, the measurements made prove that the ceramic materials are better for high temperature. The PDIV and the parallel resistance seem to stabilize over 400°C. The machines running in that range of temperature do not exist yet. The ceramic insulation provides the ability to build this kind of machine in the future.
Potential Impact:
1 Expected impacts
In the short term, this study will help the design electrical machinery of conventional construction, able of overcoming the current thermal limits. The recommendations for machine design will give a range of technological solutions based on the desired maximum temperature. It is expected that for a temperature range higher than another, some additional constraints will appear. This study will help to make sound choices leading to the construction of a reliable machine in the context of severe application. In the longer term, ceramics insulated windings will produce insulation systems able of withstanding temperatures until the Curie point of magnetic steel. When these coils will be developed, it will be possible to design radically different electrical machines able of operating in hot atmospheres. At room temperature, this solution will increase the temperature difference between hot spots and cold ones that can greatly increase the current densities, therefore the specific power. With high temperature ceramic coils, many research opportunities will be opened for the specialists in machine design; they will have to solve the mechanical problems, which will arise for higher temperatures. This technological leap will lead to patents in the field of machinery design.
2 Dissemination and exploitation of project results
The major results have been presented in international conferences of the domain of electrical machine insulation
Posters presentation have been presented:
1) IEEE International Conference on Solid Dielectrics (2013 ICSD), July 2013, BOLOGNA, Italia,
D. COZONAC, V. MIHAILA, G. VELU, S. DUCHESNE, J.Ph. LECOINTE, D.ROGER. "Opportunities for winding wires compatible with high temperatures in the range of 500 °C".
2) French conference Symposium Electrical Energy (SGE), july 2014 CACHAN, France
V. MIHAILA, M.Q. NGUYEN, S. DUCHESNE, G. VELU, D. ROGER, D. MALEC, J.P. CAMBRONNE.
"Solutions d'isolation électrique pour les machines fonctionnant à Hautes Températures".
An Oral presentation have been presented:
A wider public conference organized by a french college and an industrial and commercial Organization, Avril 2013 (where 300 peoples have assisted)
Conference on vehicles of the futurs, BOULOGNE/MER France
G. VELU.
"l’Avion du futur : l’avion plus électrique "
List of Websites:
Contact :
1) prof. Gabriel VELU
Université d'ARTOIS
LSEE
Technoparc FUTURA
62400 BETHUNE FRANCE
gabriel.velu@univ-artois.fr
2) Prof. David MALEC
Université Paul Sabatier Toulouse III
118 rue de Narbonne
31062 TOULOUSE CEDEX 9
david.malec@laplace.univ-tlse.fr
3) Dr. Philippe Molinié
SUPELEC
Plateau du Moulon
3 rue Joliot-Curie
91192 GIF-SUR-YVETTE CEDEX
Philippe.Molinie@supelec.fr
Reliability assessment of key technologies for high temperature electrical machines
HT° Motor windings (JTI-CS-2011-1-SAGE-02-008)
The project objective is to evaluate the reliability of motor winding technologies able to work at higher temperatures than those currently used in the field of aeronautics or other applications.
To achieve this goal, it is necessary to rely on existing European technologies, which offer opportunities for rapid implementations on conventional machines, while opening new opportunities towards higher temperatures with innovative technologies.
Three sets of technologies will be studied:
- The technologies based on enamelled wires associated to polymer impregnation varnishes. These technologies are conventional for small and medium electrical machines that operate at temperatures defined by current standards.
- Technologies based on wires wrapped with glass fibber associated to impregnations made with epoxy resins. These technologies are currently used for large machines.
- The insulation technologies based on ceramics, which offer potential prospects for very high temperatures. However, these technologies will be more difficult to implement in the short term because they require exploratory studies.
The three sets of technologies have been tested for increasing temperatures on representative test vehicles (twisted pairs, and motorettes) involving measurement systems able to get the main parameters of the insulation system (DC and AC leakage currents, partial discharge inception voltage, breakdown voltage,…). The analysis of changes in recorded parameters during the test campaigns were estimate the maximum permissible temperature in the heart of the winding for each technology. These data, combined with constraints of implementation of each technology are defining recommendations for making electrical machines able to operate at high temperatures for each selected technology.
The part relating to the use of ceramic insulations opens the way to a major technological leap. Indeed, with such materials, the maximum operating temperature of electrical machines will no longer be imposed by the winding insulation system but by other parts of the motor (magnetic or mechanical parts), consequently new design methods will have to be developed. It is therefore important to put this new approach against the best we might obtain from other technologies by proposing solutions applicable in the short term.
The three components of the project are estimated of equal importance; the research effort devoted to insulation technologies based on ceramics, which correspond to long-term applications with a major technological leap, is equal to one third of the overall effort.
Finally, the project gives special attention to propose one or more key technologies for high temperature electrical machines for future.
Project Context and Objectives:
Summary description of the project context and the main objectives.
The more-electric aircraft will require higher performance and more compact electric actuators (motors) which will run hotter putting severe limitations on their operation. Today's objective of the aircraft industry is to push up the temperature limit.
The context of this project is Propellers electrical de-icing system: reliability assessment of key technologies for high temperature electrical machines .It is a part of the SAGE2 Demonstration Project aims at designing, manufacturing & testing a Counter-Rotating Open-Rotor Demonstrator. It involves most of the best European Engine & Engine Modules & Subsystems Manufacturers.
The SAGE2 Demonstrator incorporates two counter-rotating propellers, which should be deiced. An electrical deicing system is studied to supply and transfer the power necessary to the deicing. For this system several type of electrical machines are considered. High reliability of such machines in a harsh environment (high temperature) is a key target for the project.
The activities of this topic concern the assessment of reliability of materials and windings for high temperature electrical machines through tests campaigns.
The project objective is to evaluate the reliability of motor winding technologies able to work at high temperatures. To achieve this goal within the time define in the Clean Sky call sheet, it is necessary to rely on existing technologies, which offer opportunities for rapid implementations on conventional machines, while opening new opportunities towards higher temperatures with innovative technologies.
This project analyses existing insulation options and advances the state of the art of high temperature electrical motor windings. All this work is new because the objective is to push up the temperature limit. There are some initial studies on ceramic wire coatings which are very promising technologies.
However, the base material of the windings must be specified and the general mechanical characteristics of the insulated wires, required for various coil geometries, needs to be investigated.
Three sets of technologies have been studied:
- The technologies based on enameled wires associated to polymer impregnation varnishes.
- Technologies based on wires wrapped with glass fibber associated to impregnations made with epoxy resins.
- The insulation technologies based on ceramics, which offer potential prospects for very high temperatures.
The three sets of technologies have been tested for increasing temperatures on representative test vehicles (twisted pairs and motorette) involving measurement systems able to get the main parameters of the insulation system (DC and AC leakage currents, partial discharge inception voltage, breakdown voltage,…). The analysis of changes in recorded parameters during the test campaigns estimated the maximum permissible temperature in the heart of the winding for each technology. These data, combined with constraints of implementation of each technology defined recommendations for the design of electrical machines able to operate at high temperatures for each selected technology.
The part relating to the use of ceramic insulations opens the way to a major technological leap. With such materials, the maximum operating temperature of electrical machines are no longer imposed by the winding insulation system but by other parts (magnetic or mechanical), consequently new design methods will have to be developed. It is therefore important to put this new approach against the best we might obtain from other technologies by proposing solutions applicable in the short term.
This project reinforces European competitiveness in the field windings of electric motors. It also provide some information about the applicability of ceramic windings. Also, the project provide material and process data useful for motor health monitoring.
In the short term, this study will help the design electrical machinery of conventional construction, able of overcoming the current thermal limits. The recommendations for machine design give a range of technological solutions based on the desired maximum temperature. For a temperature range higher than another, some additional constraints appear. This study helps to make sound choices leading to the construction of a reliable machine in the context of severe application for example in the core of an open rotor where the temperature and mechanical stress are important. In the longer term, ceramics insulated windings will produce insulation systems able of withstanding temperatures until the Curie point of magnetic steel. When these coils will be developed, it will be possible to design radically different electrical machines able of operating in hot atmospheres. At room temperature, this solution will increase the temperature difference between hot spots and cold ones that can greatly increase the current densities, therefore the specific power. However this study had showed that this wires aren’t yet optimized for winding. Magnet wire with ceramic insulation needs to be improved particularly on their breakdown voltage and reducing the radius of curvature too restrictive for winding applications
Project Results:
Description of the main Scientific and Technical results
Project CleanSky: JTI-CS-2011-1-SAGE-02-008 [Propellers electrical de-icing system: reliability assessment of key technologies for high temperature electrical machines]
Project acronym: HT° Motor windings
Project full title: Reliability assessment of key technologies for high temperature electrical machines
Project number: 296503
1 Introduction
The project objective is to evaluate the reliability of motor winding technologies able to work at higher temperatures than those currently used in the field of aeronautics or other applications.
To achieve this goal within the time define, it is necessary to rely on existing European technologies, which offer opportunities for rapid implementations on conventional machines, while opening new opportunities towards higher temperatures with innovative technologies.
Three sets of technologies are studied:
- The technologies based on enamelled wires associated to polymer impregnation varnishes. These technologies are conventional for small and medium electrical machines that operate at temperatures defined by current standards.
- Technologies based on wires wrapped with glass fibber associated to impregnations made with epoxy resins. These technologies are currently used for large machines.
- The insulation technologies based on ceramics, which offer potential prospects for very high temperatures. However, these technologies will be more difficult to implement in the short term because they require exploratory studies.
The three sets of technologies are tested for increasing temperatures on representative test vehicles (twisted pairs, transformer coils and motorettes) involving measurement systems able to get the main parameters of the insulation system (DC and AC leakage currents, partial discharge inception voltage, breakdown voltage,…). The analysis of changes in recorded parameters during the test campaigns are estimate the maximum permissible temperature in the heart of the winding for each technology. These data, combined with constraints of implementation of each technology will define recommendations for making electrical machines able to operate at high temperatures for each selected technology.
The part relating to the use of ceramic insulations opens the way to a major technological leap. Indeed, with such materials, the maximum operating temperature of electrical machines will no longer be imposed by the winding insulation system but by other parts of the motor (magnetic or mechanical parts), consequently new design methods will have to be developed. It is therefore important to put this new approach against the best we might obtain from other technologies by proposing solutions applicable in the short term.
The three components of the project are estimated of equal importance; the research effort devoted to insulation technologies based on ceramics, which correspond to long-term applications with a major technological leap, is equal to one third of the overall effort.
Finally, the project gives special attention to propose one or more key technologies for high temperature electrical machines for future.
2 Measurements and tests on material sample and twisted pairs made with enameled wires
2.1 SAMPLES
In order to test the ability of some European Commercial Polyimide (PI) enameled wires, different dielectric measurements have been performed.
Three kinds of wires existing in the market have been selected and tested (Table 1). For investigations, these wires have been prepared in the form of twisted pairs (Figure 1), according to NEMA MW 1000.
Table 1: Main characteristics of enameled wires tested
Manufacturer Diameter (mm) Grade Insulation composition
Vonroll 0,5 2 PI
Essex 0,5 2 PI
Ederfil 0,5 2 PI
Figure 1 : Twisted pair and manufacturing device
2.2 RESULTS ON POLYIMIDE WIRES
The results of PDIV measurements, dielectric strength and lifetime under pulse voltage have been reported respectively in Figures 2, 3, 4 and 5.
Figure 2: Experimental values of PIDV (T=25°C – 50% RH)
Figure 3: Experimental values of dielectric strength (T=25°C)
Figure 4: Experimental values of lifetime under pulse voltage versus temperature (V = +/-1kV, f=10 kHz)
Figure 5: Experimental values of lifetime under pulse versus temperature (V = +/- 0.5kV f=10 kHz)
In comparison with others Polyimide wires, the vonRoll’ wire has shown the best dielectric behavior. In particular, the lifetime of this wire at high temperature (60°C above its thermal class) submitted to partial discharges has been found to be the higher. Note that all the wires have the same insulation composition: i.e.: Polyimide enamel. The differences observed in the dielectric performance are therefore related not only to quality of enamel, but also to its implementation on the copper (i.e.: the wire manufacturing).
2.3 REPORTS ON LIFETIME OF COMMERCIAL NANO-FILLED WIRES
2.3.1 Results on dielectric strength measurements
The results are shown in Figure 6. These results show that there is a lot of dispersion in the different breakdown voltage values. These differences may be attributed to the enameling process which can be different from one manufacturer to another (even if they use the same enamel) and on the dispersion in the enamel thickness. There is no strong difference between standard and nano-filled enameled wires.
Figure 6: Dielectric breakdown voltage (kV rms) of standard (wire 1) and nano-filled enameled wires (wires 2 to 5). (Thermal class: 180°C, diameter: 0,5 mm, grade 2, temperature: 25°C, frequency: 50Hz, dV/dt=1kV/s)
2.3.2 Results on lifetime measurements
Figure 7: Lifetime of standard (wire 1) and nano-filled enameled wires (wires 2 to 5) versus temperature: from the left to the right: -55°C, 25°C and 180°C (frequency: 5 kHz, voltage: +/- 1kV)
These results showed on figure 7 strongly demonstrate the impact of the nano-fillers on the lifetime of these wires when they are submitted to partial discharges (PD). An increase in lifetime between one and two orders of magnitude has been found. The use of these particular wires is consequently highly recommended when motors are fed by inverters, as this kind of feeding leads to strong over-voltages at the motor terminals which can initiate PD.
2.4 TESTS MADE FOR CERAMIC MATERIALS
2.4.1 Introduction
First, the description of the wires used and the method of manufacturing the specimens are presented. Then the measurement system is described. The report ends with a paragraph presenting results and discussions. The results show the variation of various parameters of the insulation of ceramic-coated wires as a function of temperature.
2.4.2 Presentation of ceramic samples
The measurements presented in this report were carried out on specimens made with various ceramic insulated wires of different diameters. The wires used are named in this report W1, W2 and W3. The properties, given by the manufacturer, of these wires are shown in table 2.
Table 2: Properties of used ceramic insulated wires.
Designation Insulation Conductor Temperature Diameter Insulation thickness Voltage rating/Test voltage
W1, W2 Ceramic Nickel Clad Copper +500°C
(peak +1000°C) 0.5 mm
1 mm Not available (U0/U) 100/170 V
W3 PbO, TiO2, SiO2, MgO 27% Nickel Clad Copper +537°C
(peak +815°C) 0.8 mm 7.6 – 15 µm 200 VDC
Usually the dielectric properties of the insulation of the magnet wires are measured on twisted pairs according to [IEC 60851-5] standard. This standard details the different testing procedures and the method which must be used for manufacturing enameled twisted pairs. The ceramic insulation layer of the ceramic wires remains quite fragile. It implies that the ceramic layer cannot be strained beyond a quite low limit. Consequently, the wire bending can be an issue. For W1 and W2, technical data specifies that the ceramic coating does not crack when wound on a form 7x diameter of wire. The manufacturing of standard twisted pairs with ceramic wires may introduce premature failures in insulating coating, thus this type of specimen is not suitable for ceramic wires. For wires with a small radius, the same standard proposes the use of another type of specimen. This method is used to manufacture specimens for testing insulation properties of ceramic wires presented in this report. This method for manufacturing specimens is exempt of mechanical stresses in terms of small bending radius. The method consists in measuring the properties of the insulation of a wire wound around a metallic tube whose diameter may be different.
According to the wire diameter of ceramic wires the measurements were made on specimens wound on cylinders of different diameters:
• 12 mm for W1 0.5 mm,
• 50 mm for W2 1 mm and W3, 0.8 mm.
The specimens are placed in a metallic frame as shown in figure 8. This frame is made with steel bars to ensure that the ends of the specimens surpass it, to make the necessary connections for the dielectric tests. To insulate the specimens from the metallic frame, two electrical insulators are used: Neffalite11 (this insulator resists a temperature up to 1100°C) and fiber glass cloth. Due to small dimensions of oven, ten specimens have been made and placed in the metal frame. The load shown in figures 8 and 9, applies the required force on the cylinder to ensure an intimate contact between the specimen and the cylinder; the fiberglass cloth on the supporting bar gives the necessary softness. These allow measuring the dielectric parameters of the insulating layer of the full length of the wire wound around the cylinder.
Figure 8: Specimens wound on cylinder and placed on a metallic frame
2.4.3 Description of the measurement system
The capacitances, resistances and losses (tan δ) measurement system includes the impedance analyzer Agilent E4980A and a connecting cables system based on a 4 points method in order to eliminate their influences on the impedance measurement. This cable system transfers the measurement point near to the specimen which will allow us to measure the dielectric parameters of the specimens only. The measurements must be made at a given temperature, and therefore, when the temperature become too high, it is impossible to manipulate the measuring cables and connect it them to each specimens. Two high temperature (up to 600°C) cables (each cable has 12 conductors) were placed and fixed in the oven and were connected to each specimen (Figure9).
The accuracy of the capacitances measurement is given by the abacus taken in the technical data of the measuring instrument. We note that to measure the capacitance between 10 pF and 200 pF, the range of frequencies providing a measurement accuracy of 0.3% is between about 1 kHz and 100 kHz. The frequency of 10 kHz has been chosen for these measurements.
Concerning the parallel resistance of the insulation this impedance analyzer is able to measure values up to 100 MΩ with an accuracy of 10%. When an accuracy of 1% is required values are limited to 10 MΩ. For values higher than 100 MΩ measurement accuracy of the impedance analyzer goes outside the area of 10% and therefore it is only capable to give an order of magnitude of this parameter.
Figure 9: Photograph of oven and connection cables to specimens.
3 Measurements and tests made on motorettes
3.1 MEASUREMENT AND TEST PROTOCOL
3.1.1 Aim of the study
In order to evaluate the ability of different primary and secondary insulations resins to be used in windings of HT° (high temperature) rotating machines, different tests have been performed. For that purpose, 12 motorettes have been produced with the same shape and size (see Fig. 10).
Figure 10: View of one motorette under study
3.1.1 Tested solutions
Solution number Wire Ground insulation Impregnation varnish
0 Diameter: 0.5mm insulation polyimide Fiberglass textile impregnated with polyimide, thickness : 200µm polyimide
1 Diameter 0.8mm
nanocharged, class 180°C Mica fiberglass composite; thickness: 90μm for tooth + mica sheets 250μm on the lateral plates Silicon
2 Diameter 0.5mm ceramic coated Silicon impregnated fiber glass, thickness: 200µm Silicon
3 Diameter 0.5mm ceramic coated Mica fiberglass composite; thickness: 90μm for tooth + mica sheets 250μm on the lateral plates Ceramic cement
Solutions 0 correspond to a standard organic insulation solution, which is used as a reference.
Solution 1 is performed to test a nanocharged wire associated to a silicon impregnation.
Solution 2 associates a ceramic-coated wire and a silicon impregnation.
Solutions 0, 1, and 2 are limited in temperature because of the impregnation, they are tested up to 300°C for increasing and decreasing temperatures. With this procedure, it is possible to observe irreversible changes in insulation layers. The graphs are presented between 100°C and 300°C in order to eliminate variations due to difference in the ambient humidity.
Solution 3 is fully inorganic; it has been designed for very high temperatures, beyond the capabilities of polymers and silicones. Resistance measurements are made between 300°C and 500°C.
For each combination, three motorettes have been prepared in order to take into account the dispersion to find the characteristic values of both PDIV and dielectric strength.
For each motorette, the PDIV (Partial Discharge Inception Voltage) has been estimated between coils and between coils and ground from 20°C to 300°C, during both increasing and decreasing temperatures. After that, the dielectric strength has been measured at 300°C.
3.1.2 Experimental results
The experimental results of PDIV are presented in Fig. 11 to 14.
Figure 11: Evolution of PIDV between phases versus temperature from 20°C to 300°C.
Figure 12: Evolution of PIDV between phases versus temperature from 300°C to 20°C.
Whatever the combination of insulations, the PDIV decreases versus temperature, which is a classical result. There is no strong difference between the measurements made with an increasing and a decreasing temperature. We can conclude that the insulation between phases has not been deteriorated or has not moved during these heat cycles.
The solutions “no ceramic” (0 and 1) show higher PDIV values than the solutions “ceramic” (2 and 3). This can be explained by the fact that:
- Ceramic materials exhibit higher permittivity than polymer ones. Consequently, if we consider that the thickness of secondary insulation is the same in both cases, the electric field is reinforced in both embedded and vented voids, leading to lower PDIV’s.
- The primary insulation of ceramic wires contains much more voids than polymer wires ones, leading to lower PDIV’s.
The PDIV observed for ceramic solutions is relatively low, typically lower than the minimum of Paschen’s curves at high temperature. We can conclude that in ceramic wires, partial discharges may also occur at the surface of imbedded voids (surface discharges), leading to PDIV lower than those obtained with partial discharges in gaz.
Figure 13: Evolution of PIDV between phases and ground versus temperature from 20°C to 300°C. Weibull’s values with 90% confidence intervals.
Figure 14: Evolution of PIDV between phases and ground versus temperature from 300°C to 20°C. Weibull’s values with 90% confidence intervals
The solution (3) ‘all ceramic’ exhibits the lower PDIV. The assumptions proposed for inter-phases PDIV may also be used here (higher permittivity).
Solutions (1) and (2), impregnated with the same secondary insulation (silicon), have approximately the same PDIV. This can be attributed to the good penetration of silicon between wires and ground, leading to a lower impact of the higher permittivity of the primary insulation of solution (2).
The PDIV at room temperature of solution (0) has been found to be lower when measured for the second time. In this case, the ground wall insulation (polyimide reinforced with glass fiber) may have been affected by the thermal cycle (either deterioration or movement).
The experimental results of dielectric strength are presented in Fig. 15.
Measurement have been performed at 300°C.
Figure 15: Dielectric strength of motorettes at 300°C
The solutions (0) and (1) have approximately the same breakdown values. Moreover, they exhibit the higher breakdown strength values (for both inter-phases and ground wall values). This can be attributed to the fact that the dielectric strength of polymer wires is higher than those of ceramic wires.
The solution (3) has not been broken in order to keep it intact for experiments to be done at higher temperatures (i.e. 500°C).
These experiments need to be completed by lifetime measurements. Indeed, there is no strong correlation between the dielectric strength value and the lifetime. In other words, there is no evidence that both solutions (0) and (1) will present a higher lifetime at high temperature and under nominal voltage than ceramic solutions.
3.2 TEST MADE FOR CERAMIC MATERIALS
3.2.1 Introduction
This section is divided in 2 parts. The first concern the results of the measurements made until 300°C in the range of the study. The second one concern measurement made up to 500°C in order to give a first idea of the real capabilities of ceramic insulation.
3.2.2 Results up to 300°C
Figure 16 presents the variations of the parallel capacitance with the temperature. The results are plotted relative to the capacitance value at 20°C.
Figure 16: Parallel Capacitance up to 300°C on full ceramic solution
The measurements of the parallel capacitance Cp show few variations with the temperature. For the 4 testing frequency the capacitance decrease at most 20%.
The variation of resistance Rp is given in figure 17. The plotted results are always given in relation with the value at 20°C. The resistance is very difficult to measure at 50 and 100Hz due to its very high value, therefore, only the results to 1kHz and 10kHz are presented.
Figure 17: Resistance Parallel Rp up to 300°C on full ceramic solution
We can see on this figure a clear increase of the resistance until 100°C due to the evaporation of the water contained in the ceramic porosity. Then the resistance returned to values closed to initial one.
The figure 18 presents the PDIV values during a cycle during a thermal cycle of 20°C thus amount to 300°C and back to 20°C for the turn-to-turn insulation and the turn to ground insulation. The measurement is made at 50Hz.
Figure 18: PDIV up to 300°C on full ceramic solution
First the PDIV decrease slowly with the temperature increasing, as is typically seen. In addition we can see clearly that the variations are the same during the warming and the cooling.
3.2.3 Results up to 500°C
Given the capability of the ceramic material, we have made a series of measurement until 500°C to see the behaviour of the insulation at this kind of temperature. The measurements are made at 1 kHz with a thermal cycle similar to that used for PDIV measurements: from 300°C to 500°C and back. The parallel capacitance variations give in figure 19 still very stable during warming and cooling.
Figure 19: Parallel Capacitance Cp
The parallel resistance have the same dynamics as the lowest temperature and continues to fall sharply as we can see in figure 20.
Figure 20: Parallel resistance up to 500°C on full ceramic solution
Finally the PDIV tends to stabilize beyond 400°C (Figure 21). The ceramic material seems to be slightly better with the increase of the temperature. It is possible that the materials need heat treatment after the mechanical process to have its maximum potential.
Figure 21: PDIV up to 500°C on full ceramic solution
4 Conclusion
Different solutions (solutions (0) to (3)) different vehicles test as twisted pairs and motorettes have been tested to measure their PDIV and dielectric strength.
The fully organic insulation is a mature technology, its thermal class defined by standards is 240°C. The definition of the thermal class corresponds to a life span of 20,000h with a single thermal constraint. In other words, after an aging test at a constant temperature of 240°C during 20,000 hours (2 years and 4 months), half the standard samples are destroyed by the final test which imposes 1kV/50Hz during 1 minute. The definition of the thermal class is rather theoretical and corresponds to a single thermal constraints, it is far from the real life of an operating electrical machine. The tests made on this mature technology show that, over the thermal class, at 300°C (25% over the thermal class) irreversible degradations are noted for short times. Consequently it is not possible to work over the thermal class of a fully organic insulation system except for short live spans corresponding to very specific applications.
Compared to the fully organic mature winding technology (solution 0), the high temperature tested solutions (solutions 1, 2 and 3) have a much lower insulation resistance. However, the equivalent resistance at 10 kHz is over 1 M, which is an acceptable value for a low voltage electrical machine.
For solution 1, achieved tests corresponds to the ground insulation that means a mica / fiberglass with a silicon impregnation. The measurements made with increasing and decreasing temperatures up to 300°C show that there is no significant irreversible degradation of the ground insulation. Consequently, the silicon varnish is a good candidate for further explorations toward temperature slightly higher than the classical organic solutions. For the solution 1 the thermal limit will be the turn-to-turn organic layer, which is defined by the thermal class of the wire.
Solution 2 avoids this problem using a ceramic-coated wire combined to a silicon impregnation. This fully inorganic insulation has mechanical performances close to the organic one but it shows higher resistances for decreasing temperatures. More extensive investigation should be made on these phenomena; the ceramic layer is porous and interface phenomena between ceramics and silicon require deeper studies beyond the scope of the exploratory approach of the project. However the silicon varnishes have thermal limits and it will be difficult to work over 300°C for long life span.
Solution 3 is a fully ceramic solution that can resist to extreme temperatures but the coil has mechanical characteristics very different that organic ones, it hard and brittle and the insertion of such coils in a motor will be more difficult. These preliminary tests shows that it will be possible to work up to 500°C with this technology when mechanical problems will be solved.
As expected, in solutions using ceramic materials, the PDIV may be reduced due to their permittivity. Moreover, due to the presence of micro-voids in the primary insulation of ceramic wires, surface partial discharges occurring in these voids lead to PDIV down to 150V at high temperatures (i.e.: 300°C).
Solutions using ceramic wires exhibit lower values of breakdown strength at high temperature (i.e.: 300°C). Nevertheless, lifetime measurements have to be performed to really have an idea of the gain achieved when using ceramic materials instead of polymer ones.
The measurements made on the ceramic material have shown that the insulation performance are slightly poorer that for organic materials. Nevertheless, the material presents a good stability with the temperature. The capacitance is stable, the PDIV decrease slowly. A special attention is needed with the parallel resistance, which seems to decrease strongly.
That kind of insulation material cannot replace the polymer in a range of temperature inferior to 300°C. Indeed, the difficulty to process the wire and the limited capabilities make it impossible the used of the ceramic wire in industrial applications. Therefore, the measurements made prove that the ceramic materials are better for high temperature. The PDIV and the parallel resistance seem to stabilize over 400°C. The machines running in that range of temperature do not exist yet. The ceramic insulation provides the ability to build this kind of machine in the future.
Potential Impact:
1 Expected impacts
In the short term, this study will help the design electrical machinery of conventional construction, able of overcoming the current thermal limits. The recommendations for machine design will give a range of technological solutions based on the desired maximum temperature. It is expected that for a temperature range higher than another, some additional constraints will appear. This study will help to make sound choices leading to the construction of a reliable machine in the context of severe application. In the longer term, ceramics insulated windings will produce insulation systems able of withstanding temperatures until the Curie point of magnetic steel. When these coils will be developed, it will be possible to design radically different electrical machines able of operating in hot atmospheres. At room temperature, this solution will increase the temperature difference between hot spots and cold ones that can greatly increase the current densities, therefore the specific power. With high temperature ceramic coils, many research opportunities will be opened for the specialists in machine design; they will have to solve the mechanical problems, which will arise for higher temperatures. This technological leap will lead to patents in the field of machinery design.
2 Dissemination and exploitation of project results
The major results have been presented in international conferences of the domain of electrical machine insulation
Posters presentation have been presented:
1) IEEE International Conference on Solid Dielectrics (2013 ICSD), July 2013, BOLOGNA, Italia,
D. COZONAC, V. MIHAILA, G. VELU, S. DUCHESNE, J.Ph. LECOINTE, D.ROGER. "Opportunities for winding wires compatible with high temperatures in the range of 500 °C".
2) French conference Symposium Electrical Energy (SGE), july 2014 CACHAN, France
V. MIHAILA, M.Q. NGUYEN, S. DUCHESNE, G. VELU, D. ROGER, D. MALEC, J.P. CAMBRONNE.
"Solutions d'isolation électrique pour les machines fonctionnant à Hautes Températures".
An Oral presentation have been presented:
A wider public conference organized by a french college and an industrial and commercial Organization, Avril 2013 (where 300 peoples have assisted)
Conference on vehicles of the futurs, BOULOGNE/MER France
G. VELU.
"l’Avion du futur : l’avion plus électrique "
List of Websites:
Contact :
1) prof. Gabriel VELU
Université d'ARTOIS
LSEE
Technoparc FUTURA
62400 BETHUNE FRANCE
gabriel.velu@univ-artois.fr
2) Prof. David MALEC
Université Paul Sabatier Toulouse III
118 rue de Narbonne
31062 TOULOUSE CEDEX 9
david.malec@laplace.univ-tlse.fr
3) Dr. Philippe Molinié
SUPELEC
Plateau du Moulon
3 rue Joliot-Curie
91192 GIF-SUR-YVETTE CEDEX
Philippe.Molinie@supelec.fr
