Winding wires for high temperature machine

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Winding wires for high temperature machine


application/pdf Winding wires for high temperature machine Gabriel Vélu, Daniel Roger, Jean-Philippe Lecointe
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Winding wires for high temperature machine


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Winding wires for high temperature machine G. Vélu (1)(2), S. Duchesne(1)(2), D. Roger(1)(2), J. Ph. Lecointe(1)(2) 1 Univ. Lille Nord de France, F-59000 Lille, France 2 UArtois, LSEE, F-62400 Bethune, France Email : Abstract The purpose of this paper is to describe materials allowing to improve the density of electric actuators. Indeed the more electric aircraft requires to reduce the weight of the actuators. One of way is to increase the operating temperature using new insulation better able to support this new constraint. For this, the commonly used, organic polymers must be replaced by other technologies. The originality of this study is based on a test campaign of now very mature solutions for use in a machine as the polymer with nanoparticles, and more advanced solutions such as the use of ceramics. The results are used to open interesting perspectives on the development of electrical insulation systems machines tomorrow. Introduction The mass power of electrical rotating machine is nowadays limited by the winding temperatures. The working temperatures which do not exceed 250°C are imposed by the organic conductor insulation [1]. However, some typical applications could use the electrical machines beyond of these temperatures for high temperature environment, for example in the fields of aviation, spatial [2-3]. Increasing the working temperature means that all the electrical machine components have to be made with specific materials whose properties allows reaching high internal temperature, close to 500°C. Indeed, in reality, the problem is not limited to the insulation conductor. The main effects of the high temperatures concern the metal corrosion and the thermal decomposition of organic materials over 200-250°C [1]-[4]-[5]. As a consequence, a real technological leap has to be done concerning all the machine components. The slot insulating material, the bearings or plastic parts such as the fan or the connection support in the terminal box must withstand temperatures. The magnetic circuit should also be adapted for high temperatures. Two fundamental points appear for the magnetic circuit. First, the temperature must not alter the surface laminations to ensure the machine reliability. Second, the magnetic circuit must keep performances despite the temperature rise [6]. All the materials characterized are commercial available and the tests have been done in atmospheric conditions where the temperature varies from 20°C to 500°C. Constrains imposed by high temperature Increase the temperature up to 500°C for electrical rotating machines implies to exceed strongly the thermal limit of the classical machines. In order to evaluate the weakest points which appear with the temperature increase, the machine can be divided in four element families discussed below: The magnetic circuit. Classically made with magnetic sheets of FeSi, it undergoes high temperatures during its production process (≃ 800°C). Thus, 500°C is a reachable working temperature for FeSi sheets. The weakest point concerns the sheet insulation because for the non- oriented (NO) lamination, the insulation is added after the rolling process [7] It is still possible to use it but some electrical steel can provide better performance for high temperature conditions like oriented grain (GO) laminations have an inorganic insulation coating applied during of technological process[8]. The electrical conductors. Most of the time, conductor material is copper, which resistivity increases with temperature. But the major problem with copper is its oxidation [9]. At room temperature, oxidation of copper is negligible; this is not the case for a temperature above 200 ° C [10]. At high temperature, the conductive metal is diffused into the insulating layer so as to form a semi-conductor inside the insulating material layer and oxide degrades the general characteristics of the insulation. Under the influence of the temperature in the copper conductors with organic or inorganic insulators, the diffusion of copper into the insulating layer occurs in a more or less sensitive [11]. This reduces the effective thickness of the insulating layer which may cause the formation of conductive channels and therefore short- circuit between adjacent turns. The paper reference [sge10] shows that the depth of copper diffusion into the insulating layer depends on the temperature, a higher temperature corresponds to a greater diffusion depth. To avoid the problem of diffusion of copper from other metals may be used as a diffusion barrier layer for example, nickel (Ni), titanium (Ti) or platinum (Pt). The alloy of copper and nickel is the most used, because they are neighboring elements in the periodic table, and it is easy to combine them. It is still possible to use it but some metal alloys can provide better performance for high temperature conditions like Silver-Palladium alloys (AgPd). The electrical insulation of the wire. Made with organic polymer as Polyamide-imide and Polyester- imide (PEI/PAI), their working temperatures do not exceed 200°C. Other polymer materials such as Kapton, Teflon AF or PEEK, can work up to 250°C but this material technology is limited [12]. The peripheral elements like the fan or the terminal block: usually made with consumer polymer like PET, their temperature is limited to 120°C, but they can easily be replaced by elements made with steels which give better temperature capabilities. This short review shows that the weak point of the rotating electrical machine when the temperature is significantly increased is the Electrical Insulation System (EIS). That includes insulation of the wires, slot insulation and insulation of the magnetic sheets. Polymers are usually used since they have mechanical properties well adapted to form the winding of the machine. Nevertheless a material capable to endure higher temperature condition is required. Inorganic materials such mica paper, ceramics and fiber glass are good candidates for a significant increase of the temperature in the electrical rotating machines. Wire technology for high temperature In an environment of above 300 ° C, the mineral insulation are best suited for long periods of operation [13]. However, in addition to temperature, these materials must also withstand without failure the electrical and mechanical stresses. In this section, a study of non-organic insulation is made. Preferred non-organic materials in this study are mica, glass fiber, silicone and ceramics. Mica is a mineral composed primarily of aluminum and potassium silicate. Mica is stable when exposed to electricity, light, moisture and temperature extremes. It is thermally stable and is resistant to partial discharge. The main categories of mica used in electrical engineering phlogopite and muscovite. The muscovite mica is primarily used in capacitors that are ideal for high frequencies, but their temperature stability is lower than for the phlogopite mica. The phlogopite mica is stable at higher temperatures (up to 900 ° C) and is used in applications where a combination of high temperature stability and electrical properties is necessary. Fiberglass is a hard material or alloy, brittle and transparent to visible light. In most cases, the glass is made of silicon dioxide (SiO2, silica) which was added to the flux. The physical point of view, the glass is a (non-crystalline) with amorphous material with a glass transition phenomenon. At room temperature the glass is a good electrical insulator with a resistivity of about 1017 Ω.m, but at very high temperatures, it becomes conductive. Thus at 500 ° C falls to its electrical resistivity 6,3.105 m.Ω [14]. Glass has good thermal properties, but it is fragile and it is difficult to use glass to make insulation having good mechanical properties in terms of flexibility. Therefore, the glass is processed into flexible fibers. These glass fibers are used to produce fabrics which are wound around the cables. Like any inorganic material, the glass has a high resistance to the electrical stress in terms of partial discharges. Examination of insulating glass composite, after a period of exposure to partial discharges reveals erosion of polymer materials, but no damage to the glass [15]. Silicon are inorganic compounds containing a silicon-oxygen chain (-Si -...-O) on which are fixed groups on the silicon atoms. Some organic groups (carbon, hydrogen and possibly other chemicals) can be used to link several of these chains. Some of the most useful properties of silicone resins include: thermal stability, high degradation temperature and good electrical insulation characteristics. In most examples of insulation systems, silicone is used as binder for the silicone into an inorganic material when heated to high temperatures [13]. In many cases, mineral fillers may be included in these binders. Ceramics are essentially inorganic and non-metallic substances with excellent thermal properties. Ceramics generally have very high chemical inertness and resistance to attack by aggressive substances, oxidation and weather damage. Ceramic retain their strength even at high temperatures and are able to withstand thermal shocks, they have a low thermal expansion coefficient and low thermal conductivity. Technical ceramics are divided into three different categories : • oxides: Al2O3 (alumina), ZrO2 (zirconium dioxide)...; • non-oxides: carbide, boride, nitride, silicon and ceramics composed of tungsten, magnesium, titanium or platinum; • ceramic composites which are a combination of oxides and non-oxides. For the manufacture of ceramic insulated wires, typically oxides are used because a thin insulating layer may be deposited on the conductor while maintaining their good mechanical and thermal properties. Depositing a layer of conductive ceramic is very different from other conventional insulating materials such as polymers. The production procedure of isolated ceramic son is difficult and therefore expensive. Ceramics, placed on the wires remains quite porous and therefore sensitive to moisture. They also have a fairly low strength making them brittle. Under these conditions, a fundamental constraint in using this type of wires is the radius of curvature. The wires of ceramics are limited in terms of radius of curvature to ≃ 10 times the diameter of the wire. This restriction must be taken into account in the design of the coils. Moreover, the dielectric strength of the insulating ceramic layers depends on many factors such as porosity, size and uniformity of the grain size used in the deposition process, the presence of defects in the microstructure, ... The dielectric strength is even greater than the thickness of the ceramic layer is low because an increasing volume of material increases the probability of the presence of random defects. Studies by [16] have shown that the breakdown voltage decreases with increasing thickness of the insulating layer and with the decrease of the purity of the alumina. Experimental result of electrical and breakdown stength The authors have characterized two wires. The ceramic coated wire (Wire A) is a high-temperature wire of 0.518 mm diameter, with a conductor made of Cu (73%) and plated Ni (27%). The insulation layer of 9 μm thickness is made of ceramic mainly composed of Al2O3 and SiO2. The mica-fiber glass taped wires (Wire B), 0.7 mm diameter is a nickel-plated copper wire taped with a thin film of mica phlogopite and fiber glass (thickness of 0.1mm). The thickness of nickel varies between 3 and 5 μm. To the solution twisted fiberglass samples were performed in a conventional manner in accordance with IEC 60851-5 [17] (Fig. 1a) standard. However ceramics wires are not flexible enough to produce specimens twisted without damaging the ceramic insulating layer. We used the second test system proposed by the standard [17] which consists in winding the wires on a steel mandrel (Fig. 1b). (a) (b) Fig. 1: Dielectric test for wrapped wires(a) and for ceramic insulated wires (b) The identification of the dielectric characteristics of ceramics is classically done by measuring the permittivity, the leakage current, the tan δ and the breakdown voltage Ub. The typical voltage-current curve of ceramic insulation gives the range of use in the zone close to the breakdown voltage. However, measurement made on a ceramic insulated wire and presented in Fig. 2 shows that individual PD pulse appears at low voltages. It differs slightly from breakdown voltage; it can be explained by discharges due to the initiation of the conduction channel of breakdown: an increasing electronic movement in the insulation layer impacts the dielectric constant and the insulation resistance. The ceramic porosity can also influence the initiation phenomena by guiding the path of the electrons. (a) (b) Fig. 2: Inception of Partial Discharge and the quantity of the charge observed with AC voltage for: (a) Wire B (b) Wire A. The partial discharge for Wire B and the first PD observed for Wire A are presented in Fig. 3. The voltage is progressively increased until the first PD is observed. As the occurrence of the PD cannot be a good indicator for ceramics insulation due to the several factors that can influence independently the insulation structure (porosity, cracks, humidity…), a breakdown test is made. 10 100 1000 0 100 200 300 400 500 Voltage (V) Temperature ( C) Wire A Wire B Partial Discharge BreakdownVoltage Fig. 3: Temperature impact on the first PD and breakdown voltages. Test under 20xDc bend radius supplied by an AC source at 50Hz. In the temperature range from 400 to 500°C, the breakdown voltage is the same that the voltage corresponding to the first partial discharge. The variation of the parallel resistance Rp between the conductor wire and the cylinder with the temperature is shown in Fig. 4. Each point results from the average of measurements made on eleven individual samples. The insulation resistance gives information about the variation of the leakage current at a given voltage. Since the ceramic layer is sensitive to moisture and can absorb the humidity from the ambient. In results the smallest value of the resistance is observed at 20°C. 0,1 1 10 100 0 100 200 300 400 500 Parallel resistance (MΩ) Temperature ( C) Wire A Wire B Fig. 4: Variation of the parallel resistance with the temperature For temperatures inferior to 400°C, the values are relatively stable, whence the losses are increased significantly The Wire B partial discharge measurement at high-temperature is shown in Fig. 2.a. The sinusoidal voltage Urms is applied and the quantity of electrical charge which pass through the insulation at positive and negative alternation is measured. The measurement for Wire B (Fig. 2.a) can be compared with the discharge of classical organic insulation wires. However, the PD behavior at low voltages is particular for the Wire A (Fig. 2.b) with very energetic discharges, which pass through the layer insulation. The low PDIV can be explained by the high local electric fields, which appear because of the high permittivity of the ceramic insulation, especially in high-temperature environment. As the ceramic is porous, short-circuits can appear between the pores, which form plasma channels responsible for the breakdown. The performances of the studied wires at 500°C are shown in TABLE 1. The Wire A and Wire B dielectric parameters have good thermal stability. These two wires may be good candidates for an electrical machine working at 400-500°C. Wire B has the best properties concerning the partial discharge and breakdown tests. The PD and the breakdown voltages for Wire A were found between 100 and 120 V in the range 400-500°C. Parameter Wire Rp (MΩ) Cp (pF) tan δ PDIV (V) Ub (V) Wire A 2.47 68.6 0.099 119 173 Wire B 1.74 57.3 0.206 328 540 Table 1: Comparasion of the studied wires at 500°C Conclusions This paper proposes a characterization of materials which can be used to build high temperature induction machine. Candidate materials have been identified. The authors characterize the wire. The further work concerns the design of the machine and the test of the prototype References 1 F. Aymonino, T. Lebey, D. Malec, C. Petit, J. Saint Michel,A. Anton, "Dielectrics measurements of rotating machines insulation at high temperature (200-400°C)," in IEEE Conference on Electrical Insulation and Dielectric Phenomena, pp. 740-743, 2006. 2 V. R. 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