Minimisation of Losses in Permanent Magnet Synchronous Generator for High Speed Applications

Transcription

1 Minimisation of Losses in Permanent Magnet Synchronous Generator for High Speed Applications Sarumol G 1, Reeba S V 2 1 (Electrical and Electronics, Kerala University, Trivandrum, India, sarumol18@gmail.com) 2 ( Electrical and Electronics, Kerala University, Trivandrum, India, reebasv@gmail.com ) Abstract In recent years, the demand for higher efficiency and more compact machine increase rapidly, particularly in high speed application. High speed machines are being developed for many applications such as machine tools, compressors, vacuum pumps, friction welding units, turbine generators and so on. With equivalent shaft power specification high speed electrical machines present lower volume and mass compared with standard machines. The design of these machines involves loss determination. Electric machines have different losses in each of their parts. Usually copper and iron losses represent the major part. The computation of the core losses of PM machines aroused great interest in recent years. The losses of Interior permanent magnet synchronous machine (IPMSM) and Surface mount permanent magnet synchronous machine (SPMSM) are determined using Maxwell software. Core loss can be reduced by replacing core material with an amorphous alloy. Thus the efficiency and hence performance of the high speed machine can be improved. The thermal behaviour of the machine is a very important aspect. The steady-state temperature rise under rated condition is used to judge the heat distribution of the machine. A comparative study between these two machines is done along with thermal analysis using Ansys mechanical workbench. Keywords core loss; amorphous alloy; permanent magnet synchronous generator 1. INTRODUCTION Permanent magnet synchronous machines(pmsm) have become increasingly popular in the last two decades. Many new applications, which require high-performance electrical machines, are invariably linked to PM machines. Two factors may be singled out as crucial for such developments are cost competitiveness and availability of rare-earth magnets and emergence of efficiency-driven applications. Increasing the rotational speed of PM machines tends to increase the power density, but also increase losses. The major losses associated with a high speed PMSM are stator loss, rotor loss and windage loss. Through accurate loss determination and loss minimization machine efficiency can be improved. This paper deals with the design of high speed permanent magnet synchronous generator and the major losses associated with it. The usage of amorphous alloy (AA) as the core material is attracting great attention in recent decades. Here iron based AA is used to reduce the core loss of a high speed permanent magnet synchronous generator(pmsg). 2. HIGH SPEED PERMANENT MAGNET SYNCHRONOUS GENERATOR High speed machines are being developed for many applications such as machine tools, compressors, turbine generators and so on. Newly developed applications, which require high-performance electrical machines, prefer PM machines. Owing to strong magnets, PM machines can have high power densities and also achieve high efficiencies due to no excitation losses, no magnetizing currents and very low rotor losses. Virtue of high power density becomes particularly important for applications where low volume/weight and high-speeds are important. In addition, the cylindrical shape of its rotor compared to the solid rotor of the reluctance machines generates less aerodynamic losses. Undoubtedly, PM machines dominate the field of small highspeed machines. In the design of high speed PMSM, several and opposite constraints have to be considered simultaneously for an accurate design. These constraints are especially related to mechanical stress due to centrifugal force, core losses and temperature rise. 3. TYPES OF PMSM The PMSG can be divided into different types based on the magnet position on the rotor. Fig 1 shows different types of permanent magnet synchronous generator. A. Surface Mount Permanent Magnet Synchronous Machine. Here the magnets are mounted on the surface of the rotor, sometimes referred to as exterior magnet, but, more known as Surface Mounted Permanent Magnet Synchronous Machine (SPMSM). The magnets are glued andor bandaged to the rotor surface in order to withstand the centrifugal force. Usually, the magnets are oriented or magnetised in radial direction and more seldom in circumferential direction. The main advantage of these machines is the simplicity of construction and therefore, a consequently reduction of cost compared to other magnets topologies. Nevertheless, this configuration exposes the PM to demagnetization fields. Besides the direct and quadrature reactances are almost equal i.e. L d Lq, since the relative permeability of the magnet is similar to air (μ r = ), the rotor is isotropic. Hence, this configuration is not able to perform reluctance torque. Construction of the rotor core in SPMSM is the easiest among different PM configurations due to simple rotor geometry. Fig. 1.Different type of PMSM a) IPM machine b) SPM machine Volume: 03 Issue:

2 B. Interior Permanent Magnet Synchronous Machine In this configuration the magnets are embedded inside the rotor and therefore it is referred to as Interior Permanent Magnet Synchronous Machine (IPMSM). There are different ways in achieving interior magnet configuration. The magnets can be magnetised in radial direction as well as circumferential direction. The inductance in quadrature axes is different from that in direct axes direction. The main advantage of this PM configuration is that weak PM material such as ferrite can also be used, but burying magnets in production stage is a complicated process. Since the volumes of the stator and rotor in a high-speed electrical machine are much smaller than in a conventional machine that has the same power, the loss density in a high speed machine is much higher. This means that the thermal design of a high-speed machine is a very demanding task because a well designed high-speed machine operates at high temperatures that are close to the critical temperatures of its components. The problem is more serious for the rotor because the rotor cooling is more difficult than the stator since it can be performed only through the air gap. This mostly refers to a permanent-magnet (PM) high-speed machine because it has a more complex rotor construction and thermally it is more sensitive in comparison with a high speed induction machine 4. PERMANENT MAGNET SYNCHRONOUS GENERATOR LOSSES The losses in PM alternators are grouped into core loss, copper loss, and mechanical loss. A. Core Loss Eddy current loss can also be divided into classical eddy current loss and excess eddy current loss for a more accurate analysis. Therefore, at a given frequency (f), the iron loss for electrical steel can be calculated from P iron = K h B 2 f + K c (Bf) 2 + K e (Bf) 3 2 (1) where K h, K c, and K e are the coefficients of hysteresis loss, classical eddy current loss, and excess eddy current loss, respectively, and B is the peak flux density. The coefficients can be calculated using the curve fitting of iron loss data from manufacturers or from material test data. The above equation is based on the assumption of sinusoidal excitation. B. Copper Loss Copper loss is the loss due to the current going through the armature windings. The copper loss is P CU = m 1 I 2 R (2) where m 1 is the number of phases, I is the armature current, and R is the dc armature resistance. The I 2 R loss can be significant when large current flows through the conductor with large Ohmic resistance. The copper loss is temperature dependent, so the copper loss is calculated at the expected copper temperature. Copper loss increases when temperature increases due to increased winding resistance. C. Mechanical Loss Mechanical loss are relatively small in comparison to other losses especially in low speed applications. It encompasses two parts, namely windage and bearing. Windage losses are caused by mechanical friction of air and Fig 2. 2D FEM model of IPMSM Fig 3. 2D FEM model of SPMSM rotor surface. The windage loss generation is a function of shaft rotational speed and fluid properties such as temperature, pressure, density, and temperature gradients at stator and rotor walls. Mechanical loss in the bearings depends on parameters like bearing type, lubricator physical characteristics, shaft mechanical load and rotor speed, where lubricator characteristics are dependent on the temperature. 5. DESIGN AND ANALYSIS OF HIGH FREQUENCY PMSG A 4 pole IPMSM with double layer integral slot winding is designed. The design specification is given in Table I. The rotor consists of a simple core structure with 4 rectangular magnets. The rotor is usually built from the same material as the stator for the ease of construction but it can be made of any material provided it is strong enough for its function [5]. M19, 24 gauge electrical silicon steel is selected for PMSG because it is economical and widely used. After designing the machine in maxwell software, losses associated with the machine are determined. The major losses such as core loss and copper loss are determined for interior PMSG. Similarly surface mount PMSG is designed and losses are determined. Fig 2 and Fig 3 shows the model of IPMSM and surface mount PMSM obtained. Core loss account for the significant amount of total losses ranging from 15 to 25% in electrical machines. So to reduce these losses appropriate method should be adopted. Improving the efficiency by replacing the core with materials having better Volume: 03 Issue:

3 the comparison of losses obtained in IPMSM and surface mount PMSM. TABLE I DESIGN SPECIFICATIONS OF PMSG Machine Specifications Parameter Rated power Value 90 kw Fig 4. Comparison of core loss of IPMSM using silicon steel and amorphous alloy Rated line voltage 440 V Number of Poles 4 Number of slots 12 Synchronous speed 27000rpm Rated power factor 0.96 Rated Frequency 900 Hz Connection Type Star TABLE II PROPERTIES OF AMORPHOUS ALLOY Material RF Thickness 0.03mm Density 7.2gm/cm 3 Fig 5. Comparison of core loss of IPMSM using silicon steel and amorphous alloy properties is attracting great attention in recent decades. Amorphous alloy(aa) is a kind of advanced soft magnetic material which features super high efficiency especially in high frequency operation [1]. The main magentic and mechanical properties of amorphous alloy are given in Table II. It is noticeable that, the core loss of machines with AA is much lower than that of silicon steel such as M19, 24 gauge [8]. This is the most exciting feature of the AA, making it a good candidate as the core material of a high efficiency machine [6]. Since its flux density is lower than the silicon steel, the core losses of the same working condition are reduced as the direct result of the replacement. But due to the saturation, the current of the AA machine may be larger than the silicon steel machine. As a result, the copper loss of the machine increases. So an appropriate method should be adopted to reduce the copper loss [9]. The core loss curves obtained for IPMSM and surface mount PMSG using silicon steel and AA alloy is shown in Fig 4 and Fig 5 respectively. Copper loss can be reduced by using parallel strand instead of single strand in the winding [3].If the diameter of a single strand is close to skin depth, employing several thinner strands in parallel may sufficiently reduce the additional losses. The skin depth of copper is 2.19mm. Hence by using 20 strands of diameter 0.218mm instead of a single strand of diameter 4.361mm, copper losses are reduced.table III shows Core loss Copper loss B 1.57T Loss 1.62W/kg TABLE III LOSS COMPARISON BETWEEN IPMSM AND SPMSM Machine Losses Material IPMSM SPMSM Silicon Steel W 8973 W Amorphous alloy 9380 W 3939 W Silicon Steel W 1405 W Amorphous alloy 1480 W 1592 W Amorphous alloy using 22strands 1368 W 1450 W Volume: 03 Issue:

4 Fig.6. Thermal Analysis of IPMSM using silicon steel alloy Fig.8. Thermal Analysis of SPMSM using AA alloy Fig.7. Thermal Analysis of SPMSM using silicon steel alloy 6. THERMAL ANALYSIS Thermal analysis is a branch of material science where the temperature distribution and other thermal quantities of materials are studied. Core materials are often subjected to situations that may exceed their thermal capabilities. Therefore anaysis of thermal behviour of a machine is a very important aspect [1]. The different losses in machine give rise to temperature, which affects the performance and lifetime of electrical machines. Hence thermal analysis is done to analyze the heat distribution of the machine. Fig 6 and Fig 7 shows the thermal analysis of IPMSM and SPMSM. The material used here is silicon steel. Fig 8 and Fig 9 shows the thermal analysis of the machines after loss minimization using amorphous alloy. The machine uses amorphous alloy as the core material and the windings contain increased number of strands. The temperature in the core and the windings are reduced. However the temperature of the machines are still high so that it may lead to thermal damage. So to further reduce the working temperature and maintain a uniform heat distribution cooling is done. 7. COOLING IN ELECTRICAL MACHINES Cooling is very much essential for electrical machines especially high speed machines. It ensures high efficiency, increase in longevity of machines, a safe operation and protection of inner parts from thermal damages. Fig.9. Thermal Analysis of IPMSM using AA alloy In most cases the cooling of machines is done by air streams and this cooling is called ventilation. This can be divided into two sub categories depending upon how the air passes over the heated machine parts namely radial and axial ventilation. Radial ventilation is suitable for machines upto 20 kw and axial ventilation is used in medium output high speed machines. Fig. 10 and Fig. 11 show the thermal analysis of IPMSM and SPMSM respectively after providing axial ducts. It can be seen that for both the machines, the temperature reduces to a value which lies within the allowable range.. Fig.10. Thermal Analysis of SPMSM using AA alloy with axial duct Volume: 03 Issue:

5 Fig.11. Thermal Analysis of IPMSM using AA alloy with axial duct 8. CONCLUSION An interior PMSM suitable for high speed applications is designed. Along with that modeling of surface mount PMSM in maxwell is done. Then the losses are determined for both the machines. Core loss can be reduced by replacing core material of the machine by amorphous alloy. Since copper loss is increased due to the use of AA it can be reduced by using parallel strands. The thermal plot of the machine before and after loss minimization is obtained using ansys workbench. It is evident that temperature decreases with decrease in loss. Further axial ventilation is provided to minimize the temperature to an acceptable range. Finally comparative study between interior and surface mount PMSM is done after loss minimization and SPMSM seems to be more efficient for high speed applications REFERENCES [1] G. Eason, Tao Fan, Member, IEEE, Qi Li, Member, IEEE, and Xuhui, Senior Member, IEEE Development of a High Power Density Motor Made of Amorphous Alloy Cores IEEE Trans.Ind,Appl, vol. 61,no 9,sept 2014 [2] Floran Martin, Mohammed El-HadiZaim, AbdelmounaimTounzi, and Nicolas Bernard Improved Analytical Determination of Eddy Current Losses in Surface Mounted Permanent Magnets of Synchronous Machine IEEE Trans. Magn., vol. 50, no. 6, June [3] Martin van der Geest, Student member, IEEE, HenkPolinder, Senior member, IEEE, Jan A. Ferreira, Fellow, IEEE, Dennis Zeilstra Stator winding proximity loss reduction techniques in high speed electrical machines in Proc.IEEE ECCE,Sept.2013 [4] Co Huynh, LipingZheng, DipjyotiAcharya, Losses in High Speed Permanent Magnet Machines Used in Microturbine Applications, Journal of Engineering for Gas Turbines and Power MARCH 2009, Vol. 131 / [5] N. Bianchi, S. Bolognani, and F. Luise, Potentials and limits of high speed pm motors, IEEE Trans. Ind. Appl., vol. 40, no. 6, pp , Nov./Dec [6] C. C. Jensen, F. Profumo, and T. A. Lipo, A low-loss permanentmagnet brushless DC motor utilizing tape wound amorphous iron, IEEE Trans.Ind. Appl., vol. 28, no. 3, pp , May/Jun [7] K. Yamazaki and Y. Fukushima, Effect of eddy-current loss reduction by magnet segmentation in synchronous motors with concentrated windings, IEEE Trans. Ind. Appl., vol. 47, no. 2, pp , Mar./Apr [8] L. Li, L. Fang, and Q. Wei, Research on a novel Fe-based amorphous electric motor, in Proc. 3rd IEEE ICCSIT, 2010, pp [9] Thomas, Z. Zhu, and G. Jewell, Proximity loss study in high speed flux-switching permanent magnet machine, IEEE Trans. Magn., vol. 45,no. 10, pp , Oct Volume: 03 Issue: