Asenkron Motorda Eksen Kaçıklığının Analizi

Abstract

Asenkron motorlar, düşük maliyetleri, uygun boyutları ve çok fazla bakım gerektirmemeleri gibi özellikleri sayesinde en çok tercih edilen elektrik motorlarıdır ve günlük hayatta, endüstriyel birçok çalışma alanında sıkça kullanılmaktadırlar. Bu sebeple bu motorların arıza durumları da, birçok çalışmaya konu olmuştur. Eksen kaçıklığı bu çalışmaların büyük bir kısmını oluşturmaktadır. Eksen kaçıklığı, stator ve rotor arasındaki hava aralığının homojen bir şekilde dağılmadığı durumdur. Statik, dinamik ve bu iki durumu aynı anda ihtiva eden karma eksen kaçıklığı olmak üzere üç farklı tipi mevcuttur. Hava aralığının homojen olmayışı ve asimetrik çalışma durumu, tasarım öncelikleri, üretim süreci ve çalışma koşullarına bağlıdır. Bu hata sonucunda, dengesiz hava aralığı akı yoğunluğu, moment titreşimlerine artış, kayıpların artması ve verimin düşmesi, aşırı ısınma ve gürültü seviyesinde artış gibi sorunlar ortaya çıkacaktır. Sağlıklı ve asimetrik çalışma durumlarının karşılaştırılması amacıyla, hem sağlıklı hem de farklı mertebelerde statik, dinamik ve karma eksen kaçıklığı barındıran modeller kurulmuş ve sonlu elemanlar yöntemi ile analizler yapılmıştır. Aynı zamanda, modelleme esnasında parametreleri kullanılan motor için, sağlıklı durum ve asimetrik çalışma durumları deneysel açıdan incelenmiştir. Modelleme ve test sonucunda elde edilen veriler karşılaştırılmıştır. Böylece, eksen kaçıklığı hatası kapsamında, hava aralığı manyetik akı yoğunluğu, akım, moment gibi parametrelerin yanı sıra titreşim ve gürültü gibi mekanik parametrelerin nasıl yorumlanması gerektiği konusu incelenmiştir. Farklı mertebelerde eksen kaçıklığının, motorun manyetik, elektriksel ve mekanik parametrelerine nasıl tesir ettiği, hangi arıza boyutlarında, motorun çalışmasını nasıl etkilediği ayrıntılı olarak incelenmiştir. Böylece eksen kaçıklığının tanısı amacıyla hangi parametrelerin izlenmesi gerektiği benzetim ve deney sonuçları ışığında araştırılmıştır.

Induction motors are the most preferred electrical motor because of their low cost, reasonable size and low maintenance. Besides all these advantages, induction motors can be operated under many stresses such as, thermal, electrical, mechanical and environmental impacts. Hence, induction motors are used in many applications of daily life and industry. The failure analysis of these machines has been researched by many years in terms of mechanical, electrical, etc. Eccentricity constitutes a significant portion of the faults related to induction motors. Eccentricity is the condition of non-uniform air gap that exists between the stator and rotor. There are three types of eccentricity; static eccentricity, dynamic eccentricity and mixed eccentricity that is combination of former two. The stator is centered on first axis and the rotor is centered on second axis. In a healthy machine, first and second axes are coincident. Static eccentricity occurs when second axis is the center of rotation. If the first axis is center of rotation, the dynamic eccentricity occurs at this time. Besides, if the center of rotation can be anywhere between first and second axes, the mixed eccentricity appear to be a problem. The non-uniformity of the air gap of an induction motor depends on design features, manufacturing process and operating conditions. The asymmetric operational conditions of induction motors are connected with bearing fault, stator windings fault, broken rotor, cracked end ring and eccentricity related faults. The causes of the misalignment of the rotor as follows; the rotor center is not being at the center of the stator (bearing fault), the stator bore and rotor face are not perfectly cylindrical, bent rotor shaft, bearing wear or mechanical resonance at critical speed.These faults produce that the differences of characteristics of motor parameters as follows; non-stable air gap voltage and line current, unbalanced air gap flux density, increase in pulsating torque, decrease in average torque, reduction in efficiency (increase in losses), over heating and increase in noise level.In most of the literature, the studies are relevant to which parameters should be monitored for the detection of eccentricity in consideration of analytical calculations. The thesis presents the dynamic model of the induction motor and shows the deviation in the magnetic flux, the current and the torque values. Faulty conditions are investigated for static, dynamic and mixed eccentricity. The air gap flux density and its harmonic components are researched for healthy and asymmetric operational conditions. Therefore, the change in the harmonic components with increasing of eccentricity is analyzed.A 2.2 kW, 4 pole, 50 Hz, delta connected squirrel cage induction motor is used in this thesis. The finite element method (FEM) is used for the analysis as a numerical method. FEM expresses a physical system in terms of mathematical quantities. Besides, it is a model that could be decomposed into sub-elements and have material properties and applicable boundary conditions. Unknown quantities with finite number are obtained by using known quantities of system.In thesis, the dynamic model of the induction motor is created for healthy and eccentric condition by changing the air gap. After verification of the healthy model respect to the experimental results, the flux density, current and torque characteristics of the eccentric models are examined. The test motor has 3 phase, 4 poles, 36 slots and 28 rotor bars. Firstly, the motor is modeled for the rated operation condition with no eccentricity. The eccentricity is modeled by shifting the rotor position and defining the axes of rotation. The non-uniform air gap is obtained in order to analysis the asymmetric operational conditions.The uniformity of the air gap width is corrupted with eccentricity. Consequently the air gap flux density will be different than the one in the healthy condition. Therefore, the higher air gap flux density values appear in points where air gap width is smaller and lower values appear where air gap width is larger, similarly.The harmonic spectrum is obtained in order to investigate the eccentricity effect on the air gap flux density. When fundamental component is decreasing with eccentricity fault, the high frequency components increase. As the eccentricity increases, the magnitude of the harmonic permeance waves due to eccentricity will increase. The flux density distribution in the air gap is found as the product of the permeance and the magnetomotive force (MMF), so, when the harmonic permeance waves due to eccentricity increases, the magnitude of harmonic components of air gap flux density will also increase as shown in simulation results.Therefore, pulsation of torque increases. The peak values of torque increase with eccentricity about 1%. Besides, the torque ripple increases 1~14 % with increasing of eccentricity. In addition, eccentricity is investigated in terms of mechanical parameters such as vibration and noise with using experimental setup. The two different asymmetrical condition and healthy condition of motor are tested. In this way, the vibration level of asymmetric operational conditions increase %8.5 compared to the healthy conditon. In conclusion, in this thesis , the dynamic model of the induction motor is modeled with uniform and non-uniform air gaps. The results for the healthy model are verified the rated values of the motor. After the verification of the motor parameters, the motor geometry is redesigned for static, dynamic and mixed eccentricity condition. Changing of the air gap flux density is illustrated for eccentric conditions. The decreasing of fundamental component and the increasing of high frequency components with increasing the percentage of eccentricity are observed. The effects of eccentricity on the air gap flux distribution, current and torque characteristics are investigated with simulation results. Besides, the effects on vibration and noise are observed with test results. Therefore, the monitoring methods of eccentricity are studied in terms of different parameters, for instance; air gap flux density, harmonic components, current, moment and vibration.