Fault detection based on coherence analysis for stator insulation in induction motors

Asenkron makinelerde stator yalıtım arızasının koherens analizi ile belirlenmesi

Abstract

Induction motors are the most frequently used components in almost every type of industrial processes. Early detection and isolation of faults that cause catastrophic system failure are crucial for more reliable and economic operation of industrial processes. For this reason, condition monitoring studies are performed for predictive maintenance purposes of the critical systems in most areas of the industry. Hence, sudden and unscheduled downtimes can be prevented by performing correct and timely maintenance activities using the information provided by condition monitoring studies. Through the statistical studies on induction motor failure distribution, it is reported that 40 % of all induction motor failures are caused by stator related faults such as insulation faults (Bonnett and Soukup, 1992). In the last two decades, the wide use of the motor drives based on the power electronics devices, caused current harmonics to occur and resulted in stator and bearing related problems. Hence, the use of motor drives creates some undesirable effects with the eventual failure of motors. In the related literature, there are several studies based on the stator degradation using the signal analysis methods for rotating machineries in recent years. Some of these investigators have reported the role of pulse-width modulated (PWM) devices on the stator winding insulation. They also indicate the effect of the even harmonics in the stator current of induction machines by using the spectral analysis approaches (Kueck, 2002; Filippetti, 2000). In this research, stator-winding insulation damage in induction motors is considered and fault features developed during the artificial aging process of the stator insulation are extracted from the collected data by the spectral analysis methods. Even harmonic effects are determined as a common feature between the phase currents and vibration signals using the coherence analysis and this feature indicates the stator insulation fault. This research is aimed to extract certain spectral features of the stator insulation damage based on an experimental study in the laboratory environment. This was determined by a signal processing application using the collected data. In order to get a database for induction motor failures from the experimental study, 5-HP, three phases, four poles induction motors are artificially aged by electrical methods. Several aging processes are performed. After each cycle of accelerated aging, the test motor was put on a motor performance test platform. From the experimental setup, high frequency data with a sampling frequency of 12 kHz were acquired and include 3-phase motor currents and voltages, rotor speed, torque, and six vibration measurements. The spectral analysis on the current signals is not so much informative. As an alternative approach, to determine the insulation failure, the cross approach which is defined between the current and vibration signals, is used. This new approach is the calculation of coherence function between the motor current and vibration signals regarding to the initial and aged cases. Hence the following functional relationship is calculated and it shows the effective harmonics with the huge amplitudes to extract the failure information related to motor insulation. Through the coherence analysis between the current signal and torque measurements for the initial and aged cases, the existence of all the harmonic components like the even and odd harmonics of the fundamental frequency at 60 Hz was clearly observed. Thereupon, the coherence application was used to reveal the relationship between motor current and vibration and then, as a result of this application, some special frequency values represented by the even harmonics of the line frequency at 60 Hz are determined at 240, 360, 480, 600, 720, 840 and 960 Hz. These even harmonics can be interpreted as the characteristic property of the insulation aging. These harmonics are present as a result of the stator current imbalances based upon the stator insulation damage occurring after the aging processes. These current imbalances act on the stator magneto motor force (mmf) and cause motor vibrations. Hence high correlated amplitudes in coherence variations as appeared in the figure 7(a) can be interpreted as indicators of the stator insulation damage. Results obtained by using time information of one of three motors have been analysed for remaining motors and similar results have also obtained for them. At the final part of the study, extracted failure features have been monitored by Artificial Neural Networks (ANN) method.

Özet

Asenkron motorlar bütün endüstriyel süreçler içinde en sık kullanılan elemanlardır. Endüstriyel sistemlerin ekonomik ve daha güvenli çalışabilmeleri için arızaların erken tespit edilmesi ve izole edilmesi çok önemlidir. Bu nedenle, endüstrinin birçok alanında kritik sistemlerde öngörülü bakım amaçlı olarak durum izleme çalışmaları yapılmaktadır. Durum izleme sayesinde elde edilen verilerin kullanılması ile aniden ve plansız olarak meydana gelen kesintiler önlenebilmektedir. Bu çalışmada, asenkron motorlarda stator sargısı yalıtım arızası göz önünde tutulmuş, stator yalıtımında arıza özellikleri suni olarak yaşlandırma işlemi uygulanarak oluşturulmuş ve veriler spektral analiz yöntemi ile analiz edilmiştir. Stator yalıtım arızasının temel spektral özellikleri 5 BG gücünde bir asenkron motora hızlandırılmış yaşlandırma uygulanması ile elde edilmiştir. Hedefe ulaşmak üzere, tek başına akım ve titreşim işaretlerine ait verilerle yapılan spektral analizlerle sargı yalıtım arıza tanısına ait belirgin sonuçlar elde edilememiştir. Stator akımı ve titreşim işaretlerinin arasındaki koherens yaklaşımı ile ise çift harmonik değerlerinde kuvvetlenmeler olduğu belirlenmiş ve bunların stator yalıtımı arızasının göstergesi olarak kullanılabileceği belirlenmiştir. Bir motora ait akım ve titreşim verileri ile elde edilen çıkarımlar, aynı deneye tabi tutulan iki eşdeğer motora ait verilerle yapılan analizlerle de doğrulanmıştır. Moment bilgisinin stator akımı ve dolayısıyla magneto-motor kuvvet (mmk) ile olan ilişkisine dayalı olarak titreşim işaretlerine yansıması dolayısıyla, akım-titreşim ilişkisinin daha belirleyici olacağı kararına varılmıştır. Bu çalışma, bu uygulamada 4. ve 16. harmonikler arasındaki çift harmoniklerin yalıtım arızasının en önemli imzası olduğunu göstermiştir.