CONDITION MONITORING FOR PREVENTIVE MAINTENANCE OF LARGE INDUCTION MOTORS By W.K.L.PRASAD This thesis was submitted to the Department of Electrical Engineering of the University of Moratuwa in partial fulfillment of the requirements for the Degree of Master of Engineering in Electrical Engineering Department of Electrical Engineering Faculty of Engineering University of Moratuwa Sri Lanka 2004 80498 Abstract Induction motor faults, such as broken rotor bars, rotor eccentricities, bearing faults, mechanical misalignments cause harmonic changes in the permeance of the motor magnetic circuit. A motor vibration at a specific frequency will result in a current harmonic at a known frequency. The specific relationship between the vibration and the stator current harmonic magnitudes is a complex function. It is a function of the mechanical system and the magnetic system of the motor itself. The sensors used, increases it's complexity further. In the research, first, the relationship between the vibration and the stator current harmonic magnitudes are studied for a particular machine problem and the critical frequencies in the frequency spectrum of the Induction motor stator current are identified using the Micro log instrument CMVA 60, which is a property of the Mahaweli Complex of Ceylon Electricity Board. Then, the magnitude ratios of the harmonics at critical frequencies to the fundamental component of the stator current are determined by conducting a survey on previous faults found in Induction motors used in Kotmale Power Station and by carrying out an experiment on an Induction motor having a squirrel cage rotor with open rotor bars. The •defective Induction motor, which had been used in one of the gear pumps to pressurize the MIV pressure vessels of the three synchronous generators in Victoria Power Station, could be used for the measurements. Next, the results obtained from the case study and the survey are compared with the standards that have been evolved from the past studies and the researches in order to determine the feasibility of setting a limit or standard on the current harmonics at critical frequencies due to motor faults. Finally, the possibility of developing a current monitoring system to identify a rotor problem is investigated for a drainage pump in Kotmale Power Station. The results can then be extended to alarm the fault by integrating the circuit to the existing annunciation system to minimize the interruption of the drainage system, which is very important in the concept of maintenance as well as the safety of the under ground power station, caused by the failure of the machine. D ECLARATION I hereby declare that this submission is my own work and that, to the best of my knowledge and behalf, it contains no materials previously published or written by another person nor material, which to substantial exten t, has been accepted for the award of any other academic qualification of a University or Institute of higher learning except where acknowledgement is made in~ text. '!(e........_._-~ W.K.L.Prasad January 2004 ~ Dr. J. P.Karunadasa Project Supervisor January 2004 ... ... - Abstract Contents Preface List of figures Chapter 1 Introduction 1.1 General 1.2 Condition Monitoring Tools 1.3 Vibration Vs Stator Curren t 1.4 Thesis Objective & Outline Chapter 2 Instrumentation 2.1 Monitoring System Outline 2.2 Data acquisition Contents I ... lJ 11 iv v 1 1 1 2 3 5 5 7 Chapter 3 The Induction Motor monitoring system available in Kotmale 10 ~ Power Station 3.1 Vibration Monitoring System 10 3.2 Analysis of the Vibration Spectrum for Motor Fault Identification 12 3.2.1 Misalignment 12 3.2.2 Imbalance 13 3.2.3 Mechanical Looseness 13 3.2.4 Bearing defects 14 3.2.5 Broken rotor bars and end rings - 15 Chapter 4 Why a current monitoring system for IM is required 16 Chapter 5 Theory 5.1 Rotor Bar Analysis 5.2 Air gap Eccentricity Analysis 5.3 Misalignment Chapter 6 Case study 6.1 Measurements and Observations Chapter 7 Results I iii 18 18 20 23 24 25 29 7.1 Comparison of the pole pass frequencies 29 7.2 Calculation of the current ratios 30 Chapter 8 A current monitoring system for Drainage pumps at Kotmale 32 Power Station 8.1 Project description 32 8.2 Component selection 35 • Chapter 9 Simulation of the fault detection circuit using Electronics 38 Workbench 5.12 9.1 Introduction 9.2 Simulation procedure 9.3 Results Chapter 10 Conclusion Chapter 11 Discussion Reference Appendices Appendix 1 Vibration Sampling Points Appendix 2 Vibration Standardc; VDI 2056 Appendix 3 Rotor bar damage severity level chart ·--~ 38 38 50 52 53 56 58 List of Figures Figure 2.1 Block diagram of the proposed system Figure 2.2 Data Acquisition System Figure 3.1 Vibration Spectrum of UOl Thrust Bearing Pump in KPS Figure 3.2 Vibration Spectrum of UOl Cooling Water Pump No.02 in KPS Figure 3.3 Vibration Spectrum of U03 Cooling Water Pump No.02 in KPS Figure 5.1 Dynamic eccentricity Figure 5.2 Single axis air gap eccentricity I Figure 6.1 Current Spectrum of the R phase of the MN oil pump no.02 at VPS Figure 6.2 Zoomed Current SpectTum of the R phase of the MN oil pump no.02 atVPS Figure 6.3 Current Spectrum of theY phase of the MlV oil pump no.02 al VPS Figure 6.4 Zoomed Current Spectrum of lhc Y phase of the MTV oil pump no.02 atVPS Figure 6.5 Current Spectrum of the R phase of..the MIV oil pump no.01 at VPS Figure 6.6 Zoomed Current Spectrum of the R phase of the MN oil pump no.Ol atVPS Figure 8.1 Current Spectrum of the Drainage Pump No.03 of KPS Figure 8.2 Zoomed Current Spectrum of the Drainage Pump No.03 of KPS Figure 8.3 Schematic Diagram of the monitoring system Figure 8.4 Notch Filter Figure 8.5 Narrow-band RC active filter Figure 8.6 Comparator Circuit Figure 9.1 Notch Filter ·-... Figure 9.2 Output of the Bode plotter in fig 9.1: Magnitude and Phase Figure 9.3 Output of the Oscilloscope Figure 9.4 Output of the Oscilloscope in fig 9.1 for a 49.5 Hz signal v Figure 9.5 Output of the Oscilloscope in fig 9.1 for a 51.5 Hz signal Figure 9.6 Band-pass Filter Figure 9.7 Output of the Bode plotter in fig 9.6 : Magnitude and Phase Figure 9.8 Output of the Oscilloscope in fig 9.6 Figure 9.9 Output of the Oscilloscope in fig 9.6 for a 50 Hz signal Figure 9.10 Output of the Oscilloc;C'ope in fig 9.6 for a 100Hz signal Figure 9.11 Cascaded circuit Figure 9.12 Output of the Bode plotter in fig 9.11: Magnjtude and Phase Figure 9.13 Output of the Oscilloscope in fig 9.11 for a 49.5 Hz signal Figure 9.14 Output of the Oscilloscope in fig 9.11 for a 50 Hz signal Figure 9.15 Output of the Oscilloscope in fig 9.11 for a 50.5 Hz signal Figure 9.16 Output of the Oscilloscope in fig 9.11 for a 51.5 Hz signal Figure 9.18 Rectifier bridge and the comparator circuit with an indicator & a buzzer Figure 9.19 Complete fault detector circuit .. - -. 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