Case Studies In Online And Offline Motor Analysis

CASE STUDIES IN ONLINE AND OFFLINE MOTOR ANALYSIS David L. McKinnon, PdMA Corporation, Member, IEEE Abstract-In this paper we present three case studies Problem using online and offline motor analysis to prevent During routine EMAX testing in June 2003, it was catastrophic motor failures. The online and offline analysis observed that the peak dB level of the pole pass sideband in our case studies use a battery of standard electrical tests including Current Signature Analysis (CSA) and was 0.7419 dB, which exceeded the alarm set point of Demodulated Current Spectrum Analysis (DCSA). 0.3 dB (Figure 1). Rotor bar problems were suspected, however, when vibration analysis was performed the Index Terms - Current Signature Analysis (CSA), results indicated a healthy motor. Due to the conflicting Demodulated Current Spectrum Analysis (DCSA), Rotor indications between the EMAX and vibration tests, the Influence Check (RIC), DC Motors, Armature, Field, decision was made to monitor the motor and trend the Motor Testing, and Measurement. test results. I. INTRODUCTION Current Signature Analysis (CSA) has become the standard for detecting broken rotor bars by analyzing the sidebands around line frequency. Another useful tool is Demodulated Current Spectrum Analysis (DCSA) that enhances the ability to detect broken rotor bars especially on two pole motors. Our first case study will present a situation where CSA and DCSA were used to detect broken rotor bars on a two pole motor. The Polarization Index (PI) is a standard measurement in the IEEE 43-2000 standard for insulation testing. PI is a ratio of the measured resistance at ten minutes divided by the resistance measurement at one minute. This ratio can be used for analyzing the general health of the insulation system of the motor. In our second case study, we present a modification to the standard polarization index test. By plotting the resistance measurement every five seconds, a graph Figure 1 - Demodulated Current Spectrum showing the Pole Pass Sideband at 0.7419 dB. called a Polarization Index Profile (PIP) is obtained. The resulting PIP profile can then be used for additional analysis of the insulation system that cannot be obtained Action Taken from the standard polarization index. The motor was retested periodically until 5/12/2004, Power Quality is a measure of the quality of the at that time it was removed from service. The trended voltage and current supplying a motor or other load. By data indicated a 1420% increase in the peak level of the analyzing the harmonics, voltage and current unbalance, pole pass sideband from 0.1814 dB at a running speed of over or under voltage, and over current conditions a 3591 RPM on 8/15/2001 to 2.5851 dB at a running speed technician can determine what may be causing nuisance of 3592 RPM on 5/12/2004 (Figure 2). Trending the data trips, voltage swells or sags, and other power system from tests taken between 2001 and 2004 showed an related situations as we will examine in our third case exponential increase in the pole pass peak levels, which study. is typically indicative of at least one or more broken rotor bars. Additionally, there was a 275% increase in II. CASE STUDY 1 - BROKEN ROTOR BARS the load variation, from 0.855% in 2001 to 2.345% on 5/12/2004 (Figure 2). The load variation should be Motor Specifications: 3500 HP, 2 Pole, AC Induction, constant from test to test under normal operating and 4160 Volts, 3590 RPM. motor conditions. The Current Spectrum (Figure 3) of test data taken on 5/12/2004 showed an increase in sideband activity around the fundamental frequency, which also indicated broken rotor bars.
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Figure 2 - Pole pass sideband of 2.5851 dB at a running speed of 3592 RPM. Figure 4 - Rotor Influence Check (RIC) test of the motor before disassembly. Notice the repeated variations in the inductance waveforms, which are indicative of broken rotor bars.
Figure 5 - Some of the broken rotor bars found using online and offline tests.
Root Cause Figure 3 - Hi Resolution Current Signature Spectrum centered around It was determined that bad braze ... [download for more]