Vibration Monitoring and Current Analysis of AC Motors Using Motor Current and Vibration Analysis to Detect AC Motor Problems Summary Jason Mais 22 pages November 2002 SKF Reliability Systems @ptitudeXchange 4141 Ruffin Road San Diego, CA 92123 United States tel +1 858 244 2540 fax +1 858 244 2555 email: info@aptitudexchange.com Internet: www.aptitudexchange.com Use of this document is governed by the terms and conditions contained in @ptitudeXchange This article contains an extensive summary, and several examples, of analyzing AC motors using vibrational data and motor current analysis The first example references a large pulper motor in a paper mill operation, in which a shortened stator was causing the motor to fail The second example is in reference to an electric motor manufacturer in Israel that uses motor current analysis and vibration analysis to test electric motors in field applications The third example considers the monitoring of electric motors used in pumping stations The fourth example deals with a company using motor current analysis as a quality control, or testing process to establish the health of the motors being manufactured In the fifth case, a few more examples of pole pass frequencies are shown Finally, a case of motor excentricity is provided Vibration Monitoring and Current Analysis of AC Motors Introduction Apart from vibration analysis, motor current signature analysis (MCSA) is a powerful monitoring tool for electric-motor-driven equipment It provides a non-intrusive means for detecting the presence of mechanical and electrical abnormalities in motor-end driven equipment, including altered conditions in the process that may be downstream of the motor driven equipment MCSA is based upon the recognition that a conventional electric motor powering a machine also inevitably acts as a transducer of variations in the driven mechanical load, as the latter are converted into electric current variations that are transmitted along motor power cables These current variations, though very small in relation to the average current drawn by the motor, can be extracted reliably and nonintrusively at a location remote, and processed to provide indicators of condition (signatures) These signatures may be trended over time to give early warning of performance degradation or process alteration Although MCSA technology was developed for the specific task of determining the effects of aging and service wear on motor-operated values used in nuclear power plant safety systems, it is recognized as applicable to a much broader range of machinery MCSA is used to analyze pumps of various design, blowers, compressors, and air-conditioning systems powered by AC and DC motors minimal The resulting raw current signal is amplified, filtered, and further processed to provide a sensitive and selective means for extracting motor current noise information that reflects instantaneous load variation within the drive train and the ultimate load Figure SKF Microlog and AC/DC Current Clamp This article provides an extensive summary of several case studies that relate to the use of motor current signature analysis and vibration analysis The reader is also referred to the article entitled An Introduction into Motor Current Spectrum Analysis (MCSA), JM02010 on the @ptitudeXchange website Summary of AC Motor Monitoring The use of devices, such as a data analyzer Mechanical Vibration (the instrument on the left in Figure 1), and Using a standard accelerometer placed on the current clamp (the instrument on the right in bearing cap detects several unique mechanical Figure 1) are common to collect and analyze vibration signals generated by electrical faults vibration and motor current data from an AC in the motor circuits One of the more motor These types of devices obtain data in a common faults produced is a signal at twice non-invasive manner, which is important to line frequency (2FL or F.L.) If the line the operating process The use of a single frequency is 60 Hz, this signal is at 120 Hz or split-jaw current probe placed on one power 7200 CPM If the line frequency is 50 Hz, the lead is sufficient to obtain data Since there is signal is at 100 Hz or 6000 CPM Care must no electrical connection being made or be taken when testing pole motors (3600 broken, the possibility of shock hazard is © 2002 SKF Reliability Systems All Rights Reserved Vibration Monitoring and Current Analysis of AC Motors RPM or 3000 RPM) to ensure the signal is not twice rotation speed of the shaft, rather than twice line frequency This two times line frequency signal is created by any of the following faults: Uneven air gap between rotor / stator As the motor poles pass the narrow gap, the magnetic pull is greater v 180 degrees on the opposite side where the gap is the widest The number of poles (motor speed) does not change the results: an uneven air gap results in a signal at twice line frequency The cause of this uneven air gap is often due to an uneven base plate under the machine’s foundation, and is referred to as a soft foot Some empirical data seems to indicate that the twice line frequency signal appears when the gap clearance exceeds 10% variance Loosening and tightening one bolt at a time on the foundation, with the motor running, while observing the spectrum on the data analyzer, can confirm soft foot When the soft foot is loosened, the signal decreases, and then increases as the nut is tightened If soft foot occurs the foot should be shimmed during the next shutdown to the same plane as the others Damage stator windings or insulation There are numerous causes of damage to the stator: poor manufacturing, poor environment, flaws in the insulation, etc Any damage to the stator again creates an uneven magnetic field around the rotor This uneven field generates an uneven pull on the rotor, regardless of motor speed, and causes a mechanical vibration at twice line frequency It is often possible to locate the area of damage with either an infrared or thermal detector (refer to JM02008 - Thermography on the @ptitudeXchange website) Usually there is an area on the motor housing where the surface temperature is elevated 20 to 30 degrees A damaged stator also generates a mechanical vibration signal at a frequency equal to the number of rotor bars, multiplied by rotation speed Again, in the area of stator damage, the magnetic field is weakened, and stronger 180 degrees away As each rotor bar passes this area of higher strength, the bar is mechanically pulled in that direction Typically, rotor bars have between 45-55 bars in the rotor, but this can vary depending on the manufacturer For this reason, it is very important to set the Fmax at least 100 times rotation speed when troubleshooting motor vibration Please note that this Fmax value is for troubleshooting purposes only Since the number of rotor bars can vary greatly, it is most important to establish a procedure that states that at anytime a motor is down for repair, a count of the actual number of rotor bars should be made and recorded It is also important to record the bearing nomenclature so that the bearing frequencies can be accurately determined when analyzing for bearing degradation The user can verify that the vibration is electrically induced by shutting off the motor while observing the frequency spectrum in the analyzer mode The moment the power is removed, the distorted magnetic field is instantly collapsed and the twice line signal disappears If the signal does not disappear, but rather slowly degrades, then the user knows there is some type of mechanical problem When setting up an analyzer, use 100 lines, averages, and an Fmax of 2000 Hz to provide a fast cycle time If the data analyzer has enveloping circuits, and these two conditions are met, then the signal is also seen in any enveloped acceleration spectrums and will most certainly © 2002 SKF Reliability Systems All Rights Reserved Vibration Monitoring and Current Analysis of AC Motors generate harmonics of the fundamental frequency There is not an agreed upon amplitude of concern if the twice line frequency signal is present It is generally agreed that the presence of 2x line frequency is not desirable Generally accepted limits are between 0.04 0.06 IPS in the velocity spectrum at twice line frequency As an example of the amplitude levels of 2FL, a trend of an enveloped acceleration reading, in which twice line frequency was taken over a six months period, shows an increase from 0.4 gE to 1.6 gE When the motor reached a vibration level of 1.6 gE, the motor failed However, after the motor was repaired, the trend was reestablished, and the amplitude of that trend consistently remained at 0.8 gE It is suspected that the first failure was due to a damaged stator In addition to the stator problem, soft foot concerning the foundation was also present After repairing the motor, the soft foot contribution is still present, although it exhibits itself differently This is most likely due to torque on the mounting bolts in addition to machine placement and configuration Sidebands As with most vibration signals, the presence of sidebands around fundamental frequencies is a measure of an increase in severity As the sidebands increase in number and amplitude, so does the severity of the problem Some of the sideband energy is pole pass frequency and slip Pole Pass Frequency =(number of poles)(slip) Slip = (nominal speed - actual speed) may find it necessary to increase resolution to either 1600 or 3200 lines to separate these sidebands and verify the existence of this energy Analysis of AC Motor Current The effectiveness of evaluating motor condition by performing an FFT of the motor current is verified many times over in the analysis of motors And, although it is often referred to as a method of detecting broken rotor bars, it is actually detecting abnormally high resistance in the rotor circuit (bad solder joints, loose connections, and damaged rotor bars) At this point, a review of basic spectrum components is necessary to ensure a clear understanding of vibration analysis If there is a fault in the rotor circuit, then the spectrum has two prominent features when displayed Using a logarithmic scale for clarity concerning amplitude peaks with respect to the Y-axis, the display at 60 Hz or line frequency contains a large spike To the left, at a distance equal to the rotor slip, times the number of poles, pole pass frequency is another spike of energy These spikes can be labeled A for line frequency, and B for pole pass frequency Note that the amplitudes of peaks A and B have to be obtained using cursor overlays, as it is necessary to use amplitudes to four decimal places, and most data analyzers only read the value to three places To determine the health of the machine, perform the following calculation: Log (A/B) times (20) = amplitude (dB) Examples are included in Figures 2-4 Around the rotor bar pass frequency it is possible to see sidebands of twice line frequency In troubleshooting, the data analyst © 2002 SKF Reliability Systems All Rights Reserved Vibration Monitoring and Current Analysis of AC Motors Category Number dB >60 A/B Rotor Condition Corrective Action Excellent None Good None Moderate Trend Rotor Bar May be Cracked or High Resistance Joints Increase Monitoring Time Two Rotor Bars Cracked and High Resistance Joints Vibration Testing >1000 54-60 5001000 48-54 250500 42-48 125250 36-42 63125 30-36 32-63 Multiple Rotor Bars Broken, Slip Ring and Joint Problems Overhaul