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Machinery and Equipment MRO
December 2020
ELECTRIC MOTOR TROUBLESHOOTING USING VIBRATION ANALYSIS Vibration is technically defined as the oscillation of an object about its position of rest. BY L. (TEX) LEUGNER
Frequency - refers to “how many” oscillations in a given length of time (e.g., one minute), measured in cycles per minute (CPM), or cycles per second (hertz) Hz, related to 1X shaft turning speed. Displacement - refers to “how much” the object is vibrating measured in Mils (1/1000 inch) peak-to-peak. Displacement (distance or movement) is generally the best parameter to use for frequency measurements up to 600 CPM. Velocity - indicates “how fast” the object is vibrating measured in inches/second or mm/second peak. Velocity is frequently used for machinery vibration analysis where important frequencies lie in the 600 to 60,000 CPM range. Acceleration - of the object that is vibrating is related to the forces that are causing the vibration measured in “gs” (1 g = 32 ft/sec2 or 9.8 m/sec2) and is reported or shown as root mean squared (RMS). Acceleration (force) is best measured when all the troublesome vibrations occur at frequencies above 60,000 CPM. 1. How does your facility determine the correct selection and application of transducers? Logic: the quality of data gathered by vibration analysis is dependent upon proper selection and mounting of transducers. If possible, vibration readings should be taken with the transducer
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mounted perpendicular to the surface in the horizontal, vertical and axial directions. Vibration signals containing “high frequencies” must be taken with an accelerometer tightly screwed, or glued to the surface, since hand held pressure alone cannot hold it tightly enough to the surface to follow high frequency motion. Displacement non-contact proximeters are used to look directly at the rotating shafts of machinery and the frequencies obtained will be quite low. A velocity transducer’s sensitivity drops off dramatically at speeds below about 600 RPM. Accelerometers have the advantage of having adequate sensitivity over a wide range of frequencies. The low end is typically one to three Hz, while the upper range can be as high as 20 kHz. For this reason, accelerometers are the preferred device to use. 2. Can your analysts determine the causes of vibration problems in electric motors? Logic: motors use electromagnetic forces in addition to mechanical forces and exhibit some characteristics that differ from purely mechanical rotating machinery. For example, a motor shaft may “bow” or “bend” due to excessive localized heating from shorted laminations. The spectrum of this condition will appear as unbalance, but balancing the shaft and attached components will not solve the problem. Unlike a purely mechanical machine, a motor has a multitude of frequencies generated by the electromagnetic forces inherent in the machine. The magnetic flux produced by conductors in AC machines alternates at line frequency and all AC motors produce a
2 X line frequency vibration. In systems that operate at 60Hz, the vibration frequency is 120Hz. In an induction motor, the poles in the stator winding produce a rotating magnetic force that acts across the air gap between stator and rotor. This produces a 120Hz vibration of the stator and the number of poles in the winding determines the mode shape of this vibration. The number of poles and line frequency also determines the synchronous speed of the rotating magnetic field and running speed of the motor. The difference is referred to as “slip”. The number of rotor bars x rotor speed in RPM, and harmonics of the rotor bar passing frequency may indicate a problem. 3. Can your vibration analysts determine the potential causes of electric motor unbalance? Logic: AC motors may have unequal magnetic forces causing unbalance that may be caused by variations in the current in the stator or rotor, or air gap variations between the rotor and stator, or a combination of these conditions. These variations in AC current may be caused by weak or loose stator support, shorted or loose stator laminations, shorted or open windings, electrical unbalance between two consecutive conductor
Photo: Getty Images / sarinyapinngam
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efore discussing the technology as a troubleshooting tool, a review of terminology is important.
2021-01-13 8:48 AM
