Electric Motor Troubleshooting Using Vibration Analysis
Vibration is technically defined as the oscillation of an object about its position of rest.
Before discussing the technology as a troubleshooting tool, a review of terminology is important.
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 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 coils, or unbalanced resistance between any of the three current phases. These variations will affect the vibration spectra whether or not the motor is loaded. Any or all of these defects will appear on the spectrum analyzer as a high amplitude peak at 2 X line frequency with the absence of side bands around the 7,200 CPM frequency.
Broken, loose wires, or poor connectors may be seen as sidebands at ⅓X line frequency on each side of the 7,200 CPM peak. Electromagnetic force unbalance due to variations in rotor current is commonly caused by broken or cracked rotor bars, broken, cracked or poorly brazed end ring joints, high resistance end ring joints, or shorted or loose rotor laminations. These conditions will most often occur on the spectrum analyzer when the motor is under load.
This frequency may occur at 1X RPM and may be mistaken for unbalance. When pole passing frequency side bands are present at three or four x running speed harmonics with amplitudes higher than .0125 to .0150 inches per second peak, there is uneven motor heating that can cause the rotor shaft to overheat and bend. This condition will cause the electro-magnetic unbalance to increase, generating even more heat. When this condition is suspected, confirm the motor’s phase with a stroboscope. Any rotor flexing will cause temperature increases that will show phase changes. This is unlike misalignment, because vibration amplitude at 1X RPM and phase will stabilize at a particular RPM if there is a misaligned condition. The condition may increase to a point where the bowed rotor will contact the stator causing catastrophic failure.
4. Do your analysts understand faults caused by internal motor problems?
Logic: air gap between rotor and stator affects induced rotor current. The air gap should be less than five per cent of the total radial air gap between stator and rotor, and can be measured with a feeler gauge. Static eccentricity is a condition where the minimum air gap is fixed at one condition. This is commonly caused by such conditions as soft foot distortion, distorted stator core, worn sleeve bearings (where used), or non-concentric rolling element bearing housings in the motor end bells. This condition shows up as a high amplitude peak at 2 X line frequency of 7,200 CPM, with harmonics, but no side bands.
Dynamic eccentricity is a condition where the minimum air gap “moves around” the stator bore as the rotor turns caused by an eccentric rotor, bent shaft, a rotor running at resonant speed, misaligned coupling or an unbalanced overhung fan. The vibration spectrum may show high amplitude at 1X RPM and may have pole passing frequency side bands. These problems occur when the motor is partially loaded. If the stator and rotor slot teeth are not equidistant, there will be reluctance variations in magnetic forces, causing motor torque variations.
Torque pulses may excite loose or broken rotor bars or end rings, loose windings, laminations or supports in the stator. This will be indicated by 2 X line frequency (7,200) with harmonics and will appear as a mechanical looseness condition if multiples of the 2 X line frequency appear in the spectrum. Loose stator coils in synchronous motors will generate fairly high vibration at coil pass frequency (CPF) that equals the number of coils x RPM (#coils X poles X # coils/pole). The CPF will be surrounded by 1X RPM sidebands.
High amplitude peaks at 60,000 to 90,000 CPM accompanied by 2 X line frequency sidebands may indicate synchronous motor problems. Analysts should obtain at least one spectrum up to 90,000 CPM at each motor bearing housing. Loose or open rotor bars in AC induction motors are indicated by a 2 X line frequency sidebands surrounding rotor bar pass frequency and/or its harmonics (rotor bar pass frequency = number of bars X RPM), often will cause high levels at 2 X RBPF, with only a small amplitude at 1X RBPF. Electrically induced arcing between loose rotor bars and end rings will often show high levels at 2X RBPF (with 2 FL sidebands), but little or no increase in amplitudes at 1X RBPF. Electrical fault frequencies, including cracked or broken rotor bars, loose rotors, loose transformer laminations and eccentric stators or rotors are all non-synchronous.
In addition to vibration analysis, electrical equipment might include magnetic flux, motor current and circuit analysis that will prove highly beneficial in making the correct diagnosis. MRO
L. (Tex) Leugner, the author of Practical Handbook of Machinery Lubrication, is a 15-year veteran of the Royal Canadian Electrical Mechanical Engineers, where he served as a technical specialist. He was the founder and operations manager of Maintenance Technology International Inc. for 30 years. Tex holds an STLE lubricant specialist certification and is a millwright and heavy-duty mechanic. He can be reached at firstname.lastname@example.org.