Rotating machinery
Vibration analysis measures how a running motor or generator shakes at its bearings along three axes, side to side, up and down and along the shaft, to expose imbalance, misalignment, looseness and bearing wear. It is an optional running test used at acceptance and in maintenance on motors, generators and engine-generator sets, and its readings are judged against published tables and trends.
Vibration analysis on rotating electrical machines is the measurement of the mechanical motion of the machine while it is running under load. A transducer is placed on or near each bearing housing and records how fast, how far or how hard the housing moves. Readings are normally taken in three directions at each bearing location: horizontal, vertical and axial, because different faults push the structure in different directions. The measurement is applied to induction motors and generators, synchronous machines, direct-current machines and the engine-generator sets used for emergency and standby power. In the acceptance and maintenance standards the test is listed as an optional running measurement, to be performed when the specification, the size of the machine or its importance justifies it. The overall level gives a quick health indicator, while the frequency content, when the signal is analyzed as a spectrum, tells the analyst which part of the machine is responsible. The test is non-intrusive and involves no disassembly, yet it provides one of the best views of the machine's mechanical condition.
Mechanical problems in rotating equipment are a common cause of unplanned downtime, and vibration is usually the first measurable symptom. Imbalance from a shifted or fouled rotor, shaft misalignment between the machine and its load, a bent shaft, loose mounting bolts, a cracked foundation or a soft foot, worn or damaged bearings, rubbing parts and resonance with the supporting structure all raise vibration in characteristic ways. Left uncorrected they shorten bearing and seal life, fatigue the shaft, loosen connections and can damage the winding or the load. On an engine-driven generator, combustion and mounting problems add to the picture. Measuring at acceptance proves that installation, alignment and balancing were done well and gives a baseline that is unique to that machine in its real foundation. Repeating the measurement during maintenance reveals slow trends that no inspection by eye or ear would catch, so that a bearing can be replaced during a planned outage rather than after it seizes.
Every rotating machine has an ideal behavior in which the rotating mass is balanced, the shafts of the machine and its load share one centerline, and the bearings carry only the intended loads. Any departure from that ideal produces a force that repeats with each shaft revolution or at a related frequency, and the machine structure responds by moving. Imbalance appears mostly at the running speed in the radial directions, misalignment tends to add the second harmonic and axial motion, looseness creates multiples of running speed, and rolling-element bearing defects produce a set of higher frequencies that depend on the bearing geometry. Electrical effects, such as unbalanced magnetic pull or rotor bar problems, add components at line-related frequencies. The size of the motion is described by displacement, velocity or acceleration, chosen to suit the frequency range of interest; velocity is the most common choice for overall machine condition. Because the structure and its mounting influence what the transducer sees, the same fault can look different on a stiff foundation and on a flexible one, which is why baselines and trends matter as much as absolute values.
The machine is run at its normal operating speed and, if the specification asks for it, under load, and is allowed to reach a stable temperature before readings are taken. A calibrated accelerometer or velocity sensor, or a portable analyzer with a probe, is attached to each bearing housing at defined locations, using a stud, a magnet or a hand-held probe, following the manufacturer's and the instrument's guidance for repeatable placement. At each bearing, readings are taken in all three of those directions, and the same measuring points are reused on every visit so results can be compared. For a basic survey the overall vibration level is recorded; for fuller diagnosis a spectrum, and sometimes a time waveform or phase measurement, is captured. Conditions that affect the reading are noted, such as speed, load, temperature, the state of the coupling and whether neighboring machines are running, because they can transmit vibration through the structure. Technicians follow site rules for working around rotating equipment: loose clothing, exposed couplings and open guards are hazards, and no sensor cable should be allowed to approach the rotating parts.
The record identifies the machine, its rated speed, the operating and load condition during measurement, and the date and instrument used. It documents the measurement points and directions, the overall readings obtained at each, and the spectra or waveforms where they were taken, along with the units and the quantity selected. Notes cover temperature, the state of the coupling and any nearby sources of vibration, as well as the as-found condition of the mounting and foundation. Any abnormal sounds, resonances or changes with speed or load are written down so the results can be read in context.
ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 refer to published vibration guidance, including a published table of permitted levels in the acceptance document for rotating machines. The reading at each bearing and direction is compared with that table, and with the manufacturer's own limit when it is stricter. Beyond the absolute level, the shape of the spectrum, the change from the previous survey and the difference between sister machines all contribute to the judgment, because a modest reading that is climbing steadily can be more concerning than a higher but stable one. Resonance or structural effects can inflate readings and call for a closer look at the foundation. The governing numerical limits are in the named standards and the machine data, and this page deliberately does not reproduce them; consult the current edition for the values that apply to your machine.
At acceptance, vibration is a proof of installation quality: alignment, balance, grouting and mounting are all tested at once, and the results become the baseline. In maintenance, periodic surveys on a route, with the same instrument and positions, trend the machine's condition and trigger corrective action before failure. Induction and synchronous machines are measured much alike, though synchronous machines may also show effects tied to the field. Direct-current machines add commutator and brush related effects. Engine-generator sets have a rich and complicated spectrum because of the reciprocating engine, so the baseline and manufacturer guidance carry extra weight. Machines driven by variable-frequency drives are checked across their speed range, and critical or large machines may use permanently installed monitors with alarm levels. The test is optional in the standards but is nearly always worthwhile on large or important machines.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
Different faults move the machine differently. Imbalance mostly shows up in the radial directions, while misalignment often adds axial vibration. Measuring horizontal, vertical and axial motion at every bearing gives a fuller picture and helps pinpoint the cause.
It finds imbalance, shaft misalignment, bent shafts, looseness and soft foot, bearing wear and damage, rubbing, and resonance with the structure. On engine-generator sets it also reflects engine and mounting conditions. Many electrical problems also leave a vibration mark.
Where the specification calls for running under load, yes, because load changes the forces and sometimes the readings. Readings should be compared only with others taken at a similar speed and load, so the operating condition is always recorded.
It is listed as an optional running measurement in ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023, so it is performed when the specification or the importance of the machine calls for it. It is very common on large or critical equipment.
The interval is set by the machine's criticality and the owner's program, commonly from monthly checks on critical machines to longer cycles on less important ones. Consistency of method and position matters more than the exact interval.
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