Two-level monitoring and what the spectrum shows on motors and fans
Trend simple overall values first, and open the spectrum only when limits are crossed. A worked fan case and a table of motor fault frequencies.
Level 1 and Level 2
Level 1 is trend monitoring of overall values (velocity, acceleration, shock pulse) against warning and alarm limits. It is cheap and covers many machines but cannot locate the defect. Level 2 uses an FFT analyser and trained interpretation: you open the spectrum only when Level 1 limits are exceeded, and compare with a reference spectrum from when the machine was good.
A worked case: belt-driven exhaust fan
A 37 kW paint-shop exhaust fan turned unacceptably rough, with 15.2 mm/s at the motor bearing at 1475 rpm (24.6 Hz). The motor looked like the culprit. The spectrum showed the dominant peak was at the frequency of the fan sheave, carried across the belt. Balancing the fan belt wheel brought vibration down to 2.3 mm/s on the fan and 3.2 mm/s on the motor. The worst reading was not at the source.
| Fault | What the spectrum shows |
|---|---|
| Unbalance | Peak at 1x running speed; mostly radial; raised overall level |
| Misalignment | Peak at 2x running speed, sometimes 1x too; radial for offset, axial for angular |
| Loose rotating part (for example belt wheel on shaft) | 1x small, many harmonics of running speed; unchanged with the belt removed |
| Stator field asymmetry (core damage, eccentric armature) | Peak at twice mains frequency, no sidebands |
| Rotor bar faults | Bar pass frequency (running speed times number of bars) with sidebands at twice mains frequency; mains peak with slip frequency sidebands |
Useful formulas
Running frequency = rpm / 60. Slip frequency = 2 x mains frequency / number of poles minus running frequency. Bar pass frequency = running frequency times number of rotor bars.
More practical cases
Brake disc unbalance at 996 rpm was found from a typical unbalance pattern at several points; balancing it cut motor and gearbox vibration. A 1000 rpm generator showed a clear 2x peak that disappeared after alignment, leaving unbalance to correct. A motor with high vibration even with the coupling removed and a large 2x mains peak had a burnt stator core. A press drive with rising noise had almost no 1x but many harmonics; the belt wheel was loose on the motor shaft and the shaft had to be remachined.
Severity
Overall values are classed good, satisfactory, unsatisfactory or unacceptable according to the standard for the machine group. Use the limits for the machine type and its mounting, then use the spectrum to find the cause.