Motor vibration analysis

Sometime it is experienced high vibration levels. The cause may be an electromagnetic or mechanical imbalance, loose components, rubbing parts, bearing failure, or resonance.

When a problem occurs, it is critical to quickly identify the source of the trouble and take corrective action. Using techniques we can save money and downtime.

Vibration levels shown in the Table below are considered excessive. If you find these levels during inspection, quickly resolve the situation for motor reliability.

Shaft vibration vs. housing vibration

The necessity of obtaining housing vibration data vs. shaft vibration data depends on the problem. It is often desirable to obtain both, especially on a machine with sleeve bearings. If the problem originates in the housing or motor frame, it can be analyzed by vibration meter or vibration analyzer. follow OEM guideline or manual for vertical , horizontal or axial measurement.

Troubleshooting procedure

·         Ensure that all the bolts are tight.

·         Ensure that any part of the system (such as the junction box top cover or piping) is not vibrating excessively.

·         Ensure that the foundation or the motor frame is not vibrating at a higher level (more than 25%) than the motor. If so, there could be a weak motor base.

·         Are any parts of the motor or shaft loose?

·         Ensure that fan blades are not eroded or broken off?

·         If the above items are satisfactory, but vibration remains high, perform a thorough vibration analysis.

You can diagnose a vibration problem in two steps. First, obtain vibration data, but remember factors like noise, combined signals, and amplitude modulation can influence the data. Obtain vibration measurements with the motor operating under the following conditions:

·         Loaded, coupled, full voltage, all conditions stabilized. The first measurements you take should represent the machine under normal operation and may indicate the tests you should perform next.

·         Unloaded, coupled, full voltage. This condition removes some, but not all, load-related vibration.

·         Unloaded, uncoupled, full voltage. This condition removes all effects of the coupling and mechanical load driven by the motor. It isolates the motor/base system from the measurement.

analyze as under -

Acceptance of vibration levels during plant operations :

Motor vibration analysis

Type 1	-	Small industrial machines, e.g electric motors below 15 kW
Type 2	-	Medium size electric motors, 15-75 KW without special foundations.
Type 3	-	Large motors or machines consisting largely of rotating masses, mounted on rigid foundations having high rigidity in the direction of measurement.
Type 4	-	Type 3 but without rigidity in the direction of measurement.