Analyzing vibration data can be a difficult job. Using VFD (Variable Frequency Drive) motors can make it even more difficult. Pattern recognition is a huge part of this process. However, diagnosing between electrical and mechanical vibration patterns can be challenging.
Is it a Mechanical or Electrical Problem?
First, a straightforward way to distinguish between mechanical or electrical problems is to go into the “live” mode on your data collector to monitor the vibration in question then have the motor shut off. If the pattern disappears instantly, it is electrical. If not, a mechanical problem should be investigated.
Another way to check, is to look for common electrical frequencies. For instance, 7200 CPM AC and 21600 CPM DC are common frequencies to look for on steady state non VFD or DC driven motors. Taking a high-resolution reading and exact motor RPM are essential in this procedure.
Calculating for VFD frequencies is straightforward. Verify the exact motor RPM then divide by maximum motor speed.
Example: An 1800 RPM motor running on VFD at 1500 RPM. 1500/1800 = .833 or 83%. Multiply the electrical frequency of 7200 CPM by the ratio from above to get VFD frequencies: 7200 X .8333 = 6000 CPM. Look for electrical frequencies at this frequency. Be sure to use a strobe light or other device to get an accurate running speed.
These frequencies can look very much like a mechanical problem however with basic troubleshooting skills, they can be easily diagnosed.
