Consideration for Electrical Signature Analysis in Variable Frequency Drives

For some reason, variable frequency drive driven equipment seems to carry a mystique for analysis. In truth, it just requires the user of diagnostic technologies the ability to expand the way they think when they look at data. Fault signatures and peaks are just the same as a standard motor, with the following exceptions:

- Currents should be balanced within 7% because voltage must be balanced on the output of the drive. If there are voltage related unbalances, then the drive should be evaluated, if a current unbalance exists without a voltage unbalance, then the motor and connections should be evaluated. It should be noted that the allowable current unbalance is meant for certain winding designs that are common in ‘smaller’ (less than 250hp) motors that have ‘concentric’ windings versus larger machines that have ‘lap’ windings.

- Expect very high harmonic content because of the type of voltage and current waveforms that exist. In most cases, this requires the application of filters or shaft brushes to avoid shaft currents.

- The line frequency and speeds will vary from test to test or during the same test. Evaluating and confirming the line frequency and operating speed, either through the signature or separate readings, is important as virtually all faults are related to one or the other. It is also important for when performing trending readings.

- You have to look past the electrical ‘noise’ that will exist in higher frequency data analysis to identify the issues in the system. This means knowing the fault signatures that you are looking for or practiced pattern analysis (covered in Electrical Motor Diagnostics: 2nd Edition).

There are other considerations that we will cover in future newsletters.