Isolating the source of frequencies found in energized electric motor spectral data can be quite problematic. To fully analyze spectral data, we need to determine what is normal and what poses a potential problem and then make maintenance determinations. If your motor test instrument or power quality analyzer has the capability of simultaneous capture of both voltage and current with display capability in overlaying spectrums, some isolation is possible.

First, we need to understand that the power anomalies that affect voltage will affect current and power anomalies that affect current will affect voltage. Given this fundamental relationship, it is possible to provide some localization of the current or voltage source in relation to where we are connected within the system. Mechanical anomalies, in general, will affect motor torque and thus affect the magnetic coupling between the rotor and stator, which will have an effect on current and not necessarily voltage. Awareness of these properties can assist in source localization.

With an FFT of both current and voltage, look for spectral frequencies where both are present, then the following can be used to determine the source relative to your circuit connection point:

1. If the magnitude of the voltage is greater, the source is upstream and is affecting the current.

2.  If the magnitude of the current is greater, the source is downstream and is affecting the voltage.

3.  If the spectral peak is only present in current, it is a mechanical anomaly and downstream.

(Reverse if you are testing the output of a generator)

Once the source is localized, say for example the dominant peak is in voltage, we can test other loads on the common bus, and when we find the load that has that frequency with current being the dominant; that load is the source of the problem.