Fault Detection

As with other predictive maintenance technologies, trending is the best method of fault detection. Trending methodology also applies to fault detection in VFDs. By trending data over a period of time, a technician is better able to detect developing problems in motors or power systems. Occasionally, faults occur that did not exhibit any trends before the occurrence, these types of failures are considered random in nature. This article covers faults that have occurred, yet, the drive continues to operate in a mode that appears normal externally.
Upstream (Line Side) Diagnosis

Upstream (Line Side) diagnosis of the VFD (not diagnosis of the motor) refers to testing on the incoming power to the VFD as shown in Figure 1. By testing upstream of the VFD, open diodes and open phases can be detected using the MCEmax. Upstream diagnosis is only useful for diagnosing the VFD, do not perform overall motor diagnostics upstream of the VFD.

Figure 1 - Upstream Diagnosis

Figure 1

Open diode

Using the MCEmax technology, a technician is able to reliably detect an open diode fault (Figure 2) using the results from a MCEmax Power Test.

 Figure 2 - Open Diode

Figure 2

Table 1

The phase current (Current RMS) of the phase with the open diode will

decrease approximately 50% on the affected phase,and increase slightly on the other two phases (Table 1).

Table 2

Percent (%) Current Imbalance will increase to

approximately 50% on the affected phase (Table 2).

Table 3

Current THD will decrease by approximately 50% on the affected phase,

and increase slightly on the other two phases (Table 3).

Table 4

kW will decrease by approximately 50% on the affected phase,

and increase on the remaining phases (Table 4).

Table 5

kVAR will decrease by approximately 20% on the affected phase,

and increase by approximately 40% on the remaining phases (Table 5).

Table 6

kVA will decrease by approximately 20% on the affected phase, and increase by approx 40% on the remaining phases (Table 6).

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