VARIABLE FREQUENCY DRIVE (VFD) DRIVEN
AGITATOR MOTOR
| The MAT was requested to check
horsepower loading on three crystallizer agitator drive motors
powered by Variable Frequency Drives (VFD) because operations
had higher than manufacturer specified loading on the motors.
PdMA Power Analysis Testing was performed on both the input
and output of the inverter (Figure 8). The total KW on the
input side corresponded with the KW displayed on the DCS in
the control room. The total KW on the output side matched the
manufacturer’s estimate for horsepower load in this
application. |
|
|
|
Figure 8: MCE
test results on Crystallizer Motor, test 1 – input side,
test 2 – output side |
During analysis of the test results, we
determined that the instrumentation supplying the DCS was connected to
the input side of the VFD creating higher than expected readings. This
can be seen in Figure 9 where the input and output load readings are
compared. When the voltage waveforms of the VFD output were analyzed,
we also discovered one of the VFD output drivers had a distorted
waveform (Figure 9). This was due to a failing output driver on one
phase of the VFD. This distortion was not severe enough to trigger the
self-diagnostics of the VFD.
 |
|
Figure 9:
Voltage waveforms on output side of VFD feeding crystallizer
motor |
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