Rockwell Automation 1440-VAD02-01RA XM-123 Aeroderivative Module - Manual (Page 62)

Publication GMSI10-UM003D-EN-P - May 2010
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Overall Time Constant For RMS measurements, the Overall Time Constant
parameter sets the 3-DB bandwidth for the digital
filtering used to calculate the Overall Value. The
3-dB bandwidth is roughly equal to 1 / (2
π
x Overall
Time Constant). The greater the Overall Time
Constant, the slower the response of the measured
Overall Value to change in the input signal.
For example, an Overall Time Constant of 0.1
seconds may be appropriate for monitoring the
Overall Value of an input signal with a fundamental
frequency of 10 Hz and above. Although, the
response to a step change in input will take
approximately 2.2 times the Overall Time Constant to
settle. Therefore, for an Overall Time Constant of 0.1
seconds, the settling time will be approximately 0.22
seconds.
For True Peak measurements, the Overall Time
Constant sets the decay rate of the peak detection
meter. The greater the Overall Time Constant, the
slower the Peak is decayed.
Enter a value greater than 0 (zero).
Recommended Value:
The recommended values are
appropriate for a typical 50/60Hz
machine, and may need to be adjusted
depending on the application.
•
For True Peak or True
Peak-to-Peak measurements, set
the Overall Time Constant to 1.5.
•
For RMS, Calculated Peak, or
Calculated Peak-to-Peak
measurements, set the Overall Time
Constant to one of the
following:
Overall Damping Factor This parameter is used in conjunction with the
Overall Time Constant to vary the characteristics
of the response of the digital filter used in
calculating the Overall Value.
An Overall Value for a measurement with a damping
factor near 1.0 (critical damping) will slowly rise or
fall for the full settling time specified by the Overall
Time Constant before reaching the final value. An
Overall Value for a measurement with a damping
factor near 0.707 will rise or fall quickly and may
"overshoot" (measure a value greater or less than the
final value) before reaching the final value for a
given input signal.
Enter a value from 0.707 to 1.0.
Overall Filter The filter to be applied to the overall measurement
to produce the Overall Value. See Data Parameters
on page 76. Options: None
Low Pass Filter
Low Pass Filter Sets the frequency above which the input signal will
be significantly attenuated. Enter a value from 200 to 20,000 Hz.
Note: This value is used only when
the Overall Filter is set to "Low Pass
Filter."
Overall Measurement Parameters
Parameter Name Description Values/Comments
High Pass
Filter
Overall Time
Constant
1Hz 0.16
5Hz or above 0.045