ABB ACH550-UH User Manual page 221

ACH550-UH User's Manual
Point
#
85
Scaling drive feedback values
Feedback values are provided with units of percent, where 0% and 100%
correspond to the range of the sensor being used to measure the control variable.
These points have default units in Hz. If other units are required:
• Unbundle these points with appropriate slopes and intercepts.
• The new intercept equals the lowest value of the desired range.
• Calculate the new slope as follows:
Example – You are controlling water temperature from a cooling tower using the
ACH550 to control a fan. The temperature sensor has a range of 30 to 250 degrees
Fahrenheit.
To unbundle the set point (INPUT REF 2), for commanding in degrees Fahrenheit,
where 0...60 Hz is equal to 30...250
To unbundle the feedback (PRC PID FBCK) for monitoring in degrees Fahrenheit:
New Intercept = 30
Loop gains
PRC PID GAIN (Point 50) and PRC PID ITIM (Point 51) are PID parameters similar
to the P and I gains in the APOGEE TECs. Because the ABB PI loop and the
Siemens loop are structured differently, there is no a one-to-one correspondence
between the gains. The following formulas allow translation from ABB gains to
Siemens gains and vice versa:
Subpoint Name
Type
LAI AO 2 ACTUAL
(Desired Range, i.e. high - low values) x (Slope of Existing Point)
New Slope =
(60 Hz - 0 Hz) x (0.01)
=
100% - 0%
New Intercept = 30 (the temperature that corresponds to 0%)
(Desired Range) x (Slope of Existing Point)
New Slope =
(250 F - 30
°
=
100% - 0%
(Desired Range) x (Slope of Existing Point)
New Slope =
(250 F - 30
°
=
100% - 0%
FLN External PID Report
Range of Existing Point
= 0.006
F:
°
Range of Existing Point
F) x (0.1)
°
= 0.22
Range of Existing Point
F) x (0.01)
°
= 0.022
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Data
Embedded fieldbus