Fieldbus control 315
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
ACS320 to control a fan. The temperature sensor has a range of 30 to 250 degrees
Fahrenheit.
To unbundle the setpoint (INPUT REF 2), for commanding in degrees Fahrenheit,
where 0…60 Hz is equal to 30…250
° F:
To unbundle the feedback (PRC PID FBCK) for monitoring in degrees Fahrenheit:
New Slope =
(Desired Range, ie, high - low values) x (Slope of Existing Point)
Range of Existing Point
=
(60 Hz - 0 Hz) x (0.01)
100% - 0%
= 0.006
New Slope =
(Desired Range) x (Slope of Existing Point)
Range of Existing Point
=
(250° F - 30° F) x (0.1)
100% - 0%
= 0.22
New Intercept = 30 (the temperature that corresponds to 0%)
New Slope =
(Desired Range) x (Slope of Existing Point)
Range of Existing Point
=
(250° F - 30° F) x (0.01)
100% - 0%
= 0.022
New Intercept = 30
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