Operating Instructions

SED2 VFD Operation & Maintenance Manual
92 Siemens Building Technologies, Inc.
When using this strategy, the point to unbundle and command for the setpoint is FREQ REF
(Point 51). FREQ REF (Point 51) has a slope of 0.006103516, which corresponds to a value
of 16384 (4000h)=100%.
Unbundle PI FEEDBACK (Point 60) to monitor the control variable (water temperature).
These points are provided with units of percent, where 0% and 100% correspond to the
range of the sensor that measures the control variable. Therefore, if you want to read in the
proper units of the controlled variable (such as degrees Fahrenheit, WC), unbundle the
setpoint with the appropriate slope and intercept from the point database:
×
×
(Desired Range) (Slope of Existing Point)
New Slope =
Range of Existing Point
(60 - 0 HZ) (0.01)
= = 0.006
100 - 0 %
The new intercept equals the lowest value of the desired range.
Example: The following example illustrates this conversion procedure.
You are controlling water temperature from a cooling tower using the SED2 to control a fan.
The temperature sensor has a range of 30 to 250 degrees Fahrenheit.
To unbundle the setpoint (FREQ REF), for commanding in degrees Fahrenheit, where 0 to
60 Hz is equal to 30 to 250 degrees Fahrenheit:
New Intercept = 30 (the temperature that corresponds to 0%)
New Slope = (Desired Range) × (Slope of Existing Point)
Range of Existing Point
= (250 - 30 degrees Fahrenheit) × (0.006103516)
= 0.01343
100 - 0%
To unbundle the feedback (PI FEEDBACK) for monitoring in degrees Fahrenheit:
New Intercept = 30
New Slope = (Desired Range) × (Slope of Existing Point)
Range of Existing Point
= (250 - 30 degrees Fahrenheit) × (0.01)
= 0.022
100 - 0%