Pressure With A Variable Frequency Drive (Vfd) - T775U - Honeywell T775 Series 2000 Application Manual

T775 SERIES 2000 ELECTRONIC STAND-ALONE CONTROLLERS
Pressure with a Variable Frequency
Drive (VFD) – T775U
Application Description
In this application the T775U is controlling duct pressure with
a fan controlled by an NXS or NXL Variable Frequency Drive.
A P7640 pressure sensor, located in the duct, is providing
sensor input to the T775U to control the fan speed. The VFD
is looking for a 4-20 mA PID control signal from the T775U
and will drive the fan with a signal directly proportional to this
T775U output.
For additional information about the NXS or NXL variable
frequency drives (VFD), refer to the VFD Reference Guide,
form 63-9469.
Sensor Designation
This device application requires one sensor.
• Sensor A is sensing pressure at the duct (reference is at
the room).
Operation
In this example, as the sensed pressure decreases, the fan
speed increases. Assume we have a 0-10 Vdc sensor output
for 0-5 inches water column. First, set up a Sensor type of
0-10 Vdc into the T775U Sensor A settings, and a minimum
(0) and maximum (5) inches water column for the sensor
range.
Also set up a 4-20 mA PID modulating output loop (MOD 1) at
the T775U with a setpoint of 2.5 inches and a throttling range
of 2 inches, and a reverse acting action. At a sensed pressure
of 2.5 inches (in other word, at setpoint), MOD 1 will output
close to 50% or about 12 mA. At 1.5 inches water column, the
output will be 100% or about 20 mA, and at 3.5 inches water
column, the output will be 0% or about 4 mA. Keep in mind
these values are valid for proportional control and will differ in
a PID loop where integral (usually desired) and derivative
(less often required) are set to non-zero values.
Integral and derivative time may need to be adjusted, along
with the throttling range.
Programming Example
Program in Setup:
Press and hold the MENU button for 5 seconds to enter
Setup mode, and then select:
→
— Sensors Sensor A:
Type = 0-10V
Units = IN WC
Minimum Value = 0.0
Maximum Value = 5.0
→
— Outputs MOD 1 (Modulating Output):
Type = 4-20 mA
Program MOD 1 for:
— Setpoint = 2.5
— Throttling Range = 2 inches w.c.
— Action = REV ACT
63-7147—05
IMPORTANT
After the desired value is selected, be sure to press
the  or  or HOME button in order to save that
value in the controller's memory.
IMPORTANT
After programming the VFD, be sure to check the
following:
1. Verify that the VFD value P7.1.1.2 (AI2 mode) is set
to 2 (4-20 mA).
2. Verify that the jumper block X2 on the Expansion
board A is in Current Input Mode (a jumper is across
the A terminals, and a jumper is across the B
terminals).
3. Verify that the P7640A pressure sensor is set to:
Output = Voltage
Range = 0 to 5 inches w.c.
Mode = Unidirectional (default)
Volt = 10 Vdc
Wiring
See Fig. 32 for wiring connections for the T775U, the pressure
sensor, and the Variable Frequency Drive.
VFD BOARD A
5 4
MOD 1
C
B S
–
R V
+
W
T
T
Fig. 32. T775U wiring — pressure with a VFD (loop
26
SHIELDED CABLE
NOTES:
1. SHIELDED CABLE MUST BE
SENSOR A
CONNECTED TO A SEPARATE
EARTH GROUND. HOWEVER,
DO NOT GROUND SHIELDED
CABLE AT SENSOR END.
2. TO MINIMIZE NOISE PICKUP,
MAKE SENSOR CONNECTION
FROM SHIELDED CABLE AS
CLOSE AS POSSIBLE TO
SENSOR BODY.
V S C
P7640A
PRESSURE
SENSOR;
0-10 VDC
CONNECTION
powered wiring).
M28006