T775 SERIES 2000 ELECTRONIC STAND-ALONE CONTROLLERS
11 63-7147—05
Cooling Tower Control – T775B2040
Application Description
The T775B2040 is controlling a cooling tower fan, based on
two sensor inputs, to control the low and high fan speeds and
provide cold weather shutdown.
NOTE: T775R2001 reset model can also be used in this
application.
Sensor Designation
This device application requires two sensors:
• Sensor A is sensing the sump temperature.
• Sensor B is sensing the outside air temperature.
Operation
As the sump temperature increases, the low fan speed is
energized by Relay 1. On further increase in temperature,
Relay 2 closes, which energizes the high fan speed and shuts
off the low fan speed. Relay 3 provides cold weather
shutdown of the fan. Relay 3 is wired in series with the
common wire of relays 1 and 2, so when Relay 3 breaks at
55° F (13° C), power to the fan is interrupted.
Programming Example
Optional: In Setup, label sensors A and B as desired.
Relay 1: Control to the fan low speed based on the sump
temperature.
Program for:
— Setpoint = 65° F (18° C)
— Differential =5° F (12° C)
—Cool
— Sensor A
Relay 2: Control to the fan high speed based on the sump
temperature
Program for:
— Setpoint = 70° F (21° C)
— Differential = 5° F (12° C)
—Cool
— Sensor A
Relay 3: Control to the fan cutoff based on the outside air
temperature
Program for:
— Setpoint = 55° F (13° C)
— Differential = 1° F (12° C)
—Cool
— Sensor B
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
By programming the Relay 3 setpoint at 55° F (13°
C), the T775 is wired to cutoff power to the cooling
fan. Once the outside air temperature rises above
55° F (13° C), relays 1 and 2 control the fan speed
based on their setpoints for Sensor A Sump.
Wiring
All output relays should have a common power wiring source,
which may or may not be the same as the T775 power wiring.
See Fig. 6.
Fig. 6. T775B2040 wiring for cooling tower control.
Device Checkout
The T775B performance can be checked out to determine if
proper operation exists.
For example, when the outside air temperature is at 55° F
(13° C) or less, the fan should be off. Use the alternate Home
screens to verify that the fan is on or off, based on the outside
air temperature (Sensor B).
NOTE: The alternate Home screens do not show live
updates of the sensor temperature. They show the
temperature only at the moment the button is
pressed.
1. From the Home screen, use the button to verify
the setpoint temperature for Relay 3.
2. Then, press the HOME button to view the actual
outside air temperature (Sensor B).
M25513
T775B2040
C
NO
NC
C
NO
NC
C
NC
NO
C
NC
NO
T
T
120 VAC
COM
NO
LOW SPEED WINDING
COM
NC
FAN
SENSOR A - SUMP TEMPERATURE
C
+
RELAY
3
RELAY
2
RELAY
1
RELAY
4
120
COM
240
SENSOR B - OUTSIDE AIR TEMPERATURE
T
T
HIGH SPEED WINDING
MOTOR COMMON
NO
1
2
POWER WITH 24 VAC OR 120/240 VAC AT THE APPROPRIATE
TERMINAL BLOCK.
24 VAC POWER TERMINAL BLOCK.
1
2
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