Honeywell AUTOMATIC CONTROL, Engineering Manual

Page: 120 / 518

ENGINEERING MANUAL OF AUTOMATION CONTROL
ELECTRIC CONTROL FUNDAMENTALS
110
BRIDGE CIRCUIT WITH LOW-LIMIT CONTROL
In a heating application, a low-limit controller moves a valve
actuator toward open when the low-limit setting is reached. To
do this, the limit controller is wired into the left or W leg of the
bridge. An increase in resistance in the left leg of the controller
circuit drives the actuator and feedback potentiometer toward
the open position.
In Figure 23, when the controller and the low limit are
satisfied (both potentiometer wipers at the W ends), the actuator
is at the closed position and the bridge is balanced. This is
shown in the following table:
Left Leg Right Leg
Controller potentiometer 0 140
Low-limit potentiometer 0 0
Feedback potentiometer 140 0
Total 140 140
Fig. 23. Bridge Circuit with Low-Limit Control.
When the low-limit controller calls for more heat, the
potentiometer wiper R (shown dotted) moves halfway from W
to B. This causes an unbalance of 70 ohms (210 – 140) in left
leg of the bridge as shown in the following table:
Left Leg Right Leg
Controller potentiometer 0 140
Low-limit potentiometer 70 0
Feedback potentiometer 140 0
Total 210 140
The unbalance causes the electronic relay to trigger the right
triac, energize the cw motor winding, and drive the actuator
drive shaft toward open. Since half of the 70-ohm unbalance
has to go on each side of the bridge to rebalance the circuit, the
feedback potentiometer moves 35 ohms to the left. The table
then appears as follows:
Left Leg Right Leg
Controller potentiometer 0 140
Low-limit potentiometer 70 0
Feedback potentiometer 105 35
Total 175 175
When the feedback potentiometer reaches the new position
(shown dotted) the bridge is rebalanced, the right triac turns
off, and the actuator drive shaft stops in the new position
(25 percent open).
BRIDGE CIRCUIT WITH HIGH-LIMIT CONTROL
In a heating application, a high-limit controller moves a valve
actuator toward closed when the high-limit setting is reached.
To do this, the limit controller is wired into the right or B leg of
the bridge. An increase in resistance in the right leg of the
controller circuit drives the actuator and feedback potentiometer
towards the closed position.
In Figure 24, when the controller and high limit are satisfied
(both potentiometer wipers at the B ends), the actuator is at the
open position and the bridge is balanced. This is shown in the
following table:
Left Leg Right Leg
Controller potentiometer 140 0
High-limit potentiometer 0 0
Feedback potentiometer 0 140
Total 140 140
Fig. 24. Bridge Circuit with High-limit Control.
W
ELECTRONIC
RELAY
R B
W R B
DRIVE SHAFT
CONTROLLER
W R B
DISCHARGE AIR
LOW LIMIT
CONTROLLER
OPEN
CLOSE
FEEDBACK
POTENTIOMETER
C2524
W
ELECTRONIC
RELAY
R B
W R B
DRIVE SHAFT
CONTROLLER
W R B
DISCHARGE AIR
HIGH LIMIT
CONTROLLER
OPEN CLOSE
FEEDBACK
POTENTIOMETER
C2525