Control Combinations; Low-Limit Control; High-Limit Control - Honeywell AUTOMATIC CONTROL Engineering Manual

When the high-limit controller calls for less heat, the poten-
tiometer wiper R (shown dotted) moves halfway from B to W.
This causes an unbalance of 70 ohms (210 – 140) in the right
leg of the bridge as shown in the following table:
Controller potentiometer
High-limit potentiometer
Feedback potentiometer
Total
The unbalance causes the electronic relay to trigger the left
triac, energize the ccw motor winding, and drive the actuator
drive shaft toward closed. 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 right. The table
then appears as follows:
Controller potentiometer
High-limit potentiometer
Feedback potentiometer
Total
When the feedback potentiometer reaches the new position
(shown dotted) the bridge is rebalanced, the left triac turns
off, and the actuator drive shaft stops in the new position
(75 percent open).

CONTROL COMBINATIONS

The following illustrates common applications of Series 90
controls including low- and high-limit controls from an
application viewpoint.

LOW-LIMIT CONTROL

Figure 25 illustrates a typical Series 90 circuit for a heating
application with a room controller, motorized valve, and a low-
limit controller located in the discharge air to the space. The
temperature of the space can rise rapidly as a result of increased
solar radiation, occupancy, or other conditions resulting in a
sudden decrease in heating load. The room controller is then
satisfied and closes the valve to the heating coil. If the system
uses outdoor air, closing the valve to the heating coil can cause
air to be discharged into the room at a temperature lower than
desirable. To correct this, the low-limit controller causes the
valve to move toward open thus limiting the low temperature
of the discharge air.
Left Leg Right Leg
140 0
0 70
0 140
140 210
Left Leg Right Leg
140 0
0 70
35 105
175 175
ELECTRIC CONTROL FUNDAMENTALS
DISCHARGE AIR
LOW LIMIT
CONTROLLER
135
W R B
ACTUATOR
HOT WATER
Fig. 25. Series 90 Circuit with Low-Limit Control.

HIGH-LIMIT CONTROL

Figure 26 illustrates a typical Series 90 circuit for a heating
application with a room controller, motorized valve, and high-
limit controller located in the discharge air to the space. This
circuit is used when there is danger of the temperature rising
too high. The high-limit controller takes over control of the
valve to the heating coil if the discharge air temperature rises
above a comfortable level. This circuit is similar to the low-
limit circuit except that the high-limit controller is in the B
leg of the actuator circuit and drives the actuator toward the
closed position.
ROOM
CONTROLLER
135
W R
W R
ACTUATOR
HOT WATER
VALVE
Fig. 26. Series 90 Circuit with High-Limit Control.
111 ENGINEERING MANUAL OF AUTOMATION CONTROL
ROOM
CONTROLLER
135
W R B
TRANSFORMER
W R B
LINE
T1
VOLTAGE
T2
TO
COIL
VALVE
C2526
DISCHARGE AIR
HIGH LIMIT
CONTROLLER
135
B W R B
TRANSFORMER
B
LINE
T1
VOLTAGE
T2
TO
COIL
C2527