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ELECTRIC CONTROL FUNDAMENTALS
TWO-POSITION LIMIT CONTROL
Two-position limit controllers can be used in Series 90
circuits where proportioning action is unnecessary or
undesirable. They must be snap-acting, spdt. Two-position
controls should not be used where the temperature of the
controlled variable is greatly affected by the opening and
closing of the controlled valve or damper. For example, if
Series 60 high- or low-limit controllers are used as limit
controllers in discharge air, whenever the temperature of the
air is within the range of the limit controller, the steam valve
for the heating coil will cycle on and off continuously.
Figure 27 illustrates a Series 90 circuit with a temperature
controller, normally closed (spring-return actuator) cooling
valve, and a two-position, high-limit humidity controller. When
the humidity is below the setting of the high-limit humidity
controller, the R circuit is completed from the Series 90
controller to the actuator and the cooling valve is controlled
normally. If the humidity rises above the setting of the limit
controller, R to B is connected at the actuator and the cooling
valve opens fully. Reheat is advisable in a system of this kind
since opening the cooling valve to reduce humidity also lowers
the temperature of the air and may result in occupant discomfort.
ROOM
CONTROLLER
135
W
R
B
W
R
B
SPRING
T1
RETURN
ACTUATOR
T2
CHILLED
WATER
NORMALLY
CLOSED VALVE
Fig. 27. Diagram of Series 90 Circuit with
Two-Position, High-Limit, Humidity Controller.
MANUAL AND AUTOMATIC SWITCHING
Figures 28 through 31 illustrate various uses of manual
switches or relays in Series 90 circuits. Substitute a relay with
the same switching action as a manual switch where automatic
switching is desired.
Closing the Actuator with a Manual Switch
Figure 28 shows a manual switch with spdt switching action.
With the switch in auto position, the R circuit is completed
from the controller to the actuator. The actuator operates
normally under control of the controller. Placing the switch in
the closed position (dotted arrow) opens the R circuit from
HIGH LIMIT
CONTROLLER
RH RISE
B
R
W
TRANSFORMER
LINE
VOLTAGE
TO
COIL
C2528
the controller and shorts R to W at the actuator. The actuator
drives to the closed position. Such a hookup is often used in
fan heating systems to manually close a valve or damper when
operation is unnecessary.
CONTROLLER
135
W
R
W
R
ACTUATOR
HOT WATER
VALVE
Fig. 28. Series 90 Circuit with SPDT Switch
for Automatic or Manual Operation.
Transferring Actuator Control from
One Thermostat to Another
Figure 29 illustrates using a dpdt switch or time clock to
transfer control of a single Series 90 actuator from one
thermostat to another. Opening both the W and B wires prevents
interaction of the two thermostats when taking one of them out
of control. The R wire need not be opened.
THERMOSTAT 1
W
R
W
R
T1
T2
ACTUATOR
Fig. 29. Circuit for Transferring a Series 90
Actuator from One Thermostat to Another.
Reversing for Heating and Cooling Control
Figure 30 shows a thermostat used for both heating and
cooling control. With the switch in the heating position, the
thermostat and actuator are wired B to B and W to W. With
the switch in the cooling position they are wired B to W and
W to B which causes the actuator to operate the opposite of
heating control.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
112
SWITCH
CLOSED
AUTO
B
TRANSFORMER
B
T1
LINE
VOLTAGE
T2
TO
COIL
C2529
THERMOSTAT 2
B
W
R
B
DPDT
SWITCH
OR TIME
CLOCK
TRANSFORMER
B
LINE
VOLTAGE
C2530
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