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ELECTRIC CONTROL FUNDAMENTALS
LOW-VOLTAGE
CONTROL CIRCUIT
T1
T2
M
L1
L2
LINE-VOLTAGE
INPUT
Fig. 1. Typical Actuator Wiring.
Contact arrangement: The electric switch configuration of a
controller, relay, contactor, motor starter, limit switch,
or other control device. Contacts which complete
circuits when a relay is energized (pulled in) are called
normally open (N.O.) or "in" contacts. Contacts which
complete electric circuits when a relay is deenergized
(dropped out) are called normally closed (N.C.) or
"out" contacts. Many contact arrangements are
available depending on the control device. Figure 2
illustrates three contact arrangements.
A) N.O. SPST CONTACT
B) N.C. SPST CONTACT
C) SPDT CONTACTS
Fig. 2. Typical Contact Arrangements.
Control valve: A device used to control the flow of fluids such
as steam or water.
Controller: A temperature, humidity, or pressure actuated
device used to provide two-position, floating, or
proportioning control of an actuator or relay. It may
contain a mercury switch, snap-acting contacts, or a
potentiometer. Controllers can be two-wire or three-
wire devices. Two-wire controllers are spst devices.
The N.O. type (Fig. 3A) generally opens the circuit
on a fall in the controlled variable and closes the circuit
on a rise. The N.C. type (Fig. 3B) generally closes
the circuit on a fall in the controlled variable and
opens the circuit on a rise. Three-wire controllers
are spdt, floating, or potentiometer devices. The spdt
controllers (Fig. 3C) generally close R to B contacts
and open R to W contacts on a fall in the controlled
variable. The opposite occurs on a rise. The controllers
in Figures 3A through 3C do not have a true
N.O./N.C. contact arrangement but provide a
MOTOR COILS
TRANSFORMER
ACTUATOR
C2502
C2503
switching action dependent on the condition of the
controlled variable. Floating controllers (Fig. 3D) are
spdt devices with a center-off position. Refer to
SERIES 60 FLOATING CONTROL CIRCUITS for
a discussion of floating control operation.
Potentiometer controllers (Fig. 3E) move the wiper
(R) toward B on a fall and toward W on a rise in the
controlled variable. This action varies the resistance
in both legs of the potentiometer and is used for
proportional control.
A) N.O. SPST CONTROLLER
(CONTACTS OPEN ON A TEMPERATURE FALL)
B) N.C. SPST CONTROLLER
(CONTACTS CLOSE ON A TEMPERATURE FALL)
W
C) SPDT CONTROLLER
(CLOSE R TO B, OPEN R TO W ON A
TEMPERATURE FALL)
W
D) FLOATING CONTROLLER
(SEE SERIES 60 FLOATING CONTROL
CIRCUITS FOR OPERATION)
W
E) POTENTIOMETER TYPE CONTROLLER
(WIPER R MOVES FROM W TO B ON A
TEMPERATURE FALL)
Fig. 3. Typical Controller Action.
Control Modes:
Modulating Control: When an actuator is energized,
it moves the damper or valve a distance proportional
to the sensed change in the controlled variable. For
example, a Series 90 thermostat with a 5 kelvins
throttling range moves the actuator 1/5 of the total
travel for each degree change in temperature.
Two-Position Control: When an actuator is energized
it moves the valve or damper to one of the extreme
positions. The valve or damper position remains
unchanged until conditions at the controller have
moved through the entire range of the differential.
Floating Control: When an actuator is energized, it
moves the damper or valve until the controller is
satisfied. The actuator maintains that position until the
controller senses a need to adjust the output of the
valve or damper.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
98
R
B
R
B
R
B
C2504
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