Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ELECTRIC CONTROL FUNDAMENTALS
ENGINEERING MANUAL OF AUTOMATIC CONTROL
107
T1
W R B
T2
OPEN
LIMIT
SWITCH
CLOSE
LIMIT
SWITCH
ACTUATOR
LINE
VOLTAGE
TRANSFORMER
C2520-1
MOTOR
DRIVE
SHAFT
CCW
WINDING
(CLOSE)
CW
WINDING
(OPEN)
W
R
B
SENSING
ELEMENT
TEMPERATURE CONTROLLER
135 OHMS
ELECTRONIC
RELAY
FEEDBACK
POTENTIOMETER
135 OHMS
TRIAC
SWITCH
TRIAC
SWITCH
SERIES 90 CONTROL CIRCUITS
APPLICATION
The Series 90 low-voltage control circuit provides modulating
or proportional control and can be applied to:
– Motorized valves.
– Motorized dampers.
– Sequence switching mechanisms.
The Series 90 circuit can position the controlled device
(usually a motorized damper or valve) at any point between
full-open and full-closed to deliver the amount of controlled
variable required by the controller.
Proportional control, two-position control, and floating
control have different operating limitations. For example:
1. In 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.
2. In 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.
3. In 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.
Series 90 circuits combine any Series 90 controller with an
actuator usable for proportioning action. Limit controls can also
be added.
EQUIPMENT
CONTROLLERS
– Temperature controllers.
– Humidity controllers.
– Pressure controllers.
– Manual positioners.
Series 90 controllers differ from controllers of other series
in that the electrical mechanism is a variable potentiometer
rather than an electric switch. The potentiometer has a wiper
that moves across a 135-ohm coil of resistance wire. Typically
the wiper is positioned by the temperature, pressure, or humidity
sensing element of the controller.
ACTUATORS
A Series 90 actuator (Fig. 19) consists of the following:
– Reversible drive motor.
– Electronic relay.
– Switching triacs.
– Feedback potentiometer.
– Gear train and drive shaft.
– Rotation limit switches.
– Optional spring-return mechanism.
The actuator has a low-voltage, reversible-drive motor which
turns a drive shaft by means of a gear train. Limit switches
limit drive shaft rotation to 90 or 160 degrees depending on the
actuator model. The motor is started, stopped, and reversed by
the electronic relay.
The feedback potentiometer is electrically identical to the
one in the controller and consists of a resistance path and a
movable wiper. The wiper is moved by the actuator drive shaft
and can travel from one end of the resistance path to the other
as the actuator drive shaft travels through its full stroke. For
any given position of the actuator drive shaft, there is a
corresponding position for the potentiometer wiper.
All Series 90 actuators have low-voltage motors. A line-
voltage model has a built-in transformer to change the incoming
line voltage to low voltage for the control circuit and the motor.
Low-voltage models require an external transformer to supply
the actuator (Fig. 19).
Fig. 19. Series 90 Actuator Circuit.