Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATION CONTROL
ELECTRIC CONTROL FUNDAMENTALS
97
INTRODUCTION
This section provides information on electric control circuits
used in heating, ventilating, and air conditioning systems.
Electric energy is commonly used to transmit the measurement
of a change in a controlled condition from a controller to other
parts of a system and to translate the change into work at the
final control element. For these purposes, electricity offers the
following advantages:
– It is available wherever power lines can be run.
– The wiring is usually simple and easy to install.
– The signals received from sensing elements can be used
to produce one or a combination of electro-mechanical
outputs. For example, several actuators can be controlled
from one controller.
– Single controller-actuator combinations are possible
without the need for a main-air source as in pneumatic
control.
Electric controls consist of valve/damper actuators,
temperature/pressure/humidity controllers, relays, motor starters
and contactors. They are powered by low or line voltage,
depending on the circuit requirements. Controllers can be wired
to perform either primary or limit functions (high or low limit).
Electric actuators can be two position or proportioning and
either spring return or nonspring return.
The information in this section is of a general nature and
intended to explain electric control fundamentals. There are
places where Honeywell nomenclature is used, such as R,
W, B, for wiring terminals and Series 40 through 90 for
classifying control circuits. These may vary with controls
of a different manufacturer.
DEFINITIONS
Actuator:
A device used to position control dampers and
control valves. Electric actuators consist of an electric
motor coupled to a gear train and output shaft.
Typically, the shaft drives through 90 degrees or
160 degrees of rotation depending on the application.
For example, 90-degree stroke actuators are used with
dampers, and 160-degree stroke actuators are used
with valves. Limit switches, in the actuator, stop the
motor at either end of the stroke or current limiters
sense when the motor is stalled at the end of the stroke.
Actuator gear trains are generally factory lubricated
for life so no additional lubrication is necessary.
Actuators may attach to the valve stem or damper shaft
through a linkage or be direct coupled connecting
directly to the stem or shaft.
In some actuators the motor is electrically reversible
by the controller. A Solenoid brake is commonly used
on spring-return actuators to hold the actuator in the
control position. Some actuators have a return spring
which enables the output shaft to return to the normal
position on loss or interruption of power. Most two-
position actuators drive electrically to the control
position and rely on only the spring to return to the
normal position. Spring-return actuators have
approximately one-third the output torque of
comparable non-spring-return actuators since the
motor must drive in one direction against the return
spring. A return-to-normal position on power failure
function maybe provided by an integral rechargeable
battery, capacitor, or constantly wound spring so that
there is no reduction in operating force.
The direction of shaft rotation on loss of power varies
by model for spring-return actuators. The direction
can be clockwise (cw) or counterclockwise (ccw) as
viewed from the power end of the actuator. Actuator
controlled valves and dampers also vary as to whether
they open or close on a loss of power. This depends
on the specific actuator, linkage arrangement, and
valve or damper selected. Because of these factors,
the terms cw/ccw or open/close as used in this
literature are for understanding typical circuits only
and do not apply to any particular model of actuator.
Actuators are available with various timings to
drive through full stroke such as 15, 30, 60, 120, or
240 sec. In general, the timing is selected to meet
the application requirements (e.g., an actuator with
240 sec timing might be used to control the inlet vanes
to a fan in a floating control system).
Actuators designated as line voltage have line-voltage
inputs but commonly have a low-voltage control circuit
since the motor is powered by low voltage. A
transformer is used to supply power to the low voltage
motor coils. It can be built into the actuator housing
(Fig. 1) or supplied separately.