above-atmospheric pressure element
a type of control device that closes an electrical circuit on temperature rise.
actuator
a device that operates based on the signal from the controller. solenoids, contactors, and relays are examples of actuators that perform functions such as opening and closing valves or turning motors on and off.
anticipator
a device that assists the controller by causing the sensor to detect the cut-in or cut-out temperature slightly before the conditioned area actually reaches that temperature. often this is a resistor used in a thermostat.
below-atmospheric pressure element
a type of control device that opens an electrical circuit on temperature rise
bimetal coil
a thermostatic control made of a coil of two different metals that expands and contracts with changes in temperature, causing a mercury switch mounted on one end to tilt and open or close a circuit
bimetal device
a device made of two different metal bonded together and formed into a particular shape. which makes the device bend in the desired direction.
bimetal disc
a thermostatic control that consists of a concave disc composed of two different metals. the disc is dished in one direction when it is cooled and snaps into a dished position in the opposite direction as it warms.
bimetal strip
a thermostatic control with electrical contacts at either end. as the strip reacts to different heat condition, it opens and closes the contact points of an electrical circuit
blown fuse
an electrical fuse in which the conductive element inside the fuse has melted to open the circuit and prevent excess current flow.
circuit breaker
an electrical protective devise that automatically opens a circuit when the current exceeds the predetermined level. the increased magnetic effect of the current surge causes a spring-loaded switch to trip and break the circuit
close-loop control system
a type of control system in which the system's output modifies system input,which it turn helps deermine the next output. a sensor produces a signal based on the conditions in the controlled area and transmits it to the controller, which modifies the outp
conditioned area
a specific area in which an hvacr system regulates conditions, such as temperature, pressure, and humidity.
contactor
a heavy duty type of electromagnetic relay that can handle high-current loads, such as motors. contractors can have a variety of contact configurations, from single set contacts to five or six sets of contacts.
control point
the present condition as measured by a sensor in a conditioned area.
control system
a collection of interacting components that work together to regulate the operations of an hvacr system. the basic components of a control system include sensors, controllers, and actuators.
controller
a device that responds to changes in the signals from sensors and issues signals to actuators.
current relay
a relay that is activated by current running directly through it. current relays are used to close and open a motors start winding circuit to provide the motor with more starting torque.
current-limiting fuse
a fuse that open a circuit when the current exceeds its limit and the fuse heats up. current-limiting fuses use temperature-sensitive resistors that close when they cool back down.
cut-in
the temperature or pressure at which a device begins operating.
cut-out
the temperature or pressure at which a device ceases operation.
differential
the number of units (the difference) between a control system's cut-out value and cut-in value.
differential adjustment
an adjustment built into a temperature or pressure control that increases or decreases the difference between the cut-in and cut-out values
fast-acting fuse
a fuse that blows immediately after the maximum rating of the fuse is exceeded.
feedback
the information detected by a sensor in a condition area that is sent to the controller to determine what action needs to be taken.
floating control
a control action that gradually moves the operating device, such as a damper, from one position to another.
fuse
an electrical protection device containing a metal conductor in series with an electrical circuit that is design to conduct a certain amount of current before it melts to open the circuit.
impedance
the opposition to the flow of alternating current in an ac circuit.
ladder diagram
an electrical line diagram that shows the devices and connections in a circuit arranged in the order that they activate during circuit operation. the vertical lines in the diagram identify the power supply, and the horizontal lines contain the various dev
lockout relay
a special high-impadance relay that keeps a circuit from restarting when any of the safety controls in the circuit have opened. power to the lockout relay coil must be interrupted to reset the lockout relay contacts.
low-side pressure limiter
a pressure-sensative element, such as a diaphragm or bellows, that is set to shut down the compressor in the event of refrigeration loss or an evaporator freeze-up
modulating control
a control with an output signal that varies in strength. the operating device is adjusted in proportion to the control signals strength.
motor starter
a heavy-duty type of electromagnetic relay similar to a contactor that can handle high current loads, such as those for operating motors. unlike a contactor, a motor starter has built-in overload protection.
multipurpose fuse
a fuse that will not blow during small overloads lasting only a short periods of time , but will blow immediately if an extremely high overload occurs.
offset
the difference between the set point and the control point in a closed-loop control system. offset is sometimes called error.
on-off control
a type of control system action in which the control turns the operating device either on or off.
open-loop control system
a type of control system in which the controller sends commands to the actuator with no detected information from the conditioned area being retired to the controller.
pickup voltage
the voltage required in a motors start winding to energize a potential relay coil, which drops the start winding out the circuit. pickup voltage is typically generated when a motor reaches around three-forth of its normal operating speed.
pictorial diagram
an electrical wiring diagram that is used primarily in service or installation manuals to illustrate how to connect electrical devices in a unit. pictorial diagrams also show the approximate physical location of the devices in a unit.
positive temperature coefficient relay (ptc)
an electronic relay that uses ptc thermistor to control a motor circuit by increasing its resistance as it senses high ambient temperature.
potential relay
a relay that operates when a motors counter electromotive force is sufficient to energize the relays electromagnetic coil. potential relays are used to disengage a motors start winding circuit.
pressure motor control
a motor control device used to regulate a compressors motor based on low-side pressure, high-side pressure, or oil pressure.
range
the set numbers between and including the cut-in and cut-out values of a control system.
range adjustment
an adjustment that regulates the minimum and maximum temperature or pressure in a control system.
relay
an electrical switch that operates under the control of an outside an electrical signal. relays are commonly used to start or stop another system component, such as a motor.
remote temperature sensing element
a device that reacts to temperature change and sends a signal to a control device. two common types include above-atmospheric-pressure element and below-atmospheric-pressure element.
sensing bulb
a sealed fluid-filled sensing device that reacts to heat by changing its internal pressure. a sensing bulb is connected by a capillary tube to a diaphragm or bellows, such as on a TXV.
set point
the desired condition in a closed-loop control system.
solid-state relay (ssr)
a relay that uses transistors, diodes, silicon-controlled rectifiers, diacs, and triacs to switch circuits on and off. SSR's are used to start single-phase motors and are often referred to as electronic relays.
system overshoot
concerning control systems, when the temperature of a conditioned space exceeds the thermostats set point. it is a heat lag that occurs when a furnace shuts down and the amount of residual heat that has built up is sent by the blower through the ductwork
timed on-off control
a type of on-off control that turns a component on or off either before or after the cut-in or cut-out values. one type of timed on-off control uses a clock to open and close electrical contacts for specific periods of time.
time-delay fuse
a fuse that will not blow unless an over-load condition exists for a certain period of time, typically ten seconds. also called a dual-element fuse.
tripped circuit breaker
an overload protection device that has opened due to high current and must be manually reset.
volatile fluid
a fluid that vaporizes at a low temperature. volatile fluids are used in sensing bulbs due to their ability to change pressure in response to change in temperature .
wheatstone bridge
an electronic circuit used to detect changes in a thermistors resistance. a temperature change causes the resistance in the bridge to become unbalanced, which alters the bridges output voltage and sends a signal to the controller.