1050 A Tedt Flashcards
What device opens the circuit if suction pressure falls below safe levels, protecting the compressor?
Low pressure switch
Senses suction line pressure and opens its contacts when pressure drops too low, breaking the 24V control circuit to the compressor.
What switch opens when head pressure becomes dangerously high to prevent compressor damage?
High pressure switch
Monitors discharge pressure; opens its contacts when pressure exceeds a set point, stopping compressor operation.
What device controls humidity by opening/closing a circuit based on relative humidity?
Humidistat
Uses a hygroscopic element that expands/contracts with humidity, operating a set of contacts to control humidifiers or dehumidifiers.
What component adds moisture to the air in an HVAC system?
Humidifier
Uses evaporation, steam, or mist to introduce water vapor into the airstream, regulated by a humidistat.
What do the Y2 and W2 terminals on a thermostat control?
Second-stage cooling (Y2) and second-stage heating (W2)
Sends a second 24V signal to energize additional heating/cooling equipment when demand exceeds what the first stage can handle.
What older thermostat design uses a mercury-filled glass bulb that tilts with temperature change?
Mercury bulb thermostat
A bimetal coil moves with temperature changes, tilting the mercury bulb to complete or break the circuit.
What type of transformer increases voltage from primary to secondary?
Step up transformer
Electromagnetic induction transfers energy from fewer primary windings to more secondary windings, raising the output voltage.
What transformer reduces voltage to 24V for control circuits?
Step down transformer
Converts higher line voltage to 24V using fewer turns in the secondary winding compared to the primary.
What does a transformer rating represent?
Maximum power it can deliver safely
VA rating (volts x amps) defines how much current can be supplied without overheating or voltage drop.
What overload device protects low-voltage relay circuits from overheating?
Pilot duty overload
Opens its contacts when excessive current or heat is sensed in control circuits, preventing relay damage.
What device uses an electromagnet to control high-voltage current to motors or compressors?
Contactor
A 24V coil magnetizes to pull in contacts, completing a high-voltage circuit; when coil loses power, contacts open.
What thermostat terminal energizes the cooling contactor?
Y terminal
Sends 24V to the contactor coil when the thermostat calls for cooling, activating the outdoor unit.
Which thermostat terminal sends power to activate heating?
W terminal
Sends 24V to the heating control circuit or gas valve when the thermostat demands heat.
What terminal supplies 24V power from the transformer to the thermostat?
R terminal
Provides the constant 24V source to the thermostat for operation of other control terminals.
Which terminal energizes the fan relay?
G terminal
Sends 24V to the fan relay to turn on the indoor blower motor independently or during cooling.
What kit boosts starting torque for compressors using a capacitor and relay?
Hard start kit
Momentarily inserts a start capacitor into the circuit to assist the compressor in overcoming locked rotor torque, then removes it.
What are common causes of a motor not running, overheating, or stalling?
Motor failure
Can result from shorted windings, bearing seizure, open circuits, or insulation failure causing motor shutdown.
What device opens a motor’s circuit when it detects excess current or temperature?
Motor overload
Uses heat-sensitive elements to open the power circuit when excessive current causes overheating.
What uses heat to bend two metals and open a circuit?
Bi-metal overload
Two metals expand at different rates with heat, bending to mechanically open contacts in an overload condition.
What safety switch shuts off the compressor if oil pressure drops too low?
Oil pressure safety control switch
Compares crankcase pressure and oil pump pressure; opens the circuit if differential pressure is inadequate for safe lubrication.
What relay uses back EMF from the motor to disconnect the start capacitor?
Potential relay
Senses increased voltage from the running motor (EMF) and opens its contacts to remove the start capacitor.
What capacitor increases torque at startup and is removed shortly after?
Start capacitor
Creates a phase shift in current to the start winding, boosting torque until relay or motor switches it out.
What happens to the current relay coil after the motor starts?
It de-magnetizes and opens contacts
Inrush current magnetizes the coil to keep contacts closed; once current drops, the magnetic field collapses, opening the circuit.
What relay opens after inrush current drops, disconnecting the start capacitor?
Current relay
Starts as normally closed; opens due to reduced current flow through its coil after motor speed increases.
