CONTROLS Flashcards
Because most induction motors require additional starting torque _________ are implemented.
Start windings
Shaded-pole motors can most easily be identified by
A heavy copper band
The primary application for shaded-pole motors would be
When low start torque is required
When low run torque is required
One of the most popular motors used in the HVAC industry with moderate starting torque and good running efficiency is the _________ motor.
Permanent split capacitor
Identify the most rugged and reliable motor used in the industry which operates without the assistance of start components
Three phase-motor
Identify the difference between a three pole contactor and a three pole magnetic starter
A magnetic starter has an overload circuit
In identifying the common, start, run terminals of a split phase motor; terminal A to B = 10 ohms, terminal A to C = 3 ohms, and terminal B to C = 7 which terminal is common
Terminal C
Electronically commutated motors alter the output drive speed. This is accomplished by altering
Electrical frequency
To safely test a motor capacitor a technician should
Discharge the capacitor with a resistor
If a compressor with an internal motor overload opens, what should the technician do before condemning the compressor
Allow the compressor to cool down
An air conditioning system uses a 24 volt control thermostat to maintain room temperature. The same system also utilizes a low pressure switch.
What is the most likely purpose of this low pressure switch?
To cycle the compressor off if the system runs low on
charge
A roof top air conditioning system with a defective condenser fan motor would be protected by
A high pressure safety cut out switch
A roof top air conditioner operating with a pressure type fan control maintains a minimum head pressure
During low ambient conditions
When adding central air conditioning to an existing heating system a fan relay center control (or fan relay) may need to be added, this relay would be controlled by which thermostat lead?
G- Green
To prevent room temperature “overshoot” most HVAC room thermostats employ a
Heat anticipator
Solenoid valves are
Magnetically controlled
A potential magnetic relay is controlled by the back electromotive force created by
The start winding
After a defrost timer control initiates a defrost period the defrost cycle may terminate by means of
Time, pressure, or temperature
A solid state starting relay is employed with fractional horse power compressors. after the motor starts the relay
Increases in resistance disconnecting the start winding
When troubleshooting a capacitor-start-capacitor- run motor and the technician finds the start capacitor and the run capacitor defective the technician must also check the operation of
Potential magnetic relay
Most low voltage HVAC room thermostats can adjust temperature over shooting, this adjustment would be a function of
Heat anticipator
A current sensing lockout relay is an overload device that senses the current draw of the compressor through the ______________
Lockout loop
When diagnosing the compressor contactor coil a proper resistance reading would be
45 OHMS
The best method to test the condition of the contacts in a compressor contactor would be
Visual inspection
When troubleshooting a fuse with a volt meter a reading of line voltage across both ends of the fuse would indicate the fuse is
Open
Application: Residential air conditioning system
Type of Equipment: Gas furnace with air-cooled condensing unit
Complaint: No cooling
Symptoms:
1. Correct line voltage to furnace and condensing unit.
2. Thermostat calling for cooling.
3. Condensing unit receiving 24 volts.
4. All safety controls in condensing circuitry are closed.
5. Contactor coil resistance reading is 0 ohms.
Shorted contactor coil
Application: Residential conditioned air system
Type of Equipment: Fan coil unit air-cooled condensing unit
Complaint: No cooling
Symptoms:
1. Correct line voltage to furnace and condensing unit.
2. Thermostat calling for cooling.
3. Compressor contactor closed.
4. Compressor and condenser fan motor extremely hot.
5. Condenser fan motor resistance readings are C to R = 6 ohms. C to S + 28 ohms, and R to S =34 ohms.
6. Compressor run capacitor in good condition.
7. Compressor motor and bearings in good condition.
8. Condenser fan blade turns with ease.
9. Condenser fan turning slower than normal until overload cuts out.
Defective condenser fan motor capacitor
Application: Residential conditioned air system
Type of Equipment: Packaged air conditioner
Complaint: No cooling, no indoor fan operation
Symptoms:
1. Line voltage available to unit.
2. Closing of thermostat fan switch causes no action.
3. Voltage at terminals C to R on unit low-voltage terminal board is 0 volts.
Open transformer circuit
