Air Conditioning Repair Electrical Components and Controls Questions and Answers — Questions and Answers
Question 1: A technician responds to a no-cool call. The indoor blower is running, and the thermostat is calling for cooling, but the outdoor unit is silent except for a faint humming sound. The condenser fan is not spinning. Which of the following is the MOST likely cause of this specific symptom?
- A failed dual-run capacitor (Correct answer)
- A tripped circuit breaker for the condensing unit
- A faulty thermostat
- A stuck reversing valve
Correct answer: A failed dual-run capacitor
A dual-run capacitor serves both the compressor and the condenser fan motor. If it fails, neither component will have the necessary stored energy and phase shift to start, often resulting in a humming sound as the motors attempt to turn but cannot. A tripped breaker would result in no power and no humming. A faulty thermostat would likely not send the 24V signal to the outdoor unit at all. A stuck reversing valve would cause performance issues but would not prevent the compressor and fan from starting.
Question 2: Which of the following electrical safety devices is designed to open the control circuit and shut down the compressor in the event of a restricted condenser coil or a failed condenser fan motor?
- A low-pressure cutout switch
- A crankcase heater
- A high-pressure safety switch (Correct answer)
- A time-delay relay
Correct answer: A high-pressure safety switch
A high-pressure safety switch monitors the refrigerant discharge pressure. Conditions like a dirty condenser coil or a failed condenser fan motor prevent proper heat rejection, causing a rapid and dangerous increase in head pressure. The high-pressure switch is designed to detect this rise and open the 24V control circuit to the compressor contactor, stopping the compressor to prevent serious damage.
Question 3: A technician is installing a new thermostat for a heat pump system. To ensure the reversing valve is energized when the thermostat is set to 'Cool', to which terminal should the corresponding wire be connected?
- Y
- G
- W
- O/B (Correct answer)
Correct answer: O/B
The O/B terminal on a thermostat is specifically for controlling the reversing valve in a heat pump. For the majority of manufacturers, the 'O' terminal is energized to switch the valve into the cooling mode. The 'Y' terminal calls for the compressor to run, 'G' controls the indoor fan, and 'W' is for auxiliary/emergency heat.
Question 4: A technician measures 240V at the line-side terminals (L1/L2) of a contactor and 24V across the coil terminals. However, there is 0V at the load-side terminals (T1/T2) and the compressor is not running. What is the most probable fault?
- The control transformer has failed.
- The circuit breaker is tripped.
- The contactor is faulty. (Correct answer)
- The compressor windings are open.
Correct answer: The contactor is faulty.
The measurements indicate that both high voltage (240V) and control voltage (24V) are being supplied correctly to the contactor. Since the coil is energized but no voltage is passing through to the load side, the contactor's internal contacts have failed to close. This could be due to severe pitting, corrosion, or a mechanical failure of the plunger.
Question 5: In a compressor circuit that utilizes a potential relay, what is the relay's primary function?
- To protect the compressor from low voltage conditions.
- To disconnect the start capacitor from the circuit once the motor approaches full speed. (Correct answer)
- To delay the restart of the compressor after a power interruption.
- To energize the condenser fan motor simultaneously with the compressor.
Correct answer: To disconnect the start capacitor from the circuit once the motor approaches full speed.
A potential relay is used with a start capacitor to provide high starting torque for a motor. The relay's coil senses the back-EMF (voltage) generated across the motor's start winding as it speeds up. When this voltage reaches a predetermined level (typically around 75% of full speed), the relay's normally closed contacts open, taking the start capacitor out of the circuit to prevent it from overheating and failing.
Question 6: A technician is troubleshooting a system where the thermostat display is blank and there is no response from any HVAC function. Voltage measurement at the air handler's control board shows 120V entering the primary side of the control transformer, but 0V on the secondary side. What is the correct diagnosis?
- The main circuit breaker for the house is tripped.
- The thermostat is defective.
- There is a short in the low-voltage wiring somewhere in the system.
- The control transformer has failed. (Correct answer)
Correct answer: The control transformer has failed.
The control transformer's function is to step down line voltage (e.g., 120V) to low voltage (24V) to power the control circuits. The technician has confirmed that the correct input voltage is present on the primary side. Reading 0V on the secondary side under these conditions indicates an open circuit within the transformer's windings, meaning the transformer itself has failed and must be replaced.
Air Conditioning Repair Electrical Components and Controls Questions and Answers
A technician responds to a no-cool call.
The indoor blower is running, and the thermostat is calling for cooling, but the outdoor unit is silent except for a faint humming sound.
The condenser fan is not spinning.
Which of the following is the MOST likely cause of this specific symptom?