Free Air Conditioning Repair Electrical Systems and Controls Questions and Answers — Questions and Answers
Question 1: A technician measures the voltage to the coil of a contactor in a residential condensing unit and reads 0 volts, even though the thermostat is calling for cooling. Which of the following is the most likely cause?
- The high-pressure safety switch is open. (Correct answer)
- The contactor coil is shorted.
- The dual-run capacitor has failed.
- The line voltage at the disconnect is too low.
Correct answer: The high-pressure safety switch is open.
The low-voltage control circuit (typically 24V) runs in series through safety switches like the high-pressure and low-pressure switches. If a safety switch opens due to an abnormal condition (like excessive head pressure), it will break the circuit, and no voltage will reach the contactor coil.
Question 2: When testing a dual-run capacitor with a multimeter, a technician should measure the microfarad (µF) reading between which terminals to check the capacitance for the compressor motor?
- Between the 'C' (Common) and 'FAN' terminals.
- Across the 'HERM' and 'FAN' terminals.
- Between the 'C' (Common) and 'HERM' (Hermetic) terminals. (Correct answer)
- Across any two of the three terminals.
Correct answer: Between the 'C' (Common) and 'HERM' (Hermetic) terminals.
A dual-run capacitor has three terminals: Common (C), Fan, and Hermetic (HERM) for the compressor. To test the capacitance for the compressor winding, the multimeter probes must be placed on the 'C' and 'HERM' terminals. The reading should be compared to the rating stamped on the capacitor.
Question 3: What is the primary function of a sequencer in an electric furnace with multiple heating elements?
- To regulate the control circuit voltage to 24V.
- To stage the activation of heating elements and the blower. (Correct answer)
- To act as a safety limit switch in case of overheating.
- To control the speed of the indoor blower motor.
Correct answer: To stage the activation of heating elements and the blower.
A sequencer is a time-delay relay that brings heating elements on in a staggered sequence. This prevents a large, sudden inrush of current that could trip a circuit breaker. It also typically delays the blower fan until the heating elements are warm to avoid blowing cold air.
Question 4: A technician is troubleshooting a condenser fan motor that is not running. The wiring diagram shows a black wire connected to the contactor, a brown wire to the 'FAN' terminal of the capacitor, and a white wire to the 'C' terminal. Which wire provides the start winding with the necessary phase shift from the capacitor?
- The black wire
- The white wire
- The brown wire (Correct answer)
- The ground wire
Correct answer: The brown wire
In a typical PSC (Permanent Split Capacitor) motor wiring scheme, one line of power connects to the motor's common terminal. The other line of power connects to the contactor. The capacitor is wired between the run and start windings. The wire running from the motor to the 'FAN' terminal of the capacitor is connected to the start winding.
Question 5: Which of the following components is responsible for stepping down line voltage to provide the 24V AC used by the thermostat and control circuits in most residential HVAC systems?
- A relay
- A capacitor
- A contactor
- A transformer (Correct answer)
Correct answer: A transformer
A transformer is an electrical device that transfers electrical energy from one circuit to another through electromagnetic induction, and it is used to step voltage up or down. In HVAC systems, a transformer reduces the 120V or 240V line voltage to the 24V needed for the control circuit.
Question 6: A customer reports a loud buzzing or chattering noise from their outdoor unit when the air conditioner tries to start. Which electrical component is the most likely source of this noise?
- The dual-run capacitor
- The contactor (Correct answer)
- The low-pressure switch
- The crankcase heater
Correct answer: The contactor
A loud buzzing or chattering sound is a classic symptom of a failing contactor. This can be caused by a weak 24V signal to the coil, a failing coil, or pitted/corroded contacts that prevent the plunger from seating firmly.
Free Air Conditioning Repair Electrical Systems and Controls Questions and Answers
A technician measures the voltage to the coil of a contactor in a residential condensing unit and reads 0 volts, even though the thermostat is calling for cooling.
Which of the following is the most likely cause?