Electrical Systems and Controls Flashcards
6 cards from real Air Conditioning Repair practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Electrical Systems and Controls flashcards as text
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?
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.
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?
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.
What is the primary function of a sequencer in an electric furnace with multiple heating elements?
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.
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?
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.
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?
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.
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?
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.