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Electrical Controls Flashcards

7 cards from real HVAC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Electrical Controls flashcards as text

  1. A thermostat wired with an 'R', 'C', 'Y', 'G', and 'W' terminal set: which terminal, when energized alone, activates indoor fan only?

    Answer: G

    The G terminal energizes the indoor (air handler) fan relay, running the fan independently of heating or cooling calls.

  2. What is the purpose of a Time-Delay Relay (TDR) in a multi-stage commercial HVAC startup sequence?

    Answer: Stage the starting of multiple loads to reduce peak demand current

    TDRs introduce a delay between starting multiple large loads (compressors, fans) so their inrush currents don't overlap, reducing peak demand charges and stress on wiring.

  3. An electronic expansion valve (EEV) is controlled by which type of signal from the controller?

    Answer: Pulse-width modulated (PWM) or stepper motor pulses

    EEVs use a stepper motor driven by PWM pulses from the controller, allowing precise modulation of refrigerant flow in small incremental steps.

  4. Which regulation requires HVAC electrical work to follow specific wiring methods, box fill calculations, and grounding rules?

    Answer: NFPA 70 (National Electrical Code)

    NFPA 70 (NEC) governs all electrical installations including HVAC equipment wiring, disconnect sizing, conductor ampacity, and grounding requirements.

  5. A technician measures 26V AC at the transformer secondary but only 18V AC at the contactor coil. The most likely cause is:

    Answer: Overloaded control circuit drawing excessive current through wiring resistance

    Excessive current draw in the control circuit (from added loads or a partially-shorted coil) creates voltage drop across the wiring, lowering voltage at the load.

  6. In a demand-controlled ventilation (DCV) system, CO2 sensors are used to:

    Answer: Modulate outdoor air dampers based on occupancy levels

    DCV uses CO2 as a proxy for occupancy; as CO2 rises above setpoint (typically 1100 ppm), the controller opens outdoor air dampers to dilute contaminants and provide fresh air.

  7. A normally-closed (NC) low-pressure switch in a refrigeration system opens when suction pressure drops too low. This action typically:

    Answer: De-energizes the compressor contactor coil, stopping the compressor

    When the NC low-pressure switch opens due to low suction pressure, it breaks the control circuit to the contactor coil, de-energizing the contactor and stopping the compressor.