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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 motor starting capacitor (electrolytic) differs from a run capacitor (film/oil) in that it is rated for:

    Answer: Intermittent duty only — seconds per start

    Start capacitors are electrolytic and designed for intermittent use only; leaving them in circuit continuously will cause them to overheat and fail.

  2. When testing a run capacitor with a digital capacitance meter, an acceptable reading is typically within what percentage of the nameplate rating?

    Answer: ±6%

    Industry standard allows run capacitors to be within ±6% of their rated capacitance; readings outside this range indicate a failing capacitor.

  3. What is the purpose of a lockout-relay (anti-short-cycle timer) in a refrigeration control circuit?

    Answer: Prevent the compressor from restarting too quickly after shutdown

    An anti-short-cycle timer delays compressor restart (usually 3–5 minutes) to allow pressure equalization and prevent motor overload from rapid cycling.

  4. In a heat pump system, the 'O' or 'B' terminal on the thermostat controls the:

    Answer: Reversing valve solenoid

    The O (or B) terminal energizes the four-way reversing valve solenoid, switching the refrigerant flow direction between heating and cooling modes.

  5. A technician measures 0 VAC across a normally-open contactor contact set while the coil is energized. This indicates:

    Answer: The contacts are closed and conducting normally

    When contacts close and conduct properly, the voltage across them drops to near zero because there is no resistance across a good closed contact.

  6. An economizer control on a commercial HVAC unit uses which primary signal to determine if outside air can provide free cooling?

    Answer: Outside air enthalpy or dry-bulb temperature compared to return air

    Economizer controllers compare outside air enthalpy (or temperature) to return air conditions; when outside air is cooler/drier, the damper opens for free cooling.

  7. Which NEMA enclosure rating is required for electrical controls installed outdoors and exposed to rain?

    Answer: NEMA 3R

    NEMA 3R enclosures are rainproof and suitable for outdoor use, protecting controls from falling rain, sleet, and external ice formation.