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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 single-phase 240V condenser fan motor draws 3 amps. Its approximate running wattage is:

    Answer: 720 watts

    For a single-phase motor: Watts ≈ Volts × Amps = 240V × 3A = 720 watts (assuming unity power factor for this approximation).

  2. What control device is used to prevent a gas furnace from firing if the inducer blower has not proven airflow?

    Answer: Pressure switch (draft proving switch)

    The draft proving (combustion air pressure) switch verifies that the inducer is operating and creating negative pressure before the control board allows gas valve operation.

  3. In a DDC (Direct Digital Control) system, a 4–20 mA signal from a temperature sensor representing a 0–100°F range reads 12 mA. What is the temperature?

    Answer: 50°F

    The 4–20 mA span (16 mA) covers 0–100°F; 12 mA is 8 mA above the 4 mA base, so (8/16) × 100°F = 50°F.

  4. A technician suspects a shorted run capacitor on a condenser unit. Which symptom would support this diagnosis?

    Answer: All of the above

    A shorted capacitor can read over-rated on a meter, force excessive current through motor windings, or prevent starting — all are consistent with a shorted capacitor.

  5. On a variable-frequency drive (VFD), the output frequency is increased to increase motor speed. What happens to motor torque at low frequencies if V/Hz ratio is not maintained?

    Answer: Torque decreases due to magnetic flux reduction

    Maintaining a constant V/Hz ratio keeps magnetic flux constant; if voltage drops at low frequencies without compensation, flux falls and torque capacity drops.

  6. When measuring voltage on an energized 208V/3-phase disconnect, a technician finds 208V L1-L2, 208V L2-L3, but 0V L1-L3. The most likely cause is:

    Answer: Open fuse on L3

    An open L3 fuse would cause L1-L3 to read 0V while L1-L2 and L2-L3 still show voltage through the remaining phases.

  7. A crankcase heater on a compressor is energized:

    Answer: Whenever the compressor is OFF

    Crankcase heaters are wired to be energized whenever the compressor is off, keeping oil warm to prevent refrigerant migration into the crankcase.