HVAC Electrical Controls 4 — Questions and Answers
Question 1: A single-phase 240V condenser fan motor draws 3 amps. Its approximate running wattage is:
- 120 watts
- 480 watts
- 720 watts (Correct answer)
- 960 watts
Correct answer: 720 watts
For a single-phase motor: Watts ≈ Volts × Amps = 240V × 3A = 720 watts (assuming unity power factor for this approximation).
Question 2: What control device is used to prevent a gas furnace from firing if the inducer blower has not proven airflow?
- Rollout switch
- Pressure switch (draft proving switch) (Correct answer)
- Limit switch
- Ignition control module timer
Correct 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.
Question 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?
- 50°F (Correct answer)
- 60°F
- 75°F
- 80°F
Correct 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.
Question 4: A technician suspects a shorted run capacitor on a condenser unit. Which symptom would support this diagnosis?
- Motor fails to start and hums loudly
- Motor runs but trips overload from high amperage
- Capacitor reads above rated microfarads on meter
- All of the above (Correct answer)
Correct 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.
Question 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?
- Torque increases proportionally
- Torque remains constant
- Torque decreases due to magnetic flux reduction (Correct answer)
- Torque is unaffected by voltage
Correct 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.
Question 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:
- Open fuse on L3 (Correct answer)
- Open fuse on L2
- Shorted capacitor on L3
- Loose neutral connection
Correct 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.
Question 7: A crankcase heater on a compressor is energized:
- Only during the cooling season
- Whenever the compressor is OFF (Correct answer)
- Only when ambient drops below 40°F
- During the first 5 minutes of each compressor start
Correct 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.
HVAC Electrical Controls 4
A single-phase 240V condenser fan motor draws 3 amps.
Its approximate running wattage is: