HVAC Electrical Controls 4

Question 1

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

Accepted Answer

720 watts

Answer

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?

Accepted Answer

Pressure switch (draft proving switch)

Answer

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?

Accepted Answer

50°F

Answer

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?

Accepted Answer

All of the above

Answer

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?

Accepted Answer

Torque decreases due to magnetic flux reduction

Answer

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

HVAC Practice Test

HVAC Practice Test

HVAC Electrical Controls 4