HVAC Electrical Controls 3 — Questions and Answers
Question 1: A motor starting capacitor (electrolytic) differs from a run capacitor (film/oil) in that it is rated for:
- Continuous duty at full voltage
- Intermittent duty only — seconds per start (Correct answer)
- Higher capacitance but lower voltage
- Lower capacitance and higher temperature
Correct 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.
Question 2: When testing a run capacitor with a digital capacitance meter, an acceptable reading is typically within what percentage of the nameplate rating?
- ±1%
- ±6% (Correct answer)
- ±20%
- ±30%
Correct answer: ±6%
Industry standard allows run capacitors to be within ±6% of their rated capacitance; readings outside this range indicate a failing capacitor.
Question 3: What is the purpose of a lockout-relay (anti-short-cycle timer) in a refrigeration control circuit?
- Prevent the compressor from restarting too quickly after shutdown (Correct answer)
- Sense locked-rotor current and trip the circuit
- Sequence the start of multiple compressors
- Monitor refrigerant charge level
Correct 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.
Question 4: In a heat pump system, the 'O' or 'B' terminal on the thermostat controls the:
- Auxiliary heat strips
- Reversing valve solenoid (Correct answer)
- Outdoor fan motor speed
- Crankcase heater relay
Correct 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.
Question 5: A technician measures 0 VAC across a normally-open contactor contact set while the coil is energized. This indicates:
- The contacts are welded closed
- Voltage drop across open contacts confirms coil failure
- The contacts are closed and conducting normally (Correct answer)
- A blown fuse in the control circuit
Correct 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.
Question 6: An economizer control on a commercial HVAC unit uses which primary signal to determine if outside air can provide free cooling?
- CO2 level in the return air
- Outside air enthalpy or dry-bulb temperature compared to return air (Correct answer)
- Discharge air static pressure
- Refrigerant suction pressure
Correct 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.
Question 7: Which NEMA enclosure rating is required for electrical controls installed outdoors and exposed to rain?
- NEMA 1
- NEMA 3R (Correct answer)
- NEMA 5
- NEMA 7
Correct answer: NEMA 3R
NEMA 3R enclosures are rainproof and suitable for outdoor use, protecting controls from falling rain, sleet, and external ice formation.
HVAC Electrical Controls 3
A motor starting capacitor (electrolytic) differs from a run capacitor (film/oil) in that it is rated for: