Air Conditioning Repair Electrical Components and Controls 5 — Questions and Answers
Question 1: A homeowner reports the outdoor unit runs continuously even after the thermostat setpoint is satisfied. Which component is the most likely culprit?
- Welded or stuck-closed contactor contacts (Correct answer)
- Open high-pressure switch
- Failed start capacitor
- Tripped overload protector
Correct answer: Welded or stuck-closed contactor contacts
Welded contactor contacts keep the compressor and condenser fan energized regardless of the thermostat signal.
Question 2: What does the term 'MFD' (or µF) represent on a capacitor label?
- Millifarads, a unit of electrical inductance
- Microfarads, a unit of electrical capacitance (Correct answer)
- Maximum frequency deviation
- Motor flux density
Correct answer: Microfarads, a unit of electrical capacitance
MFD stands for microfarads, the unit measuring a capacitor's ability to store an electrical charge.
Question 3: A technician is checking motor winding insulation integrity. Which test instrument is used, and what minimum reading is acceptable?
- Clamp meter; minimum 1 amp
- Megohmmeter; minimum 1 megohm (Correct answer)
- Multimeter on resistance; minimum 100 ohms
- Voltmeter; minimum 24V
Correct answer: Megohmmeter; minimum 1 megohm
A megohmmeter applies high DC voltage to measure insulation resistance; readings below 1 megohm indicate degraded insulation that could lead to winding failure.
Question 4: In a 240V single-phase circuit, what is the voltage measured between either hot leg and the neutral conductor?
- 240V
- 120V (Correct answer)
- 208V
- 0V
Correct answer: 120V
A standard residential 240V circuit is center-tapped, providing 120V from each hot leg to the neutral and 240V between the two hot legs.
Question 5: What is the purpose of a ground fault circuit interrupter (GFCI) outlet required near HVAC equipment in some installations?
- To prevent voltage spikes from damaging the control board
- To trip when current imbalance between hot and neutral exceeds ~5 mA, preventing shock (Correct answer)
- To regulate voltage to the blower motor
- To replace the need for an equipment disconnect
Correct answer: To trip when current imbalance between hot and neutral exceeds ~5 mA, preventing shock
A GFCI detects small current imbalances caused by current flowing through an unintended path (like a person) and opens the circuit within milliseconds.
Question 6: A condenser fan motor runs but the compressor does not start and hums loudly. Capacitance testing shows the HERM section of the dual-run capacitor is reading 12 MFD instead of 40 MFD. What is the correct action?
- Replace only the FAN section of the capacitor
- Replace the dual-run capacitor with one matching the original MFD and voltage ratings (Correct answer)
- Add a second capacitor in parallel to make up the difference
- Replace the compressor contactor
Correct answer: Replace the dual-run capacitor with one matching the original MFD and voltage ratings
A degraded capacitor must be replaced with one of matching MFD and equal or higher voltage rating; adding capacitors in parallel is not an approved field repair.
Question 7: Which tool is best suited for measuring the actual running amperage of a compressor without breaking the circuit?
- Analog voltmeter
- Clamp-on ammeter (Correct answer)
- Megohmmeter
- Digital capacitance meter
Correct answer: Clamp-on ammeter
A clamp-on ammeter measures current by sensing the magnetic field around a conductor, allowing non-invasive current measurement on live circuits.
Air Conditioning Repair Electrical Components and Controls 5
A homeowner reports the outdoor unit runs continuously even after the thermostat setpoint is satisfied.
Which component is the most likely culprit?