Air Conditioning Repair Electrical Systems and Controls 3 — Questions and Answers
Question 1: A hard start kit (potential relay and start capacitor) is added to a compressor. What is the primary purpose of this kit?
- Improve motor efficiency during steady-state operation
- Increase starting torque to overcome high head pressure (Correct answer)
- Reduce run current below nameplate FLA
- Replace a failed run capacitor
Correct answer: Increase starting torque to overcome high head pressure
A hard start kit boosts starting torque, allowing the compressor to start against elevated head pressure or under low-voltage conditions.
Question 2: On a heat pump system, the reversing valve solenoid is energized in cooling mode. If the solenoid coil reads infinite resistance, what will happen?
- The system will operate in heating mode only (Correct answer)
- The compressor will short-cycle
- The outdoor fan will not run
- The thermostat will lose its display
Correct answer: The system will operate in heating mode only
An open reversing valve solenoid coil means the valve cannot be electrically shifted, leaving it in its de-energized (heating) position.
Question 3: What is the purpose of a time-delay relay (TDR) installed in an air conditioning control circuit?
- Prevent short cycling of the compressor by enforcing a minimum off time (Correct answer)
- Boost the starting capacitance of the compressor motor
- Regulate the 24V transformer output
- Protect the condenser fan from overcurrent
Correct answer: Prevent short cycling of the compressor by enforcing a minimum off time
A time-delay relay enforces a minimum off period between compressor cycles to protect against refrigerant pressure equalization issues and motor overheating.
Question 4: A technician uses a megohmmeter to test a compressor motor winding to ground. A reading of 2 megohms is obtained. What does this indicate?
- The winding insulation is acceptable
- The winding has failed completely
- Moisture or degraded insulation is present (Correct answer)
- The test is inconclusive without a capacitor reading
Correct answer: Moisture or degraded insulation is present
A compressor winding-to-ground reading below 1,000 megohms suggests compromised insulation, likely from moisture ingress or winding breakdown.
Question 5: Which safety control opens the circuit to the compressor contactor coil when refrigerant pressure exceeds the set point?
- Low-pressure switch
- High-pressure switch (Correct answer)
- Loss-of-charge switch
- Oil pressure switch
Correct answer: High-pressure switch
The high-pressure switch is a normally closed safety device that opens to cut power to the contactor coil when discharge pressure exceeds its cut-out setting.
Question 6: A technician finds that the outdoor unit is not running but the indoor blower is operating. The thermostat is calling for cooling. What sequence of checks is most efficient?
- Check refrigerant charge, then test capacitor, then check contactor
- Check 24V at contactor coil, verify contactor pulls in, check 240V at compressor terminals (Correct answer)
- Replace contactor coil, then run capacitor, then check compressor winding
- Reset high-pressure switch, check low-pressure switch, test transformer
Correct answer: Check 24V at contactor coil, verify contactor pulls in, check 240V at compressor terminals
Starting with 24V at the coil, verifying contactor operation, then checking 240V at the load terminals follows a logical voltage-first diagnostic sequence.
Question 7: What is the function of the 'Y' terminal on a standard residential thermostat?
- Controls the indoor blower fan only
- Signals the cooling system compressor to run (Correct answer)
- Powers the thermostat display
- Activates the emergency heat mode
Correct answer: Signals the cooling system compressor to run
The Y terminal sends the 24V cooling call signal to energize the contactor coil and start the compressor.
Air Conditioning Repair Electrical Systems and Controls 3
A hard start kit (potential relay and start capacitor) is added to a compressor.
What is the primary purpose of this kit?