Air Conditioning Repair Electrical Components and Controls 3 — Questions and Answers
Question 1: What is the primary function of a crankcase heater on a compressor?
- To keep refrigerant subcooled during the off cycle
- To prevent refrigerant migration and oil dilution during the off cycle (Correct answer)
- To warm the discharge line in cold weather
- To maintain suction superheat
Correct answer: To prevent refrigerant migration and oil dilution during the off cycle
Crankcase heaters keep the compressor oil warm so refrigerant does not migrate into the crankcase and dilute the lubricant during shutdown.
Question 2: A time-delay relay in an AC control circuit is set for 5 minutes. What is its purpose?
- To allow the condenser fan to pre-cool before the compressor starts
- To prevent short-cycle compressor restarts (Correct answer)
- To sequence the indoor blower off after the compressor
- To delay thermostat response to setpoint changes
Correct answer: To prevent short-cycle compressor restarts
Time-delay relays prevent short cycling by enforcing a minimum off time before the compressor can restart, protecting it from pressure equalization issues.
Question 3: When testing a capacitor with a capacitance meter, a reading of zero indicates what condition?
- The capacitor is within tolerance
- The capacitor is overcharged
- The capacitor is open (internally broken) (Correct answer)
- The capacitor has a shorted dielectric
Correct answer: The capacitor is open (internally broken)
A zero capacitance reading means the internal element is open and the capacitor cannot store a charge.
Question 4: A compressor is wired to a 208/230V disconnect. Which wire gauge is required for a 30-amp circuit per NEC?
- 14 AWG
- 12 AWG
- 10 AWG (Correct answer)
- 8 AWG
Correct answer: 10 AWG
NEC requires 10 AWG copper conductors for a 30-amp circuit at standard ambient temperatures.
Question 5: What type of transformer is typically used to supply 24V control voltage in a residential split system?
- Autotransformer
- Step-down transformer (Correct answer)
- Step-up transformer
- Isolation transformer with center tap
Correct answer: Step-down transformer
A step-down transformer reduces line voltage (120V or 240V primary) to 24V AC for the low-voltage control circuit.
Question 6: A technician finds a blown 3-amp control fuse repeatedly. What is the correct diagnostic approach?
- Replace with a 5-amp fuse for a permanent fix
- Disconnect control wiring sections to isolate the short before replacing the fuse (Correct answer)
- Replace the thermostat immediately
- Replace the transformer and fuse together
Correct answer: Disconnect control wiring sections to isolate the short before replacing the fuse
Isolating sections of the control circuit identifies the shorted wire or component causing the overcurrent before replacing the fuse.
Question 7: What does a locked-rotor amperage (LRA) rating indicate on a compressor nameplate?
- The maximum current the compressor draws during normal operation
- The inrush current drawn when the motor first starts (Correct answer)
- The current at which the overload protector trips
- The full-load amperage at rated conditions
Correct answer: The inrush current drawn when the motor first starts
LRA is the high initial current drawn by a stalled or starting motor before it reaches operating speed.
Air Conditioning Repair Electrical Components and Controls 3
What is the primary function of a crankcase heater on a compressor?