Air Conditioning Repair Electrical Components and Controls 2 — Questions and Answers
Question 1: A contactor's coil is rated at 24V but the unit reads only 18V at the coil terminals during a call for cooling. What is the most likely cause?
- Low voltage from the transformer secondary (Correct answer)
- Shorted contactor contacts
- Incorrect thermostat setting
- Overcharged refrigerant system
Correct answer: Low voltage from the transformer secondary
Insufficient transformer output or excessive voltage drop in the 24V control circuit causes low coil voltage, preventing the contactor from pulling in fully.
Question 2: What is the purpose of a run capacitor in a single-phase PSC compressor motor?
- To boost starting torque only
- To maintain a phase shift that improves running efficiency (Correct answer)
- To protect against short cycling
- To limit inrush current at startup
Correct answer: To maintain a phase shift that improves running efficiency
A run capacitor stays in the circuit continuously, maintaining the phase shift between run and start windings to improve motor efficiency and power factor.
Question 3: A dual-run capacitor is labeled 40/5 MFD 440V. Which terminal pair connects to the compressor?
- HERM and C (Correct answer)
- FAN and C
- HERM and FAN
- C and GND
Correct answer: HERM and C
The HERM (hermetic) terminal and C (common) terminal form the 40 MFD section used for the compressor motor.
Question 4: Which safety device opens on a rise in discharge pressure and automatically resets when pressure drops?
- Low-pressure switch
- High-pressure switch (auto-reset) (Correct answer)
- Thermistor
- Fusible plug
Correct answer: High-pressure switch (auto-reset)
An auto-reset high-pressure switch opens the control circuit when discharge pressure exceeds its setpoint and closes again once pressure falls below the reset point.
Question 5: A technician measures 240V across a contactor's load-side terminals with the contactor pulled in. What does this indicate?
- Normal operation with load connected
- Open contacts with load disconnected (Correct answer)
- Welded contacts
- Low line voltage
Correct answer: Open contacts with load disconnected
Voltage across open load-side terminals indicates the contactor is not closing; if the contacts were closed, voltage would be near zero across them.
Question 6: What happens to a start capacitor if it remains in the circuit longer than a few seconds?
- It charges to full system voltage
- It overheats and can fail or explode (Correct answer)
- It improves motor efficiency
- It reduces starting torque
Correct answer: It overheats and can fail or explode
Start capacitors are designed for short-duration use only; prolonged energization causes excessive heat buildup leading to failure.
Question 7: A potential relay used in a hard-start kit opens its contacts when which condition is met?
- Compressor discharge pressure rises
- Back-EMF across the start winding reaches approximately 75% of line voltage (Correct answer)
- Outdoor ambient temperature exceeds 95°F
- Suction pressure drops below 70 psig
Correct answer: Back-EMF across the start winding reaches approximately 75% of line voltage
The potential relay monitors the voltage induced in the start winding; once back-EMF rises to the relay's pick-up voltage, it opens to remove the start capacitor from the circuit.
Air Conditioning Repair Electrical Components and Controls 2
A contactor's coil is rated at 24V but the unit reads only 18V at the coil terminals during a call for cooling.
What is the most likely cause?