CFC Compressor & Component Diagnostics 4 — Questions and Answers
Question 1: A technician finds that a compressor trips on the high-pressure control shortly after startup. The condenser is clean and fans are running. What diagnostic step should be performed next?
- Replace the high-pressure control switch
- Check the refrigerant charge and look for non-condensables in the system (Correct answer)
- Inspect the evaporator for ice blockage
- Test the compressor capacitors
Correct answer: Check the refrigerant charge and look for non-condensables in the system
Non-condensable gases (air, nitrogen) trapped in the system cause abnormally high head pressure that triggers the high-pressure control, and overcharge also elevates head pressure.
Question 2: Which symptom would indicate a restriction in the compressor's suction strainer screen?
- High superheat and low suction pressure with normal subcooling (Correct answer)
- Low superheat and high suction pressure
- High suction and high discharge pressure
- Normal suction pressure with high discharge pressure
Correct answer: High superheat and low suction pressure with normal subcooling
A plugged suction strainer restricts refrigerant vapor flow to the compressor, causing low suction pressure and high superheat due to reduced mass flow.
Question 3: When checking a compressor for electrical faults using an ohmmeter, an open reading between the common and run terminals indicates:
- A normal motor at rest
- A short circuit in the motor windings
- An open winding requiring compressor replacement (Correct answer)
- High motor resistance from elevated temperature
Correct answer: An open winding requiring compressor replacement
An open reading (infinite resistance) between motor terminals indicates a broken winding, meaning current cannot flow and the compressor must be replaced.
Question 4: A rotary vane compressor exhibits decreasing capacity over time. Inspection reveals worn vanes. What is the primary effect of worn vanes on system performance?
- Increased suction pressure and decreased discharge pressure
- Decreased volumetric efficiency due to increased internal bypass leakage (Correct answer)
- Higher oil consumption causing evaporator flooding
- Increased power consumption with higher discharge temperature
Correct answer: Decreased volumetric efficiency due to increased internal bypass leakage
Worn vanes allow refrigerant gas to bypass from the high-pressure to the low-pressure side within the compressor, reducing volumetric efficiency and system capacity.
Question 5: A technician is diagnosing a screw compressor that has excessive vibration at startup. The refrigerant oil separator appears full. What is the most likely cause?
- Bearing failure within the screw compressor rotors
- Oil foaming due to rapid pressure drop at compressor startup
- Refrigerant migration into the oil causing flashing during startup (Correct answer)
- Misaligned coupling between the motor and compressor
Correct answer: Refrigerant migration into the oil causing flashing during startup
Refrigerant that migrates and dissolves in compressor oil during off-cycles flashes violently at startup when pressure drops, causing severe vibration and oil foaming.
Question 6: After replacing a failed hermetic compressor on a CFC system, the technician should perform which procedure to prevent premature failure of the new compressor?
- Reuse the existing filter-drier to save time and cost
- Flush the system with nitrogen and install a new filter-drier (Correct answer)
- Immediately recharge with CFC refrigerant and monitor pressures
- Perform a standing pressure test at operating pressure for 24 hours
Correct answer: Flush the system with nitrogen and install a new filter-drier
Flushing with nitrogen removes acid, debris, and moisture from the system, and installing a new filter-drier ensures these contaminants do not damage the replacement compressor.
Question 7: A compressor's actual amp draw exceeds its rated FLA by 20% with normal system pressures. What is the most likely cause?
- Low refrigerant charge
- Supply voltage below the minimum nameplate rating (Correct answer)
- Plugged condenser coil
- High suction superheat
Correct answer: Supply voltage below the minimum nameplate rating
Low supply voltage causes compressor motors to draw more current (amps increase as voltage decreases) to maintain the same power output, potentially exceeding the FLA rating.
CFC Compressor & Component Diagnostics 4
A technician finds that a compressor trips on the high-pressure control shortly after startup.
The condenser is clean and fans are running.
What diagnostic step should be performed next?