Service manual

20
COMPRESSOR TROUBLESHOOTING
Compressor Failures
Compressor failures are classified in 2 broad failure categories;
mechanical and electrical. Both types are discussed below.
Mechanical Failures
A compressor is a mechanical pump driven by an electric motor
contained in a welded or hermetic shell. In a mechanical failure,
motor or electrical circuit appears normal, but compressor does not
function normally.
ELECTRICAL SHOCK HAZARD
Failure to follow this warning could result in personal injury
or death.
Do not supply power to unit with compressor terminal box
cover removed.
!
WARNING
ELECTRICAL SHOCK HAZARD
Failure to follow this warning could result in personal injury
or death.
Exercise extreme caution when reading compressor currents
when high--voltage power is on. Correct any of the problems
described below before installing and running a replacement
compressor.
!
WARNING
Locked Rotor
In this type of failure, compressor motor and all starting
components are normal. When compressor attempts to start, it
draws locked rotor current and cycles off on internal protection.
Locked rotor current is measured by applying a clamp--on ammeter
around common (blk) lead of compressor. Current drawn when it
attempts to start is then measured. Locked rotor amp (LRA) value
is stamped on compressor nameplate.
If compressor draws locked rotor amps and all other external
sources of problems have been eliminated, compressor must be
replaced. Because compressor is a sealed unit, it is impossible to
determine exact mechanical failure. However, complete system
should be checked for abnormalities such as incorrect refrigerant
charge, restrictions, insufficient airflow across indoor or outdoor
coil, etc., which could be contributing to the failure.
Runs, Does Not
Pump
In this type of failure, compressor motor runs and turns
compressor, but compressor does not pump refrigerant. A
clamp--on ampmeter on common leg shows a very low current
draw, much lower than rated load amp (RLA) value stamped on
compressor nameplate. Because no refrigerant is being pumped,
there is no return gas to cool compressor motor. It eventually
overheats and shuts off on its internal protection.
Noisy Compr
essor
Noise may be caused by a variety of internal and external factors.
Careful attention to the “type” of noise may help identify the
source. The following are some examples of abnormal conditions
that may create objectionable noise:
1. A gurgling sound may indicate a liquid refrigerant
floodback during operation. This could be confirmed if
there is no compressor
superheat. A compressor superheat
of “0” degrees would indicate liquid refrigerant returning to
the compressor. Most common reasons for floodback are:
loss of evaporator blower, dirty coils, and improper airflow.
2. A rattling noise may indicate loose hardware. Inspect all
unit hardware including the compressor grommets.
3. A straining (hard start) or vibration occurring at start up but
clears quickly after could indicate an off cycle refrigerant
migration issue. Refrigerant migration can occur when a
compressor is off and refrigerant vapor transfers from other
areas of the system, settles into the compressor as it is
attracted to the oil, and then condenses into the oil. Upon
start up, the compressor draws suction from within itself
first and lowers the boiling point of the refrigerant that is
entrained in the oil. This can cause the liquid refrigerant
and oil to boil into the compression area or liquid refrigerant
to wipe off oil films that are critical for proper lubrication.
Migration is worsened by greater temperature differentials
and/or extra refrigerant in the system. Prevention of
migration can be reduced by various options but some of
the more common remedies is to verify proper charge and
add a crankcase heater where this situation is suspected.
4. Operational vibration could indicate a charge issue. Verify
charge and ensure proper piping and structural penetration
insulation. Tubing that is too rigid to building rafters
without proper insulation could transfer noise throughout
the structure. On some occasions a sound dampener or
mass weight (RCD part no. 328209--751) placed on the
vibrating tubing has been known to reduce this noise.
Utilizing compressor split post grommets (see Fig. 12) may
also reduce this vibration if piping cannot be remedied.
5. An operational high pitch frequency or “waa waa” sound
that appears to resonate through the suction line could
indicate a need to add more flex or muffling in the lines.
This has been occasional in scroll compressor applications
and is usually remedied by adding a field--fabricated suction
line loop (see Fig. 13). Reciprocating compressors may
have a noticeable discharge pulsation that could be
remedied with a field installed discharge muffler.
Recommend loop by continuous tubing with no more than
12 inches vertical and 6 inch horizontal loop.
6. An internal “thunking”, “thumping”, “grinding” or
“rattling” noise could indicate compressor internal failures
and may be verified by comparing the compressor
amperage to what the compressor should be drawing
according to a manufacturer’s performance data.
7. A whistling or squealing noise during operation may
indicate a partial blockage of the refrigerant charge.
8. A whistle on shut down could indicate a partial leak path as
refrigerant is equalizing from high to low side. On
occasion, an in--line discharge check valve has prevented
this sound.