Q45. When inspecting hermetic compressors, you
freezing point of water. As a result, ice can form in the
should look for what type of problems?
MAINTENANCE OF MOTORS
Learning Objective: Understand basic maintenance of
motors and methods of electrical troubleshooting of
Troubles with the electrical motors used to drive
the compressors of mechanical refrigeration systems
fall into two classesmechanical and electrical.
Some compressors are belt-driven from the
electrical motor. For proper operation, both the belt
tension and pulley alignment adjustments must be
made. Belt tension should be adjusted so a l-pound
force on the center of the belt, either up or down, does
not depress it more than one-half inch. To adjust the
alignment, loosen the setscrew on the motor pulley
after tension adjustment is made. Be sure the pulley
turns freely on the shaft; add a little oil if necessary.
Turn the flywheel forward and backward several
times. When it is correctly aligned, the pulley does not
move inward or outward on the motor shaft. Tighten
the setscrew holding the pulley to the shaft before
starting the motor.
Compressors may also be driven directly by a
mechanical coupling between the motor and
compressor shafts. Be sure the two shafts are
positioned so they form a straight line with each other.
The coupling on direct drive units should be realigned
after repair or replacement. Clamp a dial indicator to
the motor half coupling with its pointer against the
outer edge of the compressor half coupling. Rotate the
motor shaft, and observe any fluctuations of the
indicator. Move the motor or compressor until the
indicator is stationary when revolving the shaft one full
turn. Secure the hold-down bolts and then recheck.
Moisture in the System
When liquid refrigerant that contains moisture
vaporizes, the moisture separates from the vapor.
Because the vaporization of the refrigerant causes a
cooling effect, the water that has separated can freeze.
Most of the expansion and vaporization of the
refrigerant occurs in the evaporator. However, a small
amount of the liquid refrigerant vaporizes in the
expansion valve, and the valve is cooled below the
expansion valve and interfere with its operation. If the
needle in the valve freezes in a slightly off-seat
position, the valve cannot permit the passage of
enough refrigerant. If the needle freezes in a position
far from the seat, the valve feeds too much refrigerant.
In either case, precautions must be observed to assure a
A dehydrator is filled with a chemical known as a
desiccant, which absorbs moisture from the refrigerant
passing through the dehydrator. Dehydrators are
installed in the liquid line to absorb moisture in the
system after the original installation. An arrow on the
dehydrator indicates the direction of flow. Desiccants
are granular and are composed of silica gel, activated
alumina, or calcium sulfate. Do not use calcium
chloride or chemicals that form a nonfreezing solution.
These solutions may react with moisture to form
undesirable substances, such as gums, sludges, or
waxes. Follow the manufacturer's instructions as to
limitations of dehydrators, as well as operation,
recharging, replacing, and servicing.
Loose Copper Tubing
In sealed units, loose copper tubing is usually
detected by the sound of rattling or metallic vibration.
Bending the tubing carefully to the position of least
vibration usually eliminates the defect. Do not touch it
against other tubing or parts at a point of free
movement, and do not change the tubing pitch or the
tubing diameter by careless bending.
In open units, lengths of tubing must be well
supported by conduit straps or other devices attached
to walls, ceilings, or fixtures. Use friction tape pads to
protect the copper tubing from the metal of the strap.
When two tubes are together in a parallel position,
wrapping and binding them together with tape can
prevent vibration. When two lines are placed in
contact for heat exchange, they should be soldered to
prevent rattling and to permit better heat transfer.
Doors and Hardware
When hinges must be replaced because of lack of
lubrication or other reasons, the use of exact duplicates
is preferable. Loose hinge pins must be securely
braided. When thrust bearings are provided, they are
held in place by a pin.
The latch or catch is usually adjusted for proper
gasket compression. Shims or spacers may be added