Short Circuit; Refrigeration System - Carrier 38TN Service Manual

230 volts X 1000 ohms/volt = 230,000 ohms minimum.

SHORT CIRCUIT

To determine if any wires within windings have broken through
their insulation and made contact with other wires, thereby
shorting all or part of the winding(s), be sure the following
conditions are met.
1. Correct motor winding resistances must be known before
testing, either from previous readings or from manufacturer's
specifications.
2. Temperature of windings must be as specified, usually about
70° F.
3. Resistance measuring instrument must have an accuracy
within ± 5-10 percent. This requires an accurate ohmmeter
such as a Wheatstone bridge or null balance-type instrument.
4. Motor must be dry or free from direct contact with liquid
refrigerant.
MAKE THIS CRITICAL TEST
(Not advisable unless above conditions are met.)
1. Be sure all power is off.
2. Discharge all capacitors.
3. Remove wires from terminals C, S, and R.
4. Place instrument probes together and determine probe and
lead wire resistance.
5. Check resistance readings from C-R, C-S, and R-S.
6. Subtract instrument probe and lead resistance from each
reading.
If any reading is within ± 20 percent of known resistance, motor is
probably normal. Usually a considerable difference in reading is
noted if a turn-to-turn short is present.
Step 3—System Clean-Up After Burnout
Turn off all power to unit before proceeding. Wear safety
glasses and gloves when handling refrigerants. Acids formed
as a result of motor burnout can cause burns.
NOTE: To analyze level of suspected contamination from com-
pressor burnout, use RCD test kit. (See your distributor/branch.)
Some compressor electrical failures can cause motor to burn.
When this occurs, byproducts of burn, which include sludge,
carbon and acids, contaminate system. If burnout is severe enough,
system must be cleaned before replacement compressor is in-
stalled. The 2 types of motor burnout are classified as mild or
severe.
In mild burnout, there is little or no detectable odor. Compressor
oil is clear or slightly discolored. An acid test of compressor oil
will be negative. This type of failure is treated the same as
mechanical failure. Liquid line strainer should be removed and
liquid line filter drier installed.
In a severe burnout, there is a strong, pungent, rotten egg odor.
Compressor oil is very dark. Evidence of burning may be present
in tubing connected to compressor. An acid test of compressor oil
will be positive. Complete system must be reverse flushed with
refrigerant. Indoor piston or TXV must be cleaned or replaced.
Remove and discard liquid line strainer and filter drier. After
system is reassembled, install liquid and suction line R134A filter
driers. Run system for 2 hrs. Discard both driers and install new
R134A liquid line drier only.
Step 4—Compressor Removal and Replacement
Once it is determined that compressor has failed and the reason
established, compressor must be replaced.
Wear safety glasses and gloves when handling refrigerants
and when using brazing torch.
1. Shut off all power to unit.
2. Remove and recover all refrigerant from system until pressure
gages read 0 psi. Use all service ports.
3. Disconnect electrical leads from compressor. Disconnect or
remove crankcase heater and remove compressor holddown
bolts.
4. Cut compressor from system with tubing cutters. Do not use
brazing torch for compressor removal. Oil vapor may ignite
when compressor is disconnected.
5. Scratch matching marks on stubs in old compressor. Make
corresponding marks on replacement compressor.
6. Use torch to remove stubs from old compressor and to reinstall
them in replacement compressor.
7. Use copper couplings to tie compressor back into system.
8. Evacuate system, recharge, check for normal system opera-
tion.
Do not leave system open to atmosphere for a prolonged
period. Product damage could occur. R134A refrigerant is
highly susceptible to moisture absorption.

REFRIGERATION SYSTEM

Step 1—Refrigeration Cycle
In a refrigerant system, refrigerant moves heat from 1 place to
another. It is useful to understand flow of refrigerant in a system.
In a straight cooling system, compressed hot gas leaves compres-
sor and enters condensing coil. As gas passes through condenser
coil, it rejects heat and condenses into liquid. The liquid leaves
condensing unit through liquid line and enters metering device at
indoor coil. As it passes through metering device, it becomes a
gas-liquid mixture. As it passes through indoor coil, it absorbs heat
and refrigerant and is again compressed to a hot gas, and cycle
repeats.
Step 2—Leak Detection
Always wear safety glasses and gloves when handling refrig-
erants.
New installations should be checked for leaks prior to complete
charging.
If a system has lost all or most of its charge, system must be
pressurized again to approximately 150 lb minimum. This can be
done by adding refrigerant using normal charging procedures or by
pressurizing system with nitrogen (less expensive than refrigerant).
Nitrogen also leaks faster than R134A and is not absorbed by
refrigeration oil. Nitrogen cannot, however, be detected by a leak
detector. (See Fig. 16.)
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