Carrier 38TN Service Manual page 10

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POWER OFF!
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OHMMETER
0-10Ω SCALE
1
2
5.2Ω
0.6Ω
2
3
5. Clean off excess flux, check for leaks, and apply paint over
repaired area to prevent corrosion.
Do not use this method to repair a compressor leak due to severe
corrosion. Never attempt to repair a compressor leaking at electric
terminals. This type of failure requires compressor replacement.
Step 2—Electrical Failures
To avoid personal injury or death, do not supply power to unit
with compressor terminal box cover removed.
The compressor mechanical pump is driven by an electric motor
within its hermetic shell. In electrical failures, compressor does not
run although external electrical and mechanical systems appear
normal. Compressor must be checked electrically for abnormali-
ties.
Before troubleshooting compressor motor, review this description
of compressor motor terminal identification.
SINGLE-PHASE MOTORS
To identify terminals C, S, and R:
1. Turn off all unit power.
2. Short the run and start capacitors to prevent shock.
3. Remove all wires from motor terminals.
4. Read resistance between all pairs of terminals using an
ohmmeter on 0-10 ohm scale.
5. Determine 2 terminals that provide greatest resistance reading.
Through elimination, remaining terminal must be common (C).
Greatest resistance between common (C) and another terminal
indicates start winding because it has more turns. This terminal is
start (S). Remaining terminal will be run winding (R). (See Fig.
15.)
NOTE: If compressor is hot, allow time to cool and internal line
break to reset. There is an internal line break protector which must
be closed.
All compressors are equipped with internal motor protection. If
motor becomes hot for any reason, protector opens. Compressor
should always be allowed to cool and protector to close before
troubleshooting. Always turn off all power to unit and disconnect
leads at compressor terminals before taking readings.
Most common motor failures are due to either an open, grounded,
or short circuit. When a compressor fails to start or run, 3 tests can
(EXAMPLE)
TO DETERMINE INTERNAL CONNECTIONS OF SINGLE-
PHASE MOTORS (C,S,R) EXCEPT SHADED-POLE
DEDUCTION:
1 3 (GREATEST RESISTANCE)
2 3 (SMALLEST RESISTANCE)
1
1 2 (REMAINING RESISTANCE)
5.8Ω
3
Fig. 15—Identifying Internal Connections
5.8Ω (OHM)
0.6Ω
5.2Ω
help determine the problem. First, all possible external causes
should be eliminated, such as overloads, improper voltage, pres-
sure equalization, defective capacitor(s), relays, wiring, etc. Com-
pressor has internal line break overload, so be certain it is closed.
OPEN CIRCUIT
To determine if any winding has a break in the internal wires and
current is unable to pass through, follow these steps.
1. Be sure all power is off.
2. Discharge all capacitors.
3. Remove wires from terminals C, S, and R.
4. Check resistance from C-R, C-S, and R-S using an ohmmeter
on 0-1000 ohm scale.
Because winding resistances are usually less than 10 ohms, each
reading appears to be approximately 0 ohm. If resistance remains
at 1000 ohms, an open or break exists and compressor should be
replaced.
Be sure internal line break overload is not temporarily open.
GROUND CIRCUIT
To determine if a wire has broken or come in direct contact with
shell, causing a direct short to ground follow these steps.
1. Be sure all power is off.
2. Discharge all capacitors.
3. Remove wires from terminals C, S, and R.
4. Allow crankcase heaters to remain on for several hrs before
checking motor to ensure windings are not saturated with
refrigerant, using an ohmmeter on R X 10,000 ohm scale or
megohmmeter (follow manufacturer's instructions).
5. Place 1 meter probe on ground or on compressor shell. Make
a good metal-to-metal contact. Place other probe on terminals
C, S, and R in sequence.
6. Note meter scale.
7. If reading of 0 or low resistance is obtained, motor is
grounded. Replace compressor.
Compressor resistance to ground should not be less than 1000
ohms per volt of operating voltage.
Example:
10
RUN WINDING (R)
START WINDING (S)
2
IS COMMON (C)
BY ELIMINATION
2
IS COMMON,
THEREFORE, 1 IS
START WINDING (S)
3 IS RUN WINDING (R)
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