Service Manual

21
Single- Phase
Motors
To identify terminals C, S, and R:
1. Turn off all unit power.
2. Discharge 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 the start winding because it has more
turns. This terminal is the start (S). The remaining terminal
will be run winding (R).
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 help determine the problem. First, all
possible external causes should be eliminated, such as
overloads, improper voltage, pressure equalization,
defective capacitor(s), relays, wiring, etc. Compressor has
internal line break overload, so be certain it is closed.
Open
Circuit
UNIT PERSONAL INJURY HAZARD
Failure to follow this warning could result in
personal injury.
Use caution when working near compressor
terminals. Damaged terminals have the potential
to cause personal injury.
Never put face or body directly in line with
terminals.
!
WARNING
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.
UNIT DAMAGE HAZARD
Failure to follow this caution may result in equipment
damage or improper operation.
Be sure internal line break overload is not temporarily
open.
CAUTION
!
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. Allow crankcase heaters to remain on for several
hours before checking motor to ensure windings are
not saturated with refrigerant.
2. Using an ohmmeter on R X 10,000 ohm scale or
megohmmeter (follow manufacturer’s instructions).
3. Be sure all power is off.
4. Discharge all capacitors.
5. Remove wires from terminals C, S, and R.
6. Place one 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.
7. Note meter scale.
8. 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:
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.