Carrier 38QN - Motor Winding Electrical Tests; Ground Circuit Test; Short Circuit Test

POWER OFF'
OHMMETER
O-IO n SCALE
9
5.2 n
b
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0 . 61 !
6
?
5.8 ll
A
(example)
TO DETERMINE INTERNAL CONNECTIONS OF SINGLE¬
PHASE MOTORS (C,S,R) except shaded-pole
Deduction:
(D-d)
(greatest resistance)
Run Winding (R)
5.811 (ohm)
Start Winding (S)
(D—<D
(smallest resistance)
(2) is common (C)
0.611
by elimination
CD—©
(remaining resistance)
(2) is common,
5.211
therefore (T) is
Start Winding (S)
(3) is Run Winding (R)
Fig. 17 — Determining Internal Connections
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).
NOTE: If there is an internal line break protector, it
must be closed.
Three-Phase Motors — See Fig. 18. Resistance readings
between all 3 sets of windings should be the same.
All compressors are equipped with internal motor pro¬
tection. 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 com¬
pressor terminals before taking readings.
Most common motor failures are due to either an open,
grounded or short circuit. Directions below are specifi¬
cally for single-phase units, however, they also apply to
3-phase compressors. When a single-phase 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.
If compressor has internal line break overload, be sure
it is closed.
Open Circuit — To determine if any winding has a break
in the internal wires and current is unable to pass through:
Be sure all power is off. Discharge all capacitors. Remove
wires from terminals C, S and R. Use an ohmmeter on
0-1000 ohm scale to check resistance from C-R, C-S
and R-S. Because winding resistances are usually less
than 10 ohms, each reading appears to be approximately
zero ohm. If resistance remains at 1000 ohms, an open or
break exists and compressor should be replaced.
Be sure internal
porarily open.
line break overload is not tern- J
Ground Circuit — To determine if a wire has broken or
come in direct contact with shell, causing a direct short
to ground: Be sure all power is off. Discharge all capaci¬
tors. Remove wires from terminals C, S and R. On
hermetic compressors, allow crankcase heaters to remain
on for several hours before checking motor to ensure
windings are not saturated with refrigerant. Use an
ohmmeter on R x 10,000 ohm scale. A megohmmeter
may be used in place of ohmmeter (follow manufacturer’s
instructions). 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.
Note meter scale. If reading of zero or low resistance is
obtained, motor is grounded. Replace compressor.
A compressor of one-ton capacity or less is probably
grounded if resistance is below one million ohms. On
larger size single-phase compressors, resistance to ground
should not be less than 1000 ohms per volt of operating
voltage.
Example:
230-1-60 ... 230 x 1000 = 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): First, 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 accu¬
racy within ± 5% - 10%. This requires 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.) Be sure all power is off. Discharge all
capacitors. Remove wires from terminals C, S and R.
Place instrument probes together, determine probe and
lead wire resistance. Check resistance readings from C-R,
C-S and R-S. Subtract instrument probe and lead
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