Most common motor failures are due to either an open, grounded, or short circuit. Directions below are specifically 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 equal- ization, defective capacitor(s), relays, wiring, and so forth. 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:
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 ohmme- ter 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.
CAUTION: 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:
1.Be sure all power is off.
2.Discharge all capacitors.
3.Remove wires from terminals C, S, and R.
4.On hermetic compressors, allow crankcase heaters to re- main on for several hours before checking motor to ensure windings are not saturated with refrigerant.
5.Use an ohmmeter on R X 10,000 ohm scale. A megohm- meter may be used in place of ohmmeter. Follow manufac- turer’s instructions.
6.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.
7.Note meter scale.
8.If reading of zero or low resistance is obtained, motor is grounded. Replace compressor.
A 1 ton or less capacity compressor is probably grounded if resistance is below 1 million ohms. On larger-sized, single-phase compressors, 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 manufactur- er’s specifications.
2.Temperature of windings must be as specified, usually about 70°F.
3.Resistance-measuring instrument must have an accuracy within ± 5 to 10 percent. This requires an accurate ohmme- ter, such as a Wheatstone bridge or null balance-type instrument.
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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.
III. SYSTEM CLEANUP AFTER BURNOUT
CAUTION: Turn off all power to unit before proceed- ing. 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 Total Test. See your distributor/branch.
Some compressor electrical failures can cause motor to overheat. When this occurs, by-products, which include sludge, carbon, and acids, can contaminate system. If burnout is severe enough, system must be cleaned before replacement compressor is installed. 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. Check-Flo-Rater™ or TXV must be cleaned or re- placed. In a heat pump, accumulator and reversing valve are replaced. These components are also removed and bypassed during reverse-flushing procedure. Remove and discard liquid-line strainer. After system is reassembled, install liquid-line and suction-line filter driers. Run system for 2 hrs. Discard both driers and install new liquid-line drier only.
IV. COMPRESSOR REMOVAL AND REPLACEMENT
Once it is determined that compressor has failed and the reason established, compressor must be replaced.
CAUTION: Wear safety glasses and gloves when han- dling 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 zero 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.
