Mechanical Consulting Group Inc.
2208 Magnolia Lane
Elgin, IL 60120
Ph: (630) 830-2083
Fax: (630) 830-0630


TESTING FOR LIGHTNING OR POWER SURGE DAMAGE TO

SINGLE PHASE COMPRESSORS (RESIDENTIAL)

  • The motor windings of the compressor are tested for the proper winding resistance. The resistance of the run winding is approximately .5 to 1.5 ohm on 1 ½ ton (18,000 BTUH) to 3 ½ ton (42,000 BTUH) units. On 4 and 5 ton units the run winding resistance is approximately .3 to .5 ohm. The resistance of the start winding is approximately 2 ohms to 4 ohms on a 1 ½ ton to 3 ½ ton compressor. The start winding resistance should be approximately 1.5 ohms on 4 ton and 5 ton compressors. The resistance depends upon the size and make of the motor for the compressor.

  • A burned terminal or terminals at the compressor is not a likely sign of lightning or power surge damage although it is possible in cases where the lighting strike damage is severe. In these cases there is a lot of other evidence of a lightning strike. The condenser fan motor and the run capacitors for the compressor and the condenser fan motor are destroyed. The wires are burnt and there is evidence of arc marks to the cabinet where the wires run close to the cabinet. There is also considerable damage to other motors and appliances as well as to the electric panel.

The most likely cause of a burned electrical terminal on a compressor is a loose electrical connection. Most compressor manufacturers use ¼ inch quick connects for the compressor terminals. These connections are used both on the inside and outside of the compressor. These connections are used because it makes for quick assembly of units. There is a tendency for this type of connection to become loose after a period of time. The loose connection can be on either the inside or outside of the compressor. This loose connection causes a build up of heat and results in a burned terminal.

3. The resistance of the motor windings to ground (the compressor or cabinet case) is also measured. The test is run at approximately twice the operating voltage for the compressor. If the compressor operates at 230 volts, the test voltage that is used for the megohm test is 500 volts. If the insulation for the compressor motor windings is in excellent condition the resistance will read 100 megohms or more. Compressors with an insulation resistance as low as one megohm can operate. Steps should be taken to remove acid and water from the refrigerant or the compressor motor will fail.

4. If there is acid present in the compressor oil it is unlikely that the damage to the motor windings is due to lightning or a power surge. It takes time to make acid in the refrigeration system and lightning or a power surge is an instantaneous occurrence. The exception is if there was a larger power surge in which case there will be significant damage to other components of the air conditioning unit. The lack of acid in the refrigerant system does not mean that the compressor loss was due to lightning or a power surge. It just means that it is more likely, provided that there is no other evidence

5. If there is major damage to the motor windings for the compressor, ie. the motor windings are open, and or shorted, and the megohm reading is also less than .5 megohm it is likely that there were mechanical problems with the compressor. The compressor ran out of oil or some other problem caused the bearings on the compressor to seize, or the connecting rod or rods are broken and the electric motor could not turn the compressor. Another possibility is that the crankshaft is broken. An additional possibility for this kind of damage to the motor windings is acid in the compressor, due to moisture in the refrigerant or refrigerant system. Moisture in the refrigerant system is a result of poor servicing techniques.

6. Test the condenser fan motor to see if the condenser fan motor is good. The condenser fan motor is connected to the same source of electric as the compressor. If the compressor has failed due to lightning damage it is likely that the condenser fan motor would have failed also.

7. The run capacitor(s) for the compressor and condenser fan motor will be tested to see if it has the proper capacitance.

If there is a power surge or lightning that has come through the power wires, the run capacitor or capacitors will probably be damaged. The capacitor will most likely be shorted or open in which case it will not read any capacitance.

Most run capacitors have a tolerance of plus or minus 5%. If the capacitor is good it is unlikely that the damage to the compressor motor windings was due to lightning or a power surge. A run capacitor is made by two layers of extremely thin foil with a layer of extremely thin insulating paper or plastic that is placed in between the two layers of foil. The sandwich is rolled and inserted into a metal can with two terminals. In many cases the run capacitor for the compressor and the run capacitor for the condenser fan motor are combined, and there are three terminals. There is one common terminal for both the compressor and the condenser fan motor and one terminal for the compressor and one terminal for the condenser fan motor.

A high voltage spike or power surge due to lightning will burn through the thin layer of insulating paper or plastic and damage the run capacitor.

8. If the unit has a start capacitor, the start capacitor is more susceptible to lightning damage that the run capacitor is.

9. If the compressor is a hermetic compressor (welded shut) the compressor can be cut open to investigate the damage to the compressor motor windings and the mechanical part of the compressor. A power surge due to lightning would most likely cause a spot burn in the motor winding of the compressor. If there is mechanical damage to the compressor crankshaft, piston , rods or bearings, that too can be determined by cutting the case of the compressor open.

 

 

 

 

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