Check the isolation of the motor windings. Should be in the mage-ohms (you need an isolation tester / high voltage ohm meter for this). Check the voltage when the compressor is running. If the voltage drop significantly when the compressor starts/runs, you have a bad connection somewhere.
[pdf] Air compressor oil is standard or synthetic oil made specifically for air compressors. Unlike motor oil, air compressor oil does not contain detergent and typically has lower amounts of carbon, sulfur, an.
[pdf] The solution here is to check and clean the unidirectional valve, replacing it when necessary. Improper installation or blockage of the oil return pipeline will impede the normal return of the lubricating oil.
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