Try to turn the compressor by hand. It should be possible to turn it around. It should turn around smoothly, without any 'hickups' (be sure to completely shut down the air compressor!). Check the isolation of the motor windings.
[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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