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Electric Motors/Old Century motor - Replacing electric supply cord


I recently purchased a used Ingersol Rand air compressor from an old defunct gas station.  All works great but the electric supply cord insulation is dry rotted so need to replace it.  The existing plug has only two prongs and existing cord has only two wires, one is black, the other white,  These are connected via screw terminals in the Square D pressure switch which is then wired to the motor.  Should I replace with grounded 3 wire cord?   (Hot, neutral, and ground).  If so how/where do I connect the ground wire?  Or should I just replace the cord using only two wires?  The motor is made by Century, St. Louis, MO.  The IID plate on the motor is difficult to read due to the age but as best I can read, it is Model CS66-FH66-5FAS, Frame J56, HP 1/2, RPM 1725, PH 1, CY 60, V 115/230, A 7.6/3, Rating OpenCont 40 C, EM18-104425-01, Serial No. 68.  The Square D switch says it is Class 9013, Type GRG2, Form XZ  B464-262.  

My main concern is safety and whether this should be grounded.  Thanks for your help.

Ken glad to hear you are concerned with safety,  and yes you need a ground,     

Don;t make it complicated,   get the proper size SO or other cord,   2 wire with ground,   replace the existing two wires as if you were putting another two wire cord on,    except you will have the third ground only wire,  

The ground wire should follow from the source,  to switch,  to motor frame,    

All you will be doing is adding a chassis ground from motor to grounding bar,    you might have to be creative at the switch   there may be no provisions for a ground wire but there should be somewhere to connect the ground to the switch case, and continue on to the motor,    then you have a continual ground from breaker to switch to motor,    the other conductors remain the same,  

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