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I have an old air compressor (1970s/1980s vintage) that has recently started tripping the circuit breaker that it is on. I hooked up a clamp meter and found that when the compressor motor is running with no load (belt removed) it is drawing 12 amps and when the belt is in place it is drawing 27 amps.

The motor is a 1hp unit with a listed FLA of just over 17 amps @ 120v. I've checked for any loose connections as well as looked it over with a thermal camera while it's running and see nothing that stands out as obviously abnormal.

The motor can be wired for either 120 or 240 volts, and is currently wired for 120 volts. There are four wires inside of the connection box on the end of the motor.

What would cause such a high current draw and what should I check next?

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With no load on the motor you should not see or even be close to FLA at speed. Sounds like time to add grease/ oil to the motor bearings. You could also replace the bearings. It is common for older motors to start to draw more as the bearings fail. I would not wait long or the rotor may start hitting the stator then it is time for a new motor most of the time.

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  • From what I can tell this motor offers no provisions for bearing maintenance other than to tear it apart and replace them, however, it's not excessively noisy and it takes several seconds for it to run down to a stop after power has been disconnected. – John Hodge Mar 15 '16 at 19:31
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    Spin the motor by hand. If it spins freely, it's not the bearings. Sorry, gonna be the minority report here: something is wrong with the motor. Either a shorted winding in the motor that's making the motor fight itself, or something is wrong with the apparatus which allows an induction motor to start on single-phase. Either way it's making 1200W of heat, and will soon burn up the motor. IMO. – Harper - Reinstate Monica Mar 15 '16 at 20:22
  • OP said it was drawing ~70% of FLA with zero load belt off. Bad bearings or shorted stator winding are really the only 2 options to cause that high of a draw unloaded. It is a single phase motor so I hope it is single phasing. – Ed Beal Mar 15 '16 at 23:29
  • The motor spins freely and doesn't seem to make any odd noises so I guess it's probably a problem with the electrical components. I think I may need to do some research on how single phase motors work. I know my table saw seems to have a centrifugal switch that comes into play during startup/shutdown, but I haven't heard anything similar from the compressor. – John Hodge Mar 23 '16 at 17:48
  • It depends on the type of motor, some have Capacitors usually under a metal cover on the side or back. Or the switch like your table saw has. Drawing that many amps unloaded may fry the motor. If you are running at full speed, I would also think about having the motor Megged to see if the winding were damaged. Centrifugal switches I have had fail the contacts were burned and would not make it to speed. With a bad cap they usually wont start. – Ed Beal Mar 23 '16 at 18:03
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When the current is high with no load, check for proper voltage. The “Rule of Thumb” is Low Voltage = High Amps. If voltage is correct at the feed, check all connections to the motor leads. Check all connections in disconnect, motor operator, distribution panel, etc. If there is a loose connection, you might show a proper voltage, but when it is under a load it will drop. After you are confident about voltage, check for bad bearings. listen to the sound of the motor while running. Take a long screw driver, touch the bearing housing with the metal end while putting the grip end against your ear. Listen for grinding sounds. Also, while power is off, can you free spin the motor? What does it sound like? While power is off and motor is not turning, can you move the shaft up and down.
If all of that checks normal, you might have a partial ground in the winding.

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What kind of motor is it exactly? Does it have a centrifugal switch? If so, the switch might be bad, keeping the starting winding energized all the time.

If it doesn't have a centrifugal switch, but has a start/run capacitor, that could be failing.

Either will cause excessive current demand on a single phase motor.

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Doesn't really sound like an issue with the motor. The motor will have a higher draw when it has increased resistance against it's rotation. That's not abnormal.

The problem is farther down the line. What about a filter, valve, etc. that could be bad?

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    The compressor appears to work normally other than the high current draw. It spins easily by hand and is a pretty simple design using reed valves and a simple felt-like intake filter. I have a larger 1.75 hp motor on a table saw and it only pulls 3 amps with no load, so seeing 12 on the smaller compressor motor with no load surprised me. – John Hodge Mar 15 '16 at 18:34

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