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Re: Air compressor starts slowly and then trips 20 amp...
Is it 220 or 110 ? How many horse power is it but most importantly is how many amps is the motor itself? If it is 15 amps let's say then that doesn't leave much more for anything else to be running on the same circuit of which you have the compressor plugged into. While 110 is more convenient, I know that is my preferred choice, when you get to 3 HP or over size motors your looking at close to or right at 15 amps of supply needed. Could also be the breaker needs replacing. Hope this helps.
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If the pump turns easily by hand and trips breaker, most likely the start capacitor is weak or bad. If the pump is hard to turn by hand then problem is most likely the in-tank check valve is leaking pressure back to the pump causing the motor to overload. If the pump turns easily by hand and the check valve is good and capacitors are good, then use amp gauge to verify overload, if amp gauge shows normal amp draw then most likely weak reset button. Good luck
Most people recommend replacing the motor capacitor in this situation, since a faulty cap will reduce the starting torque. But in my case the check valve was the culprit - there was too much back pressure on the head.
Rmove the drive belt and check the motor. It should turn very freely by hand and if turned on should start up easily. If it turns hard check the armature bearings if it starts slowly but turns OK after you give it a push, you have a bad start capacitor. If the motor is OK check the pump. It should turn fairly freely except on the compression stroke where you should feel slightly more pressure is needed to turn it. If you can't turn the flywheel or it turns very hard, you should check the connecting rod to crank shaft joint and crank shaft bearings. These usually go bad if there is low or no oil in the crank case.
the start capacitor on the motor body is weak,the motor needs this component to over come the air pressure when cycling to re-build the low air pressure,change both the start and run capacitors when doing the start because the other run capacitor is just as old and it will soon fail
Does it trip the breaker or does the motor just hum and not turn the compressor? The motor may need a start capacitor. Do not keep tryin to start you can burn out the start winding on the motor.
TURN OFF POWER AND LOCK OUT.... try and take the belts off (if it has belts)of the motor and compressor and make sure both turn freely. IF THEY TURN FREELY THERE IS A PROBLEM WITH THE MOTOR. You said voltage, wire size and breakers are the right size.
Hope this helps you with your problem...Jim...please leave a comment
You gave a lot of information, but you didn't say if it worked when free wheeling. If not, I would suspect the inertia switch. If it is working, you should hear it click when the motor is winding down after being shut off, at least when free wheeling. A 20 amp breaker will certainly handle a 15 FLA motor, even on startup. They don't trip at 20 amps exactly, especially on a simple overload. Even at twice the rated amperage, it can take several minutes to trip it. They will trip quickly on a dead short, however, or a severe overload. I doubt the windings are shorted or it either wouldn't work (start windings) or would run at a slower speed with the inertia switch constantly tripping in and out with it coasting while tripped in (i.e., accelerate, coast, accelerate, coast, etc.)
If I did the math correctly, 5.5 HP is right at the boarderline of 20 amps, particularly for an older machine that has some deteriorating electronics.
What to do? Can you reconfig the motor to run on 220 Volts??? It would then run on 1/2 the amps.
If you have a dedicated 120 line for this compressor now ... all you have to do to change that to 220 is change the breaker (I presume you have a vacant slot to do this). Find the "neutral" for that wire (white) ane make it the "red" for your compressor line ... (and mark it as such for your future behefit) change the outlet from 110 V 20 A to 220 V 20 A. Both lines will run on #12 with G wire.
If you dont have a slot, you could change one of your standard slots to a "piggy back" remodeling style breaker (has two breakers in one slot). This will free up one slot in your panel for the new 220 (double pole) breaker.
Air compressors tend to draw more power as they age, however, will not trip breaker unless some malfunction. Here are some simple tests. Unplug compressor and turn the pump by hand, you may have to remove a cover to do this. It should turn freely; any resistance could be failing bearing (would make noise) unloader/check valve leaking allowing back pressure to pump, or failed start capacitor/start winding. Easy test for checkvalve. With pressure in tank loosen discharge hose at tank and check for air leak. Check valve should hold pressure in tank. Replace as needed. Failed capacitor/s will cause high amp draw. Remove from circuit and test. UF reading should be within 95% of printed rating. Finally, most air compressor motors have two windings, start and run. Power to windings is controlled by flyball switch inside rear of motor. This switch has contacts that can stick in start position and cause high amp draw. Examine windings for signs of overheating. Copper windings generally have color of new penny. Very dark brown windings bad news. Gook luck
It is a good sign that the motor turns freely with and without air. That indicates that you have good bearings and that the intank check valve is in good condition. Next you should check the capacitors and the condition of the windings. Some motors have start and run capacitors and others only have run cap. Remove from the circuit and test with meter having cap. test setting. Most shops will test them for free. The cap should test within 95% of stated ( uf ) value. Replace as needed. Finally check that the flyball switch at rear inside of motor is functioning correctly. At rest, switch is at start winding, then a split second after start, it shifts to run winding. Check contacts are not stuck and that switch is working and lastly that the winding are the color of new penny. Good luck.