My craftsman 5.5HP air compressor keeps tripping the breaker when it cycles on. not every time, but 1 out of 3 or so. If it doesn't trip when it kicks on, it will be ok until the beginning of the next cycle.

It's on a 20A 115V circuit by itself. The 12GA run to the sub panel is only about 20' of wire. I've tried a different breaker, and it does the same thing. It's a tandem (two breakers on one 115V pole), but it did it with a regular breaker, too. The subpanel is Square D homeline. Nothing else gives me any problems, either at the subpanel or the main. I've even run my MIG welder off the same receptacle (not at the same time) and it didn't trip it.

Any suggestions?

Check the nameplate for fla (full load amps) You may have to run a larger line and a larger circuit breaker. 5.5hp is 4103 watts at over 30 amps. Not sure if that motor actually uses that, so check the name plate and size accordingly. Hope this helps.

Probably time to call an electrician!

[This message has been edited by electure (edited 12-06-2002).]

Thanks, wirenut. I can't find the FLA anywhere. It just says 115V/60Hz/1PH/15A. Max developed HP=6. Lots of stuff about flow (cfm).

The voltage at the receptacle is 123.5, FWIW.

You should call the Manufacturer and see if they are aware of any issues with this unit.

Bill

With a 15 amp rating, this motor will draw a starting inrush current of 45 to 75 amps. It is not surprising that the 20 amp breaker trips. This motor is permitted a breaker rated at 250% of 15 amps.

2.5 X 15 = 37.5 amps. Try a 30 amp breaker.

Ed

[This message has been edited by electric-ed (edited 12-06-2002).]

Does this problem occur when the storage tank is empty? If it only occurs when there is pressure in the tank, it sounds like the "unloading" valve is not functioning correctly. This valve dumps the air from the compressor cylinders when the motor stops turning and makes the starting load much less. If the motor tries to start with pressure in the cylinders, I would expect it to trip the breaker.
Don

If this compressor is "cord and plug" connected with an "OEM" cord, then it was designed to be "plugged in" to a receptacle. Maybe the bearings in the motor are about to fail, or the internal parts in the compressor are worn, and causing the motor to be overloaded. Are there any other loads on that circuit?

Decompressing,
Doc

Is it a breaker or a GFI that's tripping. If it's a breaker then see electric ed above. If it's a GFI then plug it in on a non-GFI receptacle.

Bob

I used to have a Craftsman 5.5 HP compressor. The book says to put on dedicated 15A circut or a 20A circut. I ran it on a 15A circut and never once tripped the breaker.

The motor has a plastic box over it...so you cannont read the nameplate without taking off the cover.

(Removed long non-working url)

[This message has been edited by Webmaster (edited 12-07-2002).]

5.5hp sounds a little big for a 20 amp circuit. You may have to carefully apply article 430 here to size your wire size, breaker size and overload protection.

I may be wrong, but I recall reading somewhere (maybe here) that the horsepower ratings on home-use compressors and vacuum cleaners are grossly overstated.

CT,
You are correct, many items of this type use fictional horsepower ratings. This is the reason that Exception #3 to 430.6(A)(1) exists.
Don


[This message has been edited by resqcapt19 (edited 12-07-2002).]

Maybe a hvac type c.b might help on inrush.

r19don — You raise a good point. Used to be that discount stores sold $20 universal-motor circular saws that were labeled: "Develops 2¾ Horsepower." Kinda’ like the scam audio-amplifier “power-rating” abuse ~25 years ago in the US that required government intervention. Based on the numbers on many portable-compressor labels, either the street lights within a mile radius should extinguish, or the motor is so efficient it should make the watthour meter turn backwards—I’m not sure which.

On the subject equipment, until some basic electrical measurements are done, the problem could stem from one or more of several typically found in the troubleshooting process.

Based on 99NEC table 430-148, a 5hp 1ø 115V motor is about a 56-ampere running load or FLC. Also, a ¾hp 1ø 115V is rated ~14 amperes. That magic “5.5hp” motor is so wonderful that it should or make the building watthour meter turn backwards, or dim street lights for a mile radius—possibly both. Maybe it’s time to revise some of the laws of physics.


[This message has been edited by Bjarney (edited 12-07-2002).]

5.0 HP compressors found at Grainger.com show an amp rating in the neighborhood of 22 amps. Sounds like you have to upgrade the service.

I know this is not your exact brand, but like equipment should run alike.

FYI, you would also have to upgrade the wire and probably the conduit. I will let the real electricians on this board help you with that.
Scott

Heed Don's comment.
430.6(A)(1) Exception No. 3 makes it quite clear that the nameplate amps is used to determine the ampacity for a motor operated appliance.

Remember to look at 430.22 although doubt that many appliances are considered 'continuous' duty.

INTP,
Is it possible or practical for you to measure the current that this motor draws?

You wrote - "It just says 115V/60Hz/1PH/15A"
This indicates that the nameplate Full Load Amps(FLA) rating is 15 amps.

If the actual current drawn is not more than the nameplate rating, there doesn't appear to be anything wrong with this installation except that the circuit breaker is under-rated.
Unless you require the compressor to be portable, it could be hard-wired (no receptacle) and use a 30 amp breaker.

Ed

[This message has been edited by electric-ed (edited 12-07-2002).]

Sounds to me like you got a short
what type of cord is run to it?
Is it stranded frayed at connector or inside the termination box?
the little strands can short than clear making trouble shooting difficult
Does the motor have a capacitor?
If so have it checked or look for shorts make sure covers are not pinching wires
Make sure you disconnect power before doing any work and capacitor will still hold a charge after power
Is disconnected

I advise you to call a licensed electrician. Good luck!

The random trips on this compressor - not being the initial startup, but when tank pressure needs to be topped off [restarting motor] really sound like:

<OL TYPE=1>

[*] Faulty Unloader Valve (not allowing pressure out of pump prior to restart),

if not, next could be:


[*] Start Capacitor with random short / open problem, or some other heat related problem with the Aux. (Start) circuitry,

and the last possible cause:


[*] Bearings on either the pump or the motor (or both).
</OL>

Sounds like an unloader problem. The random start or trip situations would be from where the piston stops. If it passed Top [is Top Dead Center correct for pumps???] after motor stopped, the cylinder would be heading into a vacuum state with the piston falling to the intake stroke, so no high pressure on piston in this position.
If the piston was heading up on compression stroke, then the cylinder has pressure against the piston, making it a hard start situation for the motor.

Just rambling on with my $0.02!

Scott s.e.t.

Wow, lots of things to consider... Thanks to all for the feedback.

I'm really leaning toward somethign being wrong with the compressor, as suggested by several here. I didn't realize that there was an unloader valve that could be a factor here, but the scenario described by resqcapt19 is the same as what is happening. I don't recall the breaker tripping when the tank starts up from being empty. Also, I was aware of the startup capacitors on A/C compressors, but didn't know whether this was common on an air compressor.

My next step will be to get an ammeter and try to see what the thing is pulling. It is possible, I believe, but I will have to add the meter to my christmas wish list.

It is corded, FWIW, with a very robust looking (i.e. thick) 6 foot cord. It is made to be portable (has wheels), and I'd rather not hard-wire it if I don't have to.

Also, the receptacle is GFCI, but the GFCI has never tripped, just the breaker at the panel.

I'm going to call Sears tomorrow.

Thanks to all.

INTP, you can hear the unloader hiss when the motor shuts off, either at set pressure or if you manually turn off the compressor at the pressure switch.

stamcon: yes, I do hear it. I didn't really notice it before people here pointed it out. I bled the air to try to reproduce the problem, but after about 10 tries, it never tripped. I heard the unloader each time, and it started every time. I was hoping to not hear it and also have the breaker trip on the next try. Dang.

Here are some numbers from the panel.

Peak Start Amps:
try #1 55.6
try #2 45.3
try #3 44.8
try #4 42.7
try #5 43.1
try #6 49.6

Running amps were around 12.5-13.5A (fluctuated) while the tank was mostly empty, to 15-16A as the tank got fuller.

This is a new Craftsman clamp-type ammeter, with peak hold. As a control, I tested a general purpose circuit that was running only lights. I added up 54W fluorescent (27 + 27) + 350W incandescent (150 + 100 + 100). The meter read 3.2A. Actual voltage was 126 (this meter and another meter). 404/126 = 3.206. I know this isn't exact, but I thought it would be a sanity check.