Compressor Motor Woes...

[hold_fast]

Hey guys, long time lurker, haven't posted much. I recently acquired a hodge-podge of compressor, and I'm trying to figure out why I can't get it running. I managed to scrape together the following setup: 3HP pump (similar to harbor freight), "5HP Special" Century motor, and a 60 gallon tank. It's wired with 10/3 back to a 30A breaker with the neutral capped off.

Pump, tank, and pressure switch are in known working order, as it was my neighbors old setup until he blew the motor and bought a bigger setup. We replaced the blown motor with the century motor that I got it off craigslist for cheap. Brought the motor home, hooked it up to 240V, and it spun up fine.

Everything was mounted up, belt put on, etc. 0 PSI in the tank. Wired it up, turned the breaker on, and as soon as I moved the pressure switch to "Auto" the motor would turn slightly and the breaker would trip.

Thinking it was the pressure switch, I bypassed it by tying the hots together. Still tripped the breaker.

Thinking it was the motor pulling too many amps due to the pulley arrangement, I pulled the belt off and tried to run the motor by itself. Still tripped the breaker.

I'm a newb to electric motors, but I pulled the caps off and took a peak at them. They appear normal. Contacts look good, no corrosion, swelling, etc. What else can I do to get this working?

Motor specs:


How I have the pressure switch wired (Left feed, red & black hot, right to motor, white and black hot. Don't ask why this was done):


Thanks for your help guys.

 

[Davefr]

It sounds like you got a bad motor from CL or have a bad breaker if it trips with no load and directly wired.

The first thing I'd do is put a clamp meter on it and measure the amps.

You really can't judge caps by just looking at them. I'd test them.

After that it's a matter of ohming out the windings and inspecting the centrifical switch.

 

[Todd.Brock]

Confused as to why the insulation is clamped under the line screw. Is it even contacting? Also, you said the motor was hooked to 240v and it ran fine, how long did you run it for? My assumption is that the resistance of the belt and pump are too much for it and pops? Wouldn't a motor with bad caps just hum and not start?

 

[hold_fast]

It sounds like you got a bad motor from CL or have a bad breaker if it trips with no load and directly wired.

The first thing I'd do is put a clamp meter on it and measure the amps.

You really can't judge caps by just looking at them. I'd test them.

After that it's a matter of ohming out the windings and inspecting the centrifical switch.

I did try 3 different 30A breakers, moved some circuits around, and it did pop everyone.

As for measuring the amps, I couldn't get a good reading with how quickly it tripped. Even with the wife turning the breaker on and me on the DMM, I couldn't get a very good reading.

I took the capacitors out, and since my DMM doesn't have a capacitance setting, I was stuck ohming it. They did swing to open line, though I'm not sure how much I would trust it.

Any good resources for checking out the windings and the centrifugal switch?

Confused as to why the insulation is clamped under the line screw. Is it even contacting? Also, you said the motor was hooked to 240v and it ran fine, how long did you run it for? My assumption is that the resistance of the belt and pump are too much for it and pops? Wouldn't a motor with bad caps just hum and not start?

The little bit of insulation is from my impatience when I was switching pressure switches. Plenty of contact, trust me. I also mentioned that I tied the hots together, removed the belt, and was still popping a breaker, so the insulation and the pulley load wasn't the issue.

The motor was only run for a minute or two when it was initially bought. It sat for probably a month or two afterwards in the garage. But that's a good point, I would think the same about the motor humming, not popping the breaker, but I'm no expert either.

 

[A_Pmech]

You have a fault to ground somewhere. That's the only reason why the breaker would trip so quickly: The fault current is very high. The motor could have a winding failure and still run when tested, so long as it isn't grounded. Did you ground the motor when you tested it off the compressor?

As far as the cause of the ground fault, it could be the motor or it could be the wiring on the motor side of the pressure switch.

 

[scw1991]

perhaps fried contacts in the pressure switch? Is the pressure switch even rated for 15 amps?

 

[joel63]

You have a fault to ground somewhere. That's the only reason why the breaker would trip so quickly: The fault current is very high. The motor could have a winding failure and still run when tested, so long as it isn't grounded. Did you ground the motor when you tested it off the compressor?

As far as the cause of the ground fault, it could be the motor or it could be the wiring on the motor side of the pressure switch.

^^^^^ +1
A good place to start and easy to check.

 

[hold_fast]

You have a fault to ground somewhere. That's the only reason why the breaker would trip so quickly: The fault current is very high. The motor could have a winding failure and still run when tested, so long as it isn't grounded. Did you ground the motor when you tested it off the compressor?

As far as the cause of the ground fault, it could be the motor or it could be the wiring on the motor side of the pressure switch.

Good point. The motor was not grounded when we tested it. I checked for continuity on the wiring on the motor side and it was good. So I guess I would need to check the winding.

Having never checked windings before, I'm looking for low resistance across the motor leads, correct? If I do find that the winding has taken a ****, am I in new motor territory?

perhaps fried contacts in the pressure switch? Is the pressure switch even rated for 15 amps?

I checked continuity across the pressure switch and it's good. Also swapped it out with another one and had the same issue. I would need to check the rating again, but I believe the switch is rated for 17A. It worked fine with a 3HP motor previously.

Thanks for all of the help guys.

 

[Davefr]

Having never checked windings before, I'm looking for low resistance across the motor leads, correct? If I do find that the winding has taken a ****, am I in new motor territory?

I think PMech's theory is right on. If so, you have a very dangerous situation if the frame of the motor is ever left ungrounded. (ie Touch the motor while you're grounded and you're dead)

Remove all external wiring and then take these ohm readings at the unconnected motor and report back:

Line 1 to Line 2
Line 1 to metal frame of motor
Line 2 to metal frame of motor

(the first should be very low and the last two should be infinity)

After that you can ohm out the windings w/caps and start switch disconnected:

Run to Common
Start to Common
Run to Start
Run to frame
Start to frame
Common to frame

You might also want to take the motor to a shop for evaluation.

 

[hold_fast]

Well I did exactly what you said Dave. Disconnected all wiring and took the below readings:

Line 1 to Line 2 - ~1.0 Ohm
Line 1 to metal frame of motor - Open line
Line 2 to metal frame of motor - Open line

I'll do the windings w/ caps later. Assuming I need to pull the cover, anyone have a good picture or reference for what I'm looking for. Again, noob here.

 

[Davefr]

Well I did exactly what you said Dave. Disconnected all wiring and took the below readings:

Line 1 to Line 2 - ~1.0 Ohm
Line 1 to metal frame of motor - Open line
Line 2 to metal frame of motor - Open line

I'll do the windings w/ caps later. Assuming I need to pull the cover, anyone have a good picture or reference for what I'm looking for. Again, noob here.

I don't see any obvious path from line to ground in those readings. They look fairly normal.

Are you sure there wasn't something in your "bypassed" wiring arrangement that resulted in a short to ground? Could a wire connection direct to the motor be making an intermittent short to it's frame? (sometimes those connectors are crammed into a little recess of the motor's case and could be loose or pinched).

You want to go from the electrical panel's breaker output direct to the motor making sure the motor frame is properly/safely grounded. Let's test just the motor. Worry about the pressure switch/compressor later.

If you have a motor shop in your town you might want to take the motor in and have them bench test it for you.

 

[TM98]

Is the motor locked up?

 

[hold_fast]

I'm thinking I must have a wire shorting to the motor frame somewhere. I triple checked my pressure switch bypass wiring to make sure I didn't make a stupid mistake.

I'll pull the motor off the tank tonight, set it next to the panel, hook it up and see if she spins up. If not, I'll pull the covers and see if anything looks odd. I'll report back tonight.

I did check for a local motor shop, but the two in my area only seem to work on large motors. I fear that if I bring it in for a quick diagnosis, it'll turn into costing me as much as a new no-name motor (my luck). I'm only using this for impact guns and some light die grinding/cutting, so I don't need anything overkill.

 

[hold_fast]

So between the holiday and work, I didn't get to touch the motor until last night. Decided to start from square one again. Took the cover off the motor to peak around and see if anything looked blatantly bad, nothing. Decided to test the wiring from the panel, so with the pressure switch bypassed, I left the motor wiring disconnect, dangling in the air. Turned on the breaker, didn't trip.

"OK, so it's not the wire" (talking to myself in the garage). Then thought, OK I'm going to hook it up one last time, if it doesn't work screw it, I got stuff I need to do, new motor time. Hooked it up, motor spins, "IT'S ALIVE". Put the pulley and belt on, and she pumps. Got it up to ~50psi and killed the breaker.

I have no idea what act of god caused this to start working, but I'm happy. I left it for the night, will try and hook up the pressure switch tonight. Ordered an el cheapo $20 pressure switch off Amazon just to be on the safe side, since I really want to get some work done this weekend.

Thanks for all of your help guys, really appreciate it.