Learn me: Troubleshooting a big-arsed 3ph compressor with a rotary phase converter

Greetings Gents!

I have a big-arse compressor that I picked up out of a shop shutting down when I bought their lift. I'm not sure exactly what year it is, but it's a mid-70s USA made Quincy (I think it's a Duplex 240, but it's hard to find out for sure). It's got two 2 Hp motors to spin the compressors and it looks a lot like this (but mine has large pop-off valve expansion chamber/blow off valve things):

Even though I was assured that it was 220V single phase when I bought it, I didn't look that close because for the $400 I paid, I didn't care if it was 3 Ph.. Turns out it was, dangit. So, now I have a rotary phase converter to fire it up. The RPC is a fully-capacited unit driving a Lincoln Electric 5 Hp motor.

I got this installed last spring, and it worked awesome all summer (as in, I can turn it on from 0 PSI, open an air hose and it'll still fill up to 120 PSI in almost the same amount of time as when it's closed!) It's great!

Recently it's stopped working correctly, though. It's spinning the motors really slowly and will throw the breaker after ~20 seconds. During this time, it barely builds any pressure and the motors are pretty buzzy. I tried disconnecting one motor and starting it up. It sounded a lot better and built up to 20 PSI, but kind of stopped there.

Recent changes: The 5Ph was fairly noisy on the accell/deccell (unloaded), so I put some grease into the zerk fittings on it. Now it's practically silent on accell/decell.

My hypothesis is that I over-greased the bearings and got some grease on the brushes and now I'm not actually building any real HP on the third leg. Does that sound possible?

Thanks!

Over greasing sounds possible.Have you checked voltage or amp draw on the compressor motors? Very limited experience with rotary phase converters here,sorry.

voltage checked out okay.. I'm not sure my hand-held fluke is up to testing amp draw on this circuit, though..

3 Ph makes me a bit skittish, so I'm probably being overly cautious..

Amp draw is fairly easy,with the proper meter, and no, no such thing as overly cautious with 3 phase.

I think you need a clamp meter for amps, right? How's the HF amp meter?

No experience with HF unit but if its hefty enough you could always use it for a SFH? No more than you use one it should work properly twice? Fluke may sell an attachment for your meter?

Usually, when three phase motors sound all grumbly and don't make power, you've dropped a phase. I've only seen one rotary phase converter, and it was 8 years ago, so I'm no help there. When I've dealt with actual 3-phase spinning 3-phase motors, the culprit was generally a burned set of contacts on the contactor.

You should be able to find schematics online for your converter.

Standard electrical troubleshooting rules apply. Follow the power until you don't have it anymore. Also, make sure your meter leads are clean. Did I ever tell you the story about how I got electrocuted through the dirt on the outside of my leads? Fun stuff.

On edit: You can also check for resistance through the motor with the meter you have. With POWER REMOVED FROM THE UNIT, check resistance from 1 to 2, 2 to 3, and 1 to 3 on the motor side of the contactor closest to the motor. They should all be low and similar. A high one would indicate a motor/brush problem.

Too bad you're not still here. This would be fun.

It sounds like your capacitor may have taken a dump.

Thanks for the info, Mike! I'll probably give you a call to walk me through it when I have a chance :) Luckily power is easy to disconnect at the breaker for this system, so I can be fairly confident that I won't die inspecting it.. I would like to hear that story, sometime!

Looks like HF doesn't have an Amp Meter.. Off to clist, I've wanted one for a while anyway.

Good guess, Paranoid.. I was really hoping to not mess with those, they're all like .5-.75 farad caps, so they give me the hibbly-jibblies, too :)

They are pretty easy to check. Some of them have a dimple on the top- if the dimple is pushed out and/or the metal case is swollen that's probably it.

And yes- they hurt if you don't discharge them first. All you have to do is ground it to discharge it.

Fair warning too- new ones can be charged as well!

Edit: what is the rated voltage/amps on the compressor motors compared to the same on the phase converter?

I would consider removing the rotary phase converter and running the motors off VFDs. A Variable Frequency Drive converts single phase to 3 phase all day, solid state.

In reply to 93gsxturbo:

Yeah, considering you can get a new 5hp VFD for under $200 (Chinese import, no idea on quality, but they have a 2yr warranty) messing with a RPC doesn't make much sense. I think the VFD is more efficient too, since it will spit out proper 3 phase instead of having two of the phases 180 degrees apart.

Don't just dump the caps to ground through a wire! They can discharge very fast, and turn a wire into a fuse link. Get a fairly heavy resistor and discharge it through that. In fact, you can put a very high resistance resistor (rated for the voltage, of course) across the top of the cap, which would cause it to bleed off when there was no power present, but not effect the operation of the caps.

http://youtu.be/9xzvfsaGKKw

93gsxturbo wrote: I would consider removing the rotary phase converter and running the motors off VFDs. A Variable Frequency Drive converts single phase to 3 phase all day, solid state.

Do any of the VFD manufacturers even state that feature? If not, I'd be leery using that in such a manner. Safety first all that.

What about getting the largest 240v/1ph motor you can find on the shelf?

In reply to RossD:

Any new VFD I've seen supports 1 phase input.

Everything you ever wanted to know on RPCs and VFDs for single to three phase conversions.

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/

Well, I kinda feel like an idiot, because it seems like the issue was a ground that had wiggled loose in the junction box that goes between the 3ph motor and the switch I have to turn on the compressor. I tightened up everything in that box and it fired right up.

Before I got to there, I did end up checking the 3ph motor, and it read ~0.8ohm on all 3 legs. I discovered the disconnected ground before I got to the compressor, but I still pulled the motor wires off and checked incoming voltage, and the phase converter is putting out 240/240/238, so I'm impressed by the phase converter. It seems well built for a home-brew hacked together thing.

Here's the beast in all its' glory, and a picture of the phase converter. I really need to tidy up all the wires running around it, but it's in a back corner of the garage loft, so it isn't the biggest mess I have by far, yet. I promise that this the worst part of it, the rest is tidied up in conduit and junction boxes.

Looks like I've jumped to conclusions. Today I tried to fire it up, and it food the same thing as before.. No real pressure and a tripped breaker after ~20 seconds..

I'm thinking it's related to temperature, today was about 15-20 degrees cooler.

In reply to BrokenYugo:

Thats wierd, because I asked my electrical engineer boss and he said they cant do that. But he's bullheaded, so i assume they can do that but just not documented for UL or NEC reasons. Logically it makes sense. Ac to dc back to any hertz ac. Id be interested in some mfg data sheets or O&Ms for that type of operation.

In reply to RossD:

Probably becasue they've just never had to set one up that way, not often you need to outside of a farm or home shop situation. I've actually read into it a little deeper, many of them have to be substantially derated (50%) configured for single phase input becasue it's hard on input rectifiers and caps, others (probably not the cheap ebay units) are properly built for it with a beefed up input section.

As not to derail the thread, the cooler temp association makes me think something mechanical is acting up, like a internal start switch sticking or something, assuming 3 phase motors use them.

My experience with multi phase motors is limited, but if it was a split phase or single phase I would be checking two things. One the pressure switch contacts, and two that the motor is getting the right amperage. Too little power/amps to the (the above mentioned at least) motor will let the motor run for a while but end up doing weird stuff and slowing burning the coils out.

I have run some 1hp motors on some underrated/ghetto rigged household circuits. The breakers would sometimes trip the motor would sometimes stall prematurely at higher pressures.

Maybe it the wrong tree, but might be worth checking/double checking too.

In reply to WonkoTheSane:

Show your boss this product brochure....designed specifically for single phase the three phase output from the vfd http://www.wolfautomation.com/assets/15/EV_Micro_Drive2.pdf

eebasist - I'm the boss, since this is for my home garage :) And the boss doesn't want to spend any money if not required, but that does look pretty cool!

BrokenYugo & Jere - Reading about it, apparently a lot of people have trouble with compressors starting below ~35-40°, and they recommend switching to lighter-weight synthetic oil. I'm going to call the Quincy distributor in the area and see if they'll point me in the right direction, if not, I'll become an internet genius again :) I believe that the circuit is "okay" for running the compressor since it starts it fine in warmer weather (there is a noticeable flicker of the garage lights when it lights off), but it's on a dedicated 40 amp circuit with appropriate gauge wire. I'm thinking the cold oil (no idea what weight it is, I'm guessing 30w?) is just too much to get the old cast iron beasts spinning. Remember there's two motors here to drive (4Hp total).

Edit: The flywheels on these pumps are ~50 or 60lbs, and I know it has good compression, so I can't imagine that makes it easier to get spinning when it's that cold..

Dumb question, how does the startup work? When the pressure switch trips on and puts power to the motors, is the RPC motor already spinning?

Zero three phase experience. But, when I've run into motors doing what you're describing, I've frequently found the starting capacitor to have failed. Dunno if a three phase motor uses starting capacitors or not.