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

Thanks for the video, Toyman! That was really helpful! I'm not sure that mine is a 325, the page that I stole the first posts' image from lists it as a 320-32 duplex. The 325s don't look exactly like mine, but I cannot locate any sort of serial # or ID on my compressor.

It's definitely temperature related.. Today was great for troubleshooting that.. It was ~32° when I got home from work, so I tried to fire it up and got the same result (noisy, no real pressure built, it would have popped the breaker if I didn't turn it off). As of 11:30 tonight (when I had time to read through this post and go test something), it was ~55° and very humid and it started up perfectly fine.

So I think it really might be as simple as a slightly-too-small motor on my RPC and too thick oil.

I'm going to grab some Mobil 1 0w30 on the way home tomorrow and try to swap it out and see what happens.

Thanks to the humidity, I do see that I need some gaskets at the top end, I got to see this at the top of one of the pumps: Ewww.. foamy!

Also, I'll have to do a bit of a mod to be able to see the oil pressure gauge.. The control panel is helpfully in the way:

Parting thoughts: I think the RPC that I have is working correctly, and I'd really prefer to not get rid of it, even if I could do an even swap to single phase of the same power. Besides the 3ph being more efficient to run motors, it also gives me bad ideas about being able to pick up used machinery cheaply :)

Yeah, you've done the hard part by getting a phase converter. The Magic by which they work is pretty squiffy, too. Basically, an electric motor is a generator at the same time. It generates its own EMF, which is what you're using to power your 3-phase stuff.

Some other interesting things about "back EMF": It's not present during startup, which is why startup current is so high. As the motor spins faster, it "pushes back". This pushback is also what determines the motor's top speed.

You have unloader valves so it should start up with no load. thus spin up to speed faster. Check the ball and spring under the brass plug cap on the side of the block 90 deg from the pressure gauge.

Hey Dwarf,

I took the plug out, there's a spring in there and a piston (cylinder with a relieved slot).. The piston seemed to move freely, but I have no idea what's actually supposed to be happening there, so I just put the plug back in..

Well if it moving then It should be working on that end of it. This link might help a bit.
practical machinist Note: some units use the oil pressure to activate the unloaders. No oil pressure = unloaders held open.

I have them on mine but my 5hp motor starts it no problem with them disconnected for the past 20+ years....my system is scabbed from parts even the base is guard rail I beams.

Update to bring some closure now that we're back down below 30°. I swapped to some synthetic 0w30 in the motors. The stuff that came out of it I would guess was probably about 40w?I could practically cut it with a knife. I let it drain out all night..

After that, I put in the fresh oil and disconnected one motor at a time so I could spin them up to get oil into it.

Now it fires right up, it's currently 25°, and there's no dimming lights or anything.

I think this mystery is solved for now.. Now I gotta get to a distributor for some new gaskets and I think it'll be perfect.

Thanks all!

Glad to hear the problem is solved.

Simple solutions are a beautiful thing!

Straight weight oils really goop up when it gets cold, I bet that was generic SAE 30 compressor oil. For a fun demonstration of how spoiled we are by modern lubricants, try to start your lawnmower, tiller, etc. with the pull cord right now.

^ That's the truth.

Apparently this wasn't enough for when it's 15°, though.. It wouldn't quite light off the second motor and eventually tripped the breaker this morning, so my minimum range is somewhere between 15-24.

I think for next season I'll just install a switch at the compressor to turn off one of the motors unless I can find a good deal on a 9 or 10 hp 3 phase motor between now and then.

If there's an oil sump of sorts on the compressor units, maybe just stick one of those oil pan heater pads on each one and turn those on in the morning if you're going to need the compressor on a cold day?

heater

You can try one of these for the real cold days but you'll need a keep it on for an hour or so before turning on the compressor which would drive me nuts. You may just need to a second oil change as the old goop that was in there didn't all make it out even if it did drip over night.

Agreed with 44Dwarf, do another oil change, preferably with it warmed up.

If it comes down to heating it, you could get a pair of small pad heaters and a thermostat cube.

http://www.amazon.com/Kats-24025-Watt-Universal-Heater/dp/B000I8XDBM/ref=sr_1_4?s=automotive&ie=UTF8&qid=1457027481&sr=1-4&keywords=heater+pad

http://www.amazon.com/Farm-Innovators-TC-3-Thermostatically-Controlled/dp/B0006U2HD2/ref=pd_bxgy_263_2?ie=UTF8&refRID=0RZ25TQPJHMAEJ61AK99