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

BrokenYugo wrote: 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?

Yessir, you've got it. The start up is: Hit the contact to start the phase converter, momentarily press the "start" button to get the 5hp 3ph motor spinning, wait a second or two for it to get up to speed then flip the switch to light off the compressor motor(s). It won't start if you try to bring the 5hp and the compressor online at the same time.. It gives me a cool industrial-looking control area on the wall :)

FoxTrapper - Any idea where the starting cap would be for the individual motors? As far as I can tell the capacitors for the RPC are working fine since I've got stable voltage at all 3 legs (240/240/239), so I would imagine the starting cap would be on the 2hp motors themselves. I'm looking around for diagrams or other info, but it would be great if you knew where to find it. The motors are Baldor, Cat # M3157T, Spec 35B101T698H1, and they look like this:

Thanks gents!

Edit:This is the link to Baldor's page on the motor, I can't find anything about a starting cap on there..

Remember, zero 3-phase experience. Don't even know if 3-phase motors use a capacitor for starting.

But on the 110 and 220 single phase motors that have a capacitor, it almost always lives under that blister cover on the side or top of the motor.

Here's a picture of a generic one with the blister cover and capacitor on the top.

When the capacitor would go, the motor wouldn't start well. It would spin slowly and then trip the breaker. I was shown by some of the old coots that if you could manually spin it up before you gave it juice, many times the motor would take off and run just fine. It was a starting problem only when the capacitor would go.

But again, this is 110/220 single and double phase motors. Don't know if it applies to 3 phase motors.

Hmm.. I have little 3ph experience as well (I've hooked up CNC machines to it, that's about it..), but none of the three motors I have for this setup use a starting cap apparently. I wonder if you can add one to help with cold starts..

Hmm.. from what this says, I should be using a 10-15hp motor on my rotary phase converter.

3-phase motors don't have starter caps. The rotor chases the 3-phase around the stator. The starter cap on a single phase motor causes that phase rotation to get the rotor spinning, then falls out after the rotor is spinning fast enough that the back EMF causes it on its own.

Do your motors start together? You may be able to offset start them and get away with your current RPC motor. Put a 10 second timer relay on one of them (or separate switches, so your panel looks cooler).

Yes, they do both light off at the same time. The pressure switches are set ~5 PSI different so they don't kick back on at the same time, but that doesn't help if it's completely empty.

Any good suggestions for a timer relay? I don't think I have room for another switch, and it's annoying to run them down anyway (the switches are downstairs in the garage, the compressor is on the 2nd floor).

You are 100% right Foxtrapper. In a single phase motor, if a starting capacitor fails, the coils in the single phase motor don't see a "rotating" magnetic field. The capacitor's purpose is to "trick" the motor into seeing a rotating magnetic field to start it. Once the motor is moving the single phase magnetic field will keep it turning.

foxtrapper wrote: Remember, zero 3-phase experience. Don't even know if 3-phase motors use a capacitor for starting. But on the 110 and 220 single phase motors that have a capacitor, it almost always lives under that blister cover on the side or top of the motor. Here's a picture of a generic one with the blister cover and capacitor on the top. When the capacitor would go, the motor wouldn't start well. It would spin slowly and then trip the breaker. I was shown by some of the old coots that if you could manually spin it up before you gave it juice, many times the motor would take off and run just fine. It was a starting problem only when the capacitor would go. But again, this is 110/220 single and double phase motors. Don't know if it applies to 3 phase motors.

By code they should have a means of discharge of stored energy installed in the unit. If it's UL listed, that is.

(I will exclude advice from the trade- I forget not all individuals may be comfortable with our methods.)

snailmont5oh wrote: 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

To be thorough, does the cabinet on your compressor have fuses in it? Did you check them to see if they were all good? A blown fuse there could explain a lot.

Drives are super cool, there is no doubt about it! And they certainly can be used in an application like this. Being fed with single phase I think they have special derating rules too, but I don't know them as I've never messed with one.

Very basically speaking, drives and phase converters do the same thing. But drives can control the speed of a motor (like a motor on a lathe) whereas the phase converter is all go or all stop.

BrokenYugo wrote: In reply to RossD: Any new VFD I've seen supports 1 phase input.

A few things come to mind. 1) Are your overloads aka "heaters" all good on the output of your contactor? 2) The twin air motors should not spin up at same time there needs to be a delay or your going to over surge the rotary converter or have to get a bigger unit. Cheaper to add a time delay relay to control the bigger motor. 3) What oil is in your compressor heads? Most use 30w but at my old rental garage I had to switch to Mobil 1 0W-10 to keep from popping $12 heaters in the winter.

Yes VFD's are cool but rotary's once one motors running others can be added in with out issue and it should work fine.

WonkoTheSane wrote: Any good suggestions for a timer relay? I don't think I have room for another switch, and it's annoying to run them down anyway (the switches are downstairs in the garage, the compressor is on the 2nd floor).

If I was still a crane guy, I could tell you. Now, it would take research.

Actually, now that I think about it, a "soft start" or two probably wouldn't hurt.

Crane guy, train guy, that's close enough I thought? :) what's a "soft start" in this capacity? The pumps are supposed to be "no load starts"but I don't know how to verify that they're working. I'm thinking I need to pull the tops of the diaphragm cylinders to see what shape they're in.

Dwarf - I'm not sure about the "heaters"/contactors.. I'll have to look up what they are and get back to you. I'm thinking it's the oil.. I have no idea what viscosity is in it, I've been running it with whatever it came with.

I'll look into a timer/delay relay.

paranoid_android74 wrote: To be thorough, does the cabinet on your compressor have fuses in it? Did you check them to see if they were all good? A blown fuse there could explain a lot.

Negative on the fuses. It does have a contactor/reset block, but that has never tripped.

If you've never changed the oil it's possible these were on the deferred maintenance schedule and full of goop. Might be worth doing a flush depending on what comes out, then fill up with 0w30 or something to that effect. I've read synthetic engine oil works well in compressors.

BrokenYugo wrote: If you've never changed the oil it's possible these were on the deferred maintenance schedule and full of goop. Might be worth doing a flush depending on what comes out, then fill up with 0w30 or something to that effect. I've read synthetic engine oil works well in compressors.

I just got it recommissioned last spring, so it's only had maybe 20 hours of use on the old oil since I've had it. I was planning on changing it this spring anyway, but the oil is crystal clear and flows perfectly fine off the dipstick.

BrokenYugo wrote: If you've never changed the oil it's possible these were on the deferred maintenance schedule and full of goop. Might be worth doing a flush depending on what comes out, then fill up with 0w30 or something to that effect. I've read synthetic engine oil works well in compressors.

This is worth the quote.

Your compressor heads look like Quincy 325s. They will fill the crank case up with goop in a home shop, because they never get hot enough to burn off the moisture. The slop I dug out of mine was gross.

The un-loaders are fairly easy to check. You should hear them hissing until the oil pressure gets up to about 10 psi.

Here's a video of a cycle on my Quincy. You can just hear the un-loaders close on start up at 14 seconds on the video, and hear them dump head pressure on shut down at 2:05. That's from 40 psi to 125 psi on a 80 gallon tank.

https://www.youtube.com/embed/VNBJ9GFepYU

If you don't hear that, the un-loaders aren't working. If they aren't working, then you might have a problem where the tank is bleeding pressure back to the cylinders and the compressors won't start with pressure on the cylinders. A Quincy doesn't require a check valve and head bleed like most compressors. They use the un-loaders and the valves in the head to control back pressure to the cylinders.

Oil problems should also show up on the oil pressure gauge. There is a relief valve in the oil system that is adjustable if necessary. I think they recommend 22 psi oil pressure. I run 30W oil in mine and have never had starting problems when cold. Granted we never get below 20 degrees and I'm running a 5hp motor.

Parts are readily available for the Quincys. I got most of mine off of Ebay. You will need your serial number to figure out which build version you have. Yours look to be newer then mine is.

I would also check the voltage on all three legs on start up. There is a pretty good chance that a dirty contactor, dropping a phase, is the problem.

I add, you could probably sell the rotary phase converter and the 3p motors for more than enough to buy a couple of single phase 2hp motors.

Oh, and $400 was a steal for that setup. I paid that for mine in non-running condition.

Going the VFD route might get expensive but it should solve your dimming light problems and maybe your cold start problem. You should have 2 VFDs both rated for twice the HP of the motors you are driving.

Another thing is make sure the compressors are running the correct direction. They will work backwards but they can pump the oil wrong and lock up I believe. To switch rotation just reverse and 2 of the power wires going to the motor.

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

http://www.harborfreight.com/digital-clamp-on-multimeter-95652.html

A couple of other thoughts.

If both compressors are slow on startup and won't start, then your problem almost has to be in the RPC or the power supply. Bad contactor, bad wiring, voltage drop due to load. What is the voltage when the compressors are trying to start.

If one compressor won't start, then staging the starts with a time delay won't help. Did you try both compressor individually? Is is possible that you have a motor failing and the one you did try is the culprit?

You start the phase converter motor first correct. What happens if you let it run for several minutes before starting the compressors. Maybe greasing the bearings on it are making it slow to get up to speed on a cold day. Could that be throwing off the frequency seen by the compressor motors?

Reading through your first post carefully, if they are " buzzy" they are single phasing. Thinking. Read thru the amp and voltage posts here and check methodically. Electrons are slippery and invisible.

They hurt too.

I'm thinking the same. Its either single phasing or the motors don't have the oompf to turn the load.

Adding a timer would mean surgery. From what I recall in the RPC image there is only one contactor for both motors?

Electrically speaking I agree looking for current on all three legs per motor would tell much.

I know squat about the rest of the compressor

TRoglodyte wrote: Reading through your first post carefully, if they are " buzzy" they are single phasing. Thinking. Read thru the amp and voltage posts here and check methodically. Electrons are slippery and invisible.

Cranes are fascinating things! I learned a little about them, but never had a chance to work on them.

Big DC stuff rates high on my want list of electrical things to learn.

snailmont5oh wrote:

WonkoTheSane wrote: Any good suggestions for a timer relay? I don't think I have room for another switch, and it's annoying to run them down anyway (the switches are downstairs in the garage, the compressor is on the 2nd floor).

If I was still a crane guy, I could tell you. Now, it would take research. Actually, now that I think about it, a "soft start" or two probably wouldn't hurt.

A 3 phase motor that has power to just 2 legs will just turn a little and then reverse the same amount, hence the buzzing sound, the motor won't spin. If one of the legs is low it will have trouble starting, will never get up to speed and will be louder than normal. Single phasing will trip the overload, which it sounds like you are also missing from the circuit.

Most single phase motors have a thermal overload device that shuts the motor off if it get too hot. Most 3 phase motors do not and they need something that protects the motor from over drawing. A breaker is not an overload. You need a starter with thermal overloads set for the motor or a properly setup VFD. Without overload protection you could burn up the motor or worse, start a fire.