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Old 05-03-2010, 01:51 PM   #1
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1 phase to 3 phase


I have been trying to wire in a 3 phase air compressor to single phase.

First i started with 100 amp service box hooked to my shops 200 amp box. The 100 amp is wired with 2 2 gauge, 1 4 gauge, and 1 6 gauge aluminum wire.

Next got 20 hp phase converter box. It has 2 gauge going from service box to it. Then 6 gauge going from phase box to the 20 hp idle motor. Then 6 gauge going to the compressor thru a mag starter.

The phase converter starts up runs fine. It puts around 240 volts to the mag starter with out the compressor going. Once i start the compressor it drops to around 220.

The issue i'm having is the compressor will start up and pressure up to 175 psi fine. Then shut off like it's suppose to. Then once it bleeds off and try's to start again, It has trouble doing it. The mag starts heat strips or whatever they are called . They get glowing red hot. The weird part is if i let everything set for couple minutes and the compressor get down to around 80 psi, Then flip the power switch it will turn on just fine, no problems.

Any help with this would be great.
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Old 05-03-2010, 02:43 PM   #2
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Sounds like a problem with the decompressor. When the compressor shuts off the pressure should bleed off the compressor head so that it doesn't need to start under load.
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Old 05-03-2010, 03:45 PM   #3
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Where is that located?
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Old 05-03-2010, 04:18 PM   #4
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I think i found out it's the check valve gping from pump to the tank is bad. But if anybory has anything else they think it is please let me know. I'm going to replace check valve and will let you know
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Old 05-03-2010, 06:12 PM   #5
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Check with the unloader valve that is most common curpit and that useally found with the tank pressure switch.

by the way how many HP is your compressor is { I am not counting the rotary phase converter } ?

Merci, Marc
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Old 05-03-2010, 06:31 PM   #6
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The pressure switch should have a cable running from it to the check/unloader valve that releases the head pressure when the switch shuts off.
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Old 05-03-2010, 09:06 PM   #7
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The compressor is 7.5 hp the converter is 20 hp.

The unloader is working. I talked to a tech from company that makes the compressor. He said if hear air release after motor shuts off, that the unloader is fine.

I double checked the voltage and for some reason it has dropped. this is how it goes with no load

PHASE BOX == MAG STARTER IN

L1-L2 242= 243
L1-L3 249= 250
L2-L3 223= 224

WITH LOAD = MAG OUT

L1-L2 236= 236= 235
L1-L3 207= 207= 206
L2-L3 219= 219 =218

I'm thinking with that drop, that its not enough to restart the compressor once it got pressure. Let me know what you think. I have been working on this for weeks. It's just one issue after another. Thanks for all your help

Last edited by diesel boy; 05-05-2010 at 11:41 AM.
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Old 05-03-2010, 10:04 PM   #8
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Is the motor dual voltage, and if so, is it connected for 230 volts? (It very likely is, or it probably wouldn't build much pressure).

How far from the 200 amp panel to the 100 amp one? How far from the 100 amp panel to the phase converter, and from there to the compressor?

If there's a small (1/4" or so) line from the check valve to the pressure switch, when the compressor stops, quite a bit of air should come out. Even if it does, loosen the fitting, and see if there's any pressure.

There's a good chance that you'll need to get a smaller pulley for the motor. With a rotary phase converter, a 3 phase motor will produce about 80-90% of its normal HP. Even less on start-up.

If you go with a smaller pulley, there'll be a frame size on the motor nameplate. If it's 182T or 184T, the shaft is 1-1/8" diameter. If it's 213T (most likely) or 215T, then it's 1-3/8". There's a good chance the pulley is two-piece. If so, post back. There's a trick to getting them off, without it you'll be in for a struggle.

Rob
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Old 05-04-2010, 01:46 AM   #9
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It is wired for 230 right now.

There is about 40 feet from 200 to the 100 amp box. Everything else is within 6-7 ft.

I will check that line. What if there is no pressure?

The frame is 213t and yes it is a 2 piece ( found out hard way).

What size pully should i get?
Should i got one or two sizes smaller?

Thanks for all info.
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Old 05-04-2010, 07:40 PM   #10
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There should be no pressure in the line after the compressor is off for a minute or so. Even if the tank is at full pressure.

If there's pressure, then the check valve is leaking, or the unloader valve is stuck or gummed up with oil.

The idea here is to have no pressure at the pump when it's starting. If it tries to start against pressure, the motor won't come up to speed.

If the unloader is OK, then we need to reduce the speed of the pump. This will allow the motor to start easier. The starting torque of a 3 phase motor on a phase converter is about 50-80% of normal. Even with the pump unloaded, it might build pressure before it gets up to speed, then it'll never get up to full speed.

Usually, it's not possible to change the pump pulley, as it usually has fan blades. It's hard to say how much of a reduction is needed, I'd take a wild guess that two sizes might work, but it's a pretty wild guess. A lot of it depends on the diameters of both pulleys as it is now, and the pull-up torque of the motor. Which is completely unknown, since it's on a phase converter. Obviously, it's presently not enough though. lol.

Rob
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Old 05-04-2010, 11:14 PM   #11
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I changed the check valve yesterday. It was really gummed up. So i wonder if the underloader is also. Maybe i will look at pulling it apart tomorrow and cleaning it.

So do you think that, that is the reason why there is a voltage drop?

That all kinda makes since. Cause once it drops to around 70 psi it will start up fine, but before that it acts like it's not getting enough power.

I see you live in fernley. I had a transport company about year ago and picked up a 56 chevy wagon from there and took it to oregon. I can't remember the guys name, but he seamed like a really good guy. I think his name was billy actually. He has a bunch of cars he works on.

Thanks leroy
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Old 05-05-2010, 03:32 AM   #12
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Quote:
Originally Posted by diesel boy View Post
The compressor is 7.5 hp the converter is 20 hp.

The unloader is working. I talked to a tech from company that makes the compressor. He said if hear air release after motor shuts off, that the unloader is fine.

I double checked the voltage and for some reason it has dropped. this is how it goes with no load

PHASE BOX MAG STARTER IN

L1-L2 242 243
L1-L3 249 250
L2-L3 223 224

WITH LOAD MAG OUT

L1-L2 236 236 235
L1-L3 207 207 206
L2-L3 219 219 218

I'm thinking with that drop, that its not enough to restart the compressor once it got pressure. Let me know what you think. I have been working on this for weeks. It's just one issue after another. Thanks for all your help
I put a bold marking there and the numbers did not come up right due you have 208 volts on L1-L3 that may cause a hard start up under the load and with that much voltage spread it may affect the motor performace.

I know you mention 20 HP idler motor is that a plain jane motor or a actual phase converter motor due some of the rotary phase converter motor is wired up in delta format { the three phase motor can be wired in either wye or delta format depending on the way the coils are hooked up inside of the motor }

For your motor it should draw about 22 amps @230 volts

{ You may have to buy a clamp on ampmeter if you don't have it and do not use the standard DVM for reading the current drawage the surge will kill it( most DVM are useally rated for 10 amp the most and the motor starting current will just simpley blow the fuse or do the damage to the DVM.) }

and for overload thermal elements there should be a number on the heater itself and look inside the cover and see the number vs to the motor current drawage as I mention above you should have slightly above 22 amps.

And check all the concats to make sure they are clean if the concats are pitted really bad it may not run correct.

even with bad heater on thermal overload can afffect this as well so check the heater numbers to make sure all are the same.

I think that where the issue is at the starter contractor { make sure you have power is off complety before you mess around with starter conctractor }

Most starter contractors useally have two or four screws or clips to hold it together just make sure you follow the correct sequine.

If not sure just run the model number of the starter one of us are famuair with it and we will tell you the correct way to do this without destory it.

Merci,Marc
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Old 05-05-2010, 11:57 AM   #13
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Yea L1-L3 is at 207 all way thru then drops to 206 at bottom of the mag starter.

The heater overloads are E62. If i remember right they are eoungh amps for the motor. Two of them are discolored from heating up from hard starts.

The 20 hp motor is a baldor. I dont think that is a plain motor, but i'm not sure.

All the contacts are good everything is new.

I got some pictures i will post them later today. maybe that will help
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