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Tool Talk Discussion Forum

3 phase air comp limit switch wireing

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scott#2

03-03-2006 21:05:13




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Just wondering if anyone knows how to wire up the high limit switch on a 3 phase unit. I assume all 3 legs must break when the tank reaches 175 psi? On single phase, do boths legs break at the hi limit?

Thank You,

scott#2




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scott#2

03-04-2006 06:22:03




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 Re: 3 phase air comp limit switch wireing in reply to scott#2, 03-03-2006 21:05:13  
Thanks guys, now I remember when I wired up a big 3 phase pedestal grinder. I did use what looked like a big 3 pole reley and and it was driven by 120vac. When you threw the switch, the starter pulled in and set it off and when you released it, it broke all 3 legs. Thanks for refreshing my tired mind.

Much Obliged,

scott#2



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Charles (in GA)

03-04-2006 06:02:01




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 Re: 3 phase air comp limit switch wireing in reply to scott#2, 03-03-2006 21:05:13  
You need a magnetic starter.

Charles



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MarkB_MI

03-04-2006 05:50:02




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 Re: 3 phase air comp limit switch wireing in reply to scott#2, 03-03-2006 21:05:13  
A three phase motor is essentially three single phase motors. So if you disconnect one leg, full power is still applied to one phase of the motor. It will actually continue to run if there's no load on it.



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Bus Driver

03-04-2006 05:32:18




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 Re: 3 phase air comp limit switch wireing in reply to scott#2, 03-03-2006 21:05:13  
On a 3 phase, breaking the contact on two of the legs is OK. The one hot leg does not harm the motor. But breaking all three is better. Breaking just one contact will most likely burn up the motor.



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buickanddeere

03-04-2006 06:53:41




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 Re: 3 phase air comp limit switch wireing in reply to Bus Driver, 03-04-2006 05:32:18  
All three phaes have to be opened. Too many people have observed the motor get switched off. Then they work on the compressor or circuit and get bitten with 120,277 or 347 to ground from the remaining live line. Your local hydro inspector will have a screaming fit and rightfully so. They can cut the power to your place should they get ticked off enough. The motor has to have short circuit protection with fuses or breakers. Then it must also have overload protection with thermals on the starter contactor.

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Steve Crum

03-04-2006 03:56:08




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 Re: 3 phase air comp limit switch wireing in reply to scott#2, 03-03-2006 21:05:13  
Are you using a magnetic motor starter switch? Pretty rare not to. The three motor legs (t1,t2 ,t3) come off a set of thermal overloads (heaters) and the three input legs (L1,L2,L3) generally input into the top of the starter. engagement of the starter is by a low voltage electro magnet (sometimes 24 volt, most common 120 volt) A standard pressure switch utilizing 120 volts (or whatever your control voltage is) energizes the magnet to kick in or drop out the starter, thus starting or stopping the motor. When the starter kicks in, all three 3 phase legs are engaged at the same time, and likewise dis-engaged to stop. A 3 phase motor cannot run on single phase power without the use of a phase converter, but then it typically produces only about 70-75% of it's rated horsepower.

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buickanddeere

03-04-2006 06:54:36




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 Re: 3 phase air comp limit switch wireing in reply to Steve Crum, 03-04-2006 03:56:08  
57% of full rated when operated on single phase instead of three phase.



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scott#2

03-04-2006 04:21:37




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 Re: 3 phase air comp limit switch wireing in reply to Steve Crum, 03-04-2006 03:56:08  
I just bought this old IR T30 80 gallon for real cheap. I know its just a IR but the price couldn't be beat. I run the whole shop on a 15 hp rotary converter and it turned this pump just fine when I wanted to see if it made air, however I haven't tried to start the motor at the kick in point of say around 125 psi and dont know whether I need a mag starter to accomplish that or not. All I was trying to do was wire up the thing and make sure it shuts down @ 175 psi, as it should and I wanted to make sure it was right so I didnt burn any of the motor fields up by only switching 2 legs. An electrician friend of mine said I only needed to break 2 legs but that just didn't sit right to me. Anyone know where I can get a diagram and must I use a mag starter?

Thanks Again,

scott#2

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Steve Crum

03-04-2006 05:59:37




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 Re: 3 phase air comp limit switch wireing in reply to scott#2, 03-04-2006 04:21:37  
I'm not a real big fan of problem machinery, so I try and do things right the first time. I wouldn't touch a 3 phase machine without using a 3 pole starter and more specifically a magnetic one. Think of your pressure switch as like the ignition switch in your truck, it doesn't energize the starter motor directly, rather it energizes a solenoid which in turn energizes the starter motor due to the incredible load the motor draws.
Granted you really only need to break the 2 hot legs in a phase generated system as the third leg (wild leg) is only hot when the 2 hot legs are. But it's only good practice to have your system set up as a true line powered 3 phase system. Besides motor starters are common as dirt on fleabay and about any salvage yard that scraps industrial machinery is usually happy to have you remove these dirt cheap.

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XRogerX

03-03-2006 22:52:35




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 Re: 3 phase air comp limit switch wireing in reply to scott#2, 03-03-2006 21:05:13  
I hope you're not trying to wire up a three phase compressor through the pressure switch.



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scott#2

03-04-2006 04:23:38




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 Re: 3 phase air comp limit switch wireing in reply to XRogerX, 03-03-2006 22:52:35  
Na, its just been a while since Ive wired up something like this. Machine tools are straight forward but this thing is a little confusing to me,

Thanks,

scott#2



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van robinson

03-03-2006 21:21:47




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 Re: 3 phase air comp limit switch wireing in reply to scott#2, 03-03-2006 21:05:13  
Yes, all three legs break on three phase and both legs break on single phase. If they didn't, your compressor motor would burn up. the high limit switch should make a relay drop out when the high limit is achieved.



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scott#2

03-04-2006 04:25:44




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 Re: 3 phase air comp limit switch wireing in reply to van robinson, 03-03-2006 21:21:47  
Thats what I thought. I need to look at the contacts on that limit switch and figure out how it works.

scott#2



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