3 phase to single?

cjensen · Post by **cjensen** » Fri Feb 10, 2006 11:10 am

i have a chance to buy an big air compressor that has a 3 phase motor on it. is it feasible to convert or change the motor to single phase. is this dangerous to try?

mr. electrician CJ??

i wouldn't even consider doing this if the price wasn't so good...

hngrflyr · Post by **hngrflyr** » Fri Feb 10, 2006 12:04 pm

You can buy a frequency drive that will convert single phase power to three phase. I just happen to have one I might be ready to sell. It is rated for a 3 hp motor. It has a manual that tells how to hook it up.

Bob Severns

Post by **Spike** » Fri Feb 10, 2006 12:14 pm

Hmmm, usually Chad there is a reason that a motor is a 3 phase motor. Typically its because the load put on the motor is a large one, or has it has a large start up load. My guess would be that even if you converted it to single phase, it wouldnt have the strength anymore to run the compressor.

-- John

tshort · Post by **tshort** » Fri Feb 10, 2006 1:37 pm

Ditto what Spike said.

http://www.faqs.org/faqs/electrical-wir ... on-10.html

lots of good electrical info here (some of it pretty scientific!):
http://theoak.com/rick/Electricity_in_the_Shop.html
scroll down for the 3 phase explanations

How cheap is it? If you have to spend a bunch of time and money to get it to run it may not be worth it ... the picture looks like it was kind of old - how much life do you think it has left?

CJ can definitely provide more info...

T.

cjensen · Post by **cjensen** » Fri Feb 10, 2006 1:42 pm

thanks guys...i think i've decided against it. it's only $110, but i think the cost will go up substantially to install the 220 and switch the motor (which doesn't sound like it will work anyway.) i have no idea how old it is, but we have one of these in our shop at work, and it's probably 30 years old or so and still kickin'.

cjensen · Post by **cjensen** » Fri Feb 10, 2006 1:44 pm

good info in the links thomas!

ptrotter · Post by **ptrotter** » Fri Feb 10, 2006 1:45 pm

I'm sure that CJ can give a better answer, but there is nothing wrong with a 3-phase motor. Most industrial machines use them. 3 phase is more efficient than single phase so it is used whenever it is available. A lot of people use equipment with 3 phase motors by using a phase converter. Traditionally phase converters were basically a motor/generator that generated 3-phase power using a single phase motor. This worked well but was inefficient as you ended up losing a fairly significant amount of power. Now there are a lot of solid state phase converters available that are quite efficient and not too expensive. I know a lot of people using 3 phase equipment using phase converters very successfully. You need to know the power requirements of the equipment and size the phase converter properly and ensure that you have sufficient input power to the phase converter.

aerial · Post by **aerial** » Fri Feb 10, 2006 4:31 pm

I have a big Ing-Rand compressor that is 3 phase. I paid $500 for it so $110 sounds pretty good. I have a new 220 single phase motor I am going to put on it that was purchased from Grainger. Haven't hooked it up yet but it should be sweet especially for painting with my old Devilbiss, and I dont' think there is a air tool out there that can run it down.

captain_john · Post by **captain_john** » Fri Feb 10, 2006 5:24 pm

WOW! I get home from teaching this stuff all day and BANG! Here it is on my home computer! The internet... I hear they are gonna have it on computers soon!

Chad, ummmm, well... Where do I start? Maybe a bulletized list? Okay...

Pro's

1) Cheap (that is good 'cause hooking it up is gonna be a DOOZIE in a residential area!)
2) No other pro's

Con's

1) Need 3-phase power (in a residential situation, you just can't get it.)
2) No other con's

That was easy!

Seriously, the question is... What is 3 phase power and how do I get it?

It is where you have 3 "hot legs" 120 degrees out of synch with each other. Essentially a phase shift in the sine wave of the AC magnitude.

Each one starting a climb to max voltage after the next. This makes for a very EFFICIENT motor. Not necessarily any more powerful. You can have an equally powerful (output horsepower-wise) in a single phase motor. A three phase motor is akin to log splitting. A single man striking a maul on a wedge cannot be compared to three men with mauls 120 degrees apart, striking the same wedge. One BIG, strong man could potentially beat the three... meaning there is more "power" there. Kinda like the single/three phase "thing".

How do you get it?

Well, you could buy a phase converter. That is the UL approved, thouroughly engineered, dependable, safe way to go. Buuuut, what is so fun about that!?! We are tinkerers! Let's do some "Ghetto FAB!"

Here is a simple THREE PHASE GHETTO CONVERTER!!!

Need:
1-3 Phase load (your compressor)
1-3 Phase motor of equal size (or better)
1-Everything else to hook it up

Concept:
Make Three Phase power from Single. Single phase power is 180 degrees out of phase.

Net result due to vectoring, less leg to leg voltage.

We need to create (by induction) a THIRD leg!

How? Well, we get another three phase motor and hook it up to the TWO legs we have in a single phase system. It has higher voltage and can limp along on two legs, but not enough to really have much torque.

Soooo, we tie the three legs of the limping motor to the three legs of the compressor motor. There will be an "INDUCED" voltage present in the third leg of the liming motor. It will be roughly 20-35% lower, but nevertheless... will BE THERE!

This induced "Phantom" leg is enough to make the compressor run at nearly full power!

DANGERS:
1) No UL testing HAS EVER BEEN DONE on this!
2) Overload protection needs to be in place!
3) No manufaturer has ever made anything to set this up.
4) Depending on phase rotation, reverse operation could result.

Nuisances:
1) Limp motor may need a twist to get going. This means that you will need to stop work and give it a spin to start the motor. "HASSLE"!
2) Fabbing up the motor starting gear is complicated at best.

I HAVE done this in my lab at school for demonstrations and it DOES work!

I would only hook it up at my house or someplace where someone FULLY understands what is going on.

Disclaimer: This is TOTALLY GHETTO! Don't do it!

With that said, give it a try!

Sorry for the long read.

CJ

tshort · Post by **tshort** » Fri Feb 10, 2006 6:27 pm

I'm experiencing deja vu

Thomas

aerial · Post by **aerial** » Sat Feb 11, 2006 10:02 pm

I'm glad CJ is around so he can help with wiring our panels.

So what kind of fancy lighting are you putting in those wings? Any "trick" stuff like static electricity powered ouga horns for extra effect on those high-speed fly bys?

captain_john · Post by **captain_john** » Sat Feb 11, 2006 10:37 pm

My plane is actually going to be solar powered!

I won't have to worry about AVGas availability or fouled plugs!

CJ

cjensen · Post by **cjensen** » Sun Feb 12, 2006 7:38 pm

well, i came up with a solution to my larger air needs...

i found out a friend of mine has FOUR air compressors stashed away in his shed/barn/warehouse. i got to talking with him about the compressor i was looking at buying, but that i would need to make some changes, and end up with more money in it than i wanted. he says, well come on over to my place, i have just what you need. so, the above pic shows what i came home with today. it's REALLY quiet, and has the capacity i'm looking for right now.

captain_john · Post by **captain_john** » Sun Feb 12, 2006 7:42 pm

Chad, COOL! It's a Harley! Vee twin cylinders!

That oughtta do the trick!

Gotta love it!

CJ

jim_geo · Post by **jim_geo** » Sun Feb 12, 2006 10:03 pm

You're gonna be glad it's quiet.

cjensen · Post by **cjensen** » Sun Feb 12, 2006 11:32 pm

jim_geo wrote:You're gonna be glad it's quiet.

so will my wife!!!

aerial · Post by **aerial** » Thu Feb 23, 2006 1:48 am

OK I bought a single phase motor and need to wire my compressor for 220v and I want to use my 3 phase switch, so here is a pic of the guts

Can I use this switch? Which 2 hot legs should I use L1 & L2, or L1 & L3, L2 & L3..... The pressure reg seems to tie onto L2 ....but I need an expert (and an aspirin)

Any educated opinions?

As an aside should I run a neutral/ground thru to the motor and ground it?

captain_john · Post by **captain_john** » Thu Feb 23, 2006 6:44 am

Piece o' cake...

Tie onto any two legs you like. I assume you will be abandoning the pressure regulator and contactor portion of the system because the new compressor will be coming with it's own prewired stuff.

Sooooo if that is the case, wirenut the 2 hot wires from the new compressor to the L1 and L2 wires. Either that or you could nipple a receptacle off the side of this can with the L1 and L2 going to the receptacle. You will need to pick up a ground someplace. It looks like the can was fed with Liquidtite and it appears that there is no ground wire in the can!?! As I recall, LT is suitable for grounding if metallic (it sometimes has a metal interior) and it is in runs of 6' or less. Personally, I would run a # 12 green or bare copper back to the last box or the panel.

Does that help?

CJ

aerial · Post by **aerial** » Thu Feb 23, 2006 12:01 pm

Yes, that helps, thanks. I thought I could just hook up to 2 legs, I am using the existing pressure shut-off the only thing I am changing is the motor on this compressor,which you can see off to the left (slightly) to single phase. I'll use the exiting wire and use the 3rd as a ground/neutral (bypassing the switch obviously) I wasn't sure how the shut-off was tied into the switch.

This thing will build 300psi and I wouldn't want to test the pressure-relief valve's reliabilty if the shut-off didn't work correctly

(Maybe I would make the front page...."First Man to orbit the earth on an experimental compressor"

captain_john · Post by **captain_john** » Fri Feb 24, 2006 8:44 am

OK, sounds like you got it right...

Make sure you identify (tape) the ground green!

Glad to be of assistance.

CJ

RivetBangers.com

3 phase to single?

3 phase to single?

A lot of hot air ........ or this horse is not dead yet....