Run a 3 phase air compressor on 1 phase.

[Bert_]

A year or so ago I did a little experimenting running a 3 phase motor on single phase. You can get full horsepower using a transformer and some capacitors. https://www.garagejournal.com/forum/threads/transformer-based-phase-converter.539211/

A couple weeks ago I picked up an old US air compressor pretty cheap. Came with a 1 1/2 motor but the pump is good for 3hp and I have a 3 HP 1 phase motor laying around.

I decided I'd try a 3 phase motor and my converter first. I wired it up just like before and this time I actually got a potential relay so it's completely automatic.

Works good. Have used it a bit over the last few days for plasma cutter and other air tools.

I checked amps at different pressures as it pumps up. The current is a little high and isn't balanced perfectly. I'm fairly sure the motor pulley is too big since the single phase motor ran over nameplate amps also. I could probably improve it if I had a transformer that could produce about 330v but I can't get that out of a common, cheap transformer.

If I can scrounge up a 3 phase, 3hp motor I will make this a permanent install. I would really like to do that, but I'm not spending much money on a motor to make that happen.

 

[Bert_]

I think I'm going to turn the pressure switch down to 150psi, that would bring the motor current within spec and still plenty of pressure. Then maybe tidy up the wiring to where I'm comfortable leaving it on when I'm not standing right there watching it.

I want to use it and test it for a while. I'll try to do something that uses enough air to run it for a long period.

When the unloader bleeds off pressure as designed it always starts without issue.

Out of curiosity I tried it without the unloader and it was able to start up to 120ish psi. Higher than that and it tries for a few revolutions but once the start capacitor kicks out it loses speed. The potential relay I used picks up between 180-195 volts. A higher voltage rating would keep the start capacitor engaged longer and could probably start without the unloader at full pressure. That is pretty impressive.

 

[Bert_]

I ran it for a solid hour. I ran it down to 75psi several times and it would catch up and reached the 150psi cutout if I wasn't using air. It ran most of the time though.

Motor barely got warm. Was about 50* ambient, motor was maybe 75*. I'm using the same pulley that was on the single phase motor.

 

[Carchie]

A year or so ago I did a little experimenting running a 3 phase motor on single phase. You can get full horsepower using a transformer and some capacitors. https://www.garagejournal.com/forum/threads/transformer-based-phase-converter.539211/

A couple weeks ago I picked up an old US air compressor pretty cheap. Came with a 1 1/2 motor but the pump is good for 3hp and I have a 3 HP 1 phase motor laying around.

I decided I'd try a 3 phase motor and my converter first. I wired it up just like before and this time I actually got a potential relay so it's completely automatic.

Works good. Have used it a bit over the last few days for plasma cutter and other air tools.

I checked amps at different pressures as it pumps up. The current is a little high and isn't balanced perfectly. I'm fairly sure the motor pulley is too big since the single phase motor ran over nameplate amps also. I could probably improve it if I had a transformer that could produce about 330v but I can't get that out of a common, cheap transformer.

If I can scrounge up a 3 phase, 3hp motor I will make this a permanent install. I would really like to do that, but I'm not spending much money on a motor to make that happen.

Cool experiment, but I wouldn’t trust a homemade phase setup long term on a compressor, those things love finding the weak link and turning it into smoke lol. If it runs, great, but high and unbalanced current is basically the machine telling you it’s not happy. For a permanent setup I’d still lean toward a proper motor swap or a real converter, otherwise you’re kinda gambling every time it kicks on under load.

 

[Bert_]

Cool experiment, but I wouldn’t trust a homemade phase setup long term on a compressor, those things love finding the weak link and turning it into smoke lol. If it runs, great, but high and unbalanced current is basically the machine telling you it’s not happy. For a permanent setup I’d still lean toward a proper motor swap or a real converter, otherwise you’re kinda gambling every time it kicks on under load.

Glad to finally have some discussion. Thank you.

The single phase motor that came on this compressor ran about 8% over nameplate current as well. I do not think the motor pulley is original.

The literature I found for this compressor says max 3hp at 684 rpm. This one was built with a 1 1/2hp motor so I believe should be spinning 342rpm. The current pulley combination figures up to 410rpm pump speed, which would require 1.8hp . I assume pump speed versus horsepower is a linear relationship, correct me if I'm wrong.

I think if I had a transformer capable of producing a VT 330v I would be able to balance the current almost perfectly. But that would mean a custom transformer and that would be much more expensive, and I believe unnecessary.

If you ever work in a commercial building with a 120/240 Delta, you will see that even with real utility provided three phase, motor current is not always balanced.

 

[Bert_]

Seeing is better than reading someone's notes. I look this short video.

Before this video I made a small adjustment to the size of capacitors. I have 42.5 MFD of run capacitors and have 300 volts on VT.

No changes were made to capacitors or any other components during this test

The first clip shows startup and current with a nearly empty tank, then current with the tank about 1/2 full. The final clip shows a normal startup when the pressure switch cuts in.

Including the 1.15 service factor this motor can go to 5 amps.

 

[micromind]

I haven't worked with very many static phase converters but I've never seen one that had even close to balanced motor current.

Ya got lucky!!

 

[Bert_]

I haven't worked with very many static phase converters but I've never seen one that had even close to balanced motor current.

Ya got lucky!!

Luck's got nothing to do with it.

This autotransformer/capacitor phase converter can produce very well balanced phases. These have been commercially available since the '50s. However there is almost no information about how to build one. I'm not claiming that this is my idea. I just figured out how to build it.

This converter is not suitable for every load. The thread I linked in the first post tells more about that. But this converter does work fine on a compressor which I now think is an excellent application for one.

If you're thrifty you can scrounge up most of these parts for scrap price. Or you can go on eBay and put one together for a couple hundred bucks.

I'm not going to claim any great advantage to using this for a 1 1/2 HP motor, that just happens to be what I have. But if you want a 5-10 HP motor... Go price a single phase motor or a vfd and let me know what you think of that!

 

[Dagny]

The varying load is the problem.

 

[Dagny]

If it was in my shop I would turn it on when I needed it.

 

[Bert_]

The varying load is the problem.

If it was in my shop I would turn it on when I needed it.

I'd love it if you would expand on that.

 

[Bert_]

Still running great. Have done nothing other than turn the breaker on and off when I need it.

Have been using it about 4 days a week. Sometimes just cycles once to fill the tank but have ran it almost constantly for an hour a couple times.

Little disappointed in the lack of discussion. If you have a specific concern I want to talk about it.

 

[Bert_]

 

[William Payne]

Little disappointed in the lack of discussion. If you have a specific concern I want to talk about it.

I wish I had something to add but the need for phase converters and people trying to run 3 phase machines on single phase services seems to be a predominantly North American problem due to your guys infrastructure limitations.

3 phase stuff is often found cheap and the reason why is because most can’t run it.

 

[wyliesdiesels]

I have 3 phase at my shop so this isnt needed here but i do appreciate the content and info.

 

[mm08822]

Little disappointed in the lack of discussion.

One reason is that most of the audience does not have these components just sitting around.
Then, what are the rights specs for those components for their application....what to buy - whether new or used? Is it the right one?
Then an enclosure is needed (or should be).

vs.

Go buy a complete one component solution.....a vfd. There is little to guess at for the vfd specs: motor hp, single-phase front end or derated 3-phase. It can come in a nema1 or 4 enclosure and done. Very clean and simple.

I agree a vfd is not as robust as nema starters, caps and xformers.

 

[Firebrick43]

I wish I had something to add but the need for phase converters and people trying to run 3 phase machines on single phase services seems to be a predominantly North American problem due to your guys infrastructure limitations.

3 phase stuff is often found cheap and the reason why is because most can’t run it.

Not an infrastructure limitation so much as most don’t or won’t pay for the extra lines and transformer to the property from the road. And if few need 3 phase why would they?

There are a few rural areas that have single phase only but even in the most rural county in Indiana a lot of the roads have 3 phase on the pole.

There is a reason that Europe or even New Zealand have over double the average cost per KW hour compared to to the average rates in the US

 

[wyliesdiesels]

I wish I had something to add but the need for phase converters and people trying to run 3 phase machines on single phase services seems to be a predominantly North American problem due to your guys infrastructure limitations.

3 phase stuff is often found cheap and the reason why is because most can’t run it.

only because PoCos do not see the point in running 3 phase everywhere when only a handful of customers would actually use it especially in rural areas.... the return on investment would be very very slow....

 

[TRWham]

... And if few need 3 phase why would they?
...

Because this is GJ, where you need a Kenworth T800 and a lowboy to pick up a bale of straw at Home Depot once a year. After all, a man's gotta future proof.

 

[Bert_]

Rural lines are mostly 1 phase. The last quote I heard to rebuild primary was $80,000 per mile. Often the existing 3 phase is miles away.

Wired a site last fall, they brought 3 phase in a little over 3 miles.

 

[William Payne]

There is a reason that Europe or even New Zealand have over double the average cost per KW hour compared to to the average rates in the US

Great comment! Converted to US dollars I am paying about 20 cent USD per KW/H. What do you guys pay? I have no idea if that is good or not.

 

[Bert_]

Edit

 

[Bert_]

One reason is that most of the audience does not have these components just sitting around.
Then, what are the rights specs for those components for their application....what to buy - whether new or used? Is it the right one?
Then an enclosure is needed (or should be).

vs.

Go buy a complete one component solution.....a vfd. There is little to guess at for the vfd specs: motor hp, single-phase front end or derated 3-phase. It can come in a nema1 or 4 enclosure and done. Very clean and simple.

I agree a vfd is not as robust as nema starters, caps and xformers.

Yes a vfd is easier. Gotta get out the checkbook though. Hard to justify the price for hobby machines.

I'm pretty rural. In an area with more industry I would think this stuff would be even easier to find as surplus or scrap.

You want to know how to size the stuff? Ask away, I'll answer as best I can.

There's always a few guys wanting to run something on the cheap. This is for them. Plus it's fun to learn something.

Right now my threads about it here have more information than anywhere else on the Internet. I have spent A LOT of time searching. There is one decent thread on practical machinist, but it lacks a lot of details.

 

[William Payne]

And if few need 3 phase only a handful of customers would actually use it especially in rural areas....

Because this is GJ, where you need a Kenworth T800 and a lowboy to pick up a bale of straw at Home Depot once a year. After all, a man's gotta future proof.

Rural lines are mostly 1 phase. The last quote I heard to rebuild primary was $80,000 per mile. Often the existing 3 phase is miles away.

You guys will have to excuse me. I spend a lot of time on machining forums. The North American 3 phase problem comes up almost weekly. There was even a discussion about how much productivity is lost from people not being able to start businesses at home?

I’m not lucky though. For me down here it’s at the street I still had to pay to connect to it. Germany and Central Europe are the true lucky ones. They just plumb it into every house whether needed or not.

 

[Bert_]

If someone wants to help out with the theory I would really like that. I have a vague understanding of the theory of why it works but I don't know that I could convey it in words

 

[sparky 1971]

Rural lines are mostly 1 phase. The last quote I heard to rebuild primary was $80,000 per mile. Often the existing 3 phase is miles away.

Wired a site last fall, they brought 3 phase in a little over 3 miles.

One of my regular good customers called me to ask about the possibility of putting three phase in his shop in the middle of nowhere, about five miles of gravel road from the nearest highway. I don't know or remember what it is, but he has some pneumatic tool and a 10 HP single phase air compressor can't keep up with it and wanted to get a 15 HP. I told him anything is possible but I didn't have a clue where the nearest three phase was. He said great, he'd call Mid Am and see what they thought. He gave them my name and number as the installer and about six months later, after I'd forgotten all about it, I got a call from the POCO to ask how serious we were because he didn't have the numbers together yet, but it was looking like the price was going to be about $1,500,000.00. We aren't going to go that route.

 

[sparky 1971]

If someone wants to help out with the theory I would really like that. I have a vague understanding of the theory of why it works but I don't know that I could convey it in words

I'm clueless.

 

[mike93lx]

You guys will have to excuse me. I spend a lot of time on machining forums. The North American 3 phase problem comes up almost weekly. There was even a discussion about how much productivity is lost from people not being able to start businesses at home?

I’m not lucky though. For me down here it’s at the street I still had to pay to connect to it. Germany and Central Europe are the true lucky ones. They just plumb it into every house whether needed or not.

If your reference was us power prices, you wouldn't think that if you saw a German power bill. But compared to NZ, I believe they are "cheaper"

 

[William Payne]

If your reference was us power prices, you wouldn't think that if you saw a German power bill. But compared to NZ, I believe they are "cheaper"

Prices no no no. Everyone has a different deal based on how power is generated in their area.

The German luck. Is that if you buy a 3 phase machine on germany the most difficult thing you will have to do is maybe have an out outlet installed on a circuit. Not run a whole new supply.

 

[wyliesdiesels]

Great comment! Converted to US dollars I am paying about 20 cent USD per KW/H. What do you guys pay? I have no idea if that is good or not.

Thats kinda expensive but im on a public owned PoCo grid that is not for profit. far far cheaper than the for profit big boys.... amazingly the service is far more reliable than the big boys who burn down entire towns and have horrible customer service (looking at you PG&E Profit Gouge and Execute)

 

[35Ford]

OP, Very interesting. It’s both fun and a challenge to get it put together and have it work as planed. I did something similar but I built a rotary converter. Like you, I kept playing with the run capacitors until the load was balanced running a Bridgeport. It runs all my other three phase equipment without adjustment. Mine has been running for close to 40 years with all the same components. I’m sure yours will serve you well for a long time. Well done.

 

[86turbodsl]

Sounds about right. My uncle was less than 1 mile from a 3 phase line. He asked about bringing it in for a grain dryer. I believe he was told 1800 for 7/10 mile. This was in late 1980s.

 

[mm08822]

If someone wants to help out with the theory I would really like that. I have a vague understanding of the theory of why it works but I don't know that I could convey it in words

Here are two youtube vids from the same creator.

First one shows 3rd phase generation only using a run cap.
Second one shows 3rd phase generation (as in the first) but also needing a start cap for increased torque to get the motor running.(Similar to Bert's set-up)

Both are theory-based. Overall, nicely done, but a few points are just provided as "a given". They are both missing explanations about creating the phase shifts between the 3 phases and no mention of the fact that the phase shifts can't achieve a full +/- 120 degree displacement. Hence, start-up torque and run power is less than would be achieved with +/- 120 shifts on the same motor. Caps and inductors are only going to provide a max shift of 90 degrees.

I'll see about graphing the 120 and 90 degree phase shifts for some additional info.

 

[Bert_]

Here are two youtube vids from the same creator.

First one shows 3rd phase generation only using a run cap.
Second one shows 3rd phase generation (as in the first) but also needing a start cap for increased torque to get the motor running.(Similar to Bert's set-up)

Both are theory-based. Overall, nicely done, but a few points are just provided as "a given". They are both missing explanations about creating the phase shifts between the 3 phases and no mention of the fact that the phase shifts can't achieve a full +/- 120 degree displacement. Hence, start-up torque and run power is less than would be achieved with +/- 120 shifts on the same motor. Caps and inductors are only going to provide a max shift of 90 degrees.

I'll see about graphing the 120 and 90 degree phase shifts for some additional info.

Yikes. Am I the only one that can't stand more than a few seconds of that AI voice?

I put it on 2x speed and made it far enough to hear that it's capacitor only and will not generate full horsepower.

I think many people understand that with a capacitor, current leads voltage by 90*.

How is it possible to end up with 120° shift by increasing the voltage to the capacitor? I think it must be approximately 120° phase shift because I am able to balance the motor current very well, with the motor at full load.

 

[mm08822]

Yes a vfd is easier. Gotta get out the checkbook though. Hard to justify the price for hobby machines.

I'm pretty rural. In an area with more industry I would think this stuff would be even easier to find as surplus or scrap.

You want to know how to size the stuff? Ask away, I'll answer as best I can.

There's always a few guys wanting to run something on the cheap. This is for them. Plus it's fun to learn something.

Right now my threads about it here have more information than anywhere else on the Internet. I have spent A LOT of time searching. There is one decent thread on practical machinist, but it lacks a lot of details.

Having components at hand and for free from demo'd equipment is perfect to create a parts base to pic from. Even better is having a project in mind when you spot hardware that will be discarded. You grab it and eventually use it vs. storing ****.

I was able to scrap-pass lots of good stuff years back. Some got used, some never used and eventually painfully thrown out, some I still have. Not everyone has this benefit, space or knowledge to brain-storm a future project with a good find.

If it comes to spending money on parts for an unsure build (what sizes/ratings are needed/will it work), many would default to sure thing and do the vfd. Those with the DIY spirit, entrepreneurial creativity, time, not afraid of blue smoke types could easily be persuaded to build their own. (All I'm saying is, the audience on here isn't as big as you may be thinking. So, traffic/response on this thread is lower than you hoped. This by no means lessens what you have done, the learning, and the fun you had accomplishing it. Keep on keeping on.)

 

[mm08822]

Yikes. Am I the only one that can't stand more than a few seconds of that AI voice?

I put it on 2x speed and made it far enough to hear that it's capacitor only and will not generate full horsepower.

I think many people understand that with a capacitor, current leads voltage by 90*.

How is it possible to end up with 120° shift by increasing the voltage to the capacitor? I think it must be approximately 120° phase shift because I am able to balance the motor current very well, with the motor at full load.

C'mon, that AI voiceover was waaaaaaay more pleasant than some others.

While it wasn't your configuration entirely, it does provide insight to cap sizing, need for a start cap and some means needed to cut-out the start cap once running. Never did the OP explain why he was trying to match the impedance value of the winding with a cap or phase angles at all. To me a big miss as that's the whole purpose to start/run a 3 phase motor.

In your wiring config, the xformer you have in the circuit is providing a 180 degree phase shift (additive winding connection) and connected with the cap and windings in that leg of the circuit, there is a phase shift. Probably better than 90 degrees but without values of all the passive elements, can't say anything further. Current and voltage measurements w/o phase angles again only gives part of the story.

Without a scope or other means to measure waveforms (phase angles) it's hard to say your config is providing full output power. Not all of the phase current magnitudes are equal so I assume the phase shifts across the windings aren't equal.

Higher voltages are probably needed to provide enough current in the circuits as the winding inductance changes with larger motors and more torque (created by higher current) is needed to initiate rotation for larger connected loads.

 

[Bert_]

C'mon, that AI voiceover was waaaaaaay more pleasant than some others.

While it wasn't your configuration entirely, it does provide insight to cap sizing, need for a start cap and some means needed to cut-out the start cap once running. Never did the OP explain why he was trying to match the impedance value of the winding with a cap or phase angles at all. To me a big miss as that's the whole purpose to start/run a 3 phase motor.

In your wiring config, the xformer you have in the circuit is providing a 180 degree phase shift (additive winding connection) and connected with the cap and windings in that leg of the circuit, there is a phase shift. Probably better than 90 degrees but without values of all the passive elements, can't say anything further. Current and voltage measurements w/o phase angles again only gives part of the story.

Without a scope or other means to measure waveforms (phase angles) it's hard to say your config is providing full output power. Not all of the phase current magnitudes are equal so I assume the phase shifts across the windings aren't equal.

Higher voltages are probably needed to provide enough current in the circuits as the winding inductance changes with larger motors and more torque (created by higher current) is needed to initiate rotation for larger connected loads.

This compressor came to me with a single phase motor, the same size as the 3 phase motor I have on it currently. I used the same pulley.

The single phase motor ran about 10% over FLC and the 3 phase motor is about the same.

Isn't that reason to believe it is producing it's rated power?

In a perfect world I would have a transformer with multiple taps. But when working with surplus, close is good.

 

[wyliesdiesels]

The single phase motor ran about 10% over FLC and the 3 phase motor is about the same.

you mean nameplate FLA? if its running 10% over NEC FLC (table current) that means its horribly inefficient...

 

[TRWham]

...

How is it possible to end up with 120° shift by increasing the voltage to the capacitor? I think it must be approximately 120° phase shift because I am able to balance the motor current very well, with the motor at full load.

I modeled a phase converter in Excel a few years ago to visualize what is happening. There are two 120 Volt lines 180° apart (standard split phase supply) with a 208 V line created between those two. Remember 1) a phase is the difference in voltage between two lines, 2) both lines are always changing voltage, 3) the voltage change on the higher voltage (to ground) line is greater for a given slope in the curve, and 4) the slopes of the curves are not the same at a given point in time. Once you add all those changing things together, you get a 3 phase 240 V supply evenly spaced at 120°.

 

[Bert_]

you mean nameplate FLA? if its running 10% over NEC FLC (table current) that means its horribly inefficient...

Yes. I'm not concerning myself with anything NEC at the moment. This is a bunch of parts laying on the floor for proof of concept.