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3-phase to single phase

15KW 3-phase compressor on single phase??

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Oliver Webster27/03/2017 10:37:13
15 forum posts

Compair Motor Name Plate

So, I've got the opportunity to buy a compressor from my employer very cheaply (we're moving buildings and it's basically going to go in the bin / broken into spares if I don't want it). Obviously I'd love a 15Kw screw compressor (it's a nice one and has had little use). But, I don't have 3-phase.....I've read a little bit about rotary phase converters etc. These seem expensive, especially at the size I would need. I've also read that some 3-phase motors can be re-wired to work on single phase at lower outputs. Attached is a photo of the nameplate for the motor. It does appear that you can wire in both star and delta. But with such a big compressor is it even possible to run on 230V? I understand that it'dd suck a good deal of power on startup so maybe that's where it'll come unstuck? I'm begining to think it's more hassle than its worth.......

Appreciate any insight from people who know what they're talking about!

Chris Gunn27/03/2017 11:39:33
459 forum posts
28 photos

Oliver, If it is really really cheap, buy it, then sell it on and buy a single phase compressor.

Chris Gunn

John Stevenson27/03/2017 11:43:55
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5068 forum posts
3 photos

Ask yourself, "Why do I need all this compressed air when I don't have any at the moment ?"

Paul Lousick27/03/2017 11:58:41
2276 forum posts
801 photos

I am not an electrician but I think that the standard single phase outlet is only 15 amps at 240 volts which is only an output of 3.6 kW maximum. The compressor needs 15 kW. You will blow the fuse or start a fire.

Paul.

Edited By Paul Lousick on 27/03/2017 12:03:08

Andrew Johnston27/03/2017 12:57:30
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7061 forum posts
719 photos

The standard 3 pin UK plug is rated at 13A, so about 3kW at 230V. As part of my 3-phase installation I have a single phase 32A outlet; that can supply 7.36kW. It is possible to get 63A outlets which, at 14.5kW, might just run the compressor.

It could be tricky running a 63A outlet from a standard consumer unit; my 32A outlet comes from a 3-phase distribution box. In general the highest power circuits in a domestic environment are 45A for cookers and electric showers.

So, in theory, it might just be possible to run the compressor on a single phase supply, although how the incoming supply and consumer unit would stand up to it is another matter. It is possible to run a 3-phase motor on a single phase supply via a static converter, but when running the motor only produces about half nominal power, as one phase is essentially unpowered. That may be a problem with a compressor.

As a matter of idle curiosity I wonder how the OP is going to judge whether the responders know what they're talking about or not?

Andrew

Neil Wyatt27/03/2017 13:20:58
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19226 forum posts
749 photos
86 articles

Also a standard domestic supply has an 80A master fuse; 63A doesn't leave you much wriggle room if you have some other kit running...

Neil

Oliver Webster27/03/2017 13:47:54
15 forum posts

So, in turn....

- it's pretty cheap (£300) but tbh I don't see there being a very big market in secondhand 3-phase compressors so I might make a bit of money but not that much. Especially when you factor in time etc to shift it!

- I don't need "that" much compressed air. If it was a 5 or 7.5KW it would be a total no-brainer. I currently run 2x 2HP 50 L jobbies daisy chained together and this is not enough.

- 13A, 230V but as others have said - nothing stopping you running it on a separate circuit just like electric showers / ovens etc.

- Yes, I've read about converters. The inverter type seem like the one to go for (so they're not hungrily consuming power 100% of the time) but a 15 KW one is seriously expensive to buy. To the point where it might be cheaper to by a diesel genset. In terms of judgement, I don't know. I'm quite handy with electrics but have never dealt with 3-phase. I can easily weed out the people who don't have a clue but yeah.....not sure beyond that.

Generally I'm thinking it's more hassle than it's worth - which is where I got to last week but thoguht it would be worth a post in case someone has done similar before.

Michael Gilligan27/03/2017 13:54:46
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23121 forum posts
1360 photos

Just a thought ... Why not put a small petrol engine on it question

MichaelG.

Edited By Michael Gilligan on 27/03/2017 13:55:11

Oliver Webster27/03/2017 13:58:43
15 forum posts

That did also occur......only thing is when it's on, it's on then and the noise of air being dumped because I can't use it up quick enough would be louder than the engine!

Harry Wilkes27/03/2017 14:30:17
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1613 forum posts
72 photos

That's some machine for home use go with Chris Gunn suggestion buy and sell then buy something smaller

H

Rick Kirkland 127/03/2017 16:42:18
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175 forum posts

To clear up a couple of points. The main overcurrent protective device, (main fuse) for domestic installations supplied in particular on P M E has a value of 100 Amperes, as per the suppliers requirements. If a smaller capacity fuse is fitted it will be for a reason specific to the installation. The motor in question on this thread cannot be wired for Star or Delta. The plate clearly states the CONN, (connection) is Delta. The voltage rating backs this up by stating the voltage requirement to be 380 /420 v. The Amperage at 28.6 and 25.8 in relation to these voltages gives further weight as does the Service Factor and the Service Factor Current draw relating to the voltages, They are the between phase voltages for a Delta connected motor. Now, I would like to learn something here. I know that 3 phase machines can be run by various means from a single phase and if I'm reading the O P correctly it was stated that a 3 phase motor can be connected to run off one phase. I'd be interested to learn how this is done, How the motor actually starts and how smoothly and efficiently it runs. I love learning new stuff. I've recently discovered something called CNC. I'm going to be learning about that next.

not done it yet27/03/2017 17:26:18
7517 forum posts
20 photos

Apart from the running current, it would have to be a very soft start. Motor current in-rush, at start up, can be several multiples of full load current. Not a chance on a normal domestic supply.

How many motors of over 3 HP do you see running on domestic single phase? Not many.

Muzzer27/03/2017 17:45:47
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2904 forum posts
448 photos

You'd need a VFD that boosts the input voltage up to 700Vdc or so before generating the 415V 3-phase output. However, most VFDs that can accept single phase 230V inputs are of limited power rating. Put the whole thing together (single phase, 230V input and 415V output at 15kVA) and I suspect you are more likely to find rocking horse crap. Hardly surprising, as there are few obvious circumstances where high powered (industrial) equipment would not be used in an (industrial) environment with (industrial ie 3-ph) infrastructure - that's what it's for

Once you've done a bit of obligatory willy waving you should get it on ebay and then use the resulting beer tokens to buy something useful / usable. May not be worth much mind, especially if it requires a costly service. Has it been getting its annual service / safety inspection etc?

Murray

Oliver Webster27/03/2017 17:54:00
15 forum posts

Hey folks, so you've basically reconfirmed my conclusion from Friday night. It's too big to make sense. I really don't think it's worth buying to sell it on again. From what I can tell most people who want a 15KW screw compressor either already have one and don't have 3-phase and are therefore in the same boat as me. Add to that my geography (north-east Scotland) the fact I have to disconnect and remove it and it weighs 335 Kg and it just isn't worth it. I'm glad I asked even though it made no difference to the outcome. Had it been a 5KW I think the answer would've been different.

However, in happy news - my work have sold it back to the people that do it's annual service (they're going to use it as spares) and I called the guy up and he said he's happy to give me the 350 L air receiver for free (because they're just going to scrap it anyway). So instead of spending £300 on a compressor setup I can't use I'm getting a big air receiver for free and can buy myself a nice belt driven compressor to add to my collection of 2x 2HP direct drives. Hopefully all of this put together should easily manage to keep up with pretty much everything I do. So it's all worked out in the end.....

Bob Brown 127/03/2017 18:15:29
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1022 forum posts
127 photos

I would treat a second hand air receiver with caution, the last thing you want is it going bang. may look fine on external inspection but I would like to see inside as that is where they rust. It begs the question why are they scrapping it?

David Standing 127/03/2017 18:21:47
1297 forum posts
50 photos
Posted by Bob Brown 1 on 27/03/2017 18:15:29:

It begs the question why are they scrapping it?

I think the answer may be in line two of post one wink

Bob Brown 127/03/2017 18:32:43
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1022 forum posts
127 photos

I had read that but it didn't answer why are they scrapping it.

Oliver Webster27/03/2017 20:35:01
15 forum posts

Yeah, it's being "scrapped" because we're amalgamating a couple of buildings / workshops into 1 new building. So we have 2 compressors and 1 is newer than the other so it's being kept. Being a big oil company they're not really interested in selling it privately for a good price so offered it to the company that supplied it originally (the same company who do the annual service on both compressors). They offered £300 to take it away on the basis that it's been superceded twice so was just going to be used as spares. Hence why they offered it to me. 

I appreciate your concerns but it's a 12Bar air receiver (tested to 18Bar) has been stored inside throughout its life and there's a diet between the compressor and it. My current compressors are only good for 8Bar and the belt drive one i might get as a third one is 10Bar (would likely set it to 8 like the others) so I'm not really concerned. Furthermore I'm an engineer designing ASME boilers and pressure vessels so will have a good poke around with some of that knowledge. What i don't have a clue about us anything to do with 3-phase!

Bob Brown 127/03/2017 20:49:24
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1022 forum posts
127 photos

My concerns have now been put to bed, but as you well know it is not something to be taken lightly.

Oliver Webster27/03/2017 20:52:12
15 forum posts

Absolutely, 320L at 12 Bar letting go all at once would be one hell of a mess!

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