# 3 phase to single ?



## Scoobycarl (Aug 13, 2011)

Any sparkys available to answer a question please ? 
Can a 3 phase compressor motor be converted to single phase ? Or would another motor be needed ? We need a new compressor at work but the old one works fine but isnt keeping up with our demands,it's a 3 phase but we would like to keep it to run da's and sanders but we only have one 3 phase supply? Therefore the new compressor needs that supply but if we could make the old compressor single phase it would mean the new one would be worked less and provide better air quality. Thanks


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## Aaran (Sep 18, 2007)

you can get converter type boxes that will convert a single phase into a 3 phase but any half decent ones are not cheap, doing it this way uses more power and its limited to how many hp the 3 phase motor is

im not sure how your three phase is wired in but you need to go look inside your breaker box.

if your on 3 phase you should have in that box at the bottom a 3 pole breaker (the 3 3phase wires) and then off the bus bar all the breakers for all the single and three phase items (compressor/fans/lights etc)

all my single phase runs out of my 3 phase box as its 3 feeds of 240v. my lights use a single breaker off one of the bus bars and all my 3 phase stuff uses 3 breakers.

if its done the same way as mine all you will need is a breaker with the correct amps rating needed and 3 spare slots in the bus bar and from there 3 phase conduit to go to the new compressor.

really depends how your units set up


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## Scoobycarl (Aug 13, 2011)

Thanks for that mate real good avice ! I will have a look tomorrow but the building isnt new so it's probably not the best ellectrics lol


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## ctownshend (Sep 25, 2006)

Agreed, all you need to do is keep the existing compressor three-phase, and run in a new breaker for the new compressor if your DB (distribution box) allows it. The phase inverters are a very expensive way of getting 3 phase from single phase. And imo not worth the money if you have a 3 phase supply already available.


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## Scoobycarl (Aug 13, 2011)

Im useless when it comes to ellectrics so dont have a clue but was told we just have one three phase supply so are you guys saying depending how its wired we can run two lots from it ? Thanks


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## Aaran (Sep 18, 2007)

we have one three phase supply into our workshop from the road.

it runs just about everything. (all 3 workshops off that one feed) the limitation will be the number of cores on that main cable and its max AMP limit. (determined by the number of cores and the thickness of each core)

just find your breaker box, open the door and take a pic.

the only thing you might run into an issue is fitting a new breaker into an old board. they do change the styles every so often so newer one often fit poorly onto semi old boards, ancient ones sometimes wont take new ones at all.

also check to see what size 3 phase cable is running into your workshop! no point running 2 compressors if you only have a cable that is rated for 30amps and both compressors are running 15amphour draw (will cause no end of problems) there will be a plate on the motor of the compressor telling you how much amphours (am/h) it uses.

also note if its a big motor (like over 5hp) its likely to draw alot more current when it starts up due to the amount of current needed to get it rolling as it were.(mine 10hp compressor is a 6 pole unit and its set to delta to get it rolling, 3 poles come live to start it moving and then the other 3 belt in to keep it at full chat a few seconds later, saves on power usage and does not overload the motor or electrics etc)


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## Scoobycarl (Aug 13, 2011)

Thanks Aaran i will have a look tomorrow mate cheers


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## jdquinn (May 19, 2010)

There are a couple of options. Depending on where you plan to relocate the old compressor depends on what option is best. As with everything electrical you shouldn't be working at it unless you know what you are doing and with most things you will require some sort of formal training certificate to be able to carry out the works yourself. With that little disclaimer see the options below:

Option 1) This may not work on a compressor due to lack of starting torque. Some 3 phase motors can be wired in such a way with capacitors to make it run on a standard 230V supply at somewhere very close to the marked rpm on the motor. I believe the motor must also be marked as a 230/400V and not a 400/690V motor but I could be wrong on this one. It is a bit of trial and error and a bit of a bodge but does work if done properly. Your company insurance may not like it though!

Option 2) Add an additional 3 phase power point to plug into. It shouldn't be too costly depending where you want it and how far that is from your distribution board. This is probably the most cost effective option.

Option 3) If your existing compressor motor is below 5.5kw you can get a 230V single phase input to 230V 3 phase output frequency inverter. This will cost, as a unit on its own, approximately £400 and will also require that the existing motor is a 230/400V and not a 400/690V unit. 

If you plan to use any of the 230V options you will also need to bear in mind that a 4kw 400V motor will require something around a 20A or 32A single phase supply which means your standard 13A socket outlet won't be up to the job. For a standard 13A socket outlet the maximum motor kw is probably around 2.2kw. 

Hope this helps.

Jonny


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## Clifton94 (Feb 2, 2015)

No you cannot change a three phase motor to single phase you will have to change the motor as a whole unit


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## Scoobycarl (Aug 13, 2011)

My boss isnt up for it to be fair i showed him this and i dont think he can be asked ! Not overly surprised really as its the same with everything else in the shop ! 
Thank You for your ideas guys and its there if he does want to do it


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## Sh1ner (May 19, 2012)

Clifton94 said:


> No you cannot change a three phase motor to single phase you will have to change the motor as a whole unit


You can if it has been designed to run on either supply. My ramps and hot pressure washer will run on either when suitably wired.
It should say on the motor plate how it can be used. A 3.2kw motor will require a 16 amp supply.
As mentioned above it needs to be marked 230v/1ph/400v/3ph or such like. They usually have 6 terminals in the stator connection box along with a wiring diagram, under the lid, to show single or three phase operation.
You might need to have a 16A or 32A single phase socket fitted to cope with any extra current but it will depend on the requirements of the motor.
As you can see, there are variables so it is best to get an electrician in to advise.
You don't say the size and capacity of the current set up but DAs and air tools etc use considerable amounts of air. It sounds to me like you need a 3phase unit with a dryer and maybe 500L capacity to keep up with demand and not have the compressor running all the time.
That should give you better air quality for everything and perhaps you can take the other one home.


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## jdquinn (May 19, 2010)

Sh1ner said:


> You can if it has been designed to run on either supply. My ramps and hot pressure washer will run on either when suitably wired.
> It should say on the motor plate how it can be used
> As mentioned above it needs to be marked 230/400v or such like. They usually have 6 terminals in the stator connection box along with a wiring diagram, under the lid, to show single or three phase operation.


Just as a bit of information, when 230/400V is marked on the side of a motor with 6 terminals inside it, this normally means 230V 3 phase or 400V 3 phase. Single phase 230V is not the same as 230V 3 phase and will not run a standard 230/400V 3 phase motor unless you go out of your way to connect in a couple of capacitors. The only way you can run a 3 phase 230V/400V motor on a single phase supply without modification to the motor is to install an inline single phase to three phase inverter (VSD).

Most people think these inverters output 400V because they convert the single phase 230V input to 3 phase output which is incorrect. Infact, they only output 230V 3 phase which is why the star or delta configuration of the links inside the motor have to be changed to suit the 230V marking on the side of the motor.

A rotary or digital power converter however will output approximately 400V 3 phase when supplied with a 230V single phase but there are limitations to their use.


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