Running a single phase motor from a 3 phase supply

[DIYedboy]

I don't know whether this is possible (the 3 phase supply doesn't have a neutral), we would like to replace our old air compressor at work with another one we've got spare, however, the spare is single phase belt driven piston compressor and the three phase one is a hydrovane so we can't swap just the motors over. Is there such a thing as an inverter that works the other way round?

 

[Robin0577]

In theory you could use a transformer, but it would probably be cheaper and simpler to change either the supply to single phase or the motor to 3 phase.

 

[Iggifer]

You would need the neutral for a single phase motor.

It would be easy enough for an electrician to convert the supply from 3P to SP+N

 

[Echo the husky]

It would be easy enough for an electrician to convert the supply from 3P to SP+N

It depends on the rating of the old and new motors, a single phase motor of the same HP will draw far more current (and will be far less efficient) than the 3phase version.

 

[Iggifer]

This is very true. Best bet is still to get someone to come look at it

 

[DIYedboy]

I intend to replace the single phase motor on the compressor with a three phase one that we will purchase...........BUT.................being that I like to learn things, I just wondered IF there is an 'inverter converter' that could do a similar job instead of having to swap the motor.

 

[Robin0577]

As I said earlier, it is technically possible to derive a single phase 230v supply using a 400>230v transformer connected across 2 phases of a 3 phase supply. However in practice, there are several reasons why this is not normally done for anything bigger than supplies of a few watts to control panels, including but not limited to, unbalancing the load on the 3 phase supply, size, weight & cost issues, the output being floating with respect to earth, etc.

 

[Brigade77]

Pointless question if you ask me.

Whatever you do with the 3phase . . . it's at work, so, will need a qualified person to tinker with it.

Myself. I'd just plug the replacement compressor into one of your single phase sockets.

 

[DIYedboy]

Dork.................good name, no-one did!

 

[ericmark]

There is no reason why one could not have an inverter to step down the voltage that is the whole idea of the switch mode power supply. However because of the loads from a motor in the main they would be designed to connect direct to the motor rather than a stand alone unit. To design an inverter to run a single phase motor would mean dedicating it to a single motor as single phase motors don't have a single start method or control so if you need to pair an inverter to a motor the best option is to start with a three phase motor. So it is unlikely anyone would want to design a unit which would cost more to produce than the cost of a motor and inverter.

So no technical reason when it should not be made, but there is no market for the product so it will not happen.

 

[Spark123]

An inverter is something that creates AC from a DC supply. Way over the top for what you are doing.
All you need is a competent electrician to look at it an come up with a solution, presumably there is a cable going from a distribution board to the 3 phase compressor?
Is there a neutral in the board?
If so then it is a piece of cake for an electrician to make the supply single phase.

 

[DIYedboy]

It is my understanding that you could also get inverters that convert single phase to three phase, in order to run 3 phase motors from single phase.
Life sadly is never straightforward, I am a fully qualified sparks.............BUT, the supplies in our firm are controlled by a facilities provider, running that elusive neutral will involve considerable cost and at least a 2 month wait, much cheaper to find an alternative solution.

 

[Echo the husky]

Usually you would just swap one of the now unused phase wires to the neutral bar at the DB re-identifying it as a neutral with sleeves or whatever, and replace the 3 pole MCB with a single pole if it was going to be a permanent alteration. No need to run a separate cable. This of course assumes the existing cable is big enough to run the single phase motor.

Single phase to 3 phase inverters are very common as people sometimes need to run 3 phase equipment on supplies where only a single phase is present. Where a 3 phase supply is present it will always be possible to run single phase equipment as a neutral will be available at the supply.
(Yes I'm sure there is going to be the odd installation where the above is not true but that would be very rare indeed.)

 

[DIYedboy]

Good call Echo, as you say, the cables must be capable of carrying the extra current, BUT, sadly all wiring and DB is out of bounds!...............what a shame the commando wasn't a 5 pin, heyho

 

[Spark123]

Along the same lines as Robin said before, we have used 415v-240v transformers for 240v equipment aboard cranes where there is no neutral. I don't have a clue on cost. The floating issue can be easily solved by earthing on side of the secondary.