240v to 3 phase conversion for domestic workshop

[NickCurtis62]

Looking for both application and product advice for converting 240v to 3 phase for my home workshop.
Will be running both a screw compressor and blast cabinet (hopefully) from my domestic 240v supply.
Have no information on what is required at present so looking forward for some sound words of wisdom (experience) on this.
Many thanks in anticipation.

 

[Nozzle]

Unfortunately, that's not really how it works. Domestic 240 is the 'same' as Industrial 415 3-phase, but only 1/3 of it and a neutral derived from elsewhere... what that means is you can never make the "smaller" supply adapt to fit the bigger supply - you have to start with the bigger supply. It's also teribbly useful having a supply with 3 phases as it means motors can be simpler and more powerful for the same physical size. The issue you have though, is that plant needs POWER and it's the sort of power that a domestic supply can't provide.

However, you can run a 3HP compressor off a domestic supply, though it wont be screw type I doubt. Screw compressors are big money, and such plant wouldn't be fitted, from the factory, with single phase electrics.

You'll also get people on here complaining to you about the agreement you have with the DNO for using high power gear.

Nozzle

 

[Jackrae]

Converting 230v single phase to 3phase at either 230v or 400v is readily accomplished using an inverter unit. The limitation you will come up against is the power capability of your domestic system without causing major problems for both yourself and your supplier. Generally around 3HP will be your limit on motor size.
The big advantage of the inverter is that you can also have variable speed generally in the range from say 25% of line frequency to twice line frequency. For a 4pole motor that equates roughly from 370 to 3000RPM. You also have 'soft-start' and controlled acceleration rates.

To get sensible answers you need to advise on your anticipated motor ratings.

 

[RF Lighting]

Depending on how power hungry your appliances are, a motor generator may be an option.

 

[flameport]

screw compressor and blast cabinet

Unless it's a tiny thing for occasional hobby use, it's highly unlikely you can run it from a single phase supply even with a converter.

 

[ban-all-sheds]

You'll also get people on here complaining to you about the agreement you have with the DNO for using high power gear.

No he won't - nobody here will complain about that.

Any complaints would come from his neighbours about him messing up their supply and thence his DNO about him contravening the agreement he has with them. People here might warn him of that, but that would not be a complaint about the agreement.

 

[ericmark]

There are a number of ways to convert single phase to three phase, including motors with capacitors fitted and large flywheels.

There is nothing to stop you having a single phase motor driving a generator, what is shown is actually part of a ward leonard system, but well before static inverters we had rotary coverters, the advantage of the rotary type is it has mass, so when you connect a large load the lights don't dip to same extent.

However the big question is one large inverter or many small, using a static inverter gives one some extra options, a small 230 volt delta / 400 volt star motor in delta connected directly to an inverter allows a large amount of speed control and reduces the inrush.

So take a compressor, there are many ways to control a compressor, which include reducing motor speed as pressure is reached mainly done with refrigeration compressors, but no reason it can't be applied to an air compressor.

The problem is cost, go and buy 100 x 6mm bolts they are cheap, try to buy 100 x 1/4" whitworth and likely will cost a fortune as not many made now. It is the same with motor drives, if it is the same size as used in factories for their conveyors it is likely cheap, but if not a common size can cost arm and leg.

This is why some times cheaper to buy a 3 phase generator. Come to do same job in 6 months times and all prices have changed.

 

[JohnW2]

... the advantage of the rotary type is it has mass, so when you connect a large load the lights don't dip to same extent.

True, but I suspect that they might well 'dip' when one first switches on the motor.

Kind Regards, John

 

[Jackrae]

And then there's the inefficient transfer of 1Ph electricity to motor motion followed by the motor motion to 3Ph electricity generation followed by transfer to motor motion.

 

[Nozzle]

Indeed, you're going to have a lot of heat and some noise here, I recon 75% efficient at best. So for every 1kW of 415V 3-phase out, you're going to need 1.33kW of 230V single-phase in. Therefore to run even the smallest Machine Mart screw compressor (5.5HP) you need 5.6kW supply - with a starting power up at 17kW. That's a dedicated supply from your consumer unit, you can't plug it in to the ring final.

 

[ericmark]

If you use an inverter direct on the motor then the start current can be much reduced, however talking about a compressor, and main point with a compressor there is some form of de-load at start, there is nothing in theory to stop you using an inverter, but in practice getting an inverter which has the outputs required to operate the de-load valve is not going to be easy.

I have worked with screw compressors but these tend to be on the large side, clearly too big for home use. However the internal control was also complex, it would de-load as it reached maximum pressure and continue to run for set time, before switching off, but this was many years ago before the economy drive, I would guess today with inverter drives the compressor speed is matched to demand and only after being at minimum speed for set time would it switch off, this is how the compressors work with freezers, the motor never switched off, it just changes speed. So likely with inverter control the compressor will not need a three phase supply, however it may need a 400 volt supply.

With the old reciprocating compressor you have the same problem, it will have de-load valves, it is common for the de-load valve to be connected to an extra contact on the star contactor, using an inverter again the problem is getting an output to work the de-load valves. What I found was you could read the spec on an inverter, and it would say it had the outputs, but when I came to use them, the outputs were not in sinc with the motor, I ended up using SWF motors with the inverter and ASii control built in, it was only way to get it all to work together, not a compressor but same problem.

So although in theory possible to run three phase machines from a single phase supply, in practice best option is to buy single phase stuff to start with. Cheap inverters start at around £200 but this can rise to £750 looking at 5.5 HP even more if it needs to also convert the voltage to 400 volt.

 

[ban-all-sheds]

looking forward for some sound words of wisdom (experience) on this.

I can do the former.

Reading all the above I think it would be worth finding out what your supplier would charge to put in a 3-phase supply for your workshop.

 

[Jackrae]

But much better if you simply tell us the HP of your compressor so's you can get some reasonable opinions

 

[SimonH2]

Oh, so many questions unanswered about the OPs needs. As well as the above regarding load sizes ... Does the 3P need to be 4 wire (ie with a neutral), of will 3 wire suffice ? Do the 3 phases need to be balanced (voltage wise) around the neutral ?
Provided a 3wire system is OK, then the simplest way of getting your 415V 3P supply is to use something like a Transwave Converter. I have one of their static converters which I "acquired" when it was being thrown out at work some years ago. It's basically an auto-transformer to step up the 240V to 415V, together with a pile of large capacitors to generate a 3rd phase. There's a switch on the front to select the amount of capacitance (to suit different loads) and a voltage sensitive relay that switches in some more for starting the connected motor. It works very well.
They also do a rotary converter, which I believe is basically the same arrangement but using the windings of a 3 phase machine instead of a big transformer - ideally this needs a specially wound motor with one 415V winding tapped to inject 240V into it (dunno if this is what they have, their documents are vague on the internals). The advantage of a rotary converter is that the motor acts as a rotary transformer and evens up the balance of the phases - and indeed, you can connect a large 3 phase motor across the output to provide additional balancing.

A while ago we were experimenting at home using some of dad's "useful bits" he'd collected over the years. We'd got as far as a 5kVA transformer, an old manual star-delta starter, some capacitors, and a large (maybe 3-4 hp) 3 pahse with an old Ford Escort flywheel attached. It was working fine, but the transformer needed some adaptations - it needed some turns taking off the secondary as it wasn't the right turns ratio. Plan was to have a setup where running anything 3 phase would mean "switch on the power to the transformer, spin up the motor in star, switch to delta" which would then provide 3P to a separate distribution board.
Never went any further as my mate got divorced and lost the garage it was to go in, and the space where we'd got half way in assembling the 4t 4 poster lift it was to power, and last year most of the bits were in the loads I took to the scrapyard as "stuff I just couldn't keep due to lack of space" with dad gone and mum downsizing.

BTW - you may want to have a read of Transwave's Layman's Guide.