3 phase to single phase

[monkeymark]

I might be getting a compressor donated to me but it's a 3 phase.

I know they can be made to run on single phase via a cap. Problem is this is plated for 400v so could prove a little less effective as it would run slower.
Cant really afford a single phase motor at the moment.

Does anyone have links to any appropriate sites for a little reading up please?

 

[rikrobson]

running with a cap to make the 3rd phase is not any good for alot of power. its fine on a small suds pump for example.

You could wither swap the motor for a single phace one or get a VFD. Youll need one that bigger than the motor rating so if its a 3HP motor then you'll want a 3.5KW VFD. Also they tend to be 220V not 415V. The motor should be able to be converted to Star as opposed to delta, which will correct that problem.

You'll need to use the pressure switch to contol the motor, but you'll be able to do a soft start on the compressor.

 

[Luke]

It's the other way round, a 400V motor that has the windings connected in star can be reconnected in delta for 230V, as long as all 6 ends are brought out to the terminal box.

 

[monkeymark]

Thanks for the replies.
It's only plated 400v rather than dual voltage so not sure if it can also run 240.
I'll check the wiring to see what's in the connector box.

 

[monkeymark]

Just checked and all 6 come into the connection box currently wired in star. Should be an easy swap when i find a suitable capacitor.

 

[Chris Stevens]

I doubt very much a capacitor would run a big 3 phase motor, but if it does expect power to be reduced by maybe 50%....................

 

[monkeymark]

It's only 3hp so not that big. If it runs it should be about 70% so good enough until i get another motor or vfd.

 

[WorkshopChris]

I may be wrong but, i think you are going to have to fit a solenoid and timer to run the pump off load until it gets to speed. You might also have to lower the pressure differential setting on the pressure switch as a 3 phase motor just running with a capacitor and 230v supply will not run at full load.

 

[normspanners]

IMO This isnt going to work, sorry
no way will you have enough torque to start it, even if you fit timers and and valves

 

[normspanners]

It's only 3hp so not that big. If it runs it should be about 70% so good enough until i get another motor or vfd.

BUT IT NEEDS THE 3hp it was designed with to start it's self and 3ph has more start torque than the equivalent 1ph anyway ,----- 3hp motor on single phase at start up you will be lucky to achieve 1/6 of that ie .5hp.
My mate was trying to get a 3ph snail fan to start on single phase and we couldnt without compromising the motor, a compressor is easily 3x the initial load of that fan.

 

[brightspark]

if it was that easy and good no one would be buying converters and some of these aren't 100 percent

 

[Luke]

Agree with all of the above. Re-wiring in delta means that a suitably sized VFD should work, with the correct setting up. Hanging the third phase from a cap probably won't.

 

[normspanners]

Fans are bad enough to start, A compressor pump is ultimate hell, (bar for being totally seized) for a motor even on star delta because you have to shorten the star time to get the delta in asap to get the torque required to get to sync speed, because for every revolution the pumping load is increasing up and up.

 

[monkeymark]

Hmmm, back to the drawing board then.

 

[MattH]

HughF is selling a nice little rotary converter ;-).

 

[hareng]

Shame your so far away got a 1ph 3hp motor off a compressor and a Transwave rotary doing nowt for last 5 1/2 years.

 

[Chris Stevens]

Hmmm, back to the drawing board then.

If compressor has 3hp 3 phase motor, I think your only option is going to be a converter of some sort, which in terms of cost is probably going to be comparable to buying a good used 240v compressor...........

 

[brightspark]

about 100 quid for a 3hp single phase motor

 

[normspanners]

To give you an idea my mate wanted a bigger compressor for blasting think it was 8-10 hp and had two static phase converters that worked perfectly for every thing he had in worksdhop, BIG lathe, grinders, milling machine, welder etc. The only way to get the comp to start was to have the lathe running flat out, and then the big converter would JUST start the compressor empty, plus then we had to rewire the workshop, and also the supply and put in a cable 5x bigger, he had one of his converters adjusted by the manuf and it now starts the comp without the lathe as backup/ flywheel effect.

 

[dannyp]

with compressors i've found you take a 3hp 3hp motro off, fit a 3hp 1ph motor and you may get problems. some work fine some just stall.

usally they'll pump up from empty fine, the real test is when it cuts back in with some air in the tank