Turning two phase mains into three phase

[Squaredy]

5 minutes ago, openspaceman said:

So you have 380-440V between then and 230-250V between each and earth?

Yes, exactly.

[Paddy1000111]

7 minutes ago, openspaceman said:

So you have 380-440V between then and 230-250V between each and earth?

 

Theoretically you could stick a bigger 3ph motor wiring 2 of it's terminals with the incoming  phases and each coil to earth and then once that is spinning draw 3ph of each terminal as the rotating squirrel cage will induce the third phase.

 

Mind it is 50 years since I studied electrical engineering.

If you run a 3 phase motor on two phases I'm pretty sure you will end up toasting coils with that kw draw. Also have the issue of an imbalanced load? 3ph motors won't start on two phases either and just hum then set on fire so you would need a single phase starter motor? 

 

I think you can use transformers to create 3 phase from two phase but you need 90 degree phase rotation on the first two to get 120 degree on the outward 3? 

[Squaredy]

10 minutes ago, Paddy1000111 said:

You could get away with it on a rotary single phase converter but the 80A rotary converters can only handle single motors up to 11kw. You would need a 100A supply to give you a 15kw single motor rating. Maybe that's something that your power company can do a bit easier?

It is still an option I believe to use a rotary converter - I could drive a 30HP one using the two phases which has a maximum single motor size of 15kw.  This option though is also not true three phase but a bit of a fudge, so needs careful research to make sure it is compatible with the new mill.

[openspaceman]

25 minutes ago, Paddy1000111 said:

If you run a 3 phase motor on two phases I'm pretty sure you will end up toasting coils with that kw draw.

It's kW but as long as the motor rotates with little slip and no load very little current will be drawn until you impose the load of your saw motor onto it. Not having 3ph available to play with I am not sure but the second phase is 120 degrees out of phase with the other so a quasi rotating field exists, in much the same way a single phase induction motor uses a capacitor to create an out of phase lead coil until the rotating squirrel cages induces it's own or a copper coil creates a lag in a shaded pole motor.

25 minutes ago, Paddy1000111 said:

Also have the issue of an imbalanced load? 3ph motors won't start on two phases either and just hum then set on fire so you would need a single phase starter motor? 

See above. As for imbalanced load  I'm not at all sure but  15kW implies 62A  at a power factor of one so yes there could be a problem at low power factors

25 minutes ago, Paddy1000111 said:

 

I think you can use transformers to create 3 phase from two phase but you need 90 degree phase rotation on the first two to get 120 degree on the outward 3? 

You've lost me there, I can see using a transformer on one phase can create a 90 degree lag which would create a non symmetric rotating field with the other two but two transformers? and my maths doesn't do the sums of sines any more and it's too late for me to go back to A level maths again.

[Paddy1000111]

7 minutes ago, openspaceman said:

It's kW

I'll be more careful with my shift button use 😉

 

7 minutes ago, openspaceman said:

As for imbalanced load  I'm not at all sure but  15kW implies 62A  at a power factor of one so yes there could be a problem at low power factors

Assuming we used a single phase only at 230V then 15kW is 76.72A as induction motors have a power factor of usually ~.85 so that's pretty darn close to the limit here.  Not sure what 15kW would be split across 2 phases when those phases are also making a third phase though. That calculation is beyond me.... 

 

19 minutes ago, openspaceman said:

You've lost me there, I can see using a transformer on one phase can create a 90 degree lag which would create a non symmetric rotating field with the other two but two transformers? and my maths doesn't do the sums of sines any more and it's too late for me to go back to A level maths again.

Sorry, I meant one transformer not two! I can't remember what the transformer is called now but it was invented a long time ago for creating 2 phase power for some strange motor. It took normal 3ph and turned it into 2ph but at 90 degrees, it could also take the 2ph and turn it back to three phase. Like you say though, it was a long time ago I did electrics for my degree 😂

 

Just spent 10 minutes googling transformers and found what I was on about here:

Scott-T Transformer Connection Overview | EEP

ELECTRICAL-ENGINEERING-PORTAL.COM

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Main application of Scott-T Transformer is for for Industrial Furnace and for for traction purpose. It is is type of circuit...

 

[openspaceman]

8 hours ago, Paddy1000111 said:

 

Assuming we used a single phase only at 230V then 15kW is 76.72A as induction motors have a power factor of usually ~.85 so that's pretty darn close to the limit here.  Not sure what 15kW would be split across 2 phases when those phases are also making a third phase though. That calculation is beyond me.... 

It has to increase but the worst thing is that if the site does not have a neutral return back to the point the three phases separated  all that current travels back by earth. This is because when you put a balanced load across a 3ph supply the sum of the currents is always zero, so no separate neutral required, all the current runs in the three phase wires. If one phase is missing something has to be provided to return the current when both of the available phases are the same instantaneous polarity.

8 hours ago, Paddy1000111 said:

 

 Like you say though, it was a long time ago I did electrics for my degree 😂

 

I never got that far which is why I chopped trees down.

[Paddy1000111]

1 hour ago, openspaceman said:

I never got that far which is why I chopped trees down.

I never really enjoyed the job that I studied for, that's why I'm trying to move into chopping trees down! You chose to skip the rubbish bit of finding out you don't enjoy the work 😂

[openspaceman]

53 minutes ago, Paddy1000111 said:

I never really enjoyed the job that I studied for, that's why I'm trying to move into chopping trees down! You chose to skip the rubbish bit of finding out you don't enjoy the work 😂

Trouble is now I'm finding it all too easy to fall into being a couch potato and online warrior and this makes me wonder if I would have kept as active as I did if I had taken a desk job early on.

[Paddy1000111]

1 hour ago, openspaceman said:

Trouble is now I'm finding it all too easy to fall into being a couch potato and online warrior and this makes me wonder if I would have kept as active as I did if I had taken a desk job early on.

I think everyone is in the same boat, trapped indoors with no real outlet. You can only walk around your "local area" so many times... 

I've never done a desk job, I couldn't hack it. I have been effectively without work for about 2 weeks now and it's driving me insane. I started working stacking bricks for a local builder when I was 12 and then worked whenever I could whilst I was at college/university. I don't think I could have lived a life of 9-5 sat behind a desk firing out e-mails and only living for the annual foreign holiday 😂

[topchippyles]

Gave just do as gobby said and phone Pete at Transwave Converters which is all these guys do.They will make a bespoke unit up to run on via your 2 phase supply to maximise 3 phase output.My rotary converter came from there.10 minute phone call will answer your question.

Edited January 18, 2021 by topchippyles