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middle age spark

3 phase issue

Question

middle age spark

Hi all,

I had 2 phases down today due to a burnt up contact connections buried in the ground feeding water pressure pumps ,this was about 25 mtrs away from the distribution board so it was a major problem to rewire quickly.

 

The site agent said can you link the faulty two phases out with the good one to get these pumps running again &for a moment I thought he was joking but then I couldn't find how to answer him to explain why not apart from it won't work due to phase rotation & 2 phases make 415v so 1 phase is only 230v no matter what you link it too.

Can any of you lads enlighten me what to say to him apart from don't be a pick. 

Thanks (make me sound good boys)

 

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Andy™

2 phases have no rotation

 

a running motor may continue to run (with sid eeffects) if it loses a phase but it will not start and burn out (thermal overloads should stop this happening)

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Evans Electric

Theres no clever answer  as far as I know , you need  all three phases  to make a 3 ph motor work 

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Sidewinder

You can create "false" phases with capacitors, but you need a box full to get the right one for the load, and the two will be different to allow the correct phase delay (read create the rotation), and this is pretty much load dependent, so the caps would need changing for various loads, and it's a trial and error job to find which ones are the right ones, hence you needing a bucket full of them.

You might be able to swap the motors from delta to star and run them on a 1ph to 3ph VSD, but that would need major panel rework.

If you really need 3ph to run the pumps quick a generator may be your best bet.

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Tony S

Have a look here for the steinmetz-system for running 3Ø motors off single phase.

 

 

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Rob.
13 hours ago, Tony S said:

Have a look here for the steinmetz-system for running 3Ø motors off single phase.

 

 

 

Was it on here, or the other side we did a piece on steinmetz systems? After I ended up playing with one in the workshop. 

 

Edit: Just noticed you had posted a link to a piece. 

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Tony S

If you click on the link it will take you straight to it.

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apprentice87

I built one once!! It worker amazingly well to be honest!! Voltage between phases was correct within a few volts and motors ran beautifully. I had a 7.5 hp motor as an idler..

 

The motors i wanted to run were not dual voltage ones either. So where did i get the right voltage?? Ha, what i did, is i got a 110v power tool transformer, and i reconnected it so as to put the secondary in series with the primary so it would act as an autotransformer. This meant that instead of dropping the mains 230 down to 110, it added the 110 on top to give me 340v. A bit low, and i wondered if contactors would hold in, but they did and it worked great..

 

Took me weeks of sitting on the bog to "invent" a means of getting a suitable voltage!!! i did try running the tool transformer backwards in an attempt to get 460V, btu as soon as i turned it one the fuse blew. No idea why though, as i know that in theory at least a transformer will run backwards after a fashion..

 

john..

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Evans Electric

Just adding , for no particular reason  TBH,    If I remember correctly  our much loved standard 3 ph   DOL  motor  is a very efficient machine ...perhaps due to it's simplicity. ?

Thinking back  at looking at balancing phases  ...with a machine shop full of lathes with 3 Ph motors , all looks good until you  start turning on the SP loads .   

    

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