3 phase power question

Things at the limits.

3 phase power question

Postby John Hill » Mon Dec 03, 2012 11:33 pm

3 phase power is something (one of many really) that I have difficulty grasping.

Suppose one has a 3 phase delta connected electric motor with a need to run it from single phase, this is something that is often done when building a rotary phase converter to create 3 phase power from a single phase supply. Single phase, one neutral and one phase line from the mains supply.

There are different ways of getting the 3 phase motor up to synchronous speed. One way is a single phase pony motor, another is a pull rope to spin the motor manually and another common way is to create a phase shift by adding a capacitor in series with a connection to the third motor terminal. The capacitor is disconnected as soon as the motor is up to speed.

Now, as far as I know the desired phase angle to the third motor terminal is 120 degrees and the maximum phase angle to be had from a capacitor (in theory) is 90 degrees which I assume is always considerably less in practice.

So, I am thinking that if I used a 1:1 transformer (i.e. old isolating transformer) across the line I would have full line voltage at 180 degrees phase difference and if that was fed to the third terminal via a capacitor the resultant total phase shift would be 180 degrees minus something less than 90 degrees. Hopefully something close to 120 degrees phase angle.

My theory is that the motor would be self starting and all three coils of the motor would be directly energised from the mains. Furthermore it would appear that the rotary phase converter idler motor would not be required at all.

Any comments please?

john
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Re: 3 phase power question

Postby chrismb » Tue Dec 04, 2012 3:42 am

John,

Is this a question 'in theory' so as to discuss 3 phase power, or do you practically want to run 3 ph from a 1 ph supply?

If the latter, then just save yourself the trouble of making a song-and-dance out of it and buy yourself a 3 ph inverter. They're really not that expensive, and will probably work out cheaper than passives even if you can create some amazing passive circuit to accomplish the same.

In regards whether you can run a 3ph motor directly off of 1ph with some passive components in the loop, I guess it must be possible but only for a given set of components/motor and only at a give speed and load, and it'll also be inefficient. So it's no permanent solution. Just use an inverter!
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Re: 3 phase power question

Postby solar_dave » Tue Dec 04, 2012 8:10 am

There is also the possibility of using a VFD with single phase input and 3 phase output for a solid state solution.
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Re: 3 phase power question

Postby Jerry » Fri Dec 07, 2012 12:40 am

Are you thinking along the lines of a Scott-T transformer?

-Jerry
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Re: 3 phase power question

Postby solar_dave » Fri Dec 07, 2012 9:01 pm

Dave Shiels

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Re: 3 phase power question

Postby Jerry » Sat Dec 08, 2012 6:53 am

I was replying to John. I know for sure John knows what VFDs are.

John is merely speculating on other options, I believe.

But I dont think his idea would work, I think you would be creating something closer to three wire two phase power (Real two phase, not a center tapped transformer)

But I could be wrong. The simplest thing is get a couple small transformers, say 24vac out, and build your circuit. Even at 24v on a very small three phase motor, you should be able to see your results. A scope will come in handy if you have one.
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Re: 3 phase power question

Postby chrismb » Sat Dec 08, 2012 7:58 am

As a 'cheap mod' on a scott t, I guess maybe you could do it with a centre tapped transformer and a passive - two of the phases across the centre-tapped side of the transformer, with the third phase connected to the centre tap via a suitable inductance?

Would that work? And would it then work backwards to generate 3 ph from 1 ph?
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Re: 3 phase power question

Postby johnf » Sat Dec 08, 2012 1:00 pm

John
These products already exist for 1 to 3 phase generation.
All the ones I have seen use an idler motor to do the rotary single to 3 phase conversion and maybe a 3 phase autotransformer to get 440v
for 3kw out you would want a 5 kw idler motor.
the idler motor has capacitors to make up the phase shift on the other windings and a start capacitor to get it moving
its interesting that the idler motor should not be too efficient
do a web search for "phase convertors" "single phase to 3 phase"
or a handy how to do it here http://www.metalwebnews.com/howto/ph-conv/ph-conv.html
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Re: 3 phase power question

Postby solar_dave » Sat Dec 08, 2012 3:04 pm

johnf wrote:John
These products already exist for 1 to 3 phase generation.
All the ones I have seen use an idler motor to do the rotary single to 3 phase conversion and maybe a 3 phase autotransformer to get 440v
for 3kw out you would want a 5 kw idler motor.
the idler motor has capacitors to make up the phase shift on the other windings and a start capacitor to get it moving
its interesting that the idler motor should not be too efficient
do a web search for "phase convertors" "single phase to 3 phase"
or a handy how to do it here http://www.metalwebnews.com/howto/ph-conv/ph-conv.html


I have one to run my Boyar Schultz surface grinder. The grinder was super cheap as the 3 phase power scared most off plus it did need a good electrician to fix some of the wiring. I just had him do the whole wiring it all up.
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Re: 3 phase power question

Postby John Hill » Sun Dec 09, 2012 4:04 am

Thanks for the comments everyone...
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