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3-Phase Power? Answered

I already asked a question about three-phase power but my results only confirmed what I thought.  I don't understand how a MOTOR can run at 230v with three phases,  how do they rate power with these I guess.  230v outlets have three prongs, I think two are connected to the generator through power transmission lines, and one is a ground.  If this is wrong were do they get 230v because, if I am wrong all three wires connect to generator coils.  This would add up to 345v.  But to get 230v it would only be two phases.  Then it can also run on 480v, how in the world do you get to 480v.  Please help me with this.



Best Answer 7 years ago

You need to read up on the theory to understand the answers below. I don't know where you are in the world, but we have different voltages here in Australia, so I'll try to use those to explain.

Our single phase is 240v. Our three phase supplies 415v. The power supplied by a normal power outlet is 240v. That is one phase (live) and neutral (return). The third prong is earth (ground).

If something requires 415v to operate it will be connected between two phases (live). As frollard explains, the total of the two phases isn't 480v (240+240) because they peak at different times.

Motors that use a three phase supply (415) still only use two phases per coil (one on each end) but use different pairs of phases in sequence for each winding. That is why the rating is still 415v not 720v (240+240+240).

This explanation is very simplified and not entirely complete, or even that well presented. But as I said, if you can't understand this you need to read up on some electrical basics.

I understand this now, I just originally thought that you add them up, but apparently there is more to it. I think I finally get how this works. So this motor running 240v at three phase is running off probably three live wires hooked up bla bla, and at 480v it is just running it differently. It makes sense to me and I finally get this. After so long one person finally figures out what I wan't to know, not what I know.

The answers given below are suitable for someone who has a little bit of idea of sinusoidal curves.

In layman terms, please understand that there is nothing such as 2 phase power supply, as +/- 180 degrees for a motor is not advisable due to technical issues, although it could be possible for certain situations which I have not come across till date.

Further, the 230V value that you see is the RMS value, as well as 400V value is also the RMS of 230V. If you would check this on a calculator you would get

Sq. root of (230*230+230*230+230*230) = 230* Sq. root of (3) = approx. 400V.

Further, in order to get 480 V, which is what you want, you will have to use a transformer with 2:1 winding. The V*A will remain same in the primary and secondary windings. i.e. if you want to double the voltage in the output, then if there will be negligible loss in the transformer then the current would be halved.

It will remain in phase with the single phase input that you would use.

A motor _designed_ to run on three phases runs on three phases. Different arrangement of coils.

But HOW, in my understanding there are two hot wires at 230v That is two phase, I get the whole coil arrangement thing, and could have told you that, but how does it work at 230v and 480v, please answer my actual question

I think the point you're tripping over is that these aren't all provided via the same set of wires. If you have only two hot wires, you don't have three-phase.

American household 230VAC supply (or 220V or 240V, depending on what your local standard is) is two wires from a single transformer. That transformer also has a center tap which goes to earth. By taking power between just one of these wires and earth, you get the typical American 110/115/120VAC circuit; appliances which need the higher voltage are connected between these two power lines.

Three-phase requires three hot lines (not two) which are all the same voltage relative to earth but 120/240 degrees out of phase with each other (not 180). As you note, this means you can't use two of these and get 240VAC. While I believe it is possible to pull three-phase off the power lines with appropriate transformer/capacitor combinations, it may be simpler to instead use a standard two-phase power feed to run a motor/generator set which provides three-phase output.

480V, like three-phase, is uncommon in home applications, but is used in industry. (Most elevators used to run on 480V; I'm not sure whether that's still true.) If that's what you need, you would typically make arrangements with the power company to have an appropriate transformer set up, especially since applications which require 480V generally also require a significant amount of current.

Also a good answer, I think I get this now because of caarntedd's comment the 240v three phase still has three wires, just the voltage isn't actually timesed by three. So know I have it all figured out, thanks for your help.

> So know I have it all figured out
.  Not even close. You are only seeing the tip of the iceberg at this point. But it appears that you've made a good start towards learning more.

Three phase power has 3 DIFFERENT sine waves going on each of the 3 lines, usually PLUS a ground. The ground is not used for transmitting current unless there is a fault.

The phases are offset by 1/3rd of a waveform, as you could imagine a generator with 3 equally spaced coils and 1 magnet would produce. the three phases work that connection between any 2 phases at any given time will either be +/- the voltage rating. When waveform *a* is at its positive peak, waveform *b* and *c* will be crossing negative 'half' peak voltage. Basically what this means, is despite the power being output in a sine wave, intermittent on and off, even when one is fully off, the other 2 coils are outputting full power, or if you average the current over time it's even rather than pulsed.

I recommend reading up on wikipedia.org/wiki/3-phase-power . It explains the math and physics in laymans and very complex terms alike.

This has little to do with answering my actual question, I know what 3-phase is, just how does it apply to what I say above.

At no point do you ever get +230 and -230 volts to get 460. Connecting between any two just gives the average like I said.