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3 Phase Voltage

06/27/2014 7:56 AM

Why is there a difference in phase to phase voltage when compared to phase to neutral?

Eg: Phase to neutral = 220V

Phase to Phase = maybe 440V.

Why? because you double the phases? ie; 220+220=440?

Then 3 phase should be 660V? 220+220+220=660V?

Explain!

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#1

Re: 3 Phase Voltage

06/27/2014 8:38 AM

Think "vectors". Think " Wikipedia ".

May your camel have a thousand children.

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#2

Re: 3 Phase Voltage

06/27/2014 11:57 AM

A single phase AC voltage alternates between the maximum and minimum voltage in a sine wave. In most residences in the US, there are two phases that are opposite in phase, so that when one phase is maximum voltage, the other is negative voltage, so the difference in voltage between the two is twice the voltage between either phase and neutral.

In industrial settings, there are commonly 3 phases, separated in phase by 120 degrees, so that each phase reaches maximum voltage 1/3 cycle after the preceding one. The voltage between two of these phases is the square root of 3 times the phase to neutral voltage.

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#6
In reply to #2

Re: 3 Phase Voltage

06/28/2014 6:32 AM

A technical misconception, very common, is that its two separate phases. Its not!! It is very misleading as well....

If the power is lost for some reason, both will go. Whereas is a 3 phase system, you can lose a single phase and the other two may remain (depending upon how safety equipment is used/applied).

It is in fact a single phase, but center tapped as a neutral, which is also connected at some point to ground.....

Don't be upset with yourself as I would go as far as to say that few people on CR4 anywhere understand that fully!!

See here:-

AC_Distribution

Where the diagram Figure 2 shows the connections clearly.....The phase to neutral grounding shown will usually be at the transformer....

This is why they are at exactly 180° relative to each other and always equal and opposite....

Whereas 3 phase systems, phases have only 120° relative to each other. The are not always equal and opposite.

I hope this helped.

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#8
In reply to #6

Re: 3 Phase Voltage

06/28/2014 7:42 AM

I was aware the 220 supply comes from a center tapped secondary. I was trying to simplify the explanation as much as possible.

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#9
In reply to #8

Re: 3 Phase Voltage

06/28/2014 7:59 AM

Understood completely, it is something we should all aim for, but its better to not oversimplify to the point where accuracy is then questionable at best....

In this particular case, many really do not understand it at all well.....which is why I put accuracy always before simplicity, though as you say, we should try and put it over in an easy way to understand....

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#11
In reply to #6

Re: 3 Phase Voltage

06/28/2014 10:39 AM

Thank you, Andy. I had read that (possibly here, from you?) but I suspect I take the easy way of thinking and revert to the idea that it is separate phases.

This brings to mind a question which you might be able to answer.

In a true 3-phase system, are the generators wound such that one device actually produces the 3-phases? And are they, then, wound in similar fashion to an automobile alternator? I see from the tech diagrams that my auto alternator produces 3-phase AC, which is then rectified (full-wave bridge) through three sets of rectifiers (totaling 12) in order to produce the final DC output.

And if so, why would such a complex system (admittedly, one where the bugs have been worked out, and very stable, since my current vehicle's alternator has been running for the car's full 207K miles) be desirable over the simplicity of a single phase system?

Is the I2R efficiency of 3-phase that much higher.

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#13
In reply to #11

Re: 3 Phase Voltage

06/28/2014 2:43 PM

In a true 3 phase system, one device is so wound to produce all 3 phases...

But let us say that the poles are in multiples of 3 in a 3 phase generator, though it has been many years since I looked in one.....but all 3 phases come out of the same physical unit.

Car alternators are sometimes a single phase, though as far as I am aware/experience, (count the diodes!) most are 3 phase......and only have 6 diodes in my experience....they need 2 per phase for full rectification....

Like here:-

But possibly an alternator with 6 field coils (2 x 3 phase) would need 12 diodes.....

As to what is more efficient, I have never considered, but I would expect the 3 phases to be better overall as there would be less of a need for smoothing.... the following Wiki diagrams show this well:-

Single phase

3 Phase

With modern electronics you don't want "Lumpy" DC......so it may have more to do with the need for clean DC.....

Other opinions vary, but some say that 3 phase is more efficient in the same size unit.....I did not know that myself, but it could well be true....

I hop this is a reasonable answer for you.....

By the way, hybrids have motor/generator units with a very high efficiency, probably higher than these alternators we are discussing here....

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#14
In reply to #13

Re: 3 Phase Voltage

06/28/2014 4:25 PM

Thank you. I knew about the smoother output, having done some power supply design as a teenager.

Now that I think back on it (and take a physical look at what is on the car, instead of the mental look in what passes for my memory) I see wiring for only 6 diodes. But since they are in a monolithic block (not an IC exactly, at least not in any small size, but rather a fairly large "puck") I could not tell how many diodes were there. Of course, the external tab connections don't give away the internals, but when it says their are three pairs, I guess that makes six, total, right?

And it seems to me that it would be more efficient in a 3-phase, since it means that there are fewer degrees of rotation of the fields when at least some of the phases/fields are not in the process of delivering current.

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#15
In reply to #14

Re: 3 Phase Voltage

06/28/2014 4:32 PM

All I can say is "Correct!"

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#19
In reply to #14

Re: 3 Phase Voltage

06/29/2014 12:03 PM

Another thing that people often forget about three phase power production is the mechanical loading on the generator. A single phase generator and the arcane true two phase (90°) generator will have a relatively high amount of mechanical vibration even when the electric loads are well balanced.

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#20
In reply to #19

Re: 3 Phase Voltage

07/01/2014 1:18 AM

Is that from magnetostriction? It makes sense, but you are right. I surely never thought of it.

And in an auto, that would really play a part, along with precession effects, and the generally hostile environment under a car's hood (vibration, heat, constant running whenever the engine is on, etc.). In this case, every little bit hurts (rather than helps).

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#22
In reply to #20

Re: 3 Phase Voltage

07/01/2014 1:41 PM

Magnetostriction effect is another power loss phenomena (along with eddy currents) that happens in all magnetic cores to one degree or another.

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#23
In reply to #22

Re: 3 Phase Voltage

07/01/2014 9:35 PM

Understood.

But my question, by implication (not clearly stated), was whether magnetostriction was the (or a) reason that a single phase alternator is more prone to short life than a 3-phase alternator.

In other words, does the effect in a single phase cause more of an imbalance/distortion of the physical components than does the effect in a 3-phase?

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#24
In reply to #23

Re: 3 Phase Voltage

07/02/2014 4:33 AM

I personally go with the fact that that there is a physical "resistance" and then "push" when you turn any type of alternator/generator with a magnetic core (for a car alternator it needs to be actively powered) by hand, the easiest example being a pushbike alternator, or a stepper motor if you have one. When the poles pass each other...Its a well known effect. Like trying to push two magnets together with "like" poles....

n an alternator/generator, this effect gives rise to vibration which is bad for any mechanical device.

A possibly similar effect is seen with these large modern windmills. Some were built years ago to be driven with the wind from behind the mast (down wind), but the mast, blocks a small part of the wind flow and causes turbulence and hence vibration and flexing of each blade as it pasts the tower/mast, possibly damaging the main connections of each blade to the center hub...This is often heard as a low level noise, synchonised with the lower blade passing through BDC.

Which is why, most modern Windmills are built as "up Wind" arrangements, to reduce this effect of this noise and damage.

Noise is a good indication of some mechanical effect causing vibration, seldom positive!

I hope this explains the effects better....

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#25
In reply to #24

Re: 3 Phase Voltage

07/02/2014 3:59 PM

Yep. Thanks, Andy. Very helpful.

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#21
In reply to #19

Re: 3 Phase Voltage

07/01/2014 5:09 AM

GA

How true.

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#16
In reply to #11

Re: 3 Phase Voltage

06/29/2014 2:48 AM

I can't answer your 3Ph V 1Ph question in any technical manner, though having 3 phases rectified does give a smoother OP at the rectifiers in the automotive example, explained well by others and easy enough to comprehend...

but..

When it comes to stationary gensets I do know that KVA for KVA a 3 phase genset is significantly cheaper than a same sized single phase genset.

Motors are the same. For the same HP a 3Ph motor is a lot cheaper than a 1Ph.

Without consulting a text book I'd be betting that it is easier to wind a 3Ph machine than a 1Ph machine.

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#17
In reply to #16

Re: 3 Phase Voltage

06/29/2014 7:00 AM

Thank you.

The cost-to-produce aspect slipped right by me.

But I had no problem with the "requires less filtering" aspect, especially where D?C automobile systems are concerned.

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#3

Re: 3 Phase Voltage

06/27/2014 7:45 PM

I don't want to worry anyone, have a look at the OP's previous posts.

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#4
In reply to #3

Re: 3 Phase Voltage

06/27/2014 11:42 PM

Oh dear...

He is far enough away for me not to worry. Pity those around him.

There's a dangerous level of basic electrical principals naivety surrounding the subject of his posts.

OP, I hope you are a junior student.

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#5

Re: 3 Phase Voltage

06/28/2014 12:56 AM

Addition of Vectors is different from normal addition like addition of numbers

In a three phase system we have three vectors say

V = Va + Vb +Vc

Va= Vo e^(j. 0)

Vb= Vo e^(j. 2pi/3)

Vc= Vo e^(j. 4pi/3)

Now Phase to Phase Voltage =Vab= Va - Vb=?

Vab= Vo [ {cos(0) +jsin(0)} - {cos(2 .pi/3) +j sin (2.pi/3)} ]

Vab= Vo [{1} - {-1/2 +j (square root 3)/2}]

Further solving gives the result as

Vab =Vo (square root 3)

So where we have Phase to neutral voltage 230 V we have

Vab = 230 * 1.732 = 400 V

As Vectors are involved so it is 400V instead of 460 V.

As three phase system is Specified by Phase to Phase voltage.

For Example

1.If Phase to Neutral is 230 V , then in a Three phase system it should specified by 400 V (Phase to phase Voltage).

2. If Phase to Neutral is 400 V , then in a Three phase system it should specified by 690 V (Phase to phase Voltage).

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#18
In reply to #5

Re: 3 Phase Voltage

06/29/2014 9:58 AM

why when we get a Power( watt) in the 3 phase always mltiple with √3??
>>>> P :√3*V*I* cos Phi

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#7

Re: 3 Phase Voltage

06/28/2014 6:48 AM

As others have stated, L-N voltage needs to be multiplied by sqrt(3) or 1.732 to get L-L voltage. This is the result of a Vector calculation involving an amplitude and a phase angle.

Maybe a phasor diagram will help to understand relationship between voltages and phase angles?

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#10

Re: 3 Phase Voltage

06/28/2014 8:46 AM

How about:

Please explain?

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#12

Re: 3 Phase Voltage

06/28/2014 1:44 PM

Dear Mr.tasneemahmad,

You should not numerically add the voltage.

Actually single phase voltage to be utiplied by ROOT3 i.e. 1.732.

In your case it is 22 x 1.732 = 381.4 and not 440.

DHAYANANDHAN.S

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#26

Re: 3 Phase Voltage

07/26/2014 5:37 AM

3 phase electric power is a common method of altering the current electric power generation, something that most of the electrician never need to consider is that voltage of different values don't occur at the same time, Electrical line workers do need, power grids are paralleled for reliability.

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#27
In reply to #26

Re: 3 Phase Voltage

07/26/2014 6:29 AM

You make no sense whatsoever.

Try again.

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