steelersman

No you get 208v in 3 phase from any line to line combination whether it's A-B, A-C, or B-C. 240v is single phase line to line.

InPhase277

In a 240 V delta, with a center tap, you would get 120, 208, and 240. Usually, the coil between A and C is center tapped for a neutral (N), and you get 120 V from A to N, and from C to N. You get 240 V between any two phases, and you get 208 V from B to N.

So, between A and N or C and N, you only have 1/2 a coil. But between B and N you have 1-1/2 coils. The vectors work out such that some of the voltage is canceled and you get 208. But hat voltage is really just an artifact, and doesn't really serve any purpose. Many appliances and electronics have been smoked because someone didn't understand the 208 V high leg.

P.S.: Yes, there was such a thing as 2-phase, but it is ancient history. There was no benefit to it that couldn't be done with single phase or done better with 3-phase.

guitarboyled

Therefore, between A and N and C and N there’s 120v single phase

And Between B and N wouldn’t I presume there's 208v single phase

I also understand there’s 240v between any two phases but I'm guessing we never use only two phases but always connect all three.

InPhase277

Yes, B to N would be single phase 208, but it is virtually never used. Any device that requires 240 V single phase, like a window A/C or table saw, could be connected between any two phases. So really, with a 120/240 V delta, you get 120 V single phase, 240 V single phase, 120/240 V single phase, and 240 V three phase. Plus single phase 208 V.

frenchelectrican

Inphase277 did expain very clear on that and speaking of 208 volt line to netural that is what we called wild leg , bastard leg , red leg , hot leg , crazy leg.

That one reason why anytime you run into delta system always test and I mean test them very carefull as due not always be marked in correct spot so that something you have to watch out on that part.

{ French verison }

Inphase277 a vraiment exfait de la peine très clair et la conversation de 208 volts ligne à netural c'est ce que nous avons appelé la jambe sauvage, la jambe de bâtard, la jambe rouge, la jambe chaude, la jambe folle.

Cette une raison pourquoi n'importe quand vous vous heurtez au système de delta teste toujours et je veux dire les testent très carefull comme dû non être toujours marqué dans la tache correcte pour que quelque chose vous doive observer de sur cette partie.


Merci,Marc

__________________
Pas de problème,il marche n'est-ce pas?"(No problem, it works doesn't it?)

guitarboyled

Also how would using two phase result in 240v single phase?

Doesn’t the Delta 3 phase also follow the 120° shift between each phase?



To get true 240v single wouldn’t the two phase need to be in sync?

From wikipedia:

Another system commonly seen in North America is to have a delta-configured secondary, with a centre tap on one of the windings supplying the neutral. This allows for a 240 V three-phase supply and three different single-phase supplies from the same secondary: 120 V between either of two (non-"high leg") phases and the neutral, 208 V between the third phase (the "high leg") and the neutral, and 240 V between any two phases.

Steelersman says 208v in 3 phases and InPhase277 says 240v in 3 phases. Can we get a consensus on this one?

InPhase277

Yes, it does. But it is just convention that when two of the three phases is used it is called single phase. And when using only two of the three phases, the resulting waveform is that of a single sine wave, a composite of the two.

Quote:

Steelersman says 208v in 3 phases and InPhase277 says 240v in 3 phases. Can we get a consensus on this one?

He is talking about a 120/208 V wye connected system. Which is 208 between phases. Then we got on the subject of 3 phase deltas, which is what I described.

guitarboyled

Thanks,

And could I use that 240v on a convential oven let's say?

Toronto Sparky

You still need the neutral on the oven.. for timer/lamp/receptacle
Older ovens needed it for the range top elements.
(Remember the old 7 position controls everyone?)

Keep in mind Ovens are normally 240 but one can also buy a 208v oven for use in apartment buildings that are connected wye.

guitarboyled

What can I do with that specific 240v? I'm guessing lighting also from a previous post.

Couldn't I use the centered tap neutral for that oven connection? Just curious.

InPhase277

Yes. What Toronto was saying is that most stoves aren't just 240 V. They are 120/240, and require a neutral. So if you have just a straight 240 V delta, then you couldn't use anything that required a neutral. But if you had a 120/240 V delta with a center tap providing a neutral, then it would be fine.

Specifically, the single phase 240 could be used for anything that required it. You will usually find a 120/240 V delta in heavy commercial buildings, like machine or cabinet shops. Very often, table saws are 240 V single phase. Many window unit and through-the-wall A/C units are 240 V single phase.

guitarboyled

The delta configuration with center tap concept seems harder to grasp.

If figure A represents a 120v/208v wye configuration supplying 3 phases at 208v does figure B represent a 3 phase delta configuration with center tap supplying 3 phases at 240v?

InPhase277

Nope. Figure A represents how 208 V appears when the single phase voltage is 120 V in a wye configuration. In a wye system, the line-to-line voltage comes from multiplying the phase voltage by 1.732. In a delta system, the line-to-line voltage and the phase voltage are one in the same. In other words, the single phase voltage of a delta system is the same as the phase to phase voltage, due to how the coils are connected.

If you had only only one coil, it would be 240 V. What does change is the current. The line current divide by 1.732 is the current in the coils, because there are multiple paths for the current.

guitarboyled

Oups! I wrote Fig A 277/480 when obviously it's 120/208v

So is it possible to represent the delta 120/240 configuration with the 3 phase sine wave graph?

InPhase277

Yes, but the amplitude of each sine wave is 240 V, whereas in the 120/208 V wye, each sine wave has an amplitude of 120 V.

In the delta, coinciding with one of the sines, is a 120 V sine that represents the center tapped coil.

Larry Fine

We get 240v from two 120v secondaries in series on single phase because the positive and negative peaks occur at the same time.

In a 3-phase wye system, the positive peak of one phase occurs midway between the negative peaks of the other two, and vice versa.

The 208v voltage between phases occurs when neither polarity is at peak, but between them, when the positive and negative voltages are equal.

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slowforthecones

smells like green horn in here...

Toronto Sparky

Love this short and sweet explanation... Perfect

zappy

One more name for it, stinger leg.

__________________
"In thee abundance of water, the fool is thirsty"

zgozvrm

This is a result of vector addition.

First of all, the turns ratio of the transformers are such that 120 volts is obtained from the primary voltage. For instance if your primary voltage is 480V, 3-phase (480V can be obtained by tapping off of any two lines: leg A and leg B, leg A and leg C, or leg B and leg C), there would be a 4:1 ratio in each transformer (you need 3 to create a wye) since 480 divided by 4 is 120.

Remember that, since the primary voltages are 120 degrees out of phase, so are the secondary voltages.

In order to understand vector addition, think about how planes react to wind currents. Both the plane and the wind current have their own specific speed and direction. This can be represented by vectors, by drawing line segments of lengths that represent the speed and direction of each.

For instance, if the plane is heading, say due East at 50 mph (it's a slow plane), we would draw a line segment 50 units long going from left to right with an arrowhead on the right end. Suppose the wind was blowing due north at 40 mph, this would be represented by another vector; this one 40 units long pointing upward. These 2 vectors would be 90 degrees "out of phase." In other words, the phase angle between the 2 vectors is 90 degrees. Obviously, the wind will throw the plane off course, unless it is blowing in the same direction as the plane (or opposite). It will also cause the plane to either speed up (with a tailwind) or slow down (with a headwind). The result of this (the resultant vector) will be the vector sum of the 2.

In this case, we have a crosswind, that will cause the plane to veer off-course in a north-easterly direction. It will also cause the plane to slightly speed up, so the resultant vector, will be slightly longer than the plane's speed vector. If you put one source vector (the plane's for instance) head-to-tail to another source vector (the wind's), the resultant vector will have a length equal to the tail-to-head distance of the combination. In this example, the plane will actually travel in the speed and direction of the resultant vector, which can be shown to be 60 mph at a heading of 0 degrees, but a bearing of approximately 38.66 degrees on the compass (with East being 0 degrees, and North 90 degrees).

Now, the same thing goes for voltages. In this case, we have two 120 volt sources with a phase angle of 120 degrees. This can be represented by 2 vectors of equal length (both are the same voltage), in this case 120 units. See the picture below:




Now the resultant vector in this case would be a new, third vector with its tail at point A and its head at point C. If you were to measure this distance (which can be accurately done using trigonometry), you'll find that the resultant vector is roughly 208 units long.

A wye distribution system will have 3 such relationships (xfmr 1 to xfmr 2, xfmr 2 to xfmr 3, and xfmr 1 to xfmr 3) and therefore will have 3 different resultant vectors, all measuring 208 volts.

You should note that the 208V (line-to-line) vectors are offset from the 120V vectors (phase vectors) by +/- 30 degrees.

FYI, using vector addition, you can easily calculate the "stinger-to-ground" voltage in a center-tapped delta secondary. You can also see how an open-delta configuration works.