Kirchoff's law

[K2500]

I am trying to gain a better understanding of phase angles, like why, when you have 60A per phase, that the sum of a 3 phase 3 wire system is not 180A. Also the word potential and what it means has me confused. While I'm not looking for simple answers, somthing to tecnical could cause my head to explode .

 

[charlie b]

This is a good question, and one that is worth getting through early in your career. But it is not easy to answer. The simplest answer to the question, ?why is 60 plus 60 plus 60 not 180? is that you are adding 60 of one thing to 60 of another thing and to 60 of still another thing. It is easy to think that ?60 amps? is the same as ?60 amps.? The problem is that by only saying ?60 amps? you are not giving a complete description. In order to understand the whole picture, you need to say that you are adding ?60 amps at a phase angle of 0 degrees? to ?60 amps at a phase angle of 120 degrees,? then adding the result to ?60 amps at a phase angle of 240 degrees.?

Now, to explain what is meant by phase angle is a bit beyond what I can easily do with words. Let me just give a hint, by saying that the voltage wave form of Phase A will reach its peak value 60 times every second, and so too will the voltage wave form of Phase B, and so too with the wave form of Phase C. They just don?t do it at the same time. They are spaced apart by one third of a cycle, one third of 360 degrees, or 120 degrees. To do more, I will need pictures.

The concept of ?potential? is not so hard. But I have to get back to work now. I?ll write something up, and post it later.

 

[JKinPA]

I am trying to gain a better understanding of phase angles, like why, when you have 60A per phase, that the sum of a 3 phase 3 wire system is not 180A. Also the word potential and what it means has me confused. While I'm not looking for simple answers, somthing to tecnical could cause my head to explode .

Sometimes it helps to use water to explain so....here it goes...
If you have three guys all holding hoses that turn off and on at different times and are never squirting the maximum amount of water at the same time then the total amount of water from all three hoses at any point of time will NOT be the maximum amount of water from all three hoses.

As far as the word "potential" that usually refers to potential energy. In electronics we refer to voltage as the potential. In the physics world you would use PE = mgh to calculate potential energy.

where
PE = Energy (in Joules)
m = mass (in kilograms)
g = gravitational acceleration of the earth (9.8 m/sec2)
h = height above earth's surface (in meters)

 

[haskindm]

Don't over complicate things. For electricians we can keep electrical theory fairly simple. Let the engineers fight over the nuances of theory. Simply speaking we talk about amp loads per phase. If I have a 60 amp three phase load, an amp-meter on any phase would read 60 amps. The total wattage (volt-amps) would be the amp load on a single phase (60-amps) times the system voltage times the square root of three (1.732 approximately). So a 60 amp 3-phase 208 volt load would be 60 X 208 X 1.732 = 12,481.732 VA.
Potential simply means that:
If a surface or conductor has a higher electrical charge on it than another surface or conductor, there is a "potential" difference between them. If a conductor is placed between them, current will flow. If the potential difference increases to a point that the resistance of the air surrounding the items is overcome an arc will occur. You can see this phenomena as lightning. Potential is built up between the earth and a cloud, or between two clouds; once it is high enough to overcome the resistance of the air, an arc occurs

 

[spsnyder]

There are a lot of discussions on this website about what you asked. Here's a recent one that might help.

http://forums.mikeholt.com/showthread.php?t=87196&highlight=phase

I tried to find another recent one that had great graphical examples. Spend some time browsing this forum and you will have an abundance of examples and explainations. Regards.

 

[spsnyder]

Here's wikipedias take on it. It has a simple graphic also.

http://en.wikipedia.org/wiki/Three-phase_electrical_power

 

[dereckbc]

First thing to grasp is Kirchoffs Law using DC, which basically is whatever current is supplied must return to the source by any number of parallel paths. Once you grasp that AC is a lot easier to wrap your brain around.

 

[K2500]

.
Potential simply means that:
If a surface or conductor has a higher electrical charge on it than another surface or conductor, there is a "potential" difference between them. If a conductor is placed between them, current will flow. If the potential difference increases to a point that the resistance of the air surrounding the items is overcome an arc will occur.

Would this apply to the difference between a conductor and ground? Say a 120V difference in potential, or in a corner grounded system, a potential difference of 0 between ground and "b" phase?

Now, to explain what is meant by phase angle is a bit beyond what I can easily do with words. Let me just give a hint, by saying that the voltage wave form of Phase A will reach its peak value 60 times every second, and so too will the voltage wave form of Phase B, and so too with the wave form of Phase C. They just don?t do it at the same time. They are spaced apart by one third of a cycle, one third of 360 degrees, or 120 degrees. To do more, I will need pictures.

How does the voltage wave form(sine wave?) relate to current flow? Is the current a part of the wave, it seems like it would have to be, or maybe it is an effect of the wave??

 

[haskindm]

If the "ground" provides a return to source, then yes. Whenever a phase is referenced to "ground" then there will be no potential between that phase and "ground" because they are connected together. This is why bonding is so important. There may be "stray voltage" travelling on metal objects, such as in the vicinity of a swimming pool, but if all metal objects are bonded together there is no potential difference between the objects, and a person in the water cannot become a conductor between two objects.

 

[LarryFine]

Would this apply to the difference between a conductor and ground? Say a 120V difference in potential, or in a corner grounded system, a potential difference of 0 between ground and "b" phase?

Absolutely correct.

How does the voltage wave form(sine wave?) relate to current flow? Is the current a part of the wave, it seems like it would have to be, or maybe it is an effect of the wave??

For simple explanations and basic theory, as far as voltage and current and Ohm's Law are concerned, treat AC and DC the same.

Where AC matters the most is with 3-phase, where unlike single phase, voltage between each hot wire and neutral/earth*/ground does not reach the peak of the wave at the same time.

They take turns:

These three waves show that they begin one after another, but it is not accurate because of the spacing. I just got called to dinner.

As Ahnold would say, "A'hl be bahk!"

**********

Real quickly, this is a representation of what 240/120 single phase would look like if you measured each of the hots relative to ground (your examples above) simultaneously. Notice that the opposing peaks occur at the same time?

 

[cadpoint]

Rms

Rms

Put the Root Mean Square in that fine 3 phase diagrams - Sine wave chart.
RMS - Thats where a work is being done. In A/C.
RMS- (point where cycles cross in the other arc's -approx.)
If one takes any point off the Horizontal Line, straight up,(till they touch the the RMS Line) thats why one does have 3-phases of 60 amps in the 3P service, everything is Pop'n, move'n, snapp'n, AKA in transition.

And thats just the diagramed pulse of 1 cycle of 60Hz.
So three 3P A/C likes strangley like DC Power. Cause its bacisly a flat line of Power, DC is, A/C needs three phase to have useable power or RMS er sum of the power

 

[winnie]

Something else to consider:

The 'waves' that we are talking about are _not_ actual waves in the wire. There isn't something physically moving up and down like the pretty pictures.

Instead you simply have some measurable value (voltage between two points, current passing a point, etc.) which is _changing_ in time.

Sometimes the voltage is positive, sometimes the voltage is negative. It (hopefully) changes in a very well defined fashion. When we graph voltage versus time, we see the nice sine wave pattern.

-Jon

 

[LarryFine]

Okay, I'm back. Dinner was delicious, by the way.

Here's an image of a symmetrical three-phase waveform with proper timing intervals:

Note the even and symmetrical spacing of the timing differences. Also note that the positive peaks don't coincide with the negative peaks, which is why, on 3-phase Y systems, line-to-line voltage (208 or 480) is not twice that on line-to-neutral voltage (120 or 277).

I am trying to gain a better understanding of phase angles, like why, when you have 60A per phase, that the sum of a 3 phase 3 wire system is not 180A.

If you compare a single 120v, 60a load to three 120v, 60a loads, they do add up in the sense that you have three times the available power. 60a 3ph at 120v is equivalent to 180a 1-ph at 120v.

The difference is that the voltages add up, not the current, so that's why more phases is equal to more power. (Power = volts x amps) However, both examples are still 60a per line.

 

[chris kennedy]

Okay, I'm back. Dinner was delicious, by the way.

Whats for dessert?

 

[LarryFine]

Whats for dessert?

Shhh! It's a secret. :wink:

 

[rattus]

Strictly Speaking:

Strictly Speaking:

Nice diagram Larry, but I would argue that neither the voltages nor the currents add. In fact, the sum of the three voltages is zero, and in a balanced system, the sum of the currents is also zero.

But power, being a scalar quantity, does add algebraically. For example, in a wye, 60A x 120V = 7,200VA per phase or 21,600VA total.

 

[LarryFine]

Nice diagram Larry, but I would argue that neither the voltages nor the currents add. In fact, the sum of the three voltages is zero, and in a balanced system, the sum of the currents is also zero.

Let's not totally scare the OP away just yet.

But power, being a scalar quantity, does add algebraically. For example, in a wye, 60A x 120V = 7,200VA per phase or 21,600VA total.

"Eighty-two . . . eighty-two . . . eighty-two; two hundred forty six total." ~ Rainman

 

[K2500]

Dinner was delicious, by the way.

It took two years of marrage before I could say that.:grin:

It looks like I do, in fact, have a lot to wrap my mind around. But that will probably be tomorrow, I think my brain is starting to single phase.

BTW, a picture really is worth a thousand words, thanks to all.

 

[LarryFine]

It took two years of marrage before I could say that.:grin:

Before it was delicious, or before you could say it?

 

[K2500]

Before it was delicious, or before you could say it?

Well, I always said it was good, it just took that long to be true.

Nice diagram Larry, but I would argue that neither the voltages nor the currents add. In fact, the sum of the three voltages is zero, and in a balanced system, the sum of the currents is also zero.

But power, being a scalar quantity, does add algebraically. For example, in a wye, 60A x 120V = 7,200VA per phase or 21,600VA total.

Wow, I was still typing #18 when you two posted. I did take larry to mean power as aposed to voltage, but am intrested to know how they equal 0. I can't promise that I will understand the answer, though I think I'm making some progress here.