Biggest misunderstandings with 3 phase power......

[mwm1752]

I find many people struggle with 3 phase power.

Consider PF as 1 for all of this to keep it simple.

My method is to always revert to single phase in calculations.

On rating labels the specified Amps are always per phase. This confuses many I find.

So lets say the current is listed as 10A on a 3 phase motor label.

So if V_L-L = 480VAC (System Voltage is always V_L-L) then V_L-N= 480/1.732 = 277

Thus power per phase is 277x10 = 2770W =2.7kW.

And for the motor with all 3 phases it is 3x2770 = 8310W = 8.3kW

____________________________________________________________

Most of our folks here are experts on this. Can you list anything about 3 phase or other that you find confuses people.

Do any of you have a really good mechanical or fluid or other analogy to help people understand why 3 phase is so superb and why it was the choice across the planet. There could be four, five, even ten phases. Why not???

Also is it accurate to tell a customer that one 3 phase 200A panel is the same (even better) than 3 panels like his one at home with single split phase 200A??? Assume he does not know what an induction motor really means.

480 x 10 x 1.73 = 8.304 Kw -- can't get any simpler than that

 

[Carultch]

Do you have a water analogy for an inductor?

Yes. Think of an inductor like a paddlewheel that is free to spin on an ideal axle. It adds "inertia" to the hydraulic system, such that it applies a pressure to the water flow to oppose any change in the flow rate.

Try to speed up the water, and the water has to push the paddlewheel to a higher speed. The paddlewheel pushes back on the water, against the attempt to speed it up.
Try to slow down the water, and the water has to push the paddlewheel to a lower speed. The paddlewheel pushes foward on the water, against the attempt to slow it down.

In a sense, an inductor is a kind of "electrical inertia", in that it opposes changes in the current, and tries to keep it steady.

 

[Carultch]

I think the best way to explain 3 phase is to note that it is just three singles phase sources (or loads) that "happen" to be 120 degrees apart. That would be 6 wires, but because of the phasing difference, we can connect those 6 wires two ways (delta or wye) that reduces the number of wires to three (or four).

If one is already an expert with single phase AC, that is a great and simple way to explain it. The only missing information is what exactly "phase" is, and what it has to do with angular degrees.

I understand "phase" to be the position within the AC waveform, where the wave is at any given instant. And as for what it has to do with angular degrees, we consider the full complete cycle to be 360 degrees. So three phase consists of three AC waveforms that are delayed by 1/3 of a cycle each.


I also like to point to Smart$'s profile picture, as it depicts this very well. Imagine the three arrows rotating through a coordinate system. The projection onto the X-axis (real axis) shows what the AC voltage is in real time. While the Y-axis (imaginary axis) is the past and future.

 

[Carultch]

With DC analogies hold up very well. Voltage -> Pressure (PSI); Current -> Gal/min flow rate; Power -> Volume of water per minute.

The problem is AC. Perhaps a circulating pulsing pump like the heart pushing into the veins/arteries might serve. Now if you consider three hearts phased 120 deg apart all pumping into the same one vein/artery load. Now I am running into trouble. Anyone want to take this further???

The heart's system is DC, it is just a pulsating DC. If you have blood flowing out through the veins and back through the arteries, you've probably got a serious medical issue.

The reason why AC doesn't fit the hydraulic analogy very well, is that the analogous hydraulic system would be just about useless. So we don't really see it as a practical device in the real world. Imagine pumping water back and forth so fast, that no drop of water actually makes it from point A to point B. The only thing that this system would transport would be energy, and electricity can do this with superior performance.

It is sort of like a sound wave, where no air actually makes it from my mouth to your ear. But the pressure wave and energy does.

 

[charlie b]

Go back to the basics: Voltage is the amount of charge on an electron. . . .

Sorry, but that is not even close to the truth. The amount of charge on any electron is the same as that on any other electron, and it never changes, no matter what voltage is applied to a circuit. In simplest terms, voltage is the amount of energy that would be required to move a given amount of charge from one location to another. Its fundamental unit of measure, the volt, is equal to one joule of energy per one coulomb of charge.

. . . current is the number of electrons circulating.

This is closer to the truth, and makes for a reasonable introduction of the concept to a novice. More accurately, however, current is the amount of charge that passes by a particular point in space within a given amount of time. Its fundamental unit of measure, the amp, is equal to one coulomb of charge passing by a given location within one second of time.

 

[charlie b]

Intuitively to me it is 3 single phases.

Your intuition is not helping you here. It is very much different than 3 single phases.

The fact that they are 120 degrees staggered does not matter to the power.

It matters very much. It is the essential physical difference between 3 single phase circuits and a single 3-phase circuit. See my explanation below.

Consider water flowing through 3 pipes as an analogy. Over a period of minutes each of the three pipes will deliver the same amount of water.

The "water analogy" works well in some ways, and works poorly in others. This is one of the "works poorly" situations. The fact that the 3 phases are staggered means that when current in Phase A is at its peak value in the positive direction, current in both Phases B and C are at less than their peak values, but the Phase B and C currents are both in the negative direction. What is actually happening at that moment (and this is where the water analogy fails to explain the situation) is that the current leaving the source on the Phase A wire is returning to the source on the B and C wires. In other words, it is the same current!

You say that the three pipes are delivering the same amount of water. But to what are they delivering it? If they are connected to sprinklers, they deliver the water to the garden. They could instead be taking water from one tank and moving it to another. They could be taking water from a river, running it through some heat exchangers in an industrial plant, and delivering slightly warmer water back to a point further down the river. In none of these cases is the water that leaves its source required to make its way back to the same source. But current that leaves an electrical source (generator, transformer, UPS, battery, etc.) must necessarily make its way back to the same source that put it into motion in the first place. In a 3-phase system, you have 3 paths that current can take as it leaves the source. But whichever path it takes on its way out, it will take one or both of the other two paths on its way back.

 

[charlie b]

My method is to always revert to single phase in calculations.

I don't do that, and I don't recommend it. I am not saying that you will get the wrong answers. What I am saying is that there is a higher risk that your answers might be wrong. Put another way, there is a higher risk of incorrectly accounting for the load. The risk gets higher when there is a mix of three phase loads, single phase loads that take power from one phase and include a neutral wire in the circuit, and single phase loads that take power from two phases and may or may not include a neutral wire.

What I recommend is that all calculations be performed in units of VA. When everything is added up, then only at the end do you convert to amps.

 

[charlie b]

Do any of you have a really good mechanical or fluid or other analogy to help people understand why 3 phase is so superb and why it was the choice across the planet. There could be four, five, even ten phases. Why not???

I don't need an analogy; I have math! If anyone is interested, I can post, or send by email, two documents that contain the trigonometric derivations of power in both a single phase circuit and a three phase circuit. But here is the non-detailed math version:

• For the present purposes, let us assume that a three phase system is balanced (all three phases have the same values of peak voltage and peak current).
• Let us further assume that there are no harmonics present.
• Find a way to display (e.g., on an oscilloscope) the voltages and currents on all three phases. Plot a paper copy, if that helps you.
• Now pick (at random) a specific point in time.
  • Measure the Phase A voltage and the Phase A current at that moment, multiply those two numbers, and write the result down.
  • Measure the Phase B voltage and the Phase B current at that moment, multiply those two numbers, and write the result down.
  • Measure the Phase C voltage and the Phase C current at that moment, multiply those two numbers, and write the result down.
  • Add those three numbers together. Let's call this answer, "the power at the first point in time."
• Now pick a different point in time.
  • Take the same measurements, and do the same math.
  • Let's call this answer, "the power at the second point in time."
• Do this as often as you like.
• Make a table that has two columns.
  • In the left hand column, write down the times at which the measurements were taken.
  • In the right hand column, write down the answers to "the power at this point in time."

Can you guess what you are going to see in this table? You are going to see the exact same value in every entry in the right hand column! In simplest terms, the power drawn (or supplied) by a balanced three phase system is constant. You won't get that with any other number of phases.

 

[Carultch]

Can you guess what you are going to see in this table? You are going to see the exact same value in every entry in the right hand column! In simplest terms, the power drawn (or supplied) by a balanced three phase system is constant. You won't get that with any other number of phases.

I just attempted this calculation with 4-phase power, and you still get continuous total power delivered by the system. I think you will find this is the case in any polyphase system that is order 3 and greater.

I even tried it with 5-phase, thinking that prime number would be the real clincher. And it still gave the same result. Continuous uniform power delivered to a pure resistive load.

The reason we use 3-phase and not higher order, is that there is a diminishing return, and no significant value to increasing the number of phases.

 

[drcampbell]

Sorry, but that is not even close to the truth. The amount of charge on any electron is the same as that on any other electron, and it never changes, no matter what voltage is applied to a circuit. In simplest terms, voltage is the amount of energy that would be required to move a given amount of charge from one location to another. Its fundamental unit of measure, the volt, is equal to one joule of energy per one coulomb of charge.

D'Oh! You are correct. Energy per electron, not charge per electron. I seem to have had a brain fart.

 

[Carultch]

D'Oh! You are correct. Energy per electron, not charge per electron. I seem to have had a brain fart.

Well it is energy per unit charge, in a more general sense. But few (if any) subatomic particles that can stand alone would have a charge that is not an integer multiple of the charge of an electron.

 

[Fulthrotl]

Lets hear your vastly superior pedagogical method. Presumably if one is explaining three phase, the student already has the basics of single phase down. Seems like a straightforward and logical extension to me.

peda.... peda... wtf.....

"The word pedagogue was originally used in reference to the slave who escorted Greek children to school." -wikipedia

ah. crossing guard.
how is the 72 year old greek woman in the lawn chair, with the stop sign and safety vest, like three phase electrical power....
damn. this is gonna be hard....

ok, you analogy queens, have at it!

 

[Carultch]

peda.... peda... wtf.....

"The word pedagogue was originally used in reference to the slave who escorted Greek children to school." -wikipedia

ah. crossing guard.
how is the 72 year old greek woman in the lawn chair, with the stop sign and safety vest, like three phase electrical power....
damn. this is gonna be hard....

ok, you analogy queens, have at it!

Ped is the Greek prefix for child. Pedagogy means method of teaching (to lead a child), named as such because children are usually the people who are taught.

I'm not sure if that crossing guard is supposed to be a real analogy, or if you are joking. Are you saying that the stop sign and safety vest have duplicate yet essential purposes, just like the lines in a power system?

 

[Ingenieur]

I know by the formula you are correct.

Please help me here.

Intuitively to me it is 3 single phases. The fact that they are 120 deg staggered does not matter to the power. Consider water flowing through 3 pipes as an analogy. Over a period of minutes each of the three pipes will deliver the same amount of water.

Why am I not seeing this:dunce::dunce::dunce:

the 120 deg offset does matter
It means everything, the crux
if in phase on a single wire the 3 single ph are now a single phase i x v, NOT 3 ph 1.732 x v x i, any longer
if on 3 wires no neutral no v between them lol, so
with a neutral P is 3 x v x i
but the neutral carries 30 A
so 6 units of Cu (3 line + 3 N) vs 3 for 3 phase
73% more power yes (3/1.732) but 100% (twice 6/3) the Cu

it also has to do with eff of machine construction
1 ph per rev vs 3

 

[dionysius]

EUREKA!! EUREKA!!! EUREKA!!!! I finally am getting it...........

EUREKA!! EUREKA!!! EUREKA!!!! I finally am getting it...........

You say that the three pipes are delivering the same amount of water. But to what are they delivering it? If they are connected to sprinklers, they deliver the water to the garden. They could instead be taking water from one tank and moving it to another. They could be taking water from a river, running it through some heat exchangers in an industrial plant, and delivering slightly warmer water back to a point further down the river. In none of these cases is the water that leaves its source required to make its way back to the same source. But current that leaves an electrical source (generator, transformer, UPS, battery, etc.) must necessarily make its way back to the same source that put it into motion in the first place. In a 3-phase system, you have 3 paths that current can take as it leaves the source. But whichever path it takes on its way out, it will take one or both of the other two paths on its way back.

I now get it lucidly because of Mr. Beck's explanation, Smart $'s avatar, and all of the other inputs. That avatar is powerful. Why wasn't I focusing on this stuff so many years ago when I was in school, a teenager, and I knew it all????

.

What I was missing is that an energy transfer from electric current to magnetic field with circulation of fluid to sustain it was not being modeled. Let me elaborate.

First exit the "real" world and welcome to the "ideal" world where losses and eddy currents and friction are absent.

Consider a large frictionless hollow doughnut pipe that is filled with non-compressible fluid laying on an oval racetrack. At each oval end consider there is a perfect turbine that is 100% efficient. A little magic here works well. A mechanical shaft to drive or driven by the turbine blades is accessible outside of the pipe. Of course the seals are frictionless and never leak. Since there are no losses and we are talking 100% efficiency any rotation of drive turbine shaft (generator) will replicate the identical motion on the driven turbine shaft (Motor). Let us call the generator end G (generator). Let us call the other end M (motor).

Now I can completely understand every item I could not before. Three single phase services are three independent oval MG systems. A three phase service is the composite sum of the three 120 deg staggered sinusoidal waves applied as torque to the G end. This will have some ripple. 50 phase will have minimal ripple. Infinite phases are a continuous circular drive motor driving the G end and the M end will be a fixed RPM driven shaft just like an induction motor.

The inductive coupling is in the circulatory fed back fluid. There are reactance effects due to pipe length and diameter.

If I modify the diameter of the hollow doughnut pipe I am modelling a transformer. If I vary sine wave period of input torques I vary speed.

This is fun. Try it out. See if you can debunk it.

Please try to find a reason that I am missing something. I am here to learn.

 

[Besoeker]

For an apples to apples comparison should it not be:
240x200x1.73 = 83,040 kW ????

Actually kVA.

 

[dionysius]

Actually kVA.

I did say let the PF = 1 in the prelude.

 

[dionysius]

A further enlightenment on my fluid model above is to think of the three wires in 3 phase as a single energy conduit. This is where the hollow doughnut oval track came from. This is why there is an inherent 73.2% less energy transfer in 3 phase relative to 3 single phases. You will clearly see from the analogy why a 3 phase induction motor is so superb. Mixing the feedbacks to the G does cause some interference but also provides more controlled coupling. For highly resistive loads single phase is more than adequate which is why we settle for that in homes. But if you have heavy duty AC demand 3 phase but your POCO will not provide methinketh.

 

[Ingenieur]

100 v
10 A

3 1ph w/N
3 x 100 x 10 = 3000

3 ph
1.732 x 100 x 10 = 1732

(3000-2732)/3000 = 42% less
3 phase carries 42% LESS than 3 1 ph w/N, NOT 73% less
but the 3 x 1 ph needs 100% more Cu
the 3 x 1 ph does carry 73% MORE than the 3 ph

the mechanical model will only get you so far

 

[Saturn_Europa]

I've run into this before:

Motor is pulling 10 amps on all 3 legs. So that means that the motor is pulling 30 amps.

10+10+10=30 is completely wrong but very easy to understand