harmonics??
- Thread starter wyatt
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jtester
Senior Member
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- Las Cruces N.M.
Re: harmonics??
Harmonics are frequencies that are multiples of a fundamental frequency. If we think about 60 hertz systems, then a 3rd harmonic is 3x60 or 180 hertz.
They are often caused by reshaping voltage and current waves, which is something done in many electronic power supplies.
Jim T
Harmonics are frequencies that are multiples of a fundamental frequency. If we think about 60 hertz systems, then a 3rd harmonic is 3x60 or 180 hertz.
They are often caused by reshaping voltage and current waves, which is something done in many electronic power supplies.
Jim T
- Location
- New Jersey
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- Journeyman Electrician
Re: harmonics??
As Jim had said the power supplies can cause a distortion in the sine wave where as the current will become additive in the neutral. Generally circuits with large harmonic currents can have a neutral current up to 1.73 times the phase current. Therefore, it is not uncommon to use a neutral conductor/bus that is rated at 200% of the size of the phase conductors.
As Jim had said the power supplies can cause a distortion in the sine wave where as the current will become additive in the neutral. Generally circuits with large harmonic currents can have a neutral current up to 1.73 times the phase current. Therefore, it is not uncommon to use a neutral conductor/bus that is rated at 200% of the size of the phase conductors.
charlie tuna
Senior Member
- Location
- Florida
Re: harmonics??
original harmonic problems were caused by computerized equipment. their power supplies were big contributors of harmonics. the computer manufacturers made changes in this area to eliminate or minimize harmonics. as stated above the return or neutral current's frequencies were different and would not mix or cancel themselves out -- by not mixing they would add and cause excessive current for the design of the system.
articles stated that harmonic problems would peak out and then be reduced by the manufacturer's design changes. from what i have seen in the past ten years, i think it is on the reduction course.............. also the practice of designated circuits on computerized networks has reduced many problems....
original harmonic problems were caused by computerized equipment. their power supplies were big contributors of harmonics. the computer manufacturers made changes in this area to eliminate or minimize harmonics. as stated above the return or neutral current's frequencies were different and would not mix or cancel themselves out -- by not mixing they would add and cause excessive current for the design of the system.
articles stated that harmonic problems would peak out and then be reduced by the manufacturer's design changes. from what i have seen in the past ten years, i think it is on the reduction course.............. also the practice of designated circuits on computerized networks has reduced many problems....
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: harmonics??
Let me define a "smooth, clean sine wave" as meaning that every peak is constant in height, every valley is constant in depth, and the time intervals between peaks and valleys are the same for every cycle.
If you apply a voltage source to a component, and the voltage you apply is a smooth, clean sine wave, and if you then look at the resulting current through the component, and if the current you see is also a smooth, clean sine wave, then the component is "linear." Even if the current has been shifted in phase (or equivalently, in time), the component is still "linear."
However, if the current you see has its peaks and valleys at different heights or depths, or if the time interval between peak and valley keeps changing from cycle to cycle, then the component is "non-linear." One particular type of non-linear behavior is when the output current starts at zero, and begins to follow what looks like a normal sine wave, but when the current reaches some level it instantly goes back to zero, as if it were switched off at that point. On the next cycle, it again begins at zero, starts following a sine wave, and is again turned off at the same point in a cycle. This type of component is often used in the power supplies for computers, and is the reason that computers are non-linear loads.
When you look at the current wave form that comes from a non-linear load, you will not have to be a mathematician to say to yourself, "whatever that wave shape is, it is obviously not a sine wave." But some brilliant mathematician long ago figured out that any weird waveform can be imitated, as closely to the original as you like, by adding together the following:</font>
I think you have a spelling glitch here, and that you intended to say "capacitor." If that is what you meant, then yes, a capacitor can shift the phase. But that only means that the point in time that the voltage reaches its peak will not be the same as the point in time that the current reaches its peak. But both the voltage and the current remain smooth, clean, sine waves. A capacitor is not a source of harmonics.
Don't confuse phase shift (as described briefly above) with harmonics. They are not related in any way whatsoever.
Let me define a "smooth, clean sine wave" as meaning that every peak is constant in height, every valley is constant in depth, and the time intervals between peaks and valleys are the same for every cycle.
If you apply a voltage source to a component, and the voltage you apply is a smooth, clean sine wave, and if you then look at the resulting current through the component, and if the current you see is also a smooth, clean sine wave, then the component is "linear." Even if the current has been shifted in phase (or equivalently, in time), the component is still "linear."
However, if the current you see has its peaks and valleys at different heights or depths, or if the time interval between peak and valley keeps changing from cycle to cycle, then the component is "non-linear." One particular type of non-linear behavior is when the output current starts at zero, and begins to follow what looks like a normal sine wave, but when the current reaches some level it instantly goes back to zero, as if it were switched off at that point. On the next cycle, it again begins at zero, starts following a sine wave, and is again turned off at the same point in a cycle. This type of component is often used in the power supplies for computers, and is the reason that computers are non-linear loads.
When you look at the current wave form that comes from a non-linear load, you will not have to be a mathematician to say to yourself, "whatever that wave shape is, it is obviously not a sine wave." But some brilliant mathematician long ago figured out that any weird waveform can be imitated, as closely to the original as you like, by adding together the following:</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">One sine wave based on 60 hertz,</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Plus another sine wave based on 2 times 60 hertz,</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Plus another sine wave based on 3 times 60 hertz,</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Plus another sine wave based on 4 times 60 hertz,</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Plus another sine wave based on 5 times 60 hertz,</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Plus another sine wave based on 6 times 60 hertz,</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Plus as many others as you need to make your imitation waveform look like the original.</font>
Re: harmonics??
hi all,
it's common know that triplen harmonics make neutral overheat/overload.
But when i'm trying measure neutral wire in a building (threee phase 4 wire),there is not only triplen harmonic that present but also the other like 5th & 7th harmonic.
And when i'm trying to calculate the copper loss :
I^2 x R
it's mean the 5th and 7th harmonic also give contribution to copper loss.
Is there anyone can explain ?
hi all,
it's common know that triplen harmonics make neutral overheat/overload.
But when i'm trying measure neutral wire in a building (threee phase 4 wire),there is not only triplen harmonic that present but also the other like 5th & 7th harmonic.
And when i'm trying to calculate the copper loss :
I^2 x R
it's mean the 5th and 7th harmonic also give contribution to copper loss.
Is there anyone can explain ?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: harmonics??
The neutral will carry the imbalanced currents that arise because the loads on the three phases are not identical to each other. If the loads have harmonic currents, let us say 5th harmonic, and if the loads are not identical to each other, then there will be some 5th harmonic current flowing in the neutral.
Imagine taking a bicycle wheel, and tying three ropes to it. Lay the wheel flat on the ground, and run one rope off in a northerly direction. Run the second rope off in generally a SE direction, and the third in generally a SW direction. Actually, lay the three ropes out so that they lie on the ground at 120 degree angles from each other.
Now have three big guys take up the three ropes, and start pulling. The guy with the northerly rope tries to pull the wheel towards the North, and the other two guys try to pull the wheel towards their respective directions. Which way will the wheel actually move?
Well, that depends on which of the three guys is pulling the hardest. If all three guys exert the same force, the wheel will just stay put. This is like saying if you have balanced loads on all three phases, there will be no neutral current. That is true for the normal (60 hertz) power, and it is true for all harmonics except the triplens.
Now let's talk about the triplens. Tell the guys to stop pulling and lay down the ropes. Tell the two guys at the SE and SW ropes to walk over to where the North guy is standing. Tell all three guys to pick up the northerly rope. Tell all three guys to start pulling the wheel in the direction of that rope. Which way will the wheel move now? It will move North, and it will move faster than it would if just one guy was pulling the rope. Not only that, but if all three guys are pulling with the same strength, that is not going to "balance" the pull, but will still cause the wheel to move towards the North.
In the same way, the triplens harmonics are all flowing in the neutral in the same direction. The contribution from Phase A "pulls to the North," and the contribution from Phase B "pulls to the North," and the contribution from Phase C "pulls to the North," and the net result is like three currents flowing in the same direction. It is worse than any one current would be by itself. And it doesn't matter if the three phases have identical loads and are perfectly balanced. The triplens harmonics from all three phases are going to add together in the neutral, just like three guys pulling in the same direction.
A final note: If you ask why the triplens harmonics act this way, I will say that the math is a bit harder to explain. That is another way of saying that I don't remember how to do it.
An even more final note: I just thought of this "rope-pulling-wheel" analogy as I sat down to answer this question. If you like it, then feel free to use it. But remember to give me credit for inventing it. Say, how's about we give it a name that might be clever enough in itself to make its way into text books some day? How about we call the story, "Charlie's Rope"?
That should not be a surprise. There is simply no reason to expect that the neutral will only have the 3rd, 9th, 15th, and other triplen harmonics.
The neutral will carry the imbalanced currents that arise because the loads on the three phases are not identical to each other. If the loads have harmonic currents, let us say 5th harmonic, and if the loads are not identical to each other, then there will be some 5th harmonic current flowing in the neutral.
It is a bit harder to explain why the triplens are worse than the others. Let me try it this way.
Imagine taking a bicycle wheel, and tying three ropes to it. Lay the wheel flat on the ground, and run one rope off in a northerly direction. Run the second rope off in generally a SE direction, and the third in generally a SW direction. Actually, lay the three ropes out so that they lie on the ground at 120 degree angles from each other.
Now have three big guys take up the three ropes, and start pulling. The guy with the northerly rope tries to pull the wheel towards the North, and the other two guys try to pull the wheel towards their respective directions. Which way will the wheel actually move?
Well, that depends on which of the three guys is pulling the hardest. If all three guys exert the same force, the wheel will just stay put. This is like saying if you have balanced loads on all three phases, there will be no neutral current. That is true for the normal (60 hertz) power, and it is true for all harmonics except the triplens.
Now let's talk about the triplens. Tell the guys to stop pulling and lay down the ropes. Tell the two guys at the SE and SW ropes to walk over to where the North guy is standing. Tell all three guys to pick up the northerly rope. Tell all three guys to start pulling the wheel in the direction of that rope. Which way will the wheel move now? It will move North, and it will move faster than it would if just one guy was pulling the rope. Not only that, but if all three guys are pulling with the same strength, that is not going to "balance" the pull, but will still cause the wheel to move towards the North.
In the same way, the triplens harmonics are all flowing in the neutral in the same direction. The contribution from Phase A "pulls to the North," and the contribution from Phase B "pulls to the North," and the contribution from Phase C "pulls to the North," and the net result is like three currents flowing in the same direction. It is worse than any one current would be by itself. And it doesn't matter if the three phases have identical loads and are perfectly balanced. The triplens harmonics from all three phases are going to add together in the neutral, just like three guys pulling in the same direction.
A final note: If you ask why the triplens harmonics act this way, I will say that the math is a bit harder to explain. That is another way of saying that I don't remember how to do it.
An even more final note: I just thought of this "rope-pulling-wheel" analogy as I sat down to answer this question. If you like it, then feel free to use it. But remember to give me credit for inventing it. Say, how's about we give it a name that might be clever enough in itself to make its way into text books some day? How about we call the story, "Charlie's Rope"?
charlie tuna
Senior Member
- Location
- Florida
Re: harmonics??
that being said:
let me rewrite my explaination:
i used to find alot of transformers where the rope got caught around the neutral terminal and "pull" it in half.
i don't find as many "rope" damaged transformers neutrals terminals today as i used to.
that being said:
let me rewrite my explaination:
i used to find alot of transformers where the rope got caught around the neutral terminal and "pull" it in half.
i don't find as many "rope" damaged transformers neutrals terminals today as i used to.
rick hart
Senior Member
- Location
- Dallas Texas
Re: harmonics??
Charlies Rope it is. Very good.
Charlies Rope it is. Very good.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: harmonics??
I don't think it works the same way. I believe that all harmonic currents that are present will flow from one leg of a 120/240 system to the other leg, with the unbalanced currents going via the neutral. Three phase systems have their own quirks, and their own mathematical manipulations to analyze those quirks. In the case of the 120/240 volt system, you are looking at single phase. You don't have three separate phases each separated from the other two by one third of a circle (i.e., 120 degrees of arc).
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: harmonics??
The method involves a tool that some math wiz invented some decades ago. He proved that you can take any set of three-phase, unbalanced currents, and model them with three sets of three-phase, balanced currents. In other words, instead of having just Phase A, B, and C, you have set #1 of (A, B, C), and set #2 of (A, B, C), and set #0 of (A, B, C), for a total of nine currents. But set #1 (called the "positive sequence") is a balanced set within itself, so it is easy to analyze. Set #2 (called the "negative sequence) is also a balanced set within itself, so it too is easy to analyze. Set #0 (called the "zero sequence") is a set of three identical currents, so that is very easy to analyze.
All three currents in the zero sequence set are in phase with each other. That is the reason that triplens harmonics add up in the neutral. The first harmonic (i.e., 60 hertz) is positive sequence. The second harmonic (i.e., 2 times 60 hertz) is negative sequence. The third harmonic (i.e., 3 times 60 hertz) is zero sequence. Then it starts over with the fourth, fifth, sixth, and so on. Every third harmonic (3, 6, 9, 12, 15, 18, etc.) is a zero sequence set, and each will have its zero sequence currents in phase with each other, so they add up to three times what any one of them would be by itself.
Two more notes: First, the name of the tool I described above is "Symmetrical Components." Secondly, in a power system, all the even harmonics (2nd, 4th, 6th, etc., or equivalently, 120 hertz, 240 hertz, 360 hertz, etc.) disappear. That is the reason that "triplens" is generally defined as the 3rd, 9th, 15th, 21st, and so forth.
I actually do remember how to set up the math problem. I just never liked performing the matrix manipulations that are needed to solve the problem. Here's a brief description. I hope I can make it make some sense.
The method involves a tool that some math wiz invented some decades ago. He proved that you can take any set of three-phase, unbalanced currents, and model them with three sets of three-phase, balanced currents. In other words, instead of having just Phase A, B, and C, you have set #1 of (A, B, C), and set #2 of (A, B, C), and set #0 of (A, B, C), for a total of nine currents. But set #1 (called the "positive sequence") is a balanced set within itself, so it is easy to analyze. Set #2 (called the "negative sequence) is also a balanced set within itself, so it too is easy to analyze. Set #0 (called the "zero sequence") is a set of three identical currents, so that is very easy to analyze.
All three currents in the zero sequence set are in phase with each other. That is the reason that triplens harmonics add up in the neutral. The first harmonic (i.e., 60 hertz) is positive sequence. The second harmonic (i.e., 2 times 60 hertz) is negative sequence. The third harmonic (i.e., 3 times 60 hertz) is zero sequence. Then it starts over with the fourth, fifth, sixth, and so on. Every third harmonic (3, 6, 9, 12, 15, 18, etc.) is a zero sequence set, and each will have its zero sequence currents in phase with each other, so they add up to three times what any one of them would be by itself.
Two more notes: First, the name of the tool I described above is "Symmetrical Components." Secondly, in a power system, all the even harmonics (2nd, 4th, 6th, etc., or equivalently, 120 hertz, 240 hertz, 360 hertz, etc.) disappear. That is the reason that "triplens" is generally defined as the 3rd, 9th, 15th, 21st, and so forth.
Re: harmonics??
Let me attempt to provide an explanation of the way 3rd harmonic currents add:
The period of a 60Hz sine wave is 1/60 = 16.7 milliseconds (360 degrees). The fundamental currents are 120 degrees apart (5.6 milliseconds).
The period of the 3rd harmonics is 1/180 = 5.6 illiseconds.
Simply put, three 180 Hz waves are separated from each other by 5.6 milliseconds which is 1 period at 180 Hz. That is, they are in phase with each other (180 Hz) and add accordingly instead of cancelling each other.
Please note that phase angles only apply to sinusoids of the same frequency.
[ December 08, 2005, 09:06 AM: Message edited by: rattus ]
Let me attempt to provide an explanation of the way 3rd harmonic currents add:
The period of a 60Hz sine wave is 1/60 = 16.7 milliseconds (360 degrees). The fundamental currents are 120 degrees apart (5.6 milliseconds).
The period of the 3rd harmonics is 1/180 = 5.6 illiseconds.
Simply put, three 180 Hz waves are separated from each other by 5.6 milliseconds which is 1 period at 180 Hz. That is, they are in phase with each other (180 Hz) and add accordingly instead of cancelling each other.
Please note that phase angles only apply to sinusoids of the same frequency.
[ December 08, 2005, 09:06 AM: Message edited by: rattus ]
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: harmonics??
Not always. It depends on the component that is causing the harmonics in the first place. I have seen plots of harmonic content in which some of the higher order harmonics had a higher magnitude than some of the lower order harmonics.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: harmonics??
Theoretically it could be up to (10)(1.73)=17.3 amps.
Re: harmonics??
jb, look at it this way:
A non-linear load driven by a pure sinusoid results in a non-sinusoidal current. On a graph, the wave might appear lumpy. Some smart guy named Fourier figured out that this lumpy wave could be represented mathematically by a series (Fourier Series) of sinusoids at different multiples of the fundamental frequency. That is, 1x60, 2x60, 3x60, 4x60, 5x60,....on out to infinity. These multiples of 60Hz are called harmonic frequencies or simply harmonics.
The amplitudes of the various harmonics change with the shape of the wave. The amplitudes of some harmonics might be zero. We are especially concerned with the 3rd harmonics in a 3-phase wye because they add instead of cancel.
In a word, we break the lumpy wave into harmonic frequencies in order to perform mathematical analyses. It is more complicated than this, but perhaps you get the idea.
jb, look at it this way:
A non-linear load driven by a pure sinusoid results in a non-sinusoidal current. On a graph, the wave might appear lumpy. Some smart guy named Fourier figured out that this lumpy wave could be represented mathematically by a series (Fourier Series) of sinusoids at different multiples of the fundamental frequency. That is, 1x60, 2x60, 3x60, 4x60, 5x60,....on out to infinity. These multiples of 60Hz are called harmonic frequencies or simply harmonics.
The amplitudes of the various harmonics change with the shape of the wave. The amplitudes of some harmonics might be zero. We are especially concerned with the 3rd harmonics in a 3-phase wye because they add instead of cancel.
In a word, we break the lumpy wave into harmonic frequencies in order to perform mathematical analyses. It is more complicated than this, but perhaps you get the idea.
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