Exciting Current harmonics in Transformer

[I_am_learning]

Due to non-linearity of magnetic core of large transformer, Which is excited by pure sinusoidal voltage source, the exciting current isn't perfectly sinusoidal. It can be thought to be composed of pure sinusoidal plus mainly 3rd harmonic component.
But does this current flows in the primary side alone or both side ? If both side, what is its distribution (i.e. Primary / Secondary)

Also, when the transformer primary is driven by pure sinusoidal current source, The core flux don't be pure sinusoidal but will have major 3rd harmonics.
So, this will create non sinusoidal Voltage with major 3rd harmonics, both in the primary and secondary in this case. Isn't it?

 

[berkeman]

Due to non-linearity of magnetic core of large transformer, Which is excited by pure sinusoidal voltage source, the exciting current isn't perfectly sinusoidal. It can be thought to be composed of pure sinusoidal plus mainly 3rd harmonic component.
But does this current flows in the primary side alone or both side ? If both side, what is its distribution (i.e. Primary / Secondary)

Also, when the transformer primary is driven by pure sinusoidal current source, The core flux don't be pure sinusoidal but will have major 3rd harmonics.
So, this will create non sinusoidal Voltage with major 3rd harmonics, both in the primary and secondary in this case. Isn't it?

Are you referring to AC Mains power transformers (50/60Hz)? Because you don't want to be overdriving a signal transformer into its non-linear area.

The main non-linearity that I'm aware of is the hysteresis curve for mu. Since the value of B depends on the mu as you drive (and overdrive) the core, the B field is shared around the whole core, so yes, it affects both primary and secondary. At the primary side, I think you'll see it more as a modulation of the transformed load impedance.

 

[I_am_learning]

Are you referring to AC Mains power transformers (50/60Hz)? Because you don't want to be overdriving a signal transformer into its non-linear area.

Yes I am talking about them. No, am not talking about the non-linearity due to saturation due to overdrive, I am talking about inherent non-linearity of H Vs B due to hysteresis. So, when driven by sinusoidal current source, Voltage will have harmonics and when driven by sinusoidal voltage source current will have harmonics.

I just wanted to conform that in both case, the harmonics are present in both primary and secondary or only in primary.

 

[Carl Pugh]

If transformer does not have a load connected, then obviously the harmonic current only flows in the primary.

"So, when driven by sinusoidal current source, voltage will have harmonics" I agree. Also the transformer primary and secondary voltage will have harmonics.

"when driven by sinusoidal voltage", "source current will have harmonics" I agree. Also to a first approximation the primary and secondary voltages will not have harmonics.

 

[I_am_learning]

"when driven by sinusoidal voltage", "source current will have harmonics" I agree. Also to a first approximation the primary and secondary voltages will not have harmonics.

Everything you said was fine, but there was sloppy wording. Actually I meant like this
"when driven by sinusoidal voltage source", "currents (both Primary and Secondary) will have harmonics"
Do you agree on this too?

(quite interesting to note that the difference is created by a missing comma "...voltage source, currents...")

 

[Carl Pugh]

"when driven by sinusoidal voltage source", "currents (both Primary and Secondary) will have harmonics"

If there is no load on secondary, obviously there will be no current in secondary and so no harmonics in secondary current.

To a first approximation, the voltage on a transformer secondary is the same as the voltage on the transformer primary. So if the voltage on a transformer primary is sinusoidal, then the voltage on the transformer secondary will be sinusoidal. If the voltage on the transformer secondary is sinusoidal, then there will not be any harmonics in the transformer secondary current.
There will be harmonic currents in the transformer primary.

Disclaimer: Transformers are imperfect devices and there will actually be a small amount of harmonics in the transformer secondary current.
The harmonics in the primary current will cause a small voltage drop in the conductors used in the primary. This small voltage drop will cause the transformer primary voltage to be distorted which will cause the secondary voltage to be distorted.

 

[cabraham]

The exciting current only exists in the primary for a single phase xfmr. So the odd order harmonics in the exciting current are confined to the primary alone, regardless of whether or not the secondary is loaded.

With 3 phase units, things are different. The odd order harmonics that are not multiples of 3, i.e. 5th, 7th, 11th, etc., are present only in the primary, regardless of pri-sec winding configuration (wye-delta, delta-delta, etc.), & regardless of loading. But the 3rd harmonic, & higher multiples, i.e. 9th, 15th, etc., will exist in the delta connected secondary if a wye-connected primary does not provide a low impedance path for them.

An example would be a 3 wire wye (open neutral) primary with a delta secondary. The 5th, 7th, 11th, etc. harmonics of the exciting current can & do exist in the primary w/o a neutral connection regardless of secondary loading, since they cancel in phase at the Y neutral point of the xfmr primary. But the 3rd, 9th, etc. harmonics cannot exist on the primary side w/o the neutral of the xfmr connected to the neutral of the generator. These harmonics exist in the delta connected secondary windings, regardless of loading.

One can examine many combinations of primary & secondary winding configurations, as well as tertiaries & multiple secondary configurations. Utility companies have big manuels which detail these harmonic currents per each configuration. Hopefully what I've given you can get you started. Did I help?

Claude

 

[I_am_learning]

The exciting current only exists in the primary for a single phase xfmr. So the odd order harmonics in the exciting current are confined to the primary alone, regardless of whether or not the secondary is loaded.

With 3 phase units, things are different. The odd order harmonics that are not multiples of 3, i.e. 5th, 7th, 11th, etc., are present only in the primary, regardless of pri-sec winding configuration (wye-delta, delta-delta, etc.), & regardless of loading. But the 3rd harmonic, & higher multiples, i.e. 9th, 15th, etc., will exist in the delta connected secondary if a wye-connected primary does not provide a low impedance path for them.

An example would be a 3 wire wye (open neutral) primary with a delta secondary. The 5th, 7th, 11th, etc. harmonics of the exciting current can & do exist in the primary w/o a neutral connection regardless of secondary loading, since they cancel in phase at the Y neutral point of the xfmr primary. But the 3rd, 9th, etc. harmonics cannot exist on the primary side w/o the neutral of the xfmr connected to the neutral of the generator. These harmonics exist in the delta connected secondary windings, regardless of loading.

One can examine many combinations of primary & secondary winding configurations, as well as tertiaries & multiple secondary configurations. Utility companies have big manuels which detail these harmonic currents per each configuration. Hopefully what I've given you can get you started. Did I help?

Claude

YEAH, you have helped and got me started.
I think in open neutral, Yd connection, since the Y connection doesn't allow for 3rd harmonic current to flow, so we get third harmonic voltage in the primary instead! this voltage will be induced in the secondary winding and will create 3rd harmonic current.

Is this how it works?

 

[cabraham]

YEAH, you have helped and got me started.
I think in open neutral, Yd connection, since the Y connection doesn't allow for 3rd harmonic current to flow, so we get third harmonic voltage in the primary instead! this voltage will be induced in the secondary winding and will create 3rd harmonic current.

Is this how it works?

No, with a 3 wire Y connection (neutral open) on the primary, & a delta connected secondary, the 3rd harmonic of the exciting current is in the delta secondary windings, circulating within the closed path of the delta, without appearing on the line currents or load.

Since the 3rd harmonic current is in the delta secondary, there is no 3rd harmonic voltage on the primary nor the secondary. The core magnetic flux is reasonably free from harmonic distortion, 3rd, 5th, etc. As long as the 3rd harmonic (as well as 9th, 15th, etc.) current flows in either the primary or secondary windings, the core flux as well the the voltages on the primary & secondary will be harmonic-free.

The exception is that of "core type xfmrs", wound on a single 3-legged "E" core. This type can retain harmonic free flux/voltage w/o a neutral connection and w/o a delta. Again, power systems manuals should detail this. Make sense?

CLaude

 

[m.s.j]

The exciting current only exists in the primary for a single phase xfmr. So the odd order harmonics in the exciting current are confined to the primary alone, regardless of whether or not the secondary is loaded.

The current transformer is one of single phase transformer types, and effects of magnetizing current in secondary side (protective relays) is one of important terms in electrical protection systems. For similar discussion you can refer to http://electrical-riddles.com/topic.php?lang=en&cat=23&topic=620"

 

[cabraham]

The current transformer is one of single phase transformer types, and effects of magnetizing current in secondary side (protective relays) is one of important terms in electrical protection systems. For similar discussion you can refer to http://electrical-riddles.com/topic.php?lang=en&cat=23&topic=620"

Sorry, I beg to differ. In a single phase current xfmr, the magnetizing current cannot be in the secondary, & must be in the primary. The following illustrates the reasoning.

If the secondary is open circuited, a dangerous condition which should never be allowed in practice, the secondary voltage rises to a dangerously high value. The flux in the core as well as the primary & secondary voltages cannot exist w/o magnetizing current. The secondary is open, but the primary carries current.

When the secondary is loaded w/ a low resistance value, secondary current results in a magnetic flux which opposes the existing core flux. This is a counter-mmf which reduces the net flux in the core. The amp-turns in the secondary almost cancel that in the primary. The slight unbalance is the magnetizing current needed to provide the core flux.

If the secondary is opened (a dangerous condition), there is no secondary current. Hence the counter-mmf is zero, & all the primary current is unbalanced & becomes magnetizing current. This results in a drastic increase in core flux as well as primary & secondary voltages reaching much higher values, & shock hazard is present.

The magnetizing current in a current xfmr is in the primary winding, never in the secondary winding. Any energy conversion text will affirm the same. The "electrical riddles" site provides some interesting discussions & generally good info. However, every single answer is not subject to peer review. You cannot rely 100% on every answer. Even peer-reviewed textbooks used in the best unis around the world have errors which prompt errata sheets being published. No website is above reproach.

I would advise you to carefully examine a contrarian viewpoint before rebuking a well established principle. If the magnetizing current in a CT was really in the secondary, every peer-reviewed text would say so. The fact they don't should give you reason to have doubts about a website that states otherwise. It's just an honest mistake, I'm not dissing the web site. I've posted there, & for the most part it is a pretty good site.

Claude

 

[m.s.j]

If the magnetizing current in a CT was really in the secondary, every peer-reviewed text would say so

Please refer to attached page.

Thank you very much my dear friend.

 

[yungman]

I am no expert in transformer by any stretch. Claude is the man for that. I just want to put in my 2 cents in the eye of a guitarist. The output transformer is very important. It definitely change the characteristic of the sound by adding harmonics. I notice a very famous brand amplifiers...Mesa Boogie that made famous by Carlos Santanna which he is still using it. I looked at a few of their amps, the output transformer is very small compare to other brands. I bet they are pushing the core into saturation to generate the desired harmonics.

 

[Adjuster]

The output transformer in an electron tube audio amplifier is rather a special case. Here the impedance driving the transformer primary will be relatively high, compared to that nomally seen by a voltage transformer of similar rating used for AC mains distribution. As a result, harmonic content in the audio transformer magnetizing current can have a more appreciable effect on the primary voltage waveform, because of the relatively large voltage which can be developed in the driving impedance. One of the factors adding to the size, weight and cost of tube amplifiers is the need for a large enough output transformers to handle the required power levels, particularly at bass frequencies.

In addition, relatively small levels of harmonics will be significant for audio, because of the way in which they can affect the musical result. That said, the distortion products normally account for only a small percentage of the total power in a good design, when run within its limits. The distortion will naturally increase if the transformer core becomes saturated, possibly contributing to the sound of an amplifier when it is driven hard, as mentioned by the last contributor.

 

[I_am_learning]

m.s.j Vs Cabraham, with whom do I go with?

 

[sophiecentaur]

@cabraham
"If the secondary is opened (a dangerous condition), there is no secondary current. Hence the counter-mmf is zero, & all the primary current is unbalanced & becomes magnetizing current. This results in a drastic increase in core flux as well as primary & secondary voltages reaching much higher values, & shock hazard is present."

That's because the transformer is being driven from a valve anode (pretty much a Current Source and not a Voltage Source). This it not what the OP is discussing.

If you think of the secondary induced voltage as being proportional to the rate of change of flux in the core (related to rate of change of Primary Current) then this will consequently include harmonic content (boosted by the fact that the harmonics change at a higher rate. For small loads (i.e. high resistance), the secondary current will also contain harmonics, I reckon.

 

[cabraham]

@cabraham
"If the secondary is opened (a dangerous condition), there is no secondary current. Hence the counter-mmf is zero, & all the primary current is unbalanced & becomes magnetizing current. This results in a drastic increase in core flux as well as primary & secondary voltages reaching much higher values, & shock hazard is present."

That's because the transformer is being driven from a valve anode (pretty much a Current Source and not a Voltage Source). This it not what the OP is discussing.

If you think of the secondary induced voltage as being proportional to the rate of change of flux in the core (related to rate of change of Primary Current) then this will consequently include harmonic content (boosted by the fact that the harmonics change at a higher rate. For small loads (i.e. high resistance), the secondary current will also contain harmonics, I reckon.

Yes I know that is not what the OP is discussing. But "m.s.j." in post #10, brought up the CT as a counter-example to my assertion that in single phase xfmrs the exciting current, fundamental plus all harmonics, is present in the primary exclusively. Member "m.s.j." states that in a CT, single phase, that harmonics exist in the secondary.

This topic does stray a bit from the OP, since the OP specified constant voltage source excitation, with a low source impedance. This OP stipulation excludes CTs from the discussion. But the CT was brought into the thread, with the argument presented that the secondary of a CT carries exciting current, even though it is single phase, rebuking my statement that single phase xfmrs have all exciting current in the primary.

I stand by what I already stated. The text reference provided by m.s.j. states that the primary amp-turns of a CT must equal the secondary amp-turns including exciting current, which said reference details as "Ie", which gets multiplied by "N2", the secondary number of turns.

Sometimes, even a good text has errors. Most of my uni texts included errata sheets. The trouble with the referenced text page is as follows. If the secondary is opened, then the secondary amp-turns is zero. The ref text sums the secondary load amp-turns with the (secondary) exciting amp-turns to equal the primary amp-turns. But in the open secondary case, the secondary amp-turns is zero. Hence the primary amp-turns are zero as well.

This is not correct. Anyone can set up a CT, & measure & observe. The primary amp-turns is non-zero. The exciting current is on the primary. In addition, the load current is reflected back to the primary & adds phasorially with the exciting current.

When the secondary is open, the secondary amp-turns is zero, since load current is zero. The load component of the primary current is likewise zero. But the exciting component of the primary amp-turns is non-zero. The entire primary current is magnetizing current. Since the secondary current is zero, there is no counter-mmf from the secondary. Hence the core flux & both voltages, pri & sec, reach their maximum value.

This happens because the exciting current is in the primary. When the secondary is terminated in a low impedance, the magnetic flux due to the secondary current opposes the original core flux, reducing its value. As a result the pri & sec voltages are quite small.

Anyone intimately familiar with xfmr theory & e/m fields can affirm this. BR.

Claude

 

[sophiecentaur]

I often think that some of these threads need a 'chairperson' to maintain the direction of flow. This isn't because I'm some kind of puritan, it's because I can see the danger of people getting the wrong end of the stick as they skim through a thread, picking bits at random. We've all done it, I'm sure.
That's why I pointed out the problem and apparent inconsistency.

 

[m.s.j]

Dear Cabraham,

What is primary or secondary side in a transformer? How can you specify the primary and secondary side of a power transformer in one transmission system which the direction flow of energy can be variable in it? Whether active power direction can determine the primary/secondary of transformers or reactive power flow direction?
I think the magnetizing current exist in all windings and metallic path which can be involved to magnetic flux of transformer generally. In transformer with open circuit in secondary, the exciting current exist in primary winding and metallic body of transformer.

For similar discussion you can refer to http://electrical-riddles.com/topic.php?lang=en&cat=23&topic=620"

 

[sophiecentaur]

? Isn't the primary side regarded as being the one that energy flows in from? In most circs, it's pretty easy to determine which is which.
OK, when you are talking about a transformer bridge circuit, that may be a bit hard to define but that's not what the thread is about, is it?

At a deeper level, of course, you don't even need to introduce magnetism into any problem if you really don't want to. That can involve standing up in a hammock, though.

 

[cabraham]

Dear Cabraham,

What is primary or secondary side in a transformer? How can you specify the primary and secondary side of a power transformer in one transmission system which the direction flow of energy can be variable in it? Whether active power direction can determine the primary/secondary of transformers or reactive power flow direction?
I think the magnetizing current exist in all windings and metallic path which can be involved to magnetic flux of transformer generally. In transformer with open circuit in secondary, the exciting current exist in primary winding and metallic body of transformer.

For similar discussion you can refer to http://electrical-riddles.com/topic.php?lang=en&cat=23&topic=620"

The magnetizing current in a single phase xfmr can only exist in the primary, as far as I know. If the power source energizing the primary is constant voltage, then in order to maintain pure fundamental flux waveforms (single freq sinusoid), all necessary harmonic currents must have a low impedance path. If the power source connected to the primary is constant current, then the flux cannot be sinusoidal, as it will contain harmonics.

But in a current xfmr, we are not concerned w/ flux & voltage waveforms being harmonic free. We use the CT to measure large currents using a small ammeter, & obtain isolation from the power line to assure safety. If the CT primary carries 100 amp @ 50/60 Hz freq, the CT secondary will carry a scaled down current @ 50/60 Hz. The currents in both the primary & secondary of the CT will be low in harmonic content, but the flux & voltages will carry harmonics.

This is ok because we use the CT to read current not voltage. The voltage across the primary is extremely small, as well as the secondary.

With voltage xfmrs, exciting current must carry all harmonics to keep the flux pure, single freq sinusoid. This is needed so that the secondary voltage is harmonic free. In a single phase xfmr, the primary has 2 leads, providing a low impedance path for all harmonics. With 3 phase, however, this is not the case.

To keep pure flux, the exciting current must contain the fundamental, herein called the 1st, plus the harmonics 3rd, 5th, 7th, 9th, 11th, etc. If the primary is delta connected, all harmonics are present in the exciting current, which is carried by the primary. The 3 lines leading into the delta primary carry the 1st, 5th, 7th, 11th, etc. These are the harmonics not divisible by 3, aka the "non-triplen" harmonics.

The "triplen harmonics", which are divisible by 3, i.e. 3rd, 9th, 15th, etc. are carried by the delta primary windings. They circulate inside the closed loop of the delta primary. The flux & voltages are harmonic free.

If the pri & sec are both Y connected, & the primary Y has a 4th wire, i.e. the neutral of the source is connected to the neutral of the xfmr Y primary w/ low impedance, than the following occurs.

The non-triplens are carried in the 3 lines, & the triplens are carried in the 3 lines & the neutral. The neutral carries the sum of all 3 line triplen currents. The flux & voltages are harmonic free.

With a Y-Y with no neutral connection, or very high impedance connection, the non-triplens flow in the 3 lines, & the triplens do not flow at all, since there is no neutral path. The flux is missing these triplen components & waveform distortion exists. The distortion consists of the missing triplen frequencies from the exciting current. The exception to this behavior is the 3-legged "E core" type of 3 phase xfmr. It behaves like a Y-delta.

In a Y-delta w/o a neutral connection, the non-triplens are carried in the 3 lines, & the triplens circulate inside the delta secondary closed loop. The flux is harmonic free.

This is detailed in any utility power reference book. I've just given you what the power companies have known for almost a century or more. It's no secret. If any further explanation is needed, I can help. But I've given everyone here plenty. I ask people seeking answers to try to get them on their own. I can't just write a book here. BR.

Claude

 

[m.s.j]

I can't just write a book here.

Fact is simple, but finding it is difficult. Please go back to basic and try to answer my questions, I think you don't like to leave your traditional belief.

 

[sophiecentaur]

Fact is simple, but finding it is difficult. Please go back to basic and try to answer my questions, I think you don't like to leave your traditional belief.

Are you telling us or asking us? Is this some sort of exam question, with you as the examiner?

 

[cabraham]

Fact is simple, but finding it is difficult. Please go back to basic and try to answer my questions, I think you don't like to leave your traditional belief.

Of course I don't like to leave my traditional belief, when every utility power company, machine producer, xfmr producer, etc. has already researched this topic & published the results for all to see.

Your questions were answered over 100 years ago by people better than me. The amount of money & man-hours spent on researching xfmrs is staggering. When you say "traditional beliefs", you seem to be suggesting that there is something wrong with them.

Please enlighten me where my traditional beliefs are wrong. Take a specific question & put it up for discussion. If an answer I've already given is wrong, tell me why it's wrong. The best way to attack these questions is with Maxwell's equations.

Ampere's law, one of Maxwell's, states that:

integral H*dl = Ni

The "Ni" represents magnetomotive force, or "mmf". Thus to obtain an H field, amp-turns are required. If a load is placed across a secondary winding, the current in the sec produces an mmf/emf/flux whose polarity is opposite to the original. This is stated in the law of Lenz.

The primary voltage begins to drop, but since a voltage xfmr is excited from a constant voltage source, said source outputs more current to the primary to counter the flux of the secondary current. The secondary amp-turns is balanced by that of the primary, plus the exciting current is in the primary, & is not balanced. This exciting current is needed to keep the core flux.

In a CT, with the secondary open, the flux, pri & sec voltages are all at maximum value. Without exciting current there can be no flux. The secondary is open, so where is the exciting current?

Amperes law says that for a given integral of H*dl, the line integral of the H field around the closed magnetic path, there has to be a non-zero amp-turns. Only the primary can & does provide said amp-turns. The secondary has no current so there is no flux cancellation. All of the primary amp-turns is unbalanced making all amp-turns exciting current. Maximum flux/voltage are the result.

Every experiment has affirmed the same. I was a graduate teaching asst in the late 70's in charge of electric machines lab. I personally conducted said experiments as a senior student in EE, & as a grad asst. In the 70's this stuff was very old, established, archaic, been there done that, so what else is new. No mysteries exist here.

Again, using Ampere's law, how can the secondary of a CT provide exciting current? Please use AL and/or other Maxwell eqns like Faraday's law. I presume that the CT is the point you are disputing. If you disagree with my 3 phase explanation, let me know what needs to be clarified. BR.

Claude

 

[Carl Pugh]

If you are seriously interested in transformers, three of the best books are:

Transformer Engineering; Blume, Boyajian, Camilli, Lennox, Minneci, Montsinger, (General Electric)

Transformers for the Electric Power Industry; Richard L. Bean, Nicholas Chackan Jr., Harold R. Moore, Edward C. Wentz (Westinghouse Electric Corporation)

Electric Transformers and Circuits; Reuben Lee, Leo Wilson, Charles E. Carter

 

[cabraham]

If you are seriously interested in transformers, three of the best books are:

Transformer Engineering; Blume, Boyajian, Camilli, Lennox, Minneci, Montsinger, (General Electric)

Transformers for the Electric Power Industry; Richard L. Bean, Nicholas Chackan Jr., Harold R. Moore, Edward C. Wentz (Westinghouse Electric Corporation)

Electric Transformers and Circuits; Reuben Lee, Leo Wilson, Charles E. Carter

Agreed. The last book you listed is of interest to me in particular. From 1981-1985, I worked in the xfmr design group at Westinghouse Defense located in Balto. Md. The group I worked in was chaired by Reuben Lee for quite some time. By the time I arrived, Reuben had already retired.

Leo Wilson was my supervisor, & was a very good authority on xfmrs. Charlie Carter was my colleague, his desk was adjacent to mine. Charlie & I discussed the updates he & Leo were adding to Reuben's original book. Charlie & I attended the same church. He also was a very good authority on xfmrs.

I learned a lot from them. I hated to leave Westinghouse but I longed to go back to my native NE Ohio. Leo, Charlie, & I got along very well, & I cherish those years. BR.

Claude

 

[m.s.j]

Dear cabraham,

Which your reference book agree your idea (magnetizing current exist just in one side of loaded transformer) clearly. As you say, you know transformer inrush current. Whether it exists in primary side of transformer exclusively? I say No!
All phenomena which related to linkage flux ( magnetizing current) can be observed in both primary and secondary side of transformer, therefore magnetizing current exist in all closed circuit which involved to magnetizing flux.
Meanwhile I don't know why you and your friends in U.S or Europe don't like my website (http://electrical-riddles.com), however can I record currently discussion in it?

 

[cabraham]

Dear cabraham,

Which your reference book agree your idea (magnetizing current exist just in one side of loaded transformer) clearly. As you say, you know transformer inrush current. Whether it exists in primary side of transformer exclusively? I say No!
All phenomena which related to linkage flux ( magnetizing current) can be observed in both primary and secondary side of transformer, therefore magnetizing current exist in all closed circuit which involved to magnetizing flux.
Meanwhile I don't know why you and your friends in U.S or Europe don't like my website (http://electrical-riddles.com), however can I record currently discussion in it?

Honestly, I like your web site. It is fun, informative, friendly, & challenging. I didn't know it is yours, but I'd like to say thank you for giving us a good forum.

Due to the different languages in the world, & this being a global forum, maybe it's a language barrier/communication issue. Let's first define the term "exciting curent".

According to Ampere's law, AL for short, for a given "H" field & path length "l", the integral of H*dl must equal the net amp-turns. By *"net"* amp-turns, I mean not balanced by the other winding. In 3 phase units, this can mean that some exciting current harmonics are in the primary, with others in the secondary. I've already detailed this issue.

In single phase units, I'm not aware of this taking place. I never said it cannot happen, I just remarked that the example of a CT does not display exciting current in the secondary.

The primary is by definition the winding connected to the active source supplying power to the xfmr unit. In a CT, the primary is connected in series with the power source & load. A closed path is formed, & the CT provides a low burden, i.e. voltage drop, provided that the terminating impedance on the CT secondary presents an impedance to the CT primary much smaller than the load impedance.

Since the primary is connected to the load & power source circuit, the reactive power to energize & de-energize the core comes from the primary side. Can specific harmonics flow in the secondary, & not in the primary?

In 3 phase units, the triplen harmonics cannot flow in the primary if a high impedance at those frequencies prevents it. A Y primary w/ an open or very high impedance between source & xfmr primary neutrals presents a very high impedance to triplen harmonic currents. As soon as powered up, the flux will begin to distort w/ triplens in the flux waveform.

What happens next? The secondary has a closed path, a delta, w/ low impedance to triplen harmonic currents. The windings have a small wire resistance, plus a small reactance due to leakage inductance. The triplen flux distortion produces a triplen voltage in the secondary/primary. This small voltage across the low impedance of the delta results in triplen currents circulating in the delta.

This triplen current is exciting current, not load current, since it does not get counter-balanced in the primary. The primary cannot draw triplen current to cancel the triplen counter-flux because it has a high impedance to triplens. The triplen delta secondary current generates an mmf opposite in polarity, tending to reduce the triplen core flux.

The equilibrium condition is attained when the delta triplen current exactly cancels the triplen flux. Hence the flux is harmonic free because the triplen flux distortion is canceled by the delta secondary current triplen counter-mmf.

That is why the delta secondary current is regarded as a part of the exciting current. It does not find amp-turns balancing in the primary, & does not power the load, & is independent of loading.

With a CT this condition does not exist. A single phase CT has 2 leads into the primary. These 2 leads provide a low impedance path for exciting current. Are there current harmonics in the secondary? If so, would that be classified as "exciting current"? Maybe we need to examine this some more.

The mmf is pure sine, owing to the source-load circuit, & the CT low input impedance. The sine current results in a sine (no harmonics) mmf. The core flux will have a pure sine "H", & a "B" with harmonic distortion. So the voltages, Vsec & Vpri, will carry harmonics. But the voltage involved is well under 1% of the source voltage.

Hence the harmonics will be well under that 1%, likely 0.2% or so. This will result in very small harmonic current on the secondary side. But these harmonic secondary currents are only slightly canceled by the primary, since it is driven from a high impedance (relatively speaking) network, which looks like a constant current source. The secondary harmonics will generate a counter mmf, which tends to reduce the flux harmonics until equilibrium is reached.

But we are talking a fraction of a percent. You could argue that this secondary harmonic current is mostly unbalanced by the primary mmf, & hence can be considered "exciting current". I'll agree that there are tiny harmonics in the CT secondary unbalanced by primary mmf.

But consider the conventional voltage xfmr. The primary exciting current encounters a very low impedance path in the primary, at the fundamental freq, the 1st. But leakage inductance in said primary presents an impedance for the 3rd harmonic 3 times higher than the 1st. The 5th harmonic of the exciting current sees 5 times higher leakage reactance than the 1st.

Since harmonics of increasing order encounter a continuously increasing impedance due to increased leakage reactance in the primary, the core flux higher harmonics will be almost, but not completely suppressed. Hence the flux will be very slightly distorted. This harmonic voltage in the secondary will give rise to secondary harmonic currents when loaded. These currents are load dependent. Hence I don't think they are exciting current. They tend to cancel the flux harmonics, though, but they get balanced by primary mmf, but not completely due to secondary leakage inductance.

Hence, a small fraction of the secondary current could be classified as exciting current, but it is a very small fraction. Whan harmonic currents flow in the primary, we cannot expect the core flux to carry zero harmonic content. Every primary has wire resistance & leakage reactance. Even w/ superconducting windings, leakage reactance still exists.

How big in value must a secondary harmonic current be to be considered "exciting current"? It does exist, in the CT, as well as VT. So I see how one can make the case. I just feel that when a quantity is large enough to impact performance, safety, etc., it needs to be considered. If triplen harmonics in 3 phase xfmrs were inadequately suppressed, insulation could be stressed to the point of shortening life, & customer voltages have large peaks due to distortion.

Including secondary delta windings suppresses these harmonics, curing the ill effects. With single phase units, I'll concede that there is indeed some level of current harmonics in the secondary uncancelled by primary mmf. But is it enough to stress insulation, distort waveforms, reduce safety, etc.? If not, then it is clear that the significant current which suppresses harmonics is found in the primary. The secondary harmonics are not strong enough to adequately suppress flux distortion.

Did I get it right? Anything else that needs mentioning?

Claude

 

[Carl Pugh]

Transformer theory generally says that the exciting current only flows in the primary.
However there are some cases where it could be argued that exciting current flows in the secondary.

One interesting case of this is with transformers with a high voltage secondary. (Hundreds of kilovolts)
During test, the exciting current starts to gradually increase as expected.
However when maybe 1/2 rated voltage is reached, the exciting current starts to decrease.
The exciting current may decrease to maybe 1/2 the previously current. (Then the exciting current starts increasing normally and the transformer tests OK.
The only explanation for this is that the capacitance of the secondary is what is measured originally and when the inductive core current starts to increase, the two currents are 180 degrees out of phase and cancel.

Carl

 

[cabraham]

Transformer theory generally says that the exciting current only flows in the primary.
However there are some cases where it could be argued that exciting current flows in the secondary.

One interesting case of this is with transformers with a high voltage secondary. (Hundreds of kilovolts)
During test, the exciting current starts to gradually increase as expected.
However when maybe 1/2 rated voltage is reached, the exciting current starts to decrease.
The exciting current may decrease to maybe 1/2 the previously current. (Then the exciting current starts increasing normally and the transformer tests OK.
The only explanation for this is that the capacitance of the secondary is what is measured originally and when the inductive core current starts to increase, the two currents are 180 degrees out of phase and cancel.

Carl

Current due to capacitance is not exciting current. Exciting current is that which is necessary to excite the core & support the flux in accordance w/ the B-H curve of the magnetic core material. Capacitive displacement current does not follow the core B-H curve & is NOT exciting current. BR.

Claude

 

[m.s.j]

Please note to fundamental transformer equation as attached page. As you see, the magnetizing inductance (L1h = w1/w1.M) which related to magnetizing flux and current is related to primary and secondary mutual inductance. Therefore transformer magnetizing current effects is shared between primary and secondary windings (in closed circuit situation) and not related to primary winding exclusively.

 

[Carl Pugh]

My posting was not clear.
There was no intention to imply that capacitive current was transformer exciting current.
Please except my apology.

What was intended was that:
Transformers generally have exciting current that is partly inductive. This inductive current is much higher if the transformer core is poorly assembled or is designed with gaps in the core.
This inductive current can be canceled by capacitive current in the load.

When testing high voltage transformers, the total volt-ampere required can be less than that of the core alone.

It should be easy to do a demonstration where the exciting current of a transformer decreases when a capacitive load is applied.
The test could be done on a small transformer which has a gap deliberately placed in the core.

I wish to emphasize that I'm not implying that the secondary current in any way can supply watts to energize the transformer core.
I'm only saying that the secondary can provide some or all of the reactive current that is required to energize the transformer.

 

[sophiecentaur]

Wouldn't it be nearer to the truth to say that magnetising current is just a figment, used for calculations. Why should we expect more than that?
"What really happens?" is a question that is asked far too often, as if the answer would actually help.

 

[cabraham]

Please note to fundamental transformer equation as attached page. As you see, the magnetizing inductance (L1h = w1/w1.M) which related to magnetizing flux and current is related to primary and secondary mutual inductance. Therefore transformer magnetizing current effects is shared between primary and secondary windings (in closed circuit situation) and not related to primary winding exclusively.

But the effect of magnetizing current upon the secondary does not mean that there is magnetizing current in the secondary. The mag current establishes the core flux, making secondary voltage & current possible. But there is very little mag current in the secondary, less than 1% per my previous post.

I detailed my position exhaustively, but nobody can address where, if at all, I went wrong. I feel that since my research involves Maxwell's eqns, a valid rebuttal must do the same, pointing out where I erred. To simply say that because the primary mag current exerts an influence upon the secondary, does not infer that mag current actually flows in the secondary, not a lot anyway.

Again, if you measure the secondary current unloaded, you will indeed find a very small component of harmonics for the reasons I stated earlier. One can correctly claim that a very tiny fraction of mag current exists in the secondary. But we cannot rely on that secondary component to do much. Large flux distortion results in high insulation stress, abnormal high voltage in a consumer supply, shock hazard, etc. The mag current in the primary suppresses more than 99% of said ditortion, the secondary less than 1%.

Again, I don't dispute that a small fraction of mag current harmonics are indeed present in the secondary, but not enough to help. We rely on the primary mag current to keep the flux pure sine. BR.

Claude

 

[sophiecentaur]

How can there be ANY current in the secondary if it's unloaded? Are you introducing some self capacitance too?