Current transformer

Yes - most on-line encylopaedia facilities can do this.

A current transformer is really the same basic thing as a voltage transformer, they are just used differently and usually only have 1 or very few turns on the primary. In any transformer the voltage ratio is the inverse of the current ratio. If the truns ratio Np:Ns is 1 to 100 you the secondary voltage will be 100 times greater than the primary voltage but the secondary current will be 100 time less than the primary current making Pin = Pout a true statement. When you use a current transformer you force a current through the primary and get a multiple of the current in the secondary. That secondary current is impressed upon a load to generate a voltage that represents a multiple of the current. The primary will have a voltage equal to the secondary voltage time Np/Ns. This happens because transformers reflect the load impedance back to the primary with ZP = Zs * (Np/Ns)^2. Even though it seems like the current transformer is lossless you do get a small voltage on the primary, nothing if free.

Current Transformers have less than half turn in primary (just passing conductor wire thru hole) and thousands of turns in the secondary so if your physics is OK then you know what is it in step up or step down transformer design.

Confused? Think again.

Current transormers are typically torroids. In general a wire passing through the center of a torroid pretty much acts as a full turn, it does not have to wrap completely around the core. I have built many current transformers and I can build one with two turns if I want it will still work I just need to be aware of its primary inductance, the reflected load and how they effect the application.

If you wrap one turn in primary then will you count two turns?

Definition of tuns comes only after you really wrap around turns and not for just passing a conductor which is partial.

Most important thing here to note is that the magnetic flux generated is due to differential voltage across the conductor. Hence, current and impedance of the conductor counts a great deal. If you take this wire in primary then output will be high for same current and same turn ratio. Thick primary wire will hardly generate any output.

Physics of current transformer is very confusing one for the beginners. Best thing is to experiment as see for yourself. Easy way out is to use any step down transformer and its secondary as sense coil and primary as output. 220/110V to 6V 120W transformer will be ideal to sense up to 2A current from motor etc in series to 220/110V source. There will be drop of just few volts across the transformer.

I have designed many current transformers and they all work. Passing a single wire through the center of a torriod acts as a full turn, you do not have to wrap it all the way around the core. Yes the transfer is imperfect but if you size things correctly it works out very close to ideal.

You are repeating what you have said earlier. You did not read the first line I have written.

Will you count single primary turn as one turn or just passing a conductor as one turn. They will give different output for sure. As per your note they should not.

I think you are giving wrong information by repeatedly writing that just passing conductor is equal to single turn. Either you justify or withdraw your comment.

I am sure you are experienced person so no doubt that you must have tested CT. I teach engineers to try turn change in primary to increase or decrease the output and is a good thing to experiment with. Try out and see for yourself.

Do not feel bad that I am blunt, but there are many who will learn wrong way out. Hence, clean up things when we place it in public domain and try to be accurate as far as possible. Think again.

I re-worded what I said because it appeared that you did not understand it, you again misunderstood my re-worded answer.

Remember - I am talking about Toroid cores only - they are very commonly used for CTs

Magnetic flux forms closed loops around a current carrying conductor as well as inside the conductor. Since flux travels in the magnetic core much more easily than it does in air the shape of the flux lines will distort to follow the easiest path but they will still remain intact and around the wire. The wire passing through the center of the core can will be completely encircled by any lines of flux that are following the magnetic path of the core. The the flux following the path of the core will not encircle the wire wrapping around the outside of the core. So from a basic intuitive point of view it is easy to see that the primary contribution to the flux in the core will be the wire passing through the center of the core.

Clearly we should understand that size and wire placement will effect the quality of the magnetic coupling to the core. A 100" diameter core would not be well influenced by a small wire in the center of the window nearly as well as a 0.5" diameter core will be influenced by a wire in contacted with the surface of the hole in the core. But as long as we remain practical in our application and use good judgement the wire passing through the center of the core acts as a single turn.

Now to answer you question. If I pass a wire through the center of a toroid core I count it as one turn ( placement and size being reasonable ). If I wrap that wire around the core to complete a physical turn such that the strands pass each other somewhere on the outside perimeter of the core I still count it as one turn. If I wrap the wire such that the strands complete one physical turn but pass each-other on the inside of the core so the wire actually passes through the inside of the core twice I count it as two turns.

The physics are not that complicated and it is clear that proper winding techniques improve performance. Many factors influence specific performance but we do have to be practical at times.

Initially we were asked a simple question. Ask a simple question expect a simple answer. Not a physics lesson. If the person that wrote the initial question wants more than a simple answer they can ask another question.

If primary coil or wire forms 1V per cm and a single turn takes 10cm length then primary coil has 10V across it. It is not the same voltage formed for one cm length. Conductor current and its length linking magnetic flux is the actual cause of the induced secondary voltage. If you simply pass a wire thru hole then only 1cm wire can link the magnetic flux. If you do experiment then you will know. In normal transformers, you apply fixed voltage and coil impedance limits the current. Here current in the conductor does not change and only magnetic flux that is being linked is changing. Length of the conductor is one reason that increases linked magnetic flux. If this makes no sense then whatever you know, you can go ahead.

I admit that your argument sounds right.

Perhaps the formation of complete single turn with twice passing the conductor thru the hole causes double flux linking and part of the conductor outside the hole hardly links any magnetic flux in toroidal core, so you are right. If magnetic flux is not linked then there will be no induced emf also.

Good. Experienced one!

As you are in a pedantic and critical mode,lets look at some of your pontifications.

'The magnetic flux generated is due to the differential voltage across the conductor'

Voltage is the number of electrons available,only when these electrons start to move is a magnetic field generated,this movement of electrons is called 'current'

All that matters is Isquared t at any instant,which gives the magnitude of the field.

'current and impedance of the conductor counts a great deal'

Possibly 'current of the conductor' has lost meaning in translation,please explain.Humbly I must admit my grasp of your native language is light years behind your grasp of English.

'Impedance' is always a good word to use as it mystifies the subject,a sort of high priest of electrics incantation.

'Thick primary wire hardly gives any output'

50 sq mm busbars with CT threaded over them give exact reading of current flowing.I think this qualifies as 'thick wire'

Clamp ammeters have only one 'partial' turn and give accurate readings as long as kept away from ferrous interference.You infer that all these ammeter manufacturers are deceiving us.

You should get a 110v tool transformer,variac, CT and ammeter,short the secondary with a piece of nice thick wire, and have a good old experiment.

I hope you continue to contribute as I need a laugh now and then.

Good response!

hi,

if you notice the burden VA of the CT. U may notice that the py.voltage compared the sy.voltage is very less by the formula VA=V X I.so CT is a step up transformer.

thank u,

naveen.m

Single conductor though the toroidal make a FULL 1 Turn as the total flux links to the winding on toroidal.
To make a full turn is to see end to end of centre conductor outside the toroidal which should not enter again, so passing the centre conductor again will make it the 2nd turn.


In old Instrument CTs EI cores were used & a thicker wire "U" passed through both the slots to make a turn.

After development of Toroidal Wire [Cable] passing through is ONE turn for all purposes; and all the Secondary turns wound in a single layer.
All the CTs must be loaded to recommended Dump specified in VA for accuracy.