# Drawing current sensing transformers accurately

This is more of a stylistic question than anything else, but it important that the sketch be accurate to convey what is really going on. I'm wondering, how does one draw a current loop that is sensing the current flowing through a wire passing through the loop?

I've drawn an example below that covers three different cases that I'm interested in, all on the same current sensing loop:

-The red wire is current passing through the current loop in the normal sense.

-The green wire is current passing through the current loop twice due to the double turn.

-The blue wire is a current passing through the loop, and then turning back around to terminate elsewhere.

Is there a program, or convention for drawing this type of circuit element?

• Are you looking for a circuit element drawing for use on an electrical schematic, or just for a general diagram for a concept drawing?
– Nedd
Nov 22, 2015 at 4:20
• @Nedd It couldn't hurt to know about both. Nov 22, 2015 at 4:48
• See addition to my answer - rushing out - I could add a diagram if needed BUT tidy up your green line version and it is in my opinion as good as any. That makes drawing the N turn secondary in the same manner messy but doable (using of course just a few turns). Nov 23, 2015 at 19:57

Here is a basic current transformer shown schematically and as a more physical construction drawing. Just two windings are shown here but three would be similar. Another convention is to list the number of turns as shown or with the letter "T" as in 1000T.

• Sometimes polarity matters .Dots etc should go on the diagram. Nov 22, 2015 at 10:14
• @Nedd This is helpful, do you have construction drawings for the cases I show in the quest? *Autistic also makes a good point regarding polarity. Nov 22, 2015 at 20:41
• I wasn't able to find a good diagram with the dot notation. When used, a dot placed at one end of each coil (schematic diagram) shows the polarity for each coil (basically the direction it is wound), so if you were to view the voltage wave form at each dot end those voltages would all be in phase. As for the 3 wire cases you show, the reply from Russell M's seems very good. In the diagram I posted the right side section clearly shows that the load wire passing straight through core center is counted as the 1 turn.
– Nedd
Nov 22, 2015 at 23:37

Both of Nedd's diagrams are correct for what they aim to portray BUT your green single turn winding is the one that most portrays the concept of what is involved. A current transformer is "just" an ordinary 1 turn to N turn transformer, even though this fact is usually not obvious because of the way we use it.

Because:

There is considerable misunderstanding of what a CT (current transformer) is and how it works. What is not usually realised is that a CT is entirely a "normal" transformer for operating purposes. It will usually be optimised physically and electrically for this special role but it acts as a transformer with a one turn (or few turn) primary and an N turn secondary. If you apply normal transformer 'rules' and electrical analysis the result will be what you expect for a CT or for a "normal transformer (as they are one and the same).
For the ideal case and a 1 to N turn ratio, Rout output "burden resistor" :

Standard transformer equations:

Vin x Iin = Vout x Iout ... 1 Vout = Vin x N and ... 2 Iout = Iin / N ... 3

V = I x R (from Ohms law) so
Vout = Iout x Rout ... 4

But Iout = Iin/N ... 3 above so from 3 & 4
Vout = Iin x Rout / N ... 5

Rearranging 5 Iin = Vout x N / Rout ... 6
ie we can determine Iin from Vout & turns ration for a given burden resistor Rout. This is in fact trivial (but has snuck up on us) as in 6. Vout/Rout = Iout so 6. reduces to a rearrangement of 3.

The reason that the CT appears to walk funny is that we usually deal with Voltage ratios and are interested in the power and current into a given load.
With a CT we are interested in Iin and Vout. We set the load resistor but do not seek to measure the current in it or the power dissipated. We never ask at all about Vin (the AC drop in the single input turn.

OK - so what? - what has all that got to do with answering the question?
It's relevant because, once we establish that it is an ordinary every day transformer and nothing magic we need to establish that "proper" transformer requirements are met. In the ideal case (and as much as possible in practice) we generate magnetic flux in the primary (one turn) and "link" all that flux with the secondary (N turns). So if we have a single input turn it MUST be a "real" turn. It cannot be half a turn (which in fact cannot exist) or 75% or 99% of a turn. We may run the wire straight through the core physically but if we looked at the magnetic field it will "complete the path" around the core.
So, in your examples the whole single turn shown bu the green line comes closest to showing the single turn to N turns transformer action.

Thanks for your answer, do you think you could explain this graphically as well? That was my main question, how to show the circuit, not just describe it.

As noted above - your greenline version is, in my opinion, the most correct of any I've seen (anywhere ! :-) ) (even though "a little untidy" :-) ).

• It shows a single turn

• It shows it going round the core

• It shows it going completely round the core - so linking all the flux.

@Nedd 's left hand diagram effectively shows the same thing BUT is not so obvious. ie As we know that the N turns side goes completely round the core with N turns it implies the 1 turn does this too BUT the implication of this is probably lost on most people.

• Your red version is how it usually APPEARS in practice and how it is usually shown BUT not how it really is electrically.

• Your blue version is closer to reality electrically but also not how it usually appears OR is shown and is actually probably more confusing even though more correct :-).

• Thanks for your answer, do you think you could explain this graphically as well? That was my main question, how to show the circuit, not just describe it. Nov 23, 2015 at 15:54
• iwantmyphd: As I noted - your greenline version is, in my opinion, the most correct of any I've seen (even though "a little untidy" :-) ). It shows a single turn, it hows it going round the core, it shows it going completely round the core - so linking all the flux. @Nedd 's left hand diagram effectively shows the same thing BUT is not so obvious. ie As we know that the N turns side goes completely round the core with N turns it implies the 1 turn does this too BUT the implication of this is probably lost on most people. .... Nov 23, 2015 at 19:52
• .... Your red version is how it usually APPEARS in practice and how it is usually shown BUT not how it really is electrically. Your blue version is closer to reality electrically but also not how it usually appears OR is shown and is actually probably more confusing even though more correct :-). Nov 23, 2015 at 19:52
• Not sure I follow, the red and green are different cases of a real experiment setup, so they are both "correct". Perhaps my question is unclear, but if I wanted to draw this out, for a publication, or some other official document where a verbal description is not sufficient, is the way I drew it good enough? If not, how would I draw it correctly? Nov 23, 2015 at 20:53