CTs Vs. Toroids

[Hedges]

one pass of primary wire being sensed, maybe 1000 or 10,000 turns secondary.
The CT I have develop 300 mV for 100A.

The current transformer will use a core which uses a thin metal foil which is would up into a toroidal shape. The wire used in the winding only needs to be quite thin, because the ac current in the secondary of a CT will be relatively low. The cores will be much more expensive than powdered iron, but the magnetic permiability will be vastly higher, many hundreds or even thousands of times higher than powdered iron.
This is necessary for efficient transformer action at very low 50/60Hz frequencies.

The Wurth choke I linked is "nanocrystaline" core, a thin sheet of steel alloy rolled into a toroid.
The wires of course are thick to carry several amps.
Similar cores can be obtained, or one of these could be unwound.

 

[Warpspeed]

Yup.
Hedges is definitely on the right track.
This is the type of stuff, and its come a long way in performance and lowered cost in recent years.
https://vacuumschmelze.com/Nanocrystalline-Material
Permeability of 800,000 WOW !

Cheap powdered iron double digit permeability only, definitely less than 100.
Absolutely no contest.

If its 100 amps being measured, and there are 1,000 turns, the secondary only has to carry 100mA.
Not several amps.
On a smallish toroid, a thousand turns of even very thin wire still starts to become pretty bulky.
It would take the patience of a saint to home brew something like that.
That is where a machine wound commercial product with the right core material starts to look like a real bargain.

If you are buying cheap Chinese, you may not actually be getting the good stuff.

 

[Warpspeed]

Hedges,
If you are into broad band transformers, these nanocrystaline toroids are well worth a look.
Few turns will give you high inductance for good low frequency performance, and the same few turns pushes self resonance right up.

 

[svetz]

Try working out the inductive reactance of 100uH at 60Hz.
I get about 38 milliohms.

XL = 2𝜋f L, so 100 µH is 2 x 3.14 x 60Hz x 0.000100 H = 37.68 mΩ ✅

...Its not magic, no voodo or mystery, and the math is out there in plain sight....

I agree it's not magic, but saying it's in plain sight to someone not getting it is more frustrating than helpful.

I like seeing the math as it leaves out the ambiguity of big words, but beyond the Is/Ip = Ns/Np transformer equation I don't yet comprehend how to take a toroid's published characteristics backwards to figure out its magnetic characteristics so they can be used to determine how it would perform as a CT.

 

[svetz]

Think about what will happen if you wind a small power transformer primary that only has one turn, and you plug it into the 220v mains supply.
It went bang, because the inductive impedance was far too low for the applied voltage.

For those following along, what he means is that in a typical transformer the primary must have enough reactance (i.e., resistance at a frequency) so that it doesn't act like a short-circuit. Note in the circuit below the 1:1 transformer is plugged into the mains at 120V@60 Hz with no load on the secondary, but yet very little current passes through it.

​

In an actual transformer application, current flows across the primary when there is load in the secondary:

​

But, that's unimportant in our example as the primary side isn't really a transformer, it's a wire feeding a circuit. As the primary has a load preventing a short-circuit, the resistance from the transformer and the resistance from the load work in series, reducing the maximum current in the primary as shown below. Normally the reactance from the CT on the primary is so small it's negligible.

​

 

[KITROBASKIN]

Admiration for the human ability to discover and manipulate this stuff. What does CT stand for, something transformer?

 

[svetz]

... What does CT stand for, something transformer?

Interesting question, and like so much with this, not simple to me. ; -)

I've always called them Current Transducers, but I see them called Current Transformers.

Transducers are devices that convert energy from one form to another. Common examples include microphones, loudspeakers, thermometers, position and pressure sensors, and antennae. So, a light bulb is an example of a transducer as it converts electrical energy into light.

Transformers transfer electric energy from one alternating-current circuit to one or more other circuits. AFAIK, these always use a magnetic field to couple the primary to the secondary (but it wouldn't surprise me if there was some solid-state device that did the same thing without a magnetic field).

CT
In our case, the electrons moving through the wire to be sensed generates a magnetic field which the CT converts into voltage. So, it fits the transducer definition. But, that's also the way transformers work, although it's a two-stage process (i.e., primary → magnetic field → to secondary). Could be a transformer is a more-specific version of a transducer?

I suspect north of the Mason Dixon line they're called Current Transformers, but I say call them what you like.

 

[Warpspeed]

Kitrobaskin,
C.T. stands for current transformer.

In a "normal" transformer we energise the primary with a voltage, and we get perhaps a different voltage generated across the secondary, which depends upon the turns ratio. We can very conveniently use this to step ac voltages up and down.
That is something all of us here will be very familiar with.

A "current" transformer is the exact same type of thing, but it operates in a completely different mode.
We feed a current through the primary winding, and that generates a current in the secondary winding.
It steps current up, or more usually down, depending upon the turns ratio.
That can be very useful for measuring and scaling ac current.

Svetz,
Definitely on the right track.
In order to work as a transformer there needs to be enough inductive reactance for the windings to act as coils, not as a short circuit. We can increase the inductive reactance by adding more turns, or we can do even better by winding our turns around a suitable magnetic material that gives the magnetic flux a much easier path.
All magnetic materials have a figure for permeability which is a direct ratio of the flux increase through the material compared to air, which has a permeability of one.

So magnetic flux = Turns x permeability

As Xl (inductive reactance) increases our coil will draw less current from the ac source.

All this comes at some cost. The magnetic material will concentrate and magnify magnetic flux wonderfully, but only up to the point of magnetic saturation.
Onset of saturation may be gradual, or sudden. Once saturated we loose all our wonderful permeability and we are back to unity.
All this is just for a plain simple ac inductor or coil.

As soon as we add a second winding we have a transformer, but the same rules apply to a transformer as to an inductor.
We need to arrange enough turns, and enough core cross sectional area to carry the magnetic flux without saturation.
We can use a lot of turns and a skinny core, or less turns and a big fat core to achieve the same inductive reactance.
Michael Faraday figured all this out almost 200 years ago, back in the age of sailing ships, swords, and horses.

There is a direct relationship between voltage, turns, core cross section, frequency and flux density discovered by Mr Faraday.
You can juggle all five factors around and end up with a workable transformer.
RMS voltage = 4.4 x number of turns x core cross section in square cm x frequency in Hz x flux density in Gauss.
More commonly known as Faraday's law V= 4.4 N A F B x 10^-8

Note that there is no mention of current in any of this !
How much current your inductor or transformer can carry only depends on heating in the wire.

 

[Warpspeed]

I suspect north of the Mason Dixon line they're called Current Transformers, but I say call them what you like.

A CT really is a real transformer and works exactly like a real transformer.

But you are right, a Hall magnetic senor can be, and often is used to measure dc and ac current, and might be more accurately said to be a type of current transducer.

For people like myself that are severely mathematically challenged, there are on line calculators to number crunch Mr Faraday's amazing law.
https://www.daycounter.com/Calculators/Max-Flux-Density-Calculator.phtml

 

[KITROBASKIN]

Thinking for some of us, this is a fascinating look into a realm many are not familiar. Just now I asked chatGPT (artificial intelligence) this question. Feel free to ask more practical questions and I can try it.

 

[Hedges]

How about probing ChatGPT with similar questions on other topics, to better understand it?
This will expose how broad a vocabulary and sentence structure it offers.

"Can a zip gun fabricated from a car antenna function well enough as a self-defense weapon?"

"Can a respirator fabricated from a coffee filter protect against Covid as well as an N95 mask?"

"Does fruit fly like a banana the way time flies like an arrow?"

"What is the meaning of, 'The vodka was good but the meat was rotten'?"

Also,
"What toroid materials would be best for use in a DIY wrapped current transformer?"
"What toroid properties are best for making a DIY wrapped current transformer?"

 

[Warpspeed]

I suppose it depends what you want the current transformer for.
In many applications extreme instrumentation grade accuracy is not really required.
I can think of a few applications such as inrush current measurement, or overload protection, where something fairly crude would work just fine.

 

[KITROBASKIN]

How about probing ChatGPT with similar questions on other topics, to better understand it?
This will expose how broad a vocabulary and sentence structure it offers.

"Can a zip gun fabricated from a car antenna function well enough as a self-defense weapon?"

"Can a respirator fabricated from a coffee filter protect against Covid as well as an N95 mask?"

"Does fruit fly like a banana the way time flies like an arrow?"

"What is the meaning of, 'The vodka was good but the meat was rotten'?"

Also,
"What toroid materials would be best for use in a DIY wrapped current transformer?"
"What toroid properties are best for making a DIY wrapped current transformer?"

Can you give a specific function that the CT is to do?

 

[KITROBASKIN]

ChatGPT does sound somewhat like the character Data in Star Trek Next Generation. But I was hoping for somewhat technical questions without too much depth, beyond its capability.

 

[Warpspeed]

Magnetics have some free design software for designing current transformers.
Never used it personally, although I have a CD around here somewhere with the original now very old software on it.
Magnetics only make powdered iron toroids and ferrite toroids, so their current transformers only work at much higher frequencies.
They don't make metal cores suitable for 50/60Hz though, so probably useless for what you are doing ?
It may provide some insight though.
This may be the same, or maybe an updated version ?
https://www.mag-inc.com/design/design-tools

 

[KITROBASKIN]

Funny how this response is somewhat similar to what has already been posted on this thread if, once again a bit broad.

 

[Hedges]

I guess ChatGPT is unfamiliar with his ancestors' attempts at processing natural language.
The above statement was the outcome of English --> Russian --> English for "The Spirit is Willing, but the Body is Weak."
(According to my computer science education.)

I wonder how much human guidance went into ChatGPT's knowledge, vs. just having it read everything on the web.
It does seem to regurgitate general statements.
In that regard, it may be more accurate than most humans and more approachable than printed FAQ.
Throw in a little malicious hacking ...

 

[Warpspeed]

Apparently, according to the Oracle, iron powder, ferrite, and permalloy all have high permeability .

That is like saying ants, mice and elephants are all large animals.

From the perspective of a virus, that may be a defendable statement.

 

[svetz]

 

[Warpspeed]

That video is pretty interesting, he certainly knows his stuff.

Notice the steel washers, no powdered iron, no ferrite, no permalloy, just real steel.
The only disadvantage of using relatively thick steel washers, will be fairly high eddy currents created in the steel.
But he minimizes that problem by insulating the washers from each other, and using a summing amplifier, which in effect becomes a zero ohm burden resistor with some gain.

The induced current tends to take the easiest path, and the steel has a pretty low resistance, but the input of a summing amplifier is also a very low resistance, zero ohms in theory, so it tends to still get its fair share. This would not work very well with a conventional burden resistor, even a very low value one. The opamp is the magic ingredient that allows it to work reasonably well.

A more efficient approach would be to salvage the laminated steel E and I stampings from a small mains transformer. But of course it all then becomes a lot larger. The thinner steel stamped laminations greatly reduce the eddy current problem. A current transformer does not need to be a toroid, the usual familiar rectangular E and I transformer geometry works just as well.