Hi -

I would like to learn more about electromagnetics, I have a basic understanding and a topic that piques my interest and seems like a good next step is the difference between a general transformer like you would use in a power supply (line freq, not SMPS) vs a voltage measurement transformer vs a current transformer.

I know the basics at a macro level, i.e.

• A general power transformer will have some voltage sage with load, windings designed for the current and insulation requirements, laminated cores for reducing eddy current loss
• A voltage measurement transformer (often seems to be called "potential" transformer or PT as opposed to CT" is more tightly controlled in ratio, winding arrangment, parasitics so that some Vin = some scaled Vout across a wide range... not designed to be loaded, meant for high impedance readings with an ADC or control equipment.
• A CT is controlled in the saturation and number of (usually very high) secondary turns to step a primary current down to a scaled secondary current which when loaded with an appropriate burden resistor means the voltage across that resistor is an analog of the primary current.

I guess after typing that out my question is really - is that right, are there any major points missed, what I am trying to get straight in my head is that they are all really "just" transformers, and by controlling the build for accurate ratio with low parasitics, or going for current handling and thermals, or whatever you choose to control you get the type of transformer you want, but in the end they are all really operating on the same principles?

 

A CT is controlled in the saturation

Don't understand what the means (?).
A CT does not operate with the core saturated.
If it did then it would be inaccurate in the ratio of primary current to secondary current.

in the end they are all really operating on the same principles?

Yes, the are basically the same using the same fundamental equations, with two or more windings connected by a common magnetic flux.
The flux can be carried by a magnetic material or even air (for RF transformers).

 

Don't understand what the means (?).
A CT does not operate with the core saturated.

I pre wrote my post and miss-pasted a bit somehow!

"A CT is controlled to stay out of the saturation region so it stays linear" is along the lines of what i meant to write - I think I had saturable inductors in mind or something, been reading about them as well as ferroresonant parts.

I think what i'm trying to make sure I have right is that these things are all just transformers of various types, none of them are

 

Yes, they are all "just" transformers.
Understand the model of a real transformer and how it differs from an ideal transformer, and you will understand why they differ.
A perfect transformer has two windings on a common core. The secondary voltage is equal to the primary voltage divided by the turns ratio and the secondary current is equal to the primary current multiplied by the turns ratio, no power is lost, no current is taken by the primary when there is nothing connected to the secondary. You probably know this already.

To make a real transformer from an ideal transformer:
1. Add an inductor in parallel with the primary. This represents the magnetising inductance. You can easily work this out from the current that flows when there is nothing connected to the secondary.
2. Add a resistor in parallel with the primary. This represents the core losses. Using thinner laminations reduces the core losses and will make this resistance appear larger.
3. Add a resistor in series with the primary and another in series with the secondary. This is the winding resistance. You can easily measure this with a meter, though the secondary resistance might be too low for your meter to read.
4. Add an inductor in series with the primary and another in series with the secondary. This is the leakage inductance.You need an inductance meter to measure this.
5. Add a capacitor between primary and secondary. This is the capacitance between the two windings, which will be very small. You can measure it with a capacitance meter, or probably you can ignore it, unless you are working at high frequencies.
6. Add capacitors across the windings. These represent the capacitance between adjacent turns of wire. Again you can probably ignore it.

Now you can see that:
A power transformer will have voltage drops across the series resistances and inductances as current flows in the primary and secondary. This means that the output voltage sags under load.
You will see two types of power transformers, those with the secondary wound on top of the primary (concentrically wound), and those with primary and secondary on two sections of the bobbins side by side. The side-by-side type has much larger leakage inductance, although resistances and magnetising inductance are the same.

For a voltage measurement transformer, you don't want this to happen, so you need to run it with as little secondary current as possible. Current will flow in the magnetising inductance, but that will not affect the measurement.

For a current transformer, you need to avoid current flowing in the magnetising inductance, because this current doesn't flow in the burden resistor (that's the resistor you put across the secondary), so represents an error. To do this you need to keep the voltage across the windings as small as possible, by using the lowest possible burden resistor.

 

I think what i'm trying to make sure I have right is that these things are all just transformers of various types, none of them are

...man, I'm not having a good typing day.

"none of them are....." should have finished with : "special in terms of obeying different rules or something, they are just implementation differences. Like understanding that electrolytic caps, ceramic caps, old polystyrene caps are all electrostatic charge holding devices, and not mistaking one of them as operating like a chemical battery."

 

One important note about the CT - it only works properly when its secondary winding is loaded with a very low impedance.
Under no circumstances is it allowed to disconnect the load from the secondary winding. This is its fundamental difference from a VT or a power transformer.

 

One important note about the CT - it only works properly when its secondary winding is loaded with a very low impedance.
Under no circumstances is it allowed to disconnect the load from the secondary winding. This is its fundamental difference from a VT or a power transformer.

Yes, I've managed to resolve that by realizing (via equations) that the flux will keep increasing if the secondary is not loaded and more flux = more secondary voltage potential, but nothing pulling current on the secondary to keep that potential in check, so it jsut keeps going up! I suppose it rises until something flashes over. I note some commercial CT's have zeners across them at a much higher voltage than should ever be seen in normal measurement but best not to assume any protection is in place, better to develop a good habit since it does no harm when using a protected CT and certianly helps when using a non protected one! For the same reason I have tried to form the habit to always leave multimeter probes in the voltage sockets, scope probes on 10:1 or 100:1 switch....things with temperature controls on the lowest...... same with lab power supplies, wind the votlage down when finished with it.

 

Everyone quotes the theory that an unloaded CT has a huge (and possibly, dangerous) output voltage, but that is simply not true. An "unloaded" CT is not unloaded at all. The load is the magnetising inductance, and current will flow through that as the voltage on the secondary increases. At some point it saturates. I have measured the output voltage on an unloaded 100A:1A CT and never got more than 18V.

 

Everyone quotes the theory that an unloaded CT has a huge (and possibly, dangerous) output voltage, but that is simply not true. An "unloaded" CT is not unloaded at all. The load is the magnetising inductance, and current will flow through that as the voltage on the secondary increases. At some point it saturates. I have measured the output voltage on an unloaded 100A:1A CT and never got more than 18V.

Mmm... sorry, with respect I have seen the info in too many textbooks and articles, backed by the math, that says you can get very high voltages. I believe you got those results, but I think your experiment had a flaw, like the characteristic of that particular CT.

Maybe we can say that the high voltage hazard is not guaranteed to happen on every single CT or under all conditions, but I think it safer to warn about it and have it not happen every time. A bit like saying you crossed the road once without looking and didn't get run over, that doesn't mean the advice to always look before you cross is bad advice.

 

Mmm... sorry, with respect I have seen the info in too many textbooks and articles, backed by the math, that says you can get very high voltages. I believe you got those results, but I think your experiment had a flaw, like the characteristic of that particular CT.

Maybe we can say that the high voltage hazard is not guaranteed to happen on every single CT or under all conditions, but I think it safer to warn about it and have it not happen every time. A bit like saying you crossed the road once without looking and didn't get run over, that doesn't mean the advice to always look before you cross is bad advice.

Did you maths mention saturation? Or the core material? or the magnetising inductance? Or was it the maths for a "perfect" transformer?

 

Everyone quotes the theory that an unloaded CT has a huge (and possibly, dangerous) output voltage, but that is simply not true. An "unloaded" CT is not unloaded at all. The load is the magnetising inductance, and current will flow through that as the voltage on the secondary increases. At some point it saturates. I have measured the output voltage on an unloaded 100A:1A CT and never got more than 18V.

I once had to check CTs, these were transformers with a power rating of about 20-50VA, designed to work with 5A ammeters, protection relays and electricity meters. If the secondary winding circuit of a similar transformer is disconnected, and the current in the primary side is about equal to the nominal, then a spark jumps in the secondary side. Approximately the same as in a spark plug.

I once saw an electrician connect a long cable to the secondary winding of CT (400/5A) and short the other end of the cable. He made a mistake - he shortened the wrong cable cores.
He accidentally touched the exposed wires and got such a strong electric shock that he fell to the ground. It was good that it was over, but his hands were numb for a few more hours.

 

Did you maths mention saturation? Or the core material? or the magnetising inductance? Or was it the maths for a "perfect" transformer?

Not the point - let's pretend all of my maths is wrong and take that out of the argument; Why do all textbooks and so many learning sites including in fact the AAC textbooks warn about the hazard of CT secondary voltage if it's not a real problem?
You have to see from my point of view that I have many varied and reputable sources telling me A, and one guy on the internet who did an experiment at home once is telling me B. If you were in that position, does A or B have more compelling evidence of being correct?

 

Not the point - let's pretend all of my maths is wrong and take that out of the argument; Why do all textbooks and so many learning sites including in fact the AAC textbooks warn about the hazard of CT secondary voltage if it's not a real problem?
You have to see from my point of view that I have many varied and reputable sources telling me A, and one guy on the internet who did an experiment at home once is telling me B. If you were in that position, does A or B have more compelling evidence of being correct?

Neither. I'd measure it.

 

He accidentally touched the exposed wires and got such a strong electric shock that he fell to the ground. It was good that it was over, but his hands were numb for a few more hours.

That reminds me that in a safety briefing just earlier this year, it was mentioned that the voltages being worked on (using variacs to limit voltage - yet another "transformer" type!) although low, still had the full current behind them, and the guy mentioned that electric shocks can hurt you directly if bad enough but also indirectly e.g. you fall to the ground or worse off a ladder or platform, or you jerk your hand back in surprise and contact something hot or sharp or spinning. So it doesn't have to be a bad shock to still hurt you.

 

Neither. I'd measure it.

But still again my point - if you are an intelligent thinking person and you do seem to be, you can question if your measurement and experimental setup is correct.
There are many sources saying A and one source saying B - is it more likely that many independent sources separated by distance and time come to conclusion A and you come to conclusion B, and THEY are all wrong.....or you?
Remember I did start off above by saying i totally believe you got the result you did - but does a one off prove that CT's everywhere of every type in every circumstance don't produce high voltage in the way usually described?

 

But still again my point - if you are an intelligent thinking person and you do seem to be, you can question if your measurement and experimental setup is correct.
There are many sources saying A and one source saying B - is it more likely that many independent sources separated by distance and time come to conclusion A and you come to conclusion B, and THEY are all wrong.....or you?
Remember I did start off above by saying i totally believe you got the result you did - but does a one off prove that CT's everywhere of every type in every circumstance don't produce high voltage in the way usually described?

My colleagues often wire them up the wrong way round, so that the power measurment reads negative, so the cables need to be swapped, so I take the cables off swap them over and put them back. Many times, just out of curiosity, I have measured the voltage on the CT terminals, and never found anything hazardous.

 

But still again my point - if you are an intelligent thinking person and you do seem to be, you can question if your measurement and experimental setup is correct.
There are many sources saying A and one source saying B - is it more likely that many independent sources separated by distance and time come to conclusion A and you come to conclusion B, and THEY are all wrong.....or you?
Remember I did start off above by saying i totally believe you got the result you did - but does a one off prove that CT's everywhere of every type in every circumstance don't produce high voltage in the way usually described?

I think what I'd take from this is

Yes in theory, they can give you a big shock ,

But

As explained by people here who have used them , in reality , the voltage has a much lower limit .

Now if I was writing a book, and did not want to be sued, I'd put in that in theory this happens.

Like all reading of random internet pages , there is a big difference between being taught and reality

I still have a friend who believes in a flat earth , because he's read a lot on it. Yep a lot of his equations work on the flat model, so for him it's good enough
I have another friend who is a pilot , whos thoughts on the first friend can not be printed.

Can I suggest , if you doubt what's being said by very experienced people on here , you go out and text it yourself.

 

Find the book: Handbook of Transformer Design and Applications, by William Flanagan. There you will understand how come two different people could have had two different experiences, and they can be both right.

But as a rule of thumb, don’t push your luck and always assume that an unloaded CT can shock you badly, and act accordingly. Similarly to assuming that a gun is always loaded.
Basic safety precautions. Ignore at your own peril.

 

Can I suggest , if you doubt what's being said by very experienced people on here , you go out and text it yourself.

Coincidentally I had time in the workshop last night to do exactly this, this is how I learn and satisfy my curiosity!

Spolier alert - results matched widely published theory, I conclude that people not concerned about this have been fooled by built in protection or bad experimental method or just got lucky

I used a Model SCT-013-000 CT, datasheet here. There are variants with built in burden, my ones don't have this, i opened it and it only has a zener or TVS of some kind across the coil on a small PCB. I couldn't read the voltage marking but on a lab power supply with current limit it seemed to start clamping in either polarity at around 10-11Vdc.

CT is 100A rated, I used a resistive fan heater that pulls 10A and put multiple turns of the phase wire through the CT. Was difficult to fit 10 turns so i had to stop at 8 but that's good anyway, we are not exceeding the CT 100A limit.

Simple setup - run the heater, check the CT output voltage with true RMS Fluke 77 meter, record as below. Resistors were what I have in my kit so there are some jumps in value.

Brden R output V
4R7 0.21 V
10R 0.46 V
47 Ω 2.21 V
220 Ω 10.85 V
470 Ω 22.0 V
1k Ω 48.5 V I stopped here, I think the point is proven, 1k is way closer to open circuit than the normal CT burden and frankly i was getting nervous even though it's only <50V no need to go more!

So I can see open circuit going over 50V without the protection diode - not good, I think I will believe all the textbooks and manufacturer datasheets!

Was good to see for myself though, so thanks to all for making me question this, a great result!

 

I think that may have neatly sorted out the answer - and it's a pretty obvious answer. It depends on the turns ratio!
I was using 100:1, and the TS's device is 2000:1. More turns ratio, more voltage.
However, one thing is unexplained:
According to this datasheet the TS's device has a 9.2V bidirectional zener across the output terminals.