Current Transformers - How They Work

A look at current transformers, how they are used and how they operate differently from power transformers. We see what happens when the burden resistor is removed and also measure the current needed to drive the core into saturation. We also take a apart a GFCI receptacle to see its tiny current sense transformer and also a clamp on ammeter to see its transformer is put together.

[tsbrownie]

Another very informative video. Thank you. Editing suggestion, give yourself about 1/2 second of "silence" at the start of the clip before you start. Youtube often starts playing the clip before it opens up from the suggested video screen. Look forward to the next.

[TimPerfetto]

Correct me if I'm wrong but I don't know of any other channel that discusses these topics. Thank you very much appreciated

[knoxpruett]

Excellent video!!! It explains the function for basic users AND the Engineers like me.

[garbo]

Years ago the place I was working at had a lot of amp meters to show how much load was on equipment. Was lucky to have a large meter repair shop that stocked up to 600 amp current transformers. All of them provided a 5 amp output. A 100 amp current transformer or " Donut " would have a 20 to 1 output. Meter shop had various size 0 to 5 amp meters where they would relabel meter face to read the amps. Large hospital that I retired from had expensive 400 to 1600 amp rack in 480 volt circuit breakers that had a display to read amps & voltage on each phase. I have a 10 to 1 donut (50 amps ) along with a nice panel meter that I am going yo install on my central AC unit. Great vidio.

[Sparky-wwre]

While on the subject of transformers, it would be cool to see a video on how they are wired together to produce certain voltages for certain applications, for example, we have our typical 120/240 single phase, or sometimes called split phase because of the center tap on the secondary, then we have 120/208Y, 277/480Y and 347/600Y (very uncommon in the USA) and finally delta connected systems, which includes ungrounded delta, corner grounded delta, high leg delta, open delta and open high leg delta.

I've never seen it with my own two eyes, but it is possible to have a 240/480V delta three phase, with a high leg 415V to ground. Essentially the 120/240V with 208V high leg, but with the voltage double, this system is so rare, that the electrician I'm working alongside who has been in the trade for 43 years, only ran into this once in the early 1990s about 30 years ago in an old woodworking plant in Northern Kentucky, the large motors for the equipment ran off 480 volts, another set of transformers provided 120 volts for the office equipment, and 240 volts from two of the phases to neutral, powered the metal halide high bay lighting.

[Ivar]

Very good explanation of such a mystical device.

[anoimo]

as probably commented before, typical handheld ammeter clamps contain hall sensors, hence don't need sense coils and burden resistors. Naturally the magnetic material should be as unsaturable possible for the reasons very well explained in the video

[scottthomas]

Years ago, I had a clamp on dc ammeter meant to test the current draw if a automotive starter. It was about the size and shape of a Mason jar lid, and went from 0 to 100 amps.
I got it at a yard sale for a dollar...it was old, but hadn't been used much.
It didn't have the round clamp over the wire sensor like typical ammeters, but two clips, sort of like a large fuse holder...you clamped it to the starter cable. No idea how accurate it was. It provided some useful information...informal tests on warm weather and cold weather current consumption of starters...
I had a cheap clamp on ammeter at my last job, and it held up quite well....

[retrozmachine]

It might be interesting to do some experiments of the performance of CTs when there is a DC component to the current flowing through it, and also for different frequency in general instead of just 50/60Hz. There are significant implications for the performance of GFCIs (RCDs). It doesn't take all that much DC to significantly derate (ie make it worse) the trip current of an RCD. With a little more DC the device will not trip at all. This is why countries are moving to require RCDs that can handle a DC component in the waveform as well as a general DC bias as well as frequencies other than 50/60Hz passing through. Add arc detection into the mix and the price of these protection devices gets quite high.

[MichaelCowden]

I have a nice Fluke clamp CT for diagnosis of HVAC and appliances at home, and my house has 350 amp electric service. The entrance cable runs through a big CT to feed the electric meter, which means I can't use a meter socket surge protector. I had never taken apart a GFCI outlet before, so that was very interesting and it makes perfect sense to me how it works now. I'm curious about the flat top sine wave you mentioned was caused by power supplies in the house. Is there any way other than a power reformer device to clean up that phenomenon and maintain a sinusoidal wave?

[davesage]

How do you choose the correct value of burden resistor to give an output voltage exactly proportional to the input current. Is it by trial and error measuring the input current and output voltage across the burden resistor? Suppose you don't have an amp clamp to measure the input current.

[leetucker]

will a gfi work with split phase supply ?

[pixelpatter]

Would it be best if the burden resistor matched the resistance of the coil?

[BjornV]

Hi, i know this is a older video, but i have question. if you use a big enough iron core, would a selfmade 1 : 1 ratio CT work ?
Like 1 turn primary and 1 turn secundary, or 10 turns primary and 10 turns secundary ? Only for high currents i presume ?

[cynic]

you should test a string of 12 or 14 gauge MC cable to see if it contains the flames and smoke

[chadhiggins]

Those transformers don't look current at all, they look quite old. 🤣🤣🤣

[newmonengineering]

If i want to wind my own transformer to plug into the wall, what math do I need to do to make sure it doesn't burn up or blow a fuse? There must be some formula that tells me how many windings I need of a certain size wire without it melting right?

[jvon]

I'm working on a battery I get a volt reading out of it and yet it won't light up an led. Why? I can get 1.v on the meter. I'm a complete newbie to electronics but want to become an electrical engineer someday.

[qwertyuiop]

"Cut to the chase" next time.
1. Low number of turns (most of the time incomplete 1) on one side and many turns on other side = "ignition coil".
No need for B-H curves to explain this. So current transformer may act like ignition coil and damage the insulation. Critical in real applications like power station measurement on high voltage side. You can damage the equipment and/or shock the personnel.
2. That's why you never use/manipulate current transformer with open terminals (low amps side). This is the very basic rule.
3. I've never seen "burden resistors" on classic current transformer (they are designed to work with amp meters, so this is natural "resistance"). Common thing is a permanently installed switch or piece of wire to short the transformer when not used (or for maintenance purpose, when you have to disconnect the amp-meters, live).

[mrkattm]

You are being very loose with your language. A current transformer converts the current trough the core to a voltage. The purpose of the burden resistor is to provide a place to drop the voltage induced to be measures (you are measuring a "voltage drop"). When you removed the low valued "burden resistore" you still had a high value burden resistor attached from your meter and your scope (they are high impedance devices 1 meg ohm or more), ie you can't measure it until you put a load or a "burden" on the circuit, that is why it is called a burden resistor, again you are meausing a voltage drop.