Transformers (CURRENT transformers) in reverse

[GeorgeB]

We've beaten wiring step down transformers as step up transformers to death ... I'm not reopening that issue.

I was monitoring a system with pressure and flow transducers, with the associated motor current via a Fluke 1000:1 clamp-on CT and Action Instruments AC current to DC voltage signal converter.

My customer was interested in the differences we saw between that instrument and another clamp-on, and out of curiosity, he hooked up 2 more clamp-on meters. He was surprised at the (total 4 or 5%) variations over about a 2:1 change in current.

I was asked if there was a convenient way to create, in an electrical maintenance shop, an adjustable current carrying conductor up to about 200A for them to compare meters. I've not seen anything like this commercially.

Let's ASSUME I took a 200:5 common CT, made a single wire closed loop through the window, and hooked the 5A leads to a variable transformer with a resistive load and good quality AC multimeter. Would the single wire loop have a stable current approximately 40 times the 5A wiring current?

Are there commercial devices to accomplish this function?

 

[iceworm]

... My customer was interested in the differences we saw between that instrument and another clamp-on, and out of curiosity, he hooked up 2 more clamp-on meters. He was surprised at the (total 4 or 5%) variations over about a 2:1 change in current.

I was asked if there was a convenient way to create, in an electrical maintenance shop, an adjustable current carrying conductor up to about 200A for them to compare meters. I've not seen anything like this commercially. ...

Your customer wishes to check the calibration on various meters and one converter? If so:
1. What accuracy are you looking to achieve
2. What is the plan for the calibration standard - not the current source, the meter to cal against?

QUOTE=GeorgeB;1389129] ...Let's ASSUME I took a 200:5 common CT, made a single wire closed loop through the window, and hooked the 5A leads to a variable transformer with a resistive load and good quality AC multimeter. Would the single wire loop have a stable current approximately 40 times the 5A wiring current? ...[/QUOTE]
I've never sat down and done an analysis on what you are suggesting - so I don'tknow. However keep in mind you are discussing a current source. Voltages can really get out of hand when current sources are open circuited. You may wish to put the CT in a steel box so that if it flies apart you don't have to pick it out of your teeth. But then I have not tried this and maybe it is okay.

QUOTE=GeorgeB;1389129] ...I was asked if there was a convenient way to create, in an electrical maintenance shop, an adjustable current carrying conductor up to about 200A for them to compare meters. I've not seen anything like this commercially. ...

... Are there commercial devices to accomplish this function?[/QUOTE]

Depending on the answers for the required calibration accuracy and source stability, two come to mind.
1. A short circuited AC welding maching - just a loop between the output terminals and turn it on. Probably have to keep the output less than 50% or keep the on-time short compared to the off-time to be inside of the duty-cycle.

2. A 120V, 20A LV pipe-thaw machine. Feed it with a 20 Variac. The ones I've used have two taps, a 150A and a 300A. Again put in a short circuit loop on the output. Adjust the current with the variac.. Again pay attention with to the duty-cycle.

Good luck. It is a fun science fair project. Let us know how it comes out.

ice

 

[iceworm]

New thought -
A 200:5 CT as a 40:1 transformer. The secondary is the one turn through the CT A 10 foot loop aof 4/0 DLO through the CT is a .00006 ohm load. All this is exactly as you laid out - no new news.

As we all know about transformers, the secondary resistance is reflected back to the prikary by the square of the turns ratio. So .00006 ohms on the secondary is equavalent to .00006 x1600 = .96 ohms seen on the primary.

So, following this, with 10 foot shorted loop of 4/0 through the CT, to put 5A on the primary would take ~5V. This of course, neglects the primary resistance.

So measure the CT coil DC resistance, add to the .96 ohms reflected from the secondary, multiply by the intended 5A and that should be the required primary voltage.

As to what happens if the one turn secondary were to open - nothing. The secondary loop resistnace approaches infinity, current goes to zero. This reflects back to the primary as an open circuit. You are not forcing 5A through the primary, you are just putting a voltage source on the primary - current goes to zero.


So your idea sounds great. Great science fair project

Just a thought - Advice is worth about what you pay for it. I wouldn't trust me to have a clue about what I was saying - I would want to figure it out for myself and not take the word of some blowhard on the net. But, that is just me.

ice

 

[cadpoint]

Just a thought - Advice is worth about what you pay for it. I wouldn't trust me to have a clue about what I was saying - I would want to figure it out for myself and not take the word of some blowhard on the net. But, that is just me.

OK, that was just so funny I had to link it

If one saw the road map(one-line diagram) of the equipment in question won't it present
that the equipment can only test and present accordingly?

 

[Smart $]

...

Are there commercial devices to accomplish this function?

Pricey, but yes...

http://www.fluke.com/fluke/usen/Cal...LF-Electrical-Calibration/5500A.htm?PID=54957

http://www.fluke.com/Fluke/usen/Accessories/Options/5500A-COIL.htm?PID=54961

 

[Smart $]

Here's another...

http://www.valhallascientific.com/100_amp_current_calibrator/2555.shtml

Didn't price this one.

Probably more available out there. These are just the first two I found googling. Do not consider either as as a recommendation.

 

[gar]

120331-1659 EDT

GeorgeB:

Can you get 10 to 20 turns of #12 solid enamel insulated magnet wire thru the clamp on probes?

If so, then 10 or 20 A will provide an equivalent flux to a single turn with 200 A.

A current transformer of adequate size probably would serve as a means to get high current at low voltage for your test. In this case it would not be a current transformer. It simply would be a voltage step-down transformer. Most likely you could not use a current transformer with a built-in shunt or voltage limiter. A single wire thru the current transformer is simply a 1 turn secondary coil. You would use a large conductor shorted back on itself, and a Variac at the transformer input with possibly a step-down transformer to feed the transformer. A current transformer with 200 turns would provide 200 A in the 1 turn with 1 A input.

I took a toroid I have with a 2" dia. ID, 1 sq-in core area, probably Supermalloy or Permalloy, relatively square loop materials. Then I put 10 turns of #16 on the core for the primary, and about 2 ft of #12 as a shorted 1 turn secondary. Used half of a 28 V 2 A transformer with 15 V input to obtain about 20 A output in the 1 turn short. Voltage on the 10 turn primary was about 0.75 V. The core was bigger than needed but available. It would be satisfactory for your 200 A test. You easily could put 40 turns of #12 magnet wire around this core in one layer. 5 A thru the primary would be 200 A in the one turn short. You would use a much larger wire for the 1 turn short.

.

 

[GeorgeB]

Your customer wishes to check the calibration on various meters and one converter? If so:
1. What accuracy are you looking to achieve
2. What is the plan for the calibration standard - not the current source, the meter to cal against?

Their question was simply to compare, not calibrate. IF I were to try to calibrate, I'd use a precision shunt in series with the high current line. I can easily get class 0.5 shunts at very reasonable prices (typically 75mV) up to higher currents than they want. But if calibration is desired, I'd have it sent to a cal lab, not a DIY, untraceable, "science fair project".

And ... I suspect the setup would look like a kid's project too ... but probably discarded after 2 or 3 days when they realized what they had.

 

[GeorgeB]

120331-1659 EDT

GeorgeB:

Can you get 10 to 20 turns of #12 solid enamel insulated magnet wire thru the clamp on probes?

If so, then 10 or 20 A will provide an equivalent flux to a single turn with 200 A.

That's the direction I was thinking, too, even if the CT was the (prewound primary) core ... 0-1A primary via "variac", 2 secondary turns on the 40:1 CT, 0-20A secondary, 10 turns of #14 THHN or magnet wire. Jaws will easily handle that bundle.

One would think it would be commercially available ... maybe an old Weller soldering gun on a variac<g>.

 

[templdl]

There's a lot of good ideas presented. It is very common the change a CT's ration by instead of running the conductor of which you want to measure the current oi by simple runnung it straight through the wind of the CT you can change that rating by wrapping that conductor around the donut on the CT in one direction or the other which will either increase of decrease the ratio.
The only thing that I would like to bring up is safety. When current is being dealt with the voltage that can be generated on an open circuit can be overlooked. It can be dangerous and the insulation class of the wire can be exceeded by a voltage that skyrockets. As such caution must be used such that any time that the configuration is used that it is assured that there are no open circuits, that all circuits are a closed loop of shorted.

 

[iceworm]

... I was asked if there was a convenient way to create, in an electrical maintenance shop, an adjustable current carrying conductor up to about 200A for them to compare meters. I've not seen anything like this commercially.

That's the direction I was thinking, too, even if the CT was the (prewound primary) core ... 0-1A primary via "variac", 2 secondary turns on the 40:1 CT, 0-20A secondary, 10 turns of #14 THHN or magnet wire. Jaws will easily handle that bundle. ...

So, you don't want 200A variable. You want 20A variable. Well, that certainly changes the specs a bit.

Last one of these I did, we made up a 2 foot extension cord, with #14 THHN. Wound and tie wrapped a 10 turn loop in one lead, ran the other lead straight. Put a standard 120V 15A three prong plug on each end. Plugged one end in a wall receptacle, the other end into an adapter with a three receptacle end. We changed the current by what we plugged into the cord. We used 500W to 1000W work lights. The fine tune was not too good, but it did fit our purpose for an inexpensive adjustable current source.

ice

 

[SG-1]

When operating in reverse I have always heard the CTs called CT Drivers. The cases are vented to allow the windings & core to cool.
You are interested in doing something like this:

http://s1204.photobucket.com/albums/bb408/Finduilas1/?action=view&current=MVI_CTDRIVER.mp4