current transformer

[ptonsparky]

I have noticed on several of the large motors we are working with that one of the leads from the CT are looped several times back into and around the CT before it goes on out to the Amp meter.

Why is this done?

 

[djohns6]

If the CT doesn't have the correct ratio for the application , you can cheat by looping it back through like that .

 

[ptonsparky]

Does the direction of the loop increase or decrease the meter reading then?

 

[templdl]

I can do either depending upon the direction that you loop it through. If you look in the CT it should be a dot or other marking. It is common for the instruction book to illustrate how you would do it with reference to the mark.

 

[ptonsparky]

Had a hunch but wasn't sure.

Thanks guys.

 

[djohns6]

Are you sure that we aren't confusing CT ratio with CT polarity ?

 

[RHJohnson]

Don't know what happens here. Does this page time-out or something? I was interrupted by a phone call before I clicked on submit, then when I did submit "My carefully thought out answer disappeared into never never land" and then I was asked to log in again.
This has happened before, and so I just forgetaboudit. Quick reply below...

Some C.T.s will have a white dot to indicate polarity. Very useful when doing protective relaying in sub stations.
Small, inexpensive donut C.T.s probably do not have polarity indicated.

If you add a loop of the ammeter lead thru the C.T. it will add the current once for each extra loop.

 

[SegDog]

figured-it-out

figured-it-out

Thanks RHJ for the brief info on the additive nature of loops.

BTW, at one time I had found that when I "waxed poetic" on a long, intense response, it would just disappear. I think it was because I didn't check the automatic log-on block on the log-on page. Since then, no problems. Hope this helps.

SegDog

 

[jim dungar]

If the CT doesn't have the correct ratio for the application , you can cheat by looping it back through like that .

Actually it is very common to have multiple wraps on motor circuits for ammeters that are used to display the percent motor load. Although conductor size is one reason this practice is usually confined to circuits >50A.

CT polarity marks can be a colored lead wire, painted/molded dots, or the letters H1 and X1 just like on power transformers.

 

[mivey]

As for adding primary & secondary turns, there is a good explanation here:

http://www.branom.com/literature/actransformer.html

 

[ptonsparky]

As for adding primary & secondary turns, there is a good explanation here:

http://www.branom.com/literature/actransformer.html

and Thank You again

 

[templdl]

Yes, the mark does indicate polarity but usually is used in relationship to other CTs. The reason that I brough up that mark is that the direction of which the these turns are made to add to or subtract forn the ratio is based upon this mark otherwise it would be a guess.