CT ratio 2

[tulum]

Another quick question...

I have a ct with a nameplate ratio of 10/0.0167A (598.88:1). I want a ratio of 100:1. To get this ratio the manufacturer specifies wrapping the secondary (x1) lead through the CT (H2 to H1) 15 times?

How do you come up with this ratio? I did some tests and with the 15 turns, the ratio is now 1000:1?

Any suggestions?

 

[busbar]

Adding turns to the CT core and connecting them in series with the secondary is effectively adding or subtracting turns, depending on polarity. The factory instructions don’t sound right. Wind a temporary 10-turn ‘tertiary’ on the transformer and check its ratio against that of the in-place primary and secondary.

http://www.crmagnetics.com/pdf/ancrct-1.pdf

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

 

[alehman]

I agree with busbar. If the secondary is rated only 0.0167A, there is nothing you can do to increase its thermal rating. If you try to force 1A through it, it will probably burn out.

 

[Bung]

Or just put your own 100 turn secondary on it (as long as you take due care with insulation, wire size etc), and leave the original secondary open circuit. (This assumes a single turn primary). Be aware that the secondary output will be limited to whatever the core is capable of - you could work back to it from the class of the original secondary (at least to a first approximation), or just measure it.

But these things aren't exactly expensive - wouldn't it be better (and safer) to just buy the right one?



Bung
Life is non-linear...

 

[tulum]

Thanks for the links and the tips...

BUSBAR:

I did verify the turns ratio by putting a current in the primary and reading the secondary over a range of values.

Can you explain the test you suggested; "Wind a temporary 10-turn ‘tertiary’...

BUNG:

It would be better just to buy one...and if all else fails I will.


ALEHMAN:

I am not looking for a 100:1A... I am looking for a ratio of 100:1 more specifacally for every primary 0 - 40mA, I want to read 0 - 400uA on the secondary.

TO ALL:

From what I read in busbars links above; 1 turn added (in the proper directon) changes a ratio of X:1 to X+/-1:1 (depening on direction).

This means if I have 599:1 ct and I want a 100:1 ratio, I would need 499 turns! Is this correct? If it is...it is definitly time for a new CT.

 

[busbar]

tulum, it wasn’t clear that the primary was one turn—the ‘add 15-turns’ instruction threw me off. My suggested ’10-turn’ addition was a way to check a known winding against two seemingly unknown windings.

To meet your objective, could the single-turn primary be increased to 6 turns? [600:6] Seems a lot easier than 499, but burden would be a factor.

 

[tulum]

Busbar,

Thanks again for the tip...unfortunately, even though all we are monitoring is 40mA ground fault, the primary conductor is the neutral of a 1600kva transformer which is a #6 and too large to wrap.

 

[busbar]

In modern protective relaying, many times a shunt resistor is used on the CT secondary to convert AC current to a proportional AC voltage. If that is the case here, any chance the resistor value could be modified?

 

[jbartos]

Suggestion: Sometimes, CT ratios can properly be adjusted by using auxiliary CT transformers. Then,
1.
CT1:
I11/I21=N21/N11
2.
CT2:
I12/I22=N22/N12
3. etc.
By connecting the CT in the cascade CT1xCT2 with I21=I12

I11=I21xN21/N11=I22x(N22/N12)x(N21/N11)

 

[hsj]

Buy a new CT. To get 100/1 ratio you wull have to reduce the turns. It is not worth it