CT turns ratio

[rtronics]

Gidday All,
I fiddle with microamps all day, my only contact with real mens'business (mega-amps), is when I go to the scrap metal yard, for a bit of dumpster-diving, and I sometimes can touch those real 3000 amp copper bus-bars, and SCR's the size of dinner plates that youse guys play with all day.
Any way I picked up this CT, actually it is 3 CT's potted.
The nameplate reads:-
PURPOSE MET
GROUP
FREQ. 50~
CLASS C
ON RATIO 100/5
SPEC ASC45
MAG CURRENT
RATING 7.5 VA
OCF 230 for 3 SEC
PRIM TURNS 2
SEC TURNS 39and a half
RATIO 5/100
SYSTEM VOLTAGE 415

So, what is OCF, and why do they tell me that the sec turns is 39 and a half?
Regards, Ray.

 

[patrick7]

The turns ratio of the transformer tells you how the current is going to be changed. The ratio of 100/5 is the nominal turns ratio.
On large power bushing type transformers the primary turn is one. So for a ratio of 100 to 5 or 20 to 1 the secondary would need to have 20 turns.
This say when you have 100 amps flowing in the primary turn then you would have primary current times the turns ratio flowing in the secondary of the ct. turns ratio is 5/100 or 1/20 so 100amps *1/20 = 5 amps secondary.

now in your transformer there are 2 turns in the primary and 39.5 turns in the secondary for a ratio of 2/39.5 = 1/19.75 which is almost 1/20

So if you have 100 amps flowing in the primary of your ct.
you will hve 100*1/19.75 = 5.063 amps secondary.

It is nice that they told your exact turns ratio. that way you dont have to perform a ttr to determine the exact ratio.

Hope this wasn't too simplistic and that it helped.

 

[unclebob]

OCF ? Over-current F... something

May be the max current the CT is able to sustain for 3 sec
before it is permently damage. Other ideas?

 

[jghrist]

Probably OverCurrent Factor, meaning it will sustain 230 times nominal for 3 seconds.

 

[rtronics]

Thankyou for your replies, Gentlemen.
The thing that intrigues me, is how do they get that one half turn, assuming that the CT is a toroid.
I might have to saw this thing open to satisfy my curiosity!

Regards.

 

[davidbeach]

A half turn would seem easier than a full turn if both ends of the secondary winding exit from the same side of the core.

 

[randyman]

The simplest way to obtain 1/2 turn compensation in a toroidal CT is by winding 2 wires in hand (parallel) which, in this case is 40 T. Drop 1 turn on the compensating winding leaving 40T & 39T. If you take the average of that it is 39-1/2T. There are other ways but this, as I say, is the easiest.