Question trying to solve voltage for Burden of transformer

[Sparky Bill PE]

Having trouble connecting the dots from this example. 

 

[speakeelsy PE]

Current transformer, not regular transformer! 200A is the line current.

So Ipri/Isec = Npri/Nsec

Isec = Ipri(Nsec/Npri)

Isec = 200A(5/200) = 5A

Then Ohms Law it

V=IR

V =5A*1ohm =5V

 

[Sparky Bill PE]

Wouldn't the equation be Isec/Ipri = Npri/Nsec? 

Do CT's have the i1/i2 = n1/n2?

 

[jd5191]

When the problem states "a 200:5 CT transformer ...", that just means when you put 200A on the primary, you get 5A on the secondary. This is a simple ohms law equation on the secondary side. I wouldn't use N1 and N2 because nowhere does it tell you the turns on either side.

V = I * R = (5A) * (1ohm) = 5V

If you want to use the transformer ratio equation, note that N1 and N2 are not provided, (turns in the transformer), only I1 and I2 are provided. This will be redundant but you could say

a = I2/I1 = 5/200

so if I1 = 200 then I2 = I1 * a = (200)*(5/200) = 5

now do ohm's law

 

[Sparky Bill PE]

When the problem states "a 200:5 CT transformer ...", that just means when you put 200A on the primary, you get 5A on the secondary. This is a simple ohms law equation on the secondary side. I wouldn't use N1 and N2 because nowhere does it tell you the turns on either side.

V = I * R = (5A) * (1ohm) = 5V

If you want to use the transformer ratio equation, note that N1 and N2 are not provided, (turns in the transformer), only I1 and I2 are provided. This will be redundant but you could say

a = I2/I1 = 5/200

so if I1 = 200 then I2 = I1 * a = (200)*(5/200) = 5

now do ohm's law

So the fact that the letters say "CT" is what changes everything?

If it didn't say that, you would assume it 200:5 is the turns ratio right?

Does anyone have experience with taking the test and knows if this is common wording of the actual test or is this a spin up type wording thing. Seems weird since it says CT you just automatically know that's 200:5 current NOT 200:5 windings. 

 

[akyip]

Yes. Normally, a power transformer's (or voltage transformer's) turns ratio in terms of primary and secondary currents is:

a = I2 / I1 = I sec / I pri

BUT FOR A CURRENT TRANSFORMER:

a CT = I CT-1 / I CT-2 = I CT-pri / I CT-sec

The current transformer is the only one whose turns ratio has primary current divided by secondary current (at least as far as I know).

Keep that in mind whenever you see a current transformer or CT problem.

 

[jd5191]

So the fact that the letters say "CT" is what changes everything?

If it didn't say that, you would assume it 200:5 is the turns ratio right?

Does anyone have experience with taking the test and knows if this is common wording of the actual test or is this a spin up type wording thing. Seems weird since it says CT you just automatically know that's 200:5 current NOT 200:5 windings. 

In my experience, whenever a transformer ratio is given (PT, CT, GSU, etc.) its always given as a PrimaryValue:SecondaryValue unless specifically stated as a "turns ratio". PT/VT ratios are often provided as 13,800:120 PT Transformer. In the case of a voltage transformer, the turns ratio also happens to be the voltage ratio so there's no confusion. The only time I've seen a turns ratio shown without explicitly stating it is a turns ratio is on a one-line drawing that shows no engineering values, meaning a one-line that shows no voltages or currents but just VAs and turns ratios. Now I wait for an example to prove me wrong

 

[Sparky Bill PE]

I guess I've just been working too much on Zach Stone's website. Thanks for explaining. I am used to this saying "A 200:5 Amp CT".... which would let you know the units are in amps not a unitless ratio.