Understanding current transformer dimensioning, winding resistance and load resistance

Question

I am looking at this current transformer.

What I am trying to understand is the following:

• Can the burden/load resistor of a current transformer take an arbitrary value? What are the limits?
• Does the secondary winding resistance have to be taken into account at some point?

In this answer to a similar question, the output voltage depends only on input current, turn ratio, and burden resistor.

I am assuming, this is a simplified formula. When will I breach its limits? Would it hold even if I substitute a 80ohm burden resistor for 1ohm or 100kOhm resistor and still get accurate answers?

How do I simulate a current transformer in LTspice to explore the behaviour further?

Answer 1

The burden resistor divided by the turns ratio squared has to have an impedance that is significantly below the primary magnetization inductive reactance for a CT to have any decent measure of linearity. In other words, the current in the primary should be mainly (to a high degree) what contributes to the current in the secondary burden.

If you factor in secondary winding resistance then it, plus the burden resistance needs to be significantly below the primary magnetization inductive reactance for a CT to have any decent measure of linearity.

To simulate a CT you need to have an idea how much inductance is added to the wire that carries the current you wish to accurately measure. That inductance might be 10 uH to maybe 100 uH and it needs to be placed in parallel with an ideal transformer’s primary winding.

A CT is just an impedance transformer in parallel with an unwanted inductor that, for highest linearity, is totally swamped by the much lower impedance of the burden reflected into the primary circuit.