Following on from this question, Current transformer energy harvesting from a mid-voltage line, I've never seen any mention of primary voltage drop in a current transformer circuit.
simulate this circuit – Schematic created using CircuitLab
Figure 1. A typical CT application using a 5 A meter to measure the current through a load.
Since the burden resistance of the ammeter is reflected back onto the primary side by the inverse turns ratio squared there has to be a voltage drop on the primary side.
This raises a few questions.
- Where does the voltage drop occur? Does it fade in and out with a peak right inside the CT?
- Would we see a larger reading on VM2 compared with VM1 (10 mm and 5 mm from the plane of the CT, for example)?
- What is the relationship with the dimensions of the CT?
- What is the effect of the angle between the axis of the CT and that of the cable. (CT
Best Answer
Some answers: -
The maximum volt drop/metre (\$\frac{dv}{d\ell}\$) is in the middle of the core and rapidly becomes the normal volt drop/meter for a wire beyond the core.
Yes we would because of the normal external volt drop of a longer cable (Ω per metre and inductance per metre).
If you are asking at what point the internal higher volt drop per metre becomes regular wire volt drop per metre for a given size of CT, I can't say. I can't imagine that much beyond twice the core width it would be very much influenced by the core. Good question though.
I don't think there will be much effect given that in practice, the angle probably isn't more than about 45 °. Another good one!