161206-0852 EST
JasonCo:
Any magnetically coupled transformer is a voltage or current transformer depending upon its application, meaning external circuitry, and how appropriate the transformer's specific parameters are to the application.
If there is very tight magnetic coupling between a primary and secondary, meaning a highly magnetic core material with the windings appropriately related, then the voltage ratio between the primary and secondary voltages is approximately the primary turns to secondary turns counts. For the current ratio it is the reciprocal of the voltage ratio.
There is no transformer where all the primary flux lines link with the secondary coil, but some transformers can come very close. To the extent that this flux linkage fails determines how accurate the transformation ratio is to the turns ratio.
When you have a rather light load on the secondary, then the transformer is close to a voltage transformer. When the load is very heavy, near a short circuit, then you are close to a current transformer.
CT means a current transformer.
A current transformer in an AC circuit is a means to use a reasonable measuring device, meter or other sensor, to measure higher current than the capability of the meter or sensor. In a DC circuit this done with a resistive shunt.
In an AC circuit you might use a 5 A full scale ammeter to measure 1000 A full scale with a current transformer with one primary turn, a wire passed thru the transformer core center once, and a secondary of 200 turns wound on the core. Or you could have 1000 turns on the secondary and pass the primary wire thru the core 5 times and still be a 1000 A full scale measuring system. But pass the primary wire thru the core once and it is a 5000 A full scale instrument.
A current transformer core for 60 Hz applications is typically made in the shape of a toroid made from a continuous strip of thin high quality magnetic material wound on top of itself. The continuous winding eliminates air gaps.
You can make your own rather good current transformer. I have a 75 W wattmeter and one day I wanted to measure a somewhat greater power level. So I took a big toroid I happen to have an wound a 5 turn secondary on it. The core was bigger than necessary, but did not introduce too much error for what I needed.
.