Electrical – Current transformer for current sensing at Low Voltage AC

Current transformers normally have hundreds to thousands of turns in their secondaries, since the idea is usually to turn a sensed current into a measurable voltage.

Keep in mind that a current transformer is also a voltage transformer. With a 1:2 ratio, the output voltage will only be at most twice the input voltage. In this case, the input voltage is simply the voltage drop caused by the primary current interacting with the impedance of your 1-turn primary winding, which is very tiny.

Just to put some numbers to this, let's say that your nut has an inductance on the order of 10 µH. At 50 Hz, this will have an impedance of 2πfL = 3 milliohms. Your 2A primary current will produce a voltage drop of about 6 mV, and your secondary will produce a maximum open-circuit voltage of 12 mV.

There's no contradiction. When you limit the secondary voltage by putting a load across it, the primary voltage becomes proportionally lower, too.

You can think of it as having the secondary load resistance "appearing" in parallel with the primary, but with a value that's reduced by a factor of \$(\frac{N_2}{N_1})^2\$. When you put a given current through this combined impedance, the voltage drop across the primary is much lower than it otherwise would be.