current transformer in reverse

Question

What will happen if I connect current transformer in reverse direction, means instead of applying current in primary if I apply some mA of current in secondary(less than the rated secondary current) then what will be the current in primary (assume that in primary I have kept a thick wire with ammeter in series).

I tried this experiment with TT-50SD. I applied 5mA,60Hz current to secondary of this CT and found that current in primary wire was only 1.444mA and increasing secondary current further was not increasing this primary current. Which means core was saturated, is there a formula which will give this saturated primary current value and is it harmful for CT to do this kind of experiment.

Answer 1

In principle it would work -- you would get a current in the (original) primary greater than the forced current by a factor equal to the number of secondary turns.

However, the voltage driving this primary loop is very low, and so you would really need low resistance wire in that loop. The resistance of an ammeter would dominate the result and not be representative. If you could use a Hall sensor, or other lossless/low resistance current sensor (another current transformer !), it would work reasonably well.

Answer 2

The only real distinction between the primary and the secondary is the number of turns, you should be able to send power in either direction no problem, if you're still unsure, think about a 1:1 transformer, what would be different between the two sides if they were both wound exactly the same way? all transformers have something called their "magnetizing current", it's a small current (few mA) that they always draw even at no load - there is an iron core and it takes a little energy to flip the magnetic field back and forth, hence they draw a small bit of extra current. Having said that, current measuring transformers are designed a bit differently to regular transformers, they are more like a sensor than a transformer and sensors tend to be one way (hooking a battery to a solar panel won't make it glow). It doesn't help that current sense transformers tend to have very high resistance secondaries, they don't need to handle any real power and their voltage drop is not important in their intended applications