[SOLVED] why current transformer noy operate at low frequency?

[okman]

hi guys.......

why current transformer is not suitable for very low frequency current measurement??

 

[FvM]

What is very low frequency for you?

By nature a transformer can't work for DC. In addition, magnetizing inductance and sum of winding resistance and secondary shunt form a high-pass time constant.

A CT designed for 50 Hz can be used e.g. down to 20 or 10 Hz with reduced accuracy and possibly lower current range. It's possible to further reduce the frequency by using a second secondary winding and a compensating amplifier, but you'll rather prefer a DC capable current sensor with hall detector in this situation.

 

[madhusuthanan]

hi

CT is designedfor 50/60Hzonly.

 

[okman]

thanks to fvM & madhusuthanan

low frequency is about 10 or less than 10 hz
but if CT operate at frequency 10hz this make it operate under saturation conditions (saturation in flux )???? i expect that , what is your answer about that

 

[FvM]

but if CT operate at frequency 10hz this make it operate under saturation conditions (saturation in flux )?

I mentioned possible rated current reduction to compensate for lower operating frequency.

When analyzing the problem in detail, you'll notice that voltage across the magnetizing inductance times 1/f makes the flux. So if you reduce the shunt resistance to a minimum, only the secondary windings resistance will define the magnetizing voltage. By using a compensation circuit with separate secondary source and sense windings, magnetizing voltage can be reduced to effectively zero, also lower transformer cut-off frequency. A simulated negative resistance that compensates the windings resistance can achieve a similar effect.

If the effort is feasible, or you rather choose a commercial available hall-sensor device is up to your choice.

 

[szhighstar]

CT core is amorphous or lamination core, can use at low frequency.

 

[okman]

that 's ok

now, the good solution is the frequency is inversely proportional to the flux in CT air gab and that make CT operate under saturation conditions when operate at low frequency.