current transformers and frequency question

Any transformer will operate over a relatively large band of frequencies. The most important frequency consideration is the low frequency capability of the device. A given flux density on a given core with a give number of turns will define the volt-time product of the transformer. This defines the minimum frequency it should operate at for a given voltage. Transformers designed for 60Hz should not be operated at 50Hz unless they are operated at

20% less voltage. Transformers rated 50/60Hz are designed to accomodate the lower frequency at rated voltage. Four hundred Hz transformers have a small core and cannot be operated at lower frequencies without a corresponding reduction in voltage. Again, volt-time product is the key. Any of these transformers will operate at higher frequencies usually up to several kHz before capacitive effects and leakage inductance limits their high frequency response. Line transformers are not rated at those freuencies but that does not mean they will not operate there. You can easily use virtually any iron core transformer at say 1 kHz or there abouts with no proplem. Higher frequencies, 10kHz and above require a different design, ferrite cores, etc.

Current transformers have the same limitations, that is they will operate over several kHz of bandwidth depending on design. The volt-time rule also applies. To your specific problem, operate a 50Hz transformer at 2/5 or 40% of the rated voltage for 20Hz. This means for a given current you have to lower the burden resistor (load) and make a corresponding adjustment in detection because of the reduced voltage. Bob

As with all ferromagnetic cored transformers, the power capability is proportional to frequency. If it is designed for 60 Hz and you use it for 20 Hz, its capability falls to 1/3rd of rated. For a current transformer, that usually means that the burden resistor must go down in proportion to frequency, but it is worse than that, because the true burden includes the winding resistance of the transformer.

So, for instance, if a current transformer is rated at 60 Hz for 10 amps and a 100 ohm burden resistor, and has 20 ohms of internal resistance, its true burden is 120 ohms. At 20 Hz, and the same current, its burden would have to be reduced to 120/3= 40 ohms in order to have the same accuracy. But that leaves only 20 ohms as an external burden, or 1/5th of the 60 Hz specification.