current transformers and frequency question

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Are current transformers designed to operate on a specific frequency
??? I know filament type transformers are designed at certain
frequencies such as "normal" transformers for doorbells, ac equipment
etc... designed for 50/60 hz use and aircraft transformers designed for
400 hz... but I am having trouble with a current transformer/ switch
that seems to operate erratically at 20 hz fed from an inverter
circuit. I looked at the specs on the transformer and it is spec'd at
50/60 hz.... have not had time to go back to the job location....am
going to go back and switch the current transformer from the load side
of the inverter to the line side but in the meantime I was sorta
looking for an answer...

thanks for any replies

 

[Bob Eld]

Are current transformers designed to operate on a specific frequency
??? I know filament type transformers are designed at certain
frequencies such as "normal" transformers for doorbells, ac equipment
etc... designed for 50/60 hz use and aircraft transformers designed for
400 hz... but I am having trouble with a current transformer/ switch
that seems to operate erratically at 20 hz fed from an inverter
circuit. I looked at the specs on the transformer and it is spec'd at
50/60 hz.... have not had time to go back to the job location....am
going to go back and switch the current transformer from the load side
of the inverter to the line side but in the meantime I was sorta
looking for an answer...

thanks for any replies

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

 

[John Popelish]

Are current transformers designed to operate on a specific frequency
??? I know filament type transformers are designed at certain
frequencies such as "normal" transformers for doorbells, ac equipment
etc... designed for 50/60 hz use and aircraft transformers designed for
400 hz... but I am having trouble with a current transformer/ switch
that seems to operate erratically at 20 hz fed from an inverter
circuit. I looked at the specs on the transformer and it is spec'd at
50/60 hz.... have not had time to go back to the job location....am
going to go back and switch the current transformer from the load side
of the inverter to the line side but in the meantime I was sorta
looking for an answer...

thanks for any replies

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.