The minimum frequency of transformer

[yakut32]

Hi everyone,
I'm pretty new here, My name is yakut, very nice to meet you in advance!

My question is:

In general Transformers work only with A.C, but we know that D.C is just A.C with frequency of zero, so what is the minimum frequency that a transformer can work with??

 

[KlausST]

Hi,

it´s different for every transformer, thus you need to read it´s datasheet.

Klaus

 

[schmitt trigger]

Michael Faraday demonstrated that to induce a current the magnetic field, must be time-varying.
DC is constant, it cannot be induced.
The minimum frequency in a transformer is related to the maximum flux density B that a particular magnetic material can sustain.
And B is a function of voltage, frequency, turns and magnetic cross section.

 

[BradtheRad]

By applying a lower frequency to a transformer, it admits more current. The result is greater heat generation. Advice says we should not extract as much power from it, if we run it at a lower frequency than it was designed for.
According to articles such as this:

www.electricaltechnology.org/2019/09/can-we-operate-60hz-transformer-on-50hz.html

Along a different vein, there is a method of running DC through a transformer winding, which alters AC amplitude in the other winding. This is known as a magnetic amplifier. It can be a useful reason to apply very low frequencies to a transformer.

 

[yakut32]

Hi,

it´s different for every transformer, thus you need to read it´s datasheet.

Klaus

Thanks for your reply, but I was more concerned about the theory. I mean we have been taught that transformers to work need frequency, but what exactly? Nobody answered that!

--- Updated Dec 11, 2020 ---

Michael Faraday demonstrated that to induce a current the magnetic field, must be time-varying.
DC is constant, it cannot be induced.
The minimum frequency in a transformer is related to the maximum flux density B that a particular magnetic material can sustain.
And B is a function of voltage, frequency, turns and magnetic cross section.

Thanks, that was very helpful!

 

[KlausST]

Hi,

With ignoring DC resistance...
You may use a transformer at:
* half the frequency with half the voltage
* 1/10 the frequency with 1/10 the voltage
* 1/100 the frequency with 1/100 the voltage
and so on

There is no theretical limit.

Klaus

 

[Easy peasy]

dB / dt = V / N.Ae as long as you meet the formula at left and the Bpk does not saturate the transformer - you can go down to any low frequency, so if I want to traverse from -1.8 tesla to +1.8 tesla with an applied voltage of 200V, in 100 seconds, the N.Ae product must be 5555 ( Turns. m^2 ) so a 1 meter squared core allows 5555 turns, and etc ....

Also, for a sinusoid applied voltage: Bpk = Erms / ( 4.44 F N Ae ) Ae = core area, N = turns, F = freq, Erms = rms volts

Same example as above, Bpk = 1.8 Tesla in my high grade ( Supermendur ) electrical transformer steel, Erms = 200V, F = 1/200 sec

this implies N.Ae = 5005 ( answer a bit different as the AVE applied volts are 200 x 1.11, c.f. to 200V ave in the above ) N.B. 5005x1.11=5555

so a core 250mm x 250mm ( 0.0625 m^2 ) requires 80,080 turns for 200Vrms at 5 milli-Hz, or 400T for 1V rms at this freq,

happy designing ...