V2/v1 = n2/n1

[zesla]

Hi guys,

I am wondering why the voltage across the secondary windings of a transformer is not a function of the primary frequency?
I think that a transformer works by faraday law, so the frequency is important too the secondary voltage generated by the transformer but the below relationship does not state that! So whats wrong?:
V2/V1 = N2/N1

Thanks.

 

[ericgibbs]

Hi guys,

I am wondering why the voltage across the secondary windings of a transformer is not a function of the primary frequency?
I think that a transformer works by faraday law, so the frequency is important too the secondary voltage generated by the transformer but the below relationship does not state that! So whats wrong?:
V2/V1 = N2/N1

Thanks.

hi,
The Equ you posted is a ratio relationship, it dosn't really have specific frequency.

 

[crutschow]

The transformer flux level is indeed related to frequency and voltage. But since the primary and secondary both experience the same frequency, the voltage ratio between primary and secondary is independent of frequency and only depends upon the turns ratio.

 

[zesla]

The transformer flux level is indeed related to frequency and voltage. But since the primary and secondary both experience the same frequency, the voltage ratio between primary and secondary is independent of frequency and only depends upon the turns ratio.

Thanks but it does not make any sense. First please tell me if you are agreed with me that in the below pic which is an implementation of the farady's law, th faster turning the left magnet (which of course corresponds to increasing the frequency of the primary of a transformer), the more voltage will be generated in the right coil (which corresponds to the secondary of a transformer)? the only difference is that here in the pic there is no iron to concentrate the flux and the air will do the same job but with much more reluctance, and of course the both sides in the pic will experience the same frequency as well.

 

[zesla]

i think a transformer works by farady's law and actually the farady's law (e= d teta/dt) which states that the voltage generated is a function of changes in the current, which of course means frequency. So the higher the frequency the more output voltage we should expect.

 

[Dean Huster]

Using a normal transformer designed for use at 60 Hz, if you increase the frequency, the output voltage generally will go down. Now, there may be a point where you'll find a "sweet spot", a frequency where the L and C of the transformer will try to be a bit resonant and the voltage will peak, but that's just a fluke. In general, you'll begin to get a frequency that's high enough that the eddy currents will overpower everything else and the transformer will work about as well as a block of steel.

This is why transformers designed for higher frequencies (switch-mode power supplies, old CRT television horizontal circuits and HV supplies, etc.) use ferrite cores for their transformers. They handle the higher frequencies well with a lot lower eddy currents. However, at low frequencies, they're not so hot.

 

[Diver300]

i think a transformer works by farady's law and actually the farady's law (e= d teta/dt) which states that the voltage generated is a function of changes in the current, which of course means frequency. So the higher the frequency the more output voltage we should expect.

The primary current will not be constant if the supply frequency changes. The inductance of the primary side means that the primary side is inversely proportional to the frequency.

If you double the frequency with the same supply voltage, the current will halve. The magnetic field will halve, but it will get to that value in half the time, so the rate of change of magnetic flux will be the same, and the output voltage will be the same.

In most transformers, the winding resistances are small. The changing magnetic flux in the core generates voltage in both the primary and the secondary. The voltage generated in the primary is nearly as large as the supply voltage. If the rate of change of magnetic flux were too small, there would be a lot of current, which would increase the flux, so the voltage generate would increase, to balance the supply voltage.

So the voltage generated in the primary is just about equal to the supply voltage. The voltage generated in the secondary is larger or smaller by the turns ratio.

Two things to note. At higher frequencies, there is less flux, so the the cores can be far smaller. That is one reason why 400 Hz is used on aircraft. It is why switch mode power supplies, with transformers running at 20 kHz or more, are far smaller and lighter than linear power supplies.

Secondly, in this thread https://www.electro-tech-online.com/threads/quick-transformer-connection-question.119432/#post982194 are the results of when I reversed one half of the primary winding (with a suitable safety resistor). There was no voltage generated by the primary winding, and the current would have been huge without the resistor that was there for the test. The point is that transformers rely on the voltage generated in the primary winding.

 

[Diver300]

Thanks but it does not make any sense. First please tell me if you are agreed with me that in the below pic which is an implementation of the farady's law, th faster turning the left magnet (which of course corresponds to increasing the frequency of the primary of a transformer), the more voltage will be generated in the right coil (which corresponds to the secondary of a transformer)? the only difference is that here in the pic there is no iron to concentrate the flux and the air will do the same job but with much more reluctance, and of course the both sides in the pic will experience the same frequency as well.

In a generator, where the magnetic flux is constant, the output voltage will be proportional to the frequency.

In a transformer, the flux is not constant as the frequency changes.