In a transformer with 2 solenoids of different turn number mounted on a magnetic core powered by an AC source, why does the secondary EMF decrease as frequency of the power source is increased? What is the theoretical cause of this?

 

It should become clear if you read up on inductive reactance.. In short, the coils have an inductance, and inductors act more like resistors as the frequency increases.. (In comparison, capacitors act more like resistors as the frequency drop..)

 

Unfortunately, there are a number of reasons for this and it would be difficult to pin down the major contributor unless more was known about the particular transformer. Some of the areas you might want to look at (in no particular order) are:

· Flux saturation in the core. As the frequency goes up, given a fixed voltage, the flux in the core increases. If the transformer is already operating near saturation this can cause problems.
· Hysteresis in the core causes losses every time the flux changes direction. The more often the flux change, the higher the losses
· Skin effect in the wire at higher frequencies means the coils have effectively higher resistance.
· Induced eddy currents in the core material can cause losses due to resistance. These currents vary with the square of the frequency. They can be controlled by using ferrite type cores, but again become a problem at very high frequencies.
· While leakage inductance is not in itself lossy, it can produce losses if there are other inductive elements near the transformer (board traces, connecting wires, metal objects). Leakage inductance increases with frequency.

There may be others, but this is what I remember. You should be able to look them up and see which is going to be the biggest contributor in your particular case.

 

The primary cause of this would be "Leakage inductance" which is due to the magnetic fields which are NOT mutually coupled. This component of the equation acts as a stand-alone inductor, where the reactance is proportional to frequency.

In a perfect universe, a transformer with a perfectly matched resistive load on the secondary would present a purely resistive impedance on the primary. Such is not the case, of course.

Eric

 

The primary cause of this would be "Leakage inductance" which is due to the magnetic fields which are NOT mutually coupled. This component of the equation acts as a stand-alone inductor, where the reactance is proportional to frequency.

In a perfect universe, a transformer with a perfectly matched resistive load on the secondary would present a purely resistive impedance on the primary. Such is not the case, of course.

Eric

Leakage inductance is responsible for numerous issues with transformers (distortion, phase issues,...), but beyond what I already mentioned, is not generally a cause of energy loss. Because of this, leakage inductance is far more a concern for signal transformers than for power transmission.

If I were to pick the top 3 offenders in higer frequency situations, generally they would be:

1. hysteresis
2. eddy currents (a design problem - can be mitigated by core material choice)
3. skin effects (VHF and higher) or saturation (a design problem - can be mitigated by increasing core size)

 

Leakage inductance is responsible for numerous issues with transformers (distorsion, phase issues,...), but beyond what I already mentioned, is not generally a cause of energy loss.

I don't think anyone was claiming this was energy loss....since it is reactive power, it is essentially recoverable by means of complementary capacitance. It does make things a bit more complicated, though.

eric

 

The topic went off the rails beyond this post, so I have moved the subsequent posts nto Off Topic, under the title "Transformer nitpicking".

Sorry, OP.

 

I thought there was already a transformer nit-picking thread in that 'ideal transformer' topic..