Efficiency of a transformer with increasing frequency

[CNM]

Why does the effciency of my low voltage transformer very much
decrease (to eg 10%) when increasing the frequency to around 40000hz.
Why does this increase in frequency affect the efficiency value so
much?

 

[Lord Garth]

Why does the effciency of my low voltage transformer very much
decrease (to eg 10%) when increasing the frequency to around 40000hz.
Why does this increase in frequency affect the efficiency value so
much?

The impedance of the transformer is Xl=2*PI*F*L if F go up to 40KHz, Xl also
goes way up. Your power transformer was designed to work efficiently at
line
frequency, not 40KHz.

 

[John Larkin]

Why does the effciency of my low voltage transformer very much
decrease (to eg 10%) when increasing the frequency to around 40000hz.
Why does this increase in frequency affect the efficiency value so
much?

Core loss and skin loss. Cheap, thick steel laminations in the core
work fine at 60 Hz but have lots of eddy-current loss at hf. The wire
itself has skin and proximity effects, too. I'm guessing the core loss
is the biggie.

Besides, if it works at 60 Hz, it has far too many turns to be optimum
at 40K. A ferrite torroid or pot core would be much better up there.

John

 

[Larry Brasfield]

Why does the effciency of my low voltage transformer very much
decrease (to eg 10%) when increasing the frequency to around 40000hz.
Why does this increase in frequency affect the efficiency value so
much?

The most likely reason is that eddy current losses
in the core go up and magnetizing current goes up.
In effect, the shunt inductance of the transformer
goes down as frequency goes up when eddy
currents become signficant. Those currents are
(nearly) in phase with the applied voltage and
represent a loss.

You probably have a transformer designed for
line frequency with the lamination thickness set
accordingly. At 40 KHz, that thickness is way
too large.

 

[Don Kelly]

--
Don Kelly
[email protected]
remove the urine to answer

The most likely reason is that eddy current losses
in the core go up and magnetizing current goes up.
In effect, the shunt inductance of the transformer
goes down as frequency goes up when eddy
currents become signficant. Those currents are
(nearly) in phase with the applied voltage and
represent a loss.

--------
Actually the "magnetising" or inductive component of the current decreases
because of lower flux density for a given voltage, at higher frequencies
(The inductance won't decrease much if at all) .but the total exciting
current increases due to hysteresis and eddy current loss increases. This
increased loss current will coupled with skin effects will also lead to
higher I^R loss.
---------------

You probably have a transformer designed for
line frequency with the lamination thickness set
accordingly. At 40 KHz, that thickness is way
too large.

-----------
Right on- design a 40KHz transformer for 40KHz- don't use a 60Hz transformer
and expect good operation. It is more than just lamination thickness, it is
excessive iron.volume as well as capacitive effects. It is a wonder if a
60Hz transformer is not completely useless at 40KHz.>

 

[Larry Brasfield]

--------
Actually the "magnetising" or inductive component of the current decreases
because of lower flux density for a given voltage, at higher frequencies
(The inductance won't decrease much if at all) .but the total exciting
current increases due to hysteresis and eddy current loss increases.

I agree with your correction on terminology. I was
incorrectly referring to the shunt current term as
"magnetising current". As for inductance not being
reduced, I have to disagree. If you consider that
eddy current prevents flux from penetrating into
the interior of the laminations, you should be able to
see that the effective area of the core is reduced.

This effect shows up as a non-polynomial relation
between impedance and frequency, such that the
impedance rises approximately as sqrt(frequency).
If you take the imaginary part of the impedance in
any part of that curve, you will find that inductance
(defined as that component divided by radians/S)
is indeed decreasing with frequency.

This
increased loss current will coupled with skin effects will also lead to
higher I^R loss.
Yes.

-----------
Right on- design a 40KHz transformer for 40KHz- don't use a 60Hz transformer
and expect good operation. It is more than just lamination thickness, it is
excessive iron.volume as well as capacitive effects. It is a wonder if a
60Hz transformer is not completely useless at 40KHz.>

Agreed.

 

[Don Kelly]

I agree with your correction on terminology. I was
incorrectly referring to the shunt current term as
"magnetising current". As for inductance not being
reduced, I have to disagree. If you consider that
eddy current prevents flux from penetrating into
the interior of the laminations, you should be able to
see that the effective area of the core is reduced.

 

[Bob Eldred]

Why does the effciency of my low voltage transformer very much
decrease (to eg 10%) when increasing the frequency to around 40000hz.
Why does this increase in frequency affect the efficiency value so
much?

Core loss is a part of it and core loss goes up with frequency, but flux
density goes down with increasing frequency which helps to ameliorate core
loss. Whatever the core losses are at a given frequency, they would be much
worse if the flux density were maintained at its low frequency value.

Another important loss is driving of the winding capacitance by the
resistance of the windings and by the impedance of the leakage inductance.
This can be a very important loss factor because the AC resistance of the
copper (not impedance) goes up frequency because of skin effect and may
become excessive at 40KHz. The capacitance was most likely ignored in a line
frequency transformer design but can be significant in a 40KHz transformer.
Furthermore higher voltage windings often have higher capacitances and
smaller wire with higher resistance exacerbating the loss effects.

Leakage inductance is the inductance that does not link the primary to the
secondary and therefore no transformer coupling occurs across it. It's just
a series impedance in the way of transferring power across the transformer.
It drives the winding capacitances as mentioned but also is in series with
the load reducing the transfer. Like all inductances, it's impedance
increases with frequency.
Bob