calculating saturation current of transformer

[[email protected]]

I have designed a 1:1 transformer that will be used in a low voltage
DCC converter. I have tested the current at which the transformer
saturates by decreasing the frequency of the primary voltage while
keeping the secondary open loop.

I would like to calculate the theoretical value of the current at
which the transformer will saturate. I tried using the formula in
Flanagan's which relates Imax to Bmax and I then used the Bmax of
ferrites but that doesn't correspond to the current I measure on the
oscilloscope.

Also, please clarify the following for me since I find it very
confusing: is magnetizing inductance the same as primary inductance?
How do I correctly calculate magnetizing inductance?

 

[[email protected]]

I have designed a 1:1 transformer that will be used in a low voltage
DCC converter. I have tested the current at which the transformer
saturates by decreasing the frequency of the primary voltage while
keeping the secondary open loop.

I would like to calculate the theoretical value of the current at
which the transformer will saturate. I tried using the formula in
Flanagan's which relates Imax to Bmax and I then used the Bmax of
ferrites but that doesn't correspond to the current I measure on the
oscilloscope.

Also, please clarify the following for me since I find it very
confusing: is magnetizing inductance the same as primary inductance?
How do I correctly calculate magnetizing inductance?

I forgot to mention that my method of calculating the magnetizing
inductance was with the formula: Lprim = (Uo x Ua x N^2 x Ae) /Ie. Is
this correct? Is the effective path length the length that the flux
must travel in the core or the length that the electrical current must
travel in the winding?

 

[[email protected]]

What's Ie in the above formula? Not a current, I hope. Inductance isn't
proportional to current (well, it _is_ due to saturation, but for the
linear region, no).

If Ie is the core mean path length, then this looks OK. But I could be
leading the witness.

that is l for length yes. So that explains my other question as well,
it is the length the flux travels.

Using that formula the calculation doesn't work out on the same value
as the value simulated with Ansoft Pexprt.

 

[Phil Allison]

<[email protected]

I have designed a 1:1 transformer that will be used in a low voltage
DCC converter. I have tested the current at which the transformer
saturates by decreasing the frequency of the primary voltage while
keeping the secondary open loop.

** How is that finding a "current" ??

What you have likely found is a combination of primary voltage and frequency
that (just) fully magnetises the core.

I suppose you realise that the applied voltage and frequency that produce
any given level of core magnetisation are in inverse proportion ??

I would like to calculate the theoretical value of the current at
which the transformer will saturate.

** A verbal nonsense.

Seems you have transformers all mixed up with inductors.

What are you REALLY trying to find ??




..... Phil

 

[Eeyore]

I have designed a 1:1 transformer that will be used in a low voltage
DCC converter. I have tested the current at which the transformer
saturates by decreasing the frequency of the primary voltage while
keeping the secondary open loop.

I would like to calculate the theoretical value of the current at
which the transformer will saturate. I tried using the formula in
Flanagan's which relates Imax to Bmax and I then used the Bmax of
ferrites but that doesn't correspond to the current I measure on the
oscilloscope.

Also, please clarify the following for me since I find it very
confusing: is magnetizing inductance the same as primary inductance?
How do I correctly calculate magnetizing inductance?

Get Epcos's Magnetic Designer program. If the cores and material fit or
approximate what you're using it'll tell you all.

Graham

 

[[email protected]]

To those interested I have found the solution to my problem:

Amperes Law: A current carrying conductor produces a magnetic field of
intensity H whose SI units is in (A/m). The line integral of the
magnetic field intensity H equals the total enclosed current:

$B-s(BHdl = $B-t(Bi

The above equation can be rewritten as for most practical circuits

$B-t(B HkIk = $B-t(BNmim

Further simplifying the equation you can use:

ISAT = HSAT x l

Looking at the B-H curves for the transformer material it can be seen
that the core is saturated at a H value of around 200 A/m.

The effective path length of two E18 cores is 24mm.

Therefore Isat = 200 x 0.024 = 4.8A

Looking at my measured graphs I can see saturation starting to occur
at 90kHz with an Ip-p = 8.8A. Decreasing the frequency further the
unmistakable saturation knee starts to form and the base of the knee
stays at just below 10A p-p.