Re: Coupling coefficient of industrial transformers

On May 8, 1:03 pm, The Phantom <phan...@xxxxxxx> wrote:
On 8 May 2007 05:35:59 -0700, orvillefpike <orvillefp...@xxxxxxxxx> wrote:





On May 7, 2:35 pm, Rodger Rosenbaum <nos...@xxxxxxx> wrote:
On 7 May 2007 12:20:44 -0700, orvillefpike <orvillefp...@xxxxxxxxx> wrote:

Looking at your numbers below, it seems that you probably don't have an
inductance meter, but are deriving the inductance of a winding from the ratio of
the voltage across it and the current through it.

The scope method doesn't work as well in an inductor with a large DC
component. What you can do, however, is to measure the AC voltage across the
inductor and the AC current through it, take the ratio and get the AC impedance.
Make sure that your meter doesn't measure true RMS AC+DC; you don't want it to
see the DC component. If it does, you will need to put a capacitor in series
with it to measure only the AC component.

Start out with not much DC current and calculate the inductance by that
method. As you increase the DC current, measure and calculate the inductance
again. You can detect the onset of saturation as the value of DC current where
the inductance begins to decrease. Saturation is a gradual thing, of course,
and you will see the calculated inductance gradually decrease as the DC current
increases.
When you were making these measurements on the primary of the two
transformers, the secondaries were unloaded, right?

The MWT would seem to be well into saturation, but otherwise the numbers seem
reasonable.

M. Rosenbaum

I don't have an inductance meter, all I have is an oscilloscope, a 2
Amp adjustable DC power supply, a Wavetek digital multimeter and a
clamp on Amp meter. Do these meter measure AC+DC? Since I don't have a
large DC power supply, I guess I'll have to make these measurement in
the circuit.

The clamp on by its very nature can't measure DC, so you're all right there.
Set your Wavetek to measure AC volts and connect it to your DC power supply with
the supply set to put out maybe 5 volts (with the range of the Wavetek set
appropriately to measure 5 volts). If the Wavetek reads essentially zero, then
it's not a true RMS AC+DC meter, and you're good to go.

Yes these measurement are make with the secondary unloaded and the
impedence of the secondary is calculated according to the turn ratio.

Are you saying that the MWT is in saturation because it draws 5 Amps.
I thought so too, the value of H is around 2,600. For laminated
steel, it starts to saturate at around 1,500 I think. Before I removed
the magnetic shunt in the core of the MWT, the current was around 3
Amps which put H at around 1,500. Is there a problem if the
transformeris saturated? Does it reduce thecouplingcoefficient?
If I put the appropriate air gap in the core of thetransformer, to bring
the value of H at around 1,500, will it cause a problem in the
performance of thetransformer?

If thetransformercore is substantially into saturation, the unloaded primary
current will be high, as you have noted in the MWTtransformer. Thecoupling
coefficientwill be reduced somewhat. The additional IR drop in the wire of the
primary will reduce the efficiency of thetransformer(increased temperature
rise as a result) and will degrade its regulation. But, it will still perform
as atransformer.

Gapping the core of atransformerwill cause performance degradations similar to
those caused by saturation. The no load excitation current will go up, causing
extra losses.

Thanks- Hide quoted text -

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M. Resenbaum

From what I know, to get thetransformeraway from saturation, I would
have to take a few turns out of the primary

To reduce the flux density in the core you must *add* turns to the primary.

(which I don't want to do)
or lengthen the core

Or you must increase the *cross-sectional area* of the core. Lengthening
the core (making the magnetic path length longer) will not change the flux
density in the core much (if the primary turns and cross-sectional area
remain unchanged) but it will cause the no-load exciting current to
increase.

(which would be complicated) or reinstall the
magnetic shunt, am I right? Is there other way to achieve that?

The flux density in the core leg upon which the primary is wound is
determined by the cross-sectional area of that leg and the volts per turn
of the primary winding. Reinstalling the shunt will reduce the effective
path length in the core as seen by the primary. The core leg where the
primary is wound will still see the same flux density, but the shunt will
reduce the flux density in that part of the core beyond the shunt. Thus
there will be less volume of core with the same flux density as the primary
leg itself. That part of the core beyond the shunt will not be as far into
saturation, and this fact plus the reduced effective path length for the
primary will cause the exciting current to decrease. But, the part of the
core upon which the primary is wound will be just as far into saturation as
when there is no shunt if the primary turns remain unchanged.



I often see air gaps in large inductor, my guess was that they did
that so they didn't have to make a large steel core. I thought I could
do the same in atransformer.

Inductors have gaps because the inductor is intended to store energy and an
ungapped core doesn't store much energy. Most of the energy is stored in
the gap of an inductor.

Transformers are designed so that as little energy as possible is stored in
the core. When energy is stored in the core, the exciting current goes up.
This is what is wanted in an inductor, but not in atransformer.





Thanks again- Hide quoted text -

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M. Phantom

I took my MWT that drew about 5 amps and I removed the magnetic shunt,
the transformer still drew about 5 amps.

There was 115 turns on the primary, I added 33 turns so now it has 148
turns, now the primary of the transformer draws only .75 amp.

Since I want to make an inductor out of it, by adding the appropriate
air gap, and since I want to pass 15 to 20 amps in that inductor, is
there a way to predict it's characteristics as the current increases
or is it done by the "trial and error method".

Thanks


.

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