Re: Gaps in laminations of high-current C-core transformer


"Phil Allison" <philallison@xxxxxxxxxx> wrote in message
news:5fgrbmF3d3gefU1@xxxxxxxxxxxxxxxxxxxxx

"Paul E. Schoen"
The company I work for makes high current circuit breaker test sets,
using a transformer which can produce pulses of 60,000 amperes with an
open circuit output of about 12 VAC. Today, our production assembler
brought it to our attention that one of the C-core laminations had very
visible gaps. It appears that the tension on the steel strap that holds
the assembly together is insufficient to provide proper pressure, or
perhaps the laminations retained too much springiness. There is a jagged
opening along one edge of the core, showing where it had been cut in a
staggered pattern.



** That is not a conventional C-core.

The lams are not properly glued together and the gap is not flush but
staggered.

It is a poor man's excuse for one.

I'll inform my client of your observation. I concur completely. The earlier
versions of this tranny looked pretty good. Somebody's getting sloppy, and
we pay over $6000 each for these beauties!


The manufacturer looked at pictures we took and replied that sometimes
they opened like that during dipping and baking, but the exciting
current readings were OK.


** Who's idea of " OK " is that ?


I would not think the exciting current would be adversely affected, and
in fact would probably be less, but I think there will be other
problems.


** Imag increases with the presence of an air gap.


My reading about gaps in transformer cores shows that even small air
gaps greatly increases inductance and reduces efficiency.


** No - it reduces inductance and increases I mag.

What I read, copied at the end of this reply, mentions core flux fringing
due to air gap and increasing inductance. But later they explain how an air
gap reduces inductance. Thanks for setting me straight on this.


It is important for these transformers to be able to provide very high
peak currents and to exhibit low internal impedance. I am quite
concerned about this, and I am afraid that this transformer will
probably perform poorly, and also likely degrade in time due to thermal
and mechanical stresses. It will also probably be electrically and
acoustically noisy.


** It may work more or less OK, despite the flaw.

It is a cert to be noisy though.



I have posted a photo of the transformer:
www.smart.net/~pstech/XFMR-444b.jpg.

I'd like to send the whole batch of transformers back and make our own
from toroids. What do you think?


** The thing is, THAT is not a conventional " cut core" C-core.

Looks like strips have been simply cut to fit in bunches of about 10,
then tied in place with a lot of hope that dipping and baking would fix
all.

This the guy that designed it - shown here trying his hand at welding ?

http://www.zen40166.zen.co.uk/image005.jpg


Now *that* is an uggly tranny. I didn't know you could make one out of
clay?

Thanks, Phil. Much helpful advice. Maybe I can get them to go toroidal.

Paul

---------------------------------------------------------------------------------------


Toroidal Transformer Advantages.
Higher flux density Is possible In a toroidal transformer because the
windings are wound symmetrically Fig. 4. If an air gap is present, more
magnetizing force (H) is needed to produce a given core flux (B). I

Fig. 5. Influx fringing. a small amount of core flux escapes the core at
the air gap. decreasing the useful flux path and increasing core
inductance,
over the gapless core. That symmetry results In smaller size and weight of
the iron core. Also, because the windings completely enclose the core flux,
stray magnetic fields that could Interfere with other circuitry within the
en-closure are greatly reduced. Much less shielding Is required for use
with sensitive or high-gain electronics.
The noise (hum) In a conventional transformer Is due to core
magnetostriction, which is a very small deformation of the core iron under
the influence of the magnetic field induced by the AC primary current.
Be-cause the windings completely envelope the core in a toroidal
transformer, audible hum Is reduced to 10% to 15% of that of a conventional
transformer.
Because It has lower losses, the toroid transformer Is more efficient and
runs at a lower operating temperature. It also has better load legulatlon
than a conventional transformer of the same power rating.

The Effects of Air Gap. Much of the lower loss can be attributed to the
reduced air gap of a toroidal transformer core. While intentional air gaps
are designed into Inductors to pre-vent DC saturation, air gaps produce a
number of undesirable effects In power transformers. Par the typical E-l-or
C-core power-transformer laminations, the air gap is 0.002 Inches. In a
toroidal transformer the effective air gap is extremely small (typically
less than 0.00001 Inch) and can be Ignored for design purposes. The lack of
a discrete air gap minimizes losses, leakage. and-flux-wave -distortion,
and decreases the mmf needed to produce a given level of flux In the
transformer.
The Ac inductance Is determined by the number of turns, the Impressed
voltage, and the core cross-sectional area. The magnetic flux path has two
components, the core magnetic length and the air-gap length. Those two
components are not equal, be-cause air and Iron have vastly different
permeability's.
Permeability Is the ratio of the change In magnetic induction (B) to the
change In magnetizing force (H), and is equal to BIH. The permeability of
air is constant at 1, while the permeability of silicon Iron depends on the
degree of saturation in the core. At 80% saturation, silicon iron has a
permeability of about 4000. Because air offers 4000 times more reluctance
to flux changes, a very small air gap has a great effect on the magnetizing
volt-amperes needed to produce a given output power. An air gap In-creases
the effective length of the magnetic path, reduces the inductance and, as
shown in Fig. 4, causes more slope In the B-H curve. That re-quires more
magnetizing force, and thus more primary current, to generate a given core
flux. Once saturation Is reached, no further Increase in flux can occur
even If the magnetizing current Is increased by raising the primary
voltage.
Another disadvantage of an air gap Is flux fringing. Not all of the core
flux remains within the core cross-section adjacent to the gap. A small
per-centage curves outward near the edges of the core as shown in Fig. 5.
causing core flux fringing. That fringing decreases the useful flux path
area and increases the core Inductance


.

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