Note: Descriptions are shown in the official language in which they were submitted.
sACKGRou~D OF THE INVENTION
Field of_the Invention
This invention pertains to a device known as a ferro-
resonant trans:ormer and specifically to an improved struc-
ture therefor.
Descriptlon of the Prior Art
A ferro-resoilant transformer, sometirn-es referred to as
"Sola" regulator, is a special type of transio.rmer that
accepts an applied square wave voltage and, because of
inherent properties of the transformer, furnishes a more
si.nusoidal alternatiny current output. A capacitor is
connected across the secondary of the transformer tuned near
the resonant frequency of the ac to achieve this operating
mode.
In a conv2ll-t:ional ferro-resonant transformer, the cores
or legs of the transformer are comprised of laminations of a
steel ]cnown-for its magnetic ef:Eiciency and referred to in
the~ industry as "ferro-resonant steel". Steel grade M6 or
better have he.retofore been thought to be necessary for
obtaining fer.ro-resonarlt transformer operat:ion. Such stee
can be made i.n very thinly laminated parts and result in
very small los~-es due to eddy currents.
Now refe:rrin() to the magnetic operation of a prior ar-t
ferro-resonant trallsformer, such a transformer traditionally
includes windi.ncJs for both the primary and seconcl.lry ~ound
about a center core or leg of ferro-resonant steel. The
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external legs of -~he transformer are likewise comprised of
laminated ferro-resonant steel and the dimensions are such
that the sum of t}-le cross-sectional dimension of the ex-
terllal legs e~cc~eds the cross-sectional dimension oE the
center leg to rccluce losses.
There are also more turns on the secondary winding -than
on the primary win(3ing. In operation, the primary windin~
cloes not cause suificient flux density i~ the principal path
through relateci core to cause sa-turation and, therefore,
]0 opexates in the lirlear mode. Mowever, the secondary windillg
does cause suEficient flux density in the center l.eg of the
core related to i-ts operation and therefore does not operate
in a linear mocle. In fact, on the B-H curve (flux densi.ty-
coercive force curve), operation is such tha-t a large coer-
cive force is required for a small change of flux density
and, hence, provicles good regulation properties. The ex-
ternal legs, because o:E thei.x dimensional rela-tionship to
the center le~, do not saturate. It has heretofore been be-
lieved that i-t was necessary to avoid saturat:in~ thc e.~cter-
nal le~s because to do so would create too many losses anclthereEore such a structure would be unsatisfactorily ilte f -
:Eicien t .
Ilowever, it should be noted that ferro-resonarlt trans-
former of the t:raditional desi~n described above ls an
expensive structure to manufacture compared with a conven-
tional transforlllel-. First, the steel is much more eY~pensi.ve.
Grade Ml9 is pe:rfec-tly acceptable for a conventional trans-
former compared with Grade M6 for a ferro-resonant -transformer
~i Of traditional des;.~n. Next, a conventional transformer is
~8~
generally evenly dimensioned with respect to the flu~ paths
of both the primary and secondary windings. That is, -the
center core leg i5 usually dimensioned so that it is e~actly
twice in cross~section to the sum of the cross-scction
dimensions of the e~ternal legs. Bo-th these factors prevent
the use of conventional transformer laminations for -the
construction of a ferro-resonant transformer.
Further, it hcls no-t heen known how *o standardize
Perro-resonant transformer construction so that -the same
1~ component parts can be assembled in nearly the same fashion
for operation w:ith respeck to various input volta~es for
providing the same rated volt-ampere rating for -the trans-
former,
Therefore, it is a feature of the present invention to
provide an improved ferro-resonant-type transformer that
cloes not employ ferro-resonant steel, but the less e~pensive
steel employed in conventional transformersO
~ t is another Eeature of the present invention to
provide an improvec~ Eerro-resonant-type trarls~ol.mer tl-lat
employs core d:irnellsions common ~or conventional transformers.
It is stil:L another feature o.~ the present invention to
provile an improvecl Eerro-resonallt-type transformer that is
constructed in such a way that it is connectable to various
input driver ci.rcuits, all parts being common to a conven-
tional transformer.
~u~-,mar~ of -the Inventlon
The invent:Lon embodiment disclosed shows a transformer
core of a "T" configuration made of laminated steel commonly
employed in conventional transformers. The external legs
are each in an "L" configuration and are likewise made of
laminated stecl commonly employed in conventional trans-
:Eormers. The primal-y winding has fewer turns -than the
secondary winding arld the secondary winding has a sufficient
number oE turns to saturate the entire principal flux pa-th
re:Lated to the secondary winding, including the center leg,
the external legs and the shunts.
The windings are provided in layers for the primary so
as to have connecti.ons for operating in connecti.on with an
inverter connected to a battery or other de source ovcr a
eol~on ranc3e o:E voltacJes. The secondary is also provided
with eonvenient taps.
Thus, b:road:Ly, -the invention eontemplates a ferro-
resonant-type transEorme.r whieh eomprises a laminatecl
eenter leg and a pair of laminated external legs made of
eonventional trans:Eormer steel, with the eross-sectlonal
dimension of the center leg eclualling the sum of -the
cross-seetional dimensions of the external legs. A
primary winding is wound about the eenter leg and has
suEficient turns and resistance so as to operate within
the linear ranye with respect to applied rated s~uare
wave voltage. A secondary winding is wound about the
center leg and spaced apart from the primary winding,
with a magnetic shunt between the primary winding and
the secondary winding. The secondary winding has
sufficient turns and resistance so that the steel in
the center leg underneath the secondary winding and
the return flux path through the external legs and the
shunt Elux satura-tes :Eor rated volt ampexes, therefore
resulting in a non-s~uare wave output with good regu-
la-tion and without excessive losses in ferro-resonant-
type transformer action.
Brief Description of the ~
So that the manner in which the above-recited features,
advantages and objects of the invention, as well as others
which will become apparent, are attained and can be under-
stood in de-tail, more particular description of the inven-
tion br.iefly summari2ed above may be had by reEerencc to the
embod:iment thereo~ which is lllustrated in the appended
drawings, whi.ch drawings form a par-t oE this specifica-tion.
It i5 to be noted~ however, tha-t the appended drawings
illustrate only a preferred embodiment of the invention and
`''.`'
are therefore not to be considered limiting of its scope,
~or the inven-~ion may admit to other equally effective
embodiments.
In the Drawings: -
Fig. 1 is a plan view of a ferro-resonan-t--type -trans-
former in accordance with the presen-t invention, also showing
flux paths for both the primary winding and secondary wind-
ing .
Fig. 2 is a top view of a transformer ~obbin as i-t is
1~ positioned wi-th respect to the legs of a transformer in
accordance with -the present invention.
E'ig. 3 is a partial side view of a vertical cross-
sec-tion of a ferro-resonant-type transformer in accordance
with the present invention.
Fig. 4 is a schematic diagram of the primary winding
for a ferro-resonant-type transformer in accordance with the
presen-t invention when connected for operation with respect
to a square wave input derived from a 30-volt dc source.
Fi~. 5 is a schematic diagram of the primary winding
for a ferro-resonant-type transformer in accordance ~:ith the
presen-t invention when connec-ted Eor operation wi-th respec-t
to a s~uare wave input derived from a 60-volt dc source.
Fig. 6 is a schema-tic diagram of the primary winding
Eo.r a Eerro-reson~n-t---type -transformer in accordance wi-th -the
present invention when connected for operation with respec-t
to a square wave input derived from a 120-volt dc source.
Fig. 7 is a schematic diagram of -the secondary winding
for a ferro-resonant-type transformer connected -to a capa-
citor -for resonant operation near the desired frequency of
operation.
Fig. 8 is an overall schematic diagram of a ferro-
resonant transformer in a typical connection configura-tion.
Deseription of Preferred Embodiment
Now referring to the drawings, and first to Fig. 1, 2
and 3, it is typieal that a transformer employ a stack of
laminated parts. The number of laminates is not so material
as the dimension of the stack-. In a 1000 VA ferro-resonant-
type transformer in accordance with the present invention it
has been found convenient to provicle stacks of grade Ml9
steel laminates that is 3 3/4 inches high i.n depth dimension
10 for each the eenter leg and the -two external legs.
Width 12 of the center eore leg 11 is 1 1/2 inches and width
1~ of eaeh of the external legs lS and 17 is 3/4 inehes.
~lenee, the dimension in eross-seetion of the sum of the
e~ternal legs is ec~ual to that of the eenter leg.
It is typical to have the primary winding and the
seeondary winding of a conventional transformer wound on
bobbins made of glastic, a non-magnetie combination of
fib~r~las and plastic. For conventional transformers having
eenter eore laminates 1-1/~ inches wide, -the stack is nor-
mally 3 inches. Therefore, two bobbins are used to make one
bobbin 16~ Split 18 shows where these two bobbin parts are
joined.
Turns ~0 for the primary winding are shown in Fig. 3
Wound on bobbin 16. In a similar fashion -turns are wound on
a separate bobbi.ll for the secondary windi.ng. The windings
are sepclrated frorn each other by an air gap. Eor tl-e trans-
formel. beiny de<,cri.bed, -the air gap between the center leg
ancl external le~3 is 3/4 of an inch wide, as identi,fied by
nwlleral 22. A shunt 24 fills most of the air g.lp/ w;dth
dimc~nsion 26 being 0.660 of an inch. The depth dimension is
3 inclles and the height is 0.98 of an inch. The shunt is
also made of laminated sections, the laminations bei.ncJ shown
endwise in Fig. 3 for the shunt. A simi.lar .shun-t 28 is
prov.ided in the opposite air gap. Pieces of "NO~EX"* fill up
the remainder of the air gaps not occupied by the shunts.
The principal flux path for the primary windi.ng is
shown in Fig. l as ~p and the principal path for the flux
pa-th related to the secondary winding is shown in Fig. l as
0s It should be understood that complementary paths for
both windings also pass through the other external leg in
acldition to the l.eg illustrated. There is some flux :for both
windirlgs that travel the entire core length, and thus also
pass through the other winding from -the windi'ng creatin~ the
:~: lu~
The secJmc~llt:s or the windings which overall. ma!ie up thc
primary windi3ly just descri~ed are shown in Fiys. 4, 5 and
. In Fig. ~, wind:iTIy 30A and 30B comprise the E:irst two
.layers; windincJs 32A and 32B comprise the second two layers;
windings 3~A and 34B comprise the third two laye-s, and
windinc3s 36A and 36B comprise the fourth two layers. Each
layer set is sel:~arated by an insulatin~ layer of "NOME.X"* and
the parallel connections are made as shown. Small driver
windin~s 38 and 40 share the final layer. Each of the four
* trade mark 8
windings comprise 46 turns of number 14 wire. The driver
windings each have 5 turns of number 16 wire.
Fig. 5 shows a configuration wherein the same four
windings 30, 32, 34 and 36 are used wi-thout the ccnter tap
connections emp~oyed in Fig. 4. Fig. 6 shows a conficJura-
tion wherein winciings 30 and 32 are connected in series and
windings 34 and 36 are connected in series and the connec-
tion bekween the two series connections provides a center
tap for the ent:ire primary. Again, the driver windings are
the same as for the Fig. 4 and Fig. 5 structures.
The secondary winding comprises 238 turns of number 12
wires and is wouncl to provide seven layers of 34 turns per
layer. Taps are provided at the 72nd, 93rd and 162nd turns
to provide 120-volt, 240-volt and 277-volt taps for a 400-
volt overall secondary. Capacitor ~2 across the secondary
at 39 microfarads tunes the secondary to a resonant fre-
quency ~us-t off 60 Hz for successful ferro-resollant opera-
tlo~l.
~ typical overall schematic diagram for the ferro-
resonant-type transformer just described is shown in Fig. 8.
Such a~structure provides operation of the ~)rimary
winding in the ]inear ranye and the secondary winding in a
satisfactory saturated mode for variatlons 85 to 100 per-
cent of the voltac~e inputs shown in Figs. 4, 5 and 6.
Regula-tion can be improved with a ]arger capacitor 42, but
losscs do increase. Ilowever, with the values shown, al-
though greater -than with prior art ferro-resonan-t trans-
formers, the losses are not great.
~ regulation ratinc3 of 2-1/2 is a cor~lon design goal
~ " ,J _ g
(volts in secondary times circulating current equals 2-1/2
times the VA rating for the primary). In this case, the
circulating current is a little over 6 amperes for an
overall regulation rating of 2.8, which is quite goodO
The output waveform includes some high harmonic dis-
tortions which can be filtered out by appropriate components
(not shown), if desired. Also, the output waveform can be
further shaped to be more sinusoidal, if desired.
While a particular embodiment of the invention has been
shown and described, i-t will be understood that the inven-
-tion is not limited thereto, since many modifications may be
made and will become apparent to those skilled in the art.
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