Voltage stabilizing transformer

An improved magnetic core transformer for use as a voltage stabilizer in gas discharge lamps and tube circuits. The transformer has a magnetic stack length greater than either side of the magnetic cross-section and a floating shunt assembly constructed from stacks of magnetic strips. The stack length is optimized technically and as a function of the cost of iron and copper utilized in the transformer and when conformed with an optimum shunt a greater leakage inductance variation is achieved.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The present invention relates to voltage stabilizing magnetic core 
transformers of the type used to energize gas filled lamps and lighting 
tubes. 
2. Background Art 
Magnetic transformers have been used for voltage regulation in the ballast 
circuits of fluorescent and other gas filled discharge lamps for a number 
of years. The problems associated with the use of magnetic core 
transformers for this purpose usually involve the high cost of the iron 
and copper materials used in the manufacture of these devices. These 
problems are aggravated by the fact that proper operation of a voltage 
regulator or stabilizing transformer requires a magnetic shunt and air gap 
in the magnetic circuit of the transformer, which complicates the shape of 
the iron core elements. 
One solution to the above problems has been to assemble the magnetic core 
from magnetic sheets or stampings which include the shunt as an integral 
part of the central winding core. An example of this type of construction 
is found in Spanish Pat. No. 352,884 of Aug. 1, 1969. That patent 
discloses a transformer of essentially square cross-section with a stack 
length governed by the formula that the ratio of the stack length to twice 
the sum of the sides of the stack cross-section is equal to or greater 
than 0.25 and wherein the coils are wound in a plane parallel to the stack 
length. In addition, the shunt piece is an integral part of the winding 
core thus providing a shorter magnetic circuit length and greater 
dispersion through the shunt as opposed to the windings. This design is 
said to produce significant improvements in stabilization over previous 
designs when used with a capacitive reactance in the secondary circuit. 
FIGS. 1 and 2 show the two types of magnetic stabilizers that are presently 
used and manufactured. These two types of magnetic stabilizers are 
basically the same in concept, the use of either depending on the 
dimensions of the lamp or tube for which they are to be employed. As can 
be seen in these figures, the physical difference is in dimension "A", 
which solely affects the length of the magnetic circuit, the magnetic core 
section being the same in both models. 
The stabilizing transformer of the present invention provides a significant 
improvement over existing designs in that the stack length is much greater 
than in previous designs and is technically and economically determined by 
optimizing the stack length in terms of operation and material costs, this 
is combined with a floating magnetic shunt, both of which providing a 
greater leakage inductance variation with respect to the primary voltage 
and thus a much wider range of stabilization. 
SUMMARY OF INVENTION 
The invention is an improved voltage stabilizing magnetic core transformer 
which has a greater stack length than transformers of the prior art 
combined with a floating magnetic shunt. The greatly increased stack 
length is optimized in terms of operation and material costs thereby 
significantly reducing the weight of copper and increasing the useful 
power as a result of the concomitant reduction in winding losses. 
The unique and flexible floating magnetic shunt of the invention is formed 
from parallel stacks of magnetic strips placed between the primary and 
secondary windings and abutting the winding core. 
The stabilizing transformer of the invention provides higher useful power 
than a standard stabilizing transformer because the winding losses are 
reduced due to the optimization of the amount of copper used for a given 
transformer application. 
Greater stability under wide conditions of supply voltage variation is also 
achieved in the stabilizing transformer of the invention because the 
leakage inductance variation with respect to the supply voltage variation 
is greater as a result of the unique magnetic arrangement design. This 
arrangement provides superior flexibility due to the shape and assembly of 
sheets, allowing cost savings in materials as well as electromagnetic 
regulation of the electrical characteristics of the core/windings 
combination which permits perfect adaptation of the stabilizing 
transformer to each type of lamp. This cannot be achieved with 
conventional stabilizing transformers which have fixed shunts and shorter 
stack length.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 2 there is shown two types of magnetic stabilizers 
that are presently used and manufactured. These two types of magnetic 
stabilizers are basically the same in concept, the use of either depending 
on the dimensions of the lamp or tube for which they are to be employed. 
As can be seen in these figures, the physical difference is in dimension 
"A", which solely affects the length of the magnetic circuit, the magnetic 
core section being the same in both models. 
Referring now to FIG. 3, there is shown a perspective view of a stabilizing 
transformer according to the invention. As can be seen the stack length 
(L) is much greater in the stabilizing transformer of FIG. 3 than those of 
either FIG. 1 or 2. Thus, the magnetic core section of the stabilizing 
transformer of the invention is greater than that of the conventional 
transformer resulting in greatly improved reactance operation at 
significant savings in cost. 
In equivalent magnetic transformers, for a given magnetic induction and 
effective voltage, the number of turns N multiplied by the magnetic core 
section S is constant, hence the weight of the copper windings is 
inversely proportional to the stack length, and the opposite occurs with 
the weight of iron which is directly proportional to the stack length. 
From an economical point of view the optimum stack length is that with 
which the combined cost of the iron and copper is minimum. This stack 
length differs greatly from currently known stabilizing transformers. 
Since it is possible to save a considerable amount of copper by increasing 
that length, thus, taking into account that the increase in cost of iron 
is more than offset by the decrease in cost of copper, the reactance 
material cost is appreciably less for the optimum stack length. 
FIG. 4 which shows, a "scrapless" type magnetic sheet and strip assembly 
model for the core of a stabilizing transformer according to the 
invention, which together with the shunt strip sh, shown in FIG. 5, of the 
same length as the stack length L, has the following constructive 
advantages in the magnetic cores of the subject invention. 
As the shunt strip sh is separate, the number of strips necessary to obtain 
the optimum section can be employed, also the width of this strip can be 
precisely that required to obtain the necessary air gap in each case. As 
these shunt strips are not fixed to the core sheets, they can be floated 
at the appropriate height in order to obtain the necessary dimensions in 
the P and S window cross-sections for containing the primary and secondary 
windings. 
This flexibility, due to the shape and assembly of sheets, allows, besides 
cost savings in materials, the electromagnetic regulation of the 
electrical characteristics of the core/windings combination, which permits 
perfect adaptation of the stabilizer operation to each type of lamp. This 
can not be achieved with the conventional models as the shunt is a fixed 
part of the same piece as the sheet. 
EXAMPLE I 
With reference to FIGS. 6 and 7, it can be seen that the reactance, as well 
as the copper and iron weights for a stabilizing transformer according to 
the invention are determined by the following factors: 
Stack length; L 
Primary window height; ap 
Secondary window height; as 
Shunt stack height; b 
Vertical dimension of the magnetic sheet; A 
Core width; c 
Primary and secondary window width; d 
Horizontal dimension of the magnetic sheet; B 
Primary wire diameter; D.sub.p 
Np.times.Sp=Primary turns.times.primary core cross-section 
Ns.times.Ss=Secondary turns.times.secondary core cross-section 
##EQU1## 
Copper density; .rho.cu Iron density; .rho.fe 
The following values have been used for this example: 
L=variable 
ap=f(L) 
as=f(L) 
b=0.8 cm 
c=2.5 cm 
d=1.6 cm 
Dp=0.08 cm diameter 
Ds=0.075 cm diameter 
Np.times.Sp=8,775 
Ns.times.Ss=14,950 
.rho.cu=8.9 gr/cm.sup.3 
.rho.fe=7.6 gr/cm.sup.3 
Using those values the gross Iron weight is: 
Winding factor of the primary windings: 
##EQU2## 
Winding factor of the secondary windings: 
##EQU3## 
The height of the primary and secondary windows in function of the stack 
length L are: 
##EQU4## 
The total cross-section height will be: 
##EQU5## 
Using a theoretical stack factor of 0.9 gives: 
Gross weight Fe=0.9.times.L.times.8.2.times.A.times.7.6 gr. 
Gross weight Fe=185.0904L+3,572.8 gr. 
Using the above values the weight of copper is: 
Pcu=1 m (Np.times.primary wire section+N.sub.s .times.secondary wire 
section.times.8.9) where 1 m, the mean line length of the turn, FIG. 7 is 
the same for the primary as for the secondary windings. 
Upon substituting values: 
##EQU6## 
Using the above derived formulas the cost of copper and iron may be 
calculated as: 
Using Spanish pesetas of 54 pts/kg ($0.82/kg) as the cost of iron sheets 
and 450 pts/kg ($6.82/kg) as that of copper, we obtain: 
##EQU7## 
The minimum price therefore is: 
the L value that makes 
##EQU8## 
zero Thus: 
##EQU9## 
which corresponds to a stack length of 13.3 cm resulting in a minimum 
cost of 824 pts ($12.50). 
The results of laboratory tests of a stabilizing transformer built using 
the values of Example I are shown in FIGS. 8 and 9 which indicate losses 
and useful power, respectively, of the stabilizing transformer of Example 
I (continuous line) and a conventional stabilizing transformer (dotted 
line), as a function of the input voltage. 
EXAMPLE II 
The influence of Leakage Inductance variation on the stabilization 
characteristics of the transformer of the invention may be represented 
graphically as is shown in FIG. 10. This graph is a vector diagram of the 
secondary winding open-circuit and load voltages, as well as the voltage 
drops due to the condenser, and leakage inductance, and the angle between 
the voltage and current of the secondary under load. This graph was made 
by using the values obtained from tests performed in accordance with the 
circuit shown in FIG. 11. In these figures the symbols represent: 
Vp=primary winding terminal voltage 
Vs=secondary winding terminal voltage 
Vr=substitute resistance terminal voltage(*) 
Vc=condenser terminal voltage 
Vsh=shunt terminal voltage (independent winding)(**) 
FNT (*)A resistance is used as a substitute for the lamps in order to avoid 
distortion of the current and voltage waves as much as possible. 
FNT (**)The shunt voltage was measured in order to calculate the magnetic flux 
through its cross-section by means of a pilot winding separate from the 
primary and secondary windings. 
Eg.sub.2 =secondary winding open circuit voltage 
Is=secondary winding current 
.theta.2=angle between Is and Vs 
Ld.sub.2 =secondary leakage inductance 
w=100.pi. 
From FIG. 10 it can be seen that the leakage inductance must have a limited 
value since if it is very high, the secondary terminal voltage will also 
be high, as well as the resistance and condenser voltages, producing 
greater wave deformation and higher losses, therefore affecting the 
reactance operation. In the same figure it is seen that, if upon an 
increase in the primary voltage and consequently in the secondary 
open-circuit voltage, there is not an appreciable decrease in the value of 
the leakage inductance Ld.sub.2, the stabilization is not correct as the 
aforementioned same negative effects are produced. 
The simplified expression to calculate the leakage inductance Ld, assuming 
that the leakage magnetic circuit has a constant section is: 
##EQU10## 
where: Ld=leakage inductance (henries) 
N=number of turns 
L=stack length (cm) 
ld=leakage magnetic path length (cm) 
c=core width (cm) 
b=shunt stack height (cm) 
e=air gap (cm) 
.mu..sub.o =absolute permeability of vacuum (.OMEGA. s/cm) 
.mu.=relative permeability of core 
For sufficiently low induction values, the term 
##EQU11## 
may be disregarded compared to 
##EQU12## 
therefore simplified: 
##EQU13## 
K being constant for equal shunt stack heights. 
In a similar manner, for an equivalent reactance with the same core width, 
shunt stack height and permeability .mu., Ld.sub.1 would be: 
##EQU14## 
Making (2) and (3) equal gives: 
##EQU15## 
and as it is necessary that: 
EQU N.times.S=N.sub.1 .times.S.sub.1 (5) 
and since 
EQU S=c.times.L (6) 
and 
EQU S.sub.1 =c.times.L.sub.1 (7) 
the equation (5) will be: 
EQU N.times.L=N.sub.1 .times.L.sub.1 (8) 
substituting (8) in (4), we obtain: 
##EQU16## 
and from (6), (7) and (9) 
##EQU17## 
As a numerical example for two stabilizing transformers with stack lengths 
of L=13 cm and N.sub.2 =460 turns and L.sub.1 =3 cm (conventional 
reactance) from (10) we obtain the values: 
##EQU18## 
and therefore: 
##EQU19## 
With the core saturated, the term 
##EQU20## 
can not be neglected, since value of .mu. decreases continually as the 
induction increases. Taking into account the grain orientation and that 
1d=1d'+1d" and 1d.sub.1 =1d'.sub.1 +1d".sub.1, 
##EQU21## 
can be separated into two addends: 
##EQU22## 
and thus, 
##EQU23## 
.mu..sub.0 and .mu..sub.90 being the relative permeabilities parallel to 
the grain orientation (vertical) and perpendicular to it (horizontal), 
respectively, and C.sub.1, C.sub.2, C.sub.3 and C.sub.4 constants. 
With .mu..sub.0 &gt;&gt;.mu..sub.90, the .mu..sub.0 fractions can be disregarded, 
and as the sections and lengths are equal in those where the flux is at 
90.degree., C.sub.1 =C.sub.3 and 1d'=1d.sub.1 ', therefore the term 
##EQU24## 
becomes 
##EQU25## 
times greater than the term 
##EQU26## 
Taking into account that the leakage inductance with an unsaturated core 
would have to have a limited value and similarly for equivalent reactances 
and that 
##EQU27## 
it is deduced that the influence of the term 
##EQU28## 
is much greater in a conventional reactance of characteristics equivalent 
to the stabilizing transformer of the invention than the influence of the 
term in which the permeability is present, therefore the variation of the 
permeability due to induction would affect the Ld value much less, hence 
its decrease would be much smaller in the conventional reactance than in 
the stabilizing transformer of the invention. FIG. 12 shows the variation 
in leakage inductance with input primary voltage of the transformer of 
Example II in comparison with a transformer of the prior art design. FIG. 
13 is an expansion of the graph of FIG. 12 for selected primary voltages. 
The foregoing description will make clear to those skilled in the art the 
principles of the stabilizing transformer of the invention, the details of 
which may be modified without going beyond the scope of the invention as 
defined in the appended claims.