Three-phase autotransformer with a balancing function

A three-phase autotransformer improving balance of three-phase voltages and currents. An iron core has three legs corresponding to the three phase. A common winding and/or a serial winding of each phase includes three coils. Two of the three coils are wound on the same leg associated with the phase of the coils, and the other one coil is wound on another leg. The two coils and the other one coil are connected in series to generate magnetic flux in the opposite directions. Since the common winding and/or the series winding includes coils which are wound on different legs associated with different phases, and which generate flux in opposite directions, the balance of three-phase input voltages and currents, and output voltages and currents is automatically maintained.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a three-phase autotransformer, and 
particularly to a three-phase autotransformer with a balancing function 
which can eliminate imbalance between voltages and currents of the three 
phases, thereby improving efficiency of electric apparatuses connected to 
the autotransformer. 
2. Description of Related Art 
When a conventional three-phase autotransformer is used to supply power to 
various electric apparatuses connected to its output, imbalance between 
the three phases may occur. 
FIG. 1 is a diagram for illustrating mechanism that causes the imbalance. 
Three-phase output terminals of a distribution transformer 101 are 
connected, through distribution lines 102U, 102V and 102W, to the input 
terminals U, V and W of a three-phase autotransformer 103 including 
three-phase windings 103U, 103V and 103W which are star-connected. One end 
of each winding is connected to the neutral point N, which in turn is 
connected to the neutral point of the distribution transformer 101 through 
a distribution line 102N. Three-phase output terminals u, v and w are 
brought out of the windings, and an induction motor M is connected to the 
output terminals. In addition, an electric heater H is connected between 
the neutral point N and the output terminal v. The U-phase winding 103U 
consists of a con, non winding from the neutral point N to the output 
terminal u, and a series winding from the output terminal u to the input 
terminal U. Likewise, each of the windings 103V and 103W consists of a 
common winding from the neutral point to the output terminal, and a series 
winding from the output terminal to the input terminal. 
With this connection, although the induction motor M will keep balance of 
the three phases, the electric heater H may disturb it, thus causing 
differences in voltages and currents between the phases. In the case of 
FIG. 1, for example, the current of the V-phase is greater than the 
currents of the other phases, which will cause a voltage drop due to a 
resistance of the distribution line 102V of the V-phase. Thus, imbalance 
between voltages will occur as well as the imbalance between currents. The 
imbalance will have various harmful effects on electric apparatuses 
connected to the transformer. For example, the torque of the induction 
motor M may be reduced, and its efficiency may be decreased owing to an 
increase in the slip. In addition, the windings of the induction motor may 
be overheated, thereby shortening its life. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a three-phase 
autotransformer with a balancing function which can reduce imbalance of 
voltages and currents between the phases. 
According to one aspect of the present invention, there is provided a 
three-phase autotransformer with a balancing function, comprising: 
an iron core which includes a first leg, a second leg, and a third leg, 
which are interlinked; 
a first common winding which includes a first winding wound on the first 
leg, and a second winding wound on the third leg; 
a second common winding which includes a third winding wound on the second 
leg, and a fourth winding wound on the first leg; 
a third common winding which includes a fifth winding wound on the third 
leg, and a sixth winding wound on the second leg; 
a first series winding connected in series with the first common winding; 
a second series winding connected in series with the second common winding; 
and 
a third series winding connected in series with the third common winding, 
wherein one ends of the first, second and third common windings are 
connected in common, one ends of the first, second and third series 
windings are input terminals of a first phase, a second phase, and a third 
phase, respectively, a connecting point of the first common winding and 
the first series winding is an output terminal of the first phase, a 
connecting point of the second common winding and the second series 
winding is an output terminal of the second phase, and a connecting point 
of the third common winding and the third series winding is an output 
terminal of the third phase. 
Here, the first winding and the second winding may have the same number of 
turns, and generate magnetic flux in opposite directions; 
the third winding and the fourth winding may have the same number of turns, 
and generate magnetic flux in opposite directions; and 
the fifth winding and the sixth winding may have the same number of turns, 
and generate magnetic flux in opposite directions. 
The first series winding may include a seventh winding wound on the first 
leg, and an eighth winding wound on the third leg; 
the second series winding may include a ninth winding wound on the second 
leg, and a tenth winding wound on the first leg; and 
the third series winding may include an eleventh winding wound on the third 
leg, and a twelfth winding wound on the second leg. 
The seventh winding and the eighth winding may have the same number of 
turns, and generate magnetic flux in opposite directions; 
the ninth winding and the tenth winding may have the same number of turns, 
and generate magnetic flux in opposite directions; and 
the eleventh winding and the twelfth winding may have the same number of 
turns, and generate magnetic flux in opposite directions. 
The first common winding may comprise a first coil wound on the first leg, 
a second coil wound on the third leg, and a third coil wound on the first 
leg, the first coil and the third coil having the number of turns of N (N 
is a positive integer) and generating flux in the same direction, and the 
second coil having the number of turns of 2N and generating flux in the 
direction opposite to that of the flux of the first coil; 
the second common winding may comprise a fourth coil wound on the second 
leg, a fifth coil wound on the first leg, and a sixth coil wound on the 
second leg, the fourth coil and the sixth coil having the number of turns 
of N and generating flux in the same direction, and the fifth coil having 
the number of turns of 2N and generating flux in the direction opposite to 
that of the flux of the fourth coil; and 
the third common winding may comprise a seventh coil wound on the third 
leg, an eighth coil wound on the second leg, and a ninth coil wound on the 
third leg, the seventh coil and the ninth coil having the number of turns 
of N and generating flux in the same direction, and the eighth coil having 
the number of turns of 2N and generating flux in the direction opposite to 
that of the flux of the seventh coil. 
The first series winding may comprise a tenth coil wound on the first leg, 
an eleventh coil wound on the third leg, and a twelfth coil wound on the 
first leg, the tenth coil and the twelfth coil having the number of turns 
of M (M is a positive integer) and generating flux in the same direction, 
and the eleventh coil having the number of turns of 2M and generating flux 
in the direction opposite to that of the flux of the first coil; 
the second series winding may comprise a thirteenth coil wound on the 
second leg, a fourteenth coil wound on the first leg, and a fifteenth coil 
wound on the second leg, the thirteenth coil and the fifteenth coil having 
the number of turns of M and generating flux in the same direction, and 
the fourteenth coil having the number of turns of 2M and generating flux 
in the direction opposite to that of the flux of the thirteenth coil; and 
the third series winding may comprise a sixteenth coil wound on the third 
leg, a seventeenth coil wound on the second leg, and an eighteenth coil 
wound on the third leg, the sixteenth coil and the eighteenth coil having 
the number of turns of M and generating flux in the same direction, and 
the seventeenth coil having the number of turns of 2M and generating flux 
in the direction opposite to that of the flux of the sixteenth coil. 
According to the present invent ion, the common winding (and/or series 
winding) of each phase includes not only a coil wound on the leg of its 
own phase, but also a coil wound on the leg associated with another phase. 
As a result, even if voltage and current of a particular phase change a 
great deal, the changes are alleviated. This makes it possible to balance 
the voltages and currents between the phases, and to achieve efficient 
operation of electric apparatuses connected to the output of the 
transformer. In particular, when a three-phase induction motor is 
connected to the output of the transformer, reduction in torque is 
prevented, and a regular rotation speed can be achieved. In addition, 
overheating of coils of the induction motor can be prevented, thereby 
lengthening its life. 
The above and other objects, effects, features and advantages of the 
present invention will become more apparent from the following description 
of the embodiment thereof taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
The invention will now be described with reference to the accompanying 
drawings. 
FIG. 2 shows an embodiment of a three-phase autotransformer with a 
balancing function in accordance with the present invention, and FIG. 3 
illustrates the connection state of the embodiment. 
In these figures, a three-phase autotransformer 1 has a shell type iron 
core 3, which includes a first leg 3a, a second leg 3b and a third leg 3c. 
In addition, the three-phase autotransformer 1 has input terminals U, V 
and W, and output terminals u, v and w, which are associated with the 
three phases. 
A series winding of the U-phase includes a coil 10 wound on the first leg 
3a of the iron core 3, a coil 11 wound on the third leg 3c, and a coil 12 
wound on the first leg 3a, and the coils 10, 11, and 12 are connected in 
series. The number of turns in the coils 10 and 12 is M (M is a positive 
integer), and that of the coil 11 is double, that is, 2M. In addition, a 
current flowing through the coils 10 and 12 induces magnetic flux opposite 
to the flux induced by a current flowing through the coil 11. In other 
words, if the coils 10, 11 and 12 are wound in the same direction, and a 
current flows from the end point to the start point in the coils 10 and 
12, the coils 10-12 are connected in such a manner that a current flows 
from the start point to the end point in the coil 11. 
Likewise, a common winding of the U-phase includes a coil 19 wound on the 
first leg 3a of the iron core 3, a coil 20 wound on the third leg 3c, and 
a coil 21 wound on the first leg 3a, and the coils 19, 20, and 21 are 
connected in series. The number of turns in the coils 19 and 21 is N (N is 
a positive integer), and that of the coil 20 is double, that is, 2N. In 
addition, a current flowing through the coils 19 and 21 induces magnetic 
flux opposite to the flux induced by a current flowing through the coil 
20. 
Series windings and common windings of the other phases are arranged in a 
similar fashion. Specifically, a series winding of the V-phase includes a 
coil 13 wound on the second leg 3b of the iron core 3, a coil 14 wound on 
the first leg 3a, and a coil 15 wound on the second leg 3b, and the coils 
13, 14, and 15 are connected in series. The number of turns in the coils 
13 and 15 is M, and that of the coil 14 is double, that is, 2M. In 
addition, a current flowing through the coils 13 and 15 induces magnetic 
flux opposite to the flux induced by a current flowing through the coil 
14. 
Likewise, a common winding of the V-phase includes a coil 22 wound on the 
second leg 3b of the iron core 3, a coil 23 wound on the first leg 3a, and 
a coil 24 wound on the second leg 3b, and the coils 22, 23, and 24 are 
connected in series. The number of turns of the coils 22 and 24 is N, and 
that of the coil 23 is double, that is, 2N. In addition, a current flowing 
through the coils 22 and 24 induces magnetic flux opposite to the flux 
induced by a current flowing through the coil 23. 
A series winding of the W-phase includes a coil 16 wound on the third leg 
3c of the iron core 3, a coil 17 wound on the second leg 3b, and a coil 18 
wound on the third leg 3c, and the coils 16, 17, and 18 are connected in 
series. The number of turns in the coils 16 and 18 is M, and that of the 
coil 17 is double, that is, 2M. In addition, a current flowing through the 
coils 16 and 18 induces magnetic flux opposite to the flux induced by a 
current flowing through the coil 17. 
Likewise, a common winding of the W-phase includes a coil 25 wound on the 
third leg 3c of the iron core 3, a coil 26 wound on the second leg 3b, and 
a coil 27 wound on the third leg 3c, and the coils 25, 26, and 27 are 
connected in series. The number of turns of the coils 25 and 27 is N, and 
that of the coil 26 is double, that is, 2N. In addition, a current flowing 
through the coils 25 and 27 induces magnetic flux opposite to the flux 
induced by a current flowing through the coil 26. 
The series winding and the common winding of each phase is connected in 
series, and the output terminals u, v and w are brought out from the 
connecting points. Furthermore, one ends of the common windings are 
connected in common to the neutral point N. In this embodiment, the 
numbers of turns M=2, and N=30. 
FIG. 4 is a vector diagram illustrating the operation of the embodiment in 
comparison with that of a conventional three-phase autotransformer. The 
vector diagram is made such that it corresponds to the connection diagram 
of FIG. 3. For example, the reference numeral 21a designates a voltage 
vector of the coil 21 in a rated operation, whereas the reference numeral 
20b designates a voltage vector of the coil 21 in an imbalance operation. 
First, it is assumed that the rated input voltage U.sub.ap, and the rated 
output voltage u.sub.ap of the U-phase of a conventional autotransformer 
are as shown in FIG. 4, and that the input voltage is dropped by 30% to 
U.sub.bp of FIG. 4. In the conventional autotransformer, the output 
voltage will drop in proportion to the input voltage, and take a value 
u.sub.bp of FIG. 4. Such a drop in the U-phase input voltage is caused by 
a resistance of the distribution line 102U when a large current flows 
through the line 102U. Although the voltage drop is within 5% in practice, 
it is assumed to be 30% for the purpose of making the vector diagram 
clearer. 
Let us consider the operation of the present invention under the same 
conditions. Only, it is further assumed that the input voltage of the 
W-phase is kept at a rated voltage. When the input voltages of the three 
phases are rated one, the input voltages and the output voltages will be 
similar to those of the conventional autotransformer, as indicated by 
U.sub.a and u.sub.a for the U-phase. More specifically, the output voltage 
u.sub.a is the vector sum of the voltage vectors 21a, 20a and 19a, due to 
the common windings 21, 20 and 19, respectively, and the input voltage 
U.sub.a is the sum of the output voltage u.sub.a and the voltage vectors 
12a, 11a and 10a, due to the series windings 12, 11 and 10, respectively. 
On the other hand, the U-phase input voltage and u-phase output voltage 
when the input voltage to the U-phase is dropped by 30% are indicated by 
U.sub.b and u.sub.b of FIG. 4. More specifically, the output voltage 
u.sub.b is the vector sum of the voltage vectors 2lb, 20b and 19b, due to 
the common windings 21, 20 and 19, respectively, and the input voltage 
U.sub.b is the sum of the output voltage u.sub.b and the voltage vectors 
12b, 11b and 10b, due to the series windings 12, 11 and 10, respectively. 
As a result, drops in the input voltage and the output voltage are limited 
to approximately half of those of the conventional autotransformer, that 
is, about 15%. The reason for this is that since the coils 20 and 11 are 
wound on the leg 3c associated with the W-phase, the voltages across the 
coils 20 and 11 are not affected by the drop in the U-phase input voltage 
as shown in FIG. 4. Since an actual voltage drop is within 5%, the 
imbalance of actual voltages will be restricted within 2.5%. 
FIG. 5A shows voltages and currents of various portions in a conventional 
three-phase autotransformer 105, and FIG. 5B shows those in a three-phase 
autotransformer with a balancing function in accordance with the present 
invention. As will be seen from these figures, the imbalance between the 
three-phase input voltages of the conventional device is within 1%, and 
the imbalance between the single-phase output voltages is within 2.5%. In 
contrast, the imbalance between the three-phase input voltages and output 
voltages in the autotransformer in accordance with the present invention 
is nearly zero. In addition, the imbalance between the single-phase input 
voltages is nearly zero, and the imbalance between the single-phase output 
voltages is within 1.6%. 
Moreover, the current flowing through the neutral point N is 28 A in the 
conventional device, whereas that of the autotransformer in accordance 
with the present invention is 5.5 A, which is much smaller than the 
conventional value. This proves that the balancing function of the 
autotransformer in accordance with the present invention works 
effectively. 
Although the turn ratios of the three coils constituting each series 
winding and common winding are set as 1:2:1 in this embodiment, they are 
not restricted to the ratios. For example, the series winding or the 
common winding can be constructed by serially connecting two coils whose 
turn ratio is 1:1, and which are wound on different legs to induce flux in 
opposite directions. 
The present invention has been described in detail with respect to an 
embodiment, and it will now be apparent from the foregoing to those 
skilled in the art that changes and modifications may be made without 
departing from the invention in its broader aspects, and it is the 
intention, therefore, in the appended claims to cover all such changes and 
modifications as fall within the true spirit of the invention.