Note: Descriptions are shown in the official language in which they were submitted.
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WO 00!30130 PCT/AU99/01006
Title
ENDLESS CORE FOR A MULTIPHASE TRANSFORMER AND
A TRANSFORMER INCORPORATING SAME
Field of the Invention
The present invention relates to an endless core for multiphase transformer
and a
transformer incorporating such a core.
Background of the Invention
Multiphase transformers are well known and are used in a variety of
applications
including for stepping up or stepping down line voltage in power transmission
systems, to provide phase shifting, modulation, star-delta. converters and
general
power supplies.
A typical multiphase transformer has a planar care provided with a number of
square
or rectangular windows each window being bound by upper and lower branches of
the core, and on opposite sides by vertical legs forming part of the core.. A
primary
winding is wound through each window, either on a branch or leg of the window.
similarly a secondary winding is would through each window. Irrespective of
the
number of phases, if the core has N windows then it will have N + 1 vertical
legs.
This provides an inherent magnetic and therefore electrical imbalance between
the
phases. This arises because the magnetic flux created by current flow in the
primary
windings cannot circulate equally about the rest>ective windows because of the
additional vertical leg. As a result, assuming each primary phase voltage is
of the
same magnitude and each secondary winding has the same number of turns, then
the
secondary outputs cannot be the same. The transformation process is not
identical
between the phases due to the difference in magnetic paths surrounding each
window. In order to produce equalised outputs o:n the secondary windings, ie
the
same magnitude output on each winding, some of the primary or secondary
windings
must vary the number of turns to take account of the difference in flux
distribution
circulating about different windows of the transforrner core. Such
transformers also
have an inherent inefficiencies due to flux leakage caused by the end windows
having only a single flux return path.
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PCT/AU99/01006
~'eceived 19 December 2000
Summary of the Invention
It is an object of the present invention to provide a transformer core and an
associated
transformer that attempts to alleviate at least the above;mentioned problems
in the prior art.
According to one form of the present invention there is provided a core for a
multiphase
transformer, said core including a body made from a magnetically permeable
material
forming a closed loop about a central axis, said body further provided with a
plurality of
windows each window bound by opposite branches that extend between opposite
legs
wherein respective legs separate mutually adjacent W ndows so that said
mutually
l0 adjacent windows have a leg in common whereby the number of windows is
equal to
the number of legs, said opposite branches and opposite legs of each window
defining a
path of substantially uniform magnetic permeability about which magnetic flux
circulates.
Preferably the core includes a plurality of primary windings, one primary
winding being
provided for each electrical phase, each primary winding having at least one
turn wound
directly about a branch or a leg of a corresponding window.
Preferably said primary windings are wholly wound directly about one or more
of the
2 o branches or legs of said corresponding window.
Preferably said primary windings are evenly spaced about said central axis.
Preferably said windows are formed as an array of windows circumferentially
about and
2 5 axially along said body.
Preferably in one embodiment each primary winding; has a plurality of turns
and said
turns are wound about more than one branch or one leg of said corresponding
window.
3 0 According to another form of the present invention there is also provided
a core for a
multiphase transformer including:
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PCT/AU99/01006
Received 19 December 2000
2/ 1
a body made from a magnetically permeable material forming a closed loop
about a central axis, said body further provided with a plurality of
windows, each window bound by opposite branches that extend between
opposite legs wherein respective legs separate mutually adjacent
windows so that said mutually adjacent windows have a leg in common
whereby the number of windows is equal to the number of legs; and,
a plurality of primary windings, one primary winding being provided for each
electrical phase, each primary winding having at least one turn being
wound directly about at least one branch or leg of a corresponding
l0 window to produce lines of magnetic flux to correlate about said
corresponding window when said each primary winding is energised
within electric current.
According to the present invention there is also provided a multiphase
transformer
including at least:
a core according to said one form of the present invention;
a plurality of primary windings; and
a plurality of secondary windings;
wherein at least one primary winding for each electrical phase of said
multiphase
2 0 transformer is wound directly about at least one branch or at least one
leg of one
of said windows to produce lines of magnetic flux to circulate about said one
window when said primary winding is energised with an electric current, and at
least one secondary winding having at least one turn wound directly about a
branch or a leg of a window about which said lines of magnetic flux circulates
to
2 5 thereby induce a current in said secondary winding.
According to the present invention there if also provided a multiphase
transformer
including at least:
a core according to said other form of the invention; and,
3 0 a plurality of secondary windings, e2~ch secondary winding having at
least one turn wound directly about a branch or leg of a window about which
said lines
of magnetic circulate to thereby induce a current in said each secondary
winding.
.-,__.
i.-. .' .
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Brief Description of the Drawings
Embodiments of the present invention will now be dc;scribed by way of example
only with
reference to the accompany drawings in which:
Figure 1 is a perspective view of a core in accordance with the present
invention and
a six phase transformer incorporating that core;
Figure 2 is a perspective view of a second err~bodiment of the core and a 12
phase
transformer incorporating that core;
Figure 3 is a perspective view of a third embodiment of the core in accordance
with
the present invention;
Figure 4 is a perspective view of a fourth embodiment of the core; and,
Figure 5 is a cutaway perspective view of an electric motor incorporating a
core in
accordance with the present invention.
2 0 Detailed Description of the Preferred Embodiments
Referring to Figure l, there is illustrated an endless core 10 for a
multiphase (in this
example, six phase) transformer 12. The core 10 is formed as an endless
circular or
annular loop. A plurality of windows 14,-146 (referred to in general as
windows 14;) are
formed through and about the core 10. Adjacent windows 14; share a common
portion or
2 5 leg 16;~ where ; and ~ designate the adjacent windows. For example, leg
16,,~ is the portion
of core 10 between adjacent windows 14, and 142; and leg 164,5 is the portion
or leg of
core 10 between adjacent windows 144 and 145. It will be appreciated that
because the
core 10 is endless, there are no dead ends in so far as magnetic flux is
concerned and
therefore the core 10 facilitates the existence of
"... ;-a_..:
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symmetrical magnetic flux through the core 10.
Each window 14; is bound on opposite sides by the adjacent, core portions or
legs
16;~ and, by upper and lower branches B" and B;. Thus, for example window 14,
is
bound on the left side by common core portion 1 ~6,,2; on the right side by
common
core portion 166,,; upper branch B,;; and, lower branch B,.
Multiphase transformer 12 is constructed by winding respective primary and
secondary windings through the windows 14;. In the embodiment shown, primary
windings 181 and 186 (referred to in general as primary windings 18;) link
with
respective windows 14;. More particularly, two primary windings I8; (of the
same
phase) are provided for each window 14;, with onf; primary winding about the
upper
branch B~ and another primary winding about a IorNer branch B, of each window
14;.
For example, looking at window 14, a pair of primary windings 18, is provided,
one
of each formed about the upper branch Bt, and lower branch B, of the window
141.
When the primary windings 18; are coupled to respective phases of a six phase
AC
power supply lines of magnetic flux ~; are generated and circulate about at
least the
window through which the primary winding I8; is wound. Again taking for
example
window 14, when the primary windings 18, are connected to one phase of the six
phase AC power supply, lines of magnetic flux ~, are generated that circulate
about
window 14,. However, it must be appreciated that the magnetic flux generated
can
also circulate or return about other windows 14;. Thus a part of the magnetic
flux ~1
can circulate about both windows 14; and 142 returning through legs 162,3 and
166,;
and circulate about windows 14,, 142 and 146 returning via legs 162,3 and
165,6.
The placement of secondary windings through the windows 14; is dependent upon
the
desired output. If it is desired that the phase of the output from the
secondary
windings is to be the same as the phase of the corresponding primary winding
then
secondary windings 20s, - 20ø6 can be wound for e:Kample about the lower
branch B,
of each window 14~ - 146 respectively. (Of course in a variation, the
secondary
windings 20øi - 20s6 can be placed about the upper branches B~ of each window
or
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even alternate between the upper and lower branches.) it will be appreciated
that
because of the symmetric distribution of magnetic flux ~; about each of the
windows
14;, assuming that the primary voltage for each phase is of the same
magnitude, the
magnitude of the voltage output from the secondar'~ windings 20s; will be the
same if
each of the secandary windings 20s; have the same. number of turns. Thus, the
core
and transformer 12 provide the ability to have secondary output of equal
magnitude where the secondary windings 20s, - 2Os6 have the same number of
turns.
As discussed above in relation to the prior art, because of the inherent
magnetic
imbalance of known cores and transformers, in order to have secondary outputs
of
10 equal magnitude in a multiphase transformer one must deliberately design
some of
the coils to have different number of turns.
The core 10 and transformer 12 also allow .for an in finite possibility of
phase shifting
or combining. If one wanted to obtain a secondary output of a phase halfway
between the phase of say the primary voltages supplying primary windings 18,
and
182 then a secondary winding 20p (shown in phantom) can be wound through bath
windows 14, and 142 ie about the common care portion 161,2. Now, the second
winding 2Up links with the magnetic flux ~I and ~;t and thus the secondary
output is
of a magnitude and phase corresponding to the vector or phasor addition of the
voltage induced by fluxes ~, and ~2. This provides a 1:1 transformed
combination of
the phases feeding primary windings 181 and 182. However combinations of other
ratios and thus different amounts of phase shifting can be achieved at will by
simply
winding the secondary winding 20~ about the upper or lower branches B",B, or
common core portions 16;~ of different windows. For example, in the embodiment
shown in Figure l, the primary phases are 60° ap~~rt. To obtain a
secondary output
having a phase 15° (ie '/ the phase difference) in advance of the phase
of the
primary voltage feeding primary winding 181 a secondary winding {not shown) is
provided having a 1:4 turn ratio about branch B, of window 141 and branch B,
of
window 142, ie the secondary winding has four turns passing through window 142
for
every turn passing through window 141.
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Figure 2 illustrates a core 10' suitable for constructing a twelve phase
transformer
12'. Here, the core 10' is again in shape of a ring; or annulus but this time
provided
with twelve windows 141 - 14,,2 and twelve common core portions 16;x, one of
each
between respective adjacent windows 14;. A primary winding 18; is wound about
lower branch B, of each window 14;. A secondary winding 20; is wound about the
upper branch B~ of each window 14;. The phase of the output of any secondary
winding 20; is the same as the phase of voltage driving the corresponding
primary
winding 18;. However, as with the previous embodiment, the secondary winding
20;
can be wound partially about the upper and lower branches B~ and B, or common
core portions I6;a of different windows in any desired combination to produce
a
desired phase output in accordance with standard transformer design technics.
Figure 3 illustrates an extending (vertically stacked) core 10" and a
multiphase
transformer 12" constructed using the core 10". 'The core 10" can be
considered as
being two six window cores vertically stacked upon each other. Thus the core
10"
has a lower set of windows 141 - 146 and an upper set of windows 14~ - 14,2
with
windows 14; and 14;+6 in vertical alignment. Primary windings 181 - I86 are
wound
about the lower branches B, of windows 14, - 146 respectively; and, primary
windings 18~ - 18,,2 are wound about the upper branches B, of the upper set of
windows 14~ - 14,,2. A set of secondary windings 20 are wound about the middle
branch Bm between vertically adjacent windows 14;, 14;+6. Therefore, in this
particular illustrated embodiment, there are only ~;ix secondary coils 20. The
output
of any particular secondary winding 20 would be the transformed phasor or
vector
addition of voltages induced by the magnetic flux generated by the primary
windings
linked with the windows common to that particular secondary winding 20. In
order
2S to avoid saturation it is preferred that the volunne of core constituting
the middle
branch Bm is the sum of the volume of the core constituting the lower branch
B, and
upper branch B" of the windows 14;, 14;+6. 'This embodiment then allows the
combination of two six phase supplies that are out of phase with each other.
For
example, if there are two six phase power supplies, one providing input to
coils 18, -
I86 and another providing input to primary windings 18., - 181,2, the two
power
sources can be combined to provide a six phase output through the secondary
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windings 20. This could be particularly useful in :f'or example coupling two
multiple
phase power supplies to a common power transmission grid. The core
configuration
will also allow for the ability to have 6 primary and 12 secondary windings.
Also a
turns ratio of 1/0.5 primary to secondary, or secondary to primary, as well as
incorporating other windows will produce any fraction of volts required.
In a different configuration (not illustrated) the primary windings 18, -
18,.2 of
transformer 12" can be connected to a different phase of a twelve phase power
supply and primary windings 20 round through various windows 14; to provide a
transformed twelve phase output. Again, the phasing of the output from the
Itl secondary windings can be arranged as required in accordance with known
transformer design techniques to provide the desired secondary phase output.
Figure 4 further illustrates a further embodiment of the core 10"' and a
corresponding
12"'. In the embodiments shown in Figures 1-3 the core 10 and windows 14; are
arranged so that the core 10, 10' and 10"' is endless about a first axis that
is
I5 perpendicular to the axis of any particular window 14;. With the core 1
()"' of Figure
4, the axis of the core 10 is parallel with the axis of any window 14;. As
with all
previous embodiments, core i 0"' is configured as an endless loop having a
plurality
of windows 14; where adjacent windows share a common portion of core lb;~ so
that
they number of windows 14; equals the number of common core portions lb;~.
Mare
20 specifically, three windows 141 - 143 are formed in the core 10"' with a
primary
winding 18, - 183 respectively wound about the louver (radially outer most)
branches
B, of each window 14;. Respective secondary windings 20, - 20, axe wound
through
the windows 14! - I43 respectively about the corresponding upper (radially
inner
most) branches B". It is preferred that the core 10"'' is configured so that
the volume
25 of core in the upper and lower branch portions B,, B" of each window 14; is
the
same. This assists in avoiding saturation of the core. This can be achieved by
appropriate placement or configuration of the windows 14;.
Figure 5 illustrates an application of the core 10 shown in Figure 1. The core
10 is
used in this application in a transverse flux nnotor 26. Full operation and
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constructional details of the transverse flux motor are described in the
Applicant's
Australian Application No PP 7124 the contents of which is incorporated herein
by
way of reference. The structure of core 10 and tlhe placement of primary
windings
18, - 186 is identical to that described in the first embodiment described in
relation to
Figure 1. However, instead of mufti turn electricaHly separate secondary
windings a
single turn secondary winding between 2U; is provided about each common core
portion 16;~ with each of the single turn second~~ry windings 2(); being in
mutual
electrical connection. Thus, the single turn secondary windings 20, - 206 form
a
wheel like structure 30 having an inner rim 32 and outer rim 34 joined by
radially
extending spokes 36. The outer rim 34 is depicted as residing in the air gap
38 of a
cockcroft ring 40. Without going into the detail of operation of the motor 26,
currents are induced through the single turn secondary windings 20, - 206 that
interact with magnetic t7ux passing through the air gap 38 of the cockcroft
ring 40
thereby generating transverse forces on the outer ri~rn 34 of the wheel 30
causing it to
move. The path of motion of the wheel 30 can bf; controlled at will by
variation of
the magnitude and frequency of the primary voltages supplied to the primary
coil 18,
- 186 and the phase relationship therebetween.
Now that embodiments of the present invention have been described in detail it
will
be apparent to those skilled in the relevant arts and numerous modifications
and
variations may be made without departing from the basic inventive concepts.
For
example, in each of the embodiments shown, the core 10 is depicted essentially
as
being in a ring, annulus or circular type form. However it can assume other
shapes
provided that it is continuous or endless and is provided with equal numbers
of
windows and common core pouions. Also, the exaict number of windows provided
is
simply dependent upon the application and in particular the number of primary
phases. Also, the position and placement of the secondary windings 20; is
dictated
solely by the desired magnitude and phase of the secondary outputs. The core
10,
10', 10", 10"' can be made by casting; continuous stamping and winding of an
insulated strip of magnetically permeable materiall; winding of a strip of
material
then machininglcutting the windows. Further the core can be split through a
plane
passing through the windows 14; to facilitate me,chanical/autornatic winding
of the
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primary and/or secondary windings about the window branches B", B,, or loading
of
prewound bobbins on the common core portions 16;x. All such variations and
modifications together with others that would be obvious to a person of
ordinary skill
in the art are deemed to be within the scope of the present invention the
nature of
which is to be determined from the aforegoing descuption.
