CA 02486767 2004-11-26
PATENT APPLICATION
NAME OF INVENTION:
THREE PHASE TRANSFORMER WITH
DUAL TOROIDAL FLUX RETURN PATH
CA 02486767 2004-11-26
TITLE: THREE-PHASE TRANSFORMER WITH
DUAL TOROIDAL FLUX RETURN PATH
Technical Field ofthe Inver~iOn
The invention is related to the field of three-phase power transformers for
which a gapless tape-wound toroidal core provides the inter-phase flux
return path, allowing performance improvements compared to existing
technology. The invention increases power density with reduced materials
requirements.
F~dsfing Three Phase Technology
The existing construction of three-phase power transformers was
developed over one hundred years ago. Although raw materials such as
steel cores and winding conductor characteristics have improved since,
there have been few, if any, enhancements to design and construction.
The transformer construction technique is dependent on the selected core
configuration, which is a primary design consideration and which also
determines energy losses.
The most significant energy loss component is in the form of 12R, or
'winding losses'. The roof of the problem is the inability to utilize the core
efficiently. This inefficiency is a significant contributing factor to the
winding
resistance 'R', which contributes to the inefficiency. Such losses are large
enough to become a power design consideration for improving efFciency.
This is because existing three-phase technology makes poor utilization of
the grain structure and cannot take advantage of the high quality electrical
steels available today, Power tosses are therefore seen in the form of heat
and mechanical noise.
The construction and geometry of existing three phase transformers was
developed for ease of manufacture, and not for performance.
Figure 1a shows three-phase EI transformer construction, named for
the utilization of stamped E-shaped and I-shaped grain-oriented
electrical steel laminations, which are alternately stacked and
assembled around pre-wound bobbins. The laminations are annealed
to relieve the stresses of the stamping machine, but stresses can be
re-introduced into the material during the assembly stage. Lamination
stamping equipment is costly but transformer manufacturing costs are
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kept low as lamination stamping is a high-volume automated process,
and multi-winding bobbin capability is common.
The EI transformer makes poor use of the core because a significant
portion of magnetic grains are not aligned with the preferred flux
direction, and because gaps exist at every layer. This increases core
watt losses and requires a lower flux density, increasing cost, materials
consumption and weight. This results in reduced efficiency and wider
regulation. In addition, it is necessary to vacuum varnish the EI
assembly to control the noise associated with core losses.
Another common core geometry, similar to the EI, makes use of a
distributed gap core. Strips of electrical steel are annealed and then
assembled individually by layer around wound bobbins. Unlike the Els
described above, the grains are oriented in the preferred direction.
However, inherent in the construction are air gaps. Every layer
contains an air gap which yields negative effects, such as mechanical
noise from lamination chatter, high magnetic emissions, and most
significantly, increased exciting currents. It is common that operating
flux densities be lowered to compensate for exciting currents, which
increase materials consumption, weight, and costs. These increases
can result in reduced efficiency and regulation.
EI and Distributed Gap construction have performance issues which are
accepted to for ease
All transformers generate a flux circuit, which includes a return flux
path. Both of the two systems above use a similar style of flux return
system. The total flux generated by the current-carrying conductor
flows through the limbs (the sections of the transformer lamination
stack around which the windings are wound). This flux also passes
through the yoke (the stack of steel laminations of the transformer that
link the limbs together to close the magnetic loop). In the case of
existing technology, the yokes are straight-shape and in the invented
design the yoke is circular-shape (toroidal yokes). The cross-section of
the yoke is, therefore, required to be the same as the cross-section of
the limbs to accommodate the flux.
Invented Three-Phase Technology
Figure 1b shows the invented three-phase transformer with dual toroidal
flux return path. It is constructed from three limbs that are stacked
laminations of grain-oriented silicon steel and two tape-wound toroidal
yokes. The orientation of the magnetic grains in the yokes is aligned with
the magnetic flux lines. Each limb has both primary and secondary
windings of one phase wound around it. The three wound limbs are then
positioned, equally spaced, around the surface of one of the toroidal
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yokes, with the other toroidal yoke placed on the top. Therefore, the three
wound limbs are in fact sandwiched between the two toroidal yokes.
The number of turns and wire gauge of the primary and the secondary
windings, cross-sectional area of the limbs and material of the tape wound
toroids and stacked lamination of the limbs are all calculated through a
developed design procedure.
The basis of the design formula, which defines the relationships of
voltages to magnetic flux, is Faraday's law, which holds:
e(t) = N. d~(t)/dt Equation 1
Where "e(t)" is instantaneous voltage at the terminal of the winding, N is
the number of turns and ~(t) is instantaneous magnetic flux.
For a sinusoidal voltage E, equation1 is written in phasor form:
E = jwNd~, Equation 2
Where j~ is the phasor operator for d/dt. The equation 2 can be written in
the form of the magnitude and the angle as below;
~ E~ = wN~, and LE = c~Nc~G90 Equation 3
where w is the radian frequency
Now flux;
~ = B.A Equation 4
Where B is flux density and A is cross-sectional area of the limb.
Substituting Equation 4 in Equation 3;
~ E~ = N w B A = 2~ f N B A Equation 5
The use of tape-wound toroids as yokes provides two separate flux return
paths per phase to close its magnetic circuit across the yoke section.
Therefore, the cross-section of the yoke needs to be only half that of the
limb, and half that of existing three phase technology. This phenomenon is
illustrated in Figures 2a and 2b which show an electric circuit analog of
the magnetic circuit of the invented design as well as existing technology,
showing a comparison and analysis. In the figures, the battery voltage
represents the winding magneto-motive force (mmf or Ampere-Tum),
resistance R represents the reluctance of the air-gap between the limb
and the yoke, and the current I represents the generated magnetic flux.
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Both the current and the voltage are function of time since the excitation
source is a sinusoidal function.
The general shape of the current waveform in a three-phase application is
shown in Figure 3. This is a balanced three-phase current waveform. As
presented, the sum of the instantaneous currents of the three phases is
zero. The current in each phase goes through an excursion of zero to
positive maximum back to zero and negative maximum and again back to
zero. In order to demonstrate the difference between the two existing
technologies and the invented technology, an instant of time (wt = 90°)
where one phase (phase1 ) is at its peak current and the other two (phases
B and C) at half that current but running in an opposite direction is chosen
as shown in Figure 3. This time represents the instant of time that
maximum flux passes through one section of the yoke in both the existing
technology and the invented technology.
To investigate the distribution of current (flux) around the circuit,
KirchofPs
Current Law (KCL) is applied to both the existing technology equivalent
circuit and the invented technology equivalent circuit, shown in Figures 2a
and 2b. Kirchoffs Current Law holds that the sum of the currents at any
instant of time entering an electrical node must be equal to the sum of the
currents exiting the same node. The currents are time variant, so they are
all considered as function of time (t).
Existing Technology, Figure 2a;
KCL for:
Node A: i~~(t) = iy~(t) Equation (6)
Node B: i~2(t) + iY2(t)= iY~ (t) Equation (7)
Node C: iy2(t) = i~3(t) Equation (8)
Where I~,(t), i,~(t) and i~3(t) represents the current in Limbs 1,2 and 3. And
iY~(t) and iY2(t ) are currents in sections of the yokes between limbs1-2 and
limbs 2-3
Invented Technology, figure 2b;
KCL for:
Node A: i~,(t) = iY,(t) + iYZ(t) Equation (9)
Node B: iY~(t) = i~z(t) Equation (10)
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Node C: iY2(t) = i~3(t) Equation (11)
Where I~~ (t), i,~(t) and i~3(t) represents the current in Limbs 1,2 and 3.
And
iY~(t) and iY2(t ) are currents in sections of the yokes between limbs1-2 and
limbs 1-3.
Comparing the distribution of current at Node A for both models, it is clear
from Equation (6), in the case of the existing technology, that the current in
limb 1 is equal to the current in that section of the yoke that links limb1 to
limb 2. Given that current represents flux in the equations, it is recognized
that the total flux in limb 1 must pass through the yoke. Therefore, the
cross-section of the yoke must be equal to the cross-section of the limb. .
In the case of the invented technology, Equation (9) shows that the current
in limb1 is divided into two equal components (Equal component is derived
from Equation (10) and (11 ) since i~2 and i~3 are equal at that instant of
time.).In this case, the flux in the limb is diverted to two paths when
flowing
through the yoke. These dual paths are equal, requiring the yoke to be half
the cross-sectional area of the limb..
Heat Transfer Comparison
The ability to dissipate and remove heat from a transformer and maintain
the operating temperature within the insulation class rating is directly
related to important performance characteristics such as power rating,
temperature rise, power density, efficiency, size, weight, and ultimately
cost. An inefficient thermal circuit will result in large, oversized,
inefficient
transformers.
The cooling in the existing transformers is generally done by the natural air
circulation through the transformer or by a forced-air method. Due to the
rectilinear shape of existing transformers, there is no one position in the
transformer that a fan can be installed to remove heat uniformly from all
phases. However, in the invented technology due to the cylindrical shape
of the transformer and the presence of an air tunnel in the center of the
construction, as shown in Figure 4, heat can be removed uniformly by the
use of fans installed at both ends of the transformer. Figure 5 shows a
30KVA prototype of the invented three-phase with fans installed at both
ends. Fans are installed so that the cold air (at ambient temperature) is
pushed inside the air-tunnel from both ends and forcing the heat away
from the transformer.
CA 02486767 2004-11-26
The invention is a three-phase transformer with dual toroidal flux return,
which is capable of developing higher power density (ratio of power to
volume) in the magnitude of 3.0 times the existing three-phase technology.
The technical explanation is as follows:
1) The yokes are constructed of tape-wound toroid, which join the limbs
together,
and makes the transformer cylindrical in shape. The cylindrical shape of the
transformer allows increased power density of the form of (power / volume) to
be
achieved as compared to existing (rectilinear shape) transformers.
2) The circular shape of the toroidal yokes allow the generated flux in each
limb to
offer two discreet paths along the yoke to close its respective magnetic loop.
This
phenomenon, as explained in Section "Invented Three-Phase Technology",
translates into a lighter weight transformer as compared to the existing
technology. This is because the cross-section of the yoke can be half that of
the
limb, therefore, lighter in weight compared to the existing technology where
the
yoke and the limbs have the same cross-section.
3) The invented geometry creates a natural tunnel, which can be utilized
effectively
to increase airflow across the windings. In the invention, heat is removed
uniformly from all three phases easily, efficiently, and simultaneously. Two
fans
each positioned at an end of the transformer supply airflow. The fans are
located
such that ambient air is blown inside the tunnel in the center, and
simultaneously
cools the surface of all three phases at the same rate. This level of cooling
cannot
be achieved with the existing technology, as there is no one physical position
in
the transformer where the fan can be placed to remove heat uniformly from the
three phases. Further, as they draw cool ambient air from outside of the
transformer and direct it to the transformer's central tunnel, the fans are
not
exposed to hot air. This lengthens the working life time of the fans. Note:
Use of
fans is optional, and specific to the application.
4) A 30KVA prototype based on the invented technology was built, as shown in
Figure 5 and a full load test was conducted. A similar full load test was
performed
on a 30KVA transformer with the existing technology. Figure 6 displays the
performance comparison between the two transformers. Comparing the key
features between the two technologies, the following conclusions are observed:
- Steel weight: The invented technology uses 55.6 Kg of steel, where as the
existing technology uses 65.9 Kg. This shows approximately 16% less steel is
used by the invented technology.
- Overall active weight: This is the weight of copper and steel combined. The
invented technology weight is 78.3 Kg and the existing technology weight is
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88.2 Kg. This shows that the invented technology is lighter by approximately
11 %.
Volume of space taken, including the frame construction: The invented
technology occupies 82500 cm3 and the existing technology takes 51183 cm3
of space. Therefore, the invented model takes approximately 38% less space
than the existing model.
Power densities: Two forms of power densities are compared. 1 ) Ratio of
power to volume (VA I Volume); the invented technology has 0.59 ratio
and the existing technology has 0.36 ratio. The invented model has 64%
better power density. 2) Ratio of power to weight (VA / Kg); the invented
technology has 353 ratio and the existing technology has 275 ratio.
Therefore, the invented model has 28% more power density.
$ ._
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A detailed description of the preferred embodiment is provided below with
reference to the following:
Figures 1a and 1b show the pictorial drawings of the invented three-
phase transformer and an existing EI three-phase transformer. The
invented three-phase consists of three limbs, around which windings are
wound, two tape-wound toroidal yokes and two fans. Each wound limb
represents one phase of the three-phase system. The three wound limbs
are positioned, equally spaced, around the surface of the toroidal yokes.
Two fans that blow air inside the tunnel are placed at both ends of the
assembly.
In the existing E-I three-phase transformer, Figure 1a, the limbs are
referred to that section of the stacked EI laminations around which the
windings are wound, and yokes are referred to in the sections that join the
limbs together.
Figure 2 shows the electric circuit analog of the magnetic circuit of an
existing three-phase transformer, and the invented three-phase
transformer. The battery voltage represents the winding magneto
motive force (mmf or Ampere-Turn). Resistance R represents the
reluctance of the air-gap between the limb and the yoke and the
current I represents the generated magnetic flux.
Figure 3 shows a balanced three-phase excitation waveform. The graph
demonstrates a three-phase excitation for a three-phase transformer,
showing the level of the excitation at various moments in each cycle and
also indicating that the sum of instantaneous voltages is equal to zero.
Figure 4 shows the outline of the air-tunnel and the forced-air flow in the
invented three-phase transformer with fans placed at both ends of the
transformer.
Figure 5 shows the picture of the 30KVA prototype of the invented three-
phase transformer, which includes the transformer and the casing
assembly.
Figure 6 compares the performance of the 30kVA invented three-
phase prototype transformer with an existing off-the-shelf three-phase
transformer.
Numbering of the described elements of the drawings
1 Limb
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