Inductor devices – Core – Laminated type
Reexamination Certificate
2000-12-29
2002-09-24
Mai, Anh (Department: 2832)
Inductor devices
Core
Laminated type
C336S005000, C336S218000, C336S217000, C336S232000
Reexamination Certificate
active
06456184
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to magnetic-induction devices. More specifically, the invention is directed to a reduced-cost core for an electrical-power transformer.
BACKGROUND OF THE INVENTION
Electrical-power transformers are used extensively in electrical and electronic applications. Transformers transfer electric energy from one circuit to another circuit through magnetic induction. Transformers are utilized to step electrical voltages up or down, to couple signal energy from one stage to another, and to match the impedances of interconnected electrical or electronic components. Transformers are also used to sense current, and to power electronic trip units for circuit interrupters. Transformers may also be employed in solenoid-equipped magnetic circuits, and in electric motors.
A typical transformer includes two or more multi-turned coils of wire commonly referred to as “phase windings.” The phase windings are placed in close proximity so that the magnetic fields generated by each winding are coupled when the transformer is energized. Most transformers have a primary winding and a secondary winding. The output voltage of a transformer can be increased or decreased by varying the number of turns in the primary winding in relation to the number of turns in the secondary winding.
The magnetic field generated by the current passing through the primary winding is typically concentrated by winding the primary and secondary coils on a core of magnetic material. This arrangement increases the level of induction in the primary and secondary windings so that the windings can be formed from a smaller number of turns while still maintaining a given level of magnetic-flux. In addition, the use of a magnetic core having a continuous magnetic path ensures that virtually all of the magnetic field established by the current in the primary winding is induced in the secondary winding.
An alternating current flows through the primary winding when an alternating voltage is applied to the winding. The value of this current is limited by the level of induction in the winding. The current produces an alternating magnetomotive force that, in turn, creates an alternating magnetic flux. The magnetic flux is constrained within the core of the transformer and induces a voltage across in the secondary winding. This voltage produces an alternating current when the secondary winding is connected to an electrical load. The load current in the secondary winding produces its own magnetomotive force that, in turn, creates a further alternating flux that is magnetically coupled to the primary winding. A load current then flows in the primary winding. This current is of sufficient magnitude to balance the magnetomotive force produced by the secondary load current. Thus, the primary winding carries both magnetizing and load currents, the secondary winding carries a load current, and the core carries only the flux produced by the magnetizing current.
Modern transformers generally operate with a high degree of efficiency. All magnetic devices such as transformers, however, undergo losses because some fraction of the input energy to the device is inevitably converted into unwanted heat. The most obvious type of unwanted heat generation is ohmic heating that occurs in the phase windings due to the resistance of the windings.
Two other forms of losses occur in the transformer core as a result of hysteresis and eddy currents. These losses are collectively referred to as “core losses.” Hysteresis losses represent the energy required to overcome molecular friction within the core. This friction is caused by the many reversals that the molecules in the core undergo every second due to the effects of the alternating magnetic flux. Hysteresis losses are typically reduced by constructing the core from special materials such as textured silicon steel. Eddy current losses are ohmic losses that result from the circulation of eddy currents within the core. The eddy currents are produced as the core is cut by the magnetic flux generated in the windings. Eddy-current losses are typically reduced by forming the core from thin laminae of iron or steel.
Transformer cores commonly comprise two or more magnetic loops arranged side by side. For example,
FIG. 1
depicts a conventional three-phase transformer
98
comprising a core
100
having four magnetic loops
102
. The loops
102
are arranged side by side so as to form three winding legs
110
. A phase winding
112
is disposed around each winding leg
110
so that each phase winding
112
is inductively coupled to its respective winding leg
110
when the transformer
98
is energized.
Each of the magnetic loops
102
is wound from a narrow, thin strip of magnetic material such as textured silicon steel or an amorphous alloy. In other words, each of the magnetic loops
102
is made up of a plurality of laminae
103
formed by a single winding of magnetic material.
FIGS. 1A and 1B
are diagrammatic illustrations depicting portions of the laminae
103
of two adjacent loops
102
. The sizes of the laminae
103
are exaggerated in these figures, for clarity.
The cores losses that occur in each loop
102
, in general, are proportionate to the thickness of the strip material from which the loop
102
is formed (in particular, the thinner material provides a smaller flow-path for the loss-inducing eddy currents). The cost of the thinner material, however, is generally higher than that of the thicker material. Hence, an optimal transformer design must balance material costs against the need to minimize core losses. Manufacturers of electrical-power transformers are under constant pressure from their customers to minimize both the purchase cost and the operating costs of their products. Hence, an ongoing need exists for reduced-cost, efficient transformers.
Each of the four magnetic loops
102
is typically formed from strips of material having a substantially identical thickness (see, e.g., FIGS.
1
A and
1
B). This practice is followed in order to equalize the level of induction and the resulting core losses in each of the winding legs
110
. The noted practice is dictated by a widely-held belief among skilled transformer designers that the overall core losses in a core such as the core
100
are equal to the numerical average of the core losses in the individual magnetic loops
102
, regardless of whether the loops
102
are operating under identical conditions. Hence, a potential cost savings associated with reducing the thickness of the materials from which one or more, but not all, of the loops
102
are formed cannot be realized according to the currently-accepted teachings in the art of transformer design.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electrical-power transformer having a reduced-cost core. In accordance with this object, a presently-preferred embodiment of the invention provides an electrical-power transformer comprising a core comprising a first magnetic loop including a first winding formed by a first strip of magnetic material having a first thickness, and a second magnetic loop including a second winding formed by a second strip of magnetic material having a second thickness. The second thickness is less than the first thickness, and the first and the second magnetic loops are positioned substantially side by side so that the first and the second magnetic loops form a winding leg. The transformer also comprises a phase winding that encircles the winding leg so that the phase winding and the winding leg are inductively coupled when the transformer is energized.
Further in accordance with the above-noted object, another presently-preferred embodiment of the invention provides an electrical-power transformer comprising a core comprising a first magnetic loop including a first plurality of laminae each having a first thickness, and a second magnetic loop including a second plurality of laminae each having a second thickness. The second thickness is less than the first thickness, and the first and the se
Lanni Arthur L.
Ramanan Varagur R.
Vu Tri D.
ABB Inc.
Mai Anh
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