Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1998-04-13
2001-01-23
Nguyen, Tran (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S216055, C310S191000
Reexamination Certificate
active
06177748
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to laminated stator and rotor cores for use with electromagnetic machines and more particularly to core configurations including laminations which have disparate physical properties and operational characteristics.
One common motor type is an induction motor. All induction motors include a stator assembly and a rotor assembly. A typical stator assembly includes a stator core which forms a stator cavity and a plurality of stator windings wrapped around the core such that when current passes through the windings, the current creates an electromagnetic field inside the cavity. In addition to providing support to the windings, the core serves as a flux path for stator field flux and hence strengthens the field within the cavity. By varying the stator currents the magnetic field within the cavity is caused to rotate about the cavity.
A typical rotor assembly includes a rotor core and a plurality of rotor bars (e.g. aluminum bars) which are shorted together at their ends by two shorting rings (e.g. aluminum rings) to form a “squirrel cage” around the core, the core and bars configured so as to fit within the stator cavity. The bars and ring are typically radially constrained by rotor tooth tips or rotor slot bridge formed by the rotor core. The rotor is mounted on a shaft for rotation within the cavity, the rotor and stator together forming an air gap. The rotor bars are arranged such that when the stator magnetic field rotates about the stator cavity, the varying field within the cavity induces rotor current within the bars. The rotor current in turn causes a rotor magnetic field within the stator cavity. The stator and rotor fields interact such that, as the stator field rotates about the core, the rotor field is drawn in the same direction and the rotor rotates within the cavity about the axis of the shaft. In addition to providing support for the bars, the rotor core operates as a flux guide for rotor magnetic field flux.
Motors are typically designed with an end use in mind and therefore, each motor must be able to achieve certain minimal operating characteristics within a specific budget. For example, a motor must be able to generate at least a certain minimal amount of torque, drive at least a minimal load and achieve at least a minimal rotational velocity. To this end, core structures are typically designed such that they have at least certain minimal operating characteristics and a specific cost which can be relied upon when configuring other motor components.
In addition to cost, perhaps the two most basic design criteria for cores are size and efficiency, efficiency being the amount of flux which can be generated within a core given a specific field strength. While high efficiency is desirable, size should be minimized.
There are two primary core characteristics which affect efficiency: core permeability and core losses. The permeability of a substance is the quotient of a change in magnetic induction divided by the corresponding change in magnetizing force. For example, when a magnetic field having a specific strength induces a relatively large amount of flux within a substance, the substance is said to have a high permeability. When a field induces a relatively small amount of flux within a substance, the substance is said to have a low permeability.
The term core losses is used to refer collectively to two different types of energy loss which are generally referred to as eddy current and hysteresis losses. Hysteresis losses are caused by the reality that it takes some energy to change the magnetic state of a substance. For example, when a magnetic substance is placed within a magnetic field, field energy in the form of flux is guided through the substance. A first portion of field energy is stored and is wholly recoverable from the substance when the substance is removed from the field. A second portion of field energy is converted to heat as a result of work required to magnetize the substance and begin flux flow. Hysteresis losses comprise this second portion of field energy.
Eddy current losses result from an electric field and consequent circulatory currents induced in a core by time varying fluxes. Whenever magnetic flux within a substance changes, an electric field is generated within the substance. When the substance is conductive, the electric field causes currents within the substance which are referred to as eddy currents. Eddy currents cause substance heating and subsequent eddy current losses which are proportional to the square of the eddy current multiplied by the substance resistance.
One common core type is a core formed of powdered iron material. To form this type of core, powdered iron material is fed into a mold, compressed under extremely high pressure and then sintered or resin bonded into a finished “compact”. The sintering or resin bonding processes hold the material in the compact form. There are many advantages to powdered iron cores including minimum wasted core material and extremely high magnetic permeability. In addition, resin bonded compacts generate minimal eddy current losses. Moreover, sintering can hold powdered iron in a small core configuration.
Unfortunately, compacting force within the powdered material drops off dramatically with distance from the compacting surface, resulting in lower effective density within the core. This is especially true of resin bonded compacts. Generally, as the effective density of a core decreases the relative size of the core increases. In addition, loosely packed cores have minimal mechanical strength. Moreover, while sintered compacts are relatively small, sintered compacts generate excessive eddy current losses.
Another common core type which overcomes some of the shortcomings of the powdered iron core is a laminated core. A laminated core is formed by stacking a plurality of electrical steel laminations together along the length of the core, each lamination being a flat member having oppositely facing first and second surfaces, each surface having the general shape and area of a transaxial slice of the core. An electrically insulating core plating layer is provided on each of the first and second surfaces of each lamination. When stacked together, core plating provides an electrical barrier between adjacent laminations without impeding magnetic flux within the core. While eddy currents still exist within each lamination, the electrical barriers impede greater eddy currents from flowing throughout the core.
Because different materials have different permeabilities and different hysteresis loss characteristics, the easiest way to increase permeability and decrease hysteresis losses in a laminated core and thereby enhance core efficiency is to choose substances for forming laminations which are known to have a high permeability and low hysteresis losses. Similarly, eddy current losses can also be further minimized by choosing laminate substances having characteristically low eddy current losses. As a starting point, usually core materials are limited to metals which are generally permeable and have relatively low losses. Core materials are also usually doped with a loss inhibitor such as silicon to minimize core losses.
To enhance permeability and reduce losses even further, virtually all substances used to form laminations are subjected to various processes including at least one annealing process which is performed either by the material manufacturer or the core manufacturer. During an annealing process a sheet of metallic substance is held at an elevated temperature for the duration of a specified period in order that metastable high permeability and low loss characteristics go into thermal dynamic equilibrium. After the specified period, the substance is cooled slowly back to room temperature, the metastable conditions becoming permanent characteristics of the substance. The metallic sheet or coil may be c
Katcher Thomas E.
Rodano Thomas G.
Horn John J.
Jaskolski Michael A.
Nguyen Tran
Reliance Electronics Technologies, LLC
Walburn William R.
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