Patent Application
20210344262
The present disclosure relates to techniques for manufacturing squirrel cage rotors for induction machines.
A motor includes, in some examples, a fixed stator and a rotor positioned in the stator. The stator produces a rotating magnetic field and the rotor that produces a static magnetic field. In response to the rotating magnetic field of the stator, the rotor rotates within the stator to produce torque. In an induction motor, the rotating magnetic field of the stator may induce an electric current in the rotor, which produces the magnetic field of the rotor. An efficiency of the induction motor may be related to a magnitude and uniformity of the induced magnetic field of the rotor.
The disclosure describes, in some examples, systems and techniques for manufacturing squirrel cage rotors for an induction machine having improved efficiency and/or at improved yield. The rotor core structure may be formed form laminate layers using a glass laminate process. The laminate layers each include a binder and glass (e.g., glass particles) on a laminate. The laminate layers are stacked and fired to form a rotor core with low porosity. Aluminum, copper, and/or another conductive material may be cast into open cavities in the body of the core to form rotor bars that define the “squirrel cage” structure. The low porosity of the glass laminate rotor core body reduces or prevents the conductive material from infiltrating into the core body during the casting process and forming short circuits between adjacent rotor bars.
In some examples, the disclosure describes a method for forming a squirrel cage rotor. The method includes stacking a plurality of coated laminates to form a stacked laminate core preform. The stacked laminate core preform defines a plurality of open cavities. Each coated laminate of the plurality of coated laminates includes a laminate coated with a precursor layer. The precursor layer includes a binder and glass particles. The method further includes firing the stacked laminate core preform at a temperature above the softening point of the glass particles to form a low porosity rotor core. The method further includes casting a conductive material into the plurality of open cavities formed in the rotor core to define a conductive squirrel cage structure in the low porosity rotor core.
In some examples, the disclosure describes a squirrel cage rotor that includes a rotor core and a squirrel cage structure. The rotor core includes a plurality of laminates and a plurality of interlaminate dielectric layers interspersed or interposed with the plurality of laminates in an alternating relationship. Each laminate of the plurality of laminates includes a magnetically-permeable material. Each interlaminate dielectric layer of the plurality of interlaminate dielectric layers includes glass particles. The squirrel cage structure includes distal and proximal shorting rings and a plurality of rotor bars extending longitudinally along the rotor core between the distal and proximal end caps.
The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.
The disclosure describes, in some examples, systems and techniques for manufacturing squirrel cage rotors for an induction machine having improved efficiency and/or at improved yield.
Rotor core 14 is configured to carry a magnetic field. Rotor core 14 may include a plurality of laminates separated by a plurality of interlaminate dielectric layer, as will be described further in
Rotor core 14 may be a low porosity (e.g., volume fraction of pores) rotor core. For example, rotor core 14 may have a porosity of less than about 5%. In some examples, rotor core 14 may have sufficiently low porosity (e.g., open porosity extending between adjacent open cavities) such that a conductive material having a viscosity greater than about 1.0 mPa*s at a pressure of about 1000 psi may not substantially flow between adjacent open cavities of the plurality of open cavities 24. For example, rotor core 14 may have an open porosity less than about 5%.
Referring back to
In operation, rotor 10 may be positioned in a stator. The stator produces a rotating magnetic field that induces a voltage in the plurality of rotor bars 20 and creates short-circuit currents in the plurality of rotor bars 20. These short-circuit currents create a magnetic field that interacts with the rotating magnetic field of the stator and causes rotor 10 to rotate within the stator to produce torque. In this way, rotor 10 may create torque in response to receiving a magnetic current from the stator.
During manufacture of rotor 10, a conductive material may be cast into the plurality of open cavities 24 in rotor core 14 to define the plurality of rotor bars 20 of squirrel cage structure 16, as will be described further in
In rotor cores that include silicon steel laminations, after the conductive material cools to form the rotor bars, conductive material in the open pores or gaps may form conductive bridges between adjacent rotor bars. This interlaminar bridging may cause shorting between the adjacent rotor bars and result in an uneven magnetic field that generates a reduced amount of torque. This interlaminar bridging may result in low yield of rotors that have a high number of rotor teeth and/or a low spacing between rotor teeth. The interlaminar bridging may also limit a pressure that may be used to flow the conductive material, thereby limiting a size of rotor bars to a length of the rotor. For example, to adequately infiltrate the plurality of open cavities and flow the conductive material along an entire length of the plurality of open cavities, a relatively high pressure may be applied to the conductive material during casting. However, to limit interlaminar flow of the conductive material, a relatively low pressure may be used. As a result of this relatively low pressure, the rotor may have a relatively high ratio of open cavity width to rotor length.
To prevent or reduce conductive material from flowing into the pores or interlaminar gaps, a ceramic paint may be applied to surfaces that may be exposed to a melted conductive material. Alternatively, to remove bridges formed by the conductive material, the rotor core may be quenched to attempt to break the bridges or etched to remove interlaminar conductive material. However, these processes may add complexity and expense to manufacture of the rotors, and may have limited success in reducing or removing bridging in the rotor bars.
Example rotor cores discussed herein may be configured to reduce interlaminar flow of conductive material between the plurality of rotor bars 20 during formation of the plurality of rotor bars 20 by sealing surfaces of rotor core 14 that may be exposed to the conductive material with continuous, low porosity interlaminate dielectric layers glass prior to formation of the plurality of rotor bars 20. This low porosity glass may fill open pores or gaps in laminations of rotor core 14 to reduce interlaminar flow of conductive material, and subsequent interlaminar bridging, between adjacent rotor bars, thus enabling smaller widths of the plurality of open cavities 24 and, correspondingly, a greater number of the plurality of rotor bars 20.
Example rotor cores discussed herein may include other advantageous properties due to incorporation of interlaminate dielectric layers. In some examples, rotor cores discussed herein may provide insulation and bonding of laminates for high temperature applications. For example, the interlaminate dielectric layers may be substantially free of organic materials. As a result, rotor cores discussed herein may provide prolonged and reliable operation at highly elevated temperatures (e.g., temperatures >260° C.) at which organic materials tend to breakdown and decompose. In some examples, rotor cores discussed herein may increase a rigidity of the rotor. For example, the interlaminate dielectric layers may be substantially continuous. As a result, the rotor may have improved rotor dynamics.
In some examples, the method of
Coated laminate 50A may include a laminate 52A. Laminate 52A may have a relatively thin, plate-like shape, such as illustrated in
Laminate 52A may be formed by any suitable method. In some examples, laminate 52A may be formed by cutting a desired laminate shape from a sheet or panel of magnetically-permeable material, which may include any material removal process such as etching, Electrical Discharge Machining (EDM) cutting, laser cutting, and the like. In some examples, laminate 52A may be formed from a magnetically-permeable sheet material using a photo-etching process to impart low stress on the laminates 52 and reduce or eliminate formation of burrs. In some examples, a ferric chloride (FeCl3) etch chemistry may be employed when the magnetically-permeable sheet material is composed of an Fe—Co alloy of the type described above.
In some examples, coated laminate 50A may include an oxidation barrier layer (not shown). The oxidation barrier layer may be composed of any material that decreases a propensity of laminate 52A to oxidize when exposed to air or another oxidizing ambient at elevated temperatures. The oxidation barrier layer may be formed using a variety of methods including, but not limited to: plating metal (e.g., nickel) over surfaces of laminate 52A, such as through an electrolytic or an electroless plating process; forming a Thermally-Grown Oxide (TGO) layer over laminate 52A by heating laminate 52A to an elevated temperature in an oxidizing atmosphere, such as between about 500° C. and about 600° C. (e.g., in a pre-firing step as described in
In some examples, the method of
Top precursor layer 54A overlies a first major surface 53A of laminate 52A and bottom precursor layer 56A overlies a second major surface 55A of laminate 52A. Precursor layers 54A and 56A may contain an inorganic dielectric material in particulate form. In some examples, the inorganic dielectric particles may include low melt glass particles that have a softening temperature and/or a melting temperature that is less than the melting temperature of a magnetically-permeable material from which laminate 52A may be produced. In other examples, other types of inorganic dielectric particles may be contained within the precursor material, providing that the inorganic dielectric particles may be consolidated into interlaminate dielectric layers during a consolidative firing process described below.
In some examples, the inorganic dielectric (e.g., glass) particles contained within precursor layers 54A and 56A may be chemically compatible with laminate 52A, such that interlaminate dielectric layers produced by consolidating the inorganic dielectric particles may be resistant to laminate ion migration. In some examples, a coefficient of thermal expansion (CTE) of the inorganic dielectric particles may be matched to a CTE of laminate 52A, such as between about 10 and about 20 parts per million per degree Celsius (PPM per ° C.). In some examples, a CTE of the inorganic dielectric particles may be less than or equal to a CTE of laminate 52A, which may range from about 10 PPM per ° C. to about 20 PPM per ° C. In some examples, the CTE of the inorganic dielectric particles may be greater than about 7 PPM per ° C. In some examples, the inorganic dielectric (e.g., glass) particles may be “ceramic-on-metal dielectric” material. For examples a ceramic-on-metal dielectric material may be formulated for use with a Fe—Co alloy as the laminate material. In some examples, a ceramic-on-metal dielectric material may be modified by addition of one or more refining ingredients to produce a precursor material, which may be applied onto laminate 52A, dried, and possibly pre-fired to form precursor layers 54A and 56A.
Precursor layers 54A and 56A may be applied to laminate 52A using a variety of methods. Prior to being pre-fired, precursor layers 54A and/or 56A may include a binder and the glass particles described above. In some examples, precursor layers 54A and/or 56A may be applied to laminate 52A using a wet state application technique. A wet state coating precursor material may include inorganic dielectric particles dispersed within an organic binder, such as ethyl cellulose or an acrylic. The organic binder may make the formulation printable and provide precursor layers 54A and/or 56A with green strength during handling. Additionally, the wet state coating precursor material may contain a solvent or liquid carrier transforming the precursor material to a wet or flowable state. Suitable solvents or liquid carriers include high molecular weight alcohols resistant to evaporation at room temperature, such as alpha-terpineol or TEXINOL®. The volume of solvent or liquid carrier contained within the coating precursor material can be adjusted to tailor of the viscosity of the precursor material to the selected wet state application technique. For example, in embodiments wherein the precursor material is applied by screen printing or doctor blading, the coating precursor material may contain sufficient liquid to create a paste or slurry.
In some examples, screen printing may be used as a wet state application technique to provide thickness uniformity and reduce waste. To coat laminate 52A in precursor layers 54A and 56A, a glass-containing paste may be applied to laminate 52A at a predetermined thickness (e.g., between 10 and 20 μm), which may be approximately twice a final desired thickness of the interlaminate dielectric layers produced from the precursor layers 54A and 56A. In some examples, a paste layer can be printed in a pattern providing less than 100% surface area coverage providing that non-covered areas are small enough the inorganic dielectric (e.g., glass) particles would flow over entire substrate when wet, fired, or pressed, as described below. In some examples, wet state application techniques other than screen printing can also be employed to apply precursor layers 54A and 56A to laminate 52A including, but not limited to, spraying and drying, dipping and drying, and doctor blade application.
In some examples, precursor layers 54A and/or 56A may be applied to laminate 52A using a dry state application technique. Precursor layers 54A and/or 56A may be deposited (e.g., screen printed or doctor bladed) and dried onto a temporary substrate or carrier, such as a tape backing (e.g., a strip of Mylar®). In this case, the binder content of the coating precursor material may be increased to, for example, about 8-10 weight percent (wt. %) for additional strength. Precursor layer 54 or 56 and the tape backing may be positioned over laminate 52A and inverted to place the respective precursor layer 54 or 56 in contact with laminate 52A. Heat and/or pressure may be applied to adhere the respective precursor layer 54A or 56A to laminate 52A and allow removal of the tape backing by, for example, physically peeling the tape away.
In some examples, precursor layers 54A and/or 56A may be deposited on laminate 52A after the laminate shape has been cut from a magnetically permeable sheet or panel (referred to here as “laminate singulation”). In some examples, precursor layers 54A and/or 56A may be formed over laminate 52A prior to laminate singulation and while laminate 52A remains interconnected with the other laminates as a relatively large, continuous panel. In such examples, laminate 52A may then be cut from the panel as described in step 30 above. As a result, each coated laminate 50 of the plurality of coated laminates 50 includes a laminate 52 coated with a precursor layer 54 and/or 56.
In some examples, the method of
In some examples, the method includes pre-firing the stacked laminate core preform (36) or, alternatively, the plurality of coated laminates prior to stacking (not shown) to substantially remove the binder from the precursor layer. During pre-firing, stacked laminate core preform 58 may be subject to a pre-firing process that enables organic materials contained within precursor layers 54A and/or 56A to be decomposed or burned-out after or prior to laminate stacking. In some examples, coated laminate 50A and/or stacked laminate core preform 58 may be heated to a predetermined maximum temperature for a time period sufficient to decompose substantially all organic material from the coating precursor layers, such as at least 99 wt. % of the organic material from the coating precursor materials. In certain embodiments, pre-firing may be performed at highly elevated temperatures (e.g., from about 700° C. to about 850° C.) sufficient to glaze, sinter, or slightly melt the inorganic dielectric particles to help strengthen the post-fired coating precursor layers, which may otherwise be weakened when the organic binder is decomposed therefrom. Such highly elevated temperatures may cause sintering of the inorganic dielectric (e.g., glass) particles are referred to herein as “sintering temperatures.” However, precursor layers 54A and/or 56A may still be considered to contain inorganic dielectric particles even when the particles are partially merged or sintered together as a result of such a pre-firing process. In some examples, pre-firing coated laminate 50A at such temperatures may heat treat laminate 52A; in other examples, laminate 52A may be heat treated in an independent heat treatment step or during a consolidative firing process described below. In some examples, the pre-firing process may form an oxidation barrier layer.
In some examples, such as shown in
In some examples, such as shown in
In some examples, the method of
Stacked laminate core preform 58 may be fired to a second predetermined temperature threshold exceeding the first temperature threshold used for pre-firing to melt or sinter the inorganic dielectric particles to the laminate material. The second predetermined temperature threshold may be equivalent to or greater than a softening temperature of the inorganic dielectric (e.g., glass) particles contained within precursor layers 54 and/or 56 and less than the melting temperature of the laminate material of laminate 52. In some examples, the second predetermined temperature threshold may be greater than the melting point of the inorganic dielectric particles, which may be, for example, approximately 100° C. greater than the softening temperature of the particles. In some examples, the second predetermined temperature threshold may be from about 770° C. to about 860° C., and may be achieved in vacuum, nitrogen, or inert atmosphere.
After the second temperature threshold is reached, the compressive load exerted on stacked laminate core preform 58 may be increased to cause the inorganic dielectric particles contained within precursor layers 54 and/or 56 to flow into voids between adjacent laminates 52, merge, and ultimately form a number of coherent interlaminate dielectric layers 60 between laminates 52. Interlaminate dielectric layers 60 may be densified (less porous) as compared to precursor layers 54 and/or 56 and may be substantially void free. Interlaminate dielectric layers 60 may be interspersed or interleaved with laminates 52 in a vertically alternating relationship. Interlaminate dielectric layers 60 may provide electrical insulation between neighboring laminates 52 included within rotor core 14 and bond the adjacent laminates 52 together. Additional firing cycles may be performed, as needed. In some examples, if laminates 52 have not been subjected to a metal heat treatment step, the consolidative firing process described in Step 38 may also be controlled to heat treat the metal laminates 52 as part of the consolidative firing process.
A final thickness of the interlaminate dielectric layers may range from about 5 to about 25 μm after consolidative firing. A thickness of interlaminate dielectric layers 60 may be less than a thickness of precursor layers 54 and/or 56, which may have an initial thickness between about 10 and about 30 μm when applied utilizing a wet state application technique described above. In some examples, the compressive load and temperatures applied during the consolidative firing process may be controlled to reduce or prevent laminate contact with adjacent laminates 52 and impart the resulting interlaminate dielectric layers 60 with the desired final thickness.
In some examples, inorganic standoff particles may be added to precursor layers 54 and/or 56 to ensure that a minimum gap between the laminates is maintained. The inorganic standoff particles can be, for example, presorted, spherical particles having a softening point greater than the softening point and possibly greater than the melt point of the inorganic dielectric (e.g., glass) particles contained in the coating precursor material. Suitable materials include high melt glasses and ceramics, such as alumina. Inorganic dielectric spheres having a maximum diameter substantially equivalent to a desired vertical standoff may be embedded within precursor layers 54 and/or 56 by, for example, mixing the spheres into the coating precursor material prior to application onto laminates 52. During the consolidative firing process, the processing temperatures may be held below the softening temperature of the inorganic standoff particles to ensure the standoff particles maintain their rigidity and thus provide a physical spacer setting the vertical standoff between the laminates and defining the thickness of the resulting interlaminate dielectric layers 60.
As a result of the process of
In some examples, the method includes casting a conductive material into the plurality of open cavities formed in rotor core to define a conductive squirrel cage structure in the low porosity rotor core. For example, a conductive material, such as aluminum or copper, may be melted and flowed under pressure into the plurality of longitudinal open cavities 64 illustrated in
The conductive material in the plurality of longitudinal open cavities 64 may be cooled and solidified to form the plurality of rotor bars 20 and shorting rings 18.
In this manner, inorganic dielectric materials, such as a low melt glass, can be used to electrically insulate and bond laminates 52 into low porosity rotor core 14 and seal pores and other gaps in and between laminates 52 through which the conductive material may flow during formation of squirrel cage structure 16. The resulting low porosity rotor core 14 may be substantially devoid of bridging between rotor bars.
Various examples have been described. These and other examples are within the scope of the following claims.
