Abstract:
A rotor for a generator comprises a stack of laminate plates and conductive end caps on either side thereof. The laminate plates and the end caps have holes near a periphery thereof, and conductive rods are positioned in the holes, and secured to the end caps. The stack, the end caps and the rods are then skewed by a desired angle with respect to a centerline of the rotor. The resulting rotor core may then be mounted to a rotor shaft, and wound, with the windings also being skewed due to skewing of the core. The end caps and rods form a damper cage that aids in reducing harmonics.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Non provisional U.S. Patent Application of U.S. Provisional Application No. 61/676,709, entitled “Rotor and Generator for Reducing Harmonics”, filed Jul. 27, 2012, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to electric power generators, and more particularly to rotors used in such equipment. 
         [0003]    Electrical power generators are used in a wide variety of applications throughout the industry. For example, such generators may be driven by engines, such as internal combustion engines to generate power needed for specific applications. In a particular type of application, involving welding, plasma cutting and similar operations, an electric motor drives a rotor within a stator of the generator to generate alternating current (AC) power. This power may be rectified into direct current (DC) power, and converted and conditioned in various ways for the final application. Generators of this type may serve specific purposes, such as for welding, plasma cutting and similar operations, or may be more general in purpose, such as for providing emergency or backup power, or for applications requiring power at locations remote from the conventional power grid availability. 
         [0004]    Certain generators have been developed for these applications, including generators available commercially from Miller Electric Mfg. of Appleton, Wis., under the commercial designation Bobcat™ and Trailblazer®. Certain of these generators may include rotors with particular geometries adapted to reduce fluctuations in the power generated. 
         [0005]    Despite these improvements, further refinement in generator design and manufacture are needed. 
       BRIEF DESCRIPTION 
       [0006]    The present invention provides a generator and rotor design adapted to respond to such needs. In accordance with certain aspects of the invention, the rotor described employs a mechanism to reduce the currents induced in the rotor from the stator. The mechanism literally “shorts” the currents eliminating the voltage harmonics reflected back into the stator. Eliminating the harmonics improves the sinusoidal waveform creating a “cleaner” power for many applications. In accordance with certain embodiments, then, a rotor for an electrical generator, comprises a laminated core comprising a plurality of laminate plates stacked adjacent to one another, each laminate plate comprising a plurality of holes near a periphery thereof. Conductive end caps are disposed on front and rear sides of the laminated core, each of the end caps comprising a plurality of holes near a periphery thereof. A plurality of conductive rods extend through the holes in the laminate plates and the end caps, and secured to the end caps to form a damper cage. The laminated core and the damper cage are skewed along a length of the rotor. 
         [0007]    The invention also provides an electrical generator that comprises a stator and a rotor disposed in the stator. The rotor conforms to the construction outlined above. 
         [0008]    In accordance with other aspects, the invention comprises a method for making a rotor for an electrical generator. According to the method, a plurality of laminate plates are stacked, each laminate plate comprising a plurality of holes adjacent to a periphery thereof. Conductive end caps are disposed adjacent to front and rear sides of the stack of laminate plates, each of the end caps comprising a plurality of holes adjacent to a periphery thereof. Conductive rods are disposed in the holes of the laminate plates and the end caps. The stack of laminate plates, the end caps and the rods along a length of thereof are then skewed, and the rods are secured to the end caps. 
     
    
     
       DRAWINGS 
         [0009]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0010]      FIG. 1  is a diagrammatical representation of an exemplary application for power conversion circuitry, in the form of a welding system; 
           [0011]      FIG. 2  is a circuit diagram for a portion of the power conversion circuitry of  FIG. 1 , particularly illustrating certain functional circuit components; 
           [0012]      FIG. 3  is a diagrammatical representation of an exemplary generator coupled to an engine for use in a system of the type shown in  FIG. 2 ; 
           [0013]      FIG. 4  is a perspective view of a rotor of the machine as shown in  FIG. 3 ; 
           [0014]      FIG. 5  is an exploded view of certain components of the rotor of  FIG. 4 ; 
           [0015]      FIG. 6  is a further exploded view of certain of these components; 
           [0016]      FIG. 7  is a top view of the rotor illustrating a skew in the rotor winding and core; 
           [0017]      FIG. 8  is an end view of the rotor illustrating the skew; 
           [0018]      FIG. 9  is an end view showing only the core and end caps of the rotor; 
           [0019]      FIG. 10  is perspective view showing the core and end caps; 
           [0020]      FIG. 11  shows a damper cage formed by rods and end caps of the rotor skewed as they will be positioned in the final rotor configuration; and 
           [0021]      FIG. 12  is an end view showing the end caps and skew. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Turning now to the drawings, and referring first to  FIG. 1 , an exemplary welding system  10  is illustrated that includes a power supply  12  for providing power for welding, plasma cutting and similar applications. The power supply  12  in the illustrated embodiment comprises an engine generator set  14  that itself includes an internal combustion engine  16  and a generator  18 . The engine  16  may be of any suitable type, such as gasoline engines or diesel engines, and will generally be of a size appropriate for the power output anticipated for the application. The engine will be particularly sized to drive the generator  18  to produce one or more forms of output power. In the contemplated application, the generator  18  is wound for producing multiple types of output power, such as welding power, as well as auxiliary power for lights, power tools, and so forth, and these may take the form of both AC and DC outputs. Various support components and systems of the engine and generator are not illustrated specifically in  FIG. 1 , but these will typically include batteries, battery chargers, fuel and exhaust systems, and so forth. 
         [0023]    Power conditioning circuitry  20  is coupled to the generator  18  to receive power generated during operation and to convert the power to a form desired for a load or application. In the illustrated embodiment generator  18  produces three-phase power that is applied to the power conditioning circuitry  20 . In certain embodiments, however, the generator may produce single phase power. The power conditioning circuitry includes components which receive the incoming power, converted to a DC form, and further filter and convert the power to the desired output form. More will be said about the power conditioning circuitry  20  in the discussion below. 
         [0024]    The engine  16 , the generator  18  and the power conditioning circuitry  20  are all coupled to control circuitry, illustrated generally by reference numeral  22 . In practice, the control circuitry  22  may comprise one or more actual circuits, as well as firmware and software configured to monitor operation of the engine, the generator and the power conditioning circuitry, as well as certain loads in specific applications. Portions of the control circuitry may be centrally located as illustrated, or the circuitry may be divided to control the engine, generator and power conditioning circuitry separately. In most applications, however, such separated control circuits may communicate with one another in some form to coordinate control of these system components. The control circuitry  22  is coupled to an operator interface  24 . In most applications, the operator interface will include a surface-mounted control panel that allows a system operator to control aspects of the operation and output, and to monitor or read parameters of the system operation. In a welding application, for example, the operator interface may allow the operator to select various welding processes, current and voltage levels, as well as specific regimes for welding operations. These are communicated to a control circuitry, which itself comprises one or more processors and support memory. Based upon the operator selections, then, the control circuitry will implement particular control regimes stored in the memory via the processors. Such memory may also store temporary parameters during operation, such as for facilitating feedback control. 
         [0025]    Also illustrated in  FIG. 1  for the welding application is an optional wire feeder  26 . As will be appreciated by those skilled in the art, such wire feeders are typically used in gas metal arc welding (GMAW) processes, commonly referred to as metal inert gas (MIG) processes. In such processes a wire electrode is fed from the wire feeder, along with welding power and, where suitable, shielding gas, to a welding torch  28 . In other applications, however, the wire feeder may not be required, such as for processes commonly referred to as tungsten inert gas (TIG) and stick welding. In all of these processes, however, at some point and electrode  30  is used to complete a circuit through a workpiece  32  and a work clamp  34 . The electrode thus serves to establish and maintain an electric arc with the workpiece that aides in melting the workpiece and some processes the electrode, to complete the desired weld. 
         [0026]    To allow for feedback control, the system is commonly equipped with a number of sensors which provide signals to the control circuitry during operation. Certain sensors are illustrated schematically in  FIG. 1 , including engine sensors  36 , generator sensors  38 , power conditioning circuitry sensors  40 , and application sensors  42 . As will be appreciated by those skilled in the art, in practice, a wide variety of such sensors may be employed. For example, engine sensors  36  will typically include speed sensors, temperature sensors, throttle sensors, and so forth. The generator sensors  38  will commonly include voltage and current sensors, as will the power conditioning circuitry sensors  40 . The application sensors  42  will also typically include at least one of current and voltage sensing capabilities, to detect the application of power to the load. 
         [0027]      FIG. 2  illustrates electrical circuitry that may be included in the power conditioning circuitry  20  illustrated in  FIG. 1 . As shown in  FIG. 2 , this circuitry may include the generator windings  44 , illustrated here as arranged in a delta configuration, that output three-phase power to a rectifier  46 . In the illustrated embodiment the three-phase rectifier is a passive rectifier comprising a series of diodes that provide a DC waveform to a DC bus  48 . Power on the DC bus is then applied to filtering and conditioning circuitry  50  which aide in smoothing the waveform, avoiding excessive perturbations to the DC waveform, and so forth. The DC power is ultimately applied to a switch module  52 , which in practice comprises a series of switches and associated electronic components, such as diodes. In welding applications, particular control regimes may allow for producing pulsed output, AC output, DC output, and particularly adapted regimes suitable for specific processes. As will be appreciated by those skilled in the art, various switch module designs may be employed, and these may use available components, such as insulated gate bipolar transistors (IGBTs), silicon controlled rectifiers (SCRs), transformers, and so forth. Many of these will be available in packaging that includes both the switches and/or diodes in appropriate configurations. 
         [0028]    Finally, an output inductor  54  is typically used for welding applications. As will be appreciated by those skilled in the welding arts, the size and energy storage capacity of the output inductor is selected to suit the output power (voltage and current) of the anticipated application. Although not illustrated, it should also be noted that certain other circuitry may be provided in this arrangement, and power may be drawn and conditioned in other forms. 
         [0029]    While only certain features of the exemplary systems have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. For example, in addition to the output terminals illustrated in  FIG. 2 , power may be drawn from the DC bus for use in other conversion processes. This may allow for DC welding, for example, as well as for the supply of synthetic AC power for various auxiliary applications. The synthetic auxiliary power may be adapted, for example, for single phase power tools, lighting, and so forth. Where provided, such power may be output via separate terminals, or even conventional receptacles similar to those used for power grid distribution. 
         [0030]      FIG. 3  illustrates certain functional components of the generator for use in a system of the type described above. As mentioned above, the engine  16  is coupled to a generator  18  to produce electrical power used for the welding, plasma cutting or other applications. The generator itself comprises a housing  58  in which a stator  58  is disposed. The stator is wound with stator windings (not shown) to produce the desired output upon rotation of a rotor  60 . The rotor comprises a shaft  62  that is supported by a bearing  64 . A coupling  66  serves to transmit rotational torch to the shaft of the generator as the engine is powered. Input signals  68  are provided to a generator, such as for excitation of the winding. Power signals  70  are received from the stator as the rotor is turned. 
         [0031]    In a presently contemplated embodiment, multiple slots (not separately shown) are included in the rotor, which comprises a variety of windings used to generate the desired power. Specifically, in the illustrated embodiment the generator produces three-phase welding power output, single-phase auxiliary power output, three-phase synthetic AC power output, 24 volt output for powering a wire feeder, and includes a 200 volt excitation coil. 
         [0032]    To reduce or remove slot harmonics that could be generated by the alignment of winding slots of the stator with winding slots of the rotor, the rotor is twisted or skewed as illustrated in  FIG. 4 . Specifically, the rotor comprises a laminated core  72  illustrated as having a first side  74  and second side  76 . Windings  78  are disposed between these sides of the laminated core. The windings are separated from the core by non-conductive separators  80 . As described more fully below, a damper cage  82  is defined by a front end cap  84  and a rear end cap  86  (see, e.g.,  FIG. 6 ) and by rods that connect these conductive end caps to one another in the final assembly (shown and discussed below). The structure of  FIG. 4  is illustrated in exploded views in  FIGS. 5 and 6 . Specifically, in  FIG. 5  certain of the separators  80  are exploded away from the rotor core and windings, and the shaft  62  is removed to show the sub-assembly of the core, damper cage and windings.  FIG. 6  shows the conductive end cap laminations  84  and  86  removed. As may be seen in  FIG. 6 , these end caps, made of a non-ferrous, conductive thin plate material forming a lamination, each comprises a central aperture for the shaft and peripheral apertures  88  that accommodate rods that will form, with the end cap laminations, the desired damper cage that aids in removing or reducing slot harmonics. 
         [0033]      FIGS. 7 ,  8  and  9  illustrate the skew formed in the rotor windings. In particular, as best shown in  FIG. 7 , the shaft  90  has a center line which is displaced angularly from an orientation of the windings  78  by an angle  90 . This angle is caused by twisting of the rotor core prior to winding. In a presently contemplated embodiment, for example, a skew angle of approximately 10 degrees is employed. The skew is further shown in  FIG. 8 , which is an end view of the rotor, as well as in  FIG. 9  in which the shaft and windings have been removed. As may be seen in  FIG. 9 , the apertures  88  of the end cap lamination  84  (and similarly of the opposite end cap lamination) are provided with a series of apertures  88  through which rods will be mounted in the core. Although not separately shown, similar apertures are provided in each of the core laminations forming the sides and a bridge section  92 . That is, each lamination generally has a rounded H shape with sides  74  and  76  extending around the central bridge section  92 , to form recesses for receiving the rotor windings. The skew is further illustrated in the sub-assembly view of  FIG. 10 . 
         [0034]    As shown in  FIG. 11 , the damper cage  82  is formed by linking the front end cap lamination  84  with the rear end cap lamination  86  by means of a series of rods  94  extending between and receive in the apertures  88 . In the presently contemplated embodiment, 10 aluminum bars are positioned in these apertures, and extend through similar apertures in the laminations. The skew between the front end cap lamination and the rear end cap lamination is seen in  FIG. 11 , as well as in  FIG. 12 , which is an end-on view. 
         [0035]    In a presently contemplated embodiment, the rotor is formed by first producing the sub-components, such as the laminations and front and rear end cap laminations. These may be punched or stamped from a thin plate-like material, and are in a present embodiment are made of steel with a nominal thickness of 0.028 in. The laminations are then stacked in a straight (not skewed) configuration, with a predefined number of laminations disposed between the front and rear end cap aluminum laminations. The aluminum bars are then inserted through the end cap laminations and the core laminations. The structure is then twisted to the desired angle, such as 10 degrees of skew. The end cap laminations are then secured to the ends of the rods, such as by staking, welding, or similar operations. The already-skewed core may then be pressed onto the rotor shaft, and the windings placed on the core to complete the sub-assembly along with the other rotor components as described above. 
         [0036]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.