Patent Publication Number: US-2015084455-A1

Title: Flux Directing, Pole Span Setting, Feature on Rotor Lamination of Air-Cooled Electric Motor/Generator

Description:
This application claims the priority of U.S. Provisional Application Ser. No. 61/618,259, filed Mar. 30, 2012, the entire disclosure of which is incorporated by reference into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a pole span defining feature included in a rotor lamination forming part of a lamination stack constituting the rotor of an electric machine such as an AC motor, generator, or motor/generator with recess or channels to promote rotor cooling. 
     2. Description of Related Art 
       FIG. 1  of the present application illustrates a known interior permanent magnet rotor lamination  50 . The winged features illustrated in  FIG. 1  set a span that influences flux flow across the air gap between the winged features. During loaded conditions, when armature reaction influences the flux flow from the rotor to the stator, the pole span set by the winged features provides an optimized flow path to maximize output power from the generator. During unloaded conditions, the optimized pole span allows for a higher internal generator voltage (maximized BEMF of the machine). After testing, it was determined that inner polar cooling channels in the rotor formed by a stack of the laminations  50  should be present to allow for air flow through the generator, and, in subsequent designs, scallops were inserted in the inner polar areas of the rotor lamination, eliminating the possibility of using traditional pole span approaches such as those incorporating rotor laminations having configurations as shown in  FIG. 1 . 
     U.S. Patent Application Publication 2012/0074801 to Brown, et al. discloses a magnetic rotor with inset bridges to promote cooling.  FIG. 2  of the present application corresponds to FIG. 1 of the Brown, et al. (801) publication, and illustrates a known interior permanent magnet rotor lamination  10 , in plan view, with scallops or indentations  12  on the rotor outer diameter  13 . The rotor lamination illustrated in  FIG. 2 , again, is a single layer interior permanent magnet rotor lamination. Each indentation  12  is located between adjacent pairs of magnet receiving voids or orifices  14  and  16 ,  18  and  20 ,  22  and  24 , and  26  and  28 , and each magnet receiving void or orifice of these pairs is separated from the other such void or orifice by a thin bridge  30  of rotor lamination material. In operation, permanent magnets (not shown) are affixed within the voids or orifices to cooperate with windings disposed around poles of a stator, within which the rotor lamination  10  is rotatable. A rotor shaft (not shown) is receivable within a shaft opening  32  to impart rotational motion to the rotor. The entire disclosure of the Brown, et al. (&#39;801) publication is incorporated herein by reference as non-essential subject matter. 
     Other documents that may be of interest are U.S. Pat. No. 3,364,672 to Pfeffer, U.S. Pat. No. 5,051,634 to Overton, U.S. Pat. No. 7,057,323 to Horst, U.S. Patent Application Publication 2005/0140236 A1 to Jeong et al., U.S. Patent Application Publication 2006/0119203 A1 to Brown et al., U.S. Patent Application Publication 2007/0103024 A1 to Nakayama et al., U.S. Patent Application Publication 2008/0030108 A1 to Trago et al., U.S. Patent Application Publication 2008/0224558 A1 to Ionel, and U.S. Patent Application Publication 2009/0224624 to Kumar et al. 
     The IEEE paper  Waveform Optimization of an AC Converter Machine , published May 25, 1989, identifying F. Wang as its author, teaches the value of square wave waveforms for improving the power out of a machine for a given size. This is achieved via a six phase machine and a linearly varying gap. 
       FIG. 3  illustrates standard pole shaping for large synchronous generators that is used for creating a more sinusoidal BEMF, as opposed to increasing a magnitude of the fundamental BEMF. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a flux directing pole cap is provided at the outside diameter of the rotor lamination. This pole cap is a non-circular feature, and serves to set the pole span of the machine. The non- circular pole cap is implemented into the lamination by cutting an arc in the outside diameter directly over the magnet pole. By including such features in rotor laminations, an attempt is made to provide maximum voltage to the generator load under loaded conditions. Benefits of the flux directing pole cap are optimized internal generator voltage, lowered inductance, lowered reactance, and an increased power factor. These combinations help increase generator efficiency. 
     In the process of designing a three phase permanent magnet generator, it was observed that different pole span settings provided different output voltages under certain load conditions. One design objective is to optimize the pole span to provide the highest voltage at generator loads. This characteristic helps minimize magnet material and coil turns. Pole span selection also directly impacts other generator parameters, such as inductance and reactance. 
     In one preferred arrangement, a rotor, formed from a plurality of laminations joined together, is mountable on a shaft for rotation relative to a stator of a rotary electric machine arrangement. The multilayer laminated rotor has a solid section that surrounds an opening within which the shaft is receivable. The solid section includes an outside diameter, magnet receiving voids, and steps delimiting a plurality of pole spans from flux directing features at sides of each of the pole spans. Adjacent flux directing features between adjacent pole spans are disposed on opposite sides of scalloped areas or other such recesses permitting cooling air flow past the rotor. 
     Each of the recesses mentioned permitting cooling air flow defines a scalloped area in the solid section, and the solid section mentioned defines a plurality of rotor poles, each of which includes a pair of the flux directing features, a single pole span between the flux directing features, and one of the steps located between each of the flux directing features and the single pole span. Each of the rotor poles, moreover, includes a pair of the magnet receiving voids, and each of the steps is disposed directly over one of the magnet receiving voids. The flux directing features permit air flow past the rotor between the recesses and the pole spans. 
     Selection of the pole spans may be made based on at least one of a maximum internal generator voltage, a minimum phase inductance, and a minimum generator reactance. In one particular arrangement contemplated, the rotor includes four rotor poles that cooperate with a thirty six slot stator. The pole spans, steps, and flux directing features mentioned provide an air gap of varying cross section between the rotor and a stator associated with the rotor, with the air gap being at a minimum between the pole spans and the stator. 
     The invention also concerns a lamination, usable together with additional laminations to provide such a rotor, as well as an arrangement including a stator and a rotor configured in the manner discussed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1 , discussed above, is a plan view of a known interior permanent magnet rotor lamination having winged features that influence flux flow. 
         FIG. 2 , discussed above, illustrates a known interior permanent magnet rotor lamination, in plan view, with scallops or indentations on the rotor outer diameter to permit cooling fluid flow. 
         FIG. 3 , discussed above, illustrates standard pole shaping for large synchronous generators. 
         FIG. 4  illustrates a rotor having flux directing pole cap features according to the present invention. 
         FIG. 5  is an enlarged view of a part of  FIG. 4 . 
         FIG. 6  is a schematic illustration of the lines of magnetic flux in an arrangement having a rotor configured in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 4 , according to the present invention, a flux directing pole cap feature  100  is provided at the outside diameter  102  of a rotor lamination forming part of an overall laminated rotor  106 . This pole cap feature  100  is non circular, and serves to set the pole span  108  of the machine. The non-circular pole cap feature  100  is implemented into the lamination by cutting arcs at the outside diameter  102  directly over the magnet pole. Although such characteristics are not to be considered limiting, in one configuration of the invention, the machine is a four pole, 36 slot generator designed to operate at 45 to 60 Hz and provide a minimum of 338 VAC at 45 Hz and 449 VAC at 60 Hz. 
     In conventional arrangements, buried magnet or interior permanent magnet designs originally had small inner polar cooling channels, and a pole span could be set in a traditional manner, supported by prior art. In the known interior permanent magnet rotor lamination  50  of  FIG. 1 , for example, this approach was accomplished using wing features that extend toward one another and set an angular distance above the pole. 
     In the alternative pole span delineating approach of the present invention, non-circular pole cap arcs or other features  100  are implemented into the lamination by designing arcs above the magnet pole on the outside diameter  102  of the rotor  106  just above the magnet receiving voids, orifices, or slots  109 . Areas of the rotor  106  between the poles (inner polar) includes scalloped areas  104  that allow air to flow axially through the rotor  106 . The radial magnetic air gap  110  ( FIG. 6 ) is at a minimum on the rotor lamination outside diameter  102  in the area above the permanent magnets received in the voids  109 , and gets larger in the area of the lamination outside of a pole span  108  but between the pole cap arcs or other features  100  and the scalloped areas  104  forming air flow regions. This variable air gap influences the flux and directs it to the arced section of the rotor  106  more exactly above the magnets. This becomes particularly important for maximizing flux linkage with reactive loads; as the loads become more reactive, the current angle shifts away from the voltage in the generator system. When this occurs, armature reaction will try to divert the flux in areas that are not beneficial to voltage production. The pole cap serves to guide the flux across the gap, minimizing the effect of armature reaction and maximizing generator output voltage.  FIG. 6  is a schematic illustration of the lines of magnetic flux in an arrangement having the rotor  106  rotationally disposed within a stator  112  having stator teeth  114  and associated wire coils (not shown). 
       FIG. 5  is an enlarged view of a part of  FIG. 4  showing one of the scalloped areas  104  adjacent one of the magnet receiving voids, orifices, or slots  109  of the rotor  106 . The non-circular pole cap arcs or other features  100  are delineated with respect to the pole span  108  ( FIG. 4 ) of the illustrated rotor lamination by a step down feature  116  that can more concisely be referred to simply as a “step.” Such step down features serve to increase output voltage and, as shown in  FIG. 6 , focus the flux in the teeth aligned with the magnets received within the voids. 
     As noted above, by including such features in rotor laminations, an attempt is made to provide maximum voltage to the generator load under loaded conditions. Benefits of the flux directing pole cap are optimized internal generator voltage, lowered inductance, lowered reactance, and an increased power factor. These combinations help increase generator efficiency. 
     Pole span selection is optimized by providing a maximum internal generator voltage, a minimum phase inductance, and a minimum generator reactance. By decreasing generator reactance, the power factor of the complete system increased, increasing efficiency. The reduction of inductance due to an optimized pole span lowers the voltage drop across the generator internal impedance during loaded conditions, increasing the generator output power. 
     The invention should be readily observable, is operable as evidenced by way of its use on certain prototypes, and increases the voltage generated by 5.6% relative to arrangements without a step down feature. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, and the invention should be construed to include everything within the scope of the invention ultimately claimed.