Patent Document

BACKGROUND OF THE INVENTION 
     The present invention relates to starter generators, and more particularly to brushless wound field starter generators. 
     For high power, or high power density, brushless wound field starter generator applications, relatively low stator winding inductance is often necessary, in order to meet generate mode transient power quality requirements. The need for the relatively low stator winding inductance typically results in undesirable high peak phase currents during the start mode inverter operation. Accordingly, the overall generator system may be forced to operate at reduced current and power levels that provide insufficient start mode maximum torque. 
     Efforts to overcome the aforementioned deficiencies have included upsizing the current carrying capability of semiconductor switches in the inverter, adding an external inductor, and using feeder cables with a higher inductance. These approaches often increase overall system weight and may significantly impact the generate mode performance characteristics. Alternatively, increasing the armature leakage inductance may have significant impacts on the steady-state generate mode performance. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment, a brushless starter generator includes a wound field generator having a rotor that includes a rotor body that defines a plurality of poles. Also included is at least one pole face disposed within each of the plurality of poles, wherein the at least one pole face comprises a slot opening. Further included is a damper bar supported by at least one laminate, wherein the damper bar relatively entirely fills the slot opening. 
     According to another embodiment, provided is a method of generating electrical power in a brushless starter generator and includes providing a generator having a rotor body that defines a plurality of poles, each pole having a pole face that includes an opening. Also included is providing an armature within the generator. Further included is filling the opening with a damper bar, wherein the opening is relatively entirely filled with the damper bar. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a rotor and a stator; and 
         FIG. 2  is a laminated rotor core having a plurality of damper bars disposed within a plurality of slots connected by a pair of end plates; 
         FIG. 3  is an amortisseur circuit comprised of the plurality of damper bars and the pair of end plates; and 
         FIG. 4  is an elevational view of a rotor pole segment having a plurality of damper bars disposed within the plurality of slots. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-3 , a rotor of a starter generator (not illustrated) is generally referred to as  10 . The starter generator is a brushless, wound field synchronous generator that may be operated as a motor in a starting mode to convert electrical power supplied by an external AC power source into motive power or, alternatively, in a generate mode to convert mechanical energy into electrical power. Typically, the starter generator is one assembly of an overall generator assembly, which may include a permanent magnet generator (PMG), an exciter generator for brushless operation and a main generator mounted on a common shaft. 
     The starter generator comprises a rotor  10  that includes a laminated steel core  12  that supports an “amortisseur” or damper winding  13 . A plurality of electrically conductive damper bars  14  are disposed on the laminated steel core  12  and extend axially along the longitudinal direction of the rotor  10  to form a damper winding  13 . The damper bars  14  are disposed within a plurality of pole faces  16  that are situated proximate to, and between, a plurality of adjacent rotor slots  18  that house rotor field coils. The damper bars  14  are mechanically and electrically interconnected by electrically conductive end plates, both of which are schematically illustrated as  20 . Typically, a first and second end plate is present. The end plates  20  may be formed of any suitable electrically conductive material, with one such suitable material being copper, for example. The damper bars  14  and the electrically conductive end plates  20 , in combination, form the damper winding  13 , which partially encloses or cages the rotor  10 . 
     The illustrated damper winding  13  is a twelve pole embodiment, which is shown merely for illustrative purposes and it is to be appreciated that numerous other pole embodiments are contemplated. 
     The damper winding  13  formed by the damper bars  14  and the electrically conductive end plates  20  function to retain the components that typically form the rotor core. Additionally, the damper winding  13  functions to assist the generator during transient and steady-state operations. As a motor in a starting mode, the application of power to the armature windings of the generator causes currents to be induced in the damper bars  14  and the end plates  20 . These damper winding currents plus the rotor main field current provided by the exciter rotor windings due to application of power to the stator windings of the exciter generator produce a magnetic field which interacts with a magnetic field established by currents flowing in the armature windings to cause the rotor  10  to rotate relative to a stator and thereby produce power. 
     Referring now to  FIG. 4 , a pole segment  23  is illustrated and is disposed between adjacent rotor slots  18  that provide mechanical support for retaining the rotor field coils and proximate a pole face  16 . The pole face  16  includes a plurality of slot openings  24  that are configured to receive the above-described damper bars  14 . The damper bars  14  of the exemplary embodiment are formed of a paramagnetic or soft ferromagnetic material. For purposes of this description, a “paramagnetic material” is a material which is slightly magnetically attracted when in the presence of an externally applied magnetic field. Paramagnetic materials have a relative magnetic permeability greater or equal to unity (i.e., a positive magnetic susceptibility) and hence are attracted to magnetic fields. Any suitable paramagnetic material may be employed as the damper bars  14 , and specifically any material that contains atoms, ions or molecules with unpaired spins may be considered a paramagnet. 
     The term “soft ferromagnetic material” for purposes of this description typically refers to materials that are strongly magnetically attracted and have the capacity to strengthen the magnetic field in different parts of a magnetic circuit. 
     The paramagnetic or soft ferromagnetic damper bars  14  are configured to substantially fill, but preferably completely fill the slot openings  24 . By at least substantially filling the slot openings  24  with the paramagnetic or soft ferromagnetic damper bars  14 , a closed rotor slot assembly is achieved, which inherently leads to an increase in rotor  10  sub-transient direct-axis inductance. Additionally, based on the lower conductivity of the paramagnetic or soft ferromagnetic material, the damper bar  14  resistances are greater than those associated with copper damper bars having identical cross-sections. Therefore, transient conditions decay more rapidly in an embodiment comprised of paramagnetic and/or soft ferromagnetic damper bars  14 . 
     The damper bars  14  formed of a paramagnetic or soft ferromagnetic material increase the rotor sub-transient direct-axis leakage inductance, as described above, and this has a direct impact on the transient behavior of the overall system, as well as machine winding current peaks and rise rate during periods of inverter switching transients without effecting the steady-state generate mode performance. The relationship between the machine winding current rise rate for a given inverter DC voltage (V DC ) is related to the direct-axis inductance (L D ), and is as follows:
 
 di/dt=V   DC /[(3/2)* L   D ]
 
     Therefore, the paramagnetic and/or soft ferromagnetic damper bars  14  reduce or mitigate high peak winding phase currents during start mode with inverter operation of the starter generator. 
     The starter generator is configured with paramagnetic and/or soft ferromagnetic damper bars  14  which substantially or completely fill the slot openings  24 , thereby providing a closed rotor assembly and increasing power capability, as well as power density of wound field starter generators, based on the advantages described above that are associated with the closed rotor assembly. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Technology Category: 5