Abstract:
A drive system for a multi-phase electric machine with a permanent magnet rotor, the drive system may comprise conduction paths for each phase. Detectors on each of the conduction paths may determine electrical condition of the conduction path. At least one selectable interconnection path may be present between all of the conduction paths and at least one selectable interconnection path may be operable to connect all of the conduction paths together responsively to at least one of the detectors determining that the electrical condition of its respective electrical path is representative of a predetermined electrical fault condition so that heating of the rotor during continued rotation of the rotor is prevented.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to methods and apparatus for controlling multi-phase electric machines and, more particularly, protecting such machines from damage that may result from non-symmetrical short circuit failures. 
     In many electric machine applications it is desirable to employ a synchronous machine with a permanent magnet rotor. Such a machine may be referred to as a permanent magnet machine (PMM). While a PMM may have advantageous features, it has a disadvantageous feature that has heretofore limited its use in some applications. 
     Rotor flux in a PMM is fixed and cannot be controlled or disengaged when a short circuit occurs. Unlike other electric machines where excitation of the rotor flux can be controlled and even shutdown quickly, a PMM continues to generate electromagnetic flux until the rotor stops. Therefore, a PMM may present a hazard in some applications in which rotor speed may not be quickly reduced in the event of a short-circuit failure. 
     Some specialized PMM&#39;s have been designed so that they may tolerate some short-circuit failures with continuing rotation of a rotor. For example, a high-reactance permanent magnet machine (HRPMM) may internally limit the phase current magnitude, should it become shorted. The high reactance nature of this type of machine sustains short circuit currents either indefinitely, within the thermal limits of the system, or until the rotor speed can be reduced to zero. This fault protection method may accommodate a so called, bolted short or symmetrical short, i.e. all phases of the machine connected together electrically. However, in some instances a short circuit may develop in a single phase, between two phases or as an intermittent fault rather than as a symmetrical short. Such a failure may be referred to as a non-symmetrical short circuit failure (NSSCF). A NSSCF may create negative sequence flux that may produce overheating of the rotor assembly. 
     It may be seen that there is a need to provide a PMM which may tolerate a NSSCF with continued rotor rotation. There is a need for such a PMM which may tolerate occurrence of a NSSCF without experiencing destructive overheating of the rotor assembly. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a drive system for a multi-phase electric machine with a permanent magnet rotor, the drive system comprising conduction paths for each phase; detectors on each of the conduction paths for determining electrical condition of the conduction path; at least one selectable interconnection path between all of the conduction paths; and at least one selectable interconnection path being operable to connect all of the conduction paths together responsively to at least one of the detectors determining that the electrical condition of its respective electrical path is representative of a predetermined electrical fault condition so that heating of the rotor during continued rotation of the rotor is prevented. 
     In another aspect of the present invention, a multi-phase electric machine drive system in a non-detachable relationship with a high-inertia load and/or a prime mover comprises a rotor; a stator; detectors for detecting an electrical fault in a conduction path of each phase; and a controller for electrically interconnecting all of the conduction paths responsively to detection of a fault in any of the conduction paths so that any negative sequence flux resulting from the fault and continued rotation of the rotor is converted to positive sequence flux. 
     In still another aspect of the present invention a method for controlling a synchronous electric machine comprises the steps of; rotating a rotor of the electric machine; transferring current to or from stator windings, said transferring being performed on multiple conduction paths; monitoring each of the conduction paths individually for presence of a predetermined electrical fault condition; and electrically interconnecting all of the conduction paths in a presence of the predetermined fault condition on any one or more of the conduction paths to assure that negative sequence currents do not produce heating of the rotor. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram of a drive system in a motor configuration in accordance with an embodiment of the present invention; 
         FIG. 1B  is a block diagram of a drive system in a generator configuration in accordance with an embodiment of the present invention; 
         FIG. 2  is a detailed block diagram of the drive system of  FIGS. 1A and 1B  in accordance with an embodiment of the present invention; 
         FIG. 3A  is graph plot of motor current vs. time for a simulated three phase symmetrical short in accordance with an embodiment of the present invention; 
         FIG. 3B  is a graph plot of motor current vs. time for a simulated non-symmetrical short in accordance with an embodiment of the present invention; 
         FIG. 4  is a block diagram of a drive system in accordance with a second embodiment of the present invention; 
         FIG. 5  is a block diagram of a drive system in accordance with a third embodiment of the present invention; and 
         FIG. 6  is a flow chart of a method for controlling a drive system machine in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can each be used independently of one another or in combination with other features. However, any single inventive feature may not address any of the problems discussed above or may only address one of the problems discussed above. Further, one or more of the problems discussed above may not be fully addressed by any of the features described below. 
     Broadly, embodiments of the present invention may be useful in electric machine applications. More particularly, embodiments of the present invention may provide for control of an electric machine with a permanent magnet rotor which may allow the electric machine to experience a NSSCF without experiencing destructive overheating of the rotor. Embodiments of the present invention may be particularly useful in applications in which a rotor of the electric machine may continue its rotation even after a NCSSF occurs, e.g., a PMM connected directly to a load with high rotational inertia. 
     Referring now to  FIGS. 1A and 1B , block diagrams may portray exemplary embodiments of the invention. An electric drive system  10  may comprise an electric machine  11  and a controller  12 . The electric machine  11  may comprise a rotor  14  and a stator  16 . In  FIG. 1A , the rotor  14  of the electric drive system  10  may be directly connected to drive a load  18 . In FIG.  1 B, the rotor  14  may be connected to a prime mover  20  which may drive the electric drive system  10  as a generator. The controller  12  may control current into or out of the stator  16 . The electric machine  11  may be a high-reactance permanent magnet machine (HRPMM) which may internally limit phase current magnitude, in the even of a short circuit. One example of such a HRPMM and a method of limiting phase current is described in U.S. Pat. No. 7,116,083 (G. Kalman et al., Oct. 3, 2006) which is incorporated herein by reference. The high reactance nature of this type of machine may sustain short circuit currents in its windings either indefinitely, within the thermal limits of the system, or until the rotor speed can be reduced to zero. 
     Referring now to  FIG. 2 , a block diagram may illustrate a relationship between the controller  12  and the stator  16  of the electric machine  11  in an embodiment of the invention. The controller  12  may comprise an inverter constructed with solid state inverter switches  12 - 1  and  12 - 2  which may control current to or from the stator  16 . The inverter switches  12 - 1  and  12 - 2  may be selected from among numerous conventional solid state switching devices such as metal-oxide silicon field effect transistors (MOSFET&#39;s) or insulated gate bipolar transistor (IGBT&#39;s). In the exemplary embodiment of the invention shown in  FIG. 2 , the inverter switches  12 - 1  and  12 - 2  may be IGBT&#39;s and they may be employed to operate in a three-phase power control mode. Timing and sequence of operation of the IGBT&#39;s  12 - 1  and  12 - 2  may be controlled through software in a processor  12 - 4 . During normal or steady-state operation of the electric drive system  10 , the IGBT&#39;s  12 - 1  and  12 - 2  may be controlled in a conventional manner. In an exemplary embodiment of  FIG. 2 , the inverter may be a three-phase bridge inverter  12 - 3 . 
     A phase-specific conduction path  24  may carry current to or from a stator winding  16 - 2  of the electric machine  11  for each phase. Each of the conduction paths  24  may comprise a machine terminal  26 , a feeder  28  and one of either the IGBT&#39;s  12 - 1  or the IGBT&#39;s  12 - 2 . A detector  22  may be positioned on each of the conduction paths  24 . 
     The controller  12  may be provided with a current and voltage monitoring circuit  12 - 6  which may be connected to the feeders  28 . In the event of an electrical fault in any one or more of the conduction paths  24 , the monitoring circuit may provide a fault signal  12 - 62  to the processor  12 - 4 . The monitoring circuit  12 - 6  may be constructed to be responsive to one or more predetermined types of electrical fault conditions. The electrical fault condition may be an actual short circuit fault in one or more of the IGBT&#39;s  12 - 1  or  12 - 2 , one or more of the windings  16 - 2 , or one or more of the feeders  28 . The detected electrical fault condition may be a precursor to an actual short circuit fault. For example, the electrical fault condition may be detected as an arc fault that may be detected in a manner described in US Patent Application Publication 2008/0129309 (W. Yu, Jun. 5, 2008) which publication is incorporated herein by reference. 
     Upon detection of one or more of the predetermined electrical fault conditions, the processor  12 - 4  may provide gating instruction  12 - 42  for the IGBT&#39;s  12 - 1  and/or  12 - 2  so that all three phases may be shorted together to produce a so-called bolted short. In other words, a symmetrical short circuit fault (SSCF) may be produced responsively to detection of any of the predetermined electrical faults conditions, even a NSSCF. The processor  12 - 4  may provide gating instructions  12 - 42  which may result in simultaneous closing of all of the IGBT&#39;s  12 - 1  or all of the IGBT&#39;s  12 - 2  to produce an interconnection path  25  among all of the conduction paths  24 . The interconnection path  25  may thus be seen to comprise a selectable interconnection path. 
     If an electrical fault occurs in a current path  24  in which one of the IGBT&#39;s  12 - 1  is present, then all of the IGBT&#39;s  12 - 1  may be closed. Conversely, if an electrical fault occurs in a current path  24  in which one of the IGBT&#39;s  12 - 2  is present, then all of the IGBT&#39;s  12 - 2  may be closed. Also, the processor  12 - 4  may be programmed so that, in the event of an electrical fault in one or more of the machine terminals  26 , one or more of the feeders  28  or one or more of the stator windings  16 - 2 , the IGBT&#39;s  12 - 1  and  12 - 2  may be alternately opened and closed to optimize distribution of losses. 
     It has been found that phase currents may rise when a single phase or NSSCF is converted to a SSCF. However, any undesirable effects of such current rise may be offset by the desirable effects of eliminating negative sequence flux that may arise in the continuously rotating rotor  14  during a NSSCF. In the event of a SSCF, positive sequence flux may occur in the rotor  14 . Positive sequence flux may not produce currents in the rotating rotor  14 . Negative sequence flux, on the other hand, may produce current in the rotor  14 . In most PMM&#39;s, rotors may have a high resistivity, typically equivalent to that of iron. Current produced by negative sequence flux may produce heating in the rotor  14 . This heating may continue for as long as the rotor  14  may rotate in the presence of a NSSCF. When the NSSCF is converted to a SSCF, the negative sequence flux may be converted to a positive sequence flux and the undesirable heating of the rotor may stop; even though the rotor may continue its rotation. 
     As mentioned above, a phase current rise may occur when the NSSCF is converted to the SSCF. However, simulation results have shown that this phase current rise is well within a manageable range in a typical HRPMM. An analysis was performed on a three-phase, HRPMM machine. The machine was rated 128/222 VAC, 4 HP, 10800 rpm, and was an 8-pole, 24-slot, interior surface magnet rotor type machine. No-load, three-phase symmetrical short circuit, and line-line non-symmetrical short circuit conditions were analyzed. Faulted circuit analyses were initiated at no-load and winding resistance was included. Finite element (FE) software was used to model and analyze motor performance. Short circuit currents versus time are shown in  FIGS. 3A and 3B . Results for transient and steady state fault currents are shown in Table 1. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Peak Transient Phase 
                 Steady-state Phase 
               
               
                   
                 Current 
                 Current 
               
               
                   
                 (0-to-peak Amperes) 
                 (0-to-peak Amperes) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 3-phase symmetrical 
                 63.3 
                 35.6 
               
               
                 fault 
               
               
                 Line-line non- 
                 51.7 
                 30 
               
               
                 symmetrical fault 
               
               
                   
               
             
          
         
       
     
     It may be seen from Table 1 that peak transient phase current rose about 22% upon conversion of the NSSCF to the SSCF. The steady state phase current rose about 19%. 
     Referring now to  FIG. 4  another exemplary embodiment of the invention may be illustrated. A hardware switch controller  12 - 8  may be placed in the controller  12 . The switch controller  12 - 8  may incorporate the monitoring functions of the monitoring circuit  12 - 6  of  FIG. 2  and may also provide gating signals  12 - 82  to the IGBT&#39;s  12 - 1  and  12 - 2  in the event of a fault condition. The gating signals  12 - 82  may override conventional steady gating signals produced by the processor  12 - 4  so that a SSCF may be produced. By incorporating the hardware switch controller  12 - 8  in the controller  12 , a desirable degree of added safety may be provided as compared to the controller configuration of  FIG. 2 . The switch controller  12 - 6  may perform the function of converting a NSSCF to a SSCF without using software of the processor  12 - 4  of  FIG. 2 . Thus the electric machine  11  may be provided with protection even if a software failure in the processor  12 - 4  were to occur. 
     Referring now to  FIG. 5 , another exemplary embodiment of the invention may be illustrated. In  FIG. 5 , a fault converter  30  is shown connected to the conduction paths  28 . The fault converter  30  may be placed externally of the controller  12 . The fault converter  30  may function as a controller and may incorporate the monitoring functions of the monitoring circuit  12 - 6  of  FIG. 2 . The fault converter may provide the selectable interconnection path  25  between all of the conduction paths  28  in the event of a NSSCF. By incorporating the fault converter  30  in the electric machine  11 , a desirable degree of added safety may be provided as compared to the controller configurations of  FIG. 2  or  4 . The external fault converter  30  may perform the function of converting a NSSCF to a SSCF independently from the controller  12 . Thus the electric machine  11  may be provided with protection even if a functional failure in the controller  12  were to occur. It should be noted that the fault converter  30  may be employed in the electric machine  11  even if the electric drive system  10  is provided with fault conversion capability through the processor  12 - 4  of  FIG. 2  and/or the switch controller  12 - 8  of  FIG. 4 . Indeed, such a redundant arrangement may be desirable in critical applications such as aircraft or other aerospace vehicles. 
     In one embodiment of the present invention, a method  600  may be provided for controlling an electric machine. A flow chart of the method  600  may be illustrated in  FIG. 6 . In a step  602 , a rotor of a synchronous electric machine may be rotated (e.g., the rotor  14  of the electric machine  11  may be rotated). In a step  604 , current may be transferred on multiple conduction paths (e.g. power to or from the stator windings  16 - 2  may be transferred along the conduction paths  24 ). 
     In a step  606 , detection of electrical faults may be performed on each of the conduction paths (e.g., one or more of the monitoring circuit  16 - 6 , the switch controller  12 - 8  or the external fault converter  30  may monitor the conduction paths  24  for predetermined indications of electrical faults wherein excessive currents may be indicative of a short circuit and excessive voltage may be indicative of an arc fault). Steps  604  and  606  may be performed repeatedly unless and until an electrical fault is found. In the event of a fault condition, a step  608  may be performed to electrically interconnect all of the conduction paths (e.g., gating signals may be produced to close all of the IGBT&#39;s  12 - 1  and/or all of the IGBT&#39;s  12 - 2 ). Upon performance of step  608 , a step  610  may be performed to limit phase current rise (e.g., the method described in U.S. Pat. No. 7,116,083 may be initiated to limit phase current in the windings  16 - 2 ). In a step  612 , the rotor may be allowed to continue rotation without experiencing heating from negative sequence effects. 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.