Abstract:
The invention relates to an electrical machine and to a method for the operation thereof, particularly as a drive motor for an electrical tool or as starter generator for a motor vehicle. The electrical machine includes a rotor excited by a permanent magnet and a stator carrying a multi-phase winding, and operates in a voltage-controlled, lower rotational speed range via a transformer on a DC voltage source. The electrical machine can also be operated in a higher rotational speed range by field weakening, and the structure of the machine can be changed by reducing the flux linkage between the rotor and the stator in order to weaken the field. Preferably, the change to the structure of the machine is carried out by turning off winding parts or by switching them between series and parallel connections.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP2008/058749 filed on Jul. 7, 2008. 
     BACKGROUND OF THE INVENTION 
     1.Field of the Invention 
     The invention is based on an electrical machine or a method for operating an electrical machine. 
     2.Description of the Prior Art 
     From German Patent Disclosure DE 10 2004 027 635 A1, one such electrical machine is known as a drive for a hand-held or stationary electric tool, and its drive unit has an electronically commutatable motor with a permanent-magnet-excited rotor, and its stator is operated by means of a motor controller in such a way that the motor, in a first rpm range, operates in a voltage-controlled mode and in a second rpm range, which adjoins the first rpm range in the direction of a higher rpm, is triggered in accordance with a field attenuation mode. The field attenuation is attained by means of a phase displacement between the magnetomotive force of the rotor and of the stator, and advance commutation of the stator current is effected. This mode of operation, in which in the field attenuation rpm range the exciter current of the stator leads ahead of the pole wheel voltage, can be achieved inexpensively by comparatively simple means and can be employed when stringent demands are not made of the guidance of the electrical machine, and particularly when highly dynamic setting of the transverse axis flow forming the torque can be dispensed with. 
     In principle, the idling rpm of electronically commutated (EC) motors is defined by the design of the winding and the magnitude of the voltage of the supplying direct current source. When voltage is supplied constantly, the maximum rpm, or the idling rpm, of the motor is thus defined. To increase the motor rpm still further, a field attenuation is necessary, in which the amount of the magnetic flux linked with the stator winding is reduced. To that end, it is known to attain the field attenuation by means of a stator current component which generates a magnetomotive force in the stator winding parallel to the magnetomotive force of the exciter. In the literature, this component is called the longitudinal or d-axis current, referred to the main axis of the rotor. This d-axis component is as a rule oriented such that it is counter to the magnetomotive force of the exciter, and thus the resultant magnetomotive force of the exciter in the main exciter axis is reduced. 
     The resultant d-axis magnetomotive force induces a voltage in the armature winding of the machine that leads ahead of the magnetomotive force by 90°. This induced voltage is in phase with a current that generates an armature magnetomotive force in the transverse (q) axis of the rotor and with it converts electrical power into mechanical power. On this basis, a field attenuation or field-oriented regulation is possible in which the two stator current components can be adjusted independently of one another. With the q-axis current, the torque is adjusted, and with the d-axis current, the induced voltage in the machine can be reduced so far that rotary speeds far above the natural idling rpm of the machine are attainable. One such regulation is described for instance in German Patent DE 197 25 136 C2. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     The electrical machine according to the invention and a method for operating the machine have the advantage over the prior art described that the field attenuation mode of operation and the increase in machine rpm thus made possible are attained without additional stator currents and thus without increasing the electrical machine losses that are typical with the known methods, such as the method described at the outset in terms of DE 10 2004 027 635 A1. 
     It is especially advantageous, for reducing the flux linking in the machine, to design parts of the stator winding as switchable between a series connection and a parallel connection; by means of the parallel connection, instead of the series connection, a reduction in the copper losses of the machine is achieved simultaneously with the increase in rpm. Alternatively, instead of the switchover, it may be expedient to shut off parts of the stator winding, by means of which once again the increase in rpm is made possible while reducing or at least without increasing the losses. A further advantageous possibility for reducing the flux linking is to effect the field attenuation by switching over parts of the stator winding, which are embodied with parallel wires, to a series connection, or shutting them off in order to stop the partial coils formed by them. The structural reduction in the flux linking could, however, also be done by a modification of the air gap of the machine, for instance by means of an axial displacement of frustoconical jacket faces of the rotor and of the stator in the air gap region. 
     Preferably, the stator winding of the electrical machine of the invention is embodied in each phase with two partial coils, which can be switched over or shut off. Such an arrangement has advantages in particular in terms of the expense for the switching device, which can thus be designed inexpensively with a small number of switch contacts or switch elements. The switching device is preferably embodied here as a multi-pole structural unit. 
     In terms of the electromechanical structure of the machine of the invention, it is advantageous if it is embodied with two to six phases and if the rotor is embodied with two poles, four poles, or a multiple of that number of poles. A single-phase embodiment would be attainable only with additional and in particular capacitive components, but generating a rotary field could be optimized only with regard to a single rpm, so that the demand for a wide rpm range cannot be attained appropriately with such an embodiment. On the other hand, an embodiment of the machine with more than six phases is harder to attain in particular in terms of production and requires markedly greater expense, particularly for the converter required and for the switchover device, without attaining correspondingly great advantages in terms of reducing the magnetic noise and air gap noise. The design of the machine with two or four poles or with a multiple of that number of poles has the advantage that practically all the designs of interest technically and commercially, particularly of the stator winding, can be attained by simple repetition and multiplication of the structural form on the circumference of the stator, compared to a two-pole or four-pole machine. 
     Further details and advantageous features of the invention will become apparent from the claims and the description of the exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiment of the invention are shown in the drawings and described in further detail in the ensuing description in conjunction with the drawings, in which: 
         FIG. 1  is a circuit diagram of an electrical machine, in the embodiment of an EC motor with a three-phase stator winding with partial coils that can be shut off, the motor being supplied from a direct voltage network via a converter; 
         FIG. 2  shows an electrical machine corresponding to  FIG. 1  with partial coils that can be operated selectively in a series connection or a parallel connection; 
         FIG. 3  shows an electrical machine corresponding to  FIG. 1 , whose partial coils can be connected to the converter selectively in a series connection or can be partly shut off without shifting the star point; 
         FIG. 4  shows an electrical machine corresponding to  FIG. 1 , in which the partial coils of the stator winding are wound with parallel wires and can be operated selectively in a series connection or a parallel connection; 
         FIG. 5 , shows one exemplary embodiment of a stator winding of a four-pole electrical machine with three phases and twelve slots, with beginnings and ends of the partial coils extended to the outside; 
         FIG. 6  shows an exemplary embodiment of a stator winding of a two-pole electrical machine with three phases and twelve slots, again with beginnings and ends of the coils extended to the outside; 
         FIG. 7  shows an exemplary embodiment of a stator winding of a two-pole electrical machine with three phases and twelve slots, with beginnings and ends of the windings of the partial coils extended to the outside, and with the coils embodied with a coil width of 180° e 1 ; and 
         FIG. 8  shows an exemplary embodiment of a stator winding of a four-pole electrical machine, with three phases and twelve slots, in which the partial coils are wound with two parallel wires. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , the basic circuit arrangement of an electrical machine is shown in its embodiment as a three-phase, electrically commutated and permanent-magnet-excited motor, of the kind that can be used for instance as a drive motor for electric tools. Of the stator  10  of the motor, only the three-phase winding is shown here; the individual phases are subdivided into partial coils U 1 / 2  and U 3 / 4 , V 1 / 2  and V 3 / 4 , and W 1 / 2  and W 3 / 4 . The ends of the partial coils are accordingly identified as coil ends Ul-U 4 , V 1 -V 4 , and W 1 -W 4 . Between the partial coils of the phases U, V, W is a three-pole switching device  12 , by which the partial coils of the individual phases are selectively connected in series with a star point  14 , or alternatively, the partial coils U 3 / 4 , V 3 / 4  and W 3 / 4  are disconnected, and the partial coils U 1 / 2 , V 1 / 2  and W 1 / 2  are interconnected to a new star point  16 . 
     The permanent magnet, schematically shown rotor of the machine is marked  18  and in practice is embodied with two poles, four poles, or a corresponding multiple of these numbers of poles and is designed in a known manner, with north and south poles alternating at the rotor circumference. These north and south poles may be formed either directly by permanent magnets disposed on the circumference of the rotor  18  or by rotor iron that is present there. 
     The supply to the machine is done from a direct voltage source  20  via a converter  22 , in which the direct current is converted into a three-phase alternating current. The converter  22  is preferably designed as a transistorized full-bridge circuit, and the individual transistors are switched on by means of a controller, not shown, in accordance with the rotor position, also in a well known manner. 
     The circuit arrangements of the electrical machine of  FIGS. 2 ,  3  and  4  are fundamentally constructed identically to the arrangement of  FIG. 1 , and identical elements are identified by the same reference numerals. There are differences in terms of the type of switchover of the individual partial coils and their connection to the converter  22 . 
     In  FIG. 2 , the switchover is effected by means of a six-pole switching device  24 . In it, one switchover contact each is permanently connected to the coil ends U 2  and U 3 , V 2  and V 3 , and W 2  and W 3 . The switchable contacts of the switching device  24  either connect the coil ends U 2  and U 3 , V 2  and V 3 , and W 2  and W 3 , or else from the coil ends U 2 , V 2  and W 2  they form a new star point  26  and simultaneously connect the coil ends U 3 , V 3  and W 3  to the coil ends U 1 , V 1  and W 1 , respectively, so that the partial coils U 3 / 4 , V 3 / 4  and W 3 / 4  are connected in parallel to the partial coils U 1 / 2 , V 1 / 2  and W 1 / 2 . The star points  14  and  26  are connected to one another as indicated by the dot-dashed line  27 . 
     In the circuit arrangement of  FIG. 3 , a three-pole switching device  28  is located at the output of the converter  22  and, in the position shown in the drawing, connects the series-connected partial coils U 1 / 2  and U 3 / 4 , V 1 / 2  and V 3 / 4 , and W 1 / 2  and W 3 / 4  as a series connection with the star point  14  to the converter  22 . Between the coil end pairs U 2 -U 3 , V 2 -V 3  and W 2 -W 3 , respective taps  30 ,  32  and  34  are extended to the outside and connected to the free terminals in the drawing of the switching device  28 , so that upon their switchover, the partial coils U 1 / 2 , V 1 / 2  and W 1 / 2  are disconnected from the voltage supply, and only the partial coils U 3 / 4 , V 3 / 4  and W 3 / 4  with the star point  14  with partial coils that are reduced in terms of their partial winding numbers form the stator winding  10  in the field attenuation mode of operation. Unlike the shutoff in  FIG. 1 , here the star point  14  of the partial coils is not shifted, and only one tap each has to be mounted between the partial coils. 
       FIG. 4  shows a circuit arrangement of the electrical machine in which the partial coils U 1 / 2  and U 3 / 4 , the partial coils V 1 / 2  and V 314 , and the partial coils W 1 / 2  and W 3 / 4  are formed by parallel-wound wires that are each located in the same slot. The beginnings of the coil ends Ul, V 1  and W 1  are connected directly to the outputs of the converter  22 , while the associated outputs of the coil ends U 2 , V 2  and W 2  are connected to a fixed contact of a six-pole switching device  36 . By means of this switching device  36 , the coils U 1 / 2 , V 1 / 2  and W 1 / 2  are interconnected selectively in series with the coils U 3 / 4 , V 3 / 4  and W 3 / 4  or to a further star point  38 , while the outputs of the coil ends U 4 , V 4  and W 4  form the star point  14 . The star points  14  and  38  are connected as indicated by the dot-dashed line  39 . Hence the machine can in turn be operated selectively in a voltage-controlled mode with lower rpm in a series connection of the partial coils, or on the other hand by field attenuation in a higher rpm range with a parallel connection of the partial coils. The inductance and ohmic resistance in the series connection, given the same wire cross sections and identical partial coils, each assume the quadruple value, while the rpm is only half as great as in the parallel connection of the partial coils. Simultaneously, in the series connection, the maximum short-circuit current is reduced to one-quarter of the value, and the total characteristic of the motor is modified in accordance with the various inductances and ohmic resistances. A shutoff of parallel partial coils is not suitable, since it would not lead to any change in the rpm. 
       FIGS. 5 through 8  show various winding arrangements for the partial coils of a three-phase stator winding in accordance with the circuit arrangements of  FIGS. 1 through 4 . Here, the beginnings and endings of the coils are each extended to the outside and identified by the same reference numerals as in  FIGS. 1 through 4 , so that additional descriptions of the connections of the windings to one another can be dispensed with. They are each in accordance with the interconnections in  FIGS. 1 through 4  and can be derived from there. 
     In  FIG. 5 , the developed view of a stator  10  of a four-pole machine with twelve slots  40  is shown. Only one coil side  42  of the partial coils U 1 / 2 , U 3 / 4 , V 1 / 2 , V 3 / 4  and W 1 / 2  and W 3 / 4 , embodied as a lap winding is located in each slot  40 . In an interconnection in accordance with  FIG. 1 , the beginnings of the coil ends Ul, Vl and WI are then connected to the converter  22 . The outputs of the coil ends U 2 , V 2  and W 2  are connected to the new star point  16  via the three-pole switching device  12 , and the partial coils U 3 / 4 . V 3 / 4  and W 3 / 4  with the star point  14  are disconnected from the voltage supply. After a switchover of the three-pole switching device  12 , the coil ends U 2  and U 3 , V 2  and V 3 , and W 2  and W 3  of the coils are connected to a series connection of the partial coils with the star point  14  at the coil ends U 4 , V 4  and W 4 . 
     In an interconnection of the partial coils in  FIG. 5  in accordance with the arrangement of  FIG. 2 , the six-pole switching device  24  additionally affords the possibility of connecting the various partial coils U 1 / 2  and U 3 / 4 , V 1 / 2  and V 3 / 4 , and W 1 / 2  and W 3 / 4  parallel. The circuit variant shown in  FIG. 2  corresponds to the series connection of the partial coils already described in conjunction with  FIG. 1 . For the parallel connection of the partial coils, the switchover of the six-pole switching device  24  is necessary, as a result of which on the one hand the new star point  26  is formed and on the other, the beginnings of the coil ends U 3 , V 3  and W 3  are connected to the outputs of the converter  22 . 
     A further possibility of interconnecting the partial coils in  FIG. 5  is shown in  FIG. 3 , with a three-pole switching device  28  directly at the output of the converter  22 , which either connects the coil ends Ul, V 1  and Wl and the downstream partial coils to a series connection with the star point  14  or selectively, after the switchover, disconnects the partial coils U 1 / 2 , V 1 / 2  and W 1 / 2  from the voltage supply and together with the partial coils U 3 / 4 , V 3 / 4  and W 3 / 4  forms the new, reduced partial winding for the field attenuation mode in the stator  10 . Here, the terminals of the partial coils are shifted, but the star point  14  is preserved. 
       FIG. 6  again shows a stator  10  with twelve slots, and again one coil side  42  is located in each slot  40 . The winding here, however, is designed for a two-pole embodiment of the rotor  16 , resulting in two as the number of holes. Each two coil sides  42  of partial coils of the same phase are located in adjacent slots  40 . The winding is again embodied as a lap winding. In a departure from the designations of the windings in  FIGS. 1 through 3 , however, in a two-pole rotor the partial coils U 3 / 4 , V 3 / 4  and W 3 / 4  are to be shifted by 180° e 1 , resulting in the series connection at the terminals U 4 , V 4  and W 4  and on the other hand in the star point  14  at the coil ends U 3 , V 3  and W 3 . Accordingly, in a parallel connection as in  FIG. 2 , the coil ends U 4 , V 4  and W 4  are connected to the corresponding outputs of the converter  22 , while the coil ends U 3 , V 3  and W 3  form the star point  14 . Upon the shutoff of the group of coils having the partial coils U 3 / 4 , V 3 / 4  and W 3 / 4 , the special feature occurs that after the shutoff of the group of coils, a winding arrangement with shortened coils, with a coil width of 150° e 1 , results. It is understood that the group of coils having the partial coils U 1 / 2 , V 1 / 2  and W 1 / 2  can also be shut off in accordance with  FIG. 3 , so that upon shutoff of a group of coils, the star point  14  does not shift. 
       FIG. 7  shows a three-phase winding for a stator  10  in a two-pole version of the rotor  18 , but the partial coils are embodied with a coil width of 180° el. In this arrangement, even after the shutoff of a group of coils, the result is a winding with diameter coils, but in comparison to the winding arrangement of  FIG. 6 , longer winding heads are created, which necessitate a greater wire length, with more copper, and a greater amount of space required in the stator. Otherwise, the remarks on  FIG. 6  apply accordingly to  FIG. 7  as well. 
       FIG. 8  shows a winding design for a four-pole rotor  18  with a stator  10  having twelve slots  40 , in which two parallel wires or parallel coil sides  42  are located in each slot  40 . Such an arrangement is equivalent to the circuit diagram in  FIG. 4 , with a close magnetic coupling of the various partial coils of each phase. The beginnings of the coil ends Ul and U 3 , V 1  and V 3 , and W 1  and W 3  of the windings, like the associated ends of the coils, are located in the same slot; the individual partial coils are distributed uniformly over the stator circumference. In terms of circuitry, as in  FIG. 4 , the series connection for the lower rpm range and the parallel connection for the field attenuation mode of operation in the higher rpm range are selectively attractive, while upon a shutoff of one coil part, compared to the series connection, no change in the number of windings and accordingly no change in rpm results, but only higher losses in the machine. 
     The design according to the invention of the electrical machine, and the proposed method for operating such a machine, enable operation, by simple means without additional losses, with a markedly increased idling rpm at a constant and fixedly predetermined voltage source. In contrast to conventional arrangements with a field-attenuating stator current component in the d axis of the rotor  18 , the proposed arrangement does not lessen the efficiency of the machine, and the change in rpm can be attained selectively by switchover or shutoff of groups of coils. The arrangement can be employed especially advantageously in motors operated with rechargeable batteries, such as electric tools operated with rechargeable batteries, so that mechanical switchover gears that are usual otherwise are dispensed with. As a result, the machine can be made more compact, lighter in weight, and less expensive, and all the known switch elements are suitable as switch elements for the switching devices  12 ,  24 ,  28  and  36 , or in other words both mechanical switches or relays and electronic switch elements. The actuation of the switching devices can be done either directly by the user or by means of an electronic unit, such as a microprocessor. The switching device can continue to be designed either as an independent switchover device, similarly to the otherwise usual mechanical gear switchover means, or it can integrated structurally with the tool switch that simultaneously acts as an rpm transducer, so that the switchover is automatically jointly actuated whenever the user requires high rpm. An automatic switchover as a function of the load moment is also still possible. 
     A further advantageous possibility of use of the proposed machine design is in the automotive field, in the field of starter generators, for which the structural form as an electronically commutated machine is again very highly suitable, and the wide available rpm range can be exploited. Because of the high rpm that occur, rotor forms with a higher number of poles, such as twelve-pole or sixteen-pole arrangements, are suitable. The corresponding is true for the number of stator phases. While a three-phase embodiment in principle makes a simple, inexpensive construction of the machine possible, higher-phase stator windings offer advantages in terms of magnetic noise and air gap noise, noises that are particularly irritating in continuous operation in the motor vehicle. In each application, however, the focus is on the possibility of furnishing high load moments at a correspondingly reduced rpm, or on the availability of high rpm, if the requisite load moment allows. 
     In closing, it will also be pointed out that the shutoff of winding parts also produces good results in terms of the efficiency of the machine. This initially surprising outcome is due to the fact that, because of the asymmetries in the magnetic circuit of the machine that are created by the shutoff of winding parts, higher-harmonic components occur in the air gap of the machine, and as a result, the inductance, particularly in the upper rpm range, increases markedly and with it the efficiency of the machine. 
     The foregoing relates to the preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.