Abstract:
An improved electric motor has a rotor ( 124 ), a stator ( 125 ) having at least one phase winding strand ( 126 ), an output stage ( 122 ) for influencing the current flow in said phase winding strand, a DC link circuit ( 170 ) for supplying the output stage ( 122 ) with current, including a link circuit capacitor ( 178 ), and a control unit ( 132 ) having an arrangement ( 152 ) for sensing a value characterizing the current recharge into the link circuit capacitor ( 178 ), which control unit ( 132 ) is configured to specify commutation instants as a function of the sensed value, and to perform commutation operations in the power stage ( 122 ) at the commutation instants thus specified. Avoiding a need for prolonged “currentless intervals” permits achieving higher efficiency and power output, particularly in a motor having less than three winding phases.

Description:
CROSS-REFERENCE 
       [0001]    This application claims priority of German Application 10 2006 014 520.8 filed 24 Mar. 2006, the entire content of which is hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a method and an arrangement for sensorless operation of an electronically commutated motor (ECM). 
       BACKGROUND 
       [0003]    DE 10 2005 020 737 and corresponding U.S. application Ser. No. 11/127,856, DORNHOF, published as US 2005-025,3546-A, describe a method of sensorless operation of an electronically commutated motor having at least two phase winding strands, in which motor the voltage, induced in the non-energized stator phase winding strand of the stator, i.e. the strand not experiencing current flow at the moment, is differentiated, in order to generate a control signal for controlling commutation of the motor. Since, in stator winding arrangements having two strands, and in general in multi-strand stator windings, the stator winding strands never all experience current flow simultaneously, the sensing, of the zero transition of the induced voltage necessary for generation of the control signal, can always occur in a stator phase winding not experiencing current flow at the moment. This sensing requires, however, a relatively large outlay in terms of circuit engineering. 
         [0004]    In a single-phase-winding motor, furthermore, current flow in the single stator phase winding strand must be discontinued during the expected zero transition of the induced voltage for a sufficiently long period of time to enable sensing of that zero transition. The result of such current-flow gaps is, however, to reduce the efficiency and maximum power of the motor. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the present invention to enable improved sensorless commutation in electric motors having a single-phase winding strand. These motors are commonly, though somewhat inaccurately, called “single phase motors.” 
         [0006]    The invention is based on the recognition that the induced voltage, that occurs at each commutation of an ECM in the stator winding thereof, influences a recharge current that is recharged into a DC link circuit associated with the ECM. Sensing of this recharge current thus allows an inference as to the induced voltage, and can accordingly be used to determine and specify suitable points in time for the commutation of the ECM. With optimum commutation, this recharged current is minimal, over time and/or absolutely (e.g. maximum current magnitude or integral of the current magnitude over time). Application of this principle is not, however, limited to a specific type of motor. 
     
     
       BRIEF FIGURE DESCRIPTION 
         [0007]    Further details and advantageous refinements of the invention are evident from the exemplifying embodiments, in no way to be understood as a limitation of the invention, that are described below and depicted in the drawings. 
           [0008]      FIG. 1  is a simplified circuit diagram of an apparatus for sensorless commutation of an electronically commutated motor (ECM), according to a first embodiment of the invention; 
           [0009]      FIG. 2  is a simplified circuit diagram of an apparatus for sensorless commutation of an ECM, according to a second embodiment of the invention; 
           [0010]      FIG. 3  schematically depicts various operating parameters of the apparatus of  FIG. 1  or  2  in a context of optimum commutation; 
           [0011]      FIG. 4  schematically depicts various operating parameters of the apparatus of  FIG. 1  or  2  in a context of delayed commutation; 
           [0012]      FIG. 5  schematically depicts various operating parameters of the apparatus of  FIG. 1  or  2  in a context of greatly delayed commutation; 
           [0013]      FIG. 6  shows what is schematically depicted in  FIG. 5 , with the commutation status emphasized; 
           [0014]      FIG. 7  is a flow diagram of a method for operating the apparatus of  FIG. 1  or  2 , according to an embodiment of the invention; 
           [0015]      FIG. 8  is a flow diagram of a method for determining suitable commutation instants, according to an embodiment of the invention; 
           [0016]      FIG. 9  schematically depicts a correction variable used to determine suitable commutation instants; 
           [0017]      FIG. 10  schematically depicts the correction variable of  FIG. 9 ; 
           [0018]      FIG. 11  schematically depicts various operating parameters of the apparatus of  FIG. 1  or  2  in a context of optimization of the commutation instants; and 
           [0019]      FIG. 12  schematically depicts various operating parameters of the apparatus of  FIG. 1  or  2 , upon ramp-up of the ECM. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    In the description that follows, the terms “left,” “right”, “top,” and “bottom” refer to the respective Figure of the drawings and can vary from one Figure to the next depending on an orientation (portrait or landscape) selected in each case. Identical or identically functioning parts are labeled with the same reference characters in the various figures, and are usually described only once. 
         [0021]      FIG. 1  is a simplified circuit diagram that schematically illustrates the functioning of an apparatus  100  for operating an ECM  120 . Apparatus  100  is implemented to enable sensorless operation of ECM  120  with improved commutation, and comprises ECM  120 , a power stage  122 , a control unit  132 , and a DC link circuit  170 . ECM  120  is preferably implemented to drive a fan but may, of course, be used for other purposes as well. 
         [0022]    According to an embodiment of the present invention, ECM  120  comprises a rotor  124  and a stator  125  having at least one stator phase winding strand  126 . Rotor  124  is, for example, a permanent-magnet rotor having one or more pole pairs. Stator  125  is preferably embodied in single-phase-winding fashion, i.e. with one stator phase winding strand  126  (L 1 ). A different number of phase winding strands would also be possible. Stator phase winding strand  126  has two terminals U and V through which stator phase winding strand  126  is connected to power stage  122 . 
         [0023]    Power stage  122  serves to influence the current in stator phase winding strand  126 , and is connected on one side via a node  114  to a lead  112 , and on the other side via a node  118  to a lead  116  connected to ground (GND). Via leads  112  and  116 , a supply voltage U b  is delivered to power stage  122  from DC link circuit  170 . 
         [0024]    DC link circuit  170  is depicted in  FIG. 1  only schematically, having two terminals  184 ,  186 , a diode  182  (D 1 ) and a link circuit capacitor  178  (C 1 ). The positive pole +U b  of supply voltage source U b  is applied to terminal  184  connected to lead  112 , and its negative pole −U b  is applied to terminal  186  which is connected to lead  116 . Diode  182  (D 1 ), which is connected at its anode to terminal  184  and at its cathode to a node  172 , serves on the one hand as a polarity protector and, on the other hand, prevents a recharge current I_RC, generated by the ECM, from flowing back into DC voltage source U b . A recharge current I_RC of this kind thus causes charging of capacitor  178 . 
         [0025]    Power stage  122  is preferably implemented as a full bridge circuit (H-bridge) having four semiconductor switches  192  (T 1 , p-channel type),  194  (T 3 , n-channel type),  196  (T 2 , p-channel type),  198  (T 4 , n-channel type) that are depicted, by way of example, as field-effect transistors having free-wheeling diodes. Semiconductor switches  192 ,  194  form a first half-bridge and are connected to terminal U of stator phase winding strand  126 , terminal V of which is connected to semiconductor switches  196 ,  198  which form a second half-bridge. Semiconductor switches  192 ,  194 ,  196 ,  198  are connected to control unit  132 , and are controlled by it in order to commutate the motor current. 
         [0026]    Control unit  132  comprises a microprocessor  130  (μC) and an arrangement  152  for sensing the charge state of link circuit capacitor  178 , which latter is arranged between node  172  and a node  158 . Suitable microprocessors are available from Microchip of Chandler, Arizona and other microprocessor manufacturers. Arrangement  152  encompasses a resistor  140  (R 1 ), an npn transistor  160  (T 5 ) in an emitter circuit, and two resistors  154  (R 2 ) and  156  (R 3 ). Resistor  140  is connected to node  158  and to a node  106  in lead  116 . Base terminal B of transistor  160  is likewise connected to node  158  via resistor  154 ; its collector C is connected via resistor  156  to a node  104  in lead  112 , and its emitter E is connected to a node  108  in lead  116 . Microprocessor  130  is connected on the input side to the collector of transistor  160 , and receives therefrom a signal RCI (recharge indicator) that characterizes the charge state of link circuit capacitor  178 . On the basis of signal RCI, microprocessor  130  generates commutation signals HSL, HSR, LSL, LSR for power stage  122 . These signals are applied to semiconductor switches  192 ,  196 ,  194 ,  198  in order to produce commutation operations in power stage  122  at predetermined commutation instants. 
       Operation 
       [0027]    When apparatus  100  is in operation, ECM  120 , after being switched on, is first ramped up to a predetermined minimum rotation speed that is necessary to enable execution of the described method for specifying suitable commutation instants, as a function of the charge state of link circuit capacitor  178 . This minimum rotation speed is preferably reached, after switching-on, by a forced commutation of stator phase winding  126  with a decreasing current-flow duration, so that ECM  120  is initially accelerated in a stepping-motor mode. 
         [0028]    Commutation signals HSR, LSL, HSL, LSR generated by control unit  132  for the commutation of ECM  120  preferably assume the logical state “HIGH” or “LOW.” Commutation signal HSR (“high side right”) serves to control semiconductor switch  196 , LSL (“low side left”) to control semiconductor switch  194 , HSL (“high side left”) to control semiconductor switch  192 , and LSR (“low side right”) to control semiconductor switch  198 . 
         [0029]    At each commutation, commutation signals HSR, LSL, HSL, LSR are, as a rule, generated in such a way that those semiconductor switches  192  to  198  that are conducting, become switched off; and those semiconductor switches  192  to  198  that are switched off, become conducting. For example, in the context of a first commutation operation at a first commutation instant t COMMUT     —     1 , firstly commutation signal HSL is toggled from “LOW” to “HIGH” and commutation signal LSR is toggled from “HIGH” to “LOW.” Semiconductor switches  192  and  198  are thereby switched off, interrupting delivery of current from DC link circuit  170  in the direction from winding terminal U to winding terminal V. After a so-called “current-flow gap,” which is also referred to as a “commutation gap” or “dead time,” commutation signal HSR is toggled from “HIGH” to “LOW” and commutation signal LSL is toggled from “LOW” to “HIGH.” Semiconductor switches  196  and  194  are thereby switched on, and current can flow from winding terminal V to winding terminal U. The commutation operation at instant t COMMUT     —     1  is thereby completed. The current-flow gap, between the switching-on and switching-off operations that are performed, is produced in order to reliably prevent a short circuit of the full bridge circuit of power stage  122 . 
         [0030]    Once the minimum rotation speed n_min of ECM  120  is reached, the charge state of link circuit capacitor  178  is continuously determined. This can already be done from the time ECM  120  is switched on, but becomes necessary for specifying, after each commutation, suitable commutation instants, once the minimum rotation speed has been reached. 
         [0031]    To determine the charge state of link circuit capacitor  178 , the voltage drop at resistor  140  generated by the recharge current I_RC (I_RECHARGE) is continuously sensed. This voltage drop allows an inference as to whether, and for how long, a recharge current is flowing into capacitor  178 ; a precise determination of the amplitude of the recharge current is not absolutely necessary. The sensitivity with which the recharge current is detected, i.e. its minimum detectable amplitude, can be set by appropriately selecting resistor  140  and measurement apparatus  160 . 
         [0032]    Be it noted that the selection of resistor  140  also influences the EMC (electromagnetic compatibility) interference emission. The larger the resistor  140  that is selected, the smaller the filter effect of capacitor  178 . The size of resistor  140  also influences the service life of capacitor  178 . The higher the resistance of resistor  140 , the lower the recharge current into capacitor  178  and the longer the service life of capacitor  178 . 
         [0033]    According to a preferred embodiment, transistor  160  (functioning as a threshold value switch) becomes conductive whenever the voltage drop at resistor  140  exceeds a predetermined threshold value. As a result, the logic signal RCI present at the input of microprocessor  130  becomes logical “LOW.” Once the recharge current has decayed, transistor  160  blocks and signal RCI becomes “HIGH.” 
         [0034]    The magnitude of the threshold value corresponds, in this embodiment, to the base-emitter voltage of the switched-on transistor  160 , and is equal to approximately 0.5 V. Depending on the selection of resistor  140 , the threshold value is reached at different recharge current levels. With a smaller (lower-resistance) resistor  140 , a higher recharge current is needed to achieve a voltage drop corresponding to the switch-on voltage. 
         [0000]    This higher recharge current occurs in a context of late commutation and as a result of smaller ignition advance angles. The desired ignition advance can also be set via resistor  140 . The larger the resistor  140  that is selected, the longer the service life of capacitor  178 . 
         [0035]    By evaluating the logic signal RCI, microprocessor  130  determines a time period during which the recharge current I_RC is above a threshold value, and during which the current is accordingly charging link circuit capacitor  178 . This time period extends substantially from the end of one commutation operation to the earliest instant at which the voltage drop (dependent on the recharge process) at resistor  140  drops below the predetermined threshold value, and corresponds to the time span during which signal RCI is logically “LOW.” As a function of this time period, and other suitable actions that are described below with reference to  FIG. 8 , the microprocessor specifies suitable commutation instants t COMMUT  for ECM  120 , at which instants commutation signals HSR, LSL, HSL, LSR of semiconductor switches  196 ,  194 ,  192 ,  198  are toggled as described above. 
         [0036]    According to a preferred embodiment, the suitable commutation instants are calculated in an indirect way by determining current-flow time durations. When a recharge current occurs after a commutation operation, it is assumed that a previously ascertained current-flow time period was too long, and needs to be shortened by a predetermined amount. If no recharge occurs, the current-flow time duration was tending toward being too short, and accordingly is lengthened by a predetermined amount. As mentioned above, the predetermined amount by which the current-flow time period is shortened or lengthened is preferably specified as a function of various operating parameters such as the recharge duration, supply voltage, motor current, acceleration, deceleration, and/or a target value definition, in order to optimize the motor. 
         [0037]    The commutation of ECM  120  can thus be optimized as a function of the current I_RC recharged into link circuit  170 , with the result that the recharged current also is minimized. Commutation then automatically synchronizes itself to an optimum, or at least good, current-flow time period, without the use of additional sensors for direct measurement of rotor position. 
         [0000]    Experiments have shown that with ECM  120  in a steady state, i.e. after the recharge current has been reduced, commutation occurs in the form of a so-called time-advanced commutation (“ignition advance”) with improved power output, improved efficiency, and improved EMC properties. In particular, minimization of the recharged current reduces the current load on link circuit capacitor  178  and extends its service life, which is then influenced mainly by the ambient temperature and effective current. 
         [0038]      FIG. 2  is a simplified circuit diagram that schematically illustrates the functioning of an apparatus  200  for operating ECM  120  of  FIG. 1 , according to a further embodiment. Apparatus  200  corresponds substantially to apparatus  100  of  FIG. 1 , and it is in particular the differences that are described. 
         [0039]    Lead  112  has two nodal points  103 ,  104 , and lead  116  has two points  105 ,  108 . In contrast to apparatus  100 , however, apparatus  200  has an arrangement  152 ′ for sensing the recharge into link circuit capacitor  178 , which arrangement determines the charge state of link circuit capacitor  178  using a comparator  157 . The latter is connected to points  103  and  105  for the delivery of supply voltage U b . A resistor  153  (R 4 ) is located between point  103  and a point  159 , and a resistor  155  (R 5 ) between point  159  and point  108 . Point  159  is connected to the non-inverting input (+) of comparator  157  and specifies to the latter a threshold value defined by voltage divider  153 ,  155 . The inverting input (−) of comparator  157  is connected to nodal point  158 , and the output to the input of microprocessor  130 . 
         [0040]    Comparator  157  compares the potentials at nodal points  158  and  159 , and generates signal RCI as a function of the result of the comparison. Signal RCI becomes “LOW” when the potential at nodal point  159  is lower than the potential at nodal point  158 . This is preferably the case when the charge state of link circuit capacitor  178  is being influenced by a recharged current, and when a voltage drop is occurring at resistor  140 . Otherwise the signal RCI generated by comparator  157  is “HIGH.” 
         [0041]    With the use of comparator  157 , the threshold value can be set more precisely, and at a lower value, than when transistor  160  of  FIG. 1  is used. This enables the effective current of link circuit capacitor  178 , and the time-advanced commutation performed once a steady state is reached, to be set relatively independently of one another. Because of the greater gain of comparator  157 , resistor  140  can be dimensioned to be smaller than in the case of  FIG. 1 , so that the filter effect of capacitor  178  is increased and EMC interference emission is diminished. 
         [0042]      FIG. 3  is a schematic depiction  300  of an exemplifying time course of operating parameters  310 ,  320 ,  330 ,  340  that are measured, during the operation of apparatus  100  of  FIG. 1  or apparatus  200  of  FIG. 2 , in the context of a commutation optimized according to an embodiment. Operating parameter  310  illustrates the winding current I in stator winding  126 , which current can also be referred to as a substitute Hall signal;  320  illustrates the commutation status of ECM  120 ;  330  illustrates the voltage induced into stator winding  126 ; and  340  illustrates the (positive) current recharged into capacitor  178  (shown as positive values) or the (negative) current fed out of capacitor  178 . 
         [0043]    Commutation status  320  exhibits, by way of example, two different levels: logical “LOW” and “HIGH.” At each level change, i.e. at commutation instants  322 ,  324 ,  326 , a commutation operation is performed. After the commutation operation at commutation instants  322 ,  324 ,  326 , current  340  has positive current peaks  342 ′,  344 ′,  346 ′, i.e. a current flows from ECM  120  into link circuit capacitor  178 . Current  340  subsequently decays to zero, and a current then flows in the opposite direction (negative) from capacitor  178  to ECM  120  until said current also, after a time period, decays back to zero. In the case of current peak  342 ′, the time period from commutation instant  322  to completion of the first recharge current or to the first zero transition (recharge time duration) is labeled  350 ′, and the time period from commutation instant  322  to the second zero transition, i.e. until link circuit capacitor  178  has returned the stored energy, is labeled  350 . Time periods  350 ′ and  350  can be referred to in general as decay time periods. 
         [0044]      FIG. 3  illustrates a substantially optimum commutation of ECM  120 , which commutation is in this case a time-advanced commutation in which commutation instants  322 ,  324 ,  326  each occur with a so-called ignition advance of 20° el. This results in an approximately minimal (recharge) current  340 , in terms of both the maximum amplitude at commutation instant  322  and the decay time duration  350 ′ or  350 . This is illustrated by the uniformity and symmetry of winding current  310 , which is largely free of undesirable current peaks and thus results in reduced EMC interference. 
         [0045]      FIG. 4  shows a schematic depiction  400  of an exemplifying time course of a winding current  410 , a commutation status  420 , an induced voltage  430 , and a recharge current  440 , which are measured during the operation of apparatus  100  of  FIG. 1  or apparatus  200  of  FIG. 2  in the context of a commutation that is not advanced or retarded.  FIG. 4  accordingly illustrates winding current  410  and the recharged current  440  in the context of a commutation of ECM  120  at commutation instants  422 ,  424 ,  426  that are not optimized according to the present invention. 
         [0046]    At commutation instants  422 ,  424 ,  426 , current peaks  442 ′,  444 ′,  446 ′ occur in recharge current  440 ; in terms of current intensity, these peaks are approximately 50% higher than current peaks  342 ′,  344 ′,  346 ′ that occur in the context of the optimized time-advanced commutation according to  FIG. 3 . These current peaks  442 ′,  444 ′,  446 ′ require, for decay, a time duration  450 ′ or  450  that is approximately 50% longer than in  FIG. 3 .  FIG. 4  depicts, by way of example, a recharge time period (decay time duration)  450 ′,  450  for comparison with recharge time period  350 ′,  350  of  FIG. 3 . Winding current  410  has, at commutation instants  422 ,  424 ,  436 , current peaks  412 ,  414 ,  416  that result in undesirable EMC interference signals. 
         [0047]      FIG. 5  is a schematic depiction  500  of an exemplifying profile of a winding current  510 , a commutation status  520 , an induced voltage  530 , and a recharge current  540  that are measured during the operation of apparatus  100  of  FIG. 1  and apparatus  200  of  FIG. 2  in the context of a commutation delayed by 10° el. 
         [0048]    At commutation instant  524 , a current peak  544 ′ occurs in recharge current  540  after commutation; this peak is approximately twice as large as current peaks  342 ′,  344 ′,  346 ′ of  FIG. 3 . This current peak  544 ′ requires, for its decay, a time duration  550 ′,  550  that is approximately three times as long as the time duration required for the decay of current peaks  342 ′,  344 ′,  346 ′ according to  FIG. 3  (compare time period  350 ′,  350  of  FIG. 3 ). Winding current  510  also exhibits, at commutation instant  524 , a current peak  514  that is approximately twice as high as current peak  414  in winding current  410 , and thus results in an even stronger undesirable EMC interference. The asymmetry of recharge current  540  results from the fact that the ignition angle at commutation instants  522 ,  526  is approximately 0° el., and at commutation instant  524  is +10° el. (ignition retard). The ignition retard is also evident from the fact that induced voltage  530  has its zero transition prior to commutation instant  524 . The zero transition of the induced voltage corresponds to an ignition angle of 0° el. 
         [0049]      FIG. 6  shows a schematic depiction  600  with winding current  510 , commutation status  520 , induced voltage  530 , and recharge current  540  of  FIG. 5 , in which commutation status  520  is emphasized for elucidation of the commutation operations performed at commutation instants  522 ,  524 , and  526 . 
         [0050]    At commutation instant  522  (t COMMUT     —     1 ), a first commutation operation is initiated in which, for example, semiconductor switches  192  and  198  are switched off and semiconductor switches  194  and  196  are switched on, in which context commutation status  520  changes from “LOW” to “HIGH.” As a result, stator phase winding strand  126  experiences current flow via semiconductor switches  194  and  196 , for a current-flow time period  612  (T CF (1)) until commutation instant  524  (t COMMUT     —     2 ), at which a second commutation operation with an ignition retard of 10° el. is initiated. Here semiconductor switches  194  and  196  are switched off and semiconductor switches  192  and  198  are switched on, in which context commutation status  520  changes from “HIGH” to “LOW.” As a result, stator phase winding strand  126  experiences current flow via semiconductor switches  192  and  198  for a current-flow time period  662  (T CF (2)) until commutation instant  526  (t COMMUT     —     3) ). 
         [0051]    Because peak  544 ′ of recharge current  540  (cf.  FIG. 5 ) is very large in the context of the second commutation operation, since commutation took place too late, current-flow time period T CF (2) is decreased so as thereby to reduce a peak  646 , occurring at commutation instant  526 , in recharge current  540 . An exemplifying method for determining suitable current-flow durations and commutation instants is described below with reference to  FIG. 8 . 
         [0052]      FIG. 7  is a flow diagram of a method  700  for operating apparatus  100  of  FIG. 1  and apparatus  200  of  FIG. 2 , according to a preferred embodiment. Method  700  is executed as a main program of control unit  132  in the form of an infinite loop, the execution of which begins each time ECM  120  is started up after an initialization and after acceleration to a predetermined minimum rotation speed, and then ends only when operation is interrupted or terminated. 
         [0053]    Initialization of the main program is accomplished in an “Init” subroutine that is executed in step S 710 , control unit  132  being initialized with its inputs and outputs and the requisite control variables. In step S 710 , for example, the inputs and outputs of microprocessor  130  are initialized, and a predetermined current-flow time period (T CF (n)) is set. A “Startup” subroutine is then executed in step S 720  to ramp up ECM  120 , in order to accelerate it, for example in stepping-motor mode with forced commutations as described above, to the necessary minimum speed. After a predetermined number of forced commutations, execution leaves the “Startup” subroutine, and execution of the infinite loop of the main program begins in step S 730 . 
         [0054]    Step S 730  checks whether the present current-flow time period T CF (n) has elapsed. This is done by comparing the latter with a time variable T TIMER  that senses the respective time span from the most recently performed commutation operation up to the present point in time. The time variable T TIMER  is ascertained, for example, using a suitable timer that is implemented by control unit  132 . If T TIMER ≧T CF (n), the current-flow time period T CF (n) that was set has elapsed. In that case, the main program calls, in step S 740 , a “Commutate” subroutine that performs a commutation operation as described above. Otherwise the main program waits in step S 730 . Once commutation operation S 740  has been performed, the main program continues in step S 750 . 
         [0055]    Be it noted that any suitable subroutine can be used to initialize and ramp up ECM  120 . Because such subroutines are sufficiently known from the existing art, a detailed description of examples of subroutines is omitted here. 
         [0056]    In step S 750 , the main program calls a “Charge Check” subroutine to determine a suitable current-flow time period that defines a commutation instant for performing the next commutation operation. An example of a “Charge Check” subroutine is described below with reference to  FIG. 8 . After execution of this subroutine, the main program of  FIG. 7  returns to step S 730 . The current-flow time period is thus determined afresh after each commutation operation, and thus automatically optimizes itself as described above. 
         [0057]    The triggering of commutation operation S 740  can preferably also be achieved by way of a timer interrupt, if such an interrupt is made available by the microprocessor. 
         [0058]      FIG. 8  is a flow diagram of a method  800  with which the “Charge Check” subroutine in step S 750  of the main program of  FIG. 7  can be executed. The “Charge Check” subroutine begins, after each commutation operation is performed, with step S 810  in which time variable T TIMER  of the timer of μC  130  is reset to zero. 
         [0059]    In step S 815 , a difference ΔT CF  is determined between the preceding current-flow time duration T CF (−1) and the present current-flow time duration T CF (n), yielding ΔT CF :=T CF (−1)−T CF (n). In step S 820 , the value T CF (−1) is overwritten with the value T CF (n). 
         [0060]    Step S 825  checks whether signal RCI present at microprocessor  130  is logically “LOW,” i.e. whether a recharge current is flowing from stator winding strand  126  into DC link circuit  170 . If signal RCI is logically “LOW,” the “Charge Check” subroutine continues with step S 830 ; otherwise it goes to S 850 . 
         [0061]    Step S 830  checks whether signal RCI is logically “HIGH.” If signal RCI is logically “HIGH,” the “Charge Check” subroutine continues with step S 835 . Otherwise, it waits in step S 830  until signal RCI becomes logically “HIGH.” Step S 830  accordingly serves to determine the point in time at which the recharge current has decayed and signal RCI changes from logical “LOW” to logical “HIGH.” 
         [0062]    In step S 835 , a time duration T CT  is determined which describes the recharge duration, and thus extends from the occurrence of the recharge current to its decay. This time duration can be ascertained by determining a time span that extends substantially from the end of the most recently performed commutation operation to the earliest point in time at which a current value dependent on the charge state of link circuit capacitor  178  is below a predetermined current threshold. The determination of time period T CT  is accomplished by assigning to that earliest point in time the present value of the time variable T TIMER . 
         [0063]    Because T CT  is, in this case, greater than zero (because a recharge current was sensed), commutation was tending toward being too late, and the current-flow time duration T CF (n) is decreased in step S 840  by an amount equal to a correction value T CORRECT ; the “Charge Check” subroutine then continues with step S 860 . 
         [0064]    In step S 850  the time period T CT  is set to zero, since no return current was sensed. Because this means (as described above) that commutation was tending toward being too early, the current-flow time duration T CF (n) is increased in step S 855  by an amount equal to the correction value T CORRECT ; the “Charge Check” subroutine then continues with step S 860 . 
         [0065]    In the present embodiment, the correction value T CORRECT  is determined using a lookup table that is stored, for example, in a storage unit of control unit  132  suitable therefor. A suitable lookup table can be ascertained by performing appropriate laboratory experiments. Graphic depictions of an example of a lookup table are described with reference to  FIGS. 9 and 10 . 
         [0066]    In step  860 , the current-flow time period T CF (n) is reduced by an amount equal to the difference value ΔT CF . This produces a D component upon generation of the current-flow time duration T CF (n). In step S 865 , the current-flow time duration T CF (n) is finally reduced by an amount equal to the time period T CT ; the “Charge Check” subroutine then terminates, and the main program of  FIG. 7  returns to step S 730 . 
         [0067]      FIG. 9  shows a schematic depiction  900  of an exemplifying profile of current-flow time durations  920  and associated correction values  910  as a function of the rotation speed of ECM  120  of  FIG. 1  and  FIG. 2 . A predetermined correction value T CORRECT  is allocated to each specific current-flow time duration T CF (n). For example, a current-flow time duration  922  of 10 ms at a rotation speed n=1500 rpm has a correction value (Syncvar)  912  of approximately 15.1 μs allocated to it. 
         [0068]    As is apparent from  FIG. 9 , correction value  910  decreases monotonically as the rotation speed n of ECM  120  rises. 
         [0069]    Shown below is a table of the correction values (Syncvar) as a function of rotation speed n and the time duration KZ between two successive commutation instants, for a four-pole rotor: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 n (rpm) 
                 Time KZ (ms) 
                 Syncvar 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 500 
                 30.000 
                 8.51E−4 
               
               
                 1000 
                 15.000 
                 3.47E−4 
               
               
                 1500 
                 10.000 
                 1.51E−4 
               
               
                 2000 
                 7.500 
                 6.17E−5 
               
               
                 2500 
                 6.000 
                 2.57E−5 
               
               
                 3000 
                 5.000 
                 1.05E−5 
               
               
                 3500 
                 4.286 
                 4.37E−6 
               
               
                 4000 
                 3.750 
                 1.74E−6 
               
               
                 4500 
                 3.333 
                 7.24E−7 
               
               
                 5000 
                 3.000 
                 2.95E−7 
               
               
                 5500 
                 2.727 
                 1.26E−7 
               
               
                 6000 
                 2.500 
                 5.25E−8 
               
               
                 6500 
                 2.308 
                 2.14E−8 
               
               
                 7000 
                 2.143 
                 8.91E−9 
               
               
                 7500 
                 2.000 
                 3.80E−9 
               
               
                   
               
             
          
         
       
     
         [0070]      FIG. 10  is a schematic depiction  1000  of an exemplifying profile of rotation speeds  1020  of ECM  120  of  FIG. 1  and  FIG. 2  and associated correction values  1010  as a function of current-flow time duration T CF (n), a predetermined correction value T CORRECT (Syncvar) being allocated to each specific rotation speed n. For example, a rotation speed  1022  of approximately 1000 rpm with a current-flow time duration of 15 ms has a correction value  1012  of approximately 34.7 μs allocated to it. 
         [0071]      FIG. 11  is a schematic depiction  1100  of an exemplifying time course of operating parameters  1110 ,  1120 ,  1130 ,  1140  that are measured, during the operation of apparatus  100  of  FIG. 1  and apparatus  200  of  FIG. 2 , in the context of a commutation, according to an embodiment. Operating parameter  1120  illustrates the commutation status of ECM  120 ,  1130  the voltage induced in stator winding strand  126  in the context of the commutation operations,  1140  the current recharged into DC link circuit  170 , and  1110  a corresponding recharge status. 
         [0072]      FIG. 11  illustrates automatic optimization of the commutation of ECM  120 , also referred to as the achievement of a “steady state.” Here recharge status  1110  characterizes instants at which the voltage drop at resistor  140  exceeds a predetermined threshold value, and a recharge flow is thus sensed. A recharge current is accordingly sensed at instants  1112 ,  1114 ,  1116 ,  1118 ,  1119 , and no further recharge current is sensed after commutation instant  1122  in the context of commutation, since the predetermined threshold value is no longer being exceeded. As of instant  1122 , ECM  120  is thus being operated in the steady state. 
         [0073]      FIG. 12  is a schematic depiction  1200  of an exemplifying time course of operating parameters  1210 ,  1220 ,  1230 ,  1240  that are measured during the ramp-up of apparatus  100  of  FIG. 1  and apparatus  200  of  FIG. 2 . Operating parameter  1220  illustrates rotation speed n of ECM  120 ;  1230  illustrates the respective current-flow time periods T CF (n);  1240  illustrates the corresponding difference values ΔT CF ; and  1210  illustrates the respective recharge time durations T CT . 
         [0074]    As is apparent from  FIG. 12 , the recharge time duration T CT  becomes shorter with increasing rotation speed n. The current-flow time duration T CF (n) approaches a lower limit value that represents an optimum current-flow time duration. The difference value ΔT CF  and recharge time periods T CT  accordingly become continuously lower. 
         [0075]    Many variants and modifications are of course possible within the scope of the present invention. 
         [0076]    For example, it is possible to measure the recharge into DC link circuit capacitor  178  in different ways. Whereas in the exemplifying embodiments according to  FIG. 1  and  FIG. 2  the recharge was ascertained by way of the current flowing to capacitor  178 , the measurement can also be accomplished by way of the voltage present at capacitor  178 , which voltage rises as capacitor  178  is charged by a current I_RC flowing to it. This is done preferably by picking off the voltage present at capacitor  178  and delivering that voltage to a differentiating member. The signal resulting therefrom can be compared with a threshold value, and the result delivered to a μC  130  for evaluation. 
         [0077]    If a μC  130  having an A/D converter is used, in all the exemplifying embodiments the charge state of the capacitor ascertained in analog fashion can be delivered directly to the A/D converter of μC  130  for evaluation. A pre-evaluation, such as that occurring with transistor  160  in  FIG. 1  and with comparator  157  in  FIG. 2 , can then be omitted. 
         [0078]    It is also possible to set the ignition advance angle by, for example, defining resistor  140  ( FIGS. 1 and 2 ) or the threshold value of the comparator ( FIG. 2 ) in correspondingly variable fashion. Definition is performed preferably by means of μC  130 . In preferred fashion, it is possible to set the ignition angle in the range from −20° el. to +10° el. by means of such a circuit. This setting is preferably accomplished variably, and in additionally preferred fashion the setting of the ignition advance angle is controlled by the μC. Indirect definition of the ignition angle also allows a rotation speed control capability to be achieved.