Abstract:
The invention relates to a sensorless electric motor and a method of controlling such an electric motor, which motor comprises a permanently magnetic rotor, a stator having at least one winding, and a power stage for influencing the current flowing through the winding. As a function of a predetermined commutation duration (T_K), a commutation period is defined, during which period the direction of the magnetic field generated by current flow through the winding is not modified, during which period a commutation completion operation ( 107 ) and a commutation initiation operation ( 109 ) take place, and which period starts at a first commutation instant (t_K N ) and ends at a second commutation instant (t_K N+1 ); preferably, commutation timing is adjusted, based upon a value of induced voltage picked up at a currently non-energized one of the winding strands, during a plateau portion ( 108 ) of a winding voltage trace, located temporally between commutation instants.

Description:
CROSS-REFERENCE 
       [0001]    This application is a section 371 of PCT/EP06/01815, filed 28 Feb. 2006, published 8 Sep. 2006 as WO 2006-092 265-A, and further claims priority from German application DE 10 2005 011 263.3, the contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to an electric motor and to a method of controlling it. 
       BACKGROUND 
       [0003]    It is often required of electric motors that they be low in cost and quiet. 
         [0004]    It is therefore an object of the invention to make available a novel electric motor and a novel method of controlling it. 
         [0005]    This object is achieved, according to the invention, by distinguishing between a current rise subperiod, a current-constant middle subperiod and a current-drop subperiod during each commutation period, and measuring induced voltage only during the current-constant middle subperiod. The fact that current flow occurs with a substantially constant current enables an adaptation of the commutation duration as a function of a sensed voltage signal, in order to adapt the commutation duration to the rotation speed of the rotor. This enables commutation without additional rotor position sensors, and results in an inexpensive motor. 
         [0006]    A preferred refinement is to regulate current rise and current drop gradually so that the signal traces form ramps. Because the commutation duration is ascertained, it is possible to carry out the commutation initiation process and the commutation completion process in the form of ramps. This smooth switching-on and shutoff decreases motor noise, and makes possible a quieter motor. 
         [0007]    A further preferred embodiment is to calculate current target values in a digital controller, which applies those values to a current regulator, which in turn controls semiconductor switches in series with the windings. With such a method, rotation speed regulation with an electric motor according to the present invention is possible. 
         [0008]    According to a further aspect of the invention, the object is achieved by an electric motor with two winding strands which are energized in alternation, with induced voltage being monitored in the currently non-energized winding strand. An electric motor of this kind allows a method according to the present invention to be carried out, and results in a low-cost and quiet motor. 
     
    
     
       BRIEF FIGURE DESCRIPTION 
         [0009]    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. In the drawings: 
           [0010]      FIG. 1  is a schematic circuit diagram of an exemplifying embodiment of an arrangement according to the present invention having a two-strand stator; 
           [0011]      FIG. 2  is a circuit diagram of an evaluation device for the induced voltage; 
           [0012]      FIG. 3  is a circuit diagram of a motor current regulator; 
           [0013]      FIG. 4  depicts the motor current regulated by the motor current regulation system; 
           [0014]      FIG. 5  schematically depicts a single-phase, two-strand motor; 
           [0015]      FIG. 6  depicts currents and voltages occurring in the motor of  FIG. 1 ; 
           [0016]      FIG. 7  schematically depicts a late commutation process; 
           [0017]      FIG. 8  is an oscillogram of a late commutation process; 
           [0018]      FIG. 9  schematically depicts an early commutation process; 
           [0019]      FIG. 10  is an oscillogram of an early commutation process; 
           [0020]      FIG. 11  schematically depicts a single-phase, single-strand motor; 
           [0021]      FIG. 12  depicts currents and voltages occurring in the motor of  FIG. 11 ; 
           [0022]      FIG. 13  schematically depicts a late commutation voltage area; 
           [0023]      FIG. 14  is a flow chart of an overall program for controlling a motor according to the present invention; 
           [0024]      FIG. 15  schematically depicts a commutation period; 
           [0025]      FIG. 16  is a flow chart of a current flow during one commutation period; 
           [0026]      FIG. 17  is a flow chart for a timer interrupt routine; 
           [0027]      FIG. 18  is a flow chart for generating a rising ramp; 
           [0028]      FIG. 19  is a flow chart for generating a falling ramp; 
           [0029]      FIG. 20  is a flow chart for a rotation speed regulation system; and 
           [0030]      FIG. 21  is a block diagram of a current and rotation-speed regulator. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 1  shows an electric motor  10  having a permanently magnetic rotor  12  and a single-phase, two-strand stator  14  having a winding arrangement  15 , which arrangement comprises a first stator strand  16  and a second stator strand  18 . 
         [0032]    The respective upper ends  161 ,  181  of strands  16  and  18  are connected via lead  20  to link circuit voltage UZK, which can be picked off via a measurement node MP_UZK  24 . Link circuit voltage UZK is generated by a power supply  22  from operating voltage +UB, e.g. from an alternating line voltage or from a battery. 
         [0033]    The lower end  162  of first strand  16  is connected via a MOSFET  40  and a measuring resistor  42  to ground GND. The potential at the lower end  162  of first strand  16  is picked off via a measurement node MP 1   44 . The potential between MOSFET  40  and resistor  42  is picked off via a node  46 , and delivered through a lead  50  to a current regulator I_RGL 1   48 . Current regulator I_RGL 1   48  is connected via a lead  52  to a microprocessor μC  32  that delivers a target value signal I_SOLL 1  to current regulator I_RGL 1   48 . Current regulator  48  is connected via a lead  54  to the gate terminal of MOSFET  40  in order to control the latter. 
         [0034]    In the same fashion, the lower end  182  of second strand  18  is connected to ground GND via a MOSFET  60  and a measuring resistor  62 . The potential at the lower end  182  of second strand  18  is picked off via a measurement node MP 2   64 . The potential between MOSFET  60  and resistor  62  is picked off via a node  66 , and delivered through a lead  70  to a current regulator I_RGL 2   68 . Current regulator I_RGL 2   68  is connected via a lead  72  to microprocessor μC  32  that delivers a target value signal I_SOLL 2  to current regulator I_RGL 2   68 . Current regulator  68  is connected via a lead  74  to the gate terminal of MOSFET  60  in order to control the latter. 
         [0035]    Target current value signals I_SOLL 1  and I_SOLL 2  are preferably specified as analog voltage signals or as PWM (Pulse Width Modulated) signals. 
         [0036]    Microprocessor μC  32  is connected via a lead  80  to a rotation direction indicator circuit “DIR DIG”  82 , via a lead  84  to a “U1&gt;0?” circuit  86  for detecting the sign of voltage U 1 , and via a lead  88  to a “U2&gt;0?” circuit  90  for detecting the sign of voltage U 2 . Rotation direction indicator circuit “DIR DIG”  82  is connected to measurement node MP 1   44 , the “U1&gt;0?” circuit is connected to measurement nodes MP 1   44  and MP_UZK  24 , and the “U2&gt;0?” circuit is connected to measurement nodes MP 2   64  and MP_UZK  24 . 
         [0037]    Operating data such as, for example, a target rotation speed n_s are delivered to microprocessor μC  32  via a bidirectional data bus  92 , and the program executing in microprocessor μC  32  controls the rotation speed (n_CTRL), commutation (COMMUT), and input/output (I/O). 
         [0038]    Examples of component values: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 μC 32 
                 PIC16F873A (Microchip Inc, Chandler AZ) 
               
               
                   
                 MOSFETs 40, 60 
                 SPB47N10 (with integrated recovery diode) 
               
               
                   
                 Resistors 42, 62 
                 1.5 ohm 
               
               
                   
                   
               
             
          
         
       
     
       Operation 
       [0039]    Rotor  12  is driven by the fact that current flows alternatingly in strands  16  and  18 . The current is controlled by MOSFETs  40  and  60 , and current regulation takes place by way of current regulators  48  and  68 . Electric motor  10  according to the present invention works in sensorless fashion, i.e. no rotor position sensor such as, for example, a Hall sensor, is provided. The rotation direction is determined via rotation direction indicator circuit  82  from the potential at measurement node MP 1  or MP 2 , and commutation (i.e. the alternation between current flow in the first and the second strand) is effected by measuring and evaluating voltages U 1  and U 2 . 
         [0040]      FIG. 2  shows an exemplifying embodiment of the “U1&gt;0?” circuit  86 . Circuit  86  comprises a resistor  140  that is connected on one side to measurement node MP 1   44  and on the other side to the base of a pnp transistor  146 . A capacitor  142  and a resistor  144  are each connected on one side to the base of transistor  146  and on the other side to measurement node MP_UZK  24 . The emitter of transistor  146  is likewise connected to measurement node  24 . The collector of transistor  146  is connected via a resistor  148  to a node  150 . Node  150  is connected to ground GND via a capacitor  152  and a resistor  154 . Measurement node  150  is also connected to the base of an npn transistor  156 . The emitter of transistor  156  is connected to ground GND, and the collector of transistor  156  is connected via a resistor  158  to a voltage “+5 V” and via a resistor  160  to lead  84  that goes to μC  32 . 
         [0041]    Signal U_MP 1  picked off via measurement node MP 1  is delivered, through resistor  140  and (in order to filter interference voltage spikes) through the low-pass filter constituted by resistor  144  and capacitor  142 , to the base of transistor  146 . When signal U_MP 1  is less than signal UZK, transistor  146  conducts. Conversely, when signal U_MP 1  is greater than signal UZK, transistor  146  blocks. When transistor  146  blocks, the base of transistor  156  is pulled to ground, and the latter transistor likewise blocks. Lead  84  is thereby pulled to +5 V, and this means a High signal for μC  32 . When transistor  146  conducts, on the other hand, resistors  148  and  154  then act as a voltage divider and raise the potential at the base of transistor  156 . Transistor  156  becomes conductive as a result, and lead  84  is pulled to ground GND, which corresponds to a Low signal for μC  32 . 
         [0042]    The sign of the voltage 
         [0000]        U 1 =U   —   MP 1 −UZK    
         [0000]    is converted by circuit  86  into a digital signal U 1 _DIG. When U 1 &gt;0 V, U 1 _DIG=High, and when U 1 &lt;=0 V, U 1 _DIG=Low. This allows simple evaluation of voltage U 1  by μC  32 . 
         [0043]    The “U2&gt;0?” circuit  90  is preferably constructed in the same fashion. 
         [0044]    Examples of component values: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Resistor 140 
                 47 
                 kilohm 
               
               
                   
                 Capacitor 142 
                 470 
                 pF 
               
               
                   
                 Resistor 144 
                 470 
                 kilohm 
               
             
          
           
               
                   
                 pnp Transistor 146 
                 PMBTA92 
               
             
          
           
               
                   
                 Resistor 148 
                 68 
                 kilohm 
               
               
                   
                 Capacitor 152 
                 1 
                 nF 
               
               
                   
                 Resistor 154 
                 10 
                 kilohm 
               
             
          
           
               
                   
                 npn Transistor 156 
                 BC846B 
               
             
          
           
               
                   
                 Resistor 158 
                 4.7 
                 kilohm 
               
               
                   
                 Resistor 160 
                 1 
                 kilohm 
               
               
                   
                   
               
             
          
         
       
     
         [0045]      FIG. 3  shows an exemplifying embodiment of the “I_RGL 1 ” circuit  48 , to which a target current value signal I_SOLL 1  is delivered from μC  32 , and to which a signal I_IST 1  is delivered via lead  50  from base resistor  42 . Circuit  48  controls MOSFET  40  via lead  54 . Signal I_SOLL 1  is delivered to an operational amplifier  174  through three resistors  162 ,  166 , and  170  connected in series. A capacitor  164  is connected to ground between resistors  162  and  166 , and a capacitor  168  is connected to ground between resistors  166  and  170 . Between resistor  170  and the positive input of operational amplifier  174 , a resistor  172  is connected to ground. 
         [0046]    Signal I_IST 1  is delivered to the negative input of operational amplifier  174  through a resistor  180 . The output of operational amplifier  174  is connected via a resistor  176  to the gate terminal of MOSFET  40 . The negative input and the output of operational amplifier  174  are connected via a capacitor  178 . 
         [0047]    In this exemplifying embodiment, target value signal I_SOLL 1  is specified by μC  32  as a PWM signal pwm. The PWM signal is smoothed by the low-pass filter constituted by resistors  162 ,  166 , and  170  and capacitors  164  and  168 , and delivered to the positive input of operational amplifier  174 . Motor current I 1  is measured via base resistor  42 , and the potential at node  46  is delivered through resistor  180  to the negative input of operational amplifier  174 . Operational amplifier  174  controls the gate terminal of MOSFET  40  via resistor  176 , and thus performs a current regulation of current I 1  in such a way that the potential at node  46  corresponds to target current value I_SOLL 1 . 
         [0048]    The utilization of an analog current regulator allows the use of a simple μC  32 , since the latter needs to carry out only the calculation of target current value I_SOLL 1 . Alternatively, a digital current regulator can also be used, with which actual current value I_IST 1  is delivered to μC  32  in suitable form. 
         [0049]    Current regulator “I_RGL 2 ”  68  is preferably constructed in the same manner as current regulator “I_RGL 1 ”  48 . 
         [0050]    Examples of component values: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Resistor 162 
                 22 
                 kilohm 
               
               
                   
                 Capacitor 164 
                 10 
                 nF 
               
               
                   
                 Resistors 166, 170 
                 10 
                 kilohm 
               
               
                   
                 Resistor 172 
                 1.8 
                 kilohm 
               
             
          
           
               
                   
                 Operational amplifier 174 
                 TSH24 
               
             
          
           
               
                   
                 Resistor 176 
                 220 
                 ohm 
               
               
                   
                 Capacitor 178 
                 22 
                 pF 
               
               
                   
                 Resistor 180 
                 10 
                 kilohm 
               
               
                   
                   
               
             
          
         
       
     
         [0051]      FIG. 4  shows target current value signal I_SOLL 1  as a line  105 , and motor current I 1 , regulated by analog current regulator  48  in accordance with the target current value signal, as a line  100 . Motor current I 1  thus substantially tracks target value I_SOLL 1 , i.e. it rises in the form of a rising ramp  107  then, with a constant target current value signal I_SOLL 1 , proceeds substantially constantly in the form of a plateau  108 , and then drops toward 0 V in the form of a falling ramp  109 . A shape of this kind is also called a trapezoidal shape. As is evident from plateau  108 , current regulator  48  initially overshoots slightly and motor current I 1  drops slightly. This is usual for simple current regulators, and a motor current of this kind can nevertheless be referred to as constant, and in any case as substantially constant. The current regulator can also regulate motor current I 1  to a value of 0 A. 
         [0052]    In contrast to a “hard” switch-on and shutoff of current I 1 , switching on and shutting off current I 1  in the form of a ramp generates less noise. 
         [0053]      FIG. 5  schematically depicts stator  14  and permanently magnetic rotor  12 . External rotor  12  comprises four poles  121 ,  122 ,  123 , and  124 . Stator  14  is made of soft ferromagnetic material and likewise comprises four poles  131 ,  132 ,  133 , and  134 , whose polarity is determined by the motor current flowing through stator strand  16  or  18 . Stator strands  16  and  18  are wound in bifilar fashion for cost reduction, and oppositely directed magnetic-field generation is achieved by the fact that the link circuit voltage is applied at beginning  161  of the winding wire in the case of first strand  16 , and at end  181  of the winding wire in the case of second strand  18 . In motor  10  that is depicted, the voltage induced by a rotation of rotor  12  depends on the rotation angle. 
       Commutation 
       [0054]      FIG. 6  schematically depicts the current flow through stator  14  of  FIG. 5  over one complete revolution of rotor  12  (360° mech.). Current I 1  through stator strand  16  is depicted as a line  100 , current I 2  through stator strand  18  as line  101 , voltage U 1  present at stator strand  16  as line  102 , and voltage U 2  present at stator strand  18  as line  103 . 
         [0055]    Four commutation periods (720° el.) are depicted, which extend between commutation instants t_K 1  and t_K 2 , t_K 2  and t_K 3 , t_K 3  and t_K 4 , and t_K 4  and t_K 5 . In general, the respective first commutation instant for a commutation period will be referred to hereinafter as t_K N , and the respective second commutation instant as t_K n+1 . The commutation duration of the respective commutation periods is referred to as T_K. Only one of stator strands  16  and  18  of winding  15  experiences current flow during a commutation period, so that the direction of the magnetic field generated by the current flow of winding  15  does not change during that commutation period. Currents I 1  and I 2  flow alternatingly through stator strands  16  and  18 . 
         [0056]    During each commutation period, one commutation completion operation  107 , one operation  108  at substantially constant current flow, and one commutation initiation operation  109  take place. In this exemplifying embodiment, commutation completion operation  107  begins after commutation instant t_K N , and current I 1  or I 2  rises in the form of a ramp during commutation completion operation  107 . The duration of the commutation completion operation is labeled T_KA. Commutation completion operation  107  is followed by a time phase  108  with constant current flow, for a duration T_KK. Following time phase  108  with substantially constant current flow is commutation initiation operation  109 , during which (in this exemplifying embodiment) current I 1  or I 2  is decreased in the form of a ramp until it reaches a value of 0 V. The duration for the commutation initiation operation is labeled T_KE. 
         [0057]    Voltages U 1  at stator strand  16  and U 2  at stator strand  18  can contain, in particular, the following components: 
         [0000]        U 1= U 1 —   ind+L   11   *dI 1 /dt+I 1 *R 1 +L   12   *dI 2 /dt   (1) 
         [0000]        U 2= U 2 —   ind+L   22   *dI 2 /dt+I 2 *R 2+ IL   21   *dI 1 /dt   (2) 
         [0058]    where 
         [0059]    U 1 =voltage at stator strand  16   
         [0060]    U 2 =voltage at stator strand  18   
         [0061]    U 1 _ind=voltage induced in stator strand  16  by the rotating permanently magnetic rotor  12   
         [0062]    U 2 _ind=voltage induced in stator strand  18  by the rotating permanently magnetic rotor  12   
         [0063]    L 11 =self−inductance of stator strand  16   
         [0064]    L 22 =self−inductance of stator strand  18   
         [0065]    I 1 =current through stator strand  16   
         [0066]    I 2 =current through stator strand  18   
         [0067]    R 1 =ohmic resistance of stator strand  16   
         [0068]    R 2 =ohmic resistance of stator strand  18   
         [0069]    L 12 =mutual inductance between stator strand  18  and stator strand  16   
         [0070]    L 21 =mutual inductance between stator strand  16  and stator strand  18   
         [0071]    When a constant current I 1  flows through stator strand  16  and when current I 2 =0 (time phase  108 ), the time-dependent terms drop out of equations (1) and (2) and what remains is: 
         [0000]        U 1 =U 1 —   ind+I 1 *R 1  (3) 
         [0000]      U2=U2_ind  (4) 
         [0072]    In the same fashion, what applies when stator strand  18  has a constant current I 2  flowing through it, and current I 1 =0, is: 
         [0000]      U1=U1_ind  (5) 
         [0000]        U 2 =U 2 —   ind+I 2 *R 2  (6) 
         [0000]    For a single-phase, two-strand motor with constant current flow through a first winding strand, the induced voltage U_ind can thus be sensed at the respective winding strand through which current is not flowing. During the commutation operation, on the other hand, such sensing would generally be impossible, or possible only very inaccurately, because of the changing current I 1  or I 2 . 
         [0073]    The commutation period between commutation instant t_K 3  and commutation instant t_K 4  will be considered below. During time phase  108  with constant current flow through stator strand  16 , voltage U 1  at stator strand  16  is made up, according to equation (3), of induced voltage U 1 _ind (depicted as line  104 ) and a magnitude I 1 *R 1  that is constant because current I 1  is constant. Voltage U 1  therefore does not correspond directly to induced voltage U 1 _ind. But because current I 2  is equal to 0 A, voltage U 2  at stator strand  18  corresponds, during time phase  108  of constant current flow, to induced voltage U 2 _ind, the following being applicable because of the winding  15  selected according to  FIG. 5 : 
         [0000]        U 1 —   ind=−U 2 —   ind   (7) 
         [0074]    Voltages U 1  and U 2  rise slightly during time phase  108  of constant current flow because motor  10  is configured to generate an auxiliary reluctance torque. For a motor of this kind, induced voltages U 1 _ind and U 2 _ind are dependent (for a uniform rotation speed) on the instantaneous rotation angle phi_mech, since stator poles  131 ,  132 ,  133 , and  134  are configured asymmetrically, as indicated very schematically in  FIG. 5 . The slight slope of voltages U 1  and U 2  allows the rotation direction of rotor  12  to be detected. In the one rotation direction the respective induced voltage rises, and in the opposite rotation direction the respective voltage falls. 
         [0075]    In the exemplifying embodiment of  FIG. 6 , electric motor  10  is being driven, and after every 90° mechanical or 180° electrical, the current flow is switched from one stator strand to the other stator strand. A prerequisite for this, however, is that commutation duration T_K correspond exactly to the time required by rotor  12  for one rotation of 90° mechanical. Because commutation duration T_K becomes shorter during each commutation period upon startup or acceleration of motor  10 , changes upon a change in a load being driven or braked by motor  10 , or is modified by a change in the magnitude of current I 1  or I 2 , an adaptation of commutation duration T_K to the present operating state of motor  10  must constantly be performed. This can be accomplished, for example, with rotor position sensors. What will be described below, however, is a method in which the adaptation of commutation duration T_K is accomplished via an evaluation of voltages U 1  and/or U 2 . 
       Late Commutation Operation 
       [0076]      FIG. 7  is a schematic diagram in which current I 1  is depicted as line  100 , voltage U 2  as line  103 , and the “U 2 &gt;0” signal generated by apparatus  90  as line  111 . In this example, commutation duration T_K is too long, and commutation therefore takes place not after one revolution of 180° el., but instead too late. A change in the sign of voltage U 2  thus already takes place during time phase  108  with constant current flow, and voltage U 2 , which at this point in time corresponds to induced voltage U 2 _ind, ends up in an area U 2 &gt;0 that is unsuitable for this commutation period, resulting in braking of motor  10 . 
         [0077]    At the moment of the change in sign of voltage U 2 , the “U 2 &gt;0” signal  111  jumps from Low to High. The instant of the change is referred to hereinafter as late commutation instant t_spat. 
         [0078]    Commutation duration T_K can be corrected by subtracting late commutation duration T_spät from it. Late commutation duration T_spät is obtained from the time span between late commutation instant t_spät and the commutation instant t_K N+1  predetermined by commutation duration T_K. The correct commutation duration T_K is also obtained directly from the time span between commutation instant t_K N  and instant t_spät. 
         [0079]    An operation of this kind, in which voltage U 1  or U 2  assumes, during duration T_KK of constant current flow, a value from an area unsuitable for the selected operating mode of motor  10  for the particular commutation period, is referred to as a late commutation operation. 
         [0080]      FIG. 8  shows an example of a measurement for a late commutation operation such as the one depicted in  FIG. 7 . Current I 1  is depicted as line  100 , current I 2  as line  101 , voltage U 1  as line  102 , and signal “U 2 &gt;0” as line  111 . At point  115 , signal “U 2 &gt;0” becomes positive shortly before constant current flow through stator strand  106  ends. This means that commutation duration T_K is too long, and a late commutation operation exists. 
         [0081]    It is also apparent that voltage U 1 , and thus also voltage U 2 , exhibit large disturbances during ramps  107  and  109  because of the regulator and the changes in over time in the current, and said voltages are therefore unsuitable, or only poorly suitable for evaluating induced voltage U 1 _ind or U 2 _ind. 
       Early Commutation Operation 
       [0082]      FIG. 9  is a schematic diagram in which current I 1  is depicted as line  100 , voltage U 2  as line  103 , and the “U 2 &gt;0?” signal generated by apparatus  90  as line  111 . In this example, commutation duration T_K is too short. The result of this is that induced voltage U 1 _ind or U 2 _ind does not, as in the ideal case, exhibit a sign change at commutation instant t_K N+1 , but instead the sign change takes place only after an early commutation duration T_früh. 
         [0083]    Because both I 1 =0 and I 2 =0 after commutation initiation operation  109 , both U 1  and U 2  correspond to the induced voltage (cf. equations (3) and (4)). In this exemplifying embodiment, the induced voltage is measured via voltage U 2 . The latter exhibits a sign change at instant t_früh, and the “U 2 &gt;0?” signal  111  changes from Low to High at instant t_früh. 
         [0084]    Commutation duration T_K can be corrected by increasing it by an amount equal to early commutation duration T_früh. Early commutation duration T_früh is obtained from the time span between early commutation instant t_früh and the commutation instant t_K N+1  predetermined by commutation duration T_K. Instead of commutation instant t_K N+1  it is also generally possible to use the point in time at which commutation initiation operation  109  ends. 
         [0085]    An operation of this kind, in which voltage U 1  or U 2  assumes, at the end of commutation initiation operation  109 , a value from an area unsuitable for the device operating mode of motor  10  for the particular commutation period, is referred to as an early commutation operation. 
         [0086]      FIG. 10  shows an example of a measurement for an early commutation operation such as the one depicted in  FIG. 9 . Current I 1  is depicted as line  100 , current I 2  as line  101 , voltage U 1  as line  102 , and the “U 1 &gt;0” signal as line  112 . As explained in the description for  FIG. 9 , the induced voltage after the completion of commutation initiation operation  109  can also be measured via voltage U 1 , which performs a sign change from High to Low at early commutation instant t_früh. 
         [0087]    It is also apparent that voltage U 1  exhibits large interference spikes during commutation initiation operation  109 , making it very difficult or impossible to evaluate the induced voltage during commutation initiation operation  109 . The interference during commutation initiation operation  109  also occurs as a result of the work of the current regulator that converts current I 1  to the value I 1 =0 V in a predetermined form. 
         [0088]    At instant t_früh, a change in the “U 1 &gt;0?” signal  112  from High to Low is detected. After this detection, in this exemplifying embodiment, commutation completion operation  107  is performed, i.e. current flow through stator strand  18  begins. Instant t_früh is preferably selected as first commutation instant t_K N+1  for calculating the next commutation instant t_K N+2 . 
         [0089]      FIG. 11  shows an electric motor  310  having a single-phase, single-strand stator  314  and a two-pole rotor  312 . Rotor  312  comprises a first rotor pole  321  and an oppositely magnetized second rotor pole  322 . Stator  314  comprises a first pole  331  and a second pole  333 , as well as a winding  315 . Winding  315  comprises a stator strand  316  that is connected via two terminals  361  and  362  to a schematically indicated power stage  51 . Power stage  51  is preferably configured as a full-bridge circuit, in order to allow current flow through strand  316  in both directions. The current through strand  316  is designated I 1 , and the voltage drop at stator strand  316  as U 1 . 
         [0090]    Voltage U 1  is preferably measured via two measurement nodes MP 1   344  and MP 2   346  that are arranged at the opposite ends of stator strand  316 , and at which voltages U_MP 1  and U_MP 2  are present. Voltage U 1  is calculated as 
         [0000]        U 1 =U   —   MP 2 −U   —   MP 1  (8). 
         [0091]      FIG. 12  depicts current I 1  as line  300 , voltage U 1  as line  302 , and voltage U 1 _ind induced in stator strand  316  as line  304 . During one commutation period of length T_K, as in the exemplifying embodiment with two strands, one commutation completion operation  107 , one operation  108  with constant current flow, and one commutation initiation operation  109  take place. During operation  108  with constant current flow, voltage U 1  is made up, according to equation (3), of induced voltage U 1 _ind induced in stator strand  316  by the rotating permanently magnetic rotor  312 , and a constant factor I 1 *R 1  dependent on the magnitude of constant current I 1 . 
         [0092]    For the check as to whether a late commutation operation exists, the I 1 *R 1  component is subtracted from voltage U 1 . Either the target value for the corresponding current regulator can be used as a value of current I 1 , or it is ascertained by a measuring apparatus for current. 
       Late Commutation Voltage Area and Early Commutation Voltage Area 
       [0093]      FIG. 13  shows a late commutation operation. A current I 1  is plotted as line  100 , and a voltage U 2  as line  103 . A late commutation voltage area  140  is defined for detection of a late commutation operation. Late commutation voltage area  140  begins at 0 V and encompasses the entire positive area. To clarify as to whether a late commutation operation exists, a check is made as to whether the value of voltage signal U 2  is within late commutation voltage area  140 . This is the case at instant t_ 140 , and a late commutation operation is thus taking place. 
         [0094]    Late commutation area  140  can be defined in different ways. Two late commutation voltage areas  140 ′ and  140 ′ are presented as further exemplifying embodiments. In contrast to late commutation area  140 , late commutation area  140 ′ is not open toward the top, but ends at a maximum voltage. This allows, if applicable, a simpler evaluation circuit. Late commutation voltage area  140 ′, on the other hand, begins not at 0 V but at a negative (or positive) voltage. This can be utilized, for example, for earlier detection of a late commutation operation. Detection occurs here at instant t_ 140 ″, which is located earlier in time than instant t_ 140 . A shift of this kind can furthermore, for example, take into account an offset of voltage U 2  that can occur in a single-strand motor as a result of component I 1 *R 1 . 
         [0095]    An early commutation voltage area can be defined in the same fashion for the early commutation operation. 
       Software Control System of the Motor 
       [0096]      FIG. 14  shows the main program that executes in μC  32 . The program begins with the “POWER ON_RESET” step S 270 , to which μC  32  branches after switch-on. In the “INIT” step S 272 , variables are initialized and operating parameters are polled, for example via data line  92  of  FIG. 1 . In the “SYNCH_ROTOR” step S 274 , execution of the program is synchronized with a rotation (if present) of the rotor, so that said rotation can be utilized as applicable. In step S 276 , a “CHK_ROT( )” routine checks whether or not the rotor is rotating. If the rotor is rotating, step S 280  checks whether it is rotating in the desired direction. In the case of a motor with reluctance torque this can be ascertained as described above, for example, by way of the slope of the induced voltage. If the rotor is rotating in the correct direction, in step S 282  a BRAKE_ON variable is set to 0. This indicates that the rotor is already rotating in the correct direction, and a normal commutation can be performed. If the rotor is rotating in the wrong direction, however, then in step S 284  the BRAKE_ON variable is set to 1. This indicates that a deceleration of the rotor needs to be accomplished. This can be done, for example, by causing current to flow in the opposite direction in the corresponding commutation period. 
         [0097]    If a rotor standstill is identified in step S 276 , then in the “START_ROT” step S 278  the rotor is caused to move by current flow. The main loop begins in step S 286 , and a check is made as to whether the rotor is still moving. If this is not the case, execution branches back to step S 276 . If the rotor is rotating, however, then in the “PERIOD — 1” step S 288  current flow is performed for the first commutation period. The PERIOD — 1 routine is set forth in more detail in  FIG. 16 . 
         [0098]    In the “PERIOD — 2” step S 290 , current flow is performed for the second commutation period, i.e. in the opposite direction. In the “n_CTRL” step S 292 , the rotation speed regulation calculation operation takes place. This is presented in more detail in  FIG. 20 . 
         [0099]    In the “OTHER” step S 294 , further steps necessary for operation of the motor take place. For example, input/output is performed, and error signals are outputted in the event of an error. 
         [0100]    After step S 294 , execution branches back to step S 286  and the next current flow takes place. 
       Ramp 
       [0101]      FIG. 15  shows an exemplifying embodiment for a commutation operation in which current flow is taking place through stator strand  16  of  FIG. 1 . 
         [0102]    In a commutation completion operation  107 , current I 1  is elevated in four steps (N_KA=4) from the value I 1 =0 A to the value corresponding to target value I_SOLL. This is followed by a time phase  108  during which a constant current flow occurs at the value I 1 =I_SOLL. Following this is commutation initiation operation  109 , during which current I 1  is decreased in four steps (N_KE=4), in ramped fashion, from the value I 1 =I_SOLL to the value I 1 =0. 
         [0103]    In this exemplifying embodiment, duration T_KA of commutation completion operation  107  and duration T_KE of commutation initiation operation  109  are calculated from commutation duration T_K. Commutation completion duration T_KA and commutation initiation duration T_KE are selected so that they each occupy 10% of the total commutation duration T_K. Time phase  108  of constant current flow occupies the remaining 80% of the commutation duration. In general, the values T_KA and T_KE are selected as follows: 
         [0000]        T   —   KA=f   —   KA*T   —   K   (9) 
         [0000]        T   —   KE=f   —   KE*T   —   K   (10) 
         [0000]    where 
         [0104]    T_KA=commutation completion duration 
         [0105]    f_KA=proportional factor for the commutation completion duration 
         [0106]    T_K=total commutation duration 
         [0107]    T_KE=commutation initiation duration 
         [0108]    f_KE=proportional factor for the commutation initiation duration. 
         [0109]    The proportional factors f_KA and f_KE are preferably adapted to the particular motor type and the particular intended application of electric motor  10 , and can be specified to μC  32 , for example, by control unit  94  via interface  92  (cf.  FIG. 1 ). 
         [0110]      FIG. 16  shows the “PERIOD — 1” routine S 238 . In step S 302 , commutation completion duration T_KA and commutation initiation duration T_KE are calculated, and variable t_KA is set to the instantaneous time value t_TIMER. Commutation completion instant t_KA corresponds here to the starting instant of the commutation completion operation. In step S 304 , the “RAMP1_UP” commutation completion operation is performed. This is described in  FIG. 18 . After the end of the commutation completion operation, a timer TIMER 1  is started via a “START_TIMER1” function. Timer TIMER 1  measures the time span for the time phase of constant current flow, which is equal to the commutation duration T_K minus commutation completion duration T_KA and commutation initiation duration T_KE. After this time has elapsed, timer TIMER 1  generates an interrupt that calls an interrupt routine “TIMER 1 _INTERRUPT” S 250 , depicted in  FIG. 20 . In step S 308 , execution is delayed for a duration T_RETARD, so that the measurement as to whether a late commutation operation exists does not occur immediately. This prevents errors due to the previously performed commutation completion operation. Step S 310  checks whether the induced voltage is in the late commutation voltage area LATE_AREA. In the case of a two-strand stator this is done, for example, by evaluating signal U 2 . 
         [0111]    If a late commutation operation is not taking place in the time phase of constant current flow, execution branches respectively from step S 310  to step S 312 . Step S 312  checks whether the time phase of constant current flow should continue to be implemented. This is done by way of the PHASE_CONST variable, which is previously set to 1 and which, upon expiration of the time entered in timer TIMER 1 , is set to 0 by the “TIMER1_INTERRUPT” interrupt routine S 250  of  FIG. 17 . Execution thus branches to step S 320  upon expiration of the calculated time for the time phase of constant current flow, and the commutation initiation operation is initiated by calling the “RAMP1_DOWN” routine. 
         [0112]    If, on the other hand, a late commutation operation is taking place during the time phase of constant current flow, execution then branches from step S 310  to the “RESET_TIMER1” step S 314 . In this step, timer TIMER 1  is reset so that an interrupt is no longer triggered. In step S 316 , late commutation duration T_LATE is then calculated from the difference between the present time t_TIMER and the starting instant of commutation completion operation t_KA. A correction of commutation duration T_K additionally takes place, by subtracting therefrom the late commutation duration T_LATE. Execution thereupon branches to step S 320 , and the “RAMP1_DOWN” commutation initiation operation S 320  is initiated. 
         [0113]    After the commutation initiation operation is complete, in step S 322  a variable t_KE is set to the present time t_TIMER and an EARLY_COMMUT variable is set to 0. Step S 324  checks whether an early commutation operation exists. This is done, for example, by way of voltage U 1 , and a check is made as to whether said voltage is in the EARLY_AREA early commutation area. In the case of an early commutation operation, execution branches to step S 326  and the EARLY_COMMUT variable is set to 1 in order to indicate an early commutation operation. Execution then branches back to S 324 . As soon as voltage U 1  is outside the EARLY_AREA early commutation area, execution branches to step S 328 . In the case of an early commutation operation, execution branches to step S 330 , where early commutation duration T_EARLY is calculated from the difference between present time t_TIMER and the time t_KE stored in step S 322 . A correction of commutation duration T_K is additionally performed, by increasing it by a value equal to early commutation duration T_EARLY. Execution thereupon branches to the end S 332 . 
         [0114]      FIG. 17  shows the “TIMER1_INTERRUPT” interrupt routine S 250 . This routine is called upon expiration of the duration entered in step S 306  of  FIG. 16 . In step S 252  the PHASE_CONST variable is set to 0 in order to indicate the end of the time phase of constant current flow. Execution then branches back in step S 254 , and the main program continues. 
         [0115]      FIG. 18  shows the “RAMP1_UP” routine S 304  executing in μC  32 , which routine performs commutation completion operation  107  for stator strand  16 . 
         [0116]    In step S 202 , commutation completion duration T_KA is calculated from commutation duration T_K (cf. description of  FIG. 9 ). A loop counter i is set to 1. In step S 204 , execution is delayed for a time T_KA/N_KA. This is the time for one step of the commutation completion operation, and after N_KA steps the entire commutation completion duration T_KA has elapsed. After the delay time in step S 204 , in step S 206  target current value I_SOLL 1  for rotation speed controller I_RGL 1   48  of  FIG. 1  is increased by a value I_SOLL/N_KA. The result of this is that the desired target value I_SOLL is reached after N_KA steps. 
         [0117]    In step S 208  the loop variable i is incremented by 1, and step S 210  checks whether all N_KA steps have not yet been carried out. If Yes, execution jumps back to step S 204  and the next step of ramp  107  is generated. After all N_KA steps have been carried out, the “RAMP1_UP” routine S 200  is ended. 
         [0118]      FIG. 19  shows a corresponding “RAMP1_DOWN” routine S 320  for commutation initiation operation  109  (cf.  FIG. 9 ). Routine S 320  corresponds in terms of structure to the “RAMP1_UP” routine S 304 , but in the loop S 224  to S 230 , what takes place at each step is firstly the decrease in target value I_SOLL 1  in step S 224 , and only then the delay time in step S 226 . 
         [0119]    The corresponding “RAMP2_UP” and “RAMP2_DOWN” routines for specifying target value I_SOLL 2  for regulator I_RGL 2   68  correspond to routines S 304  of  FIG. 18  and S 320  of  FIG. 19 , but current flow occurs through second stator strand  18 . 
         [0120]      FIG. 20  shows the “n_CTRL” rotation speed regulation function S 292  of  FIG. 14 . Step S 262  calculates the actual rotation speed n_i, which is equal to the quotient of a constant const_i and commutation duration T_K. The calculated actual rotation speed n_i and target rotation speed n_s are delivered, in this exemplifying embodiment, to a PID controller PID_RGL, and the latter calculates current target value I_SOLL, which indicates the magnitude of the current during the time phase of constant current flow. The “n_CTRL” routine ends in step S 264 . 
         [0121]      FIG. 21  is a block diagram of a current regulator and rotation speed controller for an electric motor  10  according to the present invention. A block  400  supplies a target rotation speed n_s to a block  404 , and a block  402  supplies an actual rotation speed n_i to block  404 . Block  404  is configured as a PI controller; the gain factor of proportional component kp=0.0005, and the gain factor of integral component Ki=0.0001. The output signal of block  404  is delivered to a block  406 , block  406  being configured as a proportional element, in particular as an amplifier. The output signal of block  406  is delivered to blocks  408  and  428 . 
         [0122]    Also delivered to block  408  are a Kommut1 signal from a block  410 , and a Ramp signal from a block  412 . The Kommut1 signal specifies when current flow is to occur through the first strand; the Ramp signal specifies the ramp shape, which is a function of commutation duration T_K; and the signal from block  406  specifies the amplitude of the ramp-shaped commutation signal occurring in block  408 , in order to influence the rotation speed. Block  408  is configured as a multiplier. 
         [0123]    The commutation signal generated by block  408  is delivered to a block  414 . Block  416  makes available a signal that corresponds to voltage U 42  at base resistor  42  of  FIG. 1 , and thus to actual current value signal I_IST 1 . The signal of block  416  is delivered to block  418 , which is configured as a proportional element, in particular as an amplifier. Block  414  is configured as an adder, and from the difference between the target current value signal from block  408  and the actual current value signal from block  418 , a control output is generated in the block functioning as a current regulator and is outputted, via a block  420  functioning as an amplifier, as control output signal ISte 111  for first stator strand  16 . The fact that the current regulation system does not act until shortly before block  420  yields a very fast current limiting response. 
         [0124]    Blocks  428 ,  430 ,  432 ,  434 ,  436 ,  438 , and  440  correspond to blocks  408  to  420 , and control signal ISte 112  for second stator strand  18  is generated therein. 
         [0125]    Rotation speed regulation is implemented by the fact that the control output signal of PI controller  404  is delivered to multiplier  408  or multiplier  428 , thereby determining the magnitude of the ramp current. A predetermined elevation in target rotation speed n_s would then, for example, cause the signal delivered to multiplier  408  to become greater, which results in a higher target current value and thus a higher motor current I 1  or I 2 . The result is that rotor  12  rotates faster, and an adaptation of commutation duration T_K takes place until the electric motor exhibits a rotation speed n_i corresponding to target rotation speed n_s. 
         [0126]    The rotation speed of the motor is thus determined by an interaction between rotation speed controller  404  and current regulator  414 . 
         [0127]    Many modifications are of course possible within the scope of the invention. 
         [0128]    In a simpler configuration, for example, the rotation speed controller in  FIG. 21  can be omitted, by replacing blocks  400 ,  402 ,  404 , and  406  with a block that outputs an adjustable signal. This results in an open-loop rotation speed control system. 
         [0129]    An electric motor according to the present invention is preferably used to drive and/or decelerate a fan. 
       Rotation Direction Detection 
       [0130]    Because motor  10  is configured to generate an auxiliary reluctance torque, the rotation direction can be ascertained in area  108  of constant current flow from the slope of voltage U 1   102 , of voltage U 1 _ind  104 , of voltage U 2   103 , and/or of voltage U 2 _ind (cf.  FIG. 6 ). 
         [0131]    In the case of the present motor, voltage U 1   102  is rising, and the derivative of voltage U 1  (which corresponds to the slope) is likewise positive. For a rotation in the opposite direction, conversely, the slope or derivative of voltage U 1  would be negative. 
         [0132]    The rotation direction measurement can be performed at least once after or during startup, or it can also occur at predetermined intervals. 
         [0133]    The invention is not limited to the exemplifying embodiments that are depicted and described, but rather encompasses all embodiments that function identically, within the context of the invention.