Patent Publication Number: US-2018043447-A1

Title: Method for operating an electrical machine and electrical machine

Description:
This nonprovisional application claims priority under 35 U.S.C. §119(a) to German Patent Application No. 10 2016 215 174.6, which was filed in Germany on Aug. 15, 2016, and which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for operating an electrical machine with a power source and with an electric motor as well as with an intermediary power converter, in which an input current of the power source is converted by means of a pulse width modulated control of a number of semiconductor switches of the power converter into a multiphase output current for the electric motor. The invention further relates to an electric machine operated by such a method, in particular for a motor vehicle. 
     Description of the Background Art 
     Adjustment systems driven by an electric motor used as motor vehicle components, such as, for example, window regulators, seat adjusters, door and sliding roof drives or radiator fan drives, as well as pumps and interior fans, typically have an electric machine with a controlled electric motor. For example, brushless electric motors are known in which a rotor rotatably mounted relative to a stator is driven by a magnetic rotating field. For this purpose, phase windings of the stator are subjected to a corresponding electrical three-phase or motor current, which is controlled and regulated by means of a controller as part of a (motor) electronics. 
     Such electrical machines generally comprise a (high-voltage) battery as an internal energy storage device from which the electric motor is supplied with electrical energy in the form of a direct current. For converting the direct current into the motor current, a converter (inverter, power inverter) is suitably connected between the energy store and the electric motor. A (direct voltage) intermediate circuit is connected downstream of the energy store, to which a bridge circuit of the power converter is connected. The energy store and the intermediate circuit act as a power source for providing the input-side direct current (input current) for the converter. The motor current is generated by a pulse width modulated (PWM) control of semiconductor switches of the bridge circuit as a multiphase output current. By the pulses of the PWM control, the semiconductor switches are switched over in clocked fashion between a conducting state and a blocking state. 
     By means of the switching processes of the semiconductor switches, alternating currents are generated in the lines of the intermediate circuit or of the power source. These alternating currents must undergo a critical assessment in respect of compliance with EMC directives (electromagnetic compatibility). 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a particularly suitable method for operating an electrical machine. In particular, the EMC behavior of the electrical machine is to be improved during operation. The invention is also based on the object of specifying an electrical machine operating in accordance with such a method. 
     The method according to the invention is suitable and arranged for operating an electrical machine. The electrical machine in this case has an energy store with a downstream (direct current) intermediate circuit, which together are designed as a power source (voltage supply) for an electric motor of the machine. A converter, for example in the form of an inverter (power inverter), is connected between the power source and the electric motor. 
     During operation of the machine, the converter converts an input current of the power source into a multiphase, in particular three-phase, output current (motor current, three-phase current) for the electric motor. For this purpose, a number of semiconductor switches of the converter connected in the intermediate circuit are controlled in clocked fashion with a pulse width modulated (PWM) control. In each period, for at least one phase, the PWM control has a pulse-shaped signal (pulse) for switching the respective semiconductor switches, which is generated during a respective period duration. The (signal) pulse has a pulse duration, that is to say a length of time during which the semiconductor switch is switched (active time). 
     The method provides that in an activation period, a pulse is divided into a leading half-pulse, that is, a half-pulse which occurs earlier in the period duration, and into a trailing second half-pulse, that is, a half-pulse which occurs later in the period duration, having in each case half a pulse duration. The first half-pulse with a first displacement time and the second half-pulse with a second displacement time are mutually shifted in time within the period duration of the period. In other words, this results in a pulse shift within a phase. This way, the periodicity of an alternating current, which is generated in the intermediate circuit of the power source during the pulse width modulated control of the semiconductor switches, is deliberately disturbed. As a result, the alternating current component in the (PWM) clock frequency is reduced, which on the one hand reduces the load on the power or voltage source. 
     In an embodiment, the first half-pulse is delayed in time with the first displacement time in the period, which means that the first half-pulse is generated at a later point in time in the period duration. The second half-pulse is thereby accelerated in time with the second displacement time in the period, which means that the second half-pulse is generated at an earlier point in time in the period duration. In particular, the second half-pulse is generated prior to the first half-pulse. This means that the first half-pulse leading prior to displacement, then trails the second half-pulse after displacement. This ensures that the half-pulses remain with the period. In other words, the common active time in a phase does not change during the period, whereby the machine operation is not adversely affected. 
     The shift essentially changes the sequence of the half-pulses during the period. Since both half-pulses are substantially identical, it is alternatively also possible to retain the time sequence of the half-pulses within the period, wherein the respective displacement times of the first and second half-pulse are reduced in value. 
     In an embodiment, a fraction of the period duration ( 70 ) with an even-numbered denominator, in particular half the period duration ( 70 ), is set for the pulse (P V , P W ) during the period duration ( 70 ) as the first and/or second displacement time ( T1 ,  T2 ). Preferably, a power of two (2 n , nεN 0 ), is used as the denominator. In particular, n=1 is used, which means that the first and/or second displacement time equals half the period duration. By means of such a displacement by half a period duration, the amplitude of the alternating current component—and of the EMC-critical magnetic field emission thus produced—is reduced at the clock frequency of the PWM control. Accordingly, for n=2, i.e., with a displacement by a quarter of the period duration, the alternating current components (frequency components) are reduced at the doubled clock frequency. 
     In an embodiment, the first and second displacement time of the (half-)pulse are set substantially equal in magnitude during the period. In other words, the first and second displacement time have the same value, wherein the second displacement time has a different sign than the first displacement time due to the different displacement direction. This way, a simple and economical displacement of the half-pulses is realized. 
     In an embodiment, the duration of the first and/or second displacement time of the pulse is changed for successive periods. In other words, the displacement times are changed from pulse to pulse. Thus, the periodicity of the generated alternating current components is reliably reduced so that the amplitudes can be reduced in the repetition rates or clock frequencies involved. 
     In an embodiment, it is for example conceivable, that the displacement times of each second period of the PWM control are set to zero, that is, no displacement is performed. In other words, a displacement of the half-pulses only takes place every second period. For example, in a displacement with a first and a second displacement time equal to half the period duration, an inversion of the phase position is generated every second period on the alternating current component at the clock frequency. This ensures that the alternating current component is reduced at the clock frequency. 
     In an embodiment, a plurality of successive periods have, for example, the same displacement times for the respective pulses. In other words, several pulses of successive periods are, for example, displaced or not displaced. 
     In an embodiment, the duration of the first and/or the second displacement time of the pulse for each period is randomly modified. In other words, the pulses of successive periods coincidentally have different displacement times. Due to the mutual, randomly set displacement of the pulses of the periods, a particularly irregular or non-periodic disruption of the alternating current periodicity is produced. This way, the amplitude or the spectral weight of the alternating current component at the clock frequency is divided to as many different frequencies as possible. In other words, the spectrum is broadened or made broadband, wherein the alternating current components thereby generated have in each case a comparatively low amplitude, which can be dampened or reduced by means of filtering circuits of the intermediate circuit. 
     The period duration of successive periods can be varied. This way, the periodicity of the generated alternating current components is further disrupted so that the amplitudes of the relevant alternating current components are reliably reduced. 
     In an embodiment, the first and/or second displacement time for pulses of varying phases are set differently. This means that the pulses of varying phases have different displacements, or that one or more phases have no displacements. In other words, it is possible that the displacements are not applied to all phases. This has a positive effect on a further reduction of the alternating current components. 
     In an embodiment, a first and/or second displacement time for pulses of varying phases is used in periods that differ from one another. This ensures a particularly effective dampening or reduction of the alternating current components. 
     In an embodiment, the electric machine is particularly suitable and configured for the electromotive drive in a motor vehicle, for example for an adjustment system used as a motor vehicle component. The electric motor is preferably designed brushless with a stator and with a rotor rotatably mounted therein. The stator has a number of phase windings which, on the one hand, are connected to the converter and, on the other hand, are interconnected, for example, by a common connection point (star point) in a star connection. 
     The converter has a controller, which means a control unit. In this case, the controller is generally suitable and configured for the implementation of the method described above, in a programmatic and/or circuit-engineering manner. The controller is thus specifically configured to perform a modulation of the PWM control during operation, in which pulses of the phases are divided and displaced within a period. 
     In an embodiment, the controller can be formed, at least in the core, by a microcontroller with a processor and a data memory, in which the functionality for carrying out the inventive method is programmatically implemented in the form of an operating software (firmware), so that the method—possibly interacting with the user—is executed automatically when the operating software is executed in the microcontroller. 
     The controller can alternatively also be formed by a non-programmable electronic component, for example an ASIC (application-specific integrated circuit), in which the functionality for implementing the method is implemented using a circuit. 
     The electrical machine operated with the method thus has improved behavior with regard to EMC radiation as well as with regard to the noise development occurring as a result of the switching processes of the semiconductor switches. The method according to the invention is particularly suitable and adapted for use in speed-controlled systems. In principle, the application is not restricted to the automobile sector. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes, combinations, and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIG. 1  illustrates an electrical machine with a power source and with an electric motor as well as an power converter connected therebetween, 
         FIG. 2  illustrates three phase windings of a three-phase electric motor of the machine in star connection, 
         FIG. 3  illustrates a bridge module of a bridge circuit of the converter for controlling a phase winding of the electric motor, 
         FIG. 4  illustrates an equivalent circuit diagram of the power source, and 
         FIG. 5  is a diagram of a PWM control of the phase windings. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an electrical machine  2  for an electromotive adjustment system of a motor vehicle (not shown), for example a window lifter or a seat adjuster. For this purpose, the machine  2  comprises a three-phase electric motor  4 , which is connected by means of a power converter  6  to a power source (voltage supply)  8 . In this exemplary embodiment, the power source  8  comprises an energy storage device  10  inside the vehicle, for example in the form of a (motor vehicle) battery, as well as a (DC) intermediate circuit  12  which is connected to it and which at least partially extends into the converter  6 . 
     The intermediate circuit  12  is essentially formed by a feed line  12   a  and a return line  12   b , by means of which the converter  6  is connected to the energy store  10 . The lines  12   a  and  12   b  are at least partially guided into the converter  6 , in which a DC link capacitor  14  and a bridge circuit  16  are connected between the lines. 
     During operation of the engine  2 , an input current I E  supplied to the bridge circuit  16  is converted into a three-phase output current (motor current, phase current) I U , I V , I W    
     for the three phases U, V, W of the electric motor  4 . The output currents I U , I V , I W , hereafter also known as phase currents, are guided to the respective phases (windings) U, V, W ( FIG. 2 ) of a stator, not shown. 
     A star circuit  18  of the three phase windings U, V, W is shown in  FIG. 2 . The phase windings U, V and W are each connected with a respective (phase) end  22 ,  24 ,  26  to a respective bridge module  20  ( FIG. 3 ) of the bridge circuit  16  and interconnected with the respective opposite end in a star point  28  as a common connection terminal. In the illustration in  FIG. 2 , the phase windings U, V and W are each shown by means of an equivalent circuit diagram in the form of an inductor  30  and an ohmic resistance  32  as well as a respective voltage drop  34 ,  36 ,  38 . The voltage  34 ,  36 ,  38 , which drops across the phase winding U, V, W, is schematically represented by arrows, and is the sum of the voltage drops across the inductor  30  and the ohmic resistance  32  as well as the induced voltage  40 . The voltage  40  induced by a movement of the rotor of the electric motor  4  (electromagnetic force, EMF) is shown in  FIG. 2  by means of a circle. 
     The star circuit  18  is triggered by means of the bridge circuit  16 . The bridge circuit  16  with the bridge modules  20  is designed, in particular, as a B6 circuit. In this embodiment, during operation, a high (DC) voltage level of the feed line  12   a  and a low voltage level of the return line  12   b  are switched over at a high switching frequency in clocked fashion to each of the phase windings U, V, W. The high voltage level is in this case in particular an intermediate circuit voltage U ZK  of the intermediate circuit  12 , wherein the low voltage level is preferably a ground potential U G . This clocked control is implemented as a PWM control, represented in  FIG. 1  by means of arrows, by a controller  42 , with which control and/or regulation of the speed, the power and the direction of rotation of the electric motor  4  is possible. 
     The bridge modules  20  each comprise two semiconductor switches  44  and  46 , which are shown schematically and exemplarily for the phase W in  FIG. 2 . The bridge module  20  is connected on the one hand with a potential terminal  48  to the feed line  12   a  and hence to the intermediate circuit voltage U ZK . On the other hand, the bridge module  20  is contacted with a second potential terminal  50  to the return line  12   b  and thus to the ground potential U G . Via the semiconductor switches  44 ,  46 , the respective phase end  22 ,  24 ,  26  of phase U, V, W can be connected either to the intermediate circuit voltage U ZK  or to the ground potential U G . When the semiconductor switch  44  is closed (conducting) and the semiconductor switch  46  open (non-conductive, blocking), the phase end  22 ,  24 ,  26  is connected to the potential of the intermediate circuit voltage U ZK . Accordingly, the phase U, V, W contacts the ground potential U G  upon opening the semiconductor switch  44  and closing the semiconductor switch  46 . As a result, it is possible by means of the PWM control to apply two different voltage levels to each phase winding U, V, W. 
     In  FIG. 3 , a single bridge module  20  is shown in simplified form. In this exemplary embodiment, the semiconductor switches  44  and  46  are implemented as MOSFETs (metal-oxide semiconductor field-effect transistors), each of which are switched over in clocked fashion by means of the PWM control between a switched-on state and a blocking state. For this purpose, the respective gate connections are routed to corresponding control voltage inputs  52 ,  54 , by means of which the signals of the PWM control of the controller  42  are transmitted. 
       FIG. 4  shows an equivalent circuit diagram for the power source  8 . During operation, the energy storage  10  generates a battery voltage U Bat  and a corresponding battery current I bat  for the operation of the power converter  6 . In  FIG. 4 , the internal resistance of the energy storage  10  is shown as an ohmic resistor  56 , and a self-inductance of the energy storage device  10  as an inductor  58 . A shunt resistor  60  is connected in the return line  12   b , at which the intermediate circuit voltage U ZK  drops. 
       FIG. 5  subsequently shows and describes the waveform on the individual phase terminals  22 ,  24 ,  26 , and how the voltage or PWM signals can advantageously be controlled or regulated at the individual phase windings U, V, W, as well as which consequences result therefrom with respect to the currents I U , I V , I W  in the phase windings U, V, W and the input current I E  of the external power source  8 . In the embodiment in  FIG. 5 , the phase winding U is applied to a constant, low voltage potential, i.e. in particular, to ground potential U G . The phases V and W are supplied with the pulse width modulated control signals. 
     The diagram in  FIG. 5  comprises five horizontal, superimposed sections. The time is plotted horizontally, i.e., on the x-axis or abscissa axis. By way of example, three periods  63 ,  64 ,  66  of the PWM control are shown in  FIG. 5 , a period  62 ,  64 ,  66  in each case having a period duration  68 ,  70  and  72 , which here, for example, is between 20 μs (microseconds) and 50 μs. 
       FIG. 5  shows a PWM control in which the phase terminals  22 ,  24 ,  26  of the electric motor  4  are each actuated with a PWM (pulse) signal P V , P W  of a different duty cycle. The current desired voltages U U , U V  U W  of the three phases U, V and W are shown in  FIG. 5  with an instantaneous value  74 ,  76  and  78  respectively shown as a horizontal line. In this case, the desired voltage values vary over the time as a function of the rotational speed of the electric motor  4  in each case in the manner of a sinusoidal function. This causes the lines of the instantaneous values  74 ,  76  and  78  to move up and down periodically in the vertical direction, i.e., along the Y-axis or ordinate axis. 
     The saw tooth-shaped line in the upper section of the diagram represents a periodically linearly increasing and linearly decreasing counter reading  80  of a counter integrated in the controller  42 . The points of intersection between the thresholds of the individual phases U, V, W which are fixed for a specific point in time, that is to say, the instantaneous values  74 ,  76 ,  78  with the saw tooth-like counter reading  80 , represent the point in time for generating and terminating the (PWM) pulses P V , P W , with which the phase windings U, V, W are applied. This means that in the case of a high voltage threshold, the instantaneous value  74 ,  76 ,  78  is low, so that the sample time of the pulse-shaped pulse P V , P W  is long, that is to say, that the respective phase V, W is supplied with the phase current I V , I W  or applied with a voltage for a prolonged time. A counter reading  82 , which is phase shifted by 180° relative to the counter reading  80 , is shown by dashed lines in  FIG. 5 . 
     In the second section  84  and third section  86  of the diagram of  FIG. 5 , the voltage profiles at the phase terminals  22 ,  24  and  26  are shown in a time-resolved manner. 
     In the second section  84 , a PWM control is shown in which in each period  62 ,  64  and  66 , always the same pulses P V  and P W  are generated; in the following, therefore, only the first period  62  is described by way of example. In this exemplary embodiment, the phase W is switched on at the beginning of the period  62  and is switched off at a point in time  88 . Delayed in time, a voltage is then applied to the phase winding W at a point in time  90 . After a pulse duration T V , at a point in time  92 , the pulse PV is terminated. Subsequently, the phase W is switched on at a point in time  94  up to the end of the period  62 . The pulse P W  thus essentially extends over in each case two adjacent periods  62 ,  64 ,  66  during a pulse duration T W . This voltage profile is periodically repeated for the PWM control in the second section  84  with a clock frequency (base frequency) f periode . 
     In the fourth section  96  and the fifth section  98  of  FIG. 5 , a respective time profile of the alternating current I res , I res ′ resulting from the PWM control is shown in the power source, for example, in the intermediate circuit  12 . Section  96  hereby shows the alternating current I res  for the PWM control according to section  84 , and section  98  shows the alternating current I res ′ for a PWM control according to section  86 . 
     In section  96 , the alternating current I res  is shown for an operating situation in which the flow direction of the phase currents I V  and I W  of the phases V and W correspond in terms of the directions from and to the star point  28 . The amperage in phase V, for example, is 1 A, and in phase W, the amperage is 3 A. The amperage of phase U, for example, has −4 A and has a flow direction opposite phases V and W. At the beginning of period  62 , thus, an alternating current I res  with the amperage 3 A is generated up to the point in time  88 . Accordingly, during the pulse duration TV, an alternating current I res  of 1 A is generated. 
     During a period  62 ,  64 ,  66 , the alternating current I res  thus has a three-section current block or alternating current component I block , which periodically repeats. By way of example, only the middle current block I block  is described below which corresponds to the pulse P V , wherein the two lateral current blocks, generated by the pulse P W , can be similarly described because of the linearity. 
     By Fourier transform, the alternating current component I block  is mapped on a frequency spectrum F block (ω), wherein w is the angular frequency. By application of the displacement law, the following is obtained for the total spectrum F(ω) of alternating-current components I block  of several (n) periods of the period duration T periode    
         F (ω)=Σ n   e   −jωT     periode     F   block (ω),
 
     wherein j is the imaginary unit. It follows for the clock frequency f period  or the respective multiple n×f periode  that period or 
         e   −jωT     periode     =e   −j2πf     periode     nT     periode     =e   −j2πn =1. 
     This results in that the frequency or alternating current components of the AC current I res  add up for n×f periode . This results in so-called EMC needles, which adversely affect the EMC behavior of the machine  2 . 
     A method for reducing the EMV needles is described below with reference to sections  86  and  98  of  FIG. 5 . In the embodiment of  FIG. 5 , the pulses P V  and P W  in period  64  are divided in each case into two half-pulses P V1  and P V2  and P W1  and P W2  The half-pulses P V1  and P V2  and P W1  and P W2  in this case each have a pulse duration T V ′ or T W ′, which correspond to the respective half, original pulse duration T V  or T W . In principle, the method is applicable to all three phases U, V, W. In the embodiment of  FIG. 5 , phase U is, however, by way of example, permanently at low ground potential U G  so that no displacement takes place. 
     The half-pulses P V1 , P W1 , P V2 , P W2  are then displaced by a respective displacement time within the period  64 . The half-pulses P V1 , and P W1  are in this case delayed in time by means of a displacement time  T1  as compared to the unshifted pulses P V  and P W  of section  84  in this embodiment. The half-pulses P V2  and P W2  are temporally accelerated in time with a displacement time  T2  so that they lead the respectively associated half-pulse P V1  and P W1  during the period duration  70 . 
     The displacement times  T1  and  T2  are equal in magnitude in the illustrated embodiment. In particular, the displacement times  T1  and  T2  are equal in magnitude to half the period duration  70 . The following applies: 
         e   −j2πf     periode     nT     periode     /2   =e   −jπn =−1.
 
     which means that due to the time shift, a phase shift of 180° is produced by means of the displacement times  T1  and  T2 . In other words, the counter reading  80  is converted into the counter reading  82 . 
     The resulting alternating current I res ′ thus has a phase sequence, which is shifted by 180° during the period  64 . As can be seen comparatively clearly in section  98 , the periodicity of the alternating current I res ′ or its alternating-current components is hereby disturbed. Thus, the alternating-current components no longer add up for n×f periode , but instead are distributed over a plurality of frequency components. In the modulation scheme of section  86 , preferably such a pulse displacement means is performed each second period by means of the displacement times  T1  and  T2 . 
     The invention is not limited to the embodiment described above. Rather, other variants of the invention can also be derived from those skilled in the art without departing from the scope of the invention. In particular, all the individual features described in connection with the exemplary embodiment can also be combined with one another in another manner without departing from the subject matter of the invention. 
     For example, it is equally conceivable to vary the period durations  68 ,  70 ,  72  such that the periods  62 ,  64  and  72  have different period durations. It is also conceivable, for example, that a plurality of pulses P V , P W  of consecutive periods  68 ,  70 ,  72  are shifted in time. It is essential that the active time per phase U, V, W, i.e., the pulse duration, remain substantially constant during a period  62 ,  64 ,  66 . This results only in a disruption of the periodicity of the alternating current I res ′, but not of the operation of the electric motor  4 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.