Patent Application
20180043447
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.
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.
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).
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 (PV, PW) during the period duration (70) as the first and/or second displacement time (T1, T2). Preferably, a power of two (2n, nεN0), 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.
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:
The intermediate circuit 12 is essentially formed by a feed line 12a and a return line 12b, by means of which the converter 6 is connected to the energy store 10. The lines 12a and 12b 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 IE supplied to the bridge circuit 16 is converted into a three-phase output current (motor current, phase current) IU, IV, IW
for the three phases U, V, W of the electric motor 4. The output currents IU, IV, IW, hereafter also known as phase currents, are guided to the respective phases (windings) U, V, W (
A star circuit 18 of the three phase windings U, V, W is shown in
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 12a and a low voltage level of the return line 12b 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 UZK of the intermediate circuit 12, wherein the low voltage level is preferably a ground potential UG. This clocked control is implemented as a PWM control, represented in
The bridge modules 20 each comprise two semiconductor switches 44 and 46, which are shown schematically and exemplarily for the phase W in
In
The diagram in
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 PV, PW, 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 PV, PW is long, that is to say, that the respective phase V, W is supplied with the phase current IV, IW 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
In the second section 84 and third section 86 of the diagram of
In the second section 84, a PWM control is shown in which in each period 62, 64 and 66, always the same pulses PV and PW 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 TV, 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 PW thus essentially extends over in each case two adjacent periods 62, 64, 66 during a pulse duration TW. This voltage profile is periodically repeated for the PWM control in the second section 84 with a clock frequency (base frequency) fperiode.
In the fourth section 96 and the fifth section 98 of
In section 96, the alternating current Ires is shown for an operating situation in which the flow direction of the phase currents IV and IW 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 Ires with the amperage 3 A is generated up to the point in time 88. Accordingly, during the pulse duration TV, an alternating current Ires of 1 A is generated.
During a period 62, 64, 66, the alternating current Ires thus has a three-section current block or alternating current component Iblock, which periodically repeats. By way of example, only the middle current block Iblock is described below which corresponds to the pulse PV, wherein the two lateral current blocks, generated by the pulse PW, can be similarly described because of the linearity.
By Fourier transform, the alternating current component Iblock is mapped on a frequency spectrum Fblock(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 Iblock of several (n) periods of the period duration Tperiode
F(ω)=Σne−jωT
wherein j is the imaginary unit. It follows for the clock frequency fperiod or the respective multiple n×fperiode that period or
e
−jωT
=e
−j2πf
nT
=e
−j2πn=1.
This results in that the frequency or alternating current components of the AC current Ires add up for n×fperiode. 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
The half-pulses PV1, PW1, PV2, PW2 are then displaced by a respective displacement time within the period 64. The half-pulses PV1, and PW1 are in this case delayed in time by means of a displacement time T1 as compared to the unshifted pulses PV and PW of section 84 in this embodiment. The half-pulses PV2 and PW2 are temporally accelerated in time with a displacement time T2 so that they lead the respectively associated half-pulse PV1 and PW1 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
nT
/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 Ires′ 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 Ires′ or its alternating-current components is hereby disturbed. Thus, the alternating-current components no longer add up for n×fperiode, 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 PV, PW 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 Ires′, 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 215 174.6 | Aug 2016 | DE | national |
