Patent Publication Number: US-2023163711-A1

Title: Operating method and power electronics for an electric motor

Description:
BACKGROUND 
     Technical Field 
     Embodiments of the invention relate to a method for operating an electric motor, in which power electronics of a drive system operate an electric motor of the drive system by means of clock signals. Furthermore, embodiments of the invention relate to power electronics for a drive system and a vehicle. 
     Description of the Related Art 
     Operating methods are used to drive an electric motor at an operating point of maximum power of the electric motor. The electric motor, together with a traction battery and power electronics, can, in particular, be part of a drive system of an electrically drivable vehicle, for example, an electric vehicle or a hybrid vehicle. 
     During operation, the electric motor inevitably generates heat in components of the electric motor by means of a magnetic flux. Any operating method for the electric motor must therefore control a temperature of the electric motor. Temperature control must therefore comprise removal of heat. In some embodiments, heat can be removed by a selectively used fluid flowing through the electric motor. Even an electric motor-like arrangement with a stator and a rotor can be used to heat a fluid. 
     CN 101 603 728 A, for example, discloses an electromagnetic water heater having a flow-through housing, a shaft passing through the flow-through housing, a rotor arranged inside the flow-through housing and held in a rotationally fixed manner on the shaft, the rotor having a sleeved-on permanent magnet and a motor disposed outside the flow-through housing for rotating the shaft. During operation, a harmonic magnetic flux generated by one of the rotating permanent magnets heats a stator held on the inside of the flow-through housing and water flowing between the rotor and the stator. 
     Moreover, the temperature control may comprise an avoidance of heat generation by the electric motor. 
     For example, DE 10 2017 220 941 A1 discloses an electric motor having a stator and a rotor comprising permanent magnets. The power electronics controlling the stator selectively superimposes additional harmonics of a rotational frequency of the rotor on an alternating current voltage supplied to the stator. The additional harmonics heat the rotor, whereby the permanent magnets become demagnetized. This avoids a heating of the stator by the permanent magnets and prevents a defect of the electric motor resulting from overheating. 
     The temperature control can also serve to favorably adjust a physical quantity determining an efficiency of the electric motor by means of the temperature of the electric motor, by way of example, to achieve a low flux density of a rotor of the electric motor. 
     JP 2018 082 544 A discloses an operating method for an electric motor in which magnets of a rotor of the electric motor are selectively heated by superimposing harmonics of the rotor rotational frequency on an alternating current voltage supplied to the stator of the electric motor. As the temperature of the magnets increases, the flux density of the rotor decreases. 
     However, the selective superimposition of an alternating current voltage with harmonics of the rotor rotational frequency increases the complexity of the power electronics operating the electric motor. The increased complexity of the power electronics requires an increased computing power of the power electronics and/or an increased memory space of the power electronics, which is undesirable. 
     BRIEF SUMMARY 
     Some embodiments provide a method for operating an electric motor which heats a rotor without selectively superimposing additional harmonics of the rotor rotational frequency on an alternating current voltage supplied to the stator. Some embodiments provide power electronics and a vehicle. 
     One embodiment includes a method for operating an electric motor, in which power electronics of a drive system operate an electric motor of the drive system by means of clock signals. The power electronics generates an alternating current voltage that is supplied to the electric motor. In many drive systems, the power electronics generate a square-wave alternating current voltage at an operating point of the electric motor where the electric motor is providing maximum power. The supply of a square-wave alternating current voltage is usually referred to as clock signals. 
     A square-wave alternating current voltage is understood to be a periodic step-shaped alternating current voltage which has a square-wave oscillation, which is to say a square-wave fundamental oscillation, and possibly one or more superimposed square-wave oscillations. The superimposed square-wave oscillations can each differ in a frequency and/or in an amplitude from the square-wave fundamental oscillation. In this case, a frequency of the square-wave fundamental oscillation is equal to a rotational frequency of a rotor of the electric motor multiplied by a pole-pair number of a stator of the electric motor. 
     In some embodiments, the electric motor is selectively operated at each operating point of the electric motor by means of the clock signals. In other words, the power electronics do not apply the clock signals exclusively in an operating point of maximum power but can apply the clock signals arbitrarily in any power range of the electric motor. 
     A harmonic of a sinusoidal fundamental oscillation is generally understood to be any oscillation which has the same shape as the fundamental oscillation and a frequency which is an integer multiple of a frequency of the fundamental oscillation. 
     A square-wave alternating current voltage is therefore a superposition of a (in the precise limit case, infinite) plurality of harmonics of a sinusoidal fundamental oscillation. In contrast with alternating currents of other forms, the low harmonics of a square-wave alternating current voltage, for example, the fifth or the seventh harmonic, have relatively high amplitudes. 
     Since the rotor rotates with a frequency of the fundamental oscillation divided by the number of pole pairs of the stator, the harmonics rotate asynchronously to the rotor and as a consequence, in contrast to the fundamental oscillation, do not generate a constant torque acting on the rotor. The harmonics do however generate a magnetic flux proportional to the respective amplitude, which induces eddy currents in the rotor. The harmonics, which are asynchronous to the rotor, are usually called flux harmonics. The eddy currents induced by the flux harmonics very efficiently heat the rotor, which is to say both laminations forming a rotor core of the rotor and magnets of the rotor held on the rotor core. 
     As a consequence, clock signals allow rapid heating of the rotor thanks to the flux harmonics contained in the square-wave alternating current voltage, without selectively superimposing additional flux harmonics on the alternating current voltage. The power electronics apply clock signals as needed at any operating point of the electric motor, which is to say, independently of any operating point of the electric motor, to rapidly heat the rotor of the electric motor. 
     The power electronics may provide the clock signals for an operating point with a power of the electric motor below a maximum power of the electric motor. A power-related efficiency of the electric motor is relatively low for the clock signals because of the strongly contained flux harmonics. This is the case because the flux harmonics do not contribute to the torque provided by the electric motor. However, the low efficiency of the clock signals is accepted at the operating point with maximum power in order to increase the maximum power of the electric motor. 
     The low efficiency of the clock signals can be (partly) compensated or even overcompensated by an increased efficiency of the power electronics, since with clock signals an ohmic loss of the power electronics or a power loss of an inverter of the power electronics are reduced. 
     At operating points below the maximum power, on the other hand, known power electronics apply timings that differ from clock signals, which are more favorable with regard to the power-related efficiency of the electric motor. Notwithstanding the relatively unfavorable efficiency, the power electronics may selectively apply the clock signals instead of these timings in order to heat the rotor of the electric motor. 
     In particular, the power electronics provides the clock signals for an operating point having a power of the electric motor that is below a limit power rating of the electric motor determined by a base frequency of the electric motor for a pulse width modulation instead of the pulse width modulation. Below the limit power rating, which corresponds, for example, to a driving speed of approximately 70 km/h for an electrically driven vehicle, the power electronics uses pulse width modulation (PWM) as timing. In operating points of the electric motor in this lower power range, the power electronics can optionally use clock signals instead of pulse width modulation to heat the rotor of the electric motor, despite the relatively unfavorable efficiency. 
     Alternatively, or additionally, the power electronics provides the clock signals for an operating point with a power of the electric motor above a limit power rating of the electric motor for a pulse width modulation instead of an overmodulation. Above the limit power rating, which corresponds, for example, to a driving speed of about 70 km/h in an electrically driven vehicle, the power electronics applies overmodulation with relatively small blocks as timing. At operating points of the electric motor in this upper power range, the power electronics can optionally use clock signals instead of overmodulation to heat the rotor of the electric motor, despite the relatively unfavorable efficiency. 
     At least one flux harmonic of the clock signals in a d/q reference frame rotating with the rotor may have a d component. In the d/q reference frame, the d component indicates a magnetic flux that does not apply a torque to the rotor, whereas the q component indicates a torque acting on the rotor. 
     In some embodiments, the power electronics heats a rotor and/or a stator of the electric motor by means of the clock signals. By means of the clock signals, the stator of the electric motor can also be heated, in addition to the rotor. 
     The rotor and/or the stator may heat an oil arranged in the electric motor and flowing around the rotor and/or the stator for heating a traction battery of an electrically drivable vehicle, which is to say an electric vehicle or a hybrid vehicle. The oil flowing around the rotor and/or the stator can flow through a cylindrical gap between the rotor and the stator. The rotor and the stator together rapidly cause a large heat input into the flowing oil, which correspondingly leads to a rapid and strong heating of the oil and an equally rapid and strong heating of the traction battery. 
     In some embodiments, a permanently excited synchronous motor (PSM), a synchronous reluctance motor (SynRM) or a separately excited synchronous motor (SSM) is operated as the electric motor. In addition to permanently excited synchronous motors, the method described herein is also suitable for synchronous reluctance motors and separately excited synchronous motors. In this respect, there are numerous possible applications for the embodiments described herein. 
     Another embodiment includes a power electronics for a drive system. The power electronics comprises an inverter, which provides an alternating current voltage on the output side for operating an electric motor of the drive system. The power electronics is configured to provide different timings of the alternating current voltage. The timings of the alternating current voltage include pulse width modulation, overmodulation, and clock signals. Known power electronics apply clock signals only in an operating point of the electric motor at maximum power, whereas pulse width modulation is applied in operating points of a lower power range and overmodulation is applied in operating points of an upper power range. 
     In some embodiments, the power electronics is configured to operate an electric motor of a drive system in a method as described herein. Accordingly, the power electronics are configured to selectively apply clock signals at any operating point of the electric motor. In this manner, the power electronics allows a rapid heating of a rotor and/or a stator of the electric motor as well as of an oil flowing through the electric motor at any operating point. 
     Another embodiment is a vehicle with an electric drive system comprising a traction battery, an electric motor and power electronics. Such vehicles are widely used and will become much more numerous in the future. Accordingly, there are more and more possible applications of the embodiments described herein. 
     In some embodiments, the drive system comprises power electronics as described herein. Thanks to the power electronics as described herein, the traction battery is rapidly heated independently of an operating point of the electric motor. 
     A significant advantage of the method described herein is that it allows power electronics to heat a rotor very effectively without deliberately superimposing additional harmonics of the rotor rotational frequency on an alternating current voltage supplied to the stator. A further advantage of the method is that the power electronics does not need to provide a new timing of the generated alternating current voltage when executing the method, but only needs to selectively provide a controlled timing, namely clock signals, independent of an operating point of the electric motor. Correspondingly, existing power electronics can easily be adapted to the method described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Embodiments are schematically illustrated in the drawings. 
         FIG.  1    shows a vehicle according to one embodiment in a partial block diagram. 
         FIG.  2    shows a bar graph of a power range of the electric motor shown in  FIG.  1   . 
         FIG.  3    shows a spectral diagram of clock signals provided by the power electronics shown in  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a vehicle  1  according to one embodiment in a partial block diagram. The vehicle  1  is electrically drivable and comprises an electric drive system  10  having a traction battery  102 , an electric motor  101 , and power electronics  100 . The electric motor  101  comprises a rotor  1010  and a stator  1011  and may be configured as a permanently excited synchronous motor, a synchronous reluctance motor, or an externally excited synchronous motor. The rotor  1010  and the stator  1011  may have an oil  2  flowing around them. 
     The power electronics  100  are suitable for the drive system  10  and configured to operate the electric motor  101  in a method as described herein. 
     The power electronics  100  of the drive system  10  operate the electric motor  101  of the drive system  10  by means of clock signals  4 . 
       FIG.  2    shows a bar graph of a power range  3  of the electric motor  101  shown in  FIG.  1   . The power range  3  comprises a maximum power  31  of the electric motor  101  and a limit power rating  30  of the electric motor  101 , which is lower than the maximum power  31 . 
     At the maximum power  31 , the power electronics  100  operate the electric motor  101  with the clock signals  4 . Below the limit power rating  30 , the power electronics  100  may provide pulse width modulation as a timing of the alternating current voltage for normal operation of the electric motor  101 . Above the limit power rating  30 , the power electronics  100  may provide overmodulation as a timing of the alternating current voltage for normal operation of the electric motor  101 . 
     The electric motor  101  is selectively operated by means of the clock signals  4 , by the power electronics  100  at each operating point  32  of the electric motor  101 . 
     Thus, the power electronics  100  may provide the clock signals  4  for an operating point  32  with a power of the electric motor  101  below the maximum power  31  of the electric motor  101 . 
     The power electronics  100  may further provide the clock signals  4  for an operating point  32  with a power of the electric motor  101  below the limit power rating  30  of the electric motor  101  for one pulse width modulation instead of the pulse width modulation. 
     Further, the power electronics  100  may provide the clock signals  4  for an operating point  32  with a power  3  of the electric motor  101  above a limit power rating  30  of the electric motor  101  for pulse width modulation instead of overmodulation. 
     At least one flux harmonic of the clock signals  4  may have a d component in a d/q reference frame rotating with the rotor  1010 . 
     The power electronics  100  may use the clock signals  4  to heat the rotor  1010  and/or the stator  1011  of the electric motor  101 . 
     In particular, the rotor  1010  and/or the stator  1011  may heat an oil  2  disposed in the electric motor  101  and flowing around the rotor  1010  and/or the stator  1011  to heat the traction battery  102  of the vehicle  1 . 
     Depending on the embodiment of the vehicle  1 , a permanently excited synchronous motor, a synchronous reluctance motor, or an externally excited synchronous motor may be operated as the electric motor  101 . 
       FIG.  3    shows a spectral diagram  5  of clock signals  4  provided by the power electronics  100  shown in  FIG.  1   . By way of example, the clock signals  4  are shown in the form of a square-wave voltage U UV  having an amplitude U d . 
     The spectral diagram  5  comprises an abscissa  50 , on which integer multiples  500 , which is to say harmonics of order v, of a rotational frequency  501  of the rotor  1010 , which is to say a fundamental oscillation, are plotted, and an ordinate  51 , on which amplitudes  510 ,  511  respectively assigned to the orders v are plotted, and shows a bar-shaped spectrum of the clock signals  4  shown. 
     The rotational frequency  501  is assigned an amplitude  510  of magnitude 100%. The integer multiples  500  are each assigned amplitudes  500  of smaller magnitudes than 100%, which decrease inversely proportional to the order v. For example, it can be read that the two lowest harmonics of orders  5  and  7  are assigned amplitudes of respectively 20% and 14%. The lowest harmonics of the clock signals  4  provide the main contribution to the heating of the rotor  1010  of the electric motor  101 . 
     German patent application no. 10 2021 130907.7, filed Nov. 25, 2021, to which this application claims priority, is hereby incorporated herein by reference, in its entirety. 
     Aspects of the various embodiments described above can be combined to provide further embodiments. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled.