Patent Application
20190149079
Electric machines in motor vehicles that can be operated as generators or motors are known. Such electric machines usually have a rotor with an excitation winding (rotor winding) which can be energized with an excitation current, and a stator with a multi-phase stator winding to which a multi-phase phase voltage with a phase voltage vector can be applied.
Use of the electric machine in a motor vehicle may require frequent switching on and off of the electric machine. Switching off the electric machine can be problematic, especially at high speeds. If the phase voltage is switched off first, a current flow through the inverse diodes of the MOSFETs occurs due to the excitation current still flowing and thus to an undesired generator operation for this operating state. If an excitation voltage or current is first switched off, the pole wheel voltage falls below a voltage limit within a decay time of the excitation voltage or current, which results in a high current flow in the electric machine. Due to the resulting power loss, the electronic components of the electric machine in particular are subjected to unnecessary thermal stress.
DE 10 2013 215 306 A1 shows a procedure for switching off an electric machine at high speeds, wherein the excitation current is first switched off and the excitation current decays according to the time constant of the excitation winding, wherein the phase voltage is then switched off when it corresponds to the absolute value of the pole wheel voltage.
According to the invention, a method for switching off a multi-phase electric machine in a motor vehicle is proposed as well as a computing unit and a computer program for carrying out the method, having the features of the independent patent claims. Advantageous embodiments are the subject of the dependent claims and of the following description.
The electric machine has a rotor with rotor winding and a stator with a multi-phase stator winding. In particular, a power converter is connected downstream of the stator winding. The electric machine is connected via the power converter in particular to a vehicle electrical system, in particular to a DC electrical system.
In the operation of the electric machine, especially at higher speeds, typically n>3000 rpm, the electric machine is operated in a so-called block mode or block commutation. Here, the individual stator windings are acted upon with voltage blocks, wherein the temporal control width of the voltage blocks preferably corresponds to half a total revolution (180°) of the rotor and wherein the individual voltage blocks of the respective phases of the stator winding are preferably temporally offset from one another. This operating mode is particularly advantageous at high speeds of the electric machine, since no significant switching losses occur, for example in comparison to an operating mode in which the electric machine would be operated in pulse width modulation mode (PWM) with a clock frequency adapted to the speed of the electric machine and thus correspondingly high. To switch off the electric machine, it is necessary that both the excitation current and the phase voltage of the electric machine are switched off or reduced as quickly as possible to a value corresponding to approximately zero (0V or 0 A). However, the excitation current and phase voltage should be switched off in such a way as to avoid the disadvantages described above.
For this purpose, the excitation current is first reduced to such an extent that the pole wheel voltage determined i.a. by the excitation current corresponds to a first threshold value. This threshold value corresponds in magnitude and phase or direction preferably to the phase voltage. If the excitation current were further reduced, the pole wheel voltage would be lower than the phase voltage, resulting in a phase current that would result in an adverse thermal effect. However, since the phase voltage in block mode is determined by the DC voltage of the vehicle battery applied to the power converter, the phase voltage in block mode can only be varied in its phase position with respect to the pole wheel voltage, but not in its magnitude.
In order to further reduce the pole wheel voltage or the excitation current and the phase voltage, according to the invention it is switched from block mode of the electric machine to PWM mode when the first threshold value is reached. This operating mode is characterized by the fact that the control pulses have a different pulse width for each constant pulse frequency. This enables the phase voltage to be reduced simultaneously with the pole wheel voltage or excitation current, thus preventing current flow in the stator winding and thus thermal loading of the electric machine. In addition, the first threshold value is selected in such a way that no undesired current peaks occur when switching between block mode and PWM mode. This is particularly the case if the pole wheel voltage and the phase voltage are identical in magnitude and direction and are always reduced equally. This avoids high electrical and mechanical loads when changing the operating modes of the electric machine (block mode to PWM mode).
In the course of the process, an optimum time for switching off the phase voltage of the electric machine is also determined. PWM mode allows the phase voltage in magnitude and/or phase (direction) to be adapted to the pole wheel voltage in such a way that phase currents of essentially 0 A result. An optimum time for switching off the phase voltage is determined by determining at least one parameter which influences the pole wheel voltage, preferably the excitation current or the excitation voltage and/or the rotational speed of the electric machine, and by comparing the parameter with a further threshold value. The parameters influencing the pole wheel voltage can be adjusted as required, wherein the speed of the electric machine is preferably a free parameter which can assume almost any value, and the excitation current or excitation voltage can be controlled to a value which essentially corresponds to 0 A or 0V in magnitude.
In principle, it is understood that an adaptation, in particular a reduction, of the excitation current or excitation voltage can be controlled by the field controller during the entire switch-off process. Furthermore, it is also possible that the excitation current is switched off and the respective steps of the switch-off procedure are effected during a decay of the excitation current within the decay time. When the excitation current is switched off by the field controller, it can disconnect the rotor winding from the excitation voltage applied. A reduction of the voltage or current to a value of approximately 0V or 0 A can also be covered by switching off.
This makes it possible to switch off the electric machine independently of a speed of the electric machine and/or of an internal combustion engine of the motor vehicle. In particular, it makes it possible to gently switch off the electric machine at high speeds.
The process is suitable for both generator and motor operation of the electric machine and for all types of motor and commercial vehicles, especially hybrid vehicles.
In particular, it makes it possible to operate the electric machine as a motor and to support the internal combustion engine. Even at high speeds, the electric machine can be switched off easily and without great loads. For example, it is not necessary to wait until the speed falls below a permissible limit value in order to switch off the electric machine. The electric machine can be switched off at the best possible time.
Even in case that the electric machine receives a torque request during the switch-off process according to the invention, one can switch back directly into motor or generator operation by switching the excitation current or the phase voltage on again, since phase voltage and pole wheel voltage are controlled during the switch-off process in such a way that the phase current preferably corresponds essentially to 0 A. This prevents the occurrence of voltage or current peaks and enables the electric machine to be switched off or on with as little mechanical stress as possible.
A computer unit according to the invention, e.g. a control unit of a motor vehicle, is set up, particularly in terms of programming, to carry out a method according to the invention.
Also the implementation of the method in the form of a computer program is advantageous, since this causes particularly low costs, in particular if an executing control unit is still used for further tasks and therefore exists anyway.
Further advantages and embodiments of the invention result from the description and the enclosed drawing.
The invention is shown schematically in the drawing on the basis of embodiments and is described in the following with reference to the drawing.
The electric machine 110 is electrically connected to a power converter 106, with several phase connections 107 provided. The power converter 106 can be operated as a rectifier and as an inverter. The several phase connections 107 are phase connections of a multi-phase stator winding of an electric machine stator 110. On the DC side, a rotor winding 101 of the electric machine 110 is connected via a field controller 102. The field controller 102 is responsible for controlling the rotor winding 101. An energy accumulator, for example a vehicle battery 105, can be connected to the DC side of the power converter 106 via DC voltage connections 103. The vehicle battery 105 can be electrically connected to and disconnected from the DC side of the power converter 106 via a switching element 104.
A computing unit embodied as a control unit 112 is configured, in particular in terms of program technology, to carry out an embodiment of the method according to the invention. In particular, the control unit 112 controls the field controller 102 and the power converter 106 according to the invention.
If the electric machine 110 is operated in generator mode, a five-phase alternating voltage, the so-called phase voltage, is generated in the stator winding 110a. This five-phase alternating voltage is directed into a DC voltage by means of appropriate clocked control of the MOSFETs 106a. This converted DC voltage can be used, for example, to charge the vehicle battery 105.
If the electric machine 110 is operated in motor mode, the DC voltage of the vehicle battery 105 is converted into the five-phase phase voltage with a rotating phase voltage vector by means of appropriate clocked control of the MOSFETs 106a. The MOSFETs 106a are controlled by the control unit 112 in a clocked manner.
It should be noted that this invention is not intended to be limited to a five-phase electric machine, but is suitable for electric machines with an appropriate number of phase connections 107.
Preferred embodiments of the method for switching off the electric machine 110 are described below with reference to
In motor operation, the electric machine 110 is supplied with an excitation voltage Uf and the phase voltage US and converts this electrical energy into mechanical energy in order to support the internal combustion engine 109 with this mechanical energy. The magnitude or amplitude of this phase voltage US is determined in particular by the DC voltage of the vehicle battery 105 applied to the power converter 106 as the supply voltage. The excitation voltage Uf is applied to the rotor winding 101 by means of the field controller 102, wherein an excitation current If is generated in the rotor winding 101. The excitation current If is in particular controlled to a desired setpoint value by means of a PI controller. The excitation current If induces the pole wheel voltage Up in the stator winding 110a when the rotor of the electric machine is rotating. The pole wheel voltage Up is dependent on the speed ω and the magnitude of the excitation current If. The exact determination of the pole wheel voltage Up is given below.
The electric machine 110 is initially switched on and is in block mode 210, wherein the electric machine 110 can support the internal combustion engine 109. Here, a torque of the electric machine 110 or the internal combustion engine 109 can be set in step 211. The torque can be adjusted by varying a pole wheel angle, which in turn is adjusted by the direction of the phase voltage vector US, which in turn is adjusted by corresponding control of the power converter 106. Via an appropriate control loop, the pole wheel angle ϑ is varied in such a way that a desired torque is set (cf. in particular
In the idling phase 212, the electric machine 110 rotates at least at the specified speed of the internal combustion engine 109 due to the torque-locking connection with the internal combustion engine 109, provided that the electric machine 110 is not decoupled from the internal combustion engine 109, for example by a freewheel. In order to minimize the losses of the electric machine 110, it is advantageous to switch off the electric machine 110, so that the loss torque of the electric machine 110 is only reduced to the comparatively low mechanical friction torque of the electric machine 110.
The electric machine 110 is switched off in the course of the preferred embodiment of the method according to the invention.
During the idling phase 212, the electric machine 110 is initially operated in block mode 210, in which a block-shaped switching voltage is applied to the stator winding 110a. To switch off the electric machine 110 during the idle phase 212, both the excitation current If and the quantities derived therefrom, such as the pole wheel voltage Up and the phase voltage US, must be reduced to a value that is as small as possible in terms of magnitude, in particular to zero. In order to avoid the disadvantages mentioned at the beginning, which can be associated with switching off the excitation current If and/or the phase voltage US, the switching off process is effected according to the invention according to the following steps. It goes without saying that the electric machine 210 can also be switched off outside an idling phase 212.
In step 213 at least one parameter influencing the pole wheel voltage Up is first determined. Preferably a magnitude and a direction of the pole wheel voltage vector are determined as the parameters influencing the pole wheel voltage Up. Alternatively or additionally, the magnitude of the excitation current If and/or the speed ω of the electric machine 110 can preferably be determined as the at least one parameter influencing the pole wheel voltage Up. The speed ω of the electric machine 110 and the magnitude of the excitation current If are usually known anyway or are determined in the vehicle anyway. Therefore, no additional effort is required to determine the magnitude of excitation current If and/or the speed ω of the electric machine 110.
In this particular example, the magnitude of the pole wheel voltage vector is determined as a parameter influencing the pole wheel voltage Up. In particular, the magnitude is determined as a function of the excitation current magnitude If and the speed ω. In particular, this determination shall be made according to the following formula:
where
ΨR is a flux linkage generated by the excitation current If. Due to saturation effects, the relationship between excitation current If and flux linkage ΨR is not linear. In particular, this relationship between excitation current If and flux linkage ΨR is stored as a characteristic curve or in the form of a compensating polynomial, especially in the control unit 112. The direction of the pole wheel voltage vector results from the construction and current position of the rotor.
A parameter influencing the pole wheel voltage, in particular the excitation current If, is reduced in a further step 214 and it is checked whether the magnitude and/or direction of the pole wheel voltage vector Up each reach a first threshold value S1. The respective threshold value S1 is selected in such a way that the threshold value S1 corresponds to the phase voltage US in magnitude and/or direction at a respective time. If the excitation current If were reduced to such an extent that the pole wheel voltage Up would be smaller than the phase voltage US, this would result in a current flow Is in the phase winding 107, which in turn would result in a power loss of the electric machine 110. To avoid this, the magnitude and/or phase of the phase voltage US would also have to be adjusted accordingly, so that the current Is in the phase winding is always zero net during the switch-off process 209. However, this is not possible in block mode 210, since only the phase can be adjusted here, but not the magnitude of the phase voltage US. When the threshold S1 is reached, block mode 210 is deactivated and PWM mode 220 is activated. In addition, the currents Is caused in the stator phases within block mode 210 are associated with superimposed harmonics, which is why rapid switch-off or switch-on is problematic, even in case that Up=US in average, as a pole wheel voltage Up can be induced to a not inconsiderable extent as a result.
Switching from block mode 210 to PWM mode 220 is therefore advantageous, as undesirable current peaks and high electrical and mechanical loads on the electric machine 110 can be avoided—by particularly selecting the appropriate threshold value S1.
In step 221, in a PWM mode, a phase voltage US with a phase voltage vector is applied to the stator winding 110a by pulse width modulation (PWM) of the supply voltage. This phase voltage vector corresponds in magnitude and direction to a currently prevailing pole wheel voltage vector of a pole wheel voltage Up. The magnitude of the pole wheel voltage vector can be adjusted while maintaining the speed ω of the electric motor 110 or the internal combustion engine 109, in particular via the excitation current If—as a parameter influencing the pole wheel voltage. This has the advantage that it is possible to switch off the electric machine 110 without any problems even at high speeds. The excitation current If is further reduced, indicated by reference sign 223, until the magnitude of the pole wheel voltage vector disappears. Here the excitation current If reaches a further threshold value S2. Meanwhile, the phase voltage US is also adjusted accordingly, so that the phase voltage vector US continues to correspond to the pole wheel voltage vector Up in magnitude and direction (also indicated by reference symbol 223).
The magnitude and phase of the phase voltage vector US and of the pole wheel voltage vector Up are each determined in step 222 and compared with each other. If there is a deviation from the magnitude and/or phase of the phase voltage vector US and the pole wheel voltage vector Up, these can be readjusted, in particular by means of controller 112. If the parameter influencing the pole wheel voltage reaches the further threshold value S2, the phase voltage US is switched off in a further step 224. Switching off the phase voltage US after the parameter influencing the pole wheel voltage has reached the threshold value S2 (this corresponds in particular to the excitation current If=0 A) is unproblematic, as the electric machine 110 can no longer be operated as a generator due to the excitation current If being no longer present.
The phase voltage US can either be switched off by disconnecting the phase connections 107 by the power converter 106 or the phase voltage US can also be reduced to a value of approx. 0V.
A disconnection of the phase connections 107 by the power converter 106 is therefore not fundamentally necessary, since the excitation current If and the phase voltage US can also be reduced, especially simultaneously, to a value of 0 A or 0V in order to adapt the phase voltage US to the pole wheel voltage Up, and this in fact also corresponds to a switch-off.
It goes without saying that an adaptation, in particular a reduction, of the excitation current If or the excitation voltage can be controlled by the field controller 102 during the entire switch-off process. It is also possible that the excitation current If is switched off and the respective steps of the switch-off method 209 are effected during a decay of the excitation current If within the decay time. When the field controller 102 switches off the excitation current If, it disconnects the rotor winding 101 from the applied excitation voltage Uf.
A relationship between the phase voltage US, the pole wheel voltage Up, the phase current IS and the excitation current If and thus the theoretical background of the inventive methods are explained in more detail in
In
The pole wheel voltage Up is by definition on the q-axis and is shown as a first pointer. The q-axis forms the so-called excitation axis. The d-axis is electrically orthogonal to the q-axis. The phase voltage US is shown as a second pointer and is shifted by one pole wheel angle ϑ with respect to the pole wheel voltage Up. The pole wheel angle ϑ is also known as the load angle. In generator operation (see
In block mode, the magnitude of the phase voltage US is determined by the DC voltage of the vehicle battery 105 applied to the power converter 106. The phase voltage US can therefore in block mode only be varied in its phase position with respect to the pole wheel voltage Up, i.e. via its pole wheel angle ϑ with respect to the pole wheel voltage Up. The phase position or the pole wheel angle ϑ can be adjusted appropriately by means of the power converter 106.
The phase current IS is represented as a third vector in the dq coordinate system. The result is a phase current vector whose locus curve corresponds to the circle shown when the polar wheel angle changes from 0°-360°.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 207 386.9 | Apr 2016 | DE | national |
| 10 2016 216 560.7 | Sep 2016 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/079184 | 11/30/2016 | WO | 00 |
