Method for controlling an electric motor without torque to generate heat for a battery

Abstract

The invention relates to a method for acting on stator windings (L1, L2, L3) of an electric motor (3) to heat an electric battery (1) via an inverter (2), the inverter comprising six switches (S1-S6), the method comprising an activation step that comprises a main full-wave control over a pair of first and second switches and an auxiliary pulse width modulation control over a third switch, and a relaxation step, following the activation step, wherein the main full-wave control is released and the auxiliary pulse width modulation control continues to be controlled, characterised in that, during the activation step and the relaxation step, the instantaneous electrical angle of the rotor is compared with an instantaneous electrical angle of the stator, in order to deduce therefrom an open duty cycle to be applied, so that the motor torque is and / or remains zero.

Description

DESCRIPTION TITLE OF THE INVENTION: METHOD FOR CONTROLLING AN ELECTRIC MOTOR WITHOUT TORQUE CAUSING HEATING OF A BATTERY

[0001] The present invention claims priority from French application No. 2307169 filed on 05.07.2023, the content of which (text, drawings and claims) is incorporated herein by reference.

[0002] The invention relates to a method for controlling an electric motor in order to cause heating of a battery without generating engine torque.

[0003] More specifically, it is a method intended to act on stator windings of an electric motor via a control inverter, for the purpose of heating an electric traction battery of an electric or hybrid vehicle, the battery supplying electrical energy to the inverter.

[0004] Since the battery has a certain internal resistance, the flow of current causes it to heat up. This heating is desirable under certain conditions because traction batteries do not perform well when their temperature is too low. This is useful, for example, in cold-start configurations of the vehicle, particularly in low outside temperatures. This sequence is performed while the vehicle is stationary, before starting.

[0005] This type of system and method has already been proposed according to document EP3674132. However, according to the proposed method, the currents flowing through the windings lead to the generation of a motor torque. However, such a torque can create a jolt in the vehicle, which is perceived as untimely by the driver and / or the occupants of the vehicle, given that the vehicle is stationary.

[0006] The inventors therefore proposed to solve this kind of drawback.

[0007] For this purpose, the present invention provides a method for acting on stator windings of an electric motor for the purpose of heating. of an electric battery forming the source of electric currents passing to the stator windings via an inverter, the inverter comprising at least six switches, the inverter being connected to the stator windings of the electric motor via three control terminals, traversed respectively by three instantaneous terminal currents, characterized in that the method comprises: a- an activation step comprising a main full-wave command on a pair of first and second switches applied to a pair of first and second terminals and an auxiliary pulse width modulation command on a third switch applied to a third terminal, b- a relaxation step, immediately following the activation step, by releasing the main full-wave command and continuing to control the auxiliary pulse width modulation command, and characterized in that, at the start of the activation step,the pair of first and second switches and the third switch are chosen according to an instantaneous electrical angle of the rotor to minimize a motor torque, and during the activation (a-) and relaxation (b-) steps, the instantaneous electrical angle of the rotor is compared with an instantaneous electrical angle of the stator, to deduce therefrom an opening duty cycle to be applied to the third switch so that the motor torque is and / or remains zero.,

[0008] Thanks to the provisions promoted above, the generation of engine torque is avoided and the battery heating sequence remains completely transparent to the driver and / or vehicle occupants.

[0009] Technically, the current control in the stator windings is carried out so that the electrical angle of the stator coincides with the electrical angle of the rotor, which leads to a situation where no torque is generated on the rotor, while significant currents flow through the windings. These currents generate Joule heating due to the internal resistance of the battery (referred to as RB in this document).

[0010] As we will see later, the relaxation stage is the seat of energy restitution to the battery. Between 60% and 75% of the current consumed during the activation phase is returned to the battery.

[0011] We seek to maximize the ratio of effective battery current to average battery current.

[0012] Note that switches can typically be power transistors, for example MOSFETs. Other types of switches are not excluded, such as IGBT or others.

[0013] The inverter is of a known design. Three high-side switches and three high-side switches are arranged inside the inverter. Each interface terminal between the inverter and the electrical machine is connected to one high-side switch and one low-side switch.

[0014] Note that each switch is equipped with a freewheel diode which is used in the relaxation step.

[0015] It should be noted that the term 'electric motor' or 'electric machine' will be used interchangeably in this document, knowing that the latter is used as a generator in certain life cycles.

[0016] In this document, the term "instantaneous stator electrical angle" refers to the orientation of the electromagnetic field in the stator air gap around the stator / rotor axis.

[0017] According to one embodiment, the instantaneous control currents (i1, i2, i3) are measured and the instantaneous electrical angle of the stator is deduced from them. The calculation of the reconstruction of the stator electromagnetic field from the winding currents gives a precise and real-time indication of the orientation of the stator magnetic field. This calculation can be made from a table of parameters or charts characterizing the electrical and electronic banking properties of the machine.

[0018] According to one embodiment, the instantaneous electrical rotor angle is acquired from a position sensor connected to the rotor of the electric motor. Knowledge of the electrical rotor angle makes it possible to create a control loop so as to permanently cancel the motor torque by generating a stator electromagnetic field aligned with that of the rotor.

[0019] In one embodiment, an instantaneous battery current involved in the control of the electric motor by the inverter is acquired. The instantaneous battery current can be received from a sensor or can be determined from the three instantaneous terminal currents.

[0020] Knowledge of the instantaneous battery current allows for finer control of stage transitions, particularly for moving from the activation stage to the relaxation stage and also for completing the relaxation stage.

[0021] According to one embodiment, the activation step is started according to first predetermined conditions.

[0022] For example, the first predetermined conditions can be met when the following clauses are met: no pedal torque demand (vehicle accelerator pedal at rest), and battery temperature below a threshold temperature (e.g. < 15°C), and rotor rotation speed substantially zero.

[0023] According to one embodiment, the activation step is passed to the relaxation step according to second predetermined conditions, depending on a current overshoot condition.

[0024] According to one embodiment, for example, the second predetermined conditions may include: the instantaneous battery current exceeds a maximum battery current threshold, or the instantaneous inverter current exceeds a maximum inverter current threshold, or the instantaneous motor current exceeds a maximum motor current threshold. The maximum current thresholds in question may be parameterized values ​​or values ​​from mapping.

[0025] According to an alternative embodiment, the second predetermined conditions can be realized as soon as a configurable activation duration has been exceeded.

[0026] According to one embodiment, the relaxation step is stopped according to third predetermined conditions, depending on a condition on the instantaneous battery current.

[0027] According to one embodiment, for example, the third predetermined conditions include: the instantaneous battery current in absolute value is less than a low threshold and the instantaneous battery current is increasing. In other words, the relaxation stage ends when the battery current l-batt reaches 0 from below. This coincides with the end of the current return to the battery.

[0028] According to an alternative embodiment, the second predetermined conditions can be achieved as soon as a configurable duration of the relaxation step has been exceeded.

[0029] The invention further relates to a control system comprising at least one battery, at least one inverter, at least one electric motor and at least one control unit configured to implement the method as described above.

[0030] The invention further relates to a vehicle comprising a control system as described above.

[0031] In other words, the invention relates to a vehicle comprising a battery, an inverter, an electric motor and a control unit, characterized in that the control unit is configured to implement the method as described previously.

[0032] The invention will be further detailed by the description of non-limiting embodiments, and on the basis of the appended figures illustrating variants of the invention, in which: [Fig.1] is a schematic representation of an example of an electric motor control system from a battery and an inverter, in which system the present invention is implemented, [Fig.2] schematically represents a functional diagram of the control unit. [Fig.3] is a schematic representation of the process steps, [Fig.4] is a schematic representation of a stator field phase wheel, [Fig.5] schematically illustrates a timing diagram showing control signals and inverter terminal currents, Figures 6 to 9 represent an example control scenario illustrating the activation and relaxation steps with transistors S1 and S4 controlled in all-or-nothing mode, and transistor S5 in pulse width modulation, Figure 6 with transistors S1 and S4 in the ON state and transistor S5 in the ON state, Figure 7 with transistors S1 and S4 in the ON state and transistor S5 in the OFF state, Figure 8 with transistors S1 and S4 in the OFF state and transistor S5 in the OFF state, Figure 9 with transistors S1 and S4 in the OFF state and transistor S5 in the ON state, Figures 10 to 11 show an example of a schematic representation of two phase wheels of the stator electromagnetic field, Figure 10 with the viewpoint of the excited windings, and Figure 11 with the viewpoint of the activated switches, [Fig.12] schematically illustrates a timeline showing steps of the process, [Fig.13] is similar to figure 1 and represents a variant of the electrical diagram, particularly on the motor side.

[0033] In the various figures, the same references designate identical or similar elements. For reasons of clarity of the presentation, certain elements are not necessarily represented to scale.

[0034] A general diagram of an electric motor control system is now described with reference to Figure 1.

[0035] The system in question comprises a battery 1, an inverter 2 and an electrical machine 3.

[0036] In an electrically powered / tracted motor vehicle, there is a traction battery 1 which is designed to store a significant amount of electrical energy in electrochemical form.

[0037] In practice, we're talking about several dozen kWh. A 100% electric vehicle battery typically has an energy storage capacity of between 50 kWh and 100 kWh, depending on the target range, weight, and fuel consumption of the vehicle. For a hybrid vehicle, the battery capacity may be lower depending on the desired zero-emission range.

[0038] We are talking here about a battery based on Lithium-Ion electrochemistry whose performance is suboptimal if its temperature is too low. Among these, there are two most widespread types, namely NMC (Nickel Manganese Cobalt) and LFP (Lithium Iron Phosphate / LifePo4).

[0039] The battery has a distributed internal resistance, denoted RB. In practice, the distributed internal resistance RB can have a total value of a few tens or even a few hundred milliohms. Due to the Joule effect, heating occurs when the current passes, and consequently the temperature increases.

[0040] The electric machine 2 is used as a motor or generator depending on the vehicle's usage cycles.

[0041] The electrical machine considered here is typically of the permanent magnet rotor type. The magnetic field of the rotor magnets has an orientation marked 0 rotor. The electric field of the rotor has an orientation marked 0 elec rotor, more concisely denoted 0r. A sensor of the electrical machine 3 acquires the instantaneous electric rotor angle. The sensor delivers information representative of the magnetic orientation of the rotor magnets.

[0042] Regarding the electrical machine, as illustrated in Figure 1, the windings can be organized with a midpoint 35 forming a so-called 'star' connection. However, it is not excluded to have a so-called 'delta' assembly like the diagram illustrated in Figure 13, the windings being respectively coupled directly between the respective terminals U, V, W.

[0043] The electrical machine 3 may comprise a pair of poles or may comprise p pairs of poles. The principle and implementation of the present invention also applies, mutatis mutandis, with a machine having p pairs of poles, up to a multiplication factor.

[0044] Inverter 2 is of conventional construction with six switches. We have three high side switches denoted S1, S3, S5 and three low side switches denoted S2, S4, S6.

[0045] The connection point 21 of the first terminal U of the electric machine is interposed between the first high side switch S1 and the first low side switch S2. The connection point 22 of the second terminal V of the electric machine is interposed between the second high side switch S3 and the second low side switch S4. The connection point 23 of the third terminal W of the electric machine is interposed between the third high side switch S5 and the third low side switch S6. Thus the inverter is connected to the electrical machine by means of three terminals U, V, W, in a conventional manner.

[0046] We denote i1, i2, i3 the instantaneous currents passing through the terminals (called terminal currents). These instantaneous currents are known either by feedback from each control transistor (smart MOSFETs) or by means of a specific sensor element.

[0047] In the star-connected version, a first winding group L1 is coupled on one side to terminal U and on the other side to midpoint 35, a second winding group L2 is coupled on one side to terminal V and on the other side to midpoint 35, a third winding group L3 is coupled on one side to terminal W and on the other side to midpoint 35. It is noted that depending on the control of the switches, the current flows through the windings in one direction or the other.

[0048] The switches here are MOSFET (field effect transistor) power transistors. Alternatively, the switches can be LV IGs.

[0049] Each transistor has a freewheeling diode 25. The freewheeling diode can be present by construction in the switch component or can be added in parallel with the switch.

[0050] As illustrated in Figure 2, the inverter 2 is controlled by a control unit 4 responsible for controlling the bases or control grids T1, T2, T3, T4, T5, T6 of the six respective switches S1, S2, S3, S4, S5, S6.

[0051] In a known manner, when the motor is tasked with creating torque on the rotor shaft, then the control unit 4 drives the appropriate transistors in sequence to create a rotating field angularly offset by the rotor electrical angle. For example, turning to Figures 4 and 10, assuming that the current state corresponds to an electrical stator angle 0a and that the rotating field rotates counterclockwise, the control unit activates the gates of transistors T1 and T6, then at 60°, deactivates T1 and activates T3, then at 120°, deactivates T6 and activates T2, then at 180°, deactivates T3 and activates T5, then at 240°, deactivates T2 and activates T4, and so on. The transistors are activated one after the other sequentially two by two, i.e. in pairs (high side / low side). Conventional control, in motor or generator, will not be described further because it is assumed to be known in itself.

[0052] As seen in Figure 2, the control unit 4 acquires the rotor electrical angle '0 Elec rotor' 0r. The control unit 4 acquires the terminal currents i1, i2, i3 and calculates, based on these terminal currents i1, i2, i3, in a preliminary block 42, the stator field electrical angle 0s.

[0053] The control unit 4 comprises a comparator 45. One of the inputs of the comparator 45 receives the stator electrical angle while the other input receives the rotor electrical angle. The output indicates the correction to be applied to the terminal currents i1, i2, i3. The functional block marked 47 calculates the control signals to be applied to the switch gates, as will be seen in detail below, for the case of zero torque control.

[0054] The control unit 4 can receive the battery current Ibatt from a sensor or from the battery manager, or alternatively, the control unit 4 can reconstruct the battery current Ibatt from the terminal currents i1, i2, i3.

[0055] In the context of the present invention, the aim is, unlike traction control, not to create motor torque and to make the stator electromagnetic field coincide with the electrical angle of the rotor.

[0056] More precisely, we advantageously use a main control between two terminals of the machine and an auxiliary control, which will be discussed below, on the third terminal of the machine.

[0057] The proposed method comprises a sequence repetition, said sequence comprising an activation step (denoted a-) also called an excitation step and a relaxation step (denoted b-). The engagement of said sequence is subject to a combination of conditions which will be seen later.

[0058] The activation step comprises a main command, called full wave, on a pair of first and second switches applied to a pair of first and second terminals and an auxiliary command in pulse width modulation (PWM) on a third switch applied to a third terminal. The pair of first and second switches comprises a high side switch and a low side switch each connected to a terminal. The third switch is connected to the third terminal, separate from the first and second terminals.

[0059] In the example illustrated in Figures 4 to 6, transistors S1 and S4 are activated by their respective gates T1 and T4, they are controlled in all or nothing, therefore here in the ON state without modulation. Furthermore, transistor S5 is controlled in pulse width modulation via its gate T5 with a duty cycle RCO controlled by the control unit 4. Transistor S5 is therefore cyclically in the ON state or in the OFF state. The period of the PWM signal can be between 50 ms and 200 ms.

[0060] Figures 6 to 9 illustrate the direction of the currents flowing through the three stator windings L1, L2, L3, with transistor S5 in the ON and OFF states respectively. The bold solid line indicates the main control current flowing successively through the first transistor S1, the first terminal U, the first winding L1, then the second winding L2, then the second terminal V, then the fourth transistor S4. The thick dotted line indicates the auxiliary control current flowing successively through the fifth transistor S5, then the third terminal W, then the second winding L2, then the second terminal V, then the fourth transistor S4.

[0061] As can be seen in Figure 7, it is the freewheel diode 25 of the transistor S6 which allows the energy stored by the windings L3 to decrease. It is the average intensity generated by the auxiliary control in PWM / MLI which is relevant, this gives the average magnetic field produced by the winding or windings which are traversed by this current.

[0062] Controlling the pulse width modulation on transistor S5 aligns the stator electromagnetic field 0s with the rotor electrical angle 0r, which leads to a zero motor torque situation.

[0063] In Figure 5, we see that the current passing through the first terminal lU (main control) and the current passing through the third terminal I-W (auxiliary control) increase in average value from time t0 to time t1. The battery current Ibatt is also illustrated, which corresponds substantially to the sum of the currents of the main control and the auxiliary control.

[0064] Immediately following the activation step, a relaxation step takes place. The relaxation step consists of releasing (i.e. stopping) the main full-wave control and continuing to control the auxiliary control in PWM, to maintain zero motor torque. S1 and S4 go to the OFF state.

[0065] The relaxation step gives rise to a restitution of current (energy) to the battery 1; this is illustrated in Figure 8 by the current which flows back to the battery (reference 72 Figure 8), in particular via the freewheel diode of the second switch S2 and via the freewheel diode of the third switch S3. The quantity of energy returned to the battery is represented by the area Irest of the zone arranged in the negative region of the battery current as illustrated in Figure 5. We note the maximum restitution current, around -150 A in Figure 5, a little lower than the positive peak of current taken at almost 200 A at time t1.

[0066] We note in Figure 9, where transistor S5 is activated for zero torque control via the auxiliary control in pulse width modulation, that the current restitution on the main control continues.

[0067] Considering the opening duty cycle of the pulse width modulation control, the main control current is substantially higher than the average current involved in the auxiliary control.

[0068] Depending on the conditions and the various operational parameters, the percentage of current returned to the battery in the relaxation phase compared to the activation phase can be between 60% and 75%. We therefore note that this solution is much more interesting than having the battery discharge into a resistive load.

[0069] We move from the activation stage to the relaxation stage according to predetermined conditions, marked C2, depending on a current overshoot condition.

[0070] The second predetermined C2 conditions may include: the instantaneous battery current exceeds a maximum battery current threshold, or the instantaneous inverter current exceeds a maximum inverter current threshold, or the instantaneous motor current exceeds a maximum motor current threshold. Note that the current thresholds in question can be configurable values ​​or values ​​from a map.

[0071] The activation step occurs from time t1 to time t2.

[0072] The duration of the activation stage is denoted TA. The duration of the relaxation stage is denoted TB. The values ​​of TA and TB can be between 0.5 ms and 3 ms.

[0073] The recurrence period of the complete sequence is denoted PER.

[0074] It may be preferentially planned to chain without time interval a new activation step after the relaxation step which ends as illustrated in the right part of figure 12. Then PER = TC = TA + TB.

[0075] Alternatively, the sequence chaining can be done on the basis of a predefined cycle TC as illustrated on the left part of Figure 12 with a pause time interval TD between the end of the relaxation period and a new activation period. The recurrence condition is noted C4 in Figure 3. The recurrence condition C4 becomes false in particular if the driver of the vehicle presses the accelerator pedal.

[0076] The TC or PER sequence period can typically be between 1 millisecond and 10 milliseconds.

[0077] As illustrated in Figure 3, the activation step is started according to first predetermined conditions denoted C1.

[0078] For example, the first predetermined conditions C1 can be met when the following clauses are met: no pedal torque demand (vehicle accelerator pedal at rest), and battery temperature below a threshold temperature (for example 15°C), and rotor rotation speed substantially zero.

[0079] If the driver presses the accelerator pedal, then the first conditions are not met, if the battery temperature is already sufficient, then the first conditions are not met, and if the rotor already has a substantially non-zero rotational speed, then the first conditions are not met.

[0080] Regarding the rotor rotation speed, we can consider the speed substantially zero for example if it is less than 5 RPM.

[0081] The relaxation step is stopped according to third predetermined conditions noted C3, depending on a condition on the instantaneous battery current.

[0082] According to a non-limiting example, the third predetermined conditions C3 include: the instantaneous battery current in absolute value is less than a low threshold and the instantaneous battery current is increasing, which is illustrated in the portion of curve 73 at the bottom of figure 5. In other words, the relaxation phase ends when the battery current l-batt reaches 0 from below. This coincides with the end of the current return to the battery.

[0083] As seen in Figure 5, in the bottom diagram, the low threshold corresponds to the interval between IBS- and IBS+. One of the conditions for stopping the relaxation step is therefore that the instantaneous battery current is between IBS- and IBS+, with an increasing slope, i.e. a return to 0 from below.

[0084] Referring to Figure 10, the selection logic that can be implemented in the control unit 4 is illustrated. Once the rotor angle 0r is known, a radial line DR is drawn from the center of the circle (this is illustrated in dashed lines in Figures 4, 10 and 11). From the center of the phase wheel, the radial line DR crosses the small diameter central disc, then successively two rings.

[0085] In Figure 11, the smaller diameter disc corresponds to the high side switch to be activated, the ring surrounding the central disc corresponds to the low side switch control to be activated, and the peripheral ring indicates the third switch to be controlled in PWM pulse width modulation, generically noted PWM Tx, x being the index of the transistor considered.

[0086] In Figure 10, the smaller diameter disc corresponds to the two windings to be excited, the sign denoting the direction of travel, and the ring around it designates the third winding involved through which the PWM control current flows.

[0087] According to the schematic variant illustrated in Figure 13, the battery circuit comprises an inductor L9 in series with the battery current circuit.

[0088] In addition, the battery circuit includes a resistor R9 and a capacitor C9 in parallel. This does not reduce the current return to the battery.

[0089] In Figure 5, the curve located at mid-height illustrates the effectiveness of the proposed control which shows that the angular position difference between the stator electrical angle and the rotor electrical angle remains close to 0.

[0090] Furthermore, it should also be noted that even if the rotor moves a little angularly during the application of the steps or sequence of steps proposed above, it may be necessary to choose another pair of switches for the main control and another third switch for the PWM control.

[0091] Not only is the duty cycle of the PWM signal dynamic during the activation and relaxation stages, but also the choice of the switch pair for the main control and the third switch for the PWM control can be challenged at each new occurrence of the activation stage. In practice, the choice of the switch pair for the main control and the third switch for the PWM control is made according to the rotor electrical angle at the beginning of each activation stage.

Claims

CLAIMS 1. Method for acting on stator windings (L1, L2, L3) of an electric motor (3) for the purpose of heating an electric battery (1) forming the source of electric currents passing to the stator windings via an inverter (2), the inverter comprising at least six switches (S1-S6), the inverter being connected to the stator windings of the electric motor via three control terminals (U, V, W), traversed respectively by three instantaneous terminal currents (i1, i2, i3), characterized in that the method comprises: a- an activation step comprising a main full-wave command on a pair of first and second switches applied to a pair of first and second terminals and an auxiliary pulse width modulation (PWM) command on a third switch applied to a third terminal, b- a relaxation step, immediately following the activation step,by releasing the main full-wave command and continuing to control the auxiliary command in pulse width modulation, and characterized in that, at the start of the activation step, the pair of first and second switches and the third switch are chosen according to an instantaneous electrical angle of the rotor to minimize a motor torque, and during the activation (a-) and relaxation (b-) steps, the instantaneous electrical angle of the rotor is compared with an instantaneous electrical angle of the stator, to deduce therefrom an opening duty cycle (ODR) to be applied to the third switch so that the motor torque is and / or remains zero., 2. Method according to claim 1, characterized in that the instantaneous control currents (i1, i2, i3) are measured and the instantaneous electrical angle of the stator is deduced therefrom.

3. Method according to any one of claims 1 to 2, characterized in that the instantaneous electrical rotor angle (0r) is acquired from a position sensor connected to the rotor of the electric motor.

4. Method according to any one of claims 1 to 3, characterized in that the activation step is started according to first predetermined conditions (C1).

5. Method according to any one of claims 1 to 4, characterized in that one passes from the activation step to the relaxation step according to second predetermined conditions (C2), as a function of a current excess condition.

6. Method according to claim 5, characterized in that the second predetermined conditions comprise: the instantaneous battery current (Ibatt) exceeds a maximum battery current threshold, or the instantaneous inverter current exceeds a maximum inverter current threshold, or the instantaneous motor current exceeds a maximum motor current threshold.

7. Method according to claim 4 and claim 5, characterized in that the relaxation step is stopped according to third predetermined conditions (C3), as a function of a condition on the instantaneous battery current.

8. Method according to claim 7, characterized in that the third predetermined conditions comprise: the instantaneous battery current in absolute value is less than a low threshold and the instantaneous battery current is increasing.

9. Control system comprising at least one battery (1), at least one inverter (2), at least one electric motor (3) and at least one control unit (4), characterized in that the control unit is configured to implement the method according to one of claims 1 to 8.

10. Vehicle comprising a control system according to claim 9.

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