Method for limiting a direct current received by an electric motor

Abstract

The invention relates to a method for limiting a direct current (I) received by an electric motor (28), wherein the direct current (I) is monitored in such a way that, in order to maintain a predefined maximum value (Imax), a motor torque (Tq) of the electric motor (28) does not exceed a maximum permissible motor torque (Tq,max). It is proposed that, on the basis of the measured direct current (I), a first maximum motor torque (Tq,max1) is adapted to the maximum value (Imax) of the direct current (I) until the maximum value (Imax) is reached, and parallel thereto, a sum of the maximum value (Imax) of the direct current (I) and a second maximum motor torque (Tq,max2) proportional to a relaxation parameter (Δ) is calculated, wherein the maximum permissible motor torque (Tq,max) is formed by selecting the smaller value of the first and the second maximum motor torque (Tq,max1, Tq,max2). The invention also relates to a drive unit (26) comprising an electric motor (28) and an electronic unit (30) for carrying out the method, and to an electric vehicle (10) with a corresponding drive unit.

Description

[0001] R. 418188

[0002] - 1 -

[0003] Description

[0004] title

[0005] Method for limiting the direct current absorbed by an electric motor

[0006] Description

[0007] The invention relates to a method for limiting a direct current drawn by an electric motor according to the preamble of claim 1, wherein the direct current is monitored such that, in order to maintain a predetermined maximum value, the motor torque of the electric motor does not exceed a maximum permissible motor torque. The invention further relates to a drive unit comprising an electric motor and an electronic unit for carrying out the method, as well as an electric vehicle, in particular an electrically driven single- or two-wheeler, with a corresponding drive unit.

[0008] State of the art

[0009] A large number of electrically powered vehicles, hereinafter also referred to as electric vehicles, are operated with rechargeable and preferably replaceable energy storage units, which are discharged by the electric motor of the electric vehicle and can be recharged using a charger. Such energy storage units comprise a plurality of electrochemical energy storage cells connected in series and / or parallel to achieve a required battery voltage or capacity. If the energy storage cells are, for example, lithium-ion (Li-ion) cells, a very high power and energy density can be achieved. The energy storage unit provides a discharge or battery current and is usually also connected to an electronic unit that controls the electric motor via a bus system for data exchange. (R. 418188)

[0010] - 2 - The maximum possible discharge current is typically determined by the energy storage cells and the current operating conditions. A distinction must be made between a continuously possible maximum discharge current and a higher, but only briefly (in the range of a few tens of milliseconds) tolerable, absolute maximum discharge current.

[0011] The discharge current is typically measured directly in the energy storage unit and communicated to the electronics unit via the bus system with a certain time delay. The electronics unit ensures compliance with the maximum discharge current using a proportional-integral controller (PI controller) by limiting the maximum torque of the electric motor. However, reliable limitation can only be determined in situations where the torque limit is active, i.e., when the electric motor is actually delivering its maximum possible torque, resulting in the maximum permissible discharge current.

[0012] However, the torque limit can only be adjusted relatively slowly using methods known from the prior art. It is therefore an object of the invention to provide an improved control system compared to the prior art, which makes it possible, particularly in the case of large changes in the speed of the electric motor, to reduce the motor torque promptly in order to prevent the direct current, especially the discharge current of the energy storage unit, from exceeding its permissible maximum value.

[0013] Advantages of the invention

[0014] To solve the problem, it is provided that, based on the measured direct current, a first maximum motor torque is adjusted to the maximum value of the direct current until the maximum value is reached, and in parallel, a second maximum motor torque proportional to the sum of the maximum value of the direct current and a relaxation parameter is calculated, wherein the maximum permissible motor torque is determined by selecting the smaller of the first and second maximum motor torque values. The method according to the invention offers, with particular advantage, reliable current limiting both in quasi-steady-state operation and in highly dynamic cases with particularly short-term speed changes.

[0015] - 3 -gen, as it ensures compliance with the maximum current and also allows short-term current peaks for optimal motor performance. Due to the parallel calculation of two maximum permissible motor torques for the quasi-stationary and the highly dynamic case and the selection of the lower maximum permissible motor torque in each case, an overall more reliable and therefore relaxed operating environment results.

[0016] Furthermore, a drive unit comprising an electric motor and an electronics unit is provided for carrying out the method, wherein the electronics unit limits the direct current drawn by the electric motor by means of its maximum permissible motor torque. This advantageously results in a high level of protection for the relevant hardware components and ensures reliable operation. The application of the drive unit in an electric vehicle powered by a rechargeable energy storage unit leads to a concrete product improvement, which manifests itself in increased safety, improved performance, and a longer service life for the energy storage unit. The electric vehicle can be, for example, an electric bicycle (e.g., EPAC - Electrically Power Assisted Cycle, e-bike, pedelec, e-cargo bike, etc.), an electric motorcycle, a one- or two-wheeled e-scooter, an e-moped, or the like.The invention is equally applicable to other implementations in the field of micromobility, such as e-kick scooters, monowheels, tricycles, or other non-type-approved vehicles with permanently or interchangeably installed energy storage units. An electrically powered vehicle is therefore also understood to include a vehicle that has a drive unit to assist the driver or a partial electric drive.

[0017] The energy storage unit can be permanently integrated into the electric vehicle or designed as a tool-free, removable battery pack. In the case of a removable battery pack, it can also be designed to be rechargeable both when connected to and disconnected from the electric vehicle. The battery voltage of the energy storage unit is typically a multiple of the voltage of a single energy storage cell within the unit and results from the connection (parallel and / or series) of the individual energy storage cells. Preferably, the energy storage cells are lithium-based, e.g., Li-ion, Li-polymer, Li-metal, or Na-ion. R. 418188

[0018] - 4 -or the like. The invention is also applicable to energy storage units with Ni-Cd, Ni-MH cells or other suitable cell types. For common Li-ion energy storage cells with a cell voltage of 3.6 V, nominal battery voltages of 3.6 V, 18 V, 36 V, 54 V, etc., are possible. The invention is not dependent on the type and design of the energy storage cells and the energy storage unit used, but can be applied to any electrochemical energy storage units and energy storage cells, e.g., in addition to cylindrical cells, also pouch cells or the like, with battery voltages of 36 V, 48 V, 52 V or the like.

[0019] In a further development of the invention, it is provided that the second maximum motor torque is determined according to the relationship

[0020] T q ,max2 = H * U * (Imax + A) / (2 * n)

[0021] This results in a value where n defines the motor speed of the electric motor, q the efficiency of the electric motor, and U the DC link voltage applied to the electric motor. A particular advantage is the very simple and efficient implementation of the method according to the invention in existing drive units due to the straightforward calculation of the second maximum motor torque. The relaxation parameter, depending on the maximum permissible current and the maximum permissible motor torque, typically lies in a range of 2 A to 5 A.

[0022] Preferably, the relaxation parameter is set such that, in the event of a very rapid change in motor torque and / or motor speed, the maximum value of the direct current is not exceeded, and in quasi-steady-state operation of the electric motor with a constant or only slowly changing motor torque and motor speed, it is always ensured that the second maximum motor torque is greater than the first maximum motor torque. The specific adjustment of the relaxation parameter enables an optimal balance between the dynamic behavior and stable operation of the drive unit according to the invention. R. 418188

[0023] - 5 - An energy storage unit is provided for the electrical supply of the drive unit, wherein the direct current is a discharge current of the energy storage unit, which is monitored by the electronic unit to ensure compliance with its maximum value. The focus on the discharge current makes the method particularly relevant and directly applicable for battery-powered drive units such as pedelecs, e-bikes, and other electrically driven electric vehicles. Adding an energy storage unit to the drive unit and monitoring its discharge current offers comprehensive protection for the overall system and advantageously increases the service life of the energy storage unit. Preferably, the maximum value of the direct current, in particular the discharge current, is 40 A, and the maximum permissible motor torque is 6 Nm.This ensures that even with particularly strong speed gradients during highly dynamic operation, no current spikes occur that could lead to the energy storage unit shutting down or being damaged.

[0024] Examples of implementation

[0025] drawing

[0026] The invention is explained below by way of example with reference to Figures 1 to 4, where identical reference numerals in the figures indicate identical components with the same function.

[0027] They show

[0028] Fig. 1 : a schematic representation of an electric vehicle designed as a two-wheeled electric bicycle in a first variant,

[0029] Fig. 2: a schematic representation of the electric vehicle designed as a two-wheeled electric bicycle in a second variant,

[0030] Fig. 3: A block diagram for the power supply of the electric motor of the electric vehicle, designed as a three-phase EC motor, according to Figures 1 or 2 and R. 418188

[0031] - 6 - Fig. 4: a flowchart of the method according to the invention.

[0032] Description of the exemplary implementations

[0033] Figure 1 shows a schematic representation of an electric vehicle 10, designed as a two-wheeled electric bicycle 12. The electric bicycle 12 is powered by an energy storage unit 16 designed as a removable battery pack 14 and can be, for example, a pedelec, an e-bike, or the like. The electric bicycle 12 has a housing in the form of a frame 18 with two wheels 20 mounted in the frame 18. The removable battery pack 14 is detachably connected via a connecting device 22 provided on the frame 18, which interacts electromechanically with an interface (not shown) attached to an outer housing 24 of the removable battery pack 14.

[0034] The electric bicycle 12 further comprises a drive unit 26, which includes an electric motor 28, preferably designed as an EC or BLDC motor, in the form of a mid-drive motor. Alternatively, a hub motor can be used in at least one of the wheels 20 instead of a mid-drive motor. The drive unit 26 is also supplied with energy via the interchangeable battery pack 14 and includes an electronics unit 30 for controlling or regulating the electric bicycle 12, in particular the electric motor 28. The electronics unit 30 is also connected to a sensor unit (not shown), which includes, for example, several sensor elements, such as a torque sensor, a motion sensor, for example in the form of an accelerometer, and a magnetic sensor. The electric bicycle 12 also has a crank arm 32 with a crank axle 34, via which the rider can perform a pedaling motion to drive the rear wheel 20 in the manner of a conventional bicycle drive.The electronic unit 30, the drive unit 26 with the electric motor 28 and the pedal crank shaft 34 are arranged in a drive housing 36 connected to the frame 18.

[0035] The drive motion of the electric motor 28 is preferably transmitted to the pedal crank shaft 34 via a gearbox (not shown), the intensity of the support provided by the drive unit 26 being controlled or regulated by the electronic unit 30. The electronic unit 30 is designed to... R. 418188

[0036] - 7 - The drive unit 26 is controlled in such a way that the rider of the electric bicycle 12 is assisted while pedaling. Preferably, the electronic unit 30 is designed to be operable by the rider, so that the rider can adjust the level of assistance.

[0037] The electric bicycle 12 includes an on-board computer 38, which is mounted on the handlebar 40 of the electric bicycle 12. The on-board computer 38 is designed to be integrated with the electric bicycle 12 and includes a human-machine interface (HMI) for displaying information and controlling the on-board computer 38 and / or the electric bicycle 12 or the drive unit 26. The HMI is designed, for example, as a touchscreen or the like. The on-board computer 38 is connected to the drive unit 26 for the exchange of information and commands. For example, the HMI can display information such as a speed determined by the electronic unit 30 of the drive unit 26 via the sensor unit, a set level of assistance for the electric motor 28, route information from a navigation unit integrated into the on-board computer 38, the charge level of the removable battery pack 14, the temperature of the removable battery pack 14, or the like.

[0038] Figure 2 shows an alternative version of the electric bicycle 12. In contrast to Figure 1, the energy storage unit 16, including its electromechanical interfaces for energy and / or data transmission between the electric bicycle 12 and the energy storage unit 16, is fully integrated into the frame 18 of the electric bicycle 12. However, for easier replacement of the energy storage unit 16, it still has an outer housing 24. Furthermore, a charging and / or discharging interface 42 for charging or discharging the energy storage unit 16 is provided externally in the frame 18 of the electric bicycle 12. This interface is shown as a USB-C interface by way of example. However, other interfaces for wired or wireless energy and / or data transmission are also conceivable. In addition, several such interfaces 42 can be provided on the electric bicycle 12.

[0039] Figure 3 shows a simplified block diagram for the energy supply of the drive unit 26 integrated in the electric bicycle 12 according to Figures 1 or 2 via the energy storage unit 16 with the battery voltage Usatt. The electronic unit 30 of the drive unit 26 comprises a control or regulating unit. 418188

[0040] - 8 -unit 44, a power electronics unit 46 and an intermediate circuit electronics unit 48. The power electronics unit 46 is used to adjust a speed n and / or a torque T. qThe rotor 50 of the electric motor 28 is controlled by the control electronics 44 such that a pulse-width modulated motor voltage signal UM generated by the control electronics for the three phases of a stator winding 54 wound on a stator core 52 sets the rotor 50, which is rigidly connected to a motor shaft 56 and equipped with permanent magnets, into a rotational movement. A gearbox of the drive unit 26 (not shown) for assisting the propulsion of the electric bicycle 12 is driven via the motor shaft 56. The electric motor 28 also has sensors (not shown) for determining the speed, torque, and / or position of the rotor 50, which provide the corresponding sensor signals to the control electronics 40 via a bus system 58. However, the invention is not explicitly limited to three-phase, brushless DC motors, but can also be applied to other types of electric motors 28, such as...Synchronous motors or universal motors are used.

[0041] The DC link electronics 48 convert the battery voltage Usat into a DC link voltage U, which in the case of the electric bicycle 12 is typically between 20 and 60 VDC. The DC link electronics 48 are connected to the control unit 44 via the bus system 58 such that, firstly, the discharge current I of the energy storage unit 16 is detected and, secondly, its maximum value l is measured. max and a maximum engine torque Tq, max are predefinable. By means of an inverter circuit of the downstream power electronics 46 (not shown in detail), which can be implemented, for example, as an H-bridge or B6-bridge, the intermediate circuit voltage U is converted into the pulse-width modulated motor voltage UM for controlling the electric motor 28, whereby the control unit 44 communicates via the bus system 58 to vary the motor speed n and / or the motor torque T q a corresponding duty cycle is specified. The discharge current I of the energy storage unit 16 is typically approximately 20 A and can briefly reach a maximum value l. max up to 40 A. As part of performance improvements, it may be permissible for the discharge current I to increase to approximately 25 A, whereas the absolute upper limit of l max= 40 A must still be maintained. This reduces the margin for short-term current peaks from 20 A to 15 A. The method according to the invention is therefore particularly helpful in cases of steep speed gradients. 418188

[0042] - 9 -th, to avoid excessively high peaks in battery current I, which could lead to the energy storage unit 16 shutting down.

[0043] The following applies to the mechanically delivered power of the electric motor 28:

[0044] PMech = T q * 2K * n.

[0045] Typical engine speed values ​​(n) range from approximately 0 to 10,000 rpm, resulting in a speed of 0 to 200 rpm after the gearbox stage with a reduction ratio of 10 to 50. The engine torque (T) q The electric motor 28 of the electric bicycle 12 moves in the range from 0 to T q , max = 6 Nm.

[0046] The electrical power input of the electric motor 28 is as follows:

[0047] Pel = I * U,

[0048] where, taking into account an efficiency q and the relationship Pmech = q * Pei, the discharge current I can be calculated as follows:

[0049] I = T q * 2K * n / (q * U).

[0050] If the engine speed n and / or the engine torque T increase q If the discharge current is high, a correspondingly higher discharge current I is usually required.

[0051] To maintain the maximum motor torque T q , ma x can therefore be a maximum value l max of the discharge current I according to the relationship

[0052] T q ,max = q * U * lmax / (2K * n)

[0053] be specified.

[0054] However, the efficiency q is highly dependent on the operating point and is generally not well known. Therefore, a reliable implementation is only possible if the motor torque T is used. qmust be restricted more than necessary. R. 418188

[0055] - 10 - Furthermore, an inaccuracy arises from the fact that the motor speed n must be measured regularly and is frequently distorted by systematic measurement errors. As long as the actual motor torque T q the maximum engine torque Tq, max Correspondingly, these measurement errors directly affect the actual engine torque T. q and lead to an unnecessarily “restless” driving behavior.

[0056] According to the invention, it is therefore provided that a first maximum motor torque T is calculated based on the measured direct current I. q , maxi according to the above relationship, such that the maximum value l max the discharge current I is adjusted until the maximum value l max is reached, and in parallel, a sum of the maximum value l maxof the direct current I and a relaxation parameter A, a second maximum motor torque T proportional to q , maX 2 is calculated, where the maximum permissible engine torque T q , max by selecting the smaller value of the first and second maximum engine torque according to the relationship

[0057] T q ,ma X = min{T q>max i, T q ,ma X 2}

[0058] This is formed. This offers the particular advantage of reliable current limiting both in quasi-steady-state operation with slowly or not at all changing speed values ​​n and torque values ​​T. q as well as in highly dynamic cases with particularly short-term changes in speed and / or torque, since it is important to ensure compliance with the maximum current l maxThis ensures that short-term current peaks are also taken into account. The relaxation parameter A is now set such that a very rapid change in motor torque T is prevented. q the maximum value l max of the direct current I is not exceeded, and in the quasi-stationary operation of the electric motor 28 it is always ensured that the second maximum motor torque T q , maX 2 is greater than the first maximum motor torque T q , max i. In order to ensure a sufficient distance to the efficiency q of the electric motor 28, a value in a range of 2 A to 5 A can be specified for the relaxation parameter A.

[0059] Figure 4 shows a flowchart of the inventive method for maintaining the maximum discharge current l max as a subprocess of motor control using a PI controller for the drive unit 26R shown in Figures 1 to 3. 418188

[0060] - 11 -of the electric bicycle 12. The subprocess starts in step 60. In step 62, the first maximum motor torque Tq is determined. maxi after the relationship

[0061] T q ,max1 = H * U * Imax / (2 * n)

[0062] This is calculated. For this purpose, the recorded measured values ​​for the current DC link voltage U, the current motor speed n, and the current motor torque T are used in the subprocess. q and the efficiency q. In addition, the maximum permissible discharge current l is determined. max The specified value is compared with the currently measured discharge current I. Parallel to step 62, in step 64, based on the same measured values ​​and the permissible maximum value l, a further calculation is performed. max of the discharge current I the second maximum motor torque Tq, maX 2 after the relationship

[0063] Tq,max2 = n * U * (Imax + A) / (2 * n)

[0064] calculated, where the maximum value l maxThe relaxation parameter A is added. In the following step 66, the first and second maximum motor torques are compared according to the relationship

[0065] Tq,max = min{Tq.max1, Tq,max2}.

[0066] The maximum motor torque T thus determined q , ma x then serves as the basis for the subsequent PI control in step 68 to limit the discharge current I of the energy storage unit 16. This concludes the described subprocess in step 70.

[0067] Finally, it should be noted that the exemplary embodiments shown are neither limited to Figures 1 to 4 nor to the application of the method according to the invention in the electric bicycle 12. They are to be understood as merely exemplary.

Claims

R. 418188 - 12 - Claims 1. Method for limiting a direct current (I) drawn by an electric motor (28), wherein the direct current (I) is monitored in such a way as to ensure compliance with a predetermined maximum value (l). max ) a motor torque (T q ) of the electric motor (28) a maximum permissible motor torque (Tq, m ax) does not exceed, characterized in that, based on the measured direct current (I), a first maximum motor torque (Tq.maxi) is adjusted to the maximum value (l) in such a way as to max ) of the direct current (I) is adjusted until the maximum value (l) max ) is reached, and in parallel, a sum of the maximum value (l) is calculated. max ) of the direct current (I) and a relaxation parameter (A) proportional second maximum motor torque (Tq, m ax2) is calculated, where the maximum permissible engine torque (Tq, max) by selecting the smaller value of the first and second maximum engine torque (Tq, max i, Tq, maX 2) is formed.

2. Method according to claim 1, characterized in that the second maximum motor torque (Tq, maX 2) after the relationship Tq, m ax2 = n * U * (lmax + A) / (2 * n) results in, where n defines a motor speed of the electric motor (28), q an efficiency of the electric motor (28) and U a DC link voltage applied to the electric motor (28).

3. Method according to one of the preceding claims, characterized in that the relaxation parameter (A) is set such that in the event of a very rapid change in the motor torque (T) q ) and / or the motor speed (n) of the electric motor (28) the maximum value (l max) of the direct current (I) is not exceeded, and in quasi-steady-state operation of the electric motor (28) with a motor torque (T) that does not change or only changes slowly q ) and a Mo-R that does not change or changes only slowly. 418188 - 13 - gate speed (n) is always ensured that the second maximum motor torque (Tq, m ax2) is greater than the first maximum motor torque (Tq,maxl)- 4. Method according to one of the preceding claims, characterized in that a value in a range of 2 A to 5 A can be specified for the relaxation parameter (A).

5. Method according to one of the preceding claims, characterized in that the direct current (I) is a discharge current (Isatt) of an energy storage unit (16) which is used to maintain the maximum value (l). max ) is being monitored.

6. Drive unit (26) comprising an electric motor (28) and an electronic unit (30) for carrying out the method according to one of the preceding claims, wherein the electronic unit (30) converts the direct current (I) received by the electric motor (28) by means of its maximum permissible motor torque (Tq, m ax) limited.

7. Drive unit (26) according to claim 6, characterized in that an energy storage unit (16) is provided for the electrical supply of the drive unit (26), wherein the direct current (I) is a discharge current (Isat) of the energy storage unit (16), which is controlled by the electronic unit (30) to maintain its maximum value (Isat). max ) is being monitored.

8. Drive unit (26) according to one of the preceding claims 6 or 7, characterized in that the maximum value (l max ) of the direct current (I), in particular the discharge current (Ißatt), 40 A and the maximum permissible motor torque (Tq, m ax) 6 Nm.

9. Electric vehicle (10), in particular an electrically powered one- or two-wheeler (12), with a drive unit (26) according to any one of the preceding claims 6 to 8.

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