METHOD AND DEVICE FOR OPERATING AN ELECTRIC MACHINE FOR DELIVERING A PREDED TORQUE AND A PREDED SPEED

Description

[0001] The invention relates to a method and a device for operating an electric machine to deliver a predetermined torque and a predetermined speed. Furthermore, the invention relates to a logic unit, a drive train with a corresponding logic unit, a vehicle with a corresponding drive train, a computer program, and a machine-readable storage medium. State of the art

[0002] An electric drive, especially an electric motor, heats up during operation. To enable an electric motor to generate torque and rotate, magnets are embedded within it, particularly in the rotor. The material properties of magnets are temperature-dependent. The higher the temperature of the magnets, the lower the permissible opposing magnetic field strength. If the permissible opposing magnetic field strength is exceeded by an applied opposing magnetic field, the magnets become irreversibly demagnetized. An electric motor with at least partially demagnetized magnets no longer delivers the specified power or fails completely. During operation, the magnets of an electric motor are exposed to an opposing magnetic field. As long as this field strength is lower than the permissible opposing magnetic field strength, the magnets are not damaged.As the load on the electric machine increases, both as a motor and as a generator, and the phase currents through the electric machine increase accordingly, the resulting opposing magnetic field within the electric machine also increases. Very large phase current amplitudes occur briefly within the electric machine when the so-called active short circuit is activated. For this to happen, the three phases of an electric machine are switched to a common potential, in particular the negative potential of a battery, by means of an inverter. An electric machine is switched to an active short circuit when a fault occurs in the electrical drive system, so that the electric machine can transition to a safe state.Diagrams showing the torque of an electric machine versus its rotational speed, and the magnitude of the resulting current amplitudes when an active short circuit is engaged, demonstrate that the largest phase current amplitudes within the electric machine develop when the active short circuit is engaged at the generator operating point. At this operating point, the strongest opposing magnetic field develops within the electric machine, which loads the magnets. For safe operation of an electric machine, engaging the active short circuit must be possible at any time. To prevent damage or defects to the electric machine in the event of a fault, it must therefore be ensured that the resulting opposing magnetic field is always smaller than the permissible opposing magnetic field load.Since the permissible magnetic field strength decreases with increasing temperature, the operating range of the electric machine must be restricted as the temperature rises. Currently, this is achieved by limiting the torque output when the electric machine's temperature increases. Consequently, the current amplitudes and the resulting magnetic field generated when an active short circuit is engaged are lower than at lower temperatures. This ensures safe operation of the electric machine over a wide temperature range. However, a disadvantage is that the available torque is limited at higher temperatures. If such a drive is installed in a vehicle, this means that the vehicle's acceleration varies depending on the electric machine's temperature.

[0003] In the prior art, the field load occurring when the active short circuit is switched on determines the rotor limit temperature and thus directly influences the achievable continuous performance or power output of the electric machine with regard to torque or power. Besides limiting the deliverable torque, rotor cooling can be improved to reduce the magnet temperature, for example, by active liquid cooling of the rotor shaft. Alternatively, the machine can be oversized to delay rotor heating. Another solution is the use of magnet material with a higher proportion of heavy rare earth elements, which exhibit a lower tendency to demagnetize.

[0004] Therefore, there is a need for solutions that minimize the temperature-dependent limitations of an electric machine's operating range, particularly the limitation of the torque output, especially without requiring structural modifications to the machine or its design. This advantageously increases the achievable continuous torque and power output of the electric machine.

[0005] DE 10 2012 211315 A1 discloses a method for temperature compensation of field-weakening current for electric motors.

[0006] JP 2008 155683 A discloses an electric power steering device that assists steering by controlling a d-axis and a q-axis of a brushless DC motor.

[0007] JP 2002 095300 A discloses a control method for a permanent magnet synchronous motor which controls the terminal voltage of the permanent magnet synchronous motor so that it is constant.

[0008] WO 2010 / 116769 A1 discloses a field weakening control for controlling the rotation of a permanent magnet synchronous machine rotating at high speed, so that the terminal voltage of the permanent magnet synchronous machine remains constant. Disclosure of the invention

[0009] A method is defined by the features of independent claim 1.

[0010] A method for operating or controlling an electric machine is provided. The electric machine is operated such that a predetermined torque is delivered or received by the machine, and the rotational speed of the electric machine corresponds to a predetermined speed. Ideally, the actual values ​​during operation or control match the predetermined values; a minimal deviation may occur due to the limited control accuracy of the controller used. To avoid lengthening the wording, in this description, "delivery of torque" also includes "receipt of torque." For example, during the generator operation of an electric machine during deceleration of a vehicle. Similarly, the phrase "delivery of a predetermined speed" includes both positive and negative speeds resulting from the operation or control of the electric machine.Two operating modes are provided for operating the electric machine. These different operating modes can be implemented using hardware or software. This could involve different controller modules, software algorithms, parameter sets or characteristic curves, or filter modules. Furthermore, the temperature of the electric machine is monitored. This means that a temperature is measured that allows the determination of at least one component of the electric machine, such as the housing, stator, rotor, or magnets. Depending on the measured temperature, the electric machine operates in either a first or a second operating mode. If the temperature falls below a threshold value, preferably a predefined one, the electric machine operates in the first operating mode.Preferably, the electric machine is operated in the usual manner to deliver the specified torque and speed. Preferably, the electric machine is operated efficiently with minimal electrical losses. If the temperature corresponds to or exceeds this threshold, the electric machine operates in the second operating mode. In this mode, the magnetic stator flux of the electric machine is reduced compared to the magnetic stator flux of the electric machine when operating in the first mode to deliver the specified torque and speed. In particular, increased electrical losses occur in the inverter and the electric machine when operating in the second mode.However, the advantageous effect of reducing potential short-term current amplitudes outweighs the slightly increased losses during operation in the second operating mode. Advantageously, switching on an active short circuit with reduced magnetic stator flux in the electric machine causes significantly smaller short-term current amplitudes, for example, 20% smaller current amplitudes, resulting in a weaker opposing magnetic field. Thus, even if switching on the active short circuit becomes necessary, demagnetization of the electric machine's magnets is avoided, despite elevated temperatures and while maintaining the specified torque and speed. In particular, the reduction in stator flux leads to a reduction in the maximum achievable torque, for example, by 10%.However, in the approach described here, this limitation is significantly less than the pure torque limitation, for example by 30%, depending on the temperature.

[0011] In another embodiment of the invention, in the second operating mode the reduction of the magnetic stator flux of the electrical machine is carried out by means of the field weakening controller.

[0012] The reduction of the magnetic stator flux when delivering the specified torque and speed is achieved by means of the field weakening controller. Specifically, when the maximum phase voltage is reached, the field weakening controller limits the stator flux, depending on the speed, to enable the operation of a machine at high speeds. This functionality is extended in the second operating mode by additionally reducing the stator flux depending on the detected temperature. This, in particular, reduces the maximum opposing magnetic field strength when the active short circuit is switched on. Advantageously, this provides a means of reducing the stator flux of the electric machine, enabling its operation at elevated temperatures while delivering the specified torque and speed.

[0013] In another embodiment of the invention, in the second operating mode the reduction of the magnetic stator flux of the electric machine is achieved by increasing the negative d-current.

[0014] The reduction of the magnetic stator flux when delivering a specified torque and speed is achieved by increasing the negative d-current. Specifically, the magnitude of the stator flux is varied by adjusting the negative d-current. Particularly at low speeds, the negative d-current is increased to maximize the machine's efficiency and torque. This method is known in the art as MTPA (Maximum Torque per Ampere) or MMPA (Maximum Torque per Ampere). In the second operating mode, this functionality is extended by increasing the negative d-current, especially compared to the MTPA method, depending on the detected temperature, thereby reducing the stator flux. This specifically reduces the maximum opposing magnetic field strength generated when the active short circuit is engaged.Advantageously, a way is provided to reduce the stator flux of the electric machine, which enables the electric machine to operate at elevated temperatures and while delivering the specified torque and speed.

[0015] In another embodiment of the invention, in the second operating mode, with increasing detected temperature of the electric machine, a greater reduction of the magnetic stator flux of the electric machine occurs.

[0016] The higher the measured temperature, the lower the permissible opposing field strength of the magnets. To prevent potential demagnetization of the magnets, the magnetic stator flux is further reduced with increasing temperature. Advantageously, when an active short circuit is switched on, the further reduced magnetic stator flux in the electric machine results in even lower short-term current amplitudes, thus generating a weaker opposing magnetic field and preventing damage to the magnets.

[0017] In another embodiment of the invention, the temperature detection includes determining the temperature of the rotor of the electric machine or the magnets of the electric machine.

[0018] Measuring the temperature of the electric machine preferably includes determining the temperature of the rotor in which the magnets are mounted, or the temperature of the magnets themselves. For this purpose, one or more sensors can be used to directly measure the temperature at the respective components. The temperatures of the magnets or the rotor can also be determined using indirect measurements, preferably with the aid of thermal models. Advantageously, precise knowledge of the magnet temperature allows for an accurate determination of the necessary reduction in stator flux, which is sufficient to reliably prevent demagnetization of the magnets in the electric machine.

[0019] Furthermore, the invention relates to a computer program configured to execute one of the methods described above.

[0020] Furthermore, the invention relates to a machine-readable storage medium on which the described computer program is stored.

[0021] Furthermore, the invention relates to a logic unit which is defined by the features of independent claim 8.

[0022] A logic unit is provided, designed to operate or control an electric machine. The electric machine is operated such that a predetermined torque is delivered or received, and the rotational speed of the electric machine corresponds to a predetermined speed. Ideally, the actual values ​​during operation or control match the predetermined values; a minimal deviation may occur due to the limited control accuracy of the controller used. Two operating modes are provided for operating the electric machine. These different operating modes can be implemented using hardware or software. Furthermore, the logic unit monitors the temperature of the electric machine. Depending on the measured temperature, the electric machine operates in either a first or a second operating mode.If the temperature falls below a predefined threshold, the electric machine operates in the first operating mode. If the temperature equals or exceeds this threshold, the electric machine operates in the second operating mode. In this mode, the magnetic stator flux of the electric machine is reduced compared to the magnetic stator flux of the electric machine when operating in the first mode at the specified torque and speed. Advantageously, when an active short circuit is engaged with the reduced magnetic stator flux in the electric machine, significantly smaller short-term current amplitudes result, thus generating a weaker opposing magnetic field.Thus, even if it becomes necessary to switch on the active short circuit, demagnetization of the magnets of the electric machine is avoided, despite increased temperatures and when delivering the specified torque and speed.

[0023] Furthermore, the invention relates to a device with a described logic unit. This device is, in particular, an electric machine, a control unit, or an inverter.

[0024] A device is provided that includes the described logic unit. Preferably, in addition to the logic unit, the device includes, for example, power semiconductor switches by means of which a supply voltage is converted into a three-phase alternating voltage for supplying the electric machine. This device can, in particular, be a control unit, a pulse inverter, or, in an integrated case, the electric machine itself.

[0025] Advantageously, various options are provided for integrating a logic unit for optimized operation of an electric machine.

[0026] Furthermore, the invention relates to a vehicle powertrain with a described device. In particular, the powertrain comprises a control unit or an inverter for controlling the electric machine. Preferably, the powertrain further comprises a battery for supplying the electric machine with electrical energy via the inverter. Likewise, the electrical energy generated during the generator operation of the electric machine can also be fed into the battery via the inverter. Such a powertrain serves, for example, to power an electrified vehicle. The method and the device enable safe operation of the powertrain.

[0027] Furthermore, the invention relates to a vehicle with a described drive train. Advantageously, a vehicle is thus provided which can be operated safely using the method and the device, whereby demagnetization of the magnets is reliably avoided, despite increased temperatures of the electric machine and when delivering the specified torque and speed.

[0028] It is understood that the features, properties and advantages of the method according to the invention apply accordingly to the logic device, the apparatus or the drive train and the vehicle and vice versa.

[0029] Further features and advantages of embodiments of the invention will become apparent from the following description with reference to the accompanying drawings. Brief description of the drawing

[0030] The invention will be explained in more detail below using some figures, including: Figure 1 a schematic representation of a device with a logic unit for operating an electric machine, Figure 2 a schematically represented vehicle with a drivetrain Figure 3 A schematically represented flowchart for a method of operating an electric machine. Embodiments of the invention

[0031] The Figure 1Figure 140 shows a device 140 comprising a control unit 150 and a logic unit 110 for operating an electric machine 10. Depending on a detected temperature of the electric machine 10, the logic unit 110 operates or controls the electric machine 10. The logic unit 110 can be arranged separately from the electric machine 10, for example, in a control unit 150 or in an inverter. Furthermore, the logic unit 110, the control unit 150, and / or the electric machine 10 can be arranged together within a housing or within a device 140.

[0032] The Figure 2Figure 1 shows a schematic representation of a vehicle 300 with a drive train 200. The illustration shows, by way of example, a vehicle with four wheels 170, whereby the invention can be used equally in any vehicle with any number of wheels on land, water, and in the air. The drive train 200 includes, for example, a battery 160 for supplying the electric motor 10 of the drive train 200. The drive train preferably further includes a control unit 150 or an inverter, which converts a direct current from the battery 160 into an alternating current. The electric motor 10 is coupled to at least one of the wheels 170 and configured to drive it.

[0033] The Figure 3Figure 400 shows a schematic sequence of a method for operating an electric machine 10 to deliver a predetermined torque and speed. The method begins in step 410. In step 420, a first and a second operating mode for operating the electric machine 10 are provided. In step 430, the temperature of the electric machine 10 is measured. In step 440, the electric machine 10 is operated in the first operating mode if the measured temperature falls below a threshold value. In step 450, the electric machine 10 is operated in the second operating mode if the measured temperature equals or exceeds the threshold value.When operating the electric machine 10 in the second operating mode, with the resulting output of the specified torque and speed, the magnetic stator flux of the electric machine 10 is reduced compared to the magnetic stator flux of the electric machine 10 when operating the electric machine 10 in the first operating mode, with the resulting output of the specified torque and speed. The procedure ends with step 460.

Claims

1. Method (400) for operating an electric machine (10) for outputting a predefined torque and a predefined rotational speed, having the steps of: providing (420) a first and a second operating mode for operating the electric machine (10) detecting (430) a temperature of the electric machine (10); operating the electric machine (10) in the first operating mode (440) if the detected temperature falls below a threshold value and operating the electric machine (10) in the second operating mode (450) if the detected temperature is equal to or exceeds the threshold value, characterized in that when the electric machine (10) is being operated in the second operating mode (450), with the resulting output of the predefined torque and of the predefined rotational speed, the magnetic stator flux of the electric machine (10) is reduced compared to the magnetic stator flux of the electric machine (10) when the electric machine (10) is being operated in the first operating mode (440), with the resulting output of the predefined torque and of the predefined rotational speed; wherein the method further comprises: switching on an active short circuit when the electric machine (10) is being operated in the second operating mode (450) at the reduced magnetic stator flux of the electric machine, so that reduced current amplitudes arise.

2. Method (400) for controlling the electric machine (10) according to Claim 1, wherein, in the second operating mode (450), the reduction in the magnetic stator flux of the electric machine (10) is carried out by means of the fieldweakening controller.

3. Method (400) for controlling the electric machine (10) according to Claim 1 or 2, wherein, in the second operating mode (450), the reduction in the magnetic stator flux of the electric machine (10) is carried out by means of an increase in the negative d-current.

4. Method (400) for controlling the electric machine (10) according to one of the preceding claims, wherein, in the second operating mode (450), as the detected temperature of the electric machine (10) increases, the magnetic stator flux of the electric machine (10) is reduced to a greater extent.

5. Method (400) for controlling the electric machine (10) according to one of the preceding claims, wherein the detection (430) of the temperature comprises ascertaining the temperature of the rotor (20) of the electric machine (10) or of the magnets (30) of the electric machine (10).

6. Computer program which is configured to perform the method (400) according to one of Claims 1 to 5.

7. Machine-readable storage medium on which the computer program according to Claim 6 is stored.

8. Logic unit (110) for operating an electric machine (10) for outputting a predefined torque and a predefined rotational speed, wherein the logic unit (110) is configured to provide a first and a second operating mode for operating the electric machine (10), to detect a temperature of the electric machine (10); to operate the electric machine (10) in the first operating mode if the detected temperature falls below a threshold value and to operate the electric machine (10) in the second operating mode if the detected temperature is equal to or exceeds the threshold value, characterized in that when the electric machine (10) is being operated in the second operating mode, with the resulting output of the predefined torque and of the predefined rotational speed, the magnetic stator flux of the electric machine (10) is reduced compared to the magnetic stator flux of the electric machine (10) when the electric machine (10) is being operated in the first operating mode, with the resulting output of the predefined torque and of the predefined rotational speed, wherein the logic unit (110) is further configured to switch on an active short circuit when the electric machine (10) is being operated in the second operating mode (450) at the reduced magnetic stator flux of the electric machine, so that reduced current amplitudes arise.

9. Device (140), in particular an electric machine (10) or a control unit (150), having a logic unit (110) according to Claim 8.

10. Drive train (200) of a vehicle (300) having a device (140) according to Claim 9.

11. Vehicle (300) having a drive train (200) according to Claim 10.