Method for operating an electric drive device, control unit and drive device

Abstract

The invention relates to a method for operating an electric drive device (1) that comprises an electrical machine (2), power electronics (3) comprising half-bridges having multiple semiconductor switches and an intermediate circuit (5) and that is connected or connectable to an electrical energy store (4), wherein an operating state of the electrical machine (2) and a demanded target torque of the electrical machine (2) are taken as a basis for switching the semiconductor switches to a normal mode for generating the target torque, to a short-circuit mode or to a freewheeling mode. There is provision for a present operating value of the electrical machine (2), which corresponds to a magnetic field weakening of a magnetic field of the electrical machine (2), to be measured and for the freewheeling mode to be enabled or disabled on the basis of the operating value.

Description

[0001] Description

[0002] title

[0003] Method for operating an electric drive device, control unit and drive device

[0004] The invention relates to a method for operating an electric drive device which has an electric machine, power electronics with half-bridges having a plurality of semiconductor switches and an intermediate circuit, and which is connected or connectable to an electrical energy store, wherein, depending on an operating state of the electric machine and on a requested target torque of the electric machine, the semiconductor switches are switched in a control mode to generate the target torque, in a short-circuit mode or in a freewheel mode.

[0005] Furthermore, the invention relates to a control device which is designed to carry out the above-mentioned method, as well as to a drive device for a motor vehicle with such a control device

[0006] State of the art

[0007] Electric drive devices typically comprise an electric machine and power electronics connected upstream of the machine, which is designed to apply a predetermined current or voltage to individual phases of the electric machine in order to generate torque for the electric machine. For this purpose, the power electronics comprise an inverter connected between the phases of the electric machine and an electrical energy storage device in order to couple the direct voltage of the energy storage device with the alternating voltage of the electric machine. For this purpose, the power electronics or inverter comprise half-bridges, with each phase of the electric machine often being assigned a half-bridge, and with the half-bridges each comprising two controllable semiconductor switches.These semiconductor switches are typically controlled by a control unit using pulse-width modulation (PWM) to set the desired operating current or voltage to generate the specified target torque. Alternatively, they are controlled in short-circuit mode such that, in particular, either all semiconductor switches coupled to the positive DC voltage or all semiconductor switches connected to the negative DC voltage (low-side switches) are closed to create an active short circuit. Alternatively, the semiconductor switches are controlled in freewheel mode such that they are open and thus non-conductive.Because a reverse-biased diode is typically connected in parallel with each semiconductor switch, or the semiconductor switches themselves incorporate reverse-biased diodes or parasitic reverse diodes, only the forward-biased current can be interrupted. Therefore, in freewheeling mode, a current through these diodes is always to be expected whenever the electric machine exhibits behavior that directs current toward the energy storage device. Uncontrolled current injection into the energy storage device during braking, in which the machine is operating in a regenerative or regenerative mode, is undesirable.Therefore, this is prevented by freewheeling operation or limited to a minimum value by comparing the behavior of the electrical machine and the supply network and only setting freewheeling operation when no operating current or only an insignificant operating current or current flow in the direction of the energy storage device is to be expected.

[0008] To switch freewheeling mode on or off, the measured or calculated induced voltage of the electrical machine is usually compared with a limit value calculated directly from the measured intermediate circuit voltage. If the induced voltage corresponds to or exceeds the limit value, freewheeling mode is exited. If the induced voltage is close to or below the limit value, freewheeling mode is activated. The induced voltage is currently determined based on the speed of a rotor position sensor or a speed measuring device, although additional correction factors, such as temperature, can be taken into account. However, the measured or calculated induced voltage and the intermediate circuit voltage are subject to tolerances in the measurement technology and signal processing and therefore do not represent ideal values.This means that the ideal, full speed range in which freewheeling would theoretically be possible cannot be used, but only a range reduced by the tolerances or measurement and calculation errors.

[0009] Disclosure of the invention

[0010] The method according to the invention with the features of claim 1 has the advantage that the possible effective range of freewheeling operation is brought very close to the physically possible limits of the electrical machine and is increased, so that the drive device can be operated more efficiently and precisely than in the prior art. In particular, the method according to the invention ensures that freewheeling operation is only blocked later than was previously the case. In this case, measurement of the intermediate circuit voltage can be dispensed with and is preferably dispensed with. The invention provides that a current operating current, in particular phase current, of the electrical machine is detected and a d-component of the current is determined as the d-current, and that freewheeling operation is enabled or disabled depending on the d-current. Enabled here means that switching to freewheeling operation can be carried out as required.Blocked means that it is not possible to switch to freewheeling mode, or that freewheeling mode is terminated and the system switches to control mode or short-circuit mode. The decision as to whether freewheeling mode is enabled or disabled is therefore based on the d-current or the d-component of the phase current or operating current. Parameters of the electrical machine, which are usually also transformed into the d / q reference system, are often already available in the control unit. The d-parameters indicate the extent to which the magnetic field of the electrical machine is influenced at the current operating point. This field is the cause of the voltage induced by the electrical machine in the power electronics and therefore influences whether and to what extent a current flows in the freewheeling state.The evaluation of the d-values, especially the d-current, thus also indicates whether it is necessary to influence the current magnetic field of the electrical machine, and to what extent. Essentially, currents are used that aim to weaken the magnetic field, in turn reducing the induced terminal voltage of the machine and, in turn, reducing or preventing the resulting current flow toward the electrical storage device.

[0011] According to the invention, it is provided that a current operating value of the electrical machine, which corresponds to a magnetic field weakening of a magnetic field of the electrical machine, is detected, and that the freewheeling operation is enabled or disabled depending on the operating value.

[0012] According to a preferred embodiment of the invention, a current operating current, in particular phase current, of the electrical machine is detected as the operating value and a d-component of the current is determined as d-current.

[0013] Furthermore, it is preferably provided that the current magnetic flux of the electric machine is determined as a function of the d-current and a q-current of the operating current. This allows additional parameters of the electric machine to be recorded and taken into account.

[0014] According to a preferred development of the invention, freewheeling mode is enabled when there is no or only a small d-current to weaken the magnetic field of the electric machine. The absence or only a small or even insufficient d-current, which causes little or no weakening of the magnetic field of the electric machine, is decisive for enabling freewheeling mode. Freewheeling mode is preferably enabled when the d-current falls below a predeterminable d-current limit, whereby the d-current limit can be, for example, less than 1 to 5% or 10% of the rated current of the inverter.According to a preferred development of the invention, a torque-dependent limit value for the d-current is specified, and freewheeling operation is blocked if the d-current exceeds the limit value for a requested torque or for an already set torque. The d-current value is thus compared with a limit value depending on the torque. In particular, a characteristic map and a characteristic curve are stored for this purpose, which represent the limit value as a function of the torque or of different torques of the electric machine. This makes it easy to implement a simple release decision for freewheeling or a blocking decision for freewheeling during operation.This means that if a larger operating current is required during torque operation or control operation of the electric machine, and thus greater field weakening, i.e., a larger d-current, then currentless freewheeling operation is not possible, and vice versa. Compared to the state of the art, the accuracy of the prediction is also improved by preferentially using the d-component of the phase current as the feedback variable. However, this requires that the control system be implemented in such a way that the minimum field weakening current is always achieved. This is particularly advantageous when the electric drive system is used as the drive system in an electric road vehicle, in particular a motor vehicle.

[0015] Freewheeling mode is preferably blocked if the operating current is not equal to 0 (zero) amperes or outside a specified tolerance range around 0 amperes. If there is a need to weaken the magnetic field, starting with an already small d-current, this means an excessively high induced voltage in freewheeling mode, so that freewheeling mode is not enabled but blocked. This is particularly the case if the phase current deviates significantly from 0 amperes. The tolerance range is preferably selected such that it permits a deviation in the single-digit ampere range. Freewheeling mode is exited or blocked when the phase current exceeds a specified threshold or the specified tolerance range.Preferably, at an operating point of the electrical machine with a torque and / or an operating current other than 0 amperes, it is determined whether an operating current of 0 could be set at this operating point with a torque of 0 Newton meters in order to determine whether freewheeling would then be possible. In the case of torque specifications that deviate significantly from 0 Newton meters or actively controlled current flow for special operations, it is not possible to directly conclude that freewheeling is enabled or disabled. In this operating state, an existing decision regarding the possibility or impossibility of freewheeling is also advantageous in order to be able to quickly trigger the appropriate error action, namely freewheeling or an active short circuit, in the event of errors, for example.In the case of newton meters (torque) and / or currents deviating from 0, the characteristics of the machine control are preferably traced back, taking the parameters into account, to check whether a current of 0 amperes could be set for a target torque of 0 newton meters. The result of this analysis is then assigned to a target state, preferably using the aforementioned criteria, by enabling and disabling freewheeling mode.

[0016] Preferably, the speed, DC link voltage, and enable of freewheeling mode are monitored, and a relationship is determined and stored, in particular using a machine learning method, in order to enable or disable freewheeling mode even in uncontrolled operation of the electrical machine, in particular in short-circuit operation. By recording the relationship between speed, DC link voltage, and the enable of freewheeling mode, this relationship can be evaluated and taken into account even in uncontrolled operation, since the machine speed and DC link voltage are available independently of the control mode. The evaluation regarding the enable or disable of freewheeling mode remains as described above.This also makes it possible to control the change between freewheeling mode and non-freewheeling mode, for example, short-circuit mode or control mode, and to control the change from non-control mode, in particular short-circuit mode, to control mode. The method can also be coupled with other methods, in particular methods known from the prior art, so that both methods enable a change to freewheeling mode, or only the one that first detects this possibility. The method according to the invention can also be used analogously in combination with known methods for exiting or blocking freewheeling mode.

[0017] The control device according to the invention with the features of claim 10 is characterized in that it is specifically designed to carry out the method according to the invention. This results in the advantages already mentioned.

[0018] The drive device according to the invention with the features of claim 11 is characterized by the control unit according to the invention. This results in the advantages already mentioned. Further advantages and preferred features and combinations of features emerge in particular from the above description and from the drawings.

[0019] The invention will be explained in more detail below with reference to the drawings.

[0020] Figure 1 is an equivalent circuit diagram of an advantageous electric drive device and

[0021] Figure 2 is a simplified flow chart to explain an advantageous operation of the method.

[0022] Figure 1 shows a simplified equivalent circuit diagram of an advantageous electric drive device 1. This device has an electric machine 2 and power electronics 3. The power electronics 3 connects the electric machine 2 to an electrical energy storage device 4. For this purpose, the power electronics 3 has a half-bridge H1, H2 and H3 for each phase U, V, W of the electric machine 2, wherein each half-bridge H1-H3 has two controllable semiconductor switches that are connected in series with one another. Each of the semiconductor switches has a backward-biased diode that is connected in parallel to the respective semiconductor switch. Furthermore, the power electronics 3 has an intermediate circuit 5 with an intermediate circuit capacitor 6. A control unit 7 is connected to the power electronics 3 in order to control the semiconductor switches to apply a desired torque to the electric machine.In this case, the torque is adjusted in a control mode depending on a requested target torque. Control unit 7 is also connected to current sensors S1, S2, and S3, which measure the respective phase currents I u, I lv, and Iw of electric machine 2.

[0023] Figure 2 shows a flow chart based on which an advantageous method for operating the drive device 1 is described below.

[0024] The method begins in step S_1 with the commissioning of drive device 1. In a subsequent step S_2, it is checked whether a torque request is present or not. If a torque request is present, the method continues in step S_3 in normal control mode to generate the target torque by controlling the semiconductor switches.

[0025] However, if there is no torque request or a sailing request (n), a subsequent query S_4 checks whether a current operating value of the electric machine 2, which corresponds to a magnetic field weakening of a magnetic field of the electric machine 2, is within a predetermined tolerance range. If the operating value is outside the tolerance range, the method continues in step S3. However, if the operating value is within the tolerance range (j), the method continues in step S_5 by enabling freewheeling mode of the electric machine in step S_5. However, as soon as the operating value leaves the tolerance range, freewheeling mode is blocked again and the electric machine 2 is preferentially switched to active short-circuit mode.

[0026] In this case, the operating value evaluated is in particular a current operating current, in particular phase current of the electrical machine 2, which is detected by means of the current sensors Su, Sv and / or Sw, and in particular a d-component of the current in a d / q coordinate system is determined as d-current. Preferably, a torque-dependent limit value for the d-current is specified and the freewheeling operation is blocked if the d-current exceeds the limit value for a requested or set torque.

[0027] Instead of considering the d-current, it is also conceivable to determine the current magnetic flux of the electrical machine 2. This is not constant, but fluctuates, for example, due to operating temperature, aging of the electrical machine, etc. An observer approach is preferably used to determine the magnetic flux, which is derived from the d-current, q-current,

[0028] Based on the results of the alternating voltage and the speed, a decision is made as to whether the magnetic flux is low enough to establish a freewheeling state. In the event of a fault in the drive device 1, this allows for faster or earlier switching to freewheeling mode. This transition can be carried out faster and more gently than switching to the active short circuit and is therefore advantageous.

Claims

Claims 1 . Method for operating an electric drive device (1) which has an electric machine (2), power electronics (3) with half-bridges comprising a plurality of semiconductor switches and an intermediate circuit (5), and which is connected or connectable to an electrical energy store (4), wherein, depending on an operating state of the electric machine (2) and on a requested target torque of the electric machine (2), the semiconductor switches are switched into a control mode for generating the target torque, into a short-circuit mode or into a freewheeling mode, characterized in that a current operating value of the electric machine (2), which corresponds to a magnetic field weakening of a magnetic field of the electric machine (2), is detected, and in that the freewheeling mode is enabled or disabled depending on the operating value.

2. Method according to claim 1, characterized in that a current operating current, in particular phase current, of the electrical machine is recorded as the operating value and a d-component of the current is determined as d-current, 3. Method according to one of the preceding claims, characterized in that the current magnetic flux of the electrical machine is determined as a function of the d-current and a q-current of the operating current.

4. Method according to one of the preceding claims, characterized in that the freewheeling operation is released when there is no or only a small d-current for weakening the magnetic field of the electrical machine (2).

5. Method according to claim 4, characterized in that the freewheeling operation is only released when no torque is requested.

6. Method according to one of the preceding claims, characterized in that a torque-dependent limit value for the d-current is specified, and that the freewheeling operation is blocked if the d-current exceeds the limit value for a requested or set torque.

7. Method according to one of the preceding claims, characterized in that the freewheeling operation is blocked if the operating current is not equal to zero or outside a predetermined tolerance range around zero.

8. Method according to one of the preceding claims, characterized in that at an operating point of the electrical machine (2) with a torque and / or an operating current not equal to zero, it is determined whether an operating current of zero could be set at this operating point with a torque of zero Newton meters in order to determine whether freewheeling operation would then be possible.

9. Method according to one of the preceding claims, characterized in that the speed, intermediate circuit voltage and release of the freewheeling operation are monitored and a relationship is determined and stored, in particular with the aid of a machine learning method, in order to enable the freewheeling operation to be released or blocked even in uncontrolled operation of the electrical machine (2), in particular in short-circuit operation.

10. Control device (7) for operating an electric drive device (1) which has an electric machine (2), a power electronics unit (3) with several half-bridges having semiconductor switches and an intermediate circuit (5), and which is connected or connectable to an electrical energy store (4), wherein a control device (7) in dependence on a Operating state of the electrical machine (2) and a requested target torque of the electrical machine (2), the semiconductor switches are switched into a control mode for generating the target torque, into a short-circuit mode or into a free-wheeling mode, characterized in that the control device (7) is specially designed to To carry out a method according to any one of claims 1 to 9.

11. Drive device (1) for a motor vehicle, which comprises an electric machine (2), power electronics (3) with half-bridges comprising a plurality of semiconductor switches and an intermediate circuit (5), and which is connected or connectable to an electrical energy store (4), characterized by a control unit (7) according to claim 10.

Images