CONTROL DEVICE FOR AN INVERTER, INVERTER FOR AN ASYNCHRONOUS MACHINE, VEHICLE AND METHOD FOR OPERATING AN INVERTER

Abstract

A control device (2) for an inverter (1) which has a DC voltage input (3) and a power unit (5) with three half-bridges (11u, 11v, 11w) each formed by two power switching elements (13u, 13v, 13w, 15u, 15v, 15w), the control device (2) being arranged to driving the power switching elements (13u, 13v, 13w, 15u, 15v, 15w) in a normal operating mode for converting a DC voltage applied to the DC voltage input (3) into a polyphase AC current provided at an AC current output (4), wherein the control means (2) is adapted to evaluate a signal state of a signal (21) indicating a disconnection of a DC voltage source (9) from the DC voltage input (3) and to control the power switching elements (13u, 13v, 13w, 15u, 15v, 15w) in dependence on a result of the evaluation for alternately adopting a first switching pattern causing DC braking and a second switching pattern causing freewheeling.

Claims

1. Control device (2) for an inverter (1) which has a DC voltage input (3) and a power unit (5) with three half-bridges (11u, 11v, 11w) each formed by two power switching elements (13u, 13v, 13w, 15u, 15v, 15w), the control device (2) being arranged to drive the power switching elements (13u, 13v, 13w, 15u, 15v, 15w) in a normal operating mode for converting a DC voltage applied to the DC voltage input (3) into a polyphase AC current provided at an AC current output (4), characterized in that the control device (2) is adapted to evaluate a signal state of a signal (21) indicating a disconnection of a DC voltage source (9) from the DC voltage input (3), and to control the power switching elements (13u, 13v, 13w, 15u, 15v, 15w) as a function of a result of the evaluation for alternately adopting a first switching pattern which effects DC braking and a second switching pattern which effects freewheeling.

2. Control device according to claim 1, which is further arranged to evaluate, in addition to the signal state, whether the inverter (1) is in a recuperation mode.

3. Control device according to claim 1, which is further arranged to switch from the first switching pattern to the second switching pattern after a predetermined period of time has elapsed and/or to switch from the second switching pattern to the first switching pattern after a predetermined period of time has elapsed.

4. Control device according to claim 1, which is further adapted to control the alternate driving in dependence on a voltage value describing a voltage at the DC voltage input (3).

5. Control means according to claim 4, which is further adapted to switch from the first switching pattern to the second switching pattern upon detection of reaching of a voltage threshold value by the voltage value and/or to switch from the second switching pattern to the first switching pattern upon detection of reaching of a voltage threshold value by the voltage value.

6. Control means according to claim 4, which is further adapted to permanently drive the power switching elements to adopt a switching pattern effecting a safe operating condition when the voltage value does not exceed a predetermined voltage threshold when driving the power switching elements (13u, 13v, 13w, 15u, 15v, 15w) with the second switching pattern.

7. Control device according to claim 6, wherein the switching pattern causing the safe operating condition is the second switching pattern.

8. Control device according to claim 1, which is further arranged to determine the first switching pattern in dependence on at least one current value describing a current flowing at the AC voltage output (4).

9. Control device according to claim 8, wherein the first switching pattern describes switching a power switching element (13u, 13v, 13w) connected to a high potential (12) of the DC input to a conductive state when a current from the center tap of the half-bridge (11u, 11v, 11w) formed by the power switching element (13u, 13v, 13w) is positive and a change of the current after time is negative.

10. Control device according to claim 9, wherein the switching pattern describes the switching of the remaining power switching elements (13u, 13v, 13w) connected to the high potential (12) of the DC voltage input (3) to a blocking state.

11. Control device according to claim 8, wherein the first switching pattern describes switching a power switching element (15u, 15v, 15w) connected to the low potential (14) of the DC input (3) to a conductive state when a current from the center tap of the half-bridge (11u, 11v, 11w) formed by the power switching element (15u, 15v, 15w) is negative and a change in the current after time is positive.

12. Control device according to claim 11, wherein the first switching pattern describes switching the remaining power switching elements (15u, 15v, 15w) connected to the low potential (14) of the DC input (3) to a blocking state.

13. Inverter (1) for an asynchronous machine (10), comprising a DC input (3), a power unit (5) with three half-bridges (11u, 11v, 11w) formed by two power switching elements (13u, 13v, 13w, 15u, 15v, 15w) each, and a control device (2) according to claim 1.

14. Vehicle (25) comprising an asynchronous machine (10) for driving the vehicle (25) and an inverter (1) according to claim 13.

15. Method of operating an inverter (1) which has a DC input (3) and a power unit (5) having three half-bridges (11u, 11v, 11w) formed by two power switching elements (13u, 13v, 13w, 15u, 15v, 15w) each, comprising the following steps: Driving the power switching elements (13u, 13v, 13w, 15u, 15v, 15w) in a normal operating mode to convert a DC voltage applied to the DC input (3) into a polyphase AC current provided at an AC output (4); evaluating a signal state of a signal (22) indicating a disconnection of a DC voltage source (9) from the DC voltage input (3); and driving the power switching elements (13u, 13v, 13w, 15u, 15v, 15w) in dependence on a result of the evaluation to alternately adopt a first switching pattern that causes DC braking and a second switching pattern that causes freewheeling.

Description

[0028] Further advantages and details of the present invention will be apparent from the embodiments described below and from the drawings. These are schematic representations and show:

[0029] FIG. 1 a circuit diagram of an embodiment of the inverter according to the invention with an embodiment of the control device according to the invention;

[0030] FIG. 2 curves of voltage at a DC input, phase currents at an AC output, and torque over time during operation of the inverter shown in FIG. 1; and

[0031] FIG. 3 a schematic sketch of a vehicle according to the invention.

[0032] FIG. 1 is a circuit diagram of an embodiment of an inverter 1 with an embodiment of a control device 2. In addition, the inverter comprises a DC voltage input 3, an AC voltage output 4, a power unit 5, and a DC link capacitor 6 connected in parallel with the DC voltage input 3. A voltage detection unit 7 is further provided for detecting a DC voltage U applied across the DC link capacitor 6 or at the DC voltage input 3.

[0033] The DC voltage input 3 is connected to a DC voltage source 9 in the form of a high-voltage battery via a two-pole isolating device 8 formed by contactors, which provides the DC voltage U at the DC voltage input 3 when the isolating device 8 is closed. This is converted by the inverter 1 into a polyphase, here three-phase, AC voltage provided at its AC output 4. An electrical machine in the form of an asynchronous machine 10 is connected to this output.

[0034] The power unit 5 comprises three half-bridges 11u, 11v, 11w, each formed by a series connection of power switching elements 13u, 13v, 13w connected to an upper potential 12 of the DC voltage input 3 and power switching elements 15u, 15v, 15w connected to a low potential 14 of the DC voltage input 3. Each power switching element 13u, 13v, 13w, 15u, 15v, 15w comprises an insulated gate bipolar transistor (IGBT) 16 and a diode 17 connected in parallel therewith. Alternatively, a respective power switching element 13u, 13v, 13w, 15u, 15v, 15w may be implemented by a power MOSFET. A center tap 18 of a respective half-bridge 11u, 11v, 11w is connected to the AC voltage output 4, at which phase currents Iu, .sub.Iv, Iw can be provided for the asynchronous machine 10.

[0035] The control device 2 is arranged to convert the power switching elements 13u, 13v, 13w, 15u, 15v, 15w in a normal, clocked operating mode for converting the DC voltage U applied to the DC voltage input 3 into the polyphase AC current. In the normal operating mode, recuperation operation is also possible, in which the electric machine 10 is operated as a generator and feeds electrical energy back to the high-voltage battery. For control, the control device 2 is connected to a control input 19 of a respective power switching element 13u, 13v, 13w, 15u, 15v, 15w.

[0036] In the event of a fault condition detected by an external control device 20, a load shedding is initiated by the control device providing a signal 21 which triggers the disconnecting device 8 to open the contactors and thus disconnect the DC voltage source 9 from the DC voltage input 3. The signal 21, which accordingly indicates the disconnection of the DC voltage source 9 from the DC voltage input 3, is further present at an input 22 of the control device 2. The signal 21 is alternatively or additionally generated internally in the control device 2 when the latter itself detects a load shedding.

[0037] The control unit 2 evaluates whether the signal 21 is present and whether the inverter 1 is in recuperation mode at that time. If this is the case, the control unit 2 terminates the normal operation mode and initiates a load shedding operation mode in which it controls the power switching elements 13u, 13v, 13w, 15u, 15v, 15w to alternately adopt a first switching pattern that causes DC braking and a second switching pattern that causes freewheeling.

[0038] For this purpose, the control device 2 determines the first switching pattern which causes the DC braking as a function of current values which describe the phase currents I.sub.u, .sub.Iv, Iw. For this purpose, a look-up table is stored within the control device 2, which assigns corresponding switching states of the power switching elements 13u, 13v, 13w, 15u, 15v, 15w to the currents Iu, .sub.Ivv, Iw and their derivatives after the time dlu/dt, dlv/dt, dlw/dt at the moment of termination of the clocked operation. The following table shows this assignment:

TABLE-US-00001 Switching state power switching element I.sub.u [00001]$\frac{{\mathrm{dI}}_u}{dt}$ I.sub.v [00002]$\frac{{\mathrm{dI}}_v}{dt}$ I.sub.w [00003]$\frac{{\mathrm{dI}}_w}{dt}$ 13u 13v 13w 15u 15v 15w >0 <0 <0 >0 — — 1 0 0 0 1 0 >0 <0 — — <0 >0 1 0 0 0 0 1 <0 >0 >0 <0 — — 0 1 0 1 0 0 — — >0 <0 <0 >0 0 1 0 0 0 1 <0 >0 — — >0 <0 0 0 1 1 0 0 — — <0 >0 >0 <0 0 0 1 0 1 0

[0039] Here, a “1” means that the respective switching element 13u, 13v, 13w, 15u, 15v, 15w is controlled for conducting, and a “0” means that it is controlled for blocking. The second switching pattern realizing the freewheel describes that all power switching elements 13u, 13v, 13w, 15u, 15v, 15w are driven for blocking.

[0040] The alternate switching between the first switching pattern and the second switching pattern is time-controlled in the present embodiment. That is, the control device 2 controls the power switching elements 13u, 13v, 13w, 15u, 15v, 15w in each case for a predetermined period of time in accordance with the first switching pattern or the second switching pattern.

[0041] This alternating switching is terminated when the DC voltage U does not exceed a predetermined voltage threshold during switching of the second switching pattern, i.e. the freewheel. For this purpose, the control device 2 evaluates voltage values which are provided to it by the voltage detection unit 7 at an input 23. After completion of the alternate switching, the control device 2 controls the power switching elements 13u, 13v, 13w, 15u, 15v, 15w permanently according to the second switching pattern, which realizes a safe operating state.

[0042] FIG. 2 shows curves of the DC voltage U, the phase currents Iu, iv, Iw and a resulting torque M of the asynchronous machine 10 over time t in an exemplary configuration of the inverter 1. At a time t0, load shedding occurs and the control device 2 transfers the inverter from the normal operating mode to the load shedding operating mode.

[0043] Before time t0, the asynchronous machine 10 is in recuperation mode, which can be seen from the negative torque M, the essentially harmonic course of the phase currents Iu, .sub.Iv, Iw and the only slightly oscillating course of the dc voltage U. At time t0, the phase current Iu has just passed its maximum. The phase current .sub.Iv is just before the zero crossing and increases. It is therefore valid:

[00004]$I_u>0⩓\frac{{\mathrm{dI}}_u}{\mathrm{dt}}<0⩓I_v<0⩓\frac{{\mathrm{dI}}_v}{\mathrm{dt}}>0$

[0044] Consequently, the control device 2 determines the first switching pattern for DC braking according to the table shown previously, in which the power switching elements 13u, 15v conduct and the other power switching elements 13v, 13w, 15u, 15w block. The control device 2 then controls the power switching elements 13u, 13v, 13w, 15u, 15v, 15w according to the determined first switching pattern for a predetermined duration. Subsequently, the control device 2 controls all power switching elements 13u, 13v, 13w, 15u, 15v, 15w for a predetermined duration for blocking, i.e. in accordance with the second switching pattern that implements freewheeling.

[0045] Switching alternates between the two switching patterns until the DC voltage U no longer exceeds a voltage threshold value, which is indicated by a dashed line 24 in FIG. 2, at a time t1. From time t1, the power switching elements 13u, 13v, 13w, 15u, 15v, 15w are permanently driven according to the second switching pattern. As can further be seen from FIG. 2, the phase currents Iu, .sub.Iv, Iw decay between the times t0 and t1 without large current peaks and there is only a slight change in the torque M for a short time.

[0046] According to a further embodiment, the alternating switching between the first and the second switching pattern is not time-controlled, but voltage-controlled. In this case, the control device 2 switches between the switching patterns in each case when the voltage value provided by the voltage detection unit 7 reaches a voltage threshold value defined for a respective switching pattern.

[0047] FIG. 3 is a schematic sketch of an embodiment of a vehicle 25 comprising an inverter 1 according to one of the embodiments described above. Analogously to FIG. 1, the inverter 1 is connected via a disconnecting device 8 to a DC voltage source 9, to an asynchronous machine 10, which is set up to drive the vehicle 25, and to a higher-level control unit 20.