Securing a separately excited electrical machine

Abstract

A separately excited electric machine (110) comprises an excitation winding (140) and a bridge circuit (150) with a half-bridge (220). The half-bridge (220) includes an upper (225) and a lower current valve (230) connected in series to a supply voltage, such that a center tap is formed between the current valves (225, 230). The excitation winding (140) can be excited based on a voltage applied to the center tap. A control device (170) for the electric machine (110) is configured to simultaneously close both current valves (225, 230) of the half-bridge (220) to initiate a safe state of the machine (110).

Description

The present invention relates to the securing of an electric machine, preferably on board a motor vehicle. In particular, the invention relates to the bringing of the electric machine into a state in which a conversion between mechanical and electrical energy is interrupted.A motor vehicle may be driven by means of an electric machine. The electric machine comprises a stator and a rotor which are mounted rotatably with respect to one another about a common axis of rotation. A stator winding is provided on the stator, which usually comprises three phases. If the machine is designed as a permanently excited synchronous machine, permanent magnets are mounted on the rotor. An assumed rotational speed and a torque converted by the machine can be controlled by controlling matching alternating currents phase-shifted with respect to one another by the individual phases.The alternating currents can be provided by means of a bridge circuit with current valves on the basis of a driving voltage. If a fault is determined, for example a winding short circuit in the stator winding, the machine can be brought into a safe state by connecting the phases to one another by means of the current valves, which is also known as an active short circuit. Alternatively, each connection of a phase to a different potential can be opened, which can also be referred to as inverter blocking.If the electric machine is constructed as an externally excited machine, a magnetic field is actively generated at the rotor by means of an exciter winding for operation. The exciter winding can be controlled by means of a further bridge circuit. To initiate the safe state, no active short circuit is possible here, because otherwise transient currents can flow which can cause damage to components. An inverter interlock may also be problematic when energy flows from the field winding into a source of the supply voltage. A capacitor in this region can thereby be subjected to an overvoltage and breakdown.It is an object of the invention to provide an improved technique for initiating a safe state on a separately excited electrical machine which is operated by means of a bridge circuit. The invention achieves this object by means of the subject matter of the independent claims. Dependent claims represent preferred embodiments.A separately excited electric machine comprises an excitation winding and a bridge circuit having a half bridge. The half bridge comprises an upper and a lower current valve which are connected in series to a supply voltage, so that a center tap is formed between the current valves. The exciter winding can be excited on the basis of a voltage present at the center tap. A control device for the electric machine is configured to simultaneously close both current valves of the half bridge in order to initiate a safe state of the machine.Current valves of the half bridge are preferably designed as semiconductors, in particular as IGBTs or FETs. A current valve may include a diode through which current may flow in a predetermined direction even when the current valve is opened. The flow valves are usually designed to withstand a high voltage at least briefly. Since in certain operating states an appreciable power loss can occur at the current valves, these are usually sufficiently cooled so that the energy absorbed from the exciter winding can easily be dissipated in the form of heat. The cooling may include liquid cooling, phase change cooling, or a heat sink.By closing the current valves, energy stored in the form of a magnetic flux can be extracted from the exciter winding. This process can also be called deenergizing or deenergizing. As a result of the controlled bridge short circuit, a strong increase in the supply voltage can be prevented.In the region of the bridge circuit, an intermediate circuit capacitor can be provided, at which the supply voltage is present. Without further measures, the intermediate circuit capacitor can absorb energy which flows back from the excitation coil. In this case, a voltage at the intermediate circuit capacitor can rise sharply, with the result that it is damaged. In the worst case, the capacitor may break down, and may be irreversibly damaged. The controlled bridge short circuit allows the capacitor to be discharged, so that the voltage across it is reduced.The control device is preferably configured to simultaneously close the current valves of the half bridge in a first phase and to simultaneously open them in a second phase. The phases may be cyclically controlled at a predetermined frequency and a predetermined duty cycle. Thus, the energy flowing out of the excitation coil can be dissipated in a pulse-like manner. The energy can be absorbed or converted by the combination of the current valves and the intermediate circuit capacitor. Neither of the components can be subjected to excessive stress.A duty cycle of the phases may be selected such that no more than a predetermined maximum power is converted by the flow control valves. The maximum power can be predefined by a design of the flow valve or of a surrounding module. The converted power generally corresponds to a quotient of the energy change and the time. By adjusting the duty cycle, the time during which current flows through the current valves, and thus the converted power, can be varied.The bridge circuit can comprise a plurality of half bridges; the control device further preferably being configured to close both current valves of different half bridges simultaneously one after the other. Thus, the load of conversion of electrical energy into heat can be distributed more effectively to a plurality of components.A break can also be taken between closing different pairs of flow valves in order to give the flow valves the opportunity to dissipate internally occurring heat. The control device can be configured to simultaneously close both flow valves of a first half bridge and open both flow valves of a second half bridge in a first phase; to open the flow valves of both half bridges in a second phase; and to open both flow valves of the first half bridge and to close both flow valves of the second half bridge in a third phase. In the second phase, the current valves of all half bridges comprised by the bridge circuit are preferably opened.Optionally, the control device is configured to open the current valves of both or all half bridges in a fourth phase. The fourth phase may correspond to the second phase. If more than two half-bridges are present, they are activated one after the other and in a predetermined sequence in order to take a short circuit, wherein a phase in which all current valves of the half-bridges are open can be provided in each case between phases in which different half-bridges each control a short circuit.The phases are preferably controlled cyclically. If the bridge circuit comprises two half bridges, four phases can be arranged in succession; if, for example, three half bridges are provided, six phases can be controlled. A phase in which all the flow valves are open can turn out to be as short as desired and thus virtually eliminated, if required.The control device may be configured to open all current valves of the bridge circuit when an energy stored in the exciter winding has dropped below a predetermined threshold value. If the exciter winding is sufficiently de-energized, a stator and a rotor of the electric machine can no longer be magnetically coupled. A voltage can then no longer be induced by a rotation of the rotor and, conversely, a current flow through a stator winding cannot cause a rotation. The machine can then be in the safe state. In this case, it can substantially behave like an asynchronous machine. The energy present in the field winding can be determined by determining, starting from an energy stored in the field winding during normal operation of the machine, the energy drawn from it by the bridge short circuit.By terminating the bridge short circuit as early as possible after a sufficient energy dissipation from the excitation coil, it can be ensured that not unnecessarily electrical energy from the source of the supply voltage is converted into heat.The electric machine may comprise a power winding, which is usually mounted on a stator and may then also be called a stator winding. In this case, a further bridge circuit having a half bridge can be provided; wherein the half bridge comprises an upper and a lower current valve which are connected in series to a driving voltage, such that a center tap is formed between the current valves, and wherein a current through the power winding can be controlled on the basis of a voltage present at the center tap. The control device is further preferably configured to simultaneously close both current valves of the half bridge of the further bridge circuit in order to initiate a safe state of the machine.Processes carried out herein with respect to the field winding can therefore also be carried out accordingly on the power winding. Significantly higher currents may flow in the power winding during operation of the machine than in the field winding and a voltage applied to the power winding may be significantly higher than that at the field winding, however, an inductance of the power winding is usually also significantly lower than an inductance of the field winding. De-excitation of the power winding can therefore be carried out substantially more quickly than de-excitation of the exciter winding. If magnetic energy is extracted from both windings, the machine can be brought into the safe state in an improved manner.An electric final drive for a motor vehicle comprises an electric machine described herein and a control device described herein. The final drive can additionally comprise a transmission, preferably a shift transmission, which can be shifted automatically or manually.A motor vehicle includes an electric final drive and / or a control device described herein. The motor vehicle is preferably drivable by means of the electric machine and further preferably comprises a motorcycle, a passenger car, a truck or an bus.A method for bringing a separately excited electric machine described herein into a safe state comprises simultaneously closing both current valves of the half bridge.The method may be carried out by means of a control device described herein. For this purpose, the control device can comprise a processing device which is configured to execute a method described herein wholly or partly. For this purpose, the processing device can be electronically embodied and comprise a programmable microcomputer or microcontroller, and the method can be present in the form of a computer program product having program code means. The computer program product can also be stored on a computer-readable data carrier. Features or advantages of the method can be transferred to the device or vice versa.The invention will now be described in more detail with reference to the accompanying figures, in which: FIG. 1 shows a system on board a motor vehicle; FIG. 2 shows a control for a rotor winding FIG. 3 shows a bridge circuit for a stator winding; and FIG. 4 shows a flow diagram of a methodis.FIG. 1 shows a system 100 on board a motor vehicle 105. The system 100 includes an electric machine 110 having a stator 115 and a rotor 120. The electric machine 110 is preferably configured as a separately excited synchronous machine and is further preferably configured to drive the motor vehicle 105. In motor vehicle 105, an electrical energy store 125 is provided, which is usually formed by a high-voltage battery. The energy store 125 can provide a supply voltage of several 100 volts.An exciter winding 140 is provided on the rotor 120 of the electric machine 110, and a power or stator winding 145 is provided on the stator 115. For controlling the exciter winding 140, a first bridge circuit 150 is provided, which is fed from the electrical energy store 125 here. In another embodiment, the first bridge circuit may also be fed from another source. For controlling the stator winding 145, a second bridge circuit 150 is provided, which is here likewise fed from the energy source 125. On the side of the energy source 125, a first intermediate circuit capacitor 160 is provided on the first bridge circuit 150 and a second intermediate circuit capacitor 165 is provided on the second bridge circuit 155.Both bridge circuits 140, 145 can be controlled by means of a common control device 170. An alternative embodiment with separate control devices 160 is also possible. The first bridge circuit 150 may be configured to cause a predetermined direct current through the field winding 140. The second bridge circuit 155 can be configured to provide three alternating voltages phase-shifted by 120° with respect to one another. Voltages and frequencies of the AC voltages may control a torque converted by the machine 110, a speed, and / or a direction of rotation.FIG. 2 shows a control 200 for a rotor winding 140. A section 205 shown on the left in FIG. 2 is fixed to the stator, and a section 210 shown on the right is fixed to the rotor. A transformer 215 is provided between the sections 205, 210, which transformer comprises two windings, one of which is assigned to the different sections 205, 210 in each case. In another embodiment, a transmission of electrical energy between the stator 115 and the rotor 120 can also take place differently, for example by means of a sliding contact. The transformer 215 can then be stationary on one side or can be omitted altogether.The first bridge circuit 150 is situated between the energy store 125 and the stator-fixed winding of the transformer 115 and comprises two half bridges 220, which each comprise an upper current valve 225 and a lower current valve 230. The current valves 225, 230 of a half bridge 220 are connected in series as shown, so that a center tap is formed between them. The stator-fixed winding of the transformer 115 is connected between the center taps of the two half bridges 220. The half bridges 220 are connected to the supply voltage of the energy store 125 and are connected in parallel to the first intermediate circuit capacitor 160.The control device 170 can switch on and off two current valves 225, 230 of the same half bridge 220 alternately in each case, with the result that the center tap of the half bridge 220 is connected alternately to a high and a low potential of the supply voltage. A potential at the center tap can be controlled by controlling a duty cycle with which the current valves 225, 230 of the half bridge are opened and closed, respectively. By suitably varying the duty cycle, a substantially sinusoidally varying potential can thus be effected. By driving the half bridges 220 to assume alternating potentials phase-shifted by 180° with respect to one another, an alternating voltage can be controlled between the center taps, which alternating voltage acts on the stator-fixed winding of the transformer 215.The transformer 215 converts the alternating voltage present at the stator-side winding into an alternating voltage present at the rotor-side winding of the same frequency. A rectifier circuit, which is formed here in a known manner by four diodes, forms a series of half oscillations from the alternating voltage, which are smoothed by the inductance of the exciter winding 140, so that an approximately direct voltage is established at the exciter winding 140.The field winding 140 forms a magnetic field due to the current flowing through it, which magnetic field allows a magnetic coupling between the stator 115 and the rotor 120. If the stator winding 145 is now controlled with phase-shifted alternating voltages, the rotor 120 can be set in rotation with respect to the stator 115. The driving of the stator winding 145 is described in more detail herein with reference to FIG. 3.In order to put the machine 110 or the system 100 into a safe state, the magnetic coupling between the stator 115 and the rotor 120 can be canceled. For this purpose, the current valves 225 and 230 of both half bridges 220 of the first bridge circuit 150 can first be opened, so that the stator-side winding of the transformer 215 is separated from the supply voltage and no current any longer flows through the exciter winding 140. Simultaneously, current valves 225, 230 of the second bridge circuit 155 can be opened.However, until the rotor 120 and the inductance of the stator 115 are magnetically discharged, energy can still be fed back into the first intermediate circuit capacitor 160 as in the case of a permanently excited synchronous machine, with the result that its voltage can increase very rapidly. In order to prevent overloading of the first intermediate circuit capacitor 160, it is proposed to close two current valves 225, 230 of the same half bridge 220 of the first bridge circuit 150 at the same time for a short time and thus convert electrical energy from the transformer 215 or the first intermediate circuit capacitor 160 into heat. The first intermediate circuit capacitor 160 can thereby be discharged, so that damage due to an overvoltage can be prevented.In various embodiments, the current valves 225, 230 of only one half bridge 220 or of both half bridges 220 can be closed simultaneously. In a further embodiment, the active short circuit of the first intermediate circuit capacitor 160 can also be produced alternately by both half bridges 220. Optionally, times during which the half-bridges 220 each produce a short circuit may overlap.FIG. 3 shows a second bridge circuit 155 for the stator winding 145 of the electric machine 110. The second bridge circuit 155 is qualitatively similar to the first bridge circuit 150, but includes three instead of two half bridges 220 each including an upper and a lower current valve 225, 230. The control device 170 is configured to open and close the current valves 225, 230 of the half bridges 220 alternately in pairs for operation of the electric machine 110 in such a way that a predetermined potential or a predetermined voltage is established in each case at the center taps. Three voltages provided by the three half bridges 220 are applied to three phases of the stator winding 145. The voltages are sinusoidal, 120° out of phase, and form a rotating magnetic field at the stator 115 that is coupled to the magnetic field of the field winding 140 so that the rotor 120 is driven with respect to the stator 115.The second bridge circuit 155 is usually dimensioned differently than the first bridge circuit 150 because a greater current can flow through the stator winding 145 than through the exciter winding 140. The flow valves 225, 230 can be designed for this greater output and can be thermally connected to a cooling device of suitable dimensions.In order to bring the electric machine 110 into a safe state, the flow valves 225, 230 can be controlled to all open. Alternatively, only the upper flow valves 225 or only the lower flow valves 230 may be closed and the respective other flow valves 225, 230 opened, which may be referred to as an active short circuit of the electric machine 110.Prior to such a measure, energy stored in a magnetic field of the stator winding 145 can be converted into heat by closing two current valves 225, 230 of at least one half bridge 220 for a short time. The procedure and reasoning correspond qualitatively to the relationships explained with reference to FIG. 2. In order not to overload the current valves 225, 230 thermally or electrically, the current valves 225, 230 of different half bridges 220 can be activated successively to form a bridge short circuit. After the opening of the current valves 225, 230 of a first half bridge 220 and before the closing of the current valves 225, 230 of a second half bridge 220, the bridge short circuit can be briefly suspended by opening all current valves 225, 230. Overlapping operation of the half bridges 220 may also be controlled.FIG. 4 shows a flow diagram of a method 400 for controlling an electric machine 110. The method can be carried out by means of the control device 170.In a step 405, a control specification for the electric machine 110 is detected. In a step 425, it may be determined that the machine 110 is to be controlled in a driving mode 410. The control specification can indicate in which direction and / or at which rotational speed the rotor 120 is to rotate with respect to the stator 115 and how large a torque is to be provided.In this case, in a step 415, the exciter winding 140 can be energized and, in a step 420, a predetermined travel current in the form of phase-shifted alternating currents can be controlled through the stator winding 145.However, in step 405, a different control specification can also be determined. Thus, in a step 425, it can be determined that the electric machine 110 is to be brought into a safe state, for example because a defect on the system 100 or an accident of the motor vehicle 105 has been determined. For this purpose, the conversion between electric current and torque at the electric machine 110 is to be canceled as quickly as possible.In order to deenergize the exciter winding 140, all current valves 225, 230 of the first bridge circuit 150 can be opened in a step 430. After a predetermined duration, which may be a few tens to a few hundreds of nanoseconds in an exemplary embodiment, both current valves 225, 230 of the same half bridge 220 of the first bridge circuit 150 may be closed in a step 435 in order to dissipate electrical energy in the region of the first intermediate circuit capacitor 160. Optionally, both current valves 225, 230 of both half bridges 220 can also be closed in each case. The flow valves 225, 230 may be maintained closed for another predetermined duration.In a step 440, it can be checked whether the field winding 140 is already sufficiently de-energized. For this purpose, a remaining magnetic energy of the field winding 140 can be determined, for example, on the basis of a model. Alternatively, it can be determined whether a voltage of the first intermediate circuit capacitor 160 is still above a predetermined threshold value. If the energy is still above a predetermined threshold value, then the system can return to step 430.If only one of the half bridges 220 has been completely closed beforehand, the bridge short circuit can be canceled again in step 430 and can be re-established in step 435. In this case, the same half bridge 220 as before can be closed. Preferably, however, the next bridge short circuit is controlled by means of the other half bridge 220 in order to distribute a thermal load caused by the current flow as uniformly as possible to both half bridges 220. When steps 430, 435 are passed through a plurality of times, the half bridges 220 can alternately bring about the bridge short circuit at the first intermediate circuit capacitor 160. In one variant, closing times of the half-bridges 220 can overlap, so that in the event of a change, the still closed half-bridge 220 is only opened when the other half-bridge 220 is already closed. If a sufficient energy dissipation is determined in step 440, it can be determined in a step 470 that the safe state has been reached.In parallel with steps 430 to 440, steps 450 to 460 corresponding thereto may be executed to dissipate magnetic energy from the stator winding 145. In step 450, all of the current valves of the second bridge circuit 155 may be opened. In step 455, current valves of the half bridges of the second bridge circuit 155 can be controlled to temporarily bring about an active short circuit if the electric machine 115 and the second bridge circuit 155 are configured to decrease currents, voltages or temperatures occurring in the process in a compatible manner. In step 460, it may be determined whether an energy stored in the winding of the stator 115 has already sufficiently decreased. If this is not the case, this part of the method 400 can return to step 450. Otherwise, the secure state may be reached in step 470.In one embodiment, the safe state may be considered to be reached when the energy in at least one of the windings 140, 145 is sufficiently low. In another embodiment, the safe state may be deemed to be taken only when both windings 140, 145 contain sufficiently low energy.It should be noted that details, merits, or approaches described herein regarding de-energizing one of the windings 140, 145 via the associated bridge circuit 150, 155 may be transferred to de-energizing the respective other winding 140, 145 via the other bridge circuit 150, 155.Reference numerals denote reference numerals100 Control device 105 Motor vehicle 110 Electric machine 115 Stator 120 Rotor 125 Energy store 140 Exciter winding 145 Stator winding 150 First bridge circuit (for rotor winding) 155 Second bridge circuit (for stator winding) 160 First intermediate circuit capacitor 165 Second intermediate circuit capacitor 170 Control device 200 Control 205 Stator-fixed section 210 Rotor-fixed section 215 Transformer 220 Half bridge 225 Upper current valve 230 Lower current valve 235 Rectifier circuit 400 Method 405 Control specification capture 410 Specification for driving operation 415 Rotor excitation 420 Driving current by stator winding control 425 Specification for safe state 430 Half bridges of the rotor controller open 435 Half bridge short-circuiting 440 Winding energy sufficiently dropped? 450 Half bridges Stator controller open 455 Half bridge short-circuiting 460 Winding energy sufficiently dropped? 470 Safe state reached

Claims

Control device (170) for a separately excited electrical machine (110), wherein the electrical machine (110) comprises an excitation winding (140); wherein a bridge circuit (150) with a half bridge (220) is provided; wherein the half bridge (220) comprises an upper and a lower current valve (225, 230) which are connected in series to a supply voltage, such that a center tap is formed between the current valves (225, 230); wherein the excitation winding (140) can be excited on the basis of a voltage present at the center tap; wherein the control device (170) is configured to simultaneously close both current valves (225, 230) of the half bridge (220) in order to initiate a safe state of the machine (110).Control device (170) according to Claim 1, wherein an intermediate circuit capacitor is provided in the region of the bridge circuit (150), at which the supply voltage is present.Control device (170) according to Claim 1 or 2, wherein the control device (170) is configured to simultaneously close the current valves (225, 230) of the half bridge (220) in a first phase and to simultaneously open them in a second phase.The control device (170) of claim 3, wherein a duty cycle of the phases is selected such that no more than a predetermined maximum power is converted by the flow control valves (225, 230).Control device (170) according to one of the preceding claims, wherein the bridge circuit (150) comprises a plurality of half bridges (220); wherein the control device (170) is configured to close each of the two current valves (225, 230) of different half bridges (220) simultaneously in succession.Control device (170) according to claim 5, wherein the control device (170) is configured to simultaneously close both flow valves (225, 230) of a first half bridge (220) and to open both flow valves (225, 230) of a second half bridge (220) in a first phase; to open both flow valves (225, 230) of both half bridges (220) in a second phase; and to open both flow valves (225, 230) of the first half bridge (220) and to close both flow valves (225, 230) of the second half bridge (220) in a third phase.Control device (170) according to claim 6, wherein the control device (170) is configured to open the current valves (225, 230) of both half bridges (220) in a fourth phase.Control device (170) according to one of the preceding claims, wherein the control device (170) is configured to open all current valves (225, 230) of the bridge circuit (150) when an energy stored in the exciter winding (140) has dropped below a predetermined threshold value.Control device (170) according to one of the preceding claims, wherein the electric machine (110) comprises a power winding (145); wherein a further bridge circuit (155) is provided with a half bridge (220); wherein the half bridge (220) comprises an upper (225) and a lower current valve (230) which are connected in series to a driving voltage, such that a center tap is formed between the current valves (225, 230); wherein a current through the power winding (145) can be controlled on the basis of a voltage connected to the center tap; wherein the control device (170) is configured to simultaneously close both current valves (225, 230) of the half bridge (220) in order to initiate a safe state of the machine (110).An electric final drive for a motor vehicle (105), the final drive comprising an electric machine (110) and a control device (170) according to any of the preceding claims.Motor vehicle (105) comprising an electric final drive according to Claim 10.Method (400) for bringing an externally excited electric machine (110) into a safe state, wherein the electric machine (110) comprises an excitation winding (140); wherein a bridge circuit (150) with a half bridge (220) is provided; wherein the half bridge (220) comprises an upper (225) and a lower current valve (230) which are connected in series to a supply voltage, such that a center tap is formed between the current valves (225, 230); wherein the excitation winding (140) can be excited on the basis of a voltage present at the center tap; wherein the method comprises simultaneously closing (435) both current valves (225, 230) of the half bridge (220).

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