Excitation current circuit having hybrid full bridge for controlled excitation and de-excitation of an excitation winding

Abstract

The invention relates to an excitation current circuit (ES) for controlling the excitation of an excitation winding (EW), the circuit having two connections (A1, A2) for an excitation winding (EW) of an externally excited electric machine (EM), and two supply inputs (V+, V-). The excitation current circuit (ES) is also equipped with a first half bridge (H1) and a second half bridge (H2), the ends of which are connected to the supply inputs (V+, V-) and the connection points of which are connected to the connections (A1, A2). The first half bridge has a series circuit of a diode (D) and a first semiconductor switch (S1). The second half bridge has a series circuit of a second and a third semiconductor switch (S2, S3). The invention further relates to a correspondingly equipped control device (ST) and to a correspondingly equipped electric vehicle traction drive.

Description

[0001] Description

[0002] Excitation current circuit with hybrid full bridge for controlled excitation and de-excitation of an excitation winding

[0003] Electric vehicles consist of an electric motor and an associated control unit that generates the current for the motor to produce a rotating magnetic field. The electric motor (in the case of internal rotor motors) has a stator, which is energized by the control unit, and a magnetic rotor. The rotor can contain a permanent magnet; however, achieving high flux densities with permanent magnets requires expensive materials such as rare-earth compounds, and they also lose their magnetization at high temperatures.

[0004] Separately excited electric machines can also be used in the drive system, in which the rotor has a winding that generates the desired magnetic field. For this purpose, this winding must be energized, advantageously providing the additional possibility of further control intervention. However, this winding, referred to as the excitation winding, naturally exhibits an inductance that negatively affects the dynamics of the excitation current control. It is an object of the invention to demonstrate a method by which the excitation of a separately excited machine can be adjusted with sufficient precision even during dynamic control interventions.

[0005] This problem is solved by the subject matter of claim 1. Further properties, features, embodiments, applications and advantages become apparent with the dependent claims, the description and the figure.

[0006] An excitation current circuit is proposed in which the excitation winding forms the transverse element of a full bridge circuit. One of the two half-bridges is fully switchable, i.e., with two series switches, while the other half-bridge is semi-switchable, namely with one switch and a diode connected in series with it. This allows for the cost-effective activation of the excitation winding by switching on the relevant switches, thus connecting the excitation winding to two supply potentials (supply inputs). The de-excitation can be controlled (and, in particular, actively) by appropriately operating the third switch, optionally with pulses. The de-excitation current can then flow through the cost-effective diode and the third semiconductor switch, with the de-excitation current being assisted by the opposite-polarity connection of the rectification winding to the supply potentials.The magnitude of the excitation current can then exceed the magnitude of an excitation current that would occur with a simple short circuit of the excitation winding. The excitation current circuit is thus equipped with a hybrid full bridge, in which one of the two half-bridges has different current control elements, namely a semiconductor switch and a diode, which is why this half-bridge and the full bridge can be described as hybrid.

[0007] The proposed excitation circuit is used to control the excitation of the field winding of a separately excited electric machine. The circuit has two terminals for connecting the two ends of the field winding. The excitation circuit has two supply inputs for two different supply potentials of a supply voltage, which is typically a DC voltage. The excitation circuit comprises a first half-bridge and a second half-bridge. The two half-bridges form a full bridge circuit; the field winding, or rather the relevant terminals of the circuit, form the cross-connection of this full bridge. The ends of the half-bridges are connected to each other. Each half-bridge has a first end and a second end. The first ends are connected to each other and, in particular, also to a first supply input, such as a positive potential.The second ends are connected to each other and, in particular, also to a second supply input, such as a negative potential. A voltage source (DC voltage) is preferably connected between the positive and negative potentials, either directly or via a resistor, a voltage converter, or similar device. The ends of the half-bridges are thus connected to the supply inputs or to the two supply potentials.

[0008] Each half-bridge has two elements (two switches or one switch and one diode), with the two elements of each half-bridge connected via a junction point. These junction points form the connection for the two ends of the excitation winding. The junction points are connected to terminals that can also be referred to as the circuit-side excitation winding terminals. The first half-bridge features a series connection of a diode and a first semiconductor switch. The two elements of the first half-bridge are therefore the diode and the first semiconductor switch. The first half-bridge can be described as a semi-controlled half-bridge, since only the switch, but not the diode, can be controlled by an external switching signal. The second half-bridge features a series connection of a second and a third semiconductor switch. The two elements of the second half-bridge are therefore the second and the third switch.The switches are designed as semiconductor switches, in particular as transistors such as IGBTs or MOSFETs. The semiconductor switches can also be designed as a combination of two (or more than two) transistor elements, which are connected in anti-series configuration.

[0009] Preferably, the excitation current is passed through the second semiconductor switch, i.e., through the semiconductor switch of the second half-bridge. This is preferably connected to a first of the supply terminals (in particular the positive one). This connection is particularly switch-free. The excitation current is further passed through the first semiconductor switch, i.e., through the semiconductor switch of the first half-bridge. This is preferably connected to a second of the supply terminals (in particular the negative one). This connection is particularly switch-free. The terminals for the excitation winding are thus connected to semiconductor switches of different half-bridges, in particular to semiconductor switches that lead to different supply inputs.

[0010] The demagnetization current, i.e., the current impressed into the excitation winding to assist and / or control its demagnetization, passes through the diode, which preferably connects the first supply terminal to one of the excitation winding terminals. This connection is, in particular, switch-free. The demagnetization current also passes through the third semiconductor switch, which connects the other terminal provided for the excitation winding to the second supply terminal. This connection is also, in particular, switch-free. The diode and the third semiconductor switch are arranged in different half-bridges. The diode and the third semiconductor switch are connected to different supply potentials or supply inputs. The supply inputs are configured to be connected to a voltage source.The voltage direction of this voltage points in the same direction as the voltage that arises at the terminals during demagnetization of the excitation winding. The voltage of the voltage source adds to the voltage that arises during demagnetization, thereby increasing the demagnetization current (compared to a short circuit of the excitation winding); the time constant of the demagnetization is thereby reduced (compared to a short circuit of the excitation winding).

[0011] The first and second semiconductor switches are connected to different supply inputs, i.e., different supply potentials. The first and second semiconductor switches are positioned diagonally to each other in the full bridge and are connected via their terminals (intended for the excitation winding). One end of the first semiconductor switch is connected to one supply terminal. One end of the second semiconductor switch is connected to the other supply terminal. These first ends can be referred to as the outer ends. The second ends of each of these semiconductor switches (i.e., the inner ends) are connected to the two terminals intended for the excitation winding. The second end of the first semiconductor switch is also connected to the diode.The diode connects the second end of the first semiconductor switch to the supply terminal opposite the supply terminal to which the first end of the first semiconductor switch is connected. The first and second semiconductor switches are connected to different terminals for the excitation winding.

[0012] The second end of the second semiconductor switch is connected to the third semiconductor switch. Via the third semiconductor switch, the second (inner) end of the second semiconductor switch is connected to the supply terminal opposite the supply terminal to which the first (outer) end of the third semiconductor switch is connected. The diode and the third semiconductor switch are preferably connected to different supply inputs. The diode and the third semiconductor switch are connected to different terminals for connecting the excitation winding. In the full bridge circuit, the diode and the third semiconductor switch are positioned diagonally to each other. If the first and second semiconductor switches have body diodes, their reverse bias is configured (by appropriate terminal polarity of these semiconductor switches) such that the first and / or second semiconductor switch is reverse-biased when the diode conducts.If the third semiconductor switch includes a body diode, its reverse bias and the reverse bias of the diode are configured (by corresponding connection polarity of these elements) such that the third semiconductor switch and / or the diode are reverse-biased when the first and / or second semiconductor switches are conducting. The first, second, and / or third semiconductor switches may be implemented with a body diode whose forward bias preferably points towards the more positive of the two supply inputs, particularly to prevent current from flowing through the semiconductor switches in the open state. This also applies to any diode components that may be directly connected in parallel to the first, second, and / or third semiconductor switches.

[0013] The excitation current circuit preferably includes a control unit. This unit is designed as a device, in particular as (part of a) driver circuit, a microprocessor (or part thereof), or a microcontroller (or part thereof). The control unit is connected to the semiconductor switches for control purposes. For this purpose, the control unit can have an output that is connected to the control inputs of the semiconductor switches (i.e., to the gate or the base) for signal transmission. The control unit is configured to drive the first and second semiconductor switches in an excitation state according to a set excitation current. In particular, depending on the set excitation current, the first and second semiconductor switches can be continuously driven or, for field weakening, with a duty cycle of less than 100%, corresponding to the relevant setpoint.Particularly during driving or recuperation mode, the first and / or second semiconductor switches can be controlled with a duty cycle of > 0% ... 100% (corresponding to a continuous on state). The control system is designed for this.

[0014] The controller is further configured to control the third semiconductor switch in a de-excitation state according to a set de-excitation current. For this purpose, the third semiconductor switch can be kept in the ON state for the duration of the de-excitation, or a duty cycle > 0% can be set. The third semiconductor switch can be activated by the controller with an ON pulse whose duration corresponds to the duration of the de-excitation state. De-excitation can be initiated and carried out, particularly in the event of a fault. In some embodiments, the controller is configured to generate the set excitation current by continuously closing the first and second semiconductor switches for the duration of the excitation state.Furthermore, the control system may be configured to generate the desired excitation current by pulsed switching of the third semiconductor switch, for example, by switching it on with a pulse whose duration corresponds to the excitation phase. The control system may be configured to drive the semiconductor switches to generate opposite polarities at the terminals under different states. The control system may also be configured to drive the semiconductor switches under different states to generate an excitation current and an opposing excitation current flowing through the terminals.

[0015] The semiconductor switches can be designed differently. For example, the first and second semiconductor switches can each have a higher current-carrying capacity (especially continuous current-carrying capacity) than the third semiconductor switch (and / or the diode). The first and second semiconductor switches can each have a lower on-resistance (i.e., forward resistance in the ON state) than the third semiconductor switch. The semiconductor switches are preferably designed as MOSFETs. The semiconductor switches can also be designed as IGBTs. The current-carrying capacity of the first and second semiconductor switches (IGBT, MOSFET, or other transistor) is preferably each higher than that of the third semiconductor switch (IGBT, MOSFET, or other transistor). The third semiconductor switch can be a different type of transistor than the first and second semiconductor switches.The first and second semiconductor switches are preferably of the same transistor type and are identically designed (particularly with regard to current carrying capacity). A control device can be configured for controlling a separately excited electrical machine, in particular for controlling the excitation of a separately excited electrical machine. The control device preferably includes the excitation current circuit as described herein. The control device has a (two-pole) excitation winding connection that is connected to the two terminals of the excitation current circuit. The control device can also include a control for operating stator windings, or a control for operating stator windings can be provided, wherein, in particular, a higher-level control unit controls this control unit as well as the excitation current circuit.The control device may include a DC voltage source connected to the supply inputs of the excitation current circuit.

[0016] An electric vehicle traction drive can be equipped with a separately excited electric machine, the excitation winding of which is connected to an excitation current circuit as described herein. For this purpose, the terminals of the excitation current circuit are connected to the ends of the excitation winding. The drive can include a DC voltage source connected to the supply inputs of the excitation current circuit.

[0017] Figure 1 serves as an exemplary illustration of implementation forms of the excitation current control, control device and traction drive specified here, using a symbolically represented circuit diagram.

[0018] Figure 1 shows a control device ST, which can be part of an electric traction drive of a vehicle, comprising an electric machine EM, the control device ST and thus also the excitation current circuit ES.

[0019] Figure 1 shows an electric machine EM connected to an exemplary excitation circuit ES. The excitation circuit ES has two terminals A1 and A2, for example, two terminal contacts. An excitation winding EW of an electric machine EM, which may be a separately excited synchronous machine, can be connected to terminals A1 and A2. The excitation circuit ES, which is external to the electric machine EM and can be connected to it or to its excitation winding EW, is described in more detail below.

[0020] The excitation circuit ES comprises a first half-bridge H1 and a second half-bridge H2. Together, the half-bridges can be considered a full-bridge circuit. The inner ends of the half-bridges H1 and H2 serve to connect the excitation winding EW (as the transverse element of the full bridge), with terminals A1 and A2 providing connection to the ends of the excitation winding EW. The first half-bridge H1 comprises a diode D and a first semiconductor switch S1, which are connected in series. The junction connecting these two elements is connected to the first terminal A1, and the potentials are identical. The second half-bridge H2 comprises a second semiconductor switch S2 and a third semiconductor switch S3, which are connected in series. The junction connecting these two semiconductor switches is connected to the second terminal A2, and the potentials are identical.The two outer ends of the two half-bridges are connected to each other, with the upper ends of the half-bridges shown being connected to a positive supply potential V+ and the lower ends of the half-bridges shown being connected to a negative supply potential V-. The supply potentials are fed in via the supply inputs with the same reference symbols.

[0021] A controller C is connected to the semiconductor switches S1, S2, and S3, as indicated by the double arrows. The controller can have an output that transmits a signal and is connected to the control inputs of the semiconductor switches S1 to S3, for example, to their gates. To excite the excitation winding EW, switches S1 and S2 can be closed. In this case, the first terminal A1 is connected to the negative supply potential V-, and the second terminal A2 is connected to the positive supply potential V+. This results in an excitation current originating from V+, which flows via semiconductor switch S2 to the second terminal A2. From there, the current flows through the (external) excitation winding EW to the first terminal A1 and from there via the first semiconductor switch S1 to the negative supply terminal V-. (This consideration is based on conventional current direction.)

[0022] Depending on the winding system of the excitation winding EW, a specific magnetic field with a specific north direction is generated. The control unit C is configured to simultaneously switch switches S1 and S2 into conduction during an excitation phase. In this state, the control unit C is in an excitation state, during which the semiconductor switches S1 and S2 receive a control signal that causes them to conduct.

[0023] For example, if a fault is detected, the operation of the electric machine is terminated to prevent further damage, which includes, among other things, the energizing of the excitation winding EW. The aim here is to reduce the magnetic field present in the excitation winding EW (due to the previous excitation) to zero (or to a predetermined threshold value) as quickly as possible. The control unit C is in a corresponding de-excitation state in which switches S1 and S2 are open. Switch S3 is closed for de-excitation. The second terminal A2 is not connected to the positive supply potential V+, but rather to the negative supply potential V-, since switch S3 connects terminal A2 to the negative supply potential V-.In this case, diode D is conductive and connects the first terminal A1 to the positive supply potential V+ in the de-excitation state (while the diode is not conductive in the excitation state, and switch S2 connects the first terminal A1 to the opposite, negative supply potential V-). The voltage between the supply potentials V+ and V- results in a current flow that initiates and supports the de-excitation process.

[0024] In ferry or recuperation mode, i.e., in motor or generator mode of the electric machine EM, switches S1 and S2 are closed, and switch S3 is open. Diode D is not conducting. The control unit C is configured to control the semiconductor switches S1 to S3 accordingly. In the event of a fault, S1 and S2 are opened, and S3 is closed, thus generating a current (or voltage) at terminals A1 and A2 via S3 and diode D. This current has an effect in the excitation winding that opposes the effect of the excitation current. In other words, the current through S3 and diode D dissipates the magnetic field generated in the previous excitation state, while the current through S1 and S2 builds up the magnetic field in the excitation winding EW.Since the motor and generator operation of the electric machine is typically essentially continuous, switches S1 and S2 are designed for a higher current load than switch S3 (and diode D). This is possible because the de-excitation state only lasts as long as the magnetic field decays. The depicted electric machine also has stator windings SW, which are controlled by a motor controller CM. The motor controller can be configured to implement space vector-based control. In the example shown, this motor controller CM and the excitation current controller C are distinct entities. However, these can be implemented by a common drive controller, or there can be a (not shown) higher-level control unit that controls both this motor controller CM and the excitation current controller C.

[0025] An electric vehicle traction drive can be implemented using the excitation circuit ES shown in Figure 1, the external electric machine EM, and the motor controller CM, which energizes the stator (or stator windings SW) of the machine EM. The components shown in Figure 1 (reference numbers EM with EW, ES with C, and CM) can together constitute a vehicle traction drive or at least its electromechanical component. The control device ST shown comprises the motor controller CM and the excitation circuit and is connected to the electric machine EM, which is external to the control device ST.

Claims

Patent claims 1. Excitation current circuit (ES) for controlling the excitation of an excitation winding (EW) with two terminals (A1, A2) for an excitation winding (EW) of a separately excited electrical machine (EM) and with two supply inputs (V+, V-) as well as with a first half-bridge (H1) and a second half-bridge (H2), the ends of which are connected to the supply inputs (V+, V-) and the connection points of which are connected to the terminals (A1, A2), wherein the first half-bridge has a series connection of a diode (D) and a first semiconductor switch (S1) and the second half-bridge has a series connection of a second and a third semiconductor switch (S2, S3).

2. Excitation current circuit (ES) according to claim 1, wherein the first and the second semiconductor switches (S1, S2) are connected to different supply inputs (V+, V-).

3. Excitation current circuit (ES) according to claim 1 or 2, wherein the diode (D) and the third semiconductor switch (S1 , S2) are connected to different supply inputs (V+, V-).

4. Excitation current circuit (ES) according to claim 1, 2 or 3, comprising a control unit (C) which is connected to the semiconductor switches (S1 - S3) in a controlling manner, wherein the control unit (C) is configured to control the first and second semiconductor switches (S1, S2) according to a set excitation current in an excitation state, and to control the third semiconductor switch (S3) according to a set de-excitation current in a de-excitation state.

5. Excitation current circuit (ES) according to claim 4, wherein the control (C) is configured to generate the desired excitation current by permanently closing the first and second semiconductor switches (S1 , S2) for the duration of the excitation state, and to generate the desired de-excitation current by pulsed switching of the third semiconductor switch (S1 , S2).

6. Excitation current circuit (ES) according to claim 4 or 5, wherein the control (C) is configured to control the semiconductor switches to generate opposite polarities at the terminals (A1 , A2) under different states.

7. Excitation current circuit (ES) according to one of the preceding claims, wherein the first and second semiconductor switches (S1 , S2) each have a lower on-resistance than the third semiconductor switch (S3).

8. Excitation current circuit (ES) according to one of the preceding claims, wherein the semiconductor switches (S1 - S3) are designed as MOSFETs.

9. Control device (ST) configured for controlling a separately excited electrical machine (EM), wherein the control device (EM) has the excitation current circuit according to one of the preceding claims and the control device has an excitation winding connection which is connected to the two terminals (A1 , A2) of the excitation current circuit.

10. Electric vehicle traction drive with a separately excited electric machine (EM) comprising an excitation winding (EW) and an excitation current circuit (ES) according to one of claims 1 - 8, the terminals (A1 , A2) of which are connected to the excitation winding (EW).

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