BACKGROUND OF THE INVENTION

The present invention relates to a brake resistor device for use in a heavy duty vehicle including an electric drive motor, in particular for vehicles driven electrically using electric fuel cells and/or batteries as an alternative or addition to combustion engines.

Of particular interest are heavy load vehicles such as trucks. Trucks in general are used to transport heavy loads over long distances. Particularly in case of heavy duty long haulage applications so-called tractor semi-trailer combinations are used in which the tractor vehicle pulls and partly supports the payload that is packed onto the semi-trailer. In new generations of these trucks, fuel cells may also provide an important contribution to the electrification of the drive train, since they are seen as a promising and increasing factor in providing electrical energy, in addition to battery setups. Thus, a drive train is then powered by a traction e-motor that receives it’s electrical energy from a battery setup, which is typically a DC high voltage, that will be converted to a multiphase AC current used for powering the electric motor.

As long as the battery is not full and can accept all the power, the electrical return flow can be accepted by the battery as well, and regenerative braking can occur by charging the battery. In a truck or bus however there is the risk that the battery gets full and ultimately does not accept all braking energy and an alternative electrical load called brake resistor will need to accept the power from the electric motor and release the power by means of thermal heat. In such setups it is therefore important to provide a brake resistor device that is used to brake the vehicle by converting the electric energy that can be generated in the e-motor, when used as a generator, into heat that is dissipated in the brake resistor. This brake chopper controls the power flow to the brake resistor for resistive braking, also called endurance braking. As a result, the engine provides negative torque to the drive train and substantially aids in braking the vehicle.

The concept of providing a brake resistor in such a power circuit setup is in principle straightforward, since it merely involves circuiting the e-motor to a brake resistor, in particular, to the phase terminals of the e- motor control circuit that generate the different phases of electric power, in particular, a multiphase alternating current voltage generated by the e- motor in brake mode. Furthermore, the brake device, in the art known as brake chopper, needs only to function when the battery is no longer able to take up the electric energy, and the excess energy needs to be ‘chopped off.

However, since the brake resistor needs to be switched into a circuit connection with the e-motor, this poses a challenge on a switching control circuit design, since, in view of the massive flow of electrical power, this needs expensive power electronics such as high voltage switching devices, in particular power transistors, that will add to the price of the total electrical powertrain.

Thus a demand exists for providing a cost effective solution for integrating a brake chopper in the inverter circuit that provides high voltage to the e-motor of truck.

It is therefore an object to provide a circuit that is suitable for a variety of vehicle configurations without the need for specialist circuit design for the brake chopper, where it may be applicable to a classical powertrain with a central drive motor, as well as to a modern powertrain configuration using a so-called e-axle where the electric motor is directly integrated onto the rear axle of the truck.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a switching control circuit is provided for a DC high voltage battery system, to be coupled to a multiphase electric motor for use in a heavy duty vehicle, the invertor unit. The switching control unit comprises a first power switching circuit section, called invertor, having at least two first DC high voltage input terminals connectable to the high voltage battery system and controlled by a first control unit, said first control unit arranged to control the first power switching circuit to provide a multiphase AC electric current, from the two first DC high voltage terminals to first multiple AC output terminals, each switched according to a respective AC phase, to be coupled to the electric motor, in order to drive the electric motor through a multiphase AC electric current. A second power switching circuit section is provided, called brake chopper, having at least two second DC high voltage input terminals connectable to the high voltage battery system and controlled by a second control unit, said second control unit arranged to control the second power switching circuit to provide a multiphase AC electric current, from the DC high voltage terminals to second multiple AC output terminals, to be coupled to a brake resistor unit.

With the circuit design as proposed, it is possible to provide a brake chopper that is, by its design able to provide an AC output to be coupled to a brake resistor and refrain from a dedicated brake resistor design that would designed according to specific brake resistor specifications.

Preferably, first and second high voltage input terminals (HV+ and HV-) of the first and second power switching circuit sections may be shared on a common HV bus. Note that the common HV+ and HV- nodes can make use of cables, but a busbar can be used as well which is attractive for efficiency, cost and durability. Also important is that the mechanical enclosure makes sure that no high voltage cables or busbars will be accessible from the outside which is again good for safety and durability (no risk for water ingress or degradation of the connection).

More specifically, the first and second power switching circuit sections could have the same structural layout, moreover, be largely identical, which diminishes costs and design time.

In a further exemplary example, the second power switching circuit section may be provided with inductors that mimic the inductive characteristic of the electric motor, which enables a switching control of first and second switching circuits to be identical further reducing design effort and costs.

The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is not to be regarded as describing the only embodiments falling under the scope. The scope of the invention is defined in the claims, and the description is to be regarded as illustrative without being restrictive on the invention. In the drawings:

Figure 1 schematically shows an invertor design according to the prior art;

Figure 2 shows a schematic timing schedule for timing of the brake chopper circuit;

Figure 3 shows alternative embodiments of an invertor design, wherein brake chopper circuit designs are provided with a common HV terminal;

Figure 4 shows an exemplary embodiment according to the invention;

Figure 5-7 show schematic timing diagrams for showing the current flow of the brake chopper circuit, when coupled to the brake resistor.

Figure 8 shows a schematic illustration of an e-motor provided with a invertor and brake-chopper unit coupled to a single HV terminal busbar;

Figure 9 shows a generalized diagram of n modular invertor and m brake-chopper circuits, each largely identical to the other, provided in connection with a specified demand for multiphase e-motor design and specified modular design for a brake resistor configuration.

In more detail, the example of Figure 1 shows an inverter circuit 10 of a conventional setup; still without a brake chopper circuit. The inverter 10 shown controls the power flow, by ECU controller 15 in two directions: to and from the battery system 20, depending on the drive mode: power is supplied to the electric motor 30 for propulsion, but it also allows to reverse the power and generate a brake torque with the electric motor 30 to decelerate the vehicle (so called regenerative braking). The flow is controlled via paired switch devices 16 typically MOSFETS or IGBTs that are each conjugated with a freewheel diode 17, so that the switch device only blocks flow in a single direction. A capacitor 18 is provided in parallel to the high voltage bus terminals of battery system 20 to drain any alternating currents. By proper switching of paired switch devices 16, appropriate current paths are formed to provide an oscillating electrical current flow from the battery system 20 to the motor 30. Conversely, the AC current generated by the e-motor will be provided by suitable switching of the paired switches into a DC return flow.

Figure 2 shows a timing diagram, wherein an on-signal is provided on a respective base terminal of paired switch devices 16, in particular of transistors Tl- T6. For a three phase terminal, in particular respective voltages between terminals 31, this will translate as a 2/3 pi phase shifted alternating current that switches between first and second high voltage input terminals 21a,b (HV+ and HV-) of the power switching circuit 10. As can be seen, at t= 0, one voltage phase is positive, one is negative and one is zero. For currents that lag the voltage phase due to inductive characteristic, this means that the phase preceding should still allow a return current, which is reflected in a delayed closing of switches 16.

In order to provide for a brake resistor device, circuit 10 of Figure 1 should be further providing a brake chopper circuit, which, according to current state-of-art packaging concepts may fall apart into various solution categories as hsted in Figure 3a and 3b:

In Figure 3a a switching control circuit 11 for a DC high voltage battery system with power electronics are provided within a single invertor housing 11, and battery system 20, motor 30 and brake resistor unit 40 are connected via high voltage connections 2 lab, 31 and 41 respectively to the switching control circuit 11 which is now provided with an integrated brake chopper circuit design, including a brake resistor switch 111 that switches the output of the high voltage terminals 21a,b to a brake resistor unit 40 coupled circuit via terminals 41. The integrated design obviates a need for additional high voltage wiring and connectors needed between e-motor inverter and brake chopper since inverter 11 and brake chopper circuit 111 can share same ECU and synchronize power generated by electric motor with power consumed by brake resistor unit 40. This is attractive for battery lifetime of the high voltage battery because the brake chopper power will, by integrated design be able to accurately follow the power generated by the e- motor 30, preventing peak powers in/out of the battery system 20.

Alternatively in Figure 3b a switching control circuit 1012 for a DC high voltage battery system 20 is shown with a separate brake chopper circuit 12 to be coupled to the high voltage bus terminals 21a, 21b. In this design according to Figure 3b a standard inverter switching control circuit 10 may be combined with a separate brake chopper switching control circuit 12, that can be specifically developed and which should be in accordance with a specified gross combination weight of the vehicle that determines the brake resistor unit 40 and brake chopper current switch 121. This at least circumvents the need for dedicated circuitry of the standard invertor switching control circuit 10, but still poses a problem that the power circuitry of the brake chopper 12 needs to be specifically adapted for each type of heavy duty truck, in dependence of its weight characteristics.

Now turning to Figure 4, aspects of the invention will be further elucidated in an exemplary embodiment, that overcomes this problem of providing a dedicated brake chopper switching control circuit. In Figure 4 an e-motor 30 is coupled to a switching control circuit 1010 for a DC high voltage battery system 20 according to aspects of the invention. In control circuit 1010 first and second high voltage input terminals of said first and second power switching circuit sections are shared on a common HV bus 21a, 21b so that only one connection is needed. Alternatively the common HV+ and HV- nodes can make use of cables, but a busbar can be used as well which is attractive for efficiency, cost and durability. Also important is that the mechanical enclosure makes sure that no high voltage cables or busbars will be accessible from the outside which is again good for safety and durability (no risk for water ingress or degradation of the connection). Other important remark is that the cable length between traction inverter and brake chopper remains short (owing to the strong integration) which is good for efficiency and heat production. The control circuit 1010 comprises a first power switching circuit section 10a, called invertor, having at least two first DC high voltage input terminals 21a, b connectable to the high voltage battery system 20 and controlled by a first control unit 15a, said first control unit arranged to control the first power switching circuit 10a to provide a multiphase AC electric current, from the two first DC high voltage terminals 21a, b to first multiple AC output terminals 3, each switched according to a respective AC phase, to be coupled to the electric motor 20, in order to drive the electric motor 20 through a multiphase AC electric current provided by invertor circuit 10a. A second power switching circuit section 10b is provided, called brake chopper circuit, having at least two second DC high voltage input terminals 21a, b connectable to the high voltage battery system 20 and controlled by a second control unit 10b. Second control unit 10b is arranged to control the power switching circuit 10b to provide a multiphase AC electric current, from the DC high voltage terminals 21a,b to second multiple AC output terminals 41, to be coupled to a brake resistor unit. In the figure a capacitor 18 is provided in parallel to the common HV bus, for the invertor circuit and the brake chopper circuit, to filter out any high frequency noise, that may be caused by switching currents. The capacitor may be alternatively shared, or implemented in a separate circuit.

In the Figure 4 embodiment preferably, but not necessarily, inductors (coils 50 ) are provided between output terminals 31b of brake chopper circuit 10b, each on a respective phase terminal 31b of the power switching circuit 10b. The coils are designed to mimic inductive characteristics of the electric motor as will be further elucidated in Figures 5 -7, wherein in the brake chopper circuit switching circuit 10b, similar as in the invertor circuit 10a, at least one further switch 161 is switched active to enable a return flow because the current is lagging voltage, due to inductive behavior.

By having inductors 50 in series with each one of the multiple AC output terminals 31b of the brake chopper circuit 10b, the same switching scheme of a standard inverter 10a can be used that switches the AC current to the electrical motor 30, which makes use of inductive behavior that will be natural to the rotor and/or stator windings of the e-motor 30.

Accordingly, switching circuits 10a, 10b can be timed synchronously in a substantially identical fashion, which reduces control complexity and may be controlled e.g. by a master control unit (not depicted). By applying a master/slave architecture that synchronizes the switching actions of the power switching circuits Vehicle Energy Manager ECU may assign the traction inverter circuit 10a as “master” and the brake chopper circuit 10b will act as “slave”. This way, the brake chopper circuit shall follow the power from the traction inverter circuit with high accuracy and minimum delay. This control architecture may prevent unnecessary peak loads and wear of the high voltage battery system. Without a proper synchronization, the brake resistor power cannot not follow exactly the regenerative power generated by the traction inverters. In case the traction inverter decreases its regenerative power and the brake chopper does not respond quickly, high peak powers will be drawn from the battery system by the brake resistor unit, which is in fact a waste of energy.

Also, unnecessary wear of classical foundation brakes is prevented, since, by means of effective regenerative braking, the foundation brakes will be used less. If the brake chopper does not respond fast once the traction inverter starts generating regenerative braking power, this will limit how fast the traction inverters can start generating braking power.

The driver will compensative for this delta by using the foundation brakes leading to extra wear and early replacement of the brake pads.

Figure 5, 6 and 7 show timing control aspects for switching paired switching devices 16 in brake chopper circuit 10b, that is coupled to a brake resistor unit 40 provided in a triangle configuration, having a number of terminals corresponding to the multiple AC output terminals of the brake chopper.

The switches are indicated with T1 up to T6 and their corresponding control voltages are UT1 up to UT6, respectively. A possible switching scheme for these control voltages is shown in an adjacent phase diagram of the circuit scheme(blue lines). Each switching element T1..T6 is about 50% of its time open and about 50% of its time closed. To get a well- balanced 3 phase output, the switching elements are controlled with a phase shift of 60 degrees. Zero in this diagram means that the switching element is open (not passing any current), whereas one means that the switching element is closed (conducting current). The resulting three output voltages from the brake chopper circuit 10b will appear at the brake resistor unit 40 and are indicated with UAB, UBC and UCA. The diodes are indicated with D1 up to D6. One can see that with this control scheme, a sinusoidal voltage will appear on UAB, UBC and UCA.

The underlying switching scheme with the corresponding current flow is visualized in Figures 5-7 using 5 steps (a, b, c, d and e). These steps are reflecting the status of the switching elements at the following time instants:

Figure 5 Step A. t=0

Figure 5 Step B. t=l/3 n (current pulled through D2 because current is lagging voltage)

Figure 6 Step C. Shortly after t= 1/3 n (current through D2 is zero again)

Figure 6 Step D. t=2/3 n (current pulled through D3 because current is lagging voltage)

Figure 7 Step E. Shortly after t=2/3 n (current through D3 is zero again)

In the Figures 5-7 only the left part of Figure 4, i.e. only the brake chopper circuit 10b and brake resistor unit 40 are visualized. It is observed that in practical embodiments, the brake chopper circuit 10b is designed in a corresponding fashion as the invertor circuit, which may be even identical in layout. Even with a corresponding layout, particular electrical circuit elements may having differing electrical characteristics, i.e. in some instances it may be sufficient to have an invertor circuit of a certain grade quahty, while the brake chopper circuit, while being corresponding in layout, uses another grade quality, especially, as further developed below, when a varying number of brake chopper circuits is used.

Steps A, C and E would be sufficient to create the sinusoidal voltages UAB, UBC and UCA as shown in Figure 2. However for traction motors or other inductive e-motor designs the current flow lags behind the voltage for which steps B and D are added. These additional steps B and D allow the current to become zero (using the diodes) before the next switching element starts conducting. Generally, brake resistor units that are currently used in the art typically do not have any coils, which would allow to omit steps B and D since there is no lagging return flow. However, an aspect of the present invention involves using off-the-shelf inverters for traction e- motors, without modifying the low-level control algorithm for the switching elements. In such invertor circuit steps B and D will be executed which calls for inductive behaviour in the load to operate properly.

Figure 8 shows a practical embodiment of a traction motor 30, having multiple invertor circuits 10a, 10a’, e.g. where each phase is provided through multiple coil windings that calls for more than three terminals to be powered. Conveniently the first and second power switching circuits, are each housed in or on the e-motor housing as separate modules 10a, 10b integral to the electric motor 30, e.g. axial to the motor housing along its circumference. In the embodiment, the e-motor 30 is provided with a invertor and brake-chopper unit coupled to a single HV terminal busbar that connects to the high voltage system, in particular, a DC battery or fuel cell (not shown).

Where Figure 8 shows the possibility of having combined multiple inverters 10a with a single brake chopper circuit 10b, Figure 9 shows a generalized diagram of n modular invertor and m brake-chopper circuits, (n, m >1) each circuit largely identical in layout to the other, provided in connection with a specified demand for multiphase e-motor design and specified modular design for a brake resistor configuration. This allows for a modular / scalable design where the number of brake choppers to be integrated is flexible or the maximum power rating of the brake chopper 10b can be optimized depending on the required power rating of the brake resistor unit 40.

Additional to the high voltage system 20, a thermal cooling system 60 may be provided that is arranged to cool the power switching circuits, which can heat up substantially due to the high currents that are switched. To this end a coolant inlet and coolant outlet manifold may be connected to respective coolant terminals of the various modules, i.e. invertor module #l..#n and brake chopper module #l..#m, similar to the single HV bus nodes HV+ and HV-. Thus, with a common coolant manifold: brake chopper(s) and all other power electronics may share the same input and output coolant hne, such that only one connection is needed from the outside.

Accordingly, first and second power switching circuits 10a, 10b may be provided as multiple power switching circuits each having a same structural layout, wherein part of said multiple power switching circuits are provided as invertors 10a to be coupled to the electric motor 30; and another part of said multiple power switching circuits are provided as brake choppers 10b to be coupled to respective brake resistor units 40. The number of brake resistors may also vary, i.e. multiple brake chopper circuits 10b may share a single brake resistor system 401..40p.

Brake resistor configurations are preferably a star or triangle configuration that match the output terminals of a multiphase output of the brake chopper.

It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description and drawings appended thereto. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. The invention applies not only to automotive applications where the e-motor is used for traction power, but also to other industrial applications where an electric motor is used in combination with a brake chopper (and maybe no battery is present, such as industrial cranes or elevators). It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which may be considered within the scope of the appended claims. Also kinematic inversions are considered inherently disclosed and can be within the scope of the invention. In the claims, any reference signs shall not be construed as limiting the claim. The terms 'comprising' and ‘including’ when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus expression as 'including' or ‘comprising’ as used herein does not exclude the presence of other elements, additional structure or additional acts or steps in addition to those listed. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. Features that are not specifically or explicitly described or claimed may additionally be included in the structure of the invention without departing from its scope. Expressions such as: "means for ...” should be read as: "component configured for ..." or "member constructed to ..." and should be construed to include equivalents for the structures disclosed. The use of expressions like: "critical", "preferred", "especially preferred" etc. is not intended to limit the invention. To the extend that structure, material, or acts are considered to be essential they are inexpressively indicated as such. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the scope of the invention, as determined by the claims.