Method for operating an electric drive train

Abstract

The invention relates to a method for controlling an electric drive train comprising at least two electrical machines and a battery which supplies, by means of power electronics, at least two electrical machines with electric energy having a specified inverted operating voltage, with the power electronics being interconnected. The aim of the invention is to operate, when the electric drive train is operated only with part of the electrical machines, this / these electrical machine(s) at an optimized operating voltage. This aim is achieved in that, with at least one stopped electrical machine and at least one driving electrical machine, the operating voltage is applied to the at least one driving electrical machine via the at least one stopped electrical machine at a reduced operating voltage.

Description

[0001] Method for operating an electric drive train

[0002] The invention relates to a method for controlling an electric drive train with at least two electric machines and a battery, which supplies at least two electric machines with electrical energy at a predetermined alternating operating voltage by means of power electronics with the interposition of power electronics.

[0003] Electric drivetrains for motor vehicles with multiple driving electric motors are well known. For example, several axles can be electrically driven as drive axles, with a central electric motor driving a drive axle, for example, via a differential, or the individual drive wheels can each be driven by a wheel-mounted electric motor or an electric hub motor. In particular, in trucks, two or more drive axles can be provided in parallel, for example, as rear axles. During empty travel or when the truck is partially loaded, one or more drive axles can be de-energized, i.e., deactivated, to protect them and save energy.

[0004] The electric machines are powered by a battery whose DC voltage is adjusted depending on the power consumption of the electric machines and other parameters such as the age of the battery, working and ambient temperature, and the like.

[0005] From the publication “Integrated Battery Charger for EV by Using Three-Phase Induction Motor Stator Windings as Filter” in IEEE TRANSACTIONS ON TRANSPORTATION ELECTRIFICATION, VOL. 6, NO. 1 , MARCH 2020, pages 83 - 94, a charger for a battery in an electric motor vehicle is known in which the switched-off electric machine serves as a voltage converter during a charging process of the battery.

[0006] The object of the invention is to further develop an electric drive train with multiple electric motors for driving an electrically powered motor vehicle. In particular, the object of the invention is to operate the driving electric motor(s) at an optimized operating voltage during partial operation of the drive train.

[0007] The object is solved by the subject matter of claim 1. The claims dependent on claim 1 represent advantageous embodiments of the subject matter of claim 1.

[0008] The proposed method is used to control an electric drive train of a motor vehicle with at least two electric motors and a battery. The at least two electric motors drive the drive wheels of the motor vehicle and are supplied with electrical energy by the battery. During coasting, at least one electric motor can feed electrical energy back into the battery by means of recuperation.

[0009] The battery is a DC battery or DC accumulator with a high DC voltage of, for example, over 600 V, such as 850 V. The electric motors, for example, have ideal operating voltages of between 300 and 500 V AC, but can also be operated at higher operating voltages with reduced efficiency. Depending on the battery charge level and the number of activated electric motors, the operating voltage at the electric motors varies. Connected between the electric motors and the battery is a power electronics unit that can be either comprehensive or separate for each electric motor. This unit controls the electric motors and contains an AC voltage converter that converts the battery's DC voltage into a preferably three-phase AC operating voltage.This AC converter is designed to be bidirectional for a drive and recuperation mode of the motor vehicle.

[0010] The electric motors can be distributed in a corresponding number across one or more drive axles. For example, a drive axle can have one or more electric motors that act on two drive wheels of the drive axle via a transmission and a differential. Alternatively, at least one electric motor can be provided on each steering knuckle with a drive wheel of a drive axle, wherein the steering knuckles can be coupled to one another via a differential. Multiple drive axles can be provided. For example, in a truck, at least two drive axles can be provided directly next to one another, for example as rear axles.

[0011] In order to be able to protect at least some of the electric machines and, if applicable, their associated drive axle when the motor vehicle is operated under partial load, for example when a truck is running empty or partially loaded, and to be able to operate the remaining driving electric machines with an operating voltage close to an optimal operating point of the operating voltage, the operating voltage is applied to the at least one driving electric machine via the at least one stopped electric machine with a reduced operating voltage when at least one electric machine and at least one driving electric machine are switched off.This means that at least one deactivated electric machine is connected as a voltage transformer or voltage converter upstream of at least one driving electric machine in order to keep its operating voltage within the range of the ideal operating voltage if the deviation from the ideal operating point is too great, for example if this operating voltage increases due to the non-operating electric machine(s). If there are several electric machines acting on the drive wheels per drive axle, one or some of the electric machines can be deactivated and the others can be operated, with the deactivated electric machine(s) being connected upstream of the driving electric machine(s) within each drive axle.Alternatively or additionally, if there are multiple drive axles, the electric machine(s) of one or more drive axles can be deactivated and connected upstream of the driving electric machine(s). Preferably, a single deactivated electric machine is connected upstream of a single driving electric machine. To achieve a wider variation in the operating voltage, several deactivated electric machines can be connected upstream of one or more driving electric machines. If there are a plurality of electric machines, a voltage converter network comprising several deactivated electric machines can be formed, which provides an optimized operating voltage for at least one driving electric machine.

[0012] The windings of a stopped electric machine serve as voltage transformers.

[0013] To reduce induction currents due to the rotor of a stopped electric machine rotating with the associated drive wheel(s), a decoupling device, for example a separating clutch, a freewheel and / or the like, is preferably provided between the respective drive wheel(s) and the rotor. The windings of the stator are preferably used for this purpose. Alternatively or additionally, if windings are present in the rotor, these windings can be used for voltage conversion or transformation. For example, the windings of at least one stopped electric machine are connected against one another as primary and secondary coils in such a way that the operating voltage is reduced by a predetermined factor. For example, the windings are combined in such a way that an operating voltage optimized for the at least one driving electric machine results in continuous operation.

[0014] For example, the phases in a three-phase electric machine are connected in series in different ways. For example, in a three-phase, deactivated electric machine, the windings of two phases are connected in series as a primary coil, and the third phase is used as a secondary coil to generate a lower operating voltage for a corresponding driving electric machine. Despite the losses associated with voltage transformation, operating the corresponding electric machine closer to its ideal operating point can result in improved efficiency of the driving electric machine.

[0015] An advantageous embodiment includes an electric drive train for a motor vehicle, preferably for a truck or bus, with two drive axles. Each of these drive axles contains two electric motors, one of which is mounted close to the wheel on a steering knuckle of the drive axle. The steering knuckles are connected to one another by means of a rotary decoupling device, for example, a differential, a retarder, or the like. A decoupling device, for example, a separating clutch, is provided between the rotor of the electric motor and the drive wheel, for example, a drive wheel with one tire or dual tires.

[0016] The electric motors are designed as three-phase AC motors and are driven by the DC voltage of a battery via power electronics in three phases. Accordingly, each stator of the electric motor has three phases with windings, which are controlled by the power electronics.

[0017] When a first of the drive axles is shut down, only the electric motors of the other, second drive axle are energized. To reduce the operating current at the electric motors of the second drive axle, two of the stator windings of the electric motors of the first drive axle are energized. The operating voltage applied to the third winding, transformed to a lower operating voltage, is used to drive the electric motors of the second drive axle via their power electronics. In this case, an electric motor of the first drive axle can supply one of the electric motors of the second drive axle with current at a reduced operating voltage. It is understood that this method can also be carried out accordingly with a single electric motor per drive axle.During deactivation of the first drive axle, the decoupling devices are preferably open to also deactivate the rotor of the electric motors. During recuperation, the separating clutches can be closed and all electric motors can be operated as power generators. During acceleration of the motor vehicle, the driving electric motors of the second drive axle can be operated at a higher operating voltage, at least briefly, or all electric motors of all drive axles can be switched to driving mode.

Claims

Patent claims 1 . Method for controlling an electric drive train with at least two electric machines and a battery which, by means of power electronics, supplies at least two electric machines with electrical energy at a predetermined alternating operating voltage with the interposition of power electronics, characterized in that when at least one electric machine is stopped and at least one driving electric machine, the operating voltage is applied to the at least one driving electric machine via the at least one stopped electric machine with a reduced operating voltage.

2. Method according to claim 1, characterized in that the electric drive train has at least two drive axles, wherein in the case of a drive of only a part of the at least two drive axles by means of at least one electric machine in each case, the supply voltage is fed to the at least one electric machine of the at least one driven drive axle via the at least one electric machine of at least one non-driven drive axle.

3. Method according to claim 1 or 2, characterized in that windings of the at least one stopped electric machine serve as voltage converters for generating a voltage which is different from a voltage for the at least one driving electric machine.

4. Method according to claim 3, characterized in that the windings of at least one stopped electric machine are connected against each other as primary and secondary coils in such a way that a reduction of the operating voltage by a predetermined factor results.

5. Method according to claim 3 or 4, characterized in that the windings are combined with at least one stopped electric machine in such a way that in continuous operation of the at least one driving electric machine an operating voltage optimized for this results.

6. Method according to one of claims 3 to 5, characterized in that the windings of the stator of the at least one electric machine are used.

7. Method according to one of claims 3 to 6, characterized in that in a three-phase, stopped electrical machine, the windings of two phases are connected in series as a primary coil and the third phase is used as a secondary coil to generate a lower operating voltage for an associated driving electrical machine.