WO2010017794A1

WO2010017794A1 - Arrangement and method for the uninterrupted supply of a fluid to a hydraulic system

Info

Publication number: WO2010017794A1
Application number: PCT/DE2009/001011
Authority: WO
Inventors: Kai Lunau
Original assignee: Ixetic Bad Homburg Gmbh
Priority date: 2008-08-12
Filing date: 2009-07-20
Publication date: 2010-02-18
Also published as: DE112009001698A5; DE112009001698B4

Abstract

The invention proposes an arrangement (1) for the uninterrupted supply of a fluid to a hydraulic system (3), comprising a mechanically driven main pump (5) for supplying a fluid to the hydraulic system (3), comprising an auxiliary pump (7), which is driven by an electric motor (9), for maintaining a hydraulic pressure in the hydraulic system (3), and comprising a control unit (11) for controlling the electric motor (9).

Description

Arrangement and method for the uninterruptible supply of a hydraulic system to a fluid

The invention relates to an arrangement for uninterruptible supply of a hydraulic system with a fluid according to the preamble of claim 1 and a method for uninterruptible supply of a hydraulic system with a fluid according to the preamble of claim 6.

Arrangements and methods of the type discussed here are known, for example, from DE 101 62 973 A1. They have a mechanical, in particular by the internal combustion engine of a motor vehicle driven main pump for supplying a hydraulic system with a fluid, in particular with hydraulic oil. Furthermore, an auxiliary pump driven by an electric motor for maintaining a certain hydraulic pressure in the hydraulic system and a control unit for controlling the electric motor are provided. Hydraulic systems for use in motor vehicles, which thus interact with intermittent shutdown internal combustion engines often require, in addition to the main pump, the electrically driven auxiliary pump to cover operating conditions in which the drive speed of the main pump is zero or too low to maintain the supply of the hydraulic system. This is the case, for example, when the internal combustion engine is stopped and the main pump is consequently not driven. In the following, hydraulic system is understood in particular to mean a power shift transmission for use in motor vehicles, for example for a hybrid drive as an automatic transmission. For example, if automatic powershift transmissions are used for use in motor vehicles with hybrid drives, then an additional electrically driven auxiliary pump is usually required to ensure the proper operation of the transmission in the characteristic of hybrid vehicles states. Even with motor vehicles with standard drive, which automatically shut off the engine and start for reasons of fuel consumption reduction when the vehicle is stationary again (start-stop function), such auxiliary pumps are used to maintain a certain hydraulic pressure in the system when the main pump is not driven by the internal combustion engine when the vehicle is stationary. The auxiliary pump must always ensure the supply of the hydraulic system, in particular with hydraulic oil, when the amount of oil delivered by the mechanically driven main pump is insufficient to ensure uninterrupted oil supply to the hydraulic system during operation. The auxiliary pump must then be switched on and off at the right moment and operated at the appropriate speed. Particularly difficult here is the transition from the sole operation of the main pump to the sole operation of the auxiliary pump. The optimum speed of the auxiliary pump depends in addition to the operating state of the system, which is characterized by the drive speed and the momentarily required torque, especially from the oil temperature and the thus changing viscosity of the hydraulic oil. Known arrangements for supplying a hydraulic system with a fluid have the disadvantage that complicated control strategies for the speed control are required for an energy-efficient operation of such auxiliary pumps. The known control strategies for the speed control of the electric motor to drive the auxiliary pump require accurate knowledge of the hydraulic conditions in the system. Thus, the temperature of the oil must be as well known as the need for speed, hydraulic circuits, etc. changing delivery demand. It can also happen that the pressure of the hydraulic oil in the inlet to the hydraulic system has to be measured. Therefore, in addition to sensors, an intelligent control logic must be present for the detection of these parameters, which converts the information into an associated pump speed and controls the auxiliary pump accordingly. This requires a lot of effort in development and production. Particularly difficult is the speed control of the electric motor, when a brushless DC motor is used, which operates in sensorless operation. These drives have the property that they require a certain minimum speed in operation to ensure trouble-free operation. However, this limits the usable operating range of the pump. It is therefore an object of the present invention to provide an arrangement and a method for the uninterrupted supply of a hydraulic system with a fluid which can be realized simply, efficiently and inexpensively.

To solve this problem, an arrangement for uninterrupted supply of a hydraulic system with a fluid is proposed which has the features of claim 1. It comprises a mechanically driven main pump for supplying the hydraulic system with a fluid and an auxiliary pump driven by an electric motor for maintaining a certain hydraulic pressure in the hydraulic system. Furthermore, a control unit for controlling the electric motor is provided. The arrangement is characterized in that the electric motor is operated torque-controlled by means of its phase current.

The invention makes use of the knowledge that the requirement for the operation of the auxiliary pump in hydraulic systems is not the provision of a defined flow rate, but rather the generation of a defined delivery pressure. This pressure is required to maintain certain hydraulic functions in the hydraulic system. The resulting from leakage, cooling oil quantities and other hydraulic consumers flow is in principle secondary. It can change in many areas due to temperature effects or other effects.

In the present invention, in contrast to the known arrangements so not the speed is controlled, but the torque of the electric motor used for the drive of the auxiliary pump is electrically adjusted by the control unit to a desired value. In conjunction with a suitable auxiliary pump corresponds to a predetermined drive torque of the electric motor in wide operating ranges of the auxiliary pump sufficiently accurate to a specific discharge pressure. For the operation of the hydraulic system with the auxiliary pump, it is therefore sufficient to define individual operating points or nominal operating pressures of the hydraulic system and to transfer them as torque setpoint values to the control unit of the electric motor. The flow rate or the engine speed then adjusts itself according to the conditions in the system. The realization of the arrangement becomes particularly simple if the current consumed by the electric motor is used as the controlled variable. Here, a distinction is made between a control of the phase current and a control of the measurable in the feed line to the electric motor total current. It has been shown that the phase current control, in particular in brushless motors, leads to a good approximation to a regulation of the torque of the electric motor. In this way, a simple, efficient and inexpensive arrangement for supplying a hydraulic system with hydraulic fluid is created, can be dispensed with sensors or intelligent control electronics.

Particularly preferred is an embodiment of the invention, which is characterized in that the auxiliary pump is a fixed displacement pump, in particular a vane pump, a radial piston pump, a roller-cell pump or a gear pump. Constant pumps of this type are pumps which have a constant displacement. They are particularly suitable for the operation of the present arrangement, so that a predetermined torque of the electric motor in a wide operating range of the auxiliary pump sufficiently accurately corresponds to a specific delivery pressure.

Further preferred is an embodiment of the invention, which is characterized in that the electric motor is a brushless electric motor, in particular a brushless, permanent magnet excited sensorless DC motor. Such electric motors have a simple structure and therefore contribute to a cost-effective implementation of the arrangement claimed here.

Further preferred is an embodiment of the invention, which is characterized in that between the main pump and the auxiliary pump, a check valve is arranged. By the check valve, the auxiliary pump is disconnected from the supply of the hydraulic system when the delivery pressure of the main pump exceeds that of the auxiliary pump. Conversely, the auxiliary pump must build with decreasing promotion of the main pump against the closed check valve, the target pressure and then deliver an increasing flow rate at constant pressure. On In this way, a smooth transition of the supply between the two pumps is achieved by means of the check valve.

Finally, an embodiment of the invention is preferred, which is characterized in that the auxiliary pump is connected to a pressure relief valve. As a result, it is ensured in certain operating states of the arrangement that the electric motor can continue to operate under pressure.

To solve this problem, a method with the features of claim 6 is proposed. For the uninterrupted supply of a hydraulic system with a fluid, in particular with hydraulic oil, an auxiliary pump driven by an electric motor maintains the supply of the hydraulic system when the delivery provided by a mechanically driven main pump delivery to supply the hydraulic system drops. The method is characterized in that the electric motor is operated torque-controlled by means of its phase current. The method has over the known method has the advantage that can be dispensed with complex sensors, since when using a suitable auxiliary pump, in particular a constant displacement pump, the predetermined drive torque of the electric motor in a wide operating ranges of the auxiliary pump sufficiently accurately corresponds to a specific delivery pressure. The delivery rate of the auxiliary pump or the engine speed then adjusts itself according to the conditions in the system. In this way, a simple, efficient and cost-effective method for the operation of an auxiliary pump for hydraulic systems is provided.

Particularly preferred is an embodiment of the method, which is characterized in that a brushless electric motor, in particular a brushless, permanent magnet excited DC motor, is used. Such electric motors are particularly simple in construction and thus contribute to a cost-effective implementation of the method. Further preferred is an embodiment of the method, which is characterized in that a constant-displacement pump, in particular a vane pump, radial piston pump, roller-cell pump or a gear pump is used as an auxiliary pump. In these pumps, which have a fixed displacement, corresponds to a predetermined torque of the electric motor in wide operating ranges of the auxiliary pump sufficiently accurate to a specific discharge pressure.

Also preferred is an embodiment of the method, which is characterized in that the auxiliary pump delivers at a decreasing flow rate of the main pump an increasing flow rate at a constant pressure. In particular, it is also provided that the auxiliary pump delivers a decreasing flow rate at a constant pressure at an increasing flow rate of the main pump. In this way, an uninterruptible supply of the hydraulic system, in particular a transmission, by the auxiliary pump possible.

Further preferred is an embodiment of the method, which is characterized in that a sensorless electric motor is used. This electric motor is characterized by the fact that it is particularly simple and thereby contributes to a cost-effective implementation of the method. However, these engines have the disadvantage that they only run stable above a certain minimum speed. Below this speed, there is a risk of the engine simply stopping under load. If such an engine to be used as a drive for auxiliary pumps of the type mentioned here, it must be ensured that the engine speed does not fall below this minimum speed during operation. Therefore, it is preferably provided in the present invention that the maximum achievable pressure of the auxiliary pump is reached at the minimum speed of the electric motor. When the minimum rotational speed is reached, the electric motor is preferably operated for this purpose in a speed-controlled manner so as not to drop further in rotational speed or to stop. In particular, it is provided that the auxiliary pump is connected to a pressure relief valve and the maximum pressure to be reached corresponds to the opening pressure of the pressure relief valve. In this case, when the minimum speed is reached, the pressure relief valve will open, so that the auxiliary pump will pass through the minimum speed predetermines small delivery through the pressure relief valve directly into an oil sump of the auxiliary pump. As a result, the maximum torque of the electric motor is limited and avoid overloading of the electric motor. Preferably, it is provided in this case that in the control electronics of the electric motor, a minimum speed limit is deposited. Preferably, it is also provided that the speed control has a higher priority than the phase current control of the electric motor, so that it is avoided in any case that the speed of the electric motor drops below the minimum speed.

Finally, an embodiment of the method is preferred, which is characterized in that the opening pressure of the pressure limiting valve is greater than the maximum occurring during normal operation of the auxiliary pump pressure. This ensures the function of the auxiliary pump during normal operation. In the event that the check valve is closed by the hydraulic pressure generated by the main pump, the electric motor can open the pressure limiting valve, so that it does not stop.

The invention will be explained in more detail below with reference to the drawing. Show it:

Figure 1 is a schematic representation of an arrangement for supplying a

Hydraulic system with a fluid;

Figure 2 is a graph of the pressure versus delivery for two load points;

Figure 3 is a graph of torque versus speed for two load points, and

4 shows a graph of the torque over the speed for the operation of the arrangement with a sensorless electric motor.

FIG. 1 shows, by way of example, an arrangement 1 for the uninterruptible supply of a hydraulic system 3, which here by way of example is designed as an automatic transmission. det, with a fluid, in particular with hydraulic oil. The hydraulic system 3 is supplied with hydraulic oil by a mechanical main pump 5 driven by an internal combustion engine (not shown). In order to maintain a specific hydraulic pressure when the internal combustion engine is switched off in the hydraulic system 3, a further pump, namely an auxiliary pump 7, which is driven by an electric motor 9, is provided. For the control of the electric motor 9, a control unit 11 is provided.

The electric motor 9 is preferably designed as a brushless electric motor. In particular, it is provided that the electric motor 9 is designed as a permanent magnet brushless DC motor. The electric motor 9 may be designed sensor-controlled, so have position sensors that detect the position of its rotor. But it can also be carried out sensorless. In this case, the electric motor 9 does not have position sensors, but the rotor position is detected via the counter-voltage. If the electric motors described here are provided as a drive for the auxiliary pump 7, the torque control can be realized in a simple manner by regulating the phase current of the electric motor 9.

In order to prevent a backflow of the hydraulic oil at a standstill of the auxiliary pump 7, a check valve 13 is provided between the main pump 5 and the auxiliary pump 7. Preferably, as shown here, a pressure limiting valve 15 is provided, which serves to secure the auxiliary pump 7, so that in normal operation of the assembly 1, so if oil production takes place solely by the mechanically driven main pump 5, hydraulic oil of the auxiliary pump 7 flows back into an oil sump 17 ,

With regard to the auxiliary pump 7, the following should be noted at this point:

A mandatory condition for the proper functioning of the arrangement 1 is the use of an auxiliary pump 7, the drive torque over a wide speed and temperature range is sufficiently accurate proportional to the delivery pressure. This For example, applies to vane pumps. However, it is also conceivable to use a radial piston pump, a roller-cell pump or a gear pump. The decisive factor is that the hill pump 7 is a fixed-displacement pump, which therefore has no variable displacement. Although increases in these pumps with falling temperature and increasing speed required for a given delivery pressure drive torque to a small extent, in practice, however, at lower temperatures, the required flow rate and thus the speed is small because of then small leaks. Both effects offset each other within certain limits.

Figure 2 shows a graph of the pressure over the delivery rate of the auxiliary pump 7 for two exemplarily selected load points.

In Figure 2, the function of the auxiliary pump 7 is explained in the operation of the arrangement 1 with reference to two exemplarily selected load points. The load points are characterized by the respective pressure at the outlet of the auxiliary pump 7. It is understood that the type of control described here is not limited to a specific number of load points, but that the setting of the setpoint can be freely selected steplessly. As soon as the transmission control therefore forwards a certain required hydraulic oil pressure to the control unit 11, it will adjust the phase current of the electric motor 9 such that a corresponding torque results at the electric motor 9 in order to generate or maintain the hydraulic pressure required in the hydraulic system 3.

The required for operation at the respective load point flow rate of hydraulic oil is temperature dependent, the oil temperature and thus the flow rate of the system are not known. However, the demand of the hydraulic system 3 on hydraulic oil can be represented by the pressure in the P / Q diagram according to FIG. 2, which shows the pressure above the delivery rate, for the two operating points of the hydraulic system selected by way of example. It can be seen that for each load point in the graph, a characteristic curve with constant pressure and variable flow rate results. In Figure 2, the characteristic of the first load point by L ₁ and the characteristic of the second load point with L _{2 is} marked.

In order to ensure a certain pressure P at the output of the auxiliary pump 7 and thus a predetermined torque of the electric motor 9, the auxiliary pump 7, depending on the oil temperature, but also from the existing system pressure of the hydraulic and the delivery of the main pump 5 more or less Hyd - Promote rauliköl, which is clear in Figure 2 by the characteristics L ₁ and L ₂ .

For electric pump drives, here for the electric motor 9, a conversion of the graph shown in Figure 2 of the pressure P on the flow rate Q in a graph of the torque M on the rotational speed N of the electric motor 9 is possible. Although at lower oil temperatures, the viscous friction in the auxiliary pump 7 is greater, this is due to the system properties of the hydraulic system 3, the necessary flow Q and thus the pump speed N lower, which keeps the required drive torque M approximately constant. In total, the delivery rate requirement of the hydraulic system 3 can be converted into a speed and torque requirement of the electric motor 9 to a good approximation, as shown in FIG.

The characteristic curves L ₁ and L _{2 of} the load points of Figure 2 are shown in Figure 3 as areas B ₁ and B ₂ to take into account the resulting from the variable boundary conditions in the auxiliary pump 7 scattering of the conversion of pressure P in a torque M.

When the electric motor 9 is operated torque-controlled, the demand-based oil supply of the hydraulic system 3 can be ensured by specifying a torque M to be adjusted without careful consideration of the characteristics of the hydraulic system 3, the auxiliary pump 7 and the oil temperature. In practice, the predetermined torque M of the control unit 11 will be chosen to be slightly higher than the actual demand of the hydraulic system 3 on hydraulic pressure a certain safety reserve to ensure the so-called pressure reserve during operation of the arrangement 1.

As has already been stated above, brushless electric motors, in particular permanent magnet-excited brushless DC motors, are preferably used as the drive machine for the auxiliary pump 7 for the present invention. Then, the torque control can be realized in a simple manner by controlling the phase current of the electric motor 9.

The properties of these electric motors require that the torque characteristic curve has a slight inclination with a constant phase current, which is evident in FIG. 3 by the characteristic curves Li 'and L ₂ '.

The torque M of the electric motor 9 is thus slightly higher at low speeds N than at high speeds N. This inclination of the characteristics Li 'and L ₂ ' is required for a stable function of the motor control. The operating characteristics of the electric motor 9 in the phase-controlled operation can then be entered in the torque / speed diagram (M / N), as shown in Figure 3.

Without the slope of the curves, the pressure control proposed here would not be possible because the electric motor 9 could also stop at constant pressure. Although the rotational speed N is reduced as the load torque increases due to the inclination of the characteristic curve, the electric motor 9 can remain functional against the constant pressure of the system.

An important operating condition in hydraulic systems is the transition from the oil supply via the main pump 5 to the auxiliary pump 7 and vice versa. For the function of the arrangement 1, an uninterrupted oil supply is indispensable. This means that a smooth transition of supply between the two pumps must be achieved. For example, if the flow rate of the main pump 5 slowly increases until it is sufficient alone for the supply of the hydraulic system 3, the additional delivery by the auxiliary pump 7 decreases until finally the check valve 13 between the auxiliary pump 7 and the main pump 5 closes when the delivery pressure the main pump 5 exceeds that of the auxiliary pump 7.

Conversely, the auxiliary pump 7 must build the target pressure at a decreasing promotion of hydraulic oil through the main pump 5 against the closed check valve 13 and then provide an increasing flow rate at a constant pressure. For the auxiliary pump 7, this means that it must supply hydraulic oil in the range of zero to maximum delivery at constant pressure. The control range of the electric motor 9 must therefore include a very wide speed range from a few revolutions per minute up to the maximum speed. This is easily possible with the described construction of the auxiliary pump 7 with electric motor 9 by a brushless permanent magnet excited electric motor. The operation of the electric motor 9 then takes place on the characteristic curves L ₁ 'and L ₂ ' shown in FIG.

As already mentioned, the invention will be explained here only with reference to two example selected load points, however, the electric motor 9 can be continuously adjusted to a certain torque M by means of its phase current.

For applications in hydraulic systems are due to their simple design electric motors 9, which do not require special components for rotor bearing feedback, so-called sensorless electric motors, particularly suitable. However, these engines often have the disadvantage that they only run stable above a certain minimum speed. Below this speed there is a risk that the electric motor 9 simply stops at a load. If such an electric motor 9 is to be used as a drive for an auxiliary pump 7, then it must be ensured that the engine speed does not fall below this minimum speed. For this purpose, a minimum speed limit is stored in the control electronics of the electric motor 9, which preferably has a higher priority than the control of the phase current, ie the torque of the electric motor 9.

Figure 4 shows a graph of the torque over the speed for the use of a sensorless electric motor.

FIG. 4 once again shows two exemplary operating characteristics L ₁ "and L ₂ " selected for two exemplary load points, ie pressures. It becomes clear that the speed of the electric motor 9 at constant phase current along the operating characteristics Li "and L ₂ " decreases when the electric motor 9 is loaded in operation with a higher load torque, which is higher than that predetermined by the control unit 11 and by the Phase current adjusted torque M.

This operating state can occur, for example, when the electric motor 9 receives a start signal from the control device 11, because the main pump 5 will soon no longer deliver enough fluid to maintain the function of the hydraulic system 3. The auxiliary pump 7 is then, driven by the electric motor 9, promote a certain amount of hydraulic oil, but initially starts against the closed return valve 13, because the hydraulic pressure generated by the main pump 5 exceeds that of the auxiliary pump 7. In this case, the load torque increases at the output of the auxiliary pump 7.

The speed N will then decrease due to the adjusted torque of the electric motor 9 up to a minimum speed N _min . As soon as the minimum rotational speed N _{min is} reached, a speed control of the electric motor 9 intervenes and keeps the rotational speed N _min constant as the phase current increases and the torque M increases. The operating point of the electric motor 9 thus runs along the characteristic L ₃ in the diagram of Figure 4 upwards.

The arrangement 1 must then have the pressure limiting valve 15 shown in Figure 1 for the auxiliary pump 7, the opening pressure is slightly above the maximum during normal operation of the auxiliary pump 7 pressure. The thus specified Maximum pressure is indicated in Figure 4 by the characteristic U. The electric motor 9 must be able to reach this predetermined maximum pressure of the pressure limiting valve 15 in any case without first stopping.

Thus, if arranged in the output of the auxiliary pump 7 check valve 13 is closed by the pressure generated by the main pump 5, the auxiliary pump 7 promotes the predetermined by the minimum speed N _min low flow through the pressure relief valve 15 directly into the oil sump 17. The operation of the electric motor 9 after Opening the check valve then takes place along the characteristic L ₅ shown in Figure 4 for the load point Li "or along the characteristic L ₆ for the load point L ₂ ", depending on the pressure requirement of the hydraulic system.

The maximum torque M is limited by the pressure relief valve and an overload of the electric motor 9 avoided. However, the electric motor 9 must be capable of starting against the load moment resulting from the opening pressure of the pressure limiting valve 15. Only then can the auxiliary pump 7 safely take over the oil supply from the main pump 5 even at lower pressures.

Overall, it turns out that the arrangement 1 proposed here, whose electric motor 9 is operated torque controlled by means of its phase current, and the resulting advantageous method, a simple, efficient and cost-effective way and ensures an uninterruptible supply of hydraulic system 3 with hydraulic oil. For the use of brushless, permanent magnet excited sensorless DC motors, the present invention is particularly suitable. Bezuqszelchenliste

1 arrangement Li "characteristic

3 Hydraulic system L ₂ "characteristic

5 Main pump Nmin minimum speed

7 auxiliary pump L ₃ characteristic

9 electric motor U characteristic

11 Control unit L ₅ Characteristic

13 Check valve L ₆ Characteristic

15 pressure relief valve

17 oil sump

P pressure

Q delivery rate

M torque

N speed

Li characteristic

L ₂ characteristic

Li 'characteristic

L ₂ ¹ characteristic

Claims

claims

1. Arrangement (1) for the uninterrupted supply of a hydraulic system (3) with a fluid comprising

a mechanically driven main pump (5) for supplying the hydraulic system (3) with a fluid,

an auxiliary pump (7) driven by an electric motor (9) for maintaining a hydraulic pressure in the hydraulic system (3), and

a control unit (11) for controlling the electric motor (9),

characterized in that the electric motor (9) is operated torque-controlled by means of its phase current.

2. Arrangement according to claim 1, characterized in that the auxiliary pump (7) is a fixed displacement pump, in particular a vane pump, radial piston pump, roller-cell pump or a gear pump.

3. Arrangement according to claim 1 or 2, characterized in that the electric motor (9) is a brushless electric motor, in particular a brushless permanent magnet excited sensorless electric motor.

4. Arrangement according to one of the preceding claims, characterized in that between the main pump (5) and the auxiliary pump (7), a check valve (13) is arranged.

5. Arrangement according to one of the preceding claims, characterized in that the auxiliary pump (7) with a pressure relief valve (15) is connected.

6. A method for uninterruptible supply of a hydraulic system (3) with a fluid, wherein the supply of the hydraulic system (3) by means of a by an electric motor (9) driven auxiliary pump (7) is maintained when a mechanically driven main pump (5) reduces the flow to supply the hydraulic system (3), characterized in that the electric motor (9) is operated by means of its phase current torque controlled.

7. The method according to claim 6, characterized in that a brushless electric motor, in particular a brushless, permanent magnet excited DC motor is used.

8. The method according to claim 6 or 7, characterized in that a constant-displacement pump, in particular a vane pump, radial piston pump, roller-cell pump or a gear pump as an auxiliary pump (7) is used.

9. The method according to any one of the preceding claims 6 to 8, characterized in that the auxiliary pump (7) at a decreasing flow rate (Q) of the main pump (5) delivers an increasing flow rate (Q) at constant pressure (P).

10. The method according to any one of the preceding claims 6 to 9, characterized in that the auxiliary pump (7) at an increasing flow rate (Q) of the main pump (5) provides a decreasing flow rate (Q) at a constant pressure (P).

11. The method according to any one of the preceding claims 6 to 10, characterized in that a sensorless electric motor is used.

12. The method according to claim 11, characterized in that the maximum achievable pressure of the auxiliary pump (7) at the minimum speed (N _min ) of the electric motor (9) is achieved.

13. The method according to any one of the preceding claims 11 or 12, characterized in that the electric motor (9) is operated speed-controlled on reaching its minimum speed (N _m i _n ).

14. The method according to any one of the preceding claims 11 to 13, characterized in that the auxiliary pump (7) is connected to a pressure relief valve (15).

15. The method according to any one of the preceding claims 11 to 14, characterized in that the maximum pressure to be reached corresponds to the opening pressure of the pressure limiting valve (15).

16 The method according to any one of the preceding claims 11 to 15, characterized in that in the control electronics of the electric motor (9) a _minimum speed limit (N _min ) is deposited.

17. The method according to any one of the preceding claims 11 to 16, characterized in that the speed control on reaching the minimum speed (N _min ) has higher priority than the phase current control of the electric motor (9).

18. The method according to any one of the preceding claims 11 to 17, characterized in that the opening pressure of the pressure limiting valve (15) is greater than the maximum during normal operation of the auxiliary pump (7) occurring pressure.