Operating method for a mobile machine tool

Abstract

The invention relates to a method for operating a mobile machine tool which has an electric motor having a rotor for generating a magnetic field fixed to the rotor and a stator for generating a rotating magnetic field, the method having a first operating mode, which includes: actuating the stator to rotate the rotor. The method has a second operating mode, which includes: actuating the stator to cancel out a magnetic flux at least in a co-rotating portion of a gap between the rotor and the stator.

Description

[0001] 2023ID00299

[0002] Hilti Aktiengesellschaft in Schaan

[0003] Principality of Liechtenstein

[0004] Operating procedures for a mobile machine tool

[0005] AREA OF INVENTION

[0006] The present invention relates to a method for operating a mobile machine tool comprising an electric motor with a rotor for generating a stationary magnetic field and a stator for generating a rotating magnetic field, wherein the method includes an operating mode comprising: controlling the stator to rotate the rotor. The present invention further relates to a computer program, a corresponding control unit for a mobile machine tool, and a corresponding mobile machine tool.

[0007] Mobile machine tools of the type mentioned above can be used to machine metal parts. Examples include a portable metal saw or an angle grinder. During this process, fine ferromagnetic metal dust can be generated, penetrate the machine tool, and accumulate on the stator in the gap between the rotor and the stator. This accumulation is caused by a magnetic flux between the rotor and stator resulting from the rotor's rotation. The accumulated metal dust can then reduce the efficiency of the machine tool and / or cause wear on the electric motor.

[0008] Against this background, one object of the present invention is to provide a means for removing ferromagnetic metal dust from the gap between the stator and rotor of a mobile machine tool.

[0009] REVELATION OF THE INVENTION

[0010] Accordingly, a method for operating a mobile machine tool is proposed. The mobile machine tool has an electric motor. The electric motor has 2019ID00232

[0011] - 2 - a rotor for generating a rotor-fixed magnetic field and a stator for generating a rotating magnetic field. The proposed method has at least a first operating mode and a second operating mode. The first operating mode includes at least: controlling the stator to rotate the rotor. The second operating mode includes at least: controlling the stator to cancel one or all magnetic fluxes in at least a co-rotating section of a gap between the rotor and the stator. By canceling the magnetic flux in the section or in the gap, the adhesion of the metal dust is prevented, at least temporarily. This allows the metal dust to be dislodged, for example, by an airflow in the gap resulting from the rotation of the rotor. As a result, the mobile machine tool operated according to the proposed method experiences less loss and / or wear.

[0012] A mobile machine tool is, in particular, a mobile machine tool for metalworking, such as an angle grinder, a saw, a drill, or the like. The mobile machine tool is preferably portable, so that it can, for example, weigh up to 50 kg, more preferably up to 20 kg.

[0013] In other words, an operating procedure for a mobile machine tool is proposed. The first operating mode can be described as a power mode, in which the machine tool is operated to deliver power generated by the electric motor to drive a tool. The second operating mode can be described as a self-cleaning mode, in which the machine tool is operated to clean the gap between the stator and the rotor. The machine tool may have one or more additional operating modes.

[0014] Controlling the stator to rotate the rotor can be understood in particular as applying alternating current to several coils of the stator.

[0015] The procedure may include the following in the second operating mode: continued rotation of the rotor. Continued rotation could, for example, mean keeping a motor brake open. Continued rotation could also, for example, mean ending 2019ID00232.

[0016] - 3 - or interrupting the stator's control signal to rotate the rotor. Continued rotation can, for example, refer to the free rotation of the rotor, which may occur under the influence of a moment of inertia and / or a frictional torque.

[0017] The procedure may include switching to the second operating mode only if the rotor speed does not fall below a minimum threshold. Eliminating the magnetic flux in the section / gap will often result in a drop in the electric motor's power output. Therefore, a suitably selected minimum speed can ensure that the rotational speed does not decrease too much during the second operating mode. This can, for example, guarantee the removal of metal dust.

[0018] The method may include, in the second operating mode, driving a fan to generate an airflow in the gap. For example, the electric motor may have a rotor-fixed fan, so that by ensuring the minimum speed before switching to the second operating mode, the rotor's moment of inertia drives the fan. Alternatively, the electric motor may have a separately driven fan, and / or the machine tool may have a separately driven housing fan. In this case, the method may advantageously include, in the second operating mode, driving a separate fan to generate the airflow in the gap. The airflow blows away the metal dust that no longer adheres to the section of the suspended magnetic flux. This improves the cleaning effect, resulting in faster or more significant wear and / or loss.The amount will be reduced more significantly.

[0019] The procedure may include: switching to the second operating mode after a minimum operating time has elapsed in the first operating mode. The minimum operating time in the first operating mode ensures that the second operating mode, or any associated performance drop and / or load change reaction (e.g., a "jerking"), only rarely affects a user. 2019ID00232

[0020] - 4 - disturb. The minimum operating time may be, for example, at least 5 minutes, preferably at least 15 minutes, and more preferably at least 30 minutes.

[0021] The procedure may include switching to the first operating mode after a maximum operating time in the second operating mode has elapsed. The maximum operating time in the second operating mode ensures that the second operating mode, or any associated power drop and / or load change reaction (e.g., a "jerking"), only briefly disturbs the user. The maximum operating time in the second operating mode may, for example, be up to 15 seconds, preferably up to 10 seconds, and more preferably up to 5 seconds.

[0022] The method may include: monitoring whether a load-free state exists in the first operating mode; and switching to the second operating mode in a load-free state. This prevents user frustration due to a (temporary) load drop. Preferably, the switch to the second operating mode only occurs in a load-free state. The load-free state preferably corresponds to a low-load state.

[0023] A no-load state may be detected if the current supplied to the stator does not exceed a certain threshold. By monitoring or measuring the current supplied to the stator, it can be determined whether switching to the second operating mode will have any significant impact on the user. An example application is when the electric motor is operated using a speed controller to maintain a predetermined rotor speed.

[0024] The procedure may include: switching to the second operating mode upon user input to exit the first operating mode. User input to exit the first operating mode could be, for example, pressing a power switch or ending a power-on process. For instance, this could include flipping a toggle switch or releasing a pressed switch. This 2019ID00232

[0025] - 5 -

[0026] This variant has the advantage that the user is only minimally affected by switching to the second operating mode. The user input for ending the first operating mode can, for example, be used to detect the load-free state.

[0027] The stator may contain at least two controllable windings; the control of the stator to cancel the magnetic flux involves generating a stator magnetic field which, in the section of the gap, is opposite in magnitude and direction to the magnetic field of the rotor. This represents a preferred embodiment of the method because the magnetic field in the stator can be very precisely controlled by controlling the at least two windings. A controllable winding can be understood as a winding that can be controlled separately, individually, or independently. This preferably means that the respective controllable winding is connected individually, and that control electronics can selectively supply the winding with a different electrical phase than the other windings and / or switch it off.

[0028] The second operating mode of the method may include: acquiring the actual angular position of the rotor; the stator is then controlled based on this acquiring position. Alternatively, the stator may be controlled based on a calculated angular position. Another possibility is to control the stator based on a measured magnetic field orientation of the rotor. If the rotor magnetic field is known, measuring the actual angular position of the rotor represents a very reliable and easily implemented solution.

[0029] The rotor may be configured to permanently generate the rotor-fixed magnetic field. The proposed method is preferably applied to an electric motor whose rotor carries a permanent magnet for generating the rotor-fixed magnetic field.

[0030] It may be that the rotor is configured to variably generate the rotor-fixed magnetic field; wherein in the second operating mode the stator and the rotor to 2019ID00232

[0031] - 6 -

[0032] The magnetic flux can be controlled in a coordinated manner. For example, the rotor can carry at least one winding which can be energized (supplied with electric current) via an interface such as a pair of sliding contacts. This represents another preferred embodiment of the electric motor. In this embodiment, preferably in the first operating mode, the stator and the rotor are controlled in a coordinated manner to rotate the rotor.

[0033] Furthermore, a computer program product is proposed which contains commands that, when executed by a computer, cause it to perform the aforementioned method for operating a mobile machine tool. In this context, "computer" refers specifically to a control unit for the mobile machine tool.

[0034] A computer program product, such as a computer program tool, can be provided or delivered from a server on a network, for example, as a storage medium such as a memory card, USB stick, CD-ROM, DVD, or as a downloadable file. This can be done, for example, in a wireless communication network by transmitting the corresponding file containing the computer program product or tool.

[0035] To solve the aforementioned problem, a control unit for a mobile machine tool is proposed. The control unit is designed and suitable for controlling at least one stator of an electric motor of the mobile machine tool. The proposed control unit is prepared to execute the proposed method for operating the machine tool as described above. Control, in this context, refers specifically to applying an electric current to the stator or one or more windings of the stator. The embodiments and features described for the proposed method apply accordingly to the proposed control unit, so that the control unit can perform the corresponding functions.

[0036] Benefits realized. 2019ID00232

[0037] - 7 -

[0038] To solve the aforementioned problem, a drive assembly for a mobile machine tool is also proposed. The proposed drive assembly comprises the proposed control unit and the electric motor. The electric motor has a rotor for generating the fixed magnetic field and a stator for generating the rotating magnetic field. At least the stator of the electric motor is electrically coupled to the control unit in a manner suitable for control. As described above, the rotor can be configured to variably generate the fixed magnetic field, with the control unit preferably also being configured to supply the rotor with electric current and being connected to the rotor. The proposed drive assembly preferably integrates the proposed control unit and the electric motor into a single, easily handled and / or jointly mountable unit.The drive assembly may, for example, include other components such as an electrical power supply and / or a gearbox. The embodiments and features described for the proposed method and / or the proposed control unit apply accordingly to the proposed drive assembly, so that the drive assembly realizes the corresponding advantages.

[0039] To solve the aforementioned problem, a mobile machine tool is proposed. The proposed machine tool has an electric motor. The electric motor has a rotor for generating a fixed magnetic field and a stator for generating a rotating magnetic field. The machine tool is configured to operate the proposed method described above. For example, the proposed machine tool has the proposed control unit, which is coupled to the electric motor. The control unit and the electric motor are preferably combined into a single, handleable and / or mountable unit, the proposed drive assembly. The control unit and the electric motor can, for example, be distributed within the machine tool, such as by being handled and mounted separately and only then, or during, assembly.The embodiments and features described for the proposed method, for the proposed control unit and / or for the proposed drive assembly 2019ID00232.

[0040] - 8 - apply accordingly to the proposed machine tool, so that the mobile machine tool realizes the corresponding advantages.

[0041] Other possible implementations of the invention also include combinations of features or embodiments described previously or subsequently with regard to the exemplary embodiments, even if not explicitly mentioned. In such cases, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the invention.

[0042] 2019ID00232

[0043] - 9 -

[0044] Further advantageous embodiments and aspects of the invention are the subject of the dependent claims and the exemplary embodiments of the invention described below. The invention will be explained in more detail below with reference to preferred embodiments and the accompanying figures.

[0045] BRIEF DESCRIPTION OF THE FIGURES

[0046] Fig. 1 schematically shows a structure of a mobile machine tool according to an embodiment of the invention;

[0047] Fig. 2 schematically shows a control of at least one stator to cancel a magnetic flux in a gap between the stator and a rotor of an electric motor of the mobile machine tool;

[0048] Fig. 3 schematically shows a flowchart of a method for operating a mobile machine tool according to an embodiment of the invention in the case that the mobile machine tool has a rotor which carries at least one permanent magnet for generating a rotor-fixed magnetic field; and

[0049] Fig. 4 schematically shows a flowchart of a method for operating a mobile machine tool according to an embodiment of the invention in the case that the mobile machine tool has a rotor which carries at least one controllable winding for generating a rotor-fixed magnetic field.

[0050] 2019ID00232

[0051] - 10 -

[0052] In the figures, identical or functionally equivalent elements have been given the same reference symbols, unless otherwise indicated.

[0053] For example, a mobile machine tool 1 has an electrical power source 2 (actually: source 2 of electrical current) to provide electrical current for powering the mobile machine tool 1. The power source 2 can, for example, contain a battery. The power source 2 can, for example, contain a power supply unit.

[0054] The mobile machine tool 1 has, for example, a control unit 3. The control unit 3 can, for example, be configured to execute a procedure M10 and / or M14 described later for operating the mobile machine tool 1.

[0055] The mobile machine tool 1 has an electric motor 4, which has the rotor 5 and the stator 6.

[0056] The control unit 3 is preferably designed to control a rotor 5 and / or a stator 6. For this purpose, the control unit 3 is, for example, connected separately to several windings of the stator 6.

[0057] The control unit 3 can also be coupled, for example, with an operating device such as an on / off switch. Furthermore, the control unit 3 can be coupled with device sensors, such as a speed sensor for detecting the actual rotational speed of the rotor 5.

[0058] The rotor 5 is arranged to deliver rotational power. For example, the rotor 5 is coupled via a gearbox 7 to a holder 8 for a tool.

[0059] The electric motor 4 has a gap 9 between the rotor 5 and the stator 6. The gap 9 can be described as an air gap and / or an annular gap. 2019ID00232

[0060] - 11 -

[0061] Furthermore, the electric motor 4 preferably has a rotor-fixed fan 10. The rotor-fixed fan is arranged and configured to generate an airflow through the gap 9.

[0062] The electric motor 4 and the control unit 3 are supplied together as a pre-assembled drive assembly 11. The pre-assembled drive assembly 11 optionally also includes the gearbox 7.

[0063] The rotor 5 is designed to generate a rotor-fixed magnetic field 11.

[0064] For example, rotor 5 carries a permanent magnet. For example, rotor 5 carries a winding that can be supplied with electric current via a pair of sliding contacts.

[0065] The stator 6 is configured to generate a rotating magnetic field 12. For example, the stator 6 has at least two windings that can be independently supplied with electric current.

[0066] During operation of the mobile machine tool 1, metallic dust can accumulate on the inside of the stator 6 and / or on the outside of the rotor 5. In other words, metallic dust may settle in the air gap 9. To clean the air gap 9, the control unit 3 can execute the following procedure M10 or M13 to operate the electric motor 4.

[0067] The following describes, with reference to Fig. 3, the method M10, which is intended for a rotor 5 with a permanent magnet for generating the rotor-fixed magnetic field 11. The described method M10 includes optional features.

[0068] The electric motor 4 is initially operated in a first operating mode M11. This first operating mode serves to deliver power to the tool holder 8. It can be said that this first operating mode fulfills the intended task of the mobile machine tool 1. 2019ID00232

[0069] - 12 -

[0070] In step S10, the stator 6 is controlled to rotate the rotor 5. For this purpose, the stator 6 is supplied with electric current by the control unit 3. For example, the stator 6 may generate the rotating magnetic field 12 along a q-axis of the rotor 5, which is perpendicular to the orientation of the S-pole and N-pole of the rotor 5.

[0071] The rotation of the rotor 5 has several effects. First, the tool performs work at the holder 8. During this work, for example, metal dust is produced, which is deposited in the gap 9. In addition, the rotor 5 is accelerated to a relatively high speed.

[0072] In step S11, it is checked whether a minimum operating time has elapsed in the first operating mode M11. This minimum operating time is measured specifically since the last execution of a second operating mode M12. If the minimum operating time in the first operating mode M11 has elapsed, a decision is made, for example, to switch to the second operating mode M12. The switch to the second operating mode M12 may occur immediately. Further checks may also be performed to ensure reliable and / or convenient operation of the mobile machine tool 1.

[0073] In step S12, it is monitored whether a load-free state exists. For example, in sub-step S13, it is recognized that the load-free state exists if the electrical current supplied to the stator 6 does not exceed a preset threshold value.

[0074] For example, in another sub-step S14, the presence of a load-free state is detected based on user input. Specifically, the release of a pressure switch is recognized as user input. The pressure switch is wired so that a user can request power output (power above no-load, e.g., the rated power of mobile machine tool 1) by pressing the switch. When the user releases the pressure switch, this means that the user is not requesting any power output. 2019ID00232

[0075] - 13 -

[0076] Therefore, this is a suitable trigger moment to switch to the second operating mode.

[0077] M12.

[0078] In step S15, based on the presence of a load-free state, it is decided that the system can switch to, or is switched to, the second operating mode M12.

[0079] In step S16, a decision is made, depending on the rotor speed, as to whether the second operating mode M12 can be activated or is activated. This is advantageous, for example, if executing the second operating mode M12 requires a minimum rotational speed to generate an airflow through the gap 9 using the fan 10, while in the second operating mode, insufficient torque is generated to drive the fan 10. In this case, the fan 10 must therefore convert at least some of its kinetic energy into air movement. The minimum rotational speed may also be required to prevent a user from being disturbed or confused by an excessive drop in rotational speed.

[0080] Next, the second operating mode M12 is described. The second operating mode M12 ensures the cleaning of gap 9. The second operating mode thus serves to maintain the functional condition of the mobile work machine 1.

[0081] It is possible that in step S17, the fan 10 is driven to generate the airflow in the gap 9. For example, a separate fan drive is supplied with electrical current.

[0082] In step S18, the actual angular position of rotor 5 is recorded. For example, the control unit 3 is signal-coupled to an angle sensor that is rotationally coupled to rotor 5. The angle sensor can be installed, for example, on rotor 5, possibly on the fan 10 which is rigidly coupled to rotor 5, and / or in the gearbox 7.

[0083] In step S19, the stator 6 is finally activated to cancel a magnetic flux 11 and 12 in the gap 9. For this purpose, the rotating magnetic field 12 is actuated as follows: 2019ID00232

[0084] - 14 - is positioned such that the rotor-fixed magnetic field 11 is exactly canceled out, at least in a partial region 13 of the gap 9. It can be said that the co-rotating magnetic field 12 is adjusted to equal the magnitude of the rotor-fixed magnetic field 11 and to be directed oppositely, at least in section 13. In section 13, a magnetic flux is canceled out.

[0085] In step S19, the stator 6 can, for example, be controlled depending on the actual angular position recorded in S18. For example, a field strength of the rotor-fixed magnetic field 11 can be determined in advance from a simulation, and on the basis of this, a current strength required to cancel this field can be determined, which is to be supplied to the stator 6 or its windings.

[0086] Preferably, the co-rotating magnetic field 12 is aligned along a d-axis of the rotor 5.

[0087] The generation of the airflow in S17 and the cancellation of the magnetic flux in S19 preferably occur simultaneously. The measurement of the angular position in S18, for example, occurs simultaneously or at intervals during the cancellation of the magnetic flux in S19.

[0088] In step S20, a decision is made to switch to the first operating mode M11 after a maximum operating time in the second operating mode M12 has elapsed. The maximum operating time in the second operating mode M12 is, for example, designed to prevent the rotational speed of rotor 5 from dropping too low. On the other hand, the maximum operating time is also designed to ensure sufficient cleaning of the gap 9. The selected value of the maximum operating time therefore represents a compromise. The maximum operating time could, for example, be a preset value of up to 10 seconds, such as 5 seconds. The maximum operating time can also be determined, for example, as a function of the rotor speed.

[0089] The M10 method with operating modes M11 and M12 ensures a constant

[0090] Power output of mobile machine tool 1 achieved over a long service life. 2019ID00232

[0091] - 15 -

[0092] The following describes a method M13 for operating the mobile machine tool 1 with reference to Fig. 4. Method M13 is designed for a rotor 5 with a controllable winding for generating the rotor-fixed magnetic field 11. The described method M13 includes optional features. Only differences from method M10 are discussed.

[0093] In step S21 of the first operating mode M11, the stator 6 and the rotor 5 are controlled to rotate the rotor 5. For example, the rotor 5, i.e., at least one winding of the rotor 5, is supplied with an electric current under DC voltage.

[0094] For example, the stator 6, i.e. at least two windings of the stator 6, is supplied with an electric current under alternating voltage.

[0095] In step S22 in the first operating mode M11, the presence of the load-free state is detected based on an electric current of an electric current supplied to the stator 6 and / or an electric current of an electric current supplied to the rotor 5.

[0096] In step S23 of the second operating mode M12, the stator 6 and the rotor 5 are jointly controlled to eliminate the magnetic flux in section 13 of the gap 9. "Jointly controlled" preferably means that the (respective) electric current supplied to the stator 6 or windings of the stator 6, on the one hand, and the electric current supplied to the rotor 5 or at least one winding of the rotor 5, on the other, are dimensioned to generate a co-rotating magnetic field 12 and a rotor-fixed magnetic field 11, which together render section 13 magnetically free.

[0097] Furthermore, reference is made to the description of procedure M10.

[0098] Although the present invention has been described using exemplary embodiments, it can be modified in many ways. 2019ID00232

[0099] - 16 -

[0100] REFERENCE MARK LIST

[0101] 1 mobile machine tool

[0102] 2 Power supply

[0103] 3 Control unit

[0104] 4 electric motor

[0105] 5 Rotor

[0106] 6 Stator

[0107] 7 gearboxes

[0108] 8th recording

[0109] 9 columns

[0110] 10 fans

[0111] 11 Rotor-fixed magnetic field

[0112] 12 rotating magnetic field

[0113] 13 Area of ​​a suspended magnetic flux

[0114] S S-Pol

[0115] N N-pole q q-axis d d-axis

[0116] M10 Method for operating a machine tool

[0117] M11 first operating mode

[0118] 510 Controlling the stator to rotate the rotor

[0119] 511 Switching to the second operating mode after a period of time

[0120] Minimum operating time

[0121] 512 Monitor whether a load-free state exists

[0122] 513 Detecting a load-free state depending on a current intensity

[0123] 514 Detecting user input as a load-free state

[0124] 515 Switching to the second operating mode only in a load-free state

[0125] 516 Switching to the second operating mode depending on a rotor speed

[0126] M12 second operating mode

[0127] S17 Driving a fan to generate an airflow in the gap 2019ID00232

[0128] - 17 -

[0129] 518 Recording the actual angular position of the rotor

[0130] 519 Controlling the stator to cancel a magnetic flux

[0131] 520 Switching to the first operating mode after the expiry of a maximum operating time M13 Procedure for operating a machine tool

[0132] 521 Controlling the stator and rotor to rotate the rotor

[0133] 522 Detecting a load-free state as a function of current

[0134] 523 Controlling the stator and rotor together, coordinated to cancel the magnetic flux

Claims

2019ID00232 - 18 - PATENT CLAIMS 1. Method (M10, M13) for operating a mobile machine tool (1) having an electric motor (4) having a rotor (5) for generating a rotor-fixed magnetic field (11) and a stator (6) for generating a rotating magnetic field (12), wherein the method has a first operating mode (M11) comprising: Controlling (S10, S21) the stator (6) to rotate the rotor (5); characterized in that the method (M10, M13) has a second operating mode (M12) which includes: Controlling (S19, S23) the stator (6) to cancel a magnetic flux at least in a co-rotating section (13) of a gap (9) between the rotor (5) and the stator (6).

2. Method according to claim 1, characterized in that the method (M10, M13) includes: switching (S16) to the second operating mode (M12) only if the rotational speed of the rotor (5) does not fall below a minimum rotational speed.

3. Method according to one of the preceding claims, characterized in that the method (M10, M13) in the second operating mode (M12) includes: driving (S17) a fan (10) to generate an airflow in the gap (9).

4. Method according to one of the preceding claims, characterized in that the method (M10, M13) includes: switching (S11) to the second operating mode (M12) after a minimum operating time has elapsed in the first operating mode (M11).

5. Method according to one of the preceding claims, characterized in that the method (M10, M13) includes: switching (S20) to the first operating mode (M11) after a maximum operating time has elapsed in the second operating mode (M12). 2019ID00232 - 19 - 6. Method according to one of the preceding claims, characterized in that the method (M10, M13) includes: monitoring (S12) whether a load-free state exists in the first operating mode; and switching (S15) to the second operating mode in a load-free state.

7. Method according to claim 6, characterized in that a load-free state is detected (S13) if the current strength of an electric current supplied to the stator (6) does not exceed a threshold value.

8. Method according to one of the preceding claims, characterized in that the method (M10, M13) includes: switching (S14) to the second operating mode (M12) upon a user input to terminate the first operating mode (M11).

9. Method according to one of the preceding claims, characterized in that the stator (6) contains at least two controllable windings, wherein the control (S19, S23) of the stator (6) to cancel the magnetic flux comprises: generating the co-rotating magnetic field (12) such that the co-rotating magnetic field (12) in the co-rotating section (13) of the gap (9) is equal in magnitude and opposite in direction to the rotor-fixed magnetic field (11).

10. Method according to one of the preceding claims, characterized in that the method (M10, M13) in the second operating mode includes: detecting (S18) an actual angular position of the rotor (5); wherein the stator (6) is controlled on the basis of the detected actual angular position of the rotor (5).

11. Method (M10) according to one of claims 1 to 10, characterized in that the rotor (5) is configured to permanently generate the rotor-fixed magnetic field (11).

12. Method (M13) according to one of claims 1 to 10, characterized in that the rotor (5) is configured to variably generate the rotor-fixed magnetic field (11) 2019ID00232 - 20 - is, wherein in the second operating mode (M12) the stator (6) and the rotor (5) are controlled in a coordinated manner to cancel the magnetic flux (S23).

13. Computer program product, containing instructions which, when the program is executed by a computer, in particular a control unit (3) for a mobile machine tool (1), cause this computer to execute the method (M10, M13) according to one of claims 1 to 12.

14. Control unit (3) for a mobile machine tool (1), wherein the control unit (3) is configured to control at least one stator (6) of an electric motor (4) of the machine tool (1), characterized in that the control unit (3) is prepared to carry out the method (M10, M13) according to one of claims 1 to 12.

15. Machine tool (1), in particular mobile machine tool (1), which has an electric motor (4) having a rotor (5) for generating a rotor-fixed magnetic field (11) and a stator (6) for generating a rotating magnetic field (12), wherein the machine tool (1) is configured to carry out the method (M10, M13) according to one of claims 1 to 12.

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