Method for operating a field-guided electric motor

Abstract

The invention relates to a method and a device for operating a field-guided electric motor (1) on a battery pack (3). In motor operation, the motor current (7) consists of a field-generating motor current component (i d ) and a torque-generating motor current component (i q ) together, which form a current amplitude (A, A') of the motor current (7). In order to brake the electric motor quickly with the option of switching back to acceleration at any time, the braking operation is divided into two temporally separated braking sections (B1; B2). In a first braking section (B1), the current amplitude (A) of the motor current (7) is adjusted by adjusting the field-forming motor current component (i dThe motor current (7) is held constant during a second braking phase (B2), while the current amplitude (A') of the motor current (7) is variable. The braking operation switches from the first braking phase (B1) to the second braking phase (B2) when a predetermined rotational speed (n) is undershot. G ) of the electric motor (1).

Description

[0001] The invention relates to a method for operating a field-guided electric motor, which is operated by a control device connected to at least one battery pack with a supply voltage. The electric motor has a stator and a rotor, the stator carrying several field windings. During motor operation, to generate a driving electromagnetic rotating field, the field windings are energized by the control device from the battery pack depending on the rotational position of the rotor, wherein the amplitude of the motor current consists of a first, field-generating motor current component i d and a second, torque-generating motor current component i q It is composed of... In regenerative braking operation, the voltages induced in the field windings of the stator when the rotor is running cause a torque-generating motor current component i that determines the braking torque. q, from which a recuperation current can be derived to feed power back into the battery pack. The recuperation current is fed into the battery pack for charging.

[0002] Charging a battery pack depends, among other things, on its capacity and its specifications; for example, a battery pack has a maximum permissible discharge current and a maximum permissible charging current. These specifications also depend on the structure of the individual cells used in the battery pack, the type of individual cells (lithium-ion, lithium-polymer, lithium-iron, nickel-metal hydride, or similar energy storage devices), the temperature of the battery pack, and other parameters.

[0003] During regenerative operation of the electric motor, the recuperation current flowing to charge the battery pack must not exceed the maximum permissible charging current of the battery pack in use. The maximum permissible charging current of the battery pack depends on the torque-generating motor current component used to brake the rotor. At the same time, the magnitude of this torque-generating motor current component also determines the coasting time of the electric motor until it comes to a complete stop.

[0004] When an electric tool is operated with a power tool, the electric motor experiences corresponding run-down times before coming to a complete stop. During operation, the user often only interrupts their work briefly, for example, to change position. As soon as the user releases the operating element (also known as the throttle), the control device enters electric braking mode. However, before the tool or electric motor comes to a complete stop, the user engages the operating element again (revs the engine) to resume work. Because the electric motor is in braking mode, there can be a delay before it accelerates again, which the user may find disruptive.

[0005] The invention is based on the objective of providing a method for operating a field-guided electric motor that, on the one hand, enables rapid braking of the electric motor to a standstill, but on the other hand, allows switching to acceleration of the electric motor at any time during braking operation.

[0006] The problem is solved by a method according to claim 1. A device for carrying out the method is specified in claim 9.

[0007] According to the invention, the braking operation of the electric motor is divided into at least two temporally separated braking phases. In a first braking phase, the current amplitude of the motor current is kept constant by adjusting the field-generating motor current component. In a second braking phase, the current amplitude of the motor current is variable. The braking operation switches from the first braking phase to the second braking phase when the speed of the electric motor falls below a predetermined level.

[0008] In this optimized braking method, a constant motor current amplitude is maintained. The distribution between a torque-generating motor current component i q and a field-forming motor current component i dThe system is configured so that the motor current amplitude remains constant during the first braking phase. This motor current can be used as recuperation current to charge the battery pack. Only after a predetermined speed limit is reached does the system switch to the second braking phase with variable current amplitude.

[0009] According to the inventive method, a control device for operating the electric motor during braking ensures that the current amplitude remains constant during the first braking phase. Despite the limitation of the motor current amplitude, rapid and effective braking of the electric motor within a predetermined braking time is possible. Simultaneously, the electric motor can be switched to acceleration at any time during braking. This is achieved by varying the field-forming first motor current component i. d If not equal to zero, the moment-generating motor current component i is immediately qits size is adjusted, thereby simultaneously generating a field-forming current flow that causes electrical power loss in the stator. This is due to the field-forming motor current component i d Conditional ohmic power loss increases the braking power of the electric motor, so that without an increase in the torque-generating motor current component i q Depending on the recuperation current exceeding the specified limit, an increased, controlled braking power is available, ensuring rapid deceleration of the electric motor and thus of the tool within a specified braking time.

[0010] The current amplitude in the first braking phase is adjusted, in particular, as a function of the temperature of a control device and / or the electric motor. The temperature of the electric motor is determined at the winding and / or the permanent magnet. The temperature of the control device is determined at the electronic switching elements. It is also advantageous to adjust the current amplitude as a function of the supply voltage applied to the electric motor. Most importantly, the current amplitude is adjusted as a function of the electric motor's inductance and / or its magnetic flux.

[0011] In a further development of the invention, the field-forming motor current component i is in the first braking section. dThe motor current is set to a non-zero value. This can be used in particular to generate recuperative power feed-in to the battery pack by adjusting the field-forming motor current component i. d to limit. As the recuperation current for charging the battery pack approaches the specified limit, it influences the field-forming second motor current component i. d taken and this is changed. If the recuperation current for charging the battery pack tends to exceed a predetermined limit, the field-forming motor current component i d set to a value other than zero, such that the predetermined limit of the recuperation current for charging the battery pack is not exceeded. In a further development of the invention, it is provided that during braking operation the setting of the field-forming first motor current component i d such that, during braking operation of the electric motor, the same recuperative energy return power P is maintained. AkkuThe braking power of the electric motor increases in the battery pack. In particular, during braking, the field-forming, first motor current component increases as the rotational speed decreases. d ab, the second, torque-generating motor current component i q is increasing.

[0012] It can be advantageous if the first braking phase has a duration that is greater than or equal to the duration of the second braking phase. In particular, the ratio of the duration of the first braking phase to the duration of the second braking phase should be a maximum of 10:1, a maximum of 4:1, a maximum of 3:1, a minimum of 2:1, and a minimum of 1:1.

[0013] In one possible embodiment of the method, a three-phase rotating field is established during the operation of the field-guided electric motor, whereby the currents flowing in the field windings i a , i b , i cThe three-phase rotating field is captured as vectors of the rotating field. These captured vectors of the rotating field are electronically transformed into a motor current in two-dimensional representation. The first, field-forming motor current component i d and a second, torque-generating motor current component i that determines the braking torque q The composite motor current of the two-dimensional representation is adjusted such that, during braking operation of the electric motor, the field-forming second motor current component i d is not equal to zero, e.g., it is set to greater than zero or less than zero. The setting is preferably made such that the first field-forming motor current component i d is set in such a way that the recuperation current for charging the battery pack does not exceed a predefined limit. After setting the field-forming second motor current component i dIn the two-dimensional representation, the values ​​are transformed back into the three-phase rotating field and applied to the electric motor via the control device.

[0014] A device for carrying out the method for braking a field-guided electric motor consisting of a stator and a rotor includes a battery pack for operating the electric motor via a control device provided between the electric motor and the battery pack for adjusting the motor current. The stator of the electric motor carries several field windings, in particular three field windings offset from each other at an electrical angle of 120°, which are arranged to form a rotating electromagnetic field. The control device is designed to drive the field windings of the stator in the direction of rotation during motor operation, depending on the rotational position of the rotor, and furthermore, to supply the motor current resulting from the voltages induced in the field windings of the stator as regenerative power to the battery pack for charging during braking operation. The control device includes a converter which is configured to measure the currents flowing in the field windings.a , i d , i c to capture the multiphase rotating field as vectors of the rotating field and to electronically transform them into a motor current in a two-dimensional representation. The motor current in the two-dimensional representation consists of a field-forming first motor current component i d and a second torque-generating motor current component i that determines the braking torque qThe control device includes a control element suitable for adjusting the motor current components of the two-dimensional motor current depending on the operating state of the electric motor and on predetermined limit values ​​such that, during braking operation of the electric motor, the current amplitude of the motor current is constant in at least a first braking section by adjusting the field-forming motor current component, and the current amplitude of the motor current is variable in at least a second, temporally separated braking section, wherein a switching device is provided which is suitable for switching from the at least one first braking section to the at least one second braking section of the braking operation when a predetermined speed of the electric motor is undershot.

[0015] In particular, the control device is configured to adjust the current amplitude as a function of the temperature of the control device and / or the electric motor. Advantageously, the control device is configured to adjust the current amplitude as a function of the supply voltage applied to the electric motor. Most importantly, the control device is configured to adjust the current amplitude as a function of the inductance of the electric motor and / or the magnetic flux of the electric motor.

[0016] In a further development of the device, the control device is designed in such a way as to set the field-forming motor current component of the motor current to a non-zero level by means of the control element in at least a first braking section.

[0017] It is advantageous to design the control device such that the recuperative power fed back into the battery pack is limited by adjusting the field-generating motor current component using the control element. It can be provided that, during braking operation, the control device adjusts the field-generating first motor current component in such a way that, during braking operation of the electric motor, the braking power of the electric motor increases with the same recuperative power fed back into the battery pack.

[0018] In a further development of the invention, the control device is designed to operate the electric motor for a certain period of time in the first braking section, wherein the duration of the first braking section is greater than or equal to the duration of the second braking section.

[0019] The field-guided electric motor is designed to generate a three-phase rotating magnetic field during operation. The currents flowing in the field windings of this three-phase field are detected as vectors of the rotating field and electronically transformed into a motor current in two-dimensional representation. This motor current, composed of the first, field-generating motor current component and the second, torque-generating motor current component, is adjusted by the control element such that the field-generating motor current component is not zero during braking operation of the electric motor.

[0020] The electric motor can be a synchronous motor or an asynchronous motor.

[0021] An electric motor is advantageous as the drive motor in a handheld power tool, especially a portable one. A handheld power tool, particularly a ground-based tool, can be, for example, a lawnmower, a rotary tiller, an angle grinder, or similar equipment. Examples of handheld, portable power tools include chainsaws, angle grinders, brush cutters, and similar tools, especially battery-powered ones.

[0022] Further features of the invention will become apparent from the further claims, the description, and the drawing, in which an exemplary embodiment of the method and the apparatus is subsequently described. The features of the claims and those disclosed in the description and in the drawings can be combined with one another within the scope of the invention, in particular in any way.

[0023] They show: Fig. 1. A schematic representation of the basic structure of a circuit arrangement for operating a field-guided electric motor on a battery pack. Fig. 2 a schematic representation of a field-guided electric motor with field windings arranged in the stator offset by 120° to each other, Fig. 3 in schematic representation a brake circuit preferably provided in the control device for adjusting the braking current, Fig. 4 in schematic representation the electric motor with a motor current as recuperative braking current with a torque-generating motor current component i q and with a field-forming motor current component set to zero i d , Fig. 5 a schematic representation accordingly Fig. 4 with a regenerative braking current with a torque-generating motor current component i q and with a non-zero field-forming motor current component i d , Fig. Figure 6 shows a schematic representation of a device for adjusting the motor current components i q and i d of the motor current during braking operation, Fig. Figure 7 shows a schematic representation of a braking operation consisting of two braking sections with different amplitudes of the motor current, Fig. 8 in schematic representation the motor current flowing during braking operation, which consists of a motor current component i q and a motor current component i d is composed of...

[0024] In Fig. Figure 1 shows a field-guided electric motor 1, which is controlled via a control device 2 from a battery pack 3 with a supply voltage U VThe control device 2 comprises a control unit 5 and electronic switching elements 6. The battery pack 3 consists of a plurality of individual cells 4, which are electrically interconnected within the battery pack 3 to form a cell assembly. The individual cells 4 can be lithium-ion cells, lithium-polymer cells, lithium-iron cells, or individual cells of other chemical compositions, e.g., NiCd, NiMh, or similar cells.

[0025] The control device 2 is connected to the supply voltage U. V of the battery pack 3, which is available as a DC voltage. A control unit 5, in particular a microprocessor, controls a control circuit consisting of electronic switching elements 6, in particular MOSFETs. By appropriately controlling the switching elements 6, operating currents i flow into the electric motor 1, which is advantageously designed as a field-guided, three-phase electric motor. a , i b and i c existing motor current 7 to.

[0026] As in Fig. As shown in Figure 2, the electric motor 1 has a stator 8 and a rotor 9. The stator 8 carries field windings a, b, and c, which are arranged around the circumference of the stator 8 with an angular spacing w of 120°. The Fig. 1 shown operating currents i a , i b and i c are assigned to the respective field windings a, b and c. The rotor 9 carries at least one permanent magnet with magnetic poles N and S. During operation of the electric motor 1, the control unit 5 energizes the field windings a, b and c, depending on the rotational position of the rotor 9, to generate a driving electromagnetic rotating field in the direction of rotation 10.

[0027] The motor current 7 can, in principle, be divided into a first field-forming motor current component i, both in motor operation and in braking operation. d as well as a second, torque-generating motor current component i qdivide. The field-forming motor current component i d This causes the electromagnetic rotating field to be built up by the field windings a, b and c, while the motor current component i q a driving torque of the rotor 9 is produced. In a broader sense, the motor current component i causes q an active power and the motor current component i d A reactive power of electric motor 1 during operation. In a two-dimensional vector representation of the motor current components i d , i q , the motor current 7 with a current amplitude A is obtained as a vector.

[0028] In a preferred embodiment of the control device 2, a brake circuit 20 is provided therein, as exemplified in Fig. 3 is shown. The brake circuit 20 can also be provided as a circuit arrangement separate from the control device 2. In Fig. 3 is a schematically represented brake circuit 20 connected to the electric motor 1. It includes a control circuit 21 for setting a recuperation current 27, which can also be referred to as negative motor current 7. The control circuit 21 is connected to a monitoring circuit 22 of the battery pack 3, which monitors the voltage V Akku as well as the performance P Akku The battery pack 3 is informed. From these values, the monitoring circuit 22 determines the magnitude of the permissible recuperation current 27, which may be supplied to the battery pack at most. The recuperation current 27 of the electric motor 1 (negative motor current 7) is divided into a torque-generating motor current component i q and a loss-generating, field-generating motor current component i d The field-forming motor current component i d is dependent on the engine speed n Motorof the electric motor 1 and the recuperation current 27 determined from the parameters of the battery pack 3, which is derived from the torque-generating motor current component i q is derived, given. Thus, the input variables of motor speed n can be used. Motor and the motor current component i determining the size of the recuperative recuperation current 27 q from a characteristic map or a memory 23 the field-forming motor current component to be set i d The data is read out and specified to the control circuit 21.

[0029] In Fig. Figure 4 shows a schematic representation of the electric motor 1 in braking mode. The recuperation current 27, shown as a negative motor current, is exclusively dependent on the torque-generating motor current component i. q determined, since the field-forming motor current component i dis set to zero. The braking time of the electric motor 1 is essentially determined by the recuperation current 27, which corresponds to the torque-generating motor current component i. q corresponds to the torque-generating motor current component i q determines the recuperation current 27 for charging the battery pack 3. The recuperative energy return power P resulting during braking operation Akku The capacity of battery pack 3 can be roughly estimated using the following formula: PAkku=const=1.5(iq*p*ΨPM*ωmech−R*iq2−R*Id2)with M~iq and Id=0 and the variables: j q = torque-generating motor current component p = number of pole pairs Ψ PM = linked magnetic flux ω mech = mechanical angular velocity R = ohmic resistance of the field windings I d = field-forming motor current component

[0030] For the current amplitude I designated with reference numeral A ampof the electric motor 1 applies Iamp<ΨPML where L denotes an inductance of the electric motor 1 and Ψ PM the linked magnetic flux. The recuperative power is determined by PAkku=const.=1.5*(p*iq*ΨPM*ωmech−R*Iamp2) determined, where ω mech specifies a mechanical angular velocity.

[0031] Since the torque-generating motor current component iq is limited by the maximum permissible recuperation current 27 for charging the battery pack 3, the torque-generating motor current component i can be increased to enhance the braking performance of the electric motor 1. q It cannot be increased arbitrarily. This would result in an increase in the recuperation current (charging current) flowing to battery pack 3 and could therefore lead to an electrical overload of battery pack 3.

[0032] The field-forming motor current component i dto be set in such a way that, without exceeding the specified limit value of the recuperation current 27 for charging the battery pack 3, the torque-generating motor current component i q The braking power of electric motor 1 is not too large, yet it is increased. This is shown schematically in Fig. 5 shown.

[0033] Due to the braking operation of the electric motor 1, the torque-generating motor current component i q and the field-forming motor current component i d in the negative axis range of the schematic representation in Fig. 5. The amplitude A of the recuperation current 27 is a composite vector of the torque-generating motor current component i. q and the field-forming motor current component i d . How Fig. As 5 clearly shows, the vector of the recuperation current 27 is significantly larger than that in Fig. 4. Vector of the recuperation current shown 27. In Fig. 5 is the field-forming motor current component i dchosen to be so large that the torque-generating motor current component i q and thus the recuperation current 27 for charging the battery pack does not exceed a permissible limit. Nevertheless, the braking current 27 is significantly larger than in Fig. 4. By adjusting the size of the field-forming motor current component i d With high braking power, a precise adjustment of the torque-generating motor current component i is necessary. q derived recuperation current 27 for charging the battery pack 3 is ensured without exceeding the permissible maximum charging current in the battery pack.

[0034] A limit value of 30 for the torque-generating motor current component i can be advantageous. q such that the recuperation current 27 corresponds approximately, and in particular exactly, to a maximum permissible charging current in the battery pack 3.

[0035] In the embodiment according to Fig. 5 consists of the torque-generating motor current component iq and the field-forming motor current component i d a phase shift of 33° from 90°. The setting of the torque-generating motor current component i q can be achieved by adjusting the size of the field-forming motor current component i d take place.

[0036] In Fig. Figure 6 shows a device 40 for carrying out the method according to the invention. The control device 2 controls the rotating field of the electric motor 1 by controlling the field windings a, b, c with the control voltages u. a , u b , u c , which changes the operating currents i a , i b , i cthe field windings a, b, c result in, depending on the rotational position of the rotor, to drive it rotating in direction 10. The device 40 is equipped with a correspondingly designed control device 2 such that, during braking operation, the voltages induced in the field windings a, b, c of the stator 8 cause a recuperation current 27 to brake the electric motor 1, whose torque-generating motor current component i q a recuperative feed-in power P Akku in battery pack 3. For controlling the motor current components i q and i d For the purpose of achieving high braking performance, a converter 41 is provided for the braking current 27, which is designed to convert the currents flowing in the field coils a, b, c i a , i b , i c to capture the vectors of the rotating field of a multi-phase field, in particular a three-phase field, and to electronically convert them into a motor current with the motor current components i d , iq to be transformed into a two-phase representation. The braking current or the motor current is composed of the first, field-forming motor current component i. d and the second, torque-generating motor current component i, which determines the braking torque q The circuit arrangement includes a control element 42, which regulates the motor current components i d , i q of the two-dimensional braking current 27 as a function of the operating state of the electric motor 1 and of specified setpoint values ​​i dsoll , i qsoll such that, during braking operation of the electric motor 1, the first, field-forming motor current component i d the recuperation current 27 (braking current) is not equal to zero, such that at high regenerative power P Akku The braking power of the electric motor 1 increases in battery pack 3.

[0037] The setpoint values ​​of the two-dimensional representation specified by the control element 42 are transformed back into the three-dimensional representation via a further converter 43 and – e.g. – as voltage values ​​u aref , u bref , u cref - supplied to the control device 2 for controlling the electric motor 1. According to the specified voltage values, the control device 2 receives control voltages u a , u b , u c adjust which corresponds to the desired recuperation current 27 with the specified motor current component i that causes the braking torque q and the field-forming motor current component i d This setting of the motor current components i d , i q The values ​​supplied to the control element 42 by the converter 41 are continuously monitored and corrected. The recuperation current 27 used to charge the battery pack is different from the torque-generating motor current component i. qdetermined, whereby the limit of the recuperation current 27 is set to the maximum charging current of the connected battery pack 3.

[0038] The rotational speed of the electric motor or the rotational position of the rotor 9 of the electric motor is detected and fed to the converters 41 and 43 for processing.

[0039] As in Fig. As shown in Figure 7, the braking operation is divided into two operating sections B1 and B2. In the first operating section B1, the amplitude A of the recuperation current 27 is constant over a period of time T1. This is shown in Figure 7. Fig. 8 shown, by adjusting the field-forming motor current component I d executed. During braking operation of the electric motor 1, the rotational speed n will drop until a predetermined limiting speed n is reached. G is reached or falls below the limiting speed n. GIf the setpoint is reached or falls below the setpoint, the control device 2 or, advantageously, the brake circuit 20 integrated in the control device 2 switches via a switching device 100 ( Fig. 3) from the first braking section B1 to a second braking section B2. In the second braking section B2, the current amplitude A' is variable.

[0040] The specified rotational speed n G The speed is selected from a range of values ​​between a minimum of 10% and a maximum of 40% of the no-load speed of electric motor 1. Before braking begins, electric motor 1 is driven at a working speed. The specified speed n G The specified speed is undershot after the first braking phase B1 has elapsed. During the first braking phase B1, the rotational speed n decreases until the specified rotational speed n is reached. GThe specified speed is reached, or in particular, undershot. Specifically, the duration T1 of the first braking phase B1 is determined by the time from braking from the operation of the electric motor 1 at its operating speed until the specified speed n is undershot. G The duration T2 of the second braking phase B2 is determined by the duration of the switch from the first braking phase B1 when the specified rotational speed n is undershot. G until the electric motor comes to a standstill.

[0041] The first braking phase B1 has a duration T1 that is greater than or equal to the duration T2 of the second braking phase B2. In particular, the ratio of the duration T1 of the first braking phase B1 to the duration T2 of the second braking phase B2 is at most 10 to 1, in particular at most 4 to 1, in particular at most 3 to 1, in particular 2 to 1 and in particular at least 1 to 1.

[0042] The size of the motor current components i q and i d The set current amplitude A of the recuperation current 27 is adjusted depending on the temperature of the control device 2 or the brake circuit 20 and / or the electric motor 1. The temperature of the electric motor 1 is measured at the winding and / or the permanent magnet. The temperature of the control device 2 or the brake circuit 20 is measured at the electronic switching elements.

[0043] Additionally or alternatively, the magnitude of the current amplitude A can be set as a function of the magnitude of the supply voltage applied to the electric motor 1. The current amplitude A can also be set as a function of the inductance of the electric motor 1 and / or the line-linked magnetic flux of the electric motor 1. In particular, the field-generating motor current component i d The motor current is set depending on the characteristics of the electric motor 1.

[0044] In the first braking section B1, the field-forming motor current component is i d of the recuperation current 27 set to non-zero, as in Fig. Figure 8 shows the recuperative power P. Akku In battery pack 3, the field-forming motor current component i is adjusted. d limited by the torque-generating motor current component i q The braking torque is limited.

[0045] During braking operation, the adjustment of the field-forming first motor current component i d such that during braking operation of the electric motor 1, with the same recuperative energy return power P Akku The braking power of the electric motor 1 increases in the battery pack 3. The limits and braking power of the method according to the invention are shown. Fig.8. Reference numeral 14 indicates the voltage limit. Curve 24 represents a braking force of 0.5 Nm. Curve 34 represents a braking force of 1 Nm. Curve 44 represents a braking force of 2 Nm. Curve 54 represents a braking force of 3 Nm. The amplitude A of the recuperation current 27 (braking current) is adjusted by modifying the motor current components i. q and i d adjusted accordingly.

Claims

[1] Method for operating a field-guided electric motor (1), - with a control device (2) for operating the electric motor (1) on at least one battery pack (3) with a supply voltage (U V ), - wherein the electric motor has a stator (8) and a rotor (9), and the stator (8) carries several field windings (a, b, c), - and in motor operation to form a driving electromagnetic rotating field, the field windings (a, b, c) are energized from the battery pack (3) by the control device (2) depending on the rotational position of the rotor (9), - wherein the flowing motor current (7) consists of a first, field-forming motor current component (i d ) and a second, a torque-generating motor current component (i q ) is composed of, and form a current amplitude (A, A'), - and in braking operation of the electric motor (1) with rotating rotor (9) induced voltages in the field windings (a, b, c) of the stator (8) a torque-generating motor current component (i q ) to brake the rotor (9), characterized by , - that the braking operation includes at least two temporally separated braking sections (B1; B2), - that in a first braking section (B1) the current amplitude (A) of the motor current (7) is adjusted by adapting the field-forming motor current component (i d ) is constant, - that in a second braking section (B2) the current amplitude (A') of the motor current (7) is variable, - wherein a switch in braking operation from the first braking section (B1) to the second braking section (B2) occurs when a predetermined rotational speed (n) is undershot G ) of the electric motor (1) is carried out. [2] Method according to claim 1, characterized by, that a quantity of the current amplitude (A) is set as a function of the temperature of a control device (2) and / or the electric motor (1). [3] Method according to any one of the preceding claims, characterized by , that the magnitude of the current amplitude (A) depends on the magnitude of a supply voltage (U) applied to the electric motor (1). V ) is set. [4] Method according to any one of the preceding claims, characterized by , that the current amplitude (A) is set depending on an inductance of the electric motor (1) and / or a linked magnetic flux of the electric motor (1). [5] Method according to any one of the preceding claims, characterized by , that in the first braking section (B1) the field-forming motor current component (i d ) of the motor current (7) is set to non-zero. [6] Method according to any one of the preceding claims, characterized bythat a recuperative feed-in power (P Akku ) into the battery pack (3) by adjusting the field-forming motor current component (i d ) is limited. [7] Method according to claim 6 characterized by , that during braking operation the setting of the field-forming first motor current component (i d ) such that during braking operation of the electric motor (1) at the same recuperative energy return power (P Akku ) in the battery pack (3) a braking power of the electric motor (1) increases. [8] Method according to any one of the preceding claims, characterized by , that the first braking section (B1) has a duration (T1) that is greater than or equal to the duration (T2) of the second braking section (B2). [9] Method according to any one of the preceding claims, characterized by, that in the operation of the field-guided electric motor (1) a three-phase rotating field is established, that the currents flowing in the field windings (a, b, c) (i a , i b , i c ) of the three-phase rotating field are captured as vectors of the rotating field and electronically transformed into a motor current (7) in two-dimensional representation, such that the motor current component from the first, field-forming component (i d ) and the second, torque-generating motor current component (i q ) the compound motor current (7) of the two-dimensional representation is adjusted such that, during braking operation of the electric motor (1), the field-forming motor current component (i d ) is not equal to zero. [10] Device for carrying out a method for operating a field-guided electric motor (1) with a stator (8) and a rotor (9), with at least one battery pack (3) for operating the electric motor (1) with a motor current (7) with a current amplitude (A), and with a control device (2) electrically connected to the electric motor (1) and the at least one battery pack (3) for adjusting the current amplitude (A) of the motor current (7), wherein the stator (8) carries several field windings (a, b, c) arranged to form an electromagnetic rotating field, and the control device (2) is configured to drive the field windings (a, b, c) of the stator in the direction of rotation (10) depending on the rotational position of the rotor (9), and the control device (2) is configured to adjust the motor current (7) flowing in the field windings (a, b, c) of the stator (8) in braking operation, characterized by, that the device (40) has a converter (41) which is configured to convert the currents (i) flowing in the field windings (a, b, c) a , i b , i c ) of the multiphase rotating field as vectors of the rotating field and electronically transform them into a motor current (7) in two-dimensional representation, wherein the motor current (7) of the two-dimensional representation consists of a field-forming, first motor current component (i d ) and a torque-generating, second motor current component (i q ) is composed, and that the device (40) has a control element (42) suitable for controlling the motor current components (i d , i q) in a two-dimensional representation of the motor current (7) as a function of an operating state of the electric motor (1) such that in braking operation of the electric motor (1) in at least a first braking section (B1) a current amplitude (A) of the motor current (7) is achieved by adjusting the field-forming motor current component (i d ) is constant, and at least in a second, temporally separated braking section (B2) the current amplitude (A) of the motor current (7) is variable, wherein a switching device (100) is provided which is suitable to switch from the at least one first braking section (B1) to the at least one second braking section (B2) of the braking operation when a predetermined speed (n) of the electric motor (1) is undershot. [11] Device according to claim 10, characterized by, that the control device (2) is configured to adjust a quantity of the current amplitude (A) as a function of a temperature of the control device (2) and / or the electric motor (1). [12] Device according to claim 10 or 11, characterized by , that the control device (2) is designed to determine the magnitude of the current amplitude (A) as a function of the magnitude of a supply voltage (U) applied to the electric motor (1). V to adjust. [13] Device according to any one of claims 10 to 12, characterized by , that the control device (2) is designed to adjust the current amplitude (A) depending on an inductance of the electric motor (1) and / or a linked magnetic flux of the electric motor (1). [14] Device according to any one of claims 10 to 13, characterized by , that the control device (2) is designed in which at least a first braking section (B1) the field-forming motor current component (i d) of the motor current (7) to be set to a non-zero value using the control element (42). [15] Device according to any one of claims 10 to 14, characterized by , that the control device (2) is configured to provide a recuperative energy return (P) Akku ) into the battery pack (3) by adjusting the field-forming motor current component (i d ) to limit by means of the control element (42). [16] Device according to claim 15, characterized by , that the control device (2) is designed to, in braking operation, control the field-forming first motor current component (i d ) such that in braking mode of the electric motor (1) at the same recuperative energy return power (P Akku ) in the battery pack (3) a braking power of the electric motor (1) increases. [17] Device according to any one of claims 10 to 16, characterized by, that the control device (2) is configured to operate the electric motor (1) for a time period (T1) of the first braking section (B1), wherein the time period (T1) of the first braking section is greater than or equal to a time period (T2) of the second braking section (B2). [18] Device according to any one of claims 10 to 17, characterized by , that the field-guided electric motor (1) is designed to generate a three-phase rotating field during operation, that the currents (ia, ib, ic) flowing in the field windings (a, b, c) of the three-phase rotating field are detected as vectors of the rotating field and electronically transformed into a motor current (7) in two-dimensional representation, that the motor current component (i) resulting from the first, field-generating motor current component (i) d ) and the second, torque-generating motor current component (i q) the compound motor current (7) of the two-dimensional representation is adjusted by means of the control element (42) such that in braking operation of the electric motor (1) the field-forming motor current component (i d ) is not equal to zero.

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