METHOD FOR BRAKING A POWER TOOL, AND POWER TOOL

Abstract

A method for braking a power tool (10) is provided, the power tool (10) being a battery-operated or mains-operated power tool and including a brake chopper, and at least pert of the electrical energy that is released when the power tool (10) is braked being fed back to a power supply device (14) or a DC link of the power tool (10).

Claims

1. A method for braking a power tool, the power tool being a battery-operated or mains-operated power tool and having a brake chopper, at least part of electrical energy released when the power tool is braked being fed back to a power supply device or a DC link of the power tool, the method comprising the following steps: a) regeneratively braking a motor-driven drive of the power tool; b) feeding back the electrical energy released when the power tool is braked to the power supply device or the DC link of the power tool; c) determining whether the electrical energy released when the power tool is braked exceeds at least one limit value of the power supply device or of the voltage of the DC link; and d) absorbing a share of the electrical energy by way of the brake chopper if the electrical energy released during braking exceeds the at least one limit value of the power supply device or of the voltage of the DC link.

2. The method as recited in claim 1 wherein a correction factor (k.sub.red) is determined on the basis of a difference between the electrical energy released during braking and the limit value of the power supply device or of the voltage of the DC link, and wherein a duty factor D of the brake chopper is determined on the basis of the correction factor (k.sub.red).

3. The method as recited in claim 2 wherein the correction factor (k.sub.red) is in a range from 0 to 1.

4. The method as recited in claim 2 wherein the correction factor (k.sub.red) is made up of a first correction parameter (k.sub.U_red) and a second correction parameter (K.sub.I_red).

5. The method as recited in claim 4 wherein the correction factor (k.sub.red) is a product of the first correction parameter (k.sub.U_red) and the second correction parameter (k.sub.I_red).

6. The method as recited in claim 4 wherein the first correction parameter (k.sub.U_red) is determined by a voltage controller of the power supply device, and wherein the second correction parameter (K.sub.I_red) is determined by a current controller of the power supply device.

7. The method as recited in claim 2 wherein the duty factor D of the brake chopper is determined on the basis of a ratio of the correction factor (k.sub.red) to a mapping limit (k.sub.Mapping), the mapping limit (k.sub.Mapping) corresponding to a limit value of the correction factor (k.sub.red) from which the braking power of the power tool is reduced.

8. The method as recited in claim 7 wherein the duty factor D of the brake chopper has the value 1 if the correction factor (k.sub.red) is less than or equal to the mapping limit (k.sub.Mapping).

9. The method as recited in claim 7 wherein the duty factor D of the brake chopper is determined using the following formula while the correction factor (k.sub.red) is above the mapping limit (k.sub.Mapping): $D=k_{\mathrm{red}}/\left(k_{\mathrm{Mapping}}-1\right)-1/\left(k_{\mathrm{Mapping}}-1\right)$

10. The method as recited in claim 7 wherein the mapping limit (k.sub.Mapping) has a value between 0 and 1.

11. The method as recited in claim 7 wherein the mapping limit (k.sub.Mapping) is a constant, predetermined value.

12. The method as recited in claim 7 wherein the mapping limit (k.sub.Mapping) is determined dynamically on the basis of a mechanical braking power (P.sub.mech) of the power tool and a chopper braking power (P.sub.Chopper).

13. The method as recited in claim 12 wherein the mapping limit is determined using the following formula: $k_{\mathrm{Mapping}}=P_{\mathrm{mech}}/\left(P_{\mathrm{mech}}+P_{\mathrm{Chopper}}\right)$

14. The method as recited in claim 7 wherein the braking power of the power tool is reduced by way of the following method step: e) rotating a current space vector I.sub.S,max in the space vector representation by applying a modified brake angle .sub.brems, with the result that a length of the current space vector I.sub.s, max remains essentially unchanged, or f) applying a current correction factor k.sub.S,red to a maximum motor current I.sub.S, max, as a result of which a reduced setpoint value I.sub.S,red for the motor current is obtained.

15. The method as recited in claim 14 wherein the current correction factor K.sub.S,red is determined on the basis of a ratio of the correction factor k.sub.red to the mapping limit (k.sub.Mapping).

16. The method as recited in claim 7 wherein the current correction factor k.sub.S,red is determined using the following formula while the correction factor k.sub.red is below the mapping limit (k.sub.Mapping): $k_{S,\mathrm{red}}=k_{\mathrm{red}}/k_{\mathrm{Mapping}}$

17. A power tool comprising a motor and a brake chopper for carrying out the method as recited in claim 1, the motor being a brushless motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Further advantages will become apparent from the description of the figures that follows. The figures, the description and the claims contain numerous features in combination. A person skilled in the art will expediently also consider the features individually and combine them to form useful further combinations.

[0044] In the figures, identical and similar components are denoted by the same reference signs.

[0045] In the figures:

[0046] FIG. 1 shows an operating range of a possible stator current space vector in a space vector representation during efficient regenerative braking operation;

[0047] FIGS. 2a and 2b show an operating range of a possible stator current space vector in a space vector representation during supremely lossy regenerative braking operation;

[0048] FIG. 3 shows a possible recuperation block diagram with efficient regenerative braking operation without a brake chopper;

[0049] FIG. 4 shows a possible recuperation block diagram with efficient regenerative braking operation with a brake chopper;

[0050] FIG. 5 shows a possible recuperation block diagram with supremely lossy regenerative braking operation without a brake chopper;

[0051] FIG. 6 shows a possible recuperation block diagram with supremely lossy regenerative braking operation with a brake chopper;

[0052] FIG. 7 shows a schematic plot of speed against time to illustrate braking time and acceleration time;

[0053] FIG. 8 shows a schematic plot of speed against time to illustrate braking time and acceleration time;

[0054] FIG. 9 shows a schematic representation of a configuration of the power tool;

[0055] FIG. 10 shows an inventive recuperation block diagram with efficient regenerative braking operation with a brake chopper and reduction of the motor current as a result of rotation of the current vector;

[0056] FIG. 11 shows an inventive recuperation block diagram with efficient regenerative braking operation with a brake chopper and reduction of the motor current as a result of shortening of the current vector; and

[0057] FIG. 12 shows a schematic representation of a mapping algorithm.

DETAILED DESCRIPTION

[0058] FIG. 1 shows a possible space vector representation that depicts the operation of a power tool 10. In particular, FIG. 1 shows the operating range AB of a possible stator current space vector I_S in such a space vector representation during efficient regenerative braking operation of the power tool 10. The x-axis of the space vector representation shows the I_d value of the current that flows through the motor 12 of the power tool 10. A power tool 10 is shown schematically in FIG. 9.

[0059] The y-axis of the space vector representation shows the I_q value of the current that flows through the motor 12 of the power tool 10. The value I_q signifies the torque-forming component of the current, while the value I_d signifies the field-forming current component.

[0060] FIG. 1 shows the four quadrants 1, 2, 3 and 4 of a space vector representation. The first quadrant 1 is characterized by a negatively rising torque M and generator operation G. The second quadrant 2 is characterized by a positively rising torque M and motor operation M. The third quadrant 3 is characterized by a negatively rising torque M and motor operation M. The fourth quadrant 4 is characterized by a positively rising torque M and generator operation G. The rising or falling torques of the torque hyperbolae are symbolized by dashed arrows in FIGS. 1 and 2. The torque hyperbolae are preferably formed by operating points with the same torque. FIG. 1 shows a circle K, the circle K representing the current limit of an inverter of the power tool 10. The circle K, or the current limit of the inverter, has equal parts located in the four quadrants 1, 2, 3, 4 of the space vector representation, which is synonymous with a centre of the circle K coinciding with the intersection of the y and x axes of the space vector representation. The operating range AB of the stator current space vector I_S is shown in the third quadrant 3 of the space vector representation. In the space vector representation shown in FIG. 1, the operating range AB of the stator current space vector I_S coincides with the MTPA characteristic curve of the power tool 10. The motor 12 of the power tool 10 is thus advantageously operated at an efficiency-optimized operating point at which the electrical heat losses are minimal.

[0061] The 0 Nm characteristic curve N, which runs substantially parallel to the y-axis of the space vector representation, runs through the first quadrant 1 and the fourth quadrant 4. Moreover, a second 0 Nm characteristic curve N2, which runs on the x-axis, or coincides with the x-axis (therefore not shown), is obtained. The space vector representation depicted in FIG. 1 shows a power tool 10, or operation thereof, for which the braking power of the power tool 10 is reduced as a result of at least one correction factor k being determined and applied to a maximum motor current I_S, max of the power tool 10, so that a reduced setpoint value I_S, red for the motor current is obtained. This corresponds to efficient regenerative braking operation of the power tool 10 (see FIG. 9).

[0062] FIGS. 2a and 2b show a space vector representation of supremely lossy regenerative braking operation of a power tool 10. Contrary to FIG. 1, the braking methods shown in FIG. 2 involve a current space vector I_S, max being rotated in the space vector representation by applying a modified braking angle _brems, so that a length of the current space vector I_S, max remains essentially unchanged and the braking power of the power tool 10 is reduced. The current space vector I_S, max is initially in the third quadrant 3 of the space vector representation, but ends up at the border between the second quadrant 2 and the third quadrant 3 as a result of the rotation in FIG. 2a. The current space vector I_S, max is initially in the third quadrant 3 of the space vector representation, but ends up in the fourth quadrant 4 as a result of the rotation in FIG. 2b. The braking angle _brems is also shown in the fourth quadrant 4 of the space vector representation. The rotated current space vector I_S, max in the fourth quadrant 4 intersects the circular area K representing the current limit of the inverter of the electronics of the power tool 10, this intersection of the rotated current space vector I_S, max and the circular area K in the example depicted in FIGS. 2a and 2b coinciding with the intersection of the circular area K and the 0 Nm characteristic curve N. The operating range AB is therefore limited to operating points between the states maximum braking torque and no braking torque, i.e. 0 Nm.

[0063] It should be noted that the block diagrams shown in FIGS. 3 to 6 show, by way of illustration, braking methods for a power tool 10 that involve the motor 12 being operated using field-oriented control. The method can of course also be carried out with a power tool 10 in which the motor 12 is controlled using block commutation. The motor 12 of the power tool 10 is a brushless motor that can preferably deliver a power of more than 1800 watts (W).

[0064] FIG. 3 shows a possible recuperation block diagram with efficient regenerative braking operation of a power tool 10 without a brake chopper 18. In the braking methods depicted in FIG. 3, the battery voltage controller 20 is configured to output a correction factor k_U, red for the voltage of the power supply device 14, while the battery current controller 22 is configured to output a correction factor k_I, red for the current of the power supply device 14. Moreover, there is provision for a speed controller 30 that controls the speed n of the power tool 10. Setpoint current values that can be relayed to the motor current controllers 34, 36 are obtained as output variables or manipulated variables of the braking method. The setpoint current values may preferably be setpoint current values for the d- and q-axes of the space vector representation. The motor current controllers 34, 36 are in particular a d-current controller 34 and a q-current controller 36, the motor current controllers 34, 36 relaying their control commands to the pulse width modulation PWM.

[0065] FIG. 4 shows a possible recuperation block diagram with efficient regenerative braking operation of a power tool 10 with a brake chopper 18. The block diagram shown in FIG. 4 depicts the battery current controller 22, the speed controller 30 and the motor current controllers 34, 36 that are already known from FIG. 3 and the block diagram shown therein. Moreover, FIG. 4 shows a duty ratio controller 28, which can also be referred to as third controller and is used to limit the duty ratio to a setpoint value D_soll of, for example, 95%. The output value of this third controller is a correction factor k_red that is between the values 0 and 1. This correction factor k_red can be multiplied by the current I_S, max to obtain the reduced setpoint motor stator current I_S, red.

[0066] Moreover, the block diagram shown in FIG. 4 shows a two-level controller 24 with hysteresis, which is referred to as first controller. This first controller 24 is preferably used to actuate the braking resistor 18 referred to as brake chopper. The first controller 24 may, for example, be in the form of a comparator with switching hysteresis and be configured to compare a DC link voltage with a reference voltage. The DC link voltage can preferably also be referred to as battery voltage u_Akku, while the reference voltage in FIG. 4 is referred to by the name U_Chopper. The first controller 24 outputs as output signal a PWM signal that outputs a high level if the battery voltage is greater than the reference voltage, i.e. u_Akku>u_Chopper. A duty ratio of the PWM signal can be formed, for example, by the low pass filter 32 shown in FIG. 4. Values or variables that vary over time are denoted by lower case letters in this specification, while constant values or variables, such as limit values or other specified values, are denoted by upper case letters.

[0067] FIG. 5 shows a possible recuperation block diagram with supremely lossy regenerative braking operation of a power tool 10 without a brake chopper 18. Similarly to the block diagram of FIG. 3, the braking method described in FIG. 5 involves correction factors k_U, red and k_I, red being output, which can be combined to form a common correction factor k_red. The correction factor k_red used in FIG. 5 results preferably from a combination of the correction factors k_U, red and k_I, red for the voltage and the electrical current. The correction factor k_red in the example shown in FIG. 5, in which a brake chopper 18 is preferably not used, has no portion that is attributable to a duty ratio of the brake chopper 18.

[0068] In particular, the correction factor k_U, red can be determined by the battery voltage controller 20 and the correction factor k_I, red can be determined by the battery current controller 22. The correction factors k_U, red and k_I, red can be used to calculate the combined correction factor k_red, which can be used to calculate the stator current space vector angle or braking angle _brems. This is accomplished by subtracting the angles _brems, MTPA and _brems, 0 Nm from one another and multiplying them by the combined correction factor k_red. The _brems, 0 Nm can be added to the product again, and so the braking angle _brems is obtained. The parameter _brems, MTPA preferably signifies the angle on the MTPA characteristic curve, while the parameter _brems, 0 Nm corresponds to the stator current space vector angle in the fourth quadrant 4 of the space vector representation or to the stator current space vector angle on the x-axis between the second quadrant 2 and the third quadrant 3, at which no torque is generated. This parameter _brems, 0 Nm in the fourth quadrant 4 is preferably obtained from the intersection of the current limit circle K with the 0 Nm characteristic curve N. The braking angle _brems and the current I_S, max can be used to calculate the d- and q-setpoint values that can be forwarded to the d-current controller 34 and the q-current controller 36.

[0069] FIG. 6 shows a possible recuperation block diagram with supremely lossy regenerative braking operation of a power tool 10 with a brake chopper 18, the block diagram shown in FIG. 6 broadly showing a combination of elements of the block diagrams from FIGS. 4 and 5.

[0070] FIG. 7 shows a schematic plot of the speed n of the motor 12 of the power tool 10 against time t to illustrate the braking time t_down and the acceleration time t_up. It shows a possible response of the speed n with an interruption between times t1 and t2. In the period of time between times to and t1, the speed n of the power tool 10 rises in order to reach a maximum value n_max at time t1. This period of time between the limits t1 and t2 is referred to as acceleration time t_up in accordance with this specification. From the maximum speed value n_max, the power tool 10 can be brought to a standstill by way of a slowing operation. In the case of conventionally operating power tools, such a braking operation can take place as per the dashed line in FIG. 7 and, for example, can take longer than the acceleration time t_up. In this case, a ratio of braking time and acceleration time may be greater than 1, because the braking time t_down is greater than the acceleration time t_up. In the plot of speed n against time t shown in FIG. 7, the braking time t_down is limited by times t2 and t3.

[0071] By preference, the braking of the power tool 10 has a response as per the two solid lines in FIG. 7. Slowing down according to a normal slowing operation takes place as per the right-hand solid line, a braking time t_down being in a range between 2 and 4 seconds. In such a case, the ratio of braking time and acceleration time may be less than 1, i.e. the braking time t_down is less than the acceleration time t_up. After a kickback situation has been detected, the power tool 10, or its tool 16, can be slowed down in less than 1.5 seconds. A fast or kickback braking operation such as this is represented by the left-hand solid line in FIG. 7. In such a case, the ratio of braking time and acceleration time may be less than 0.7, preferably less than 0.5, i.e. the braking time t_down is significantly less than the acceleration time t_up. By way of example, the braking time t_down may be less than 70%, preferably less than 50%, of the acceleration time t_up.

[0072] FIG. 8 also shows a schematic plot of the speed n of the motor 12 of the power tool 10 against time t to illustrate the braking time t_down and the acceleration time t_up. The inventors have recognized that at reduced speed the rotating cutting disk 16 now has only very little rotational energy and is therefore a low risk for the user of the power tool 10. It may be sufficient that the tool 16 of the power tool 10 is not brought to a complete standstill, but rather the braking operation ends before, for example when the tool 16 of the power tool 10 is rotating at a speed n of less than 70%, preferably less than 63%, 55%, 45%, 32% or 22%, of the original speed n_max of the tool 16. Of course, the tool 16 of the power tool 10 can also rotate at an even lower proportion of the original speed n_max before it is slowed down to a standstill, for example. The end of the braking time t_down is then determined accordingly by end times t3_70%, t3_63%, t3_55%, t3_45%, t3_32%, or t3_22%. Of course, all intermediate values between 70 and 0% are also possible, such as for example 67%, 57.5%, 43.33%, 2.56%, etc. It is preferred that the tool 16 of the power tool 10 is deprived of the greater share of its rotational energy, which is fed back to the power supply device 14 of the power tool 10. As a result of the large share of the rotational energy being taken away, the tool 16 of the power tool 10 can be slowed down so sharply that the rotating cutting disk 16 is no longer a danger to the user of the power tool 10. If the braking time t_down first ends not when the tool 16 of the power tool 10 is at a standstill, but rather when the tool 16 of the power tool 10 is only rotating at less than 70%, preferably less than 63%, 55%, 45%, 32%, 22%, of the original speed n_max of the tool 16, the period of time before the tool of the power tool is actually at a standstill is advantageously no longer important, because the braking time t_down, or the end t3 thereof, is defined by the attainment of the lower speed.

[0073] It is possible that the power tool 10 or its tool 16 is brought to a standstill with a reduced gradient having a different slope.

[0074] The plot of speed n against time t shown in FIG. 8 shows in particular constant straight lines for the values of 90% of the maximum speed n_max and 10% of the maximum speed n_max. The value of 90% of the maximum speed n_max forms a value pair together with the associated time value t1_90%, said value pair lying on the graph n (t). Analogously, the value of 10% of the maximum speed n_max forms a value pair together with the associated time value t3_10%, said value pair lying on the graph n (t).

[0075] FIG. 9 shows a schematic representation of a preferred configuration of the power tool 10. The power tool 10 has a motor 12, which is preferably in the form of a brushless motor. The power tool 10 can have a tool 16, which may, for example, be in the form of a disk-shaped cutting tool. The power tool 10 depicted in FIG. 9 is preferably a cut-off grinder that can be used to make cuts in a substrate, such as concrete. The tool 16 of the power tool 10 may be surrounded by a blade guard (without a reference sign) in order to protect the user of the power tool 10 from flying chips and sparks. The power tool 10 can be connected to at least one power supply device 14 in order to supply the power tool 10 with electrical energy. Of course, the power tool 10 can also have two or more power supply devices 14. In the context of the present invention, electrical energy can be fed back to the at least one power supply device 14, in particular when the power tool 10 is slowed down. Excess electrical energy that could damage the power supply device can be removed by the brake chopper and, if necessary, prevented by reducing the motor current. The power tool 10 can moreover have one or more handles as shown that the user of the power tool 10 can use to transport the power tool 10 or guide it during work.

[0076] FIG. 10 shows an inventive block diagram of an inventive method for braking a power tool. In the braking method depicted in FIG. 10, the battery voltage controller 20 is configured to output a correction factor k_U, red for the voltage of the power supply device (14, FIG. 9), while the battery current controller 22 is configured to output a correction factor k_I, red for the current of the power supply device 14. Moreover, there is provision for a speed controller 30 that controls the speed n of the power tool 10. Setpoint current values that can be relayed to the motor current controllers 34, 36 are firstly obtained as output variables or manipulated variables of the braking method. The setpoint current values may preferably be setpoint current values for the d- and q-axes of the space vector representation. The motor current controllers 34, 36 are in particular a d-current controller 34 and a q-current controller 36, the motor current controllers 34, 36 relaying their control commands to the pulse width modulation PWM. Secondly, the braking method according to FIG. 10 provides a manipulated variable for the duty factor D of the brake chopper.

[0077] The inventive braking method shown in FIG. 10 differs from the braking method according to FIG. 4 in particular in that no additional controller is required for controlling the brake chopper. Rather, the braking method according to FIG. 10 involves control of the brake chopper being achieved exclusively by way of the current controller 20 and the voltage controller 22. For this purpose, a mapping algorithm 38 is proposed, in particular, which takes the first correction parameter k_U, red and the second correction parameter k_I, red (e.g. takes a product of the first and second correction parameters k_U, red; k_I, red) as a basis for firstly outputting a first manipulated variable k_, red for (potentially) reducing the motor current via a rotation of the current vector. Secondly, the mapping algorithm 38 determines the duty factor D of the brake chopper as a second manipulated variable. In regard to rotation of the current space vector, reference should also be made to the explanations relating to FIG. 5.

[0078] The braking method shown in the block diagram according to FIG. 11 differs from the braking method shown in FIG. 10 in particular in that the proposal is to shorten the current space vector I_S, max in order to reduce the motor current. In regard to shortening of the current space vector I_S, max, reference should also be made to the explanations relating to FIGS. 3 and 4.

[0079] FIG. 11 also shows a mapping algorithm 38 that takes the first correction parameter k_U, red and the second correction parameter k_I, red (e.g. takes the correction factor k_red, which is the product of the first and second correction parameters k_U, red; k_I, red) as a basis for firstly outputting a first manipulated variable k_S, red for (potentially) reducing the motor current via a shortening of the current vector. Secondly, the mapping algorithm 38 determines the duty factor D of the brake chopper as a second manipulated variable.

[0080] Each of the braking methods shown in FIGS. 10 and 11 involves a mapping algorithm being used that is designed to use the product of the two correction parameters, i.e. to take the correction factor k_red as a basis, for outputting firstly a manipulated variable for reducing the motor current and secondly the duty factor D of the brake chopper. An illustrative mapping algorithm is shown in FIG. 12.

[0081] According to the mapping algorithm of FIG. 12, the required reduction of the energy released by braking, to protect the power supply device, is easily divided between the brake chopper and a reduction in the motor power. Specifically, high correction factors (i.e. only small amounts of excess energy) merely result in the duty factor of the brake chopper being changed. The motor current is not yet reduced here, and so the power supply device can continue to be charged with maximum permissible feedback power in this range without having to reduce the motor braking power. If the correction factor k_red is too low (i.e. large amounts of excess energy are generated), then besides using the brake chopper (under full load) the motor current is also reduced. The transition between removal of the excess energy solely by the brake chopper (first range 40 in FIG. 12) and reduction of the motor current (i.e. reduction of the regenerative braking power) to limit the excess energy (second range 42 in FIG. 12) is referred to as the mapping limit k_Mapping.

[0082] In FIG. 12, the mapping limit is reached, by way of illustration, at k_red=0.75. A first range 40 of the correction factor k_red, above the mapping limit k_Mapping, is used to control the brake chopper. A second range 42 of the correction factor k_red, below the mapping limit k_Mapping, is used to control (reduce) the motor current. It should be mentioned at this point that the mapping limit k_Mapping shown is only an illustration and can generally assume any value of the correction factor k_red between 0 and 1. In the example of the mapping limit at k_red=0.75 shown here, excess energy that is released during braking is absorbed by the brake chopper if the correction factor k_red is in the first range 40, i.e. between 0.75 and 1. In this first range 40, the behaviour of the duty factor D of the brake chopper is directly proportional to that of the correction factor k_red. The brake chopper duty factor D is therefore in a duty factor range 44 between 0 and 1 when the correction factor k_red is in the first range 40. In other words, a correction factor k_red that is in the middle of the first range 40 (here k_red=0.875) corresponds to a duty factor of the brake chopper of 0.5. When a correction factor k_red is less than or equal to k_Mapping (here 0.75), the duty factor of the brake chopper is 1, i.e. the brake chopper operates at full braking power.

[0083] When the correction factor k_red is below the mapping limit k_Mapping (here below 0.75), the mapping algorithm 38 (depending on the embodiment) outputs either a current correction factor k_S, red or a brake angle correction factor k_B, red in order to reduce the motor current and thus the energy generated during braking. The discussion below will merely concern determination of the current correction factor k_S, red, the sequence being able to be transferred equivalently to the determination of the brake angle correction factor k_, red.

[0084] As indicated in FIG. 12, the current correction factor k_S, red decreases proportionally with the correction factor k_red as soon as the correction factor k_red is below the mapping limit k_Mapping. In the example cited above, i.e. if the mapping limit has been set to 0.75, this means that the current correction factor k_S, red is in the current correction factor range 46 between 1 and 0 if the correction factor k_red is in the second range 42 between the mapping limit k_Mapping (e.g. 0.75) and 0. In other words, a correction factor k_red that is in the middle of the second range 42 (here k_red=0.375) means a current correction factor k_S, red of 0.5, i.e. the motor current is lowered by 50%. When a correction factor k_red is greater than or equal to k_Mapping (here 0.75), the current correction factor k_S, red=1, i.e. there is no lowering of the motor current, etc. Throughout the second range 42 of the correction factor k_red, the duty factor of the brake chopper=1, i.e. the brake chopper operates under full load.

[0085] The determination of the current correction factor k_S, red and the duty factor D of the brake chopper by way of the mapping algorithm 38 can be expressed using the following logic:

[00004]$\mathrm{if}\left(k_{\mathrm{red}}{k}_{\mathrm{Mapping}}\right)$$\{$$k_{S,\mathrm{red}}\mathrm{or}{k}_{,\mathrm{red}}=\frac{k_{\mathrm{red}}}{k_{\mathrm{Mapping}}};$$D=1;$$\}$$\mathrm{else}$$\{$$D=\frac{1}{\left(k_{\mathrm{Mapping}}-1\right)}.Math.k_{\mathrm{red}}-\frac{1}{\left(k_{\mathrm{Mapping}}-1\right)};$$k_{S,\mathrm{red}}\mathrm{or}{k}_{,\mathrm{red}}=1;$$\}$

[0086] As already indicated above, the mapping limit k_Mapping may be a constant (e.g. manufacturer-defined) value or dynamically alterable. It is particularly advantageous if the mapping limit k_Mapping is calculated dynamically, so that a load line without a bend is always approximated. A preferred value for the mapping limit k_Mapping can accordingly be calculated as follows:

[00005]$k_{\mathrm{Mapping},\mathrm{opt}}=\frac{P_{\mathrm{mech}}}{P_{\mathrm{mech}}+P_{\mathrm{Chopper}}}$

where P.sub.mech=mechanical braking power of the power tool, and P.sub.Chopper=braking power of the chopper.

[0087] For an example of P.sub.mech=3000 W and P.sub.Chopper=1000 W, the mapping limit of k_Mapping=3000 W/(3000 W+1000 W)==0.75 mentioned by way of illustration above is therefore obtained.

LIST OF REFERENCE SIGNS

[0088] 1 first quadrant of the space vector representation [0089] 2 second quadrant of the space vector representation [0090] 3 third quadrant of the space vector representation [0091] 4 fourth quadrant of the space vector representation [0092] 10 power tool [0093] 12 motor [0094] 14 power supply device [0095] 16 tool [0096] 18 brake chopper [0097] 20 voltage controller of the power supply device [0098] 22 current controller of the power supply device [0099] 24 first controller [0100] 26 second controller [0101] 28 third controller, in particular duty ratio controller [0102] 30 speed controller [0103] 32 low pass filter [0104] 34 d-current controller [0105] 36 q-current controller [0106] 38 mapping [0107] 40 first range [0108] 42 second range [0109] 44 duty factor range [0110] 46 range of reduced braking power [0111] M motor operation [0112] G generator operation [0113] I_d torque [0114] I_q torque [0115] M positively rising torque [0116] M negatively rising torque [0117] K circular area as the current limit of the inverter [0118] AB operating range of the stator current space vector [0119] N 0 Nm characteristic curve [0120] MTPA MTPA (maximum torque per ampere) [0121] characteristic curve [0122] _brems braking angle [0123] PWM pulse width modulation [0124] n speed [0125] t time