Method for operating an electric motor and corresponding electric motor device

Abstract

A method for operating an electric motor connectable via a switching device to a current source and connected in parallel with a freewheeling device, the switching device having control applied to it at a specific activation ratio within each working cycle to establish a desired effective operating voltage of the electric motor, so that in at least one operating mode each working cycle is includes an activation and a freewheeling time period. The freewheeling device has a freewheeling transistor and a freewheeling diode, connected in parallel with the transistor and reverse-biased as to the current source; and control is applied to the freewheeling transistor in the freewheeling time period as follows: identifying the freewheeling voltage dropping across the electric motor; and switching the freewheeling transistor for a specific switching time period if the freewheeling voltage is different from zero, particularly less than zero. Also described is an electric motor device.

Claims

1. A method for operating an electric motor, which is connectable via a switching device to a current source and which is connected in parallel with a freewheeling device, the method comprising: applying control to the switching device at a specific activation ratio within each working cycle to establish a desired effective operating voltage of the electric motor, so that in at least one operating mode each working cycle is made up of an activation time period and a freewheeling time period, wherein the applying control to the switching device includes switching the switching device to a conductive state during the activation period in order to supply current to the motor via the switching device, wherein the freewheeling device includes a freewheeling transistor and a freewheeling diode connected in parallel with the freewheeling transistor, wherein the freewheeling diode is reverse-biased with respect to the current source; at a conclusion of the activation time period, interrupting a connection between the motor and the current source by switching the switching device into a nonconductive state; and while the connection between the motor and the current source is interrupted during the freewheeling time period, applying control to the freewheeling transistor in the freewheeling time period by identifying a freewheeling voltage dropping across the electric motor when the freewheeling transistor is in a nonconductive state, and by switching the freewheeling transistor into a conductive state for a specific switching time period if the freewheeling voltage is less than zero.

2. The method of claim 1, wherein the duration of the switching time period is defined as a constant.

3. The method of claim 1, wherein the duration of the switching time period is selected to be equal to a duration of less than 50% of the duration of the working cycle.

4. The method of claim 1, wherein the identification of the freewheeling voltage is accomplished in a measurement time period during which the freewheeling transistor is not switched.

5. The method of claim 1, wherein the measurement time period is selected to be shorter than the switching time period.

6. The method of claim 1, wherein a MOSFET is used as a freewheeling device, and the freewheeling diode is a body diode of the MOSFET.

7. The method of claim 1, wherein at least one of application of control to the switching device, application of control to the freewheeling device, and identification of the freewheeling voltage are accomplished by a control unit or by an integrated circuit of the control unit.

8. The method of claim 1, wherein the switching device includes a power transistor.

9. The method of claim 1, wherein in the at least one operating mode, the motor current flowing through the electric motor is completely dissipated during the freewheeling time period.

10. The method of claim 1, wherein the duration of the switching time period is selected to be equal to a duration of at most 40%, of the duration of the working cycle.

11. The method of claim 1, wherein the duration of the switching time period is selected to be equal to a duration of at most 30%, of the duration of the working cycle.

12. The method of claim 1, wherein the duration of the switching time period is selected to be equal to a duration of at most 25%, of the duration of the working cycle.

13. The method of claim 1, wherein the duration of the switching time period is selected to be equal to a duration of at most 20%, of the duration of the working cycle.

14. The method of claim 1, wherein the duration of the switching time period is selected to be equal to a duration of at most 15%, of the duration of the working cycle.

15. The method of claim 1, wherein the duration of the switching time period is selected to be equal to a duration of at most 10%, of the duration of the working cycle.

16. The method of claim 1, wherein the duration of the switching time period is selected to be equal to a duration of at most 5%, of the duration of the working cycle.

17. The method of claim 1, wherein the duration of the switching time period is selected to be equal to a duration of at most 1%, of the duration of the working cycle.

18. The method of claim 1, wherein the freewheeling voltage is different from zero and in particular is less than zero.

19. The method of claim 1, further comprising: if the switching time period ends before an end of the working cycle: determining the freewheeling voltage anew, and switching the freewheeling transistor if the freewheeling voltage differs from zero.

20. An electric motor device, comprising: an electric motor, which is connectable via a switching device to a current source and which is connected in parallel with a freewheeling device; a control unit to apply control to the switching device at a specific activation ratio within each working cycle to establish a desired effective operating voltage of the electric motor, so that each working cycle is made up, in at least one operating mode, of an activation time period and a freewheeling time period, wherein the applying control to the switching device includes switching the switching device to a conductive state during the activation period in order to supply current to the motor via the switching device, wherein the freewheeling device has a freewheeling transistor and, connected in parallel therewith and reverse-biased with respect to the current source, a freewheeling diode; wherein, while a connection between the motor and the current source is interrupted by switching the switching device into a nonconductive state during the freewheeling time period, the control unit is configured to apply control to the freewheeling transistor in the freewheeling time period by identifying a freewheeling voltage dropping across the electric motor when the freewheeling transistor is in a nonconductive state, and by switching the freewheeling transistor for a switching time period if the freewheeling voltage is less than from zero.

21. The electric motor device of claim 20, wherein: if the switching time period ends before an end of the working cycle: the freewheeling voltage anew is determined, and the freewheeling transistor is switched if the freewheeling voltage differs from zero.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically depicts an electric motor device.

(2) FIG. 2 is a diagram in which a motor voltage and a motor current are plotted over time.

DETAILED DESCRIPTION

(3) FIG. 1 schematically depicts an electric motor device 1. The latter has an electric motor 2 and a switching device 3 that is connected in series with electric motor 2. Electric motor 2 is connectable via switching device 3 to one pole 4 of a current source 5. Conversely, electric motor 2 is connected directly to a pole 6. Electric motor 2 can consequently, by corresponding switching of switching device 3 which may have a power transistor 7, be connected to or disconnected from current source 5 or its pole 4. Switching device 3 can also have, besides power transistor 7, a diode 8 that in this case is disposed in parallel with power transistor 7 and is reverse-biased with respect to current source 5.

(4) A freewheeling device 9, which has a freewheeling transistor 10 and a freewheeling diode 11 connected in parallel therewith, is electrically connected in parallel with electric motor 2. Particularly, freewheeling device 9 may be present in the form of a MOSFET. The latter possesses a body diode which is employed as a freewheeling diode 11. Control is applied to freewheeling device 9 or to freewheeling transistor 11 by way of an integrated circuit 12 that is, for example, a constituent of a control unit 13. Switching device 3 can also have control applied to it with the aid of control unit 13 and in particular by way of circuit 12.

(5) While electric motor device 1 is in operation, control is applied to switching device 3 in such a way that an effective operating voltage is established at electric motor 2, i.e. in particular is periodically switched and switched off. This effective operating voltage can deviate from a voltage U.sub.S supplied from current source 5, in particular can be less than it. Provision is made, for this purpose, for clocked operation of switching device 3 within immediately successive working cycles. For each working cycle, a specific activation ratio is selected and is established at switching device 3. This activation ratio describes the ratio between an activation time period in which switching device 3 is switched (made conductive) and the duration of the respective working cycle. At a ratio of unity, switching device 3 is thus switched during the entire working cycle. Provision is made in at least one operating mode of electric motor device 1, however, that the ratio is less than unity, so that besides the activation time period a freewheeling time period exists in the working cycle. The voltage present at electric motor 2 is referred to as U.sub.Motor. This corresponds, during the activation time period, substantially to the voltage U.sub.S of current source 5.

(6) Because electric motor 2 or its motor windings, which are not separately characterized here, have a high inductance, it is necessaryafter the electrical connection between electric motor 2 and current source 5 has been interrupted by way of switching device 3to dissipate the electrical current I.sub.Motor flowing through electric motor 2. Freewheeling device 9 is provided for this purpose. The motor current is dissipated by switching freewheeling transistor 10 while switching device 3 is simultaneously open.

(7) In order to prevent the current direction of the current flowing through electric motor 2 from rotating and thereby braking the latter, freewheeling transistor 10 is to have control applied to it during the freewheeling time period as follows: Provision is made that a measurement time period and a specific switching time period, the duration of which latter is specified as a constant, alternate during the freewheeling time period. During the measurement time period, the freewheeling voltage that is dropping across the electric motor is identified. If the voltage is different from zero, in particular less than zero, the switching time period is then initiated; in this, freewheeling transistor 10 is switched so that the two poles of electric motor 2 are electrically connected to one another. During the measurement time period, conversely, or in principle outside the switching time period, freewheeling transistor 10 is not switched, i.e. is not conductive. The identification of the freewheeling voltage can, for example, likewise be performed with the aid of circuit 12 or control unit 13.

(8) In addition to the procedure described above, provision can also be made that before identification of the freewheeling voltage firstly the specific switching time period, during which freewheeling transistor 9 is switched, is initiated. This switching time period is then followed by the procedure described, in which, alternately during the measurement time period, the freewheeling voltage is identified and then freewheeling transistor 10 is switched for the specific switching time period as a function of the identified freewheeling voltage. In at least one operating mode of electric motor device 1, provision is made to dissipate the motor current flowing through electric motor 2 completely during the freewheeling time period.

(9) The time course of the method described above is illustrated with reference to FIG. 2. This is a diagram in which a curve 14 shows the change in the motor voltage U.sub.Motor over time, and a curve 15 shows the change in the motor current I.sub.Motor over time. The duration of the working cycle is labeled t.sub.T, the duration of the activation time period t.sub.E, and the duration of the freewheeling time period t.sub.F, where t.sub.T=t.sub.E+t.sub.F. At the beginning of the working cycle, i.e. at time t.sub.0, the motor current I.sub.Motor is, for example, equal to zero. Starting at time t.sub.0, electric motor 2 is connected to current source 5 by way of switching device 3. The motor voltage U.sub.Motor is correspondingly equal to the voltage U.sub.S of current source 5, and the motor current I.sub.Motor rises. At the end of the activation time period, at time t.sub.1, the electrical connection is interrupted. In order then to dissipate the motor current I.sub.Motor, in the freewheeling time period freewheeling transistor 10 is made conductive at least once for a switching time period. In the diagram depicted, several such switching time periods are provided. They are labeled t.sub.S, but only in part. The first switching time period can be provided with a time delay after the end of the activation time period. It can also, however, of course immediately follow the activation time period.

(10) As depicted, switching time periods and measurement time periods (labeled only in part as t.sub.M) alternate in the freewheeling time period. A reverse sequence, in which firstly a measurement time period is initiated and a switching time period is initiated only later at a corresponding motor voltage U.sub.Motor, is also possible. The alternation may be provided for at least until a time t.sub.2 at which the motor current I.sub.Motor has been dissipated to zero. In principle, however, the fact that the motor current I.sub.Motor has been dissipated in one of the switching time periods can only be ascertained in the measurement time period following it. This measurement time period begins at time t.sub.3. Ideally, (but, as depicted, not necessarily), the times t.sub.2 and t.sub.3 coincide. Because the motor current I.sub.Motor is equal to zero starting at time t.sub.2, the motor voltage U.sub.Motor that is ascertained in the measurement time period is positive if electric motor 2 is still running, and equal to zero if that is not the case. With a positive motor voltage U.sub.Motor and/or a motor voltage U.sub.Motor equal to zero, a further switching time period is not initiated after the measurement time period until the freewheeling time period or the working cycle has ended.