Device for rotational speed-dependent braking torque control for electrical machines excited by permanent magnets

Abstract

A device for speed-dependent braking torque control for electrical machines excited by permanent magnets includes a switching device deliberately short-circuits the electrical machine. The switching device is controlled as a function of an induced voltage generated by the electrical machine. If the induced voltage falls below a defined value, the switching device reverses the short-circuited state of the electrical machine to cause the braking torque to reduce to zero and the speed to increase again. The speed can thus be controlled within a range by alternately opening and closing the switching device.

Claims

1. A device for rotational speed-dependent braking torque control for electrical machines excited by permanent magnets, comprising: a switching device which is arranged and configured in a conductive state to short-circuit an electrical machine, wherein the switching device is controlled as a function of an induced voltage generated by the electrical machine, when a first threshold value of a smoothed and/or a delayed output voltage is exceeded, the switching device short-circuits the electrical machine, and when a second threshold value of the delayed and/or smoothed output voltage, which is lower than the first threshold voltage, is undershot, the switching device cancels the short-circuited state of the electrical machine.

2. The device of claim 1, wherein the switching device is arranged and configured in the conductive state to short-circuit motor phases of the electrical machine to ground, to a voltage supply, or to one another.

3. The device of claim 1, further comprising: a rectifier to rectify the induced voltage generated by the electrical machine to generate a rectified voltage.

4. The device of claim 3, further comprising: a delay circuit to transmit the rectified voltage in the delayed and/or the smoothed fashion.

5. The device of claim 3, further comprising: a voltage-limiting device arranged downstream of the rectifier to voltage limitation.

6. The device of claim 5, wherein the voltage-limiting device is a Zener diode.

7. The device of claim 4, wherein the delay circuit is a PT1 circuit.

8. The device of claim 4, wherein the output of the delay circuit is electrically connected to a control input of the switching device.

9. The device of claim 7, wherein the PT1 circuit is an RC circuit.

10. The device of claim 1, wherein the control of the switching device is active as a function of the induced voltage only when a voltage supply fails.

11. The device of claim 1, wherein the switching device further comprises: a threshold value controller to set the threshold values of the output voltage in a variable fashion as a function of a braking command.

12. An electrically-activated clutch actuator system comprising the device of claim 1.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

(1) FIG. 1 shows a schematic design of an embodiment of the present subject matter, and

(2) FIG. 2 shows a diagram explaining the method of functioning of the present subject matter through the illustration of the relationships between the input voltage, the output voltage and the rotational speed of the electrical machine.

BACKGROUND AND SUMMARY OF THE INVENTION

(3) The explained embodiment of the present subject matter is configured to keep the rotational speed of an electrical machine in a defined corridor after a drop in the voltage even though power is fed to the electrical machine through an external effect, and therefore the rotational speed of the electrical machine would increase without the braking torque control. An application example of this scenario would be, for example, the failure of the power supply of an electrical-machine-powered vehicle which is travelling downhill.

(4) FIG. 1 shows the schematic design of this embodiment of the present subject matter. A voltage supply 6 is provided and supplies three motor phases 1 of an electrical machine 3 with a voltage. A switching element 2 short-circuits the electrical machine 3 as a function of an output voltage U.sub.a of an upstream delay element 5 composed of a resistor R and a capacitor C, by producing an electrical connection between the electrical machine and the ground 7. An input voltage U.sub.e is connected to the delay element 5. A blocking element 4 is provided for rectifying the voltage induced by the electrical machine.

(5) If the voltage supply 6 is interrupted, a voltage is induced by the phases 1 of the electrical machine 3. This voltage is rectified in the blocking element 4 and is subsequently connected to the delay element 5. Owing to the transmission ratio of the delay element 5 in the form of a PT1 element, the output voltage U.sub.a of the delay element 5 runs behind the input voltage U.sub.e and is smoothed.

(6) FIG. 2 shows a diagram explaining the method of functioning of the present subject matter through the illustration of the relationships between the input voltage U.sub.e, the output voltage U.sub.a and the rotational speed n of the electrical machine. The explanation of the first embodiment of the present subject matter is given on the basis of the consideration of the two FIGS. 1 and 2.

(7) If the voltage supply of the system collapses and the electrical machine 3 is simultaneously subject to an external power supply, the rotational speed of the electrical machine 3 increases. Therefore, the voltage which is induced by the electrical machine 3 also increases, as a result of which the input voltage U.sub.e and the output voltage U.sub.a rise, the latter delayed by the delay element 5. If U.sub.a reaches a defined threshold value U.sub.SC, the switching element closes and therefore brings about an electrical connection between the electrical machine 3 and the ground 7. The voltage to the control device tends to zero and the rotational speed of the motor drops as a result of the braking torque which is generated in this way.

(8) The voltage is reduced by the delay element 5 as a function of the capacitance of the capacitor and the size of the resistance in the RC element over a defined period of time until the output voltage U.sub.a reaches a previously defined low threshold value U.sub.SO, and the switching element 2 subsequently disconnects again the electrical connection between the ground 7 and the electrical machine 3. The braking torque tends to zero and the rotational speed of the electrical machine increases again, while the input voltage U.sub.e rises suddenly. This process repeats, as a result of which the rotational speed can be kept in a desired corridor. The braking torque which is applied on average can be set in advance for the entire system through the corresponding selection of the resistance and capacitance.

(9) In another embodiment, in the case of collapse of the voltage supply the device can be used to reduce the rotational speed of the electrical machine 3 in a targeted fashion by applying a braking torque. The design of the system is basically the same as in the exemplary embodiment specified above, except that the switching element 2 has a threshold value controller which can change the threshold values U.sub.SC and U.sub.SO.

(10) In contrast to the example specified above, after the drop in the voltage owing to the lack of an external effect, the rotational speed n of the electrical machine 3 does not increase but rather remains approximately constant minus relatively small frictional losses and remagnetization losses of the windings.

(11) Nevertheless, as in the above embodiment, a voltage is induced in the windings by the constant rotational speed n. Therefore, the threshold value U.sub.SC is also reached by the output voltage U.sub.a here, and the electrical machine is short-circuited. In a way analogous to the above embodiment, a braking torque on the rotor is generated and therefore the rotational speed n is reduced. If the output voltage U.sub.a drops to the lower threshold value U.sub.SO, the switching element cancels the short-circuited state again, and the braking torque at the rotor tends towards zero, as a result of which the rotational speed n stabilizes again and as a result remains constant, but at a lower level than before the switching element first closed.

(12) The voltage which is induced in the windings is then present at the delay element again, as a result of which the input voltage U.sub.e rises suddenly, and the output voltage U.sub.a rises with a delay. Since the input voltage U.sub.e is then lower than before the first closing of the switching element owing to the relatively low rotational speed, it takes longer for the capacitor C of the delay element 5 to be charged, and consequently also for the output voltage U.sub.a to reach the upper threshold value U.sub.SC, and therefore for the switching element 2 to close a second time.

(13) So that this effect is prevented and a time delay does not occur, the upper threshold value is adapted downward and therefore the switching element is made to close earlier. This setting is performed by the threshold value control unit which in this embodiment is part of the switching element. By analogy to this, the lower threshold value U.sub.SO is also reduced, in order also to ensure that the capacitor of the delay element 5 discharges for the same period and therefore that the braking torque is applied for the same period.

(14) By changing the threshold values it is therefore possible, as shown, to set in a variable way the intensity of the reduction in the rotational speed of the electrical machine, which is averaged over a time period, and likewise to implement braking as far as a stationary state. This setting of the threshold value control unit is performed as a function of the user's braking command.

(15) Therefore, a device for rotational speed-dependent braking torque control for electrical machines excited by permanent magnets is disclosed, wherein in the event of an interruption of the voltage supply 6 a switching element 2 short-circuits the electrical machine 3 in a targeted fashion as a function of the induced voltage of the electrical machine 3, and therefore generates a braking torque on the rotor of the electrical machine 3. If the induced voltage undershoots a defined value, the switching element 2 cancels the short-circuited state of the electrical machine 3, as a result of which the braking torque tends to zero and the rotational speed is not reduced. As a result of alternate opening and closing of the switching element 2, the rotational speed n can therefore be throttled in a targeted fashion, or in the case of an external effect on the electrical machine it can be held in a defined corridor, as in the first exemplary embodiment.

LIST OF REFERENCE SYMBOLS

(16) 1 Motor phases 2 Switching element 3 Electrical machine 4 Blocking element 5 Delay element 6 Voltage supply 7 Ground U.sub.e Input voltage U.sub.a Output voltage U.sub.SC Upper threshold value U.sub.SO Lower threshold value n Rotational speed of electrical machine