Circuit for operating an electric motor

A circuit configuration for operating an electric motor is described, the circuit configuration containing a timer. As a function of a first differential signal between a setpoint signal and an actual value signal, a regulator supplies a first switch signal for feeding current to the electric motor. Simultaneously, the timer is started which limits the current feed signal to a specified maximum time. A detection system detects a change in the setpoint signal and thereupon outputs a second switch signal that also starts the timer and is able to trigger the current feed to the electric motor. The circuit configuration both protects the electric motor from a thermal overload and also makes it possible to feed current to the electric motor if a setpoint changes. The circuit configuration is suitable to be used in a variable speed drive.

FIELD OF THE INVENTION

The present invention is directed to a circuit configuration for operating an electric motor.

BACKGROUND INFORMATION

A circuit configuration for starting an electric motor is described in German Published Patent Application No. 197 44 729, the circuit configuration limiting the motor current or the motor output to a specified maximum value. The described circuit configuration includes a timer, which cancels the limitation of the motor current or the output for the time specified by the timer, starting from the time the electric motor starts.

The object of the invention is to specify a circuit configuration for operating an electric motor, which protects the electric motor against thermal overload.

SUMMARY OF THE INVENTION

According to the present invention, a regulator is provided, which, as a function of a difference between a setpoint and an actual value, supplies a first switch signal as a start signal to feed current to the electric motor. In addition, the first switch signal starts a timer, which limits the current feed to the electric motor to a maximum time specified by the timer.

According to the present invention, the maximum time is available to the electric motor within which time it is possible to balance the difference between the setpoint and the actual value. If at the end of the maximum time, a difference or system deviation continues to exist, the timer stops the current feed to the electric motor and prevents a thermal overload and possible destruction of the electric motor.

If the operating voltage of the electric motor drops, it is possible that the electric motor may no longer be able to produce the necessary starting torque or may stop during operation after the occurrence of the start signal to feed current. The timer then interrupts the current feed after the end of the specified maximum time. The actual value then no longer agrees with the setpoint. This continues to be the case when the operating voltage has again assumed its nominal value.

According to the present invention, a detection system is provided, which supplies a second switch signal after a change in the setpoint, this second switch signal also starting the timer so that in the operating state described above and with another change in the setpoint, it is again possible to feed current to the electric motor. Current is fed to the electric motor based on the second switch signal each time the setpoint is changed until either the timer stops the current feed on reaching the maximum time or the position actual value has reached the position setpoint.

An advantageous embodiment of the circuit configuration according to the present invention provides that a change in the sign of the difference between the setpoint and the actual value supplies a third switch signal, which causes the feed of current to the electric motor to be cut off. This measure makes a simple implementation of the regulator possible, making it possible to stop the feed of current to the electric motor if the system deviation reaches zero.

An advantageous embodiment of the circuit configuration according to the present invention provides that the timer is designed as a retriggerable timer. The second switch signal, which the detection system supplies after a change in the position setpoint, starts the timer again independently of a time that may already have expired. The measure ensures that the maximum time for the feed of current to the electric motor is present after each setpoint change.

The electric motor is used advantageously in a variable speed drive, the setpoint corresponding to a position setpoint and the actual value corresponding to a position actual value. An example of a variable speed drive is a choke valve variable speed drive, which is situated, for example, in a motor vehicle. The regulator makes continuous positioning of the choke valve possible.

In this application, the maximum time is selected in such a way that the variable speed drive is able to run through the adjustment range in all operating states. If the sign of the system deviation is reversed, it is possible to reset the timer.

Additional advantageous refinements and embodiments of the circuit configuration according to the present invention for operating an electric motor are found in additional dependent claims and the following description.

DETAILED DESCRIPTION

According to the FIGURE, a setpoint generator10outputs a setpoint signal11to a subtractor12, which determines the difference between setpoint signal11and an actual value signal14supplied by an actual value sensor13and supplies it as first differential signal15.

After passing through a first absolute value generator16, first differential signal15is compared with a first hysteresis value18in a first comparator17.

After the passage of a first time delay TI19, first comparator17supplies a first switch signal20, which is fed as a start signal to a first start input21of a timer22and to a first start input23of a motor control24.

Further, after passing through a zero crossing detector25and a second time delay T226, differential signal15reaches a first reset input28of timer22and a first cutoff input29of motor control24as a cutoff signal27.

Setpoint signal11further reaches a second subtractor30, which is further fed a delayed setpoint signal31, which is obtained from setpoint signal11after passing through a third time delay T332.

Second subtractor30supplies a second differential signal33, which is compared with a second hysteresis value36in a second comparator35after passing through a second absolute value generator34. Components30–36are contained in a detection system37, which signals a change in setpoint signal11by supplying a second switch signal38.

Second switch signal38is fed to a second start input39of timer22and to a second start input40of motor control24.

Timer22outputs a second cutoff signal41to a second cutoff input42of motor control24and to a fourth time delay43. Fourth time delay43outputs delayed second cutoff signal41to a second reset input44of timer22.

Motor control24outputs a current feed signal45to an electric motor46, which is located, for example, in a variable speed drive (not described in greater detail), which in this application contains actual value sensor13.

The circuit configuration according to the present invention for operating an electric motor46operates as follows:

Electric motor46is contained in a control loop that attempts to set first difference signal15, which reflects the difference between setpoint signal11and actual value signal14, to zero. Setpoint generator10supplies setpoint signal11and actual value sensor13outputs actual value signal14. If the electric motor is situated in a variable speed drive (not shown in greater detail), the setpoint generator is used to specify a position setpoint, the present position actual value being supplied by actual value sensor13.

First differential signal15reaches first absolute value generator16, which frees differential signal15of its sign, differential signal15being capable of having both positive and negative signal values. First comparator17situated downstream of first absolute value generator16compares first differential signal15freed of its sign with first hysteresis value18. If first differential signal15is outside of the hysteresis area specified by first hysteresis value18, first comparator17, after passing through first time delay19, outputs first switch signal20, corresponding to a start signal, both to first start input21of timer22and first start input23of motor control24. First switch signal20at first start input23of motor control24results in the supply of first current feed signal45for electric motor46. In operating electric motor46, it is attempted to bring actual value signal14in conformity with setpoint signal11so that first differential signal15is set to zero.

In the case of a variable speed drive, electric motor46actuates the variable speed drive so that the position actual value detected by actual value sensor13is changed. As soon as actual value signal14conforms with specified setpoint signal11, zero crossing detector25detects a zero crossing of first differential signal15and, after passing though second time delay26, it outputs first cutoff signal27both to first reset input28of timer22and to first cutoff input29of motor control24. Thereupon, motor control24cancels current feed signal45and shuts down electric motor control46. If first hysteresis value18is exceeded, a new change in setpoint signal18results in a recurrence of first switch signal20, which prompts motor24to again output current feed signal45.

Simultaneous with the occurrence of first switch signal20, timer22is started via first start input21. Timer22is set to a specified maximum time Tmax. Maximum time Tmaxmust be determined in such a way that electric motor46has an opportunity to bring actual value signal14into conformity with setpoint signal11. If the electric motor is located in a variable speed drive, maximum time Tmaxis set in such a way that electric motor46is able to pass through the entire adjustment range of the variable speed drive with the lowest operating voltage and maximum load torque. If the adjustment operation was carried out successfully within the maximum time, first switch signal27resets timer22via first reset input28. If, however, maximum time Tmaxhas expired, timer22outputs second cutoff signal41, which is fed to motor control24via second cutoff input42. Thereupon, motor control24cancels current feed signal45for electric motor46.

If necessary, second cutoff signal41is returned to second reset input44of timer22via fourth time delay43. This measure causes timer22to be reset.

If electric motor46is unable to bring actual value signal14into conformity with setpoint signal11within specified maximum time Tmax, motor control24cancels current feed signal45for electric motor46. A subsequent change in the setpoint signal using setpoint generator10in the same direction in which it was not possible to achieve any conformity of actual value signal14with setpoint signal11results in no further occurrence of first switch signal20so that it is no longer possible to feed current to electric motor46in the event of such an additional setpoint change.

To ensure a further current feed if setpoint signal11is changed in the same direction that previously resulted in the cancellation of current feed signal45as a consequence of the expiration of maximum time Tmax, detection system37is provided, which detects any change in setpoint signal11and subsequently outputs second switch signal38.

A change in setpoint signal11may, for example, result through a comparison of setpoint signal11with delayed setpoint signal31. To this end, setpoint signal11is delayed by a specified time quantum T3in third time delay32. Second subtractor30determines the difference between present setpoint signal11and time-delayed setpoint signal31and outputs second differential signal33to second absolute value generator34.

Second comparator35compares second differential signal33, which is freed of its sign, with second hysteresis value36and, if necessary, outputs second switch signal38. Second absolute value generator34, second comparator35and second hysteresis value36ensure that a hysteresis is produced which prevents oscillations.

Second switch signal38starts timer22via second start input39and via second start input40prompts motor control24to supply current feed signal45for electric motor46.

A reset or a new start of timer22in a retrigger operation of timer22makes it possible to make the full length of maximum time Tmaxspecified by timer22available after each change in setpoint signal11using setpoint generator10, irrespective of the operating state before the setpoint change.

First and second time delay19,26make correct timing of the circuit configuration according to the present invention possible by compensation of signal delays that occur in the individual function blocks. Fourth time delay43for delaying second cutoff signal41ensures that second cutoff signal41, if it occurs, is not immediately suppressed again due to the return to second reset input44, so that an analyzable second cutoff signal41is available to second cutoff input42of motor control24. The time settings for first and second time delay19,26may be in the microseconds or milliseconds range. The time setting for fourth time delay43is preferably in the milliseconds range. If electric motor46is situated in a variable speed drive, maximum time Tmaxspecified by timer22will be in the seconds range, which is adequate for an adjustment operation under severe conditions. The time setting for third time delay32is, for example, in the seconds range. This time delay must be matched to the expected signal change of setpoint signal11, which is, for example, in the seconds range in a manually actuated setpoint generator10.

Zero crossing detector25may be implemented, for example, with the signum function. If the sign of first differential signal15is changed, first cutoff signal27occurs, it being necessary to convert the negative signal changes at the output of zero crossing detector25into positive signal changes, for example.