Abstract:
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.

Description:
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. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE shows a block diagram of a circuit configuration according to the present invention for operating an electric motor. 
   

   DETAILED DESCRIPTION 
   According to the FIGURE, a setpoint generator  10  outputs a setpoint signal  11  to a subtractor  12 , which determines the difference between setpoint signal  11  and an actual value signal  14  supplied by an actual value sensor  13  and supplies it as first differential signal  15 . 
   After passing through a first absolute value generator  16 , first differential signal  15  is compared with a first hysteresis value  18  in a first comparator  17 . 
   After the passage of a first time delay TI  19 , first comparator  17  supplies a first switch signal  20 , which is fed as a start signal to a first start input  21  of a timer  22  and to a first start input  23  of a motor control  24 . 
   Further, after passing through a zero crossing detector  25  and a second time delay T 2   26 , differential signal  15  reaches a first reset input  28  of timer  22  and a first cutoff input  29  of motor control  24  as a cutoff signal  27 . 
   Setpoint signal  11  further reaches a second subtractor  30 , which is further fed a delayed setpoint signal  31 , which is obtained from setpoint signal  11  after passing through a third time delay T 3   32 . 
   Second subtractor  30  supplies a second differential signal  33 , which is compared with a second hysteresis value  36  in a second comparator  35  after passing through a second absolute value generator  34 . Components  30 – 36  are contained in a detection system  37 , which signals a change in setpoint signal  11  by supplying a second switch signal  38 . 
   Second switch signal  38  is fed to a second start input  39  of timer  22  and to a second start input  40  of motor control  24 . 
   Timer  22  outputs a second cutoff signal  41  to a second cutoff input  42  of motor control  24  and to a fourth time delay  43 . Fourth time delay  43  outputs delayed second cutoff signal  41  to a second reset input  44  of timer  22 . 
   Motor control  24  outputs a current feed signal  45  to an electric motor  46 , which is located, for example, in a variable speed drive (not described in greater detail), which in this application contains actual value sensor  13 . 
   The circuit configuration according to the present invention for operating an electric motor  46  operates as follows: 
   Electric motor  46  is contained in a control loop that attempts to set first difference signal  15 , which reflects the difference between setpoint signal  11  and actual value signal  14 , to zero. Setpoint generator  10  supplies setpoint signal  11  and actual value sensor  13  outputs actual value signal  14 . 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 sensor  13 . 
   First differential signal  15  reaches first absolute value generator  16 , which frees differential signal  15  of its sign, differential signal  15  being capable of having both positive and negative signal values. First comparator  17  situated downstream of first absolute value generator  16  compares first differential signal  15  freed of its sign with first hysteresis value  18 . If first differential signal  15  is outside of the hysteresis area specified by first hysteresis value  18 , first comparator  17 , after passing through first time delay  19 , outputs first switch signal  20 , corresponding to a start signal, both to first start input  21  of timer  22  and first start input  23  of motor control  24 . First switch signal  20  at first start input  23  of motor control  24  results in the supply of first current feed signal  45  for electric motor  46 . In operating electric motor  46 , it is attempted to bring actual value signal  14  in conformity with setpoint signal  11  so that first differential signal  15  is set to zero. 
   In the case of a variable speed drive, electric motor  46  actuates the variable speed drive so that the position actual value detected by actual value sensor  13  is changed. As soon as actual value signal  14  conforms with specified setpoint signal  11 , zero crossing detector  25  detects a zero crossing of first differential signal  15  and, after passing though second time delay  26 , it outputs first cutoff signal  27  both to first reset input  28  of timer  22  and to first cutoff input  29  of motor control  24 . Thereupon, motor control  24  cancels current feed signal  45  and shuts down electric motor control  46 . If first hysteresis value  18  is exceeded, a new change in setpoint signal  18  results in a recurrence of first switch signal  20 , which prompts motor  24  to again output current feed signal  45 . 
   Simultaneous with the occurrence of first switch signal  20 , timer  22  is started via first start input  21 . Timer  22  is set to a specified maximum time T max . Maximum time T max  must be determined in such a way that electric motor  46  has an opportunity to bring actual value signal  14  into conformity with setpoint signal  11 . If the electric motor is located in a variable speed drive, maximum time T max  is set in such a way that electric motor  46  is 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 signal  27  resets timer  22  via first reset input  28 . If, however, maximum time T max  has expired, timer  22  outputs second cutoff signal  41 , which is fed to motor control  24  via second cutoff input  42 . Thereupon, motor control  24  cancels current feed signal  45  for electric motor  46 . 
   If necessary, second cutoff signal  41  is returned to second reset input  44  of timer  22  via fourth time delay  43 . This measure causes timer  22  to be reset. 
   If electric motor  46  is unable to bring actual value signal  14  into conformity with setpoint signal  11  within specified maximum time T max , motor control  24  cancels current feed signal  45  for electric motor  46 . A subsequent change in the setpoint signal using setpoint generator  10  in the same direction in which it was not possible to achieve any conformity of actual value signal  14  with setpoint signal  11  results in no further occurrence of first switch signal  20  so that it is no longer possible to feed current to electric motor  46  in the event of such an additional setpoint change. 
   To ensure a further current feed if setpoint signal  11  is changed in the same direction that previously resulted in the cancellation of current feed signal  45  as a consequence of the expiration of maximum time T max , detection system  37  is provided, which detects any change in setpoint signal  11  and subsequently outputs second switch signal  38 . 
   A change in setpoint signal  11  may, for example, result through a comparison of setpoint signal  11  with delayed setpoint signal  31 . To this end, setpoint signal  11  is delayed by a specified time quantum T 3  in third time delay  32 . Second subtractor  30  determines the difference between present setpoint signal  11  and time-delayed setpoint signal  31  and outputs second differential signal  33  to second absolute value generator  34 . 
   Second comparator  35  compares second differential signal  33 , which is freed of its sign, with second hysteresis value  36  and, if necessary, outputs second switch signal  38 . Second absolute value generator  34 , second comparator  35  and second hysteresis value  36  ensure that a hysteresis is produced which prevents oscillations. 
   Second switch signal  38  starts timer  22  via second start input  39  and via second start input  40  prompts motor control  24  to supply current feed signal  45  for electric motor  46 . 
   A reset or a new start of timer  22  in a retrigger operation of timer  22  makes it possible to make the full length of maximum time T max  specified by timer  22  available after each change in setpoint signal  11  using setpoint generator  10 , irrespective of the operating state before the setpoint change. 
   First and second time delay  19 ,  26  make 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 delay  43  for delaying second cutoff signal  41  ensures that second cutoff signal  41 , if it occurs, is not immediately suppressed again due to the return to second reset input  44 , so that an analyzable second cutoff signal  41  is available to second cutoff input  42  of motor control  24 . The time settings for first and second time delay  19 ,  26  may be in the microseconds or milliseconds range. The time setting for fourth time delay  43  is preferably in the milliseconds range. If electric motor  46  is situated in a variable speed drive, maximum time T max  specified by timer  22  will be in the seconds range, which is adequate for an adjustment operation under severe conditions. The time setting for third time delay  32  is, for example, in the seconds range. This time delay must be matched to the expected signal change of setpoint signal  11 , which is, for example, in the seconds range in a manually actuated setpoint generator  10 . 
   Zero crossing detector  25  may be implemented, for example, with the signum function. If the sign of first differential signal  15  is changed, first cutoff signal  27  occurs, it being necessary to convert the negative signal changes at the output of zero crossing detector  25  into positive signal changes, for example.