Abstract:
Asynchronous motors are controlled by way of two- or three-phase controllers that include at least two pair of thyristors connected in antiparallel and fired at certain intervals. According to the inventive method, a fundamental wave with the desired frequency is defined for the first phase and in the other phase a respective fundamental wave is defined that has the same frequency as that in the first phase, but phase-shifted by corresponding values. For every phase, firing intervals are marked that have the same polarity as the respective fundamental wave. Those intervals of the potential firing intervals are used for every phase at which there is a potential firing interval in one of the two other phases. These intervals are used as the actual firing intervals.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/DE00/04444 which has an International filing date of Dec. 13, 2000, which designated the United States of America and which claims priority on German patent application number 199 62 667.7 filed Dec. 23, 1999, the entire contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to a method and an associated device for operating asynchronous motors. More preferably, it may relate to a method for finding triggering times of, for example, two- or three-phase, power controllers being connected to a three-phase network without connecting the star points of the stator winding of the asynchronous motor and which include at least two pairs of antiparallel-connected thyristors which are triggered at specific times. 
     BACKGROUND OF THE INVENTION 
     In various drive tasks, it is desired to operate an asynchronous motor with a rotational speed which is lower as compared with the nominal rotational speed and, in the process, to operate different directions of rotation, without changing the direction of rotation of the feeding network. For this purpose, conventional three-phase power controllers, such as electronic motor switch gear or soft starters, as they are known, can cost-effectively be provided with additional functionalities. Potential applications are, for example, in the positioning of transported goods or in electrically operated gates. 
     U.S. Pat. No. 4,791,341 A, U.S. Pat. No. 4,524,361 A and U.S. Pat. No. 4,461,985 A already disclose equipping a three-phase power controller with a total of five pairs of antiparallel thyristors. Using this, without changing the direction of rotation of the feeding network, a reversal of the direction of rotation of the connected asynchronous machine can be achieved. Furthermore, it is known from U.S. Pat. No. 4,481,456 A to use a total of nine pairs of antiparallel thyristors in such a way that each external conductor of the feeding network can be connected to each terminal of the three-phase asynchronous machine via a pair of the thyristors. With this arrangement, a matrix direct inverter, as it is known, is implemented. 
     In addition, EP 0 408 045 B1 and EP 0 512 372 A2 disclose a method and associated devices for motor control in which a pulse pattern is specified, with which fundamental waves can be produced at a frequency which corresponds to 1/(6n+1) of the main frequency of the feeding network, n being a natural number. Here, therefore, fundamental waves can be produced whose frequencies correspond to {fraction (1/7)}, {fraction (1/13)}, {fraction (1/19)} etc., of the main frequency. However, an option to reverse the direction of rotation is not associated with this. Finally, DE 25 58 113 A1 already discloses the proposal of producing 1/(6n+1) of the mains frequency by means of specific pulses. Here, in order to reverse the direction of rotation, either two further pairs of antiparallel thyristors or mechanical changeover devices are needed. 
     SUMMARY OF THE INVENTION 
     It is an object of an embodiment of the invention to specify a method with which suitable triggering times for a three-phase power controller for operating an asynchronous motor. At the same time, without additional outlay on power- electronic components, in particular inverter valves, and additional switching elements, merely by use of a three-phase power controller having three or two pairs of antiparallel thyristors, an operation of an asynchronous machine with any desired fundamental waves can be produced. In particular, the fundamental wave is to correspond to 1/k the main frequency, k being a number ≧3. Furthermore, at the same time any desired direction of rotation is to be possible. 
     In an embodiment of the invention, firstly, all the potentially possible triggering pulses are determined which result in the zero crossing of the concatenated voltage minus an angle φ. For the triggering of the thyristor pair in the external conductor A, potential triggering pulses result, for example, when the phase angle of the feeding network is at the angle φ before the zero crossing of the concatenated voltage (U AB ) or of the concatenated voltage (U CA ). In this case, the angle φ has a magnitude preferably between 30 and 60°. 
     In the method according to an embodiment of the invention, firstly, for all three external conductors, the fundamental waves are defined which correspond to the desired rotational speed and direction of rotation of the motor. In particular in the case of a three-phase power controller which contains three pairs of antiparallel thyristors, only the potential triggering pulses in pairs are used. Therefore in each case two pairs of thyristors are triggered, whose resultant current flow has a polarity corresponding to the defined fundamental waves. In an entirely corresponding manner, in the case of a three-phase power controller which contains only two pairs of antiparallel thyristors, in which therefore thyristors are used only in two external conductors and the third external conductor is bridged, in addition those pairs of triggering pulses which cause a current flow only between the external conductors that are not bridged are ruled out. In this way, an uncontrollable current in the bridged external conductor whose polarity does not correspond to that of the defined fundamental wave is avoided. 
     In an embodiment of the invention, it is particularly advantageous that any desired rotational speeds with 1/k of the nominal rotational speed can be produced, k being a number ≧3. It is also advantageous that merely by varying the angle φ, the torque output at the frequency of the defined fundamental wave can be influenced. 
     The method described is implemented in particular by a software program. It can therefore be implemented simply in existing three-phase power controllers without additional expenditure on components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantages of the invention emerge from the following figure description of an exemplary embodiment, using the drawings in conjunction with further subclaims. 
       In the drawings: 
         FIG. 1  shows a device for controlling an asynchronous motor with application of the method according to an embodiment of the invention, 
         FIG. 2  shows graphs to illustrate the method in the case of a motor rotating clockwise, 
         FIG. 3  shows a detail from  FIG. 2  to illustrate the influence of the triggering angle, 
         FIG. 4  shows a graph to illustrate the method in the case of a motor running counterclockwise, 
         FIG. 5  shows a device corresponding to  FIG. 1  specifically for the two-phase control of a motor, 
         FIG. 6  shows a graph of the method with two pairs of antiparallel thyristors and 
       FIG.  7  and  FIG. 8  show flow diagrams for the determination by software of the triggering times for the individual phases. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG.  1  and  FIG. 5  in each case an induction machine  2 , for example a three-phase asynchronous machine, is connected to the phases of the main via a three-phase alternating current power controller  4  as a three-phase power controller, as it is known. In  FIG. 1 , the main is the phases A, B and C of a three-phase network and, in  FIG. 4 , the phases A and B of a two-phase network. 
     Each of the phases is assigned a valve arrangement, for example in  FIG. 1  each phase A, B and C is assigned a valve arrangement V 1 , V 2 , V 3  and, in  FIG. 5 , the phases A and B are assigned a valve arrangement V 1  and V 2 . The valve arrangements in each case include two antiparallel-connected thyristors  6 . The triggering electrodes of the thyristors  6  are connected to a control device, with which the triggering signals required to trigger the thyristors  6  are provided in a predefined chronological sequence. 
     Between two external conductors of the network, for example between the terminals A and B of the network in  FIG. 1 , a voltage measuring device  10  is connected, at the output of which the main voltage U AB  occurring between these two terminals A and B is available. Furthermore, there is a control device  8  for controlling the phase gate angle for the purpose of stopping the motor softly. A control device of such a type is preferably implemented by a microcontroller. 
     In the present case, the control device  8  is used to process a suitable program, with which the operation of the device can be carried out exclusively by means of software. In this case, the basis is a standardized method in which any desired parts of the nominal rotational speed (&gt;=3) with any desired direction of rotation of the motor can be achieved for two- and three-phase soft starters. The control device can also be a microcontroller already provided for the motor. 
     In  FIGS. 2 and 4 , the individual signals s at a rotational speed of {fraction (1/9)} of the nominal rotational speed are illustrated. In particular,  FIG. 2  reproduces the situation in the case of a motor rotating clockwise and, in particular,  FIG. 4  reproduces the situation in the case of a motor rotating counterclockwise. 
     In the graphical representations, the signals have the following meaning:
         VAB=voltage of the phase A-B from FIG.  1 . The signal is used as a reference for the calculated times for triggering the thyristors.   IA=current in the phase A in the event of triggering 30° before the zero crossing of the concatenated voltage.       

     Since a current must always flow in two phases, for each current pulse in one phase there is a current with opposite polarity in another phase, which are designated by IB and IC. The fundamental wave of the current at 1/k of the nominal rotational speed is designated by FA, FB and FC for the individual phases. In each case potential triggering times PFA, PFB and PFC can be derived from the fundamental wave. The final triggering times are designated by DFA, DFB and DFC. 
     To generate the triggering times, the following is done in individual steps: firstly, for the phase A, a fundamental wave at the desired frequency corresponding to 1/k of the final rotational speed of the motor is defined. In this case, the phase shift is unimportant. In the case of clockwise rotation, a fundamental wave is defined for the phase B which has the same frequency as that of the phase A but is time-delayed with respect to the latter by 120° —based on the divided frequency. For the phase C, the same is true as for the phase A, but here the shift is 240°. 
     In the second step, for each phase the triggering times whose associated current has the same polarity as that of the respective fundamental wave are marked. In the following step, from the potential triggering times for each phase, those are used at which there is a potential triggering time in one of the two other phases. These triggering times are used as the actual triggering times to operate the three-phase power controller. 
       FIG. 3  shows how the triggering angle φ determines the torque of the asynchronous motor. With any desired devisors of the nominal rotational speed, the triggering angle φ&gt;0 can preferably be adjusted between 30° and 60°, which results in a torque that can be preset. 
     In order to achieve a rotational movement of the motor in the inverse direction, in accordance with the method described by using  FIG. 2 , the position of the fundamental waves of the external conductors B and C is interchanged. The selection and determination of the triggering times otherwise proceeds in a way identical to that in  FIG. 2 , which is reproduced by using FIG.  4 . 
     In  FIG. 5 , a stator for a two-phase network having two pairs of antiparallel thyristors  6  is constructed, the third external conductor being permanently bridged. When the two thyristors pairs are driven, a current will also flow in this external conductor. For this reason, those triggering pulses which relate only to the two external conductors fitted with thyristors are removed. 
     The latter is illustrated by using  FIG. 6 , in which again {fraction (1/9)} of the nominal rotational speed is assumed. For the case in which the phase A is bridged, the triggering pulses which relate only to the external conductors B and C are removed. 
     Using the control device  8  from FIG.  1  and  FIG. 5 , the respective thyristors are driven at suitable times in order to set a predefined rotational speed. For this purpose, the control device  8  comprises a computing unit, which in the following text is also designated by  8   a  and can be a microcontroller MC which is normally present in the case of an up-to-date three-phase power controller, for the purpose of determining the triggering times by means of software. Here, reference is made to the graph of  FIG. 4 , in which a complete period of nine individual periods is illustrated. There are counters SC, FC and CC, which count the individual times. 
     Each individual main period is subdivided into 60-degree sections. These are counted by the counter SC. The counter FC counts the 60-degree sections within one fundamental period. The counter CC is used to count the mains periods within one fundamental period. During each 60-degree section, 2 functions are used which calculate the polarity of the current in the respective phase and that of the fundamental wave. 
     The determination of the polarity of the current (function signOfCurrent) is illustrated in the following table: 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Result: −1 Sign negative 
               
               
                 0 Zero 
               
               
                 1 Sign positive 
               
             
          
           
               
                   
                 CC 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
                   
               
             
          
           
               
                   
                 Phase 
                   
                   
                   
                   
                   
                   
               
               
                   
                 A 
                 −1 
                 −1 
                 0 
                 1 
                 1 
                 0 
               
               
                   
                 B 
                 1 
                 0 
                 −1 
                 −1 
                 0 
                 1 
               
               
                   
                 C 
                 0 
                 1 
                 0 
                 0 
                 −1 
                 −1 
               
               
                   
                   
               
             
          
         
       
     
     Using  FIG. 7 , the determination of the polarity of the fundamental wave is illustrated, a speed factor (Speed-factor)SF=9 being used: in position  50 , a counter value x=FC is assumed which corresponds to 60° sections within a fundamental period.  100  designates the phase A,  200  the phase B and  300  the phase C. In the decision elements  101 ,  201  and  301 , a check is made on the value of the variable x. If the value is satisfied, a corresponding value based on the output value FC is specified at the positions  102 ,  202  and  302 . In the other case, a value increased by the speed 6×SF is output at the position  103 ,  203  and  303 . In each case a corresponding shift takes place in the individual phases. In the positions  104 ,  204  and  304 , the values are superimposed and the sum signal is passed onto the position at  305 . At  310 , a decision is made about the speed value and the polarity of the currents is output by using the sign. 
     For the triggering of the thyristors  6 , the procedure is such that the above procedure is called every 60 degrees, based on VAB. The exact time of the call lies an adjustable time before the 60-degree mark. The magnitude of this time interval determines the power supplied to the motor and therefore the torque developed by the motor. By using CC, SC, and FC, a decision is made for each phase pair as to whether triggering is to take place in the respective phases. 
     In  FIG. 8 , by using the decision diamonds, a decision is made on the basis of the signs of the individual phases as to which thyristors are to trigger. Position  400  relates to the phases A and B, position  410  relates to the phases B and C, and position  420  relates to the phases A and C. Depending on the sign determined using  FIG. 7 , in the positions  401 ,  411  and  421  in each case the signal for suitable triggering of the thyristors associated with the individual phases is provided. 
     In the examples according to the figures, it is assumed that the triggering lies 30° before the zero crossing of the respectively concatenated voltage. In order to increase the motor torque, the triggering can be advanced, a longer current flow time being achieved for each triggering. The triggering can preferably be carried out 30 to 60° before the zero crossing of the concatenated voltage. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention,and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.