Abstract:
A method with which the load current and thus the load moment of an asynchronous motor that is controlled via a phase-controlled two-phase thyristor power controller, can be easily influenced so as to allow a smooth starting operation. According to a first embodiment, the ignition point is determined in the controlled phase in order to adapt the flow angles of the subsequent current half waves. According to a second embodiment, the ignition point of the subsequent current half waves is brought forward in both controlled phases.

Description:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/DE02/04091 which has an International filing date of Nov. 4, 2002, which designated the United States of America and which claims priority on German Patent Application number DE 101 56 216.0 filed Nov. 15, 2001, the entire contents of which are hereby incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention is generally directed to a method for reducing the influence of a DC component in the load current of an asynchronous motor. 
   BACKGROUND OF THE INVENTION 
   Three-phase controllers use a principle of phase gating to control the amount of energy supplied to an electrical load. This allows the starting currents and torques in asynchronous machines to be reduced. The phase gating angle is used as a measure of the amount of energy supplied. Current half-cycles of alternate polarity flow through the load which is connected to the output of the three-phase controller with there being a time interval during which no current flows and which is determined by the phase gating—also referred to as the delay—between each two successive current half-cycles. 
   Three-phase controllers are normally equipped with three pairs of thyristors which are connected back-to-back in parallel. However, since the thyristors become the cost-determining factor as the rating of the three-phase controller increases, three-phase controllers having only two pairs of thyristors which are connected back-to-back in parallel are also used. In these so-called two-phase three-phase controllers, the remaining third phase is in the form of a conductor which cannot be switched. A drive such as this is known from DE 30 09 445 A1. 
   SUMMARY OF THE INVENTION 
   An embodiment of the invention is based on an object of specifying at least one method by which the load current, and hence the load torque, of an asynchronous motor which is controlled via a two-phase thyristor three-phase controller with phase gating can be influenced with little complexity, for soft starting purposes. 
   According to an embodiment of the invention, an object is achieved by a first method, and/or by a second method. 
   An embodiment of the invention is based on the knowledge that, by virtue of their principle of operation, when phase gating takes place in the range of 75°±10° in two-phase three-phase controllers, these controllers have a tendency to produce asymmetric current half-cycles of positive and negative polarity. In consequence, the successive positive and negative current half-cycles which alternate with one another each have a different time duration and amplitude. This leads to a DC component in the load current which, for example in the case of asynchronous machines, produces braking torques and thus makes it harder or completely impossible to start them softly. 
   Thus the phase-gated load current, which has current half-cycles of alternate polarity and of a different duration and amplitude, and accordingly DC components which produce braking torques, can be influenced such that the DC components are reduced without using measurement devices to determine the current/time integrals which are enclosed by the current half-cycles, thus ensuring that all the effective currents have a uniform rise and profile in order to achieve the desired soft starting. 
   According to a second method of another embodiment, only current flow information and in general no control loop information is required. Thus, this method can be implemented with a simpler and possibly restricted control device in comparison to the first method. Furthermore, the turn-off times are not recorded, so that it is possible to save software and/or hardware modules. 
   According to one advantageous development of the first method, it is also advantageously possible, inter alia, to save current transformers for determination of the current flow angles, provided that the respective turn-on and turn-off time can be determined on the basis of a voltage rise across the associated thyristor in the thyristor three-phase controller. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various aspects of the invention as well as advantageous refinements according to the features of the dependent claims will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated schematically in the drawing, in which: 
       FIG. 1  shows a diagram with three phases of a load current before and after the use of a first method for reducing the influence of a DC component, and 
       FIG. 2  shows a diagram with three phases of a load current before and after the use of a second method for reducing the influence of a DC component. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  uses a diagram to show the time profile of a three-phase current during starting of an asynchronous machine using a two-phase three-phase controller, before and after the use of a first method for reducing the influence of a DC component. This illustration shows a current i 1  and i 2  in a first and a second controlled phase L 1  and L 2 , respectively, as well as a current i 3  in a third, uncontrolled phase L 3 , in each case having pronounced DC components. 
   Furthermore, the currents i 1  and i 2  have a respectively associated current i 1K  and i 2K  corrected according to the method, and the current i 3  has an associated current i 3K , which is influenced by the correction according to the method, all in phase. In order to control the phases L 1  to L 3 , the phase gating is supplied by a function that can be determined via a ramp, in particular a linear ramp. 
   The first method for reducing the influence of the DC component provides that, first of all, the current flow angle γ n−1  in a first current half-cycle S 1L1  is detected in the first controlled phase L 1  from its turn-on time t Ign−1 , and its turn-off time t Ext−1 . The current flow angle γ n  in a subsequent second current half-cycle S 2L1  is then recorded in the first controlled phase L 1  from its turn-on time t Ign  and its turn-off time t Ext . The respective turn-on and turn-off time is recorded on the basis of a voltage rise across the associated thyristor in the thyristor three-phase controller, and conventional control means can be used for this purpose. 
   Following this, then, the difference between the current flow angle γ n−1  in the first current half-cycle S 1L1  and the current flow angle γ n  in the second current half-cycle S 2L1  is recorded. Finally, the turn-on time t Ign+1  in a subsequent, corrected, third current half-cycle S 3L1  is determined in the first controlled phase L 1  on the basis of the recorded difference, in the sense of matching the current flow angles in the current-half cycles. 
   The turn-on time is determined continuously for subsequent current half-cycles on the basis of the recorded difference between the current flow angles in the respective preceding current half-cycles. This results, inter alia, in the turn-on time t Ign+2  for a subsequent, corrected, fourth current half-cycle S 4L1 . 
   Both the third current half-cycle S 3L1  which follows the second current half-cycle S 2L1 , together with the associated turn-on and turn-off times t Ign+1  and t Ext+1 , respectively, as well as the fourth current half-cycle S 4L1  which follows the third current half-cycle S 3L1 , together with the associated turn-on and turn-off times t Ign+2  and t Ext+2 , respectively, for the first controlled phase L 1  are superimposed in  FIG. 1 , in order to illustrate the matching according to the method, with the current flow angles in the first current half-cycle S 1L1  and the second current half-cycle S 2L1  in the first controlled phase L 1 . 
   In consequence, in a schematic comparison, the corrected turn-on time t Ign+1  in the third current half-cycle S 3L1  occurs, according to the method, later than the original turn-on time t Ign−1  in the first current half-cycle S 1L1  and, according to the method, the corrected turn-on time t Ign+2  in the fourth current half-cycle S 4L1  occurs earlier than the original turn-on time t Ign  in the second current half-cycle S 2L1 . 
   At the same time as the application of the first method, which is related to a single phase, to the first controlled phase L 1 , this method can also be used without any interactions for the second controlled phase L 2 , that is to say with the first phase L 1  being controlled independently of the second phase L 2 . 
     FIG. 1  in this case shows the commutation process KV on the basis of the falling flank F AB  in a current half-cycle S 1L2  in the second controlled phase L 2 , and on the basis of a rising flank FAN in the fourth current half-cycle S 4L1  in the first controlled phase L 1 . 
   Since the two controlled phases L 1  and L 2  have current half-cycles S 1L2  and S 4L1  with matched respective current flow angles γ s  and γ n+2  as a result of the first method, the current profile of the third, uncontrolled, phase L 3  is also advantageously influenced so that this results in all of the effective currents having a uniform rise and profile. In principle, reducing the DC components in a load current also has a positive effect on any inductive measurement device which may be used, since this makes it possible to prevent saturation and thus to take precautions against incorrect measurements. 
   In contrast to symmetrical polarity driving and turning-on, turn-on times are advanced or delayed continuously by the use of a correction factor C so as to effectively provide compensation for different current flow angles in positive and negative current half-cycles. Symmetrical polarity means that the time period during which no current flows—the delay or phase gating angle α—between two successive current half-cycles of alternate polarity during a transition from a positive current half-cycle to a negative current half-cycle is exactly of the same magnitude as that for a transition from a negative to a positive current half-cycle. 
   Normally, symmetrical polarity driving and turning-on are associated with symmetrical phase driving and turning-on. Symmetrical phase means that the time period during which no current flows is also of the same magnitude in the sets of thyristor valves—three pairs of thyristors which are connected back-to-back in parallel—in the three phases L 1  to L 3 . Depending on the motor and the load state, the correction factor C may have a value of 0.1 to 0.4, in particular 0.2. 
   In a corresponding manner, when a thyristor valve set is turned off in the second current half-cycle S 2L1 , the corrected turn-on time γ n+1  in each subsequent third current half-cycle S 3L1  is determined using the following equation: 
               t     Ign   +   1       =       t   Ext     +         α   C       360   ∘       *   T               (   1   )             
 
   In this case, t Ext  indicates the turn-off time in the current half-cycle which precedes the third current half-cycle S 3L1 , T indicates the period duration, and α c  indicates the corrected phase gating angle. With an alternating current waveform at a frequency of 50 Hz, the period duration T is, for example, 20 ms. The corrected phase gating angle α c  is accordingly determined using the following equation:
 
α c =α T   ±C*Δγ   (2)
 
   In this case, α T  indicates the averaged phase gating angle, which is determined using the following equation: 
               α   T     =         α   n     +     α     n   -   1         2             (   3   )             
 
   In this case, α n−1 , indicates the phase gating angle in a first current half-cycle S 1L1 , and α n  indicates the phase gating angle in a second current half-cycle S 2L1 . 
   Furthermore, C indicates the constant correction factor, which is between 0.1 and 0.4, and is in particular 0.2, and Δγ indicates the difference between successive current flow angles using the following equation:
 
Δγ=γ n −γ n−1   (4)
 
   Owing to the alternating polarity between successive current half-cycles, Δγ always describes the difference between the duration of a positive current half-cycle and a negative current half-cycle. 
   In this case, γ n−1  indicates the current flow angle in a first current half-cycle S 1L1  in accordance with the following equation: 
               γ     n   -   1       =           t     Ext   -   1       -     t     Ign   -   1         T     *     360   ∘               (   5   )             
 
   In this case, t Ext−1  indicates the turn-off time in the current half-cycle S 1L1  which precedes the second current half-cycle S 2L1 , and t Ign−1  indicates the turn-on time in the first current half-cycle S 1L1  which precedes the second current half-cycle S 2L1 . 
   Furthermore, γ n  indicates the current flow angle in a subsequent second current half-cycle S 2L1 , in accordance with the following equation: 
               γ   n     =           t   Ext     -     t   Ign       T     *     360   ∘               (   6   )             
 
   In this case, t Ign  indicates the turn-on time in the second current half-cycle S 2L1  which follows the first current half-cycle S 1L1 . 
   In order to make it possible to carry out the correction after the next turn-off time, the current flow angle γ n  can be stored, in the sense of a further first current half-cycle, for continuous matching, using the following equation:
 
γ n−1 =γ n   (7)
 
   Advantageously, only time information and in general no control loop information is required for the first method, with the time information being available in a conventional controller for three-phase controllers in any case, in order to calculate the turn-on times, so that the first method can be implemented with little complexity. 
   An asynchronous motor which is operated in this manner develops from the start of its drive a torque which increases continuously as the phase gating decreases, so that this asynchronous motor is accelerated to the respective rated rotation speed within a time period of about 2 s to 4 s, for soft starting purposes. Furthermore, in comparison to conventional drives with a two-phase three-phase controller, the effective values of the currents i 1  to i 3  are reduced by reducing DC components. 
   An idea of an embodiment of the present invention is to equalize the duration of successive current half-cycles of alternate polarity in one and the same phase, in order to suppress DC components when the phase gating values are in the region of 75°±10°. The correction is generally maintained until the end of the ramp function. 
   An embodiment of the invention as explained above may be summarized as follows: 
   In order to make it possible to influence the load current, and accordingly influence the load torque, with little complexity for an asynchronous motor which is controlled by phase gating via a two-phase thyristor three-phase controller, in order to provide soft starting, the first method provides for the turn-on time (t Ign+1 ) in the controlled phases (L 1  and L 2 ) to be determined in order to match the current flow angles in the subsequent current half-cycles. 
     FIG. 2  uses a diagram to show the time profile in approximately one period of a three-phase current during starting of an asynchronous machine using a two-phase three-phase controller, before and after the use of a second method for reducing the influence of a DC component. The illustration shows a current i 1  and i 2  in a first and a second controlled phase L 1  and L 2 , respectively, and a current i 3  in a third, uncontrolled, phase L 3  in each case with pronounced DC components. 
   Furthermore, the currents i 1  and i 2  respectively have an associated current i 1K  and i 2K , corrected according to the method, and the current i 3  has a current i 3K , which is influenced by the correction according to the method, all in phase. In order to control the phases L 1  to L 3 , the phase gating is supplied by means of a function which can be determined via a ramp, in particular via a linear ramp. 
   The second method for reducing the influence of the DC component is based first of all on recording the turn-on time t Ign−1  in a first current half-cycle S 1L1  in a first controlled phase L 1 . Then, the current flow in the first current half-cycle S 1L2  in a second controlled phase L 2  is recorded at the turn-on time t Ign−1  of the first current half-cycle S 1L1  in the first controlled phase L 1 . Finally, the turn-on time t Ign  in the subsequent second current half-cycle S 2L2  is advanced in the second controlled phase L 2  on the basis of the recorded current flow, to a turn-on time t Ign+1  which is subsequent to this, in a third current half-cycle S 3L2 . 
     FIG. 2  shows the third current half-cycle S 3L2  superimposed in comparison to the second current half-cycle S 2L2 , in which case, in principle, the third current half-cycle S 3L2  follows the second current half-cycle S 2L2 . The advancing process is carried out continuously by means of a definable correction factor C, with non-compliance with the condition resulting in no correction during the respective ramp function of the phase gating. Depending on the motor and the load state, the correction factor C may have a value from −9° to −15°, in particular −12°, so that the second method is accordingly no longer phase symmetrical. 
   The detection of these so-called trigger times for the correction of the phase gating values and turn-on times is carried out continuously during the ramp function for phase gating, thus resulting in a correction which is independent of time and which is generally maintained until the end of the ramp function. 
   At the same time as the check of the condition as to whether current is flowing in the second controlled phase L 2 , the second method, which relates to two phases, can likewise check the condition as to whether current is flowing in the first controlled phase L 1 , thus allowing them to in each case be driven as a function of one another. 
   The advancing of the corresponding turn-on time means that a greater commutation current occurs, so that this results in a greater current rise so that the corresponding turn-off time accordingly occurs later so that, in the end, the associated current flow angles are increased or decreased. 
   By way of example, when carrying out the second method, two input signals, two output signals and a total of four interrupt routines may be used on the control side. The two input signals are current zero-crossing signals. Further, the output signals are thyristor turn-on signals for the two controlled phases L 1  and L 2 . The interrupt routines are in each case triggered by the negative flank—which occurs every 10 ms at a main frequency of 50 Hz—of a current zero-crossing signal. In this case, that particular phase gating is loaded into a so-called timer register, and the timer is started. 
   When or if the timer overflows, a change is made to the interrupt routine in order there to generate a turn-on pulse for the respective phase by reloading and starting of the timer. According to the method, the signal level of the corresponding input signal to the first controlled phase L 1  and second controlled phase L 2  is checked throughout the entire ramp function for phase gating to the start of the generation of the turn-on pulse. If current is flowing, a first and/or a second flag is set. 
   The respective flag is checked before that particular phase gating value is loaded into the timer register. If one or both flags is or are set, the respective phase gating angle is changed by the constant correction factor C before being loaded. If one flag or no flag is set, the respective phase gating angle for the first controlled phase L 1  and/or for the second controlled phase L 2  is loaded into the timer register without being changed, and conventional control means can be used for this purpose. 
   An asynchronous motor which is operated in this way generates a torque which increases continuously as the phase gating decreases from the start of the drive, so that it is accelerated to the respective rated speed within a time period of about 2 s to 4 s, for soft starting purposes. Furthermore, in comparison to conventional drives with a two-phase three-phase controller, the effective values of the currents i 1  to i 3  are reduced by reducing the DC components. 
   An idea of an embodiment of the present invention is to advance the turn-on times of successive current half-cycles of alternate polarity in two controlled phases as a function of a current flow condition, in order to suppress DC components for phase gating values in the region of 75°±10°. 
   An embodiment of the invention as explained above can be summarized as follows: 
   In order to make it possible to influence the load current and accordingly the load torque of an asynchronous motor, which is controlled via a two-phase thyristor three-phase controller with phase gating, with little complexity for soft starting purposes, a second method provides for the turn-on time (t ign ) in the subsequent current half-cycles to be advanced in two controlled phases (L 1  and L 2 ). 
   Exemplary embodiments 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 present 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.