PROCESS FOR DETERMINING THE LOAD OF AN ASYNCHRONOUS MOTOR FED VIA A FREQUENCY CONVERTER AND A DEVICE FOR CARRYING OUT THE PROCESS

Abstract: In an asynchronous motor fed from an AC current network (R; Mp) via a frequency converter (5; 6) and having an electronic control system (2; 3) for controlling the operating voltage as a function of a comparison of a load-specific current signal, which can be detected by the electronic control system (2; 3), with a corresponding control of the asynchronous motor (1) can be achieved with a low electronic expenditure by providing the current decay time (AI) as a load-specific current signal during which time the free-wheeling current following after voltage interruption in a motor winding path decreases by a specific degree.

Description: "PROCESS FOR DETERMINING THE LOAD OF AN ASYNCHRONOUS MOTOR FED VIA A FREQUENCY CONVERTER AND A DEVICE FOR CARRYING OUT THE PROCESS" The invention relates to a process for determining the load, in accordance with the preamble of claim 1, of an asynchronous motor fed via a frequency converter, and a device for carrying out this process; a process of the aforesaid type is known from DE-A1-31 01 511. In the aforesaid known process for determining the load of devices, in particular domestic appliances which are driven, inter alia, by an asynchronous motor and which carry out different operating programmes under different load states and in which the operating voltage of the asynchronous motor is controlled by means of an electronic control system con- taining a microcumputer, the load state determining the re¬ spective operating programme is determined by comparing at least one variable supplied by the electronic control system controlling the operating voltage of the asynchronous motor and at least one comparison value predetermined for the asyn- chronous motor; the phase difference between the operating voltage zero crossover point is used as the variable supplied by the electronic control system and the width of the pulse width signal, supplied by the electronic control system, for the power component of the converter-fed asynchronous motor is used as the manipulated value. In the known case, the zero crossover point of the operating voltage is detected by a zero crossover point detector and the zero crossover point of the operating current is detected by a Hall generator; the phase shift angle which is representative of the load is then de- tected by the electronic control system which is already pres¬ ent and required even without detection of the load. A process which manages with a low expenditure in terms of components in comparison with the above and neverthe- less ensures a reliable determination of the load is possible, starting from a process of the type according to the invention mentioned at the beginning, by means of the teaching of claim 1; advantageous embodiments of this process are the subject of claims 2 to 10; an advantageous device for carrying out this process is the subject of claim 11. Starting from the knowledge of the design - known for example from the customary equivalent circuit diagram - of an asynchronous motor with, inter alia, a main inductor and a slip-dependent ohmic rotor resistor, which is parallel thereto, according to the invention the decay time of the free-wheeling current of one phase, which decay time is dif¬ ferent in each case depending on the slip variable and thus load variable, is utilised after the said current is discon¬ nected from the supply voltage in order to transmit a load- specific signal to the electronic control system, which is present per se, and to use it further there for the closed- loop or open-loop control of the asynchronous motor. The ex¬ penditure in terms of components for detecting the current decay time can be kept particularly small by defining the start of the current decay time measurement by the time of the voltage interruption since this time is already defined by the electronic control system which is present per se. The end of the current decay time is expediently de¬ fined by the time of the end of the free-wheeling current, a voltage transistor which is initially switched off or switched on being connected as voltage decay measuring element in a particularly simple manner via voltage divider resistors solely to the phase provided for the measurement, which tran¬ sistor switches over when the zero point of the free-wheeling current is reached and passes on this time as time-specific variable for the end of the current decay time to the elec¬ tronic control system as an actual value. The invention and further advantageous embodiments of the invention are explained in greater detail below with reference to an exemplary embodiment illustrated diagrammat¬ ically in the drawings, in which: Fig. 1 shows an overall basic circuit diagram of a converter-fed asynchronous motor with current decay time detection in one phase. Fig. 2 shows the clocking diagram of the three phases of the asynchronous motor with non-complimentary clocking. Fig. 3 shows the voltage characteristic and current characteristic of a phase with current decay time measurement with a small load. Fig. 4 shows the voltage characteristic and current characteristic of a phase with current decay time meansurement of a large load. Fig. 1 shows an asynchronous motor 1 fed from a sin¬ gle phase AC current network R;Mp via a frequency converter with a three-phase bridge 5 having two transistor motor actuators T.. and Tz. per phase and via an intermediate DC cir¬ cuit 6. A detailed description of the function of the asyn- chronous motor, known for example from EP-A2-0 254 709 and fed via a frequency converter, can be dispensed with here; instead of the circuit with three phase-shifted voltages, such a cir¬ cuit with only two-phase shifted voltages and correspondingly a total of only four transistor motor actuators in only two controlled half bridges can be provided without changing the inventive idea. The clocking, provided according to an embodiment of the invention, of the three phases of the asynchronous motor 1 via transistor motor actuators T, and -z. assigned in each case to each phase can be seen in Fig. 2. Fig. 2 describes a so- called non-complementary clocking in which the motor actuators of a phase do not change cyclically but rather in clocking sections of 180° el in each case. This means that the return of the energy stored in the inductors of the motor occurs as a free-wheeling current via the completely rectified voltage. The load detection according to the invention by measuring the current decay time of the free-wheeling current is described below in particular with reference to Figs. 3 and 4. In a zero crossover point of the phase provided for current decay time measurement, i.e. when changing over from its first transistor motor actuators T, to its second transis¬ tor motor actuators Tj. or vice versa, the current of this phase returns to zero, as a function of the respective slip of the preceding load state, with a greater or lesser degree of steepness. This time is in the region of micro-seconds, for example between 50 and 1000 j*s for a typical asynchronous mo¬ tor provided for driving an automatic washing machine. In the idling load state the slip is zero so that the slip-dependent ohmic rotor resistance according to the equivalent circuit di¬ agram tends towards the infinite; a relatively long current decay time t r according to Fig. 3 corresponds to this. With full load and thus a large degree of slip, the slip-dependent ohmic rotor resistance tends towards a substantially smaller value so that now a corresponding shorter current decay time brx according to Fig. 4 represents the load state. The clock pauses of the individual transistor motor actuators Ti and T^according to Fig. 2 can be used for the re¬ spective measurement of the current decay time, however, in each case the current decay time can also be measured during an interruption of the clock pulses of a clock block and after the end of the measurement the further clocking of the previ- ously disconnected transistor motor actuator can be returned to. By means of the switch-on time of the second and third transistors T j R; T,T which is extended in comparison with the shorter switch-on time of the first transistor, the discharge voltage is reduced whilst the discharge time is cor- respondingly extended. The measurement of the current decay time or the re¬ formation time of the voltage in a phase provided for measur¬ ing, after interruption of the clocking, occurs in an advantageous manner according to Fig. 1 with only a single voltage transistor τ u between voltage divider resistors 7;8 in one current measuring path. The time t, of the start of the current decay time meansurement is known in the electronic control system by virtue of the switching over of the first transistor motor actuator T, to the second transistor motor actuator j.; the time t of the end of the current decay time measurement is signalled by the voltage transistor T u to the electronic control system 2,3 consisting at least of one microcontroller 2 and one logic component 3 by the voltage at the previously switched off or switched on voltage transistor Tu now, in reverse fashion, switching on or switching off the said voltage transistor when the free-wheeling current reaches the zero point. A current transistor Tj with voltage divider resistors and a current resistor R4- , also illustrated in Fig. 1, serves solely for current limiting in a control process of the asynchronous motor, following for example due to the cur¬ rent decay time detection, in the case of a mechanical over¬ load as a protection against a current overload. The individual current decay times detected by the voltage transistor T υ are expediently averaged according to known methods in order, if required, to be able to eliminate disturbing influences; the possible clock and measuring cycle times are so short that an adequate elimination of undesired load states is ensured at any time. The load detection according to the invention which can be achieved with a low expenditure in terms of components by measuring the current decay times is used according to a first embodiment of the invention for the voltage control or speed of rotation control of the asynchronous motor as a func¬ tion of undesired load states; this is provided in particular for an unbalance detection or unbalance-compensating voltage control of the asynchronous motor as a function of a current decay time deviation representing an unbalance. The current decay time measurement according to the invention can also be advantageously used for detecting the load of the washing drum by comparing the current decay times in a first, in particular application speed of rotation of the wash with a second higher speed of rotation subsequently achieved with a constant acceleration.

Claims: CLAIMS 1 - Process for determining the load of an asyn¬ chronous motor (1) fed via a frequency converter (5; 6), in particular for driving an automatic washing machine, having an 5 electronic control system (2;3) for controlling the operating voltage as a function of a comparison of a load-specific cur¬ rent signal, which can be detected by the electronic control system (2;3), with a predeterminable reference value, characterised in that the current decay time (tj) is provided 10 as a load-specific current signal while the free-wheeling cur¬ rent following after voltage interruption in a motor winding path decreases by a specific degree. 2 - Process according to claim 1, characterised in that the start of the current decay time measurement is de- 15 fined by the time (t, ) of the voltage interruption. 3 - Process according to claim 1 and/or 2, characterised in that the end of the current decay time meas¬ urement is defined by the time ( t_ ) when the zero crossover point of the free-wheeling current is reached. 20 4 - Process having a supply voltage, which is al¬ ternately clocked per phase by a first or a second transistor motor actuator (T\ or T2-), according to one of claims 1-3, characterised in that the start of the current decay time measurement is defined by the time (t, ) of the transistor mo- 25 tor actuator (Tl ) to the second transistor motor actuator (T D- 5 - Process according to claim 4, characterised by a non-complementary clocking of the transistor motor actuators (Tl and T2.) clocked alternately per phase over in each case 30 180° el. 6 - Process according to claim 4, characterised by a non-complementary clocking with a switch-on time of a first transistor (Tz.S) feeding the motor winding paths, which switch-on time is different from the second or third transis- 35 tors (T R and T T) simultaneously switching the free-wheeling path of the previously switched-off motor winding path with the effect of a switch-on time for the second and third tran¬ sistor (T, R and T|T) which is extended with respect to the switch-on time of the first transistor (T^S). 7 - Process according to claim 4, characterised by a non-complementary clocking of the transistor motor actuators (T\ and TZ) during the measurement of the current decay time with an otherwise complementary clocking. 8 - Process according to one of claims 1-7, characterised by a voltage control and/or speed of rotation control of the asynchronous motor (1) as a function of the current decay time (t,I). 9 - Process for an automatic washing machine ac- cording to one of claims 1-8, characterised by an unbalanced detection or unbalance-compensating voltage control of the asynchronous motor (1) as a function of a current decay time deviation representing an unbalance. 10 - Process for an automatic washing maching ac- cording to one of claims 1-9, characterised by a washing drum load detection by comparing the current decay times or corre¬ sponding voltage values of the asynchronous motor (1) in the case of a first constant speed of rotation, in particular ap¬ plication speed of rotation, with a second speed of rotation subsequently achieved with constant acceleration. 11 - Device for carrying out the process according to one of claims 1-4, characterised by an asynchronous motor (1) fed by a frequency-converter via an electronic control system (2;3) and having current decay measuring element (volt- age transistor Tu voltage divider resistors 7;8) which is as¬ signed to at least one phase, can be set in this phase after voltage interruption and connected to an actual value input of the electronic control system (2;3).

Inventors: BUCHER GETTSCHALD VIKTOR MATHI, WERLE PAULO, BUCHER GETTSCHALD, VIKTOR, MATHIEAS, FRANZ, WERLE, PAULO
Applicants: ICOTRON SA, ICOTRON S.A.
Application year: 1991
Main classification: D06F37
IPC codes: D06F37, H02P23, H02P27