SE504746C2 - Asynchronous motor control device - Google Patents

Abstract

Control means for asynchronous electrical engines, driven by an alternating 3-phase current, which creates a rotating electrical field in the engine, which at a certain lag in rotation from synchronous rotation in the rotor induces a field, which creates the field necessary for rotation of the rotor. An oscillator (8) and a frequency controlled electrical bridge (1) for the alternating current combine to create the driving current for the engine (2). The control means is arranged to receive that pulse which during operation of the engine is created in its feeder circuits for alternating current, and is designed so as to detect changes in the pulse shape of the driving voltage, which are caused by electromotive forces induced by the engine windings when these are passed by the induced field of the rotor, where the size of the changes vary with the size of the lag. The bridge (1) is fed from a power regulator (6) which through changes in said electromotive forces is controlled to output to the bridge (1) a voltage which varies according to the size of the lag so that a variation in torque load on the engine (2) and an accompanying tendency to change in the lag is compensated in a control process.

Description

504 746 10 U 20 25 30 the duration of this counter-EMF increases with increasing lag, thus with increasing load. This is ascertained when driving with a square pulse, while when driving with a sine wave the ratio is not as detectable. By electronic means, the magnitude and duration of the change of counter-EMF can be measured and used to control the motor. The change has a certain proportionality to the lag and thus to the torque load. If a control equipment is set up to detect and check the value of the counter-EMF by changing the voltage power to the motor windings, a motor is obtained which develops a torque adapted to the load on the motor shaft.

In asynchronous motors with strongly varying drive resistances, this circuit will produce variations in speed depending on the applied torque and within limits which are far greater than what results from the lag in asynchronous motors operating at a fixed frequency of the drive current. This can be advantageous in some cases, for example for the operation of pumps operating at varying back pressure. In other cases, however, it is desired to maintain a constant speed as far as possible.

OBJECTS AND SOLUTION OF THE INVENTION The object of the invention is to provide a control circuit for operating asynchronous motors. The aim is to overcome said disadvantages and to provide an engine operation which provides the engine with the necessary amount of energy required for a constant speed at varying loads on the shaft of the engine. In case of overload or difficult starting conditions for the engine, the speed is adjusted proportionally to the degree of overload. These properties ensure energy-efficient, quiet and reliable operation of the engine.

This is achieved by means of a control circuit according to the invention, which up to a certain torque load through voltage changes in the windings maintains a constant speed of the motor, thus a constant lag. When this drive resistance is reached and exceeded, the control circuit will lower the engine speed by changing the drive frequency, so that the engine does not risk falling out and stopping but can overcome the resistance torque, with reduced speed in relation to the set speed. This effect is also favorable when starting the engine at a high starting torque, where it can be controlled to work its way up from a standstill to a set speed.

DRAWINGS REGARDING THE INVENTION: In the following, an embodiment of the invention is described with reference to the accompanying drawings, in which: Fig. 1 is a diagram of the preferred embodiment of the control circuit; and in Figs. 2-8 a number of diagrams showing pulse ratios during motor operation. 504 746 10 Ü 20 25 30 PREFERRED EMBODIMENT: Fig. 1 shows a diagram in which the constituent components are combined by enclosing lines into a number of function blocks. In this case, a bridge is indicated at 1 for generating a 3-phase drive voltage, which is supplied to starter windings in a short-circuited asynchronous motor 2 via connection lines 3, 4 and 5. For controlling the motor voltage, a voltage-controlled power regulator 6 is connected to the bridge 1 via a line 7. The driving voltage to the motor 2 is in the form of square pulses whose frequency is controlled by an oscillator 8. The frequency of the oscillator is controlled by a pulse width detector 9 as will be explained later, via a converter 10 from pulse width to the corresponding direct voltage, which by a line from to the oscillator 8 as the control voltage a converter is supplied for the frequency. The connection to the power source, battery or DC network, is indicated at ll.

Between the oscillator 8 and the bridge 1 a sequence generator 12 is inserted, followed by a drive stage 13.

A pulse separator 14 is connected partly to the connecting lines 31 between the sequence generator 12 and the drive stage 13 and partly to the connecting lines 3, 4, 5 emanating from the bridge 1 to the motor 2.

The pulse separator 14 is in turn connected via output lines 15 and 16 to a window comparator 17. This can, as shown within this block, consist of two and 19, and to the output line 7 comparator elements 18 connected to lines 15 and 16 and 10 respectively. 504 746 from the power regulator 6 via a resistance chain 20 as shown. Output lines from the comparator elements 21 and 22 are connected to a summing circuit 23 from which a line 24 emits, which transmits output pulses from the summing circuit 23 for the motor control to the pulse width detector 9 and a converter 25 from pulses to a direct voltage for controlling the power regulator 6 via a connecting cable 26.

Fig. 1 shows electronic components with customary standard designations included in some of the individual blocks. The choice of type of components, their performance and interconnection can within the scope of the invention be varied depending on requirements for operating conditions, cost limits and component availability and may also depend on the development of new types of components suitable for use in the present context. The detailed construction of the device should therefore not be described in more detail other than when this is necessary for understanding the central inventive concept.

Thus, in the case of the circuits for generating drive voltage, the oscillator 8, sequence generator for the motor 2, 12, the drive stage 13 and the motor bridge 1 are constructed according to known principles for generating a 3-phase current by supply from a power regulator. In the present construction with the voltage-controlled power regulator, it applies that the magnitude of the driving voltage depends on the voltage of the control voltage, which is transmitted to the power regulator from the converter 25 at 50 50 746 10 15 20 25 30.

With a change in the control voltage, the engine torque also changes.

In the task to be solved by the invention and stated initially, the torque of the motor by changing the driving voltage from the power regulator 6 must be adapted to the load of the motor shaft, in such a way that the lag does not change significantly regardless of the applied load. This is achieved according to the invention by the driving voltage being adapted to the torque conditions and in the construction shown this is done by controlling the power regulator 6 closest from the converter 25 and on the basis of a detection of the motor load by reading the pulse course in some / some supply lines for motor voltage and by means of the window comparator 17 via the pulse separator 14. This then gives rise to a changed control voltage from the converter 25, which at 26 is transmitted to the power regulator 6 and controls it to change its output voltage.

The diagram in Fig. 2 shows pulse shapes of the signals which are drained from the supply lines 3, 4, 5 or at least one of them and fed to the pulse separator 14. It is shown that within and after each drive pulse 33 a pulse 34 and a pulse 32 are generated by an EMF when the stator field passes the induced field from the short-circuited rotor windings at the lag necessary for generating the rotor field. Both of these deviations from the pure pulse have a duration / voltage level which depends on the magnitude of the lag. The greater the lag, the greater the induced rotor currents and thus the EMF also increases.

These voltage level variations are marked with thin lines within the portions 32 and 34. In the pulse separator 14, the voltages 32 and 34 are separated from the drive pulses 33, which are indicated to the pulse separator from the sequence generator 13, and the separated pulses are transmitted to the window comparator 17. This is shown in FIG. 4. In the comparator 17, the voltages 35 and 36 change to square pulses 37 and 38 at the outputs 21 and 22.

The width of the pulses is directly proportional to the voltage levels of the voltages from the pulse separator 14. The smaller the voltage level with less lag, the shorter the pulse width. According to Fig. 5, the pulses 37 and 38 in the summing circuit 23 are summed to a pulse train 39 emanating from the window comparator.

In order for the pulse width to be an adequate expression of the lag regardless of the voltage level in the supply lines 3, 4, 5, a reference voltage taken from the power regulator 6 via the line 7 and the resistance chain 20 is input to the comparator elements 18 and 19 at 28 and 29.

This pulse train to the converter 25 is converted therein into a direct voltage 40, see Fig. 6, which is supplied to the power regulator 6. Variations in the direct voltage are directly proportional to the pulse width of the pulse train 39, see Fig. 5. As shown in Fig. 6, a larger lag is still indicated by a thick line and a smaller one by a thin line, which lines thus represent the output voltage level from the converter 25 to the power regulator 6.

The variable control signal to the voltage-controlled power regulator 6 has thus been obtained. The control thereof is arranged so that the increasing voltage due to increasing lag gives an increase in the voltage from the power regulator 6 to the bridge 1 and thus an increase in power in the motor windings.

In operating conditions where the voltage is dependent on torque load, with increasing external load, the power can thus be increased by increasing the voltage if the engine speed is to remain constant. If this can be done automatically, as achieved by the invention, one thus obtains an adaptation of the engine power to varying loads without appreciable effect of the set fixed speed. However, as with control processes in general, a completely constant speed is not maintained. If an increased torque load is imposed, the speed will first decrease and then be readjusted to the set, but slightly below the same.

This difference is required in order to maintain the required change in the control signal. At 10 20 25 30 504 746 each control sequence, a turn-in takes place to the new control position. The higher the gain of the control signal that is selected, the lower the speed difference, but at the same time the tendency to prolong the swing-in process increases and if the gain is too high, the system can become unstable with constant or prolonged speed fluctuations as a result. The oscillation tendency is stronger at a lower speed than at a higher one. The gain of the control signal must, however, be chosen in the direction that as close a constant speed as possible at different torque loads is to be maintained, so that the oscillation process during regulation becomes short-lived.

At maximum nominal load on the motor, the power regulator delivers the maximum available voltage at point 7. With continued increased load on the motor, the lag also increases as the controller cannot compensate with increased voltage at point 7. At further increased load, the lag can be so large that the rotor field cannot be maintained by induction from the rotating stator field. If this happens, the engine falls Similarly, when torque is high, no rotor field for starting the engine can fall out and stop. if the engine is started-induced. According to the invention, this must thus be solved in such a way that when a certain torque load is exceeded during operation or at start-up, the engine speed is adjusted in such a way that its operation and start-up are ensured. The task of the pulse width detector 9 is that, when the predetermined lag is exceeded, it is activated in such a way that the pulse train 41, shown in Fig. 7, is connected via the pulse width detector 9 to the converter 10, see 42 in Fig. 8. The obtained direct voltage 41 at point 11 controls the oscillator 8 so that a reduction of the motor speed is obtained. The higher the voltage, the smaller the output frequency of the oscillator and consequently the speed of the motor is reduced. With continued load on the motor, this voltage increases further with a continued reduction of the motor speed with operationally adapted lag as a result.

The reduction of the engine speed must take place at the latest at a point where increased torque load can not be compensated by increased drive voltage. However, it may be desirable that this point, when the engine speed is reduced, be located at a lower torque load than the maximum, whereby a more advantageous adaptation to different operating cases can be achieved. The control circuit can then be designed for such control by changing the detection point of the pulse width detector 9 by means of a control voltage from a control means via a line 30. As mentioned, the down regulation of the oscillator frequency and thus the motor speed is activated by detecting in the pulse width detector that a certain maximum pulse width in the signal from line 24 is exceeded. When the pulse width is further increased, the said control signal is generated to the oscillator, which leads to a reduced frequency.

The pulse width detector can then be set and, if a control means is provided, is switched to activate the down-regulation of the speed at the desired pulse width, thus at a predetermined lag; the pulse width is a proportional expression of the lag.

The said control means can be designed for manual control or automatic control depending on the operating conditions, which can then be sensed, for example, by means of some form of sensors. During this control process, the pulse DC converter 25 also receives its control signal via the line 24 for regulating the voltage level from the power regulator 6. The output voltage from the power regulator is then maximized to the maximum voltage for which the motor is arranged.

A control of an asynchronous motor with the aid of the control device according to the invention is particularly advantageous in the case of load changes, where the need for torque development and thus the energy requirement decreases with reduced load and increases with increasing load.

Thanks to an automatic adaptation of the motor's energy requirements to the load at a certain speed, a more economical drive condition is achieved than in the case of conventional frequency converters, where e.g. at lower speeds than the nominal speed of the driven motor, the energy consumption of the inverter and the motor is disproportionately greater than that indicated by the actual energy requirement for a given load. 12 504 746 W Under difficult starting conditions, the control device according to the invention ensures a safe start of the motor and / or a controlled reduction of the motor speed in the event of temporary or permanent overloads or at a predetermined torque less than that specified at maximum power output for the motor.

Claims (5)

10 U 20 25 30 13 504 746 Patent claims A control device for electric motors of the asynchronous motor type driven by a 3-phase alternating current, which creates a rotating magnetic field in the motor, which at a certain speed lag from synchronous speed in the rotor induces a field which provides the field necessary for the rotation of the rotor. of the drive current of the motor (2) is arranged an oscillator (8) and one of the frequency controlled and presumably supplied from (11) current bridge (1) voltage, the control device being arranged to (33), its supply lines for alternating voltage, a direct current source for alternating voltage. receiving the pulse which occurs in the operation of the motor and is thereby arranged to detect changes (32, 34) in the pulse shape of the driving voltage, which arise by electromotive forces induced from the motor windings when they are passed by the induced field of the rotor, which changes in size varies with the size of the lag, that the bridge (1) is fed from a power regulator (6) arranged characteristic v, that by means of changes in said electromotive forces it is regulated to emit a post-trailing load so that changing torque load on the motor (2) and thereby varying voltage to the bridge (1), tendency to changed lag is compensated in a control process by changes in motor power. said varying voltage from the power regulator (6), whereby the engine speed at M 504 746 10 15 20 25 30 operation up to a certain, maximum torque load is kept substantially constant. 2. The control device in claim 1, characterized in that the oscillator (8) is arranged that when a determined, maximum torque load of the motor (2) is achieved, indicated by the changes (32, 34) generated from said electromotive forces. ) in the pulse form of the drive voltage, is controlled to change its frequency to a lower one with increasing torque load above said maximum torque load and increase the frequency when lowering torque load within the range above said maximum torque load, so that the motor is controlled at loads that can not be compensated by increased voltage from the power regulator (6), is controlled to operate at a predetermined normal speed, lower speed at its predetermined, maximum power and enable a starting process even at a starting moment exceeding the above-mentioned maximum resistance torque. Control device according to claim 2, characterized in that it comprises a (14, 17) (32, 34) in the pulse shape of the drive voltage to these counter-devices for converting said changes corresponding to square pulses (37-39) whose width changes in proportion to the changes in the pulse shape of the driving voltage, and that the oscillator (8) is arranged to be controlled by a pulse width detector (9), which is arranged to activate a decrease of a pulse width when exceeding a certain maximum pulse width. the output frequency of the oscillator (8) and thus a decrease in the speed of the motor (2). Control device according to claim 3, characterized in that the pulse width detector (9) is drawn from it, is arranged that by means of an operating device (30) adjustable with respect to the maximum pulse width, at which the pulse width detector activates lowering of the frequency of the oscillator (8). . Control device according to claim 3 or 4, characterized in that voltage regulator (6) to the bridge is arranged to deliver said variable (1) by control by means of a varying control voltage (40), and that a converter (25) ) is arranged to convert said square pulses (37-39) to a direct voltage proportional to the integrated pulse width, which is transmitted to the power regulator (6) as control voltage.