SE426003B - DEVICE FOR CONTROL OF THE SPEED AND DIRECTION OF AN ASYNCHRON MOTOR - Google Patents

Description

"7s0o9š¿Äo is exceeded. The speed is determined by means of a frequency sensor independent of the voltage control, which is combined with an ignition signal generator and an extinguishing signal generator. The extinguishing signal generator emits extinguishing pulses with six times the desired frequency. with the inverter switching elements in three branches, whereby the connections at two pairs can be switched between each other by means of a reversing connection.In normal operation, in each branch each time an inverter switching element, eg a controlled rectifier element, is controlled to conductive state e.g. If the control lines are now reversed or switched, without the first-mentioned switching element being switched off, the second switching element in the branch is also brought to a conducting state at the moment, as a result of which a short circuit flows, while the current short-circuit current is admittedly in the first moment. gene is still acceptable due to the short-circuit impedance but rises rapidly to impermissibly high values. Although the short-circuit current occurs, the voltage of the DC voltage source is down-regulated, but the energy stored in the smoothing capacitor and in the short-circuit impedance may already be sufficient to destroy the switching elements. Since the asynchronous motor can not emit any energy, it continues to rotate unbraked, so that the reversal is delayed.

It is further known to relate the frequency of the frequency sensor to the regulated voltage at the output of the direct voltage source, since for optimal operation of the asynchronous motor its voltage and frequency must be in a substantially constant relationship to each other. If the above-mentioned reversing coupling were to be used in such a device, even greater problems would arise. If the regulated voltage is reduced when a short-circuit current occurs, the predetermined frequency decreases at the same time. This means that the time until the next extinguishing occurs is longer, so that the short-circuit current flows even longer.

The invention is based on the object of developing a device of the type described initially, in which the short-circuit problems which occur in connection with the reversal are eliminated or can be neglected, and the switching can be carried out at any arbitrary speed without prior down-regulation of the speed.

According to the invention, this task is solved by coordinating a blocking step with the reversing coupling, which prevents initial reversal until the time of the next switch-off, and by controlling the frequency of the frequency sensor in proportion to the regulated direct voltage.

The blocking step ensures that the reversal process each time coincides with an extinguishing process or takes place immediately before.

The state, when both switching elements in a branch are simultaneously conductive, therefore does not arise or at most within an innocuously short time.

Destruction of the switching elements is therefore excluded. In addition, immediately after the reversing process, the motor is connected to live clamps, which give rise to a rotating field corresponding to the new direction of rotation. Since the motor still rotates in the old direction of rotation, it acts as a generator and is braked quickly accordingly.

The high currents which occur during the reversal process, which lead to a reduction of the controllable direct voltage, also lead to a reduction of the frequency. This enables the start of the motor in the opposite direction, without higher currents having to flow than is permitted due to the maximum current limitation, which as a rule limits the motor current almost to the normal full load current. The risk that short-circuit current flows through the individual branches for too long as a result of the delay of the extinguishing process is excluded due to the use of the blocking step.

Particularly simple coupling is achieved when the frequency sensor emits a clock pulse each time at the switch-off time, which releases the blocking step. In this way, synchronism is achieved between the extinguishing process and the reversing process. In the simplest case, the clock pulse is the same as the extinguishing pulse.

In a particularly suitable embodiment it is ensured that the blocking step emits one of two alternatively occurring switching signals depending on an applied phase change signal but enables signal switching only at the next switch-off and that the reversing switch for each branch intended for switching comprises two electronic switching stages. from the ignition signal generator are logically combinable according to an AND function with each of the two switching signals. Due to the logical combination of the ignition signals with the switching signals and due to the timely output of the switching signals, the reversing process is synchronized with the extinguishing process.

The invention is described in more detail below in exemplary form with reference to the accompanying drawing, the only figure of which shows a simplified wiring diagram of the device according to the invention.

A controllable DC supply 1 is supplied with a constant DC voltage U =, a series regulator 2 comprising a controlled rectifier element 3 acting as a DC chopper, the control signals of which are supplied from a control unit 4 in such a way that direct current pulses of different widths. and / or frequency is transmitted. After this, a smoothing circuit is connected, which comprises a longitudinal damping coil 5 and a transverse capacitor 6, so that as a result a controllable direct voltage U1 is formed. This DC voltage is normally attributed to a setpoint, which is predetermined by means of a setting device 7 at the control unit 4. However, if the current in the system reaches a predetermined limit value, which can be determined by the voltage drop across a measuring resistor 8, the control unit 4 downregulates the voltage U1 so far , that some excessive currents can not occur. A frequency sensor 9 also depends on the voltage U1, which emits clock pulses t at six times the desired frequency for the voltage U1. This frequency sensor 9 can be designed as a conventional voltage-controlled oscillator.

Via a short-circuit impedance 10, which by means of a transformer winding 11 and a rectifier element 12 also serves to recover energy during shut-off, an inverter 15 is connected, which supplies a three-phase asynchronous motor 14. The inverter comprises three branches a, b and c, which in turn each comprises its two series-connected switching elements 15a and 16a, 15b and 16b and 15c and 16c, between which the three phase connections U, V and W are branched. The control electrodes in the switching elements designed as controlled rectifier elements are supplied via high-frequency ignition pulses z via the control line 17.

Additional parts of the inverter such as idle diodes and the like are excluded for simplicity.

An extinguishing stage comprises a capacitor 18, a recharging attenuation coil 19, an extinguishing switch element 20 in the form of a controlled rectifier element and a diode 21. The control electrode in the switching element 20 is supplied via a control line 22 from an extinguishing pulse generator 25 extinguishing pulses 1, which each time is triggered by a clock pulse t, and thus has a frequency which corresponds to six times the desired frequency. The clock pulses t further control an ignition signal generator which can be constructed in an arbitrary manner and can, for example, comprise bistable rocker stages, frequency dividers, rails and so on. At the outputs, ignition signals u, v and w occur, which correspond to the phase voltages desired at the outputs U, V and W. The ignition signal w is transmitted directly, 7soo9s¿ÄóiM while the ignition signals u and v are transmitted via a reversing coupling 50. The ignition signals are then transmitted either directly or via an IGKE stage 32a, 52b resp. 52c to lines 51a to o and šša to c for outputting the ignition pulses to lines 17. They can, for example, be combined with a constantly emitted high frequency signal with a frequency of eg 200 kHz, this high frequency signal therefore constantly appearing on lines 17, when also one input signal dependent on the ignition signals u, v and w is present on lines 51a to c and šša to c.

The reversing coupling 50 comprises for each of two phases two first HAND stages 57, 58 and 59, 40 together with a second NAND stage 41 and 42. Furthermore, a socket stage 45 is included in the form of a bistable D-type rocker stage, whose D input via a switch 44 is optionally connected to earth or to a voltage source 45, so that the D input is applied to a phase change signal p, which has the value 1 for one direction of rotation and the value 0 for the other direction of rotation. With each of these two values is a switching signal S1 at the output Q and a switching signal S2 at the output Q. However, switching to the outputs only takes place when a clock pulse p occurs at the clock input 01. This means that the phase change signal p can be emitted at any time, while the switching signals S1 and S2, on the other hand, only change when the clock pulse t occurs, which simultaneously causes switching off via the switch-off signal generator 25. Depending on these switching signals S1 and S2, either steps 57 and 59 or 58 and 40 are active, so that branch a optionally controlled by the ignition signal u or v and the branch b optionally by the ignition signal v or u, which corresponds to reversal.

The switching signal S1 is assumed to be present. Then the switching works in such a way that the phase voltages appear in the order U, V and W. If the switch 44 is switched, the switching signal is switched to S2 at the next clock pulse t. Nowadays the phase voltages are controlled in the order V, U, W, which gives rise to reversal of the rotating field of the motor 14. Since the switching takes place at the moment of switching off, there is no risk of any short circuit in the branches a, b and c. Immediately after this switching, the motor acts as a generator and is therefore braked. As a result of the occurring currents, the voltage U1 is down-regulated, so that the frequency of the frequency sensor 9 also decreases.

As soon as the engine 14 has stopped, it immediately starts in the opposite direction; whereby the voltage U1 and the frequency again increase to the desired setpoint. 7800956-0 (ß A number of modifications are possible. Thus, for example, the blocking step can also be read out by means of an extinguishing pulse and the HAND steps can also be replaced by AND steps and vice versa, if the coupling is adapted in a corresponding manner. \ .__ _B '

Claims (4)

7 1800956-o Patent claim Device for controlling the speed and direction of rotation of a three-phase synchronous motor, wherein the motor is supplyable from a controllable DC voltage source via a short-circuit impedance and an inverter and the regulated DC voltage is reducible upon reaching a current limit value, with a speed-determining frequency transmitter. which are connected via control lines to inverters switching elements connected in series in at least three branches, and to an extinguishing signal generator also actuated by the sensor for supplying a common extinguishing switch element and a reversing connection for influencing the direction of rotation and switching control lines for inverter switching. characterized in that with the reversing coupling (50) a blocking step (45) is coordinated, which prevents initiated reversal until the time of the next switch-off, and that the frequency of the frequency sensor (9) is controlled proportionally to the regulated DC voltage (U1). Device according to Claim 4, characterized in that the frequency sensor (9) emits a clock pulse (t) each time at the time of switching off, which releases the blocking step (45). Device according to Claim 1 or 2, characterized in that the blocking step (45) emits one of two alternatively occurring switching signals (S4, S2) in dependence on a applied phase change signal (p), but enables signal switching only on the next switch-off. and that the reversing coupling (50) for each gear intended branch (a, b, c) comprises two electronic switching stages (57-40), in which ignition signals (u, v) for each phase of the ignition signal generator (24) are logically compatible according to an AND function with each of the two switching signals (S4, S2). Device according to claim 5, characterized in that the blocking step is a bistable D-type flip-flop (45) with a D-input for supplying the phase change signal (p), a clock input for supplying clock pulses (t) from the frequency sensor (9) and two outputs for outputting the two switching signals (S1, S2). MENTION PUBLICATIONS: