DE2856574A1 - ARRANGEMENT FOR ADJUSTING THE SPEED OF AN INDUCTION MACHINE - Google Patents

Description

The invention relates to an electronic arrangement with thyristors for adjusting the speed of induction machines with squirrel cage rotors. Such arrangements are already known, namely devices that adjust the speed via the frequency, so-called frequency converters, and devices that adjust the speed via the stator voltage, so-called three-phase controllers. Speed adjustment via frequency is almost lossless; however, the previously known designs of frequency converters are very complex. Speed adjustment via the voltage is lossy and not suitable for continuous operation due to the high slip losses; however, the construction costs for a three-phase current controller are very low.

The arrangement according to the invention connects in a simple manner Wise the advantages of the two mentioned control devices! Low-loss speed adjustment via the frequency with low Construction costs.

This is achieved by modifying the ignition angle specification that is customary for thyristor three-phase current controllers. As is well known, the size of the voltage-time areas on the load and thus the load current can be set via the ignition angle. The relationship between the ignition angle and the rms value of the output voltage or output current of the three-phase current controller is approximately linear over a wide control range. In the previously known three-phase power controller operation, the ignition angle, output voltage and load current are constant in a balanced state; the frequency corresponds _J declares the frequency of the supply Netzest In the inventive arrangement will now be amended to with a suitable control device, the hitherto customary Zündschema at three-phase controllers that negative during a certain number of network oscillations ignition only in the positive half cycles and then only ignition at the Halb vibrations takes place. This can be done, for example, in such a way that a small positive load current flows during a first positive half-oscillation of the mains voltage, a slightly larger load current flows during the second positive half-oscillation, a larger one during the third and the load current with the 4th, 5th and 6th half-oscillation " again to zero

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decreases. From the 6th half-shot, in the same way the thyristors for the negative half-oscillations of the mains voltage are ignited. Overall, one output voltage is obtained or a load current with an envelope whose frequency is lower than the mains frequency and whose curve shape can be influenced by modulating the ignition angle is. Three such arrangements are to be provided for three-phase systems, whereby the ignition angle is guided in this way be that the envelopes of the three load currents form a three-phase system with each 12o ° phase shift. The phase shift of 12o relates to the envelope frequency.

It can be shown that in the present arrangement, the amplitude and frequency of the first harmonic of the output voltage and current are approximately proportional to the amplitude and frequency of the envelope are. As a result, the modulation of the ignition angle allows amplitude and frequency adjust the fundamental oscillation of the output voltage. The main advantage of the arrangement according to the invention is here that the small number of 6 thyristors is required for a three-phase converter system.

The converter system according to the invention belongs in principle to the class of direct converters. With this converter principle, the frequency setting range is limited upwards. The converter is thus well suited for driving machines that have a single high or several low ones Demand speeds. The high speed can be achieved by direct connection to the mains or the usual manual operation and the low speed realized by simply switching in the control device to the control method described will. Depending on the requirements, driving or braking operation is possible. The arrangement is used, for example for elevators, hoists, transport equipment, centrifuges, machine tools and many more.

The arrangement according to the invention and its mode of operation are explained in more detail with reference to figures:

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show it:

1 shows a schematic diagram of the arrangement according to the invention with a control device, power section and connected asynchronous machine.

Fig. 2 shows the timing of control voltage, a Lock signal, mains voltage, ignition pulses and output current for phase R.

The block diagram in FIG. 1 is based on analog control variables U. for the amplitude and U for the frequency of the fundamental oscillation of the converter output voltage. These two variables are fed to a setpoint generator 1. The setpoint generator generates three sinusoidal output signals U _{0, each shifted by .. 1 2o.}

voltage sizes U., U _{p are} adjustable. The setpoint generator signals can be generated with analog or digital circuit elements. Such arrangements are known in the literature. (Kahlen W: Setpoint generator for three-phase inverters based on the undershoot method, International Electronic Review 1970 No. 2)

The three-phase setpoint signal is rectified with three diode circuits 2 _na ". The rectified signals JU ₂ R _s mf represent the input signals for the ignition pulse generators 3 _{σ m} . The ignition pulse generators generate the ignition pulses for the thyristors of the three-phase power controller 6. Usually, ignition pulse generators are designed so that the relationship between input voltage and ignition angle is approximately linear. In three cutting triggers 4_ _em , three blocking signals S are formed from the setpoint _{signals U 00 o _.} These signals are used to detect whether positive or negative half-oscillations are present. The locking signals control the release of the ignition pulses Z _{1R s T} ° of the Z _{2R g} ^ via three electronic switching devices 5_ _{o _.} In the case of positive half oscillations of the setpoint signal, the ignition pulses ^Z -] _R s T ^for

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the thyristors n _{1T5 c φ} released and the Zühdimpulse ^Z 2R ST blocked ^{for the} thyristors n _{2R s T.} The opposite is true for negative half oscillations. This means that only positive load currents in the counting direction shown are possible during positive half oscillations and only negative load currents during negative half oscillations. It follows that the frequency of the load currents corresponds to the frequency of the setpoint signals.

For further explanation, Fig. 2 shows images a, b, c, d, e, f setpoint output _{signal U-R} f, the associated blocking signal S ", curve of the mains voltage U, ignition pulses ^Z 1R ' ^Z 2R ^ ^{for the thyristors n} iR ' ⁿ 2R ^{un <} ä associated load current i "· shown. It can be seen that due to the linearity between control voltage, ignition angle and load current, the envelope curve of the load current is similar to the curve of the control voltage. The fundamental _{oscillation frequency f 2 of} the load current is in the present example 5 Hz with an assumed network frequency of 50 Hz. ■

Ruhstorf, December 15, 1978
ET 2 - dr.schö-gr

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Claims (1)

36 Arrangement for adjusting the speed of an induction machine Loher GmbH, Elektromotorenwerke, 8399 Ruhstorf / Rott Patent claims: Arrangement for speed adjustment of a three-phase induction machine with a three-phase thyristor three-phase current controller, characterized in that the firing pulses of the thyristors are generated in such a way that the output voltages of the three-phase power controller form a three-phase three-phase voltage system, the first of which Harmonic has a lower frequency than the frequency of the feeding network. Arrangement according to Claim 1, characterized in that the envelope curve of the controller output voltage or of the controller output current via the ignition pulses sinusoidal, trapezoidal, sinusoidal, triangular or rectangular. Arrangement according to Claims 1 and 2, characterized in that the frequency of the first harmonic is synchronous or can be specified asynchronously with the mains frequency. 4. Arrangement according to claims 1, 2, 3, characterized in that the speed of the connected machine can be adjusted via the frequency of the first harmonic. 5. Arrangement according to claims 1, 2, 3, 4, characterized in that that the speed of the connected machine is adjusted via the amplitude of the first harmonic can be. 6. Arrangement according to claims 1, 2, 3, 4, 5, characterized in that that the harmonic content of the machine current is low if you have an induction machine with a special rotor for small inrush currents used. 030029/0109 ORIGINAL- INSPECTED