Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1998-06-30
1999-08-10
Salata, Jonathan
Electricity: motive power systems
Induction motor systems
Primary circuit control
36447435, H02P 700
Patent
active
059363770
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for correcting the flux direction of a model flux vector down to zero frequency.
BACKGROUND INFORMATION
In order to regulate or control a rotating-field machine, it is advantageous to control the stator current as a function of the respective direction of the magnetic flux passing through the machine. As a result, the component of the machine current that is parallel to the flux is used to set the desired field strength, and the component of the machine current at right angles (i.e. perpendicular) to the flux is used to set the torque or the speed. Such field-oriented operation of the rotating-field machine thus requires a control element for the stator current and a control device which is connected upstream of the stator current control element. The control device receives information about the direction of the flux. This information may be supplied from a machine model which, as an electrical model of the rotating-field machine, uses operating data to determine the flux direction of a simulated modelled flux. If in each case one nominal (i.e. command) value for the field-forming (field-producing) component (parallel to the flux) and for the torque-forming (field-producing) component (perpendicular to the flux) of the machine stator current is now predetermined (field-oriented command values), then the control device can regulate or control the machine current to a stator-oriented stator current vector composed of the two command values with respect to the flux direction of the modelled flux.
In order to execute the control algorithms for dynamically high-quality, (i.e., high performance) field-oriented control concepts with an asynchronous machine running at low speeds, a mechanical sensor is required in order to detect the rotor position or the rotor speed. Asynchronous machines can be operated dynamically and with high quality, without any position sensors or tachogenerators, when the speed is higher than a certain minimum value, so that, from the induced voltage, the magnetic flux can be calculated from electrical variables. Until now, it has not been possible to do this in the low speed range. The aim of many research activities has therefore been to replace the mechanical sensor by mathematical models and/or by using physical effects.
In a field-oriented asynchronous machine without any sensors, the "voltage model" (which uses the machine current and stator voltage of the asynchronous machine as state variables to calculate the magnitude and phase of the magnetic flux in this machine) is used as a machine model. The voltage model calculates the flux as an integral of the electromotive force, that is, as an integral of the voltage less the resistive and inductive voltage drops in the stator. However, the voltage model gives an inaccurate signal at low speeds: errors in the estimated stator resistance, DC voltage components which occur as measurement errors in the voltage measurements, and integration errors in the technical integrators lead to incorrect calculations. Consequently, field-oriented operation that uses a voltage model is only possible at relatively high frequencies. In addition, for the electromotive force integrators, the integration constant must be set by predetermining an initial value.
In some cases, the axis of the electromotive force vector may be used as a direction vector instead of the flux direction, this axis being rotated over 90.degree. electric from the field axis in the steady state. Although this allows the integration to be avoided, accurately controlled operation is not possible in this case at low frequencies. Instead of this, another option for detecting the direction vector is required.
A "current model" simulates the processes in the rotor which lead to the formation of the flux from instantaneous values of the current and rotor position. However, to do this a mechanical sensor is required to determine the rotor position. This complexity is admittedly reduced by an incremental rotat
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Blaschke Felix
Vandenput Andre Jean Adolf
Salata Jonathan
Siemens Aktiengesellschaft
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