Vector control, also known as field-oriented control (FOC), is a control concept in which sinusoidal – or assumed to be largely sinusoidal – alternating variables (e.g. alternating voltages and alternating currents) are not regulated directly in their temporal instantaneous value, but in an instantaneous value adjusted for the phase angle within the period. For this purpose, the recorded alternating variables are transferred to a coordinate system rotating with the frequency of the alternating variables. Within the rotating coordinate system, the alternating variables then result in equal quantities to which all common methods of control engineering can be applied.
For practical reasons, when controlling electrical quantities for the rotating coordinate system, one with two perpendicular axes almost always chosen. This has the advantage that it is identical to the space vector of alternating voltages and currents and their relation to each other, which means that the corresponding models of the electrical machines can be used directly.
Through this vector control, which is also called field-oriented control in reference to the rotational field of an electric machine, a frequency converter for electric motors achieves an extended speed and positioning accuracy compared to a control that only uses RMS values of currents and voltages of one or more period(s) filtered by a low-pass.
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However, the application of vector control is not limited to drives, the principle can also be used for power converters for feeding into grids. Frequency and phase can either be specified by the grid, in which case they are measured, as is the case with drive control, or the power converter itself specifies these variables, e.g. in an off-grid grid.
The vector control reaches the limit of its applicability when the quantities to be controlled and measured are no longer sufficiently sinusoidal. In this case, the transfer to the rotating coordinate system can only eliminate the influence of the fundamental oscillation, the harmonics remain behind and cannot be distinguished by the regulator from the application of disturbances. For practical applications in drive technology, however, filtering the harmonics is usually sufficient. However, this must be taken into account for converter applications that do not act on or are not supported by mechanical drives, but in turn work directly on other power converters (e.g. battery converters in stand-alone mode supplies 230 V~ compact fluorescent lamps).
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