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The purpose of this “Cahier Technique” is to explain the specific phenomena observed in LV installations when there is an overload or electrical fault in circuits equipped with variable speed drives. Various recommendations are given to ensure that persons and property are protected, and to provide optimum continuity of service.


The purpose of “frequency converter” type variable speed drives is to supply 3-phase asynchronous motors in such a way as to obtain operating characteristics which are radically different from those obtained in normal use (motors supplied directly from the power supply), with constant amplitude and frequency. The table in figure 1 lists the advantages of these devices.


This consists of supplying the motor with a voltage wave with variable amplitude and frequency, while keeping the voltage/frequency ratio more or less constant. This voltage wave is generated by an electronic power device whose simplified schematic is shown in figure 4.

AUTHOR: Schneider Electric experts | Jacques SCHONEK, Yves NEBON

Title: LV protection devices and variable speed drives (frequency converters)
Format: PDF
Size: 0.5MB
Pages: 34
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LV protection devices and variable speed drives

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