Generally, the Variable Frequency Drives (VFDs) are equipped with over-voltage, under-voltage, and momentary power-off protection functions. When a VFD experiences undervoltage or a power outage, the inverter element, which uses IGBT technology, will allow the VFD to continue operating for a predefined period (td). If the undervoltage or power interruption lasts for less than td (to < td), the VFD

will smoothly recover and continue running. However, if the duration exceeds td (to > td), the VFD will activate its protection mechanism and stop operation. Therefore, significant AC power fluctuations or prolonged undervoltage conditions can cause the VFD to cease operation. Currently, most common VFDs are AC-DC-AC voltage-source inverters, where the intermediate DC circuit is generated through three-phase AC rectification, resulting in a voltage of approximately 510-620V. Therefore, supplying a VFD with a DC power source in the range of 510-620V will enable the it to operate continuously. Based on this principle, we have developed the

LD/DC-BANK, an uninterrupted power supply system that provides DC support for VFDs.