1.) Online UPS VFI, (Voltage and Frequency Independent)

For online technology the UPS system is connected in series between the mains supply and the consumer. For normal operation the load is supplied via the chain rectifier-inverter according to the double conversion principle, with a double conversion of the alternating voltage into direct voltage and then back to alternating voltage again. If the mains voltage is outside the tolerance limits specified by the UPS system or if it fails completely, then the consumers continue to be supplied without interruption, and this means without any switching measures, via the battery and the inverter. The duration of operation then depends on the backup time provided by the installed battery.

2.) Line-interactive UPS VI (Voltage Independent)

For line-interactive UPS systems, the converter is connected as a stand-by power source parallel to the mains and simultaneously guarantees that the battery is charged. The converter therefore works in two operating modes together with the mains supply. For normal operation, the load is supplied with alternating voltage from the mains and the converter branch. The converter regulates the output voltage and guarantees that the battery is charged when required. With this system, the output frequency is determined by the mains frequency. If the voltage is outside the tolerance limits specified for the UPS system or if it fails completely, then the load supply is separated from the mains by a static switch and supplied exclusively and without interruption by the battery and the converter branch.

3.) Offline UPS VFD (Voltage and Frequency Dependent)

The converter is connected as the stand-by power supply parallel to the mains and only active when needed. In normal operation the UPS system only provides for the correct charging of the battery. The load is supplied via filters straight from the mains. The converter only works in a state of passive readiness. If the mains supply is not available or the mains voltage is outside the specified tolerance limits, the load supply is assumed by the battery and by the converter after a transfer time defined by the manufacturer. The battery operation will continue until the battery’s backup period has expired.