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Chapter 12 Helpful information
Line interactive or VI UPS units
VI stands for Voltage Independent from mains supply. Other names for this UPS types are line-
interactive, single-conversion or delta-conversion UPS units.
A Bidirectional inverter generates from the AC input voltage the DC voltage for charging the
batteries as required and AC voltage from the UPS batteries in case of utility power failure.
The frequency of the input voltage, if it is present, is identical to that of the output voltage. The
switchover time from mains to battery operation is between 2 and 4 ms.
The Line-Interactive UPS is the most common UPS used for small business, Web, and
departmental servers. In this design, the battery-to-ac power converter (inverter) is always
connected to the output of the UPS. Battery charging is provided by operating the inverter in
reverse during times when the input AC power is normal. When the input power fails the
transfer switch opens and the power flow is from battery to the UPS output. The fact that the
inverter is always connected to the output provides additional filtering and yields reduced
switching transients
Line-interactive UPS systems use automatic voltage regulation (AVR) to correct abnormal
voltages without switching to battery. (Regulating voltage by switching to battery drains your
backup power and can cause batteries to wear out prematurely.) The UPS detects when
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VF
6U Rack or Tower UPS RT8kVA and RT11kVA IUOMG
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