Thomas & Betts 9395 - Manual (Page 42)

UPS 225 - 275 kVA
User’s and Installation Guide
1027212
Revision B
42
The inverter produces a three-phase AC output to a customer’s load without the use of a
transformer. The inverter derives regulated DC from the rectifier and uses IGBT devices and
pulse-width modulation (PWM) to produce a regulated and filtered AC output. The AC output of
the inverter is delivered to the system output through the output contactor K3.
If the utility AC power is interrupted or is out of specification, the UPS automatically switches to
Battery mode to support the critical load without interruption. When utility power returns, the
UPS returns to Normal mode.
If the UPS becomes overloaded or unavailable, the UPS switches to Bypass mode. The UPS
automatically returns to Normal mode when the overload condition is cleared and system
operation is restored within specified limits.
If the UPS suffers an internal failure, it switches automatically to Bypass mode and remains in
that mode until the failure is corrected and the UPS is back in service.
4.2.2 Bypass mode
The UPS automatically switches to bypass mode if it detects an overload, load fault, or internal
failure. The bypass source supplies the commercial AC power to the load directly. Figure 4-3
shows the path of electrical power through the UPS system when operating in Bypass mode.
CAUTION
The critical load is not protected while the UPS is in Bypass mode.
Figure 4-3. Path of current through the UPS in bypass mode