Page 156 - Uninterruptible Power Supplies

Static Uninterruptible Power Supplies
154 Chapter Five

CONTACTOR
K50

ISOLATOR
Q51
U2

V2

W2

N

Figure 5.12

frequency of the inverter. Thus, the mains and inverter output are in
synchronism.
If mains frequency is out of tolerance, in other words, outside 3 per-
cent, then the inverter frequency is controlled by its natural frequency,
thus the static switch is inoperable.
The static switch transfers load from inverter to mains under the fol-
lowing conditions:

1. Surge loads causing an inverter overload will induce static switch
operation, on departure of the surge the static switch will transfer
load to inverter operation.
2. Failure of the inverter will immediately induce static switch operation.
3. Manual operation of the switch may occur, for example, during main-
tenance operations.
4. Operates at end of battery backup time

Various figures are quoted for the operational performance of
the static switch. Transfer can be assumed to occur when voltage is
out of tolerance by 10 percent and transfer time is in the order of
500 s.
The brief descriptions of the UPS subassemblies above are applied to
a variety of circuits all of which are identified as UPS systems. Block

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