Emerson Liebert NX User Manual download pdf (Page 88)

Operation

78 Liebert

™

9.1.1 Bypass Supplies

The Liebert NX contains an electronically controlled switching circuit that enables the critical load to

be connected to either the inverter output or to a bypass power source via the static bypass line.

During normal system operation the load is connected to the inverter and the inverter contactor is

closed; but in the event of a UPS overload or an inverter failure, the load is automatically transferred

to the static bypass line.

To provide a clean (no-break) load transfer between the inverter output and static bypass line, the

bypass static switch activates, connecting the load to bypass. To achieve this, the inverter output and

bypass supply must be fully synchronized during normal operating conditions. This is achieved

through the inverter control electronics, which make the inverter frequency track that of the static

bypass supply, provided that the bypass remains within an acceptable frequency window.

A manually controlled, maintenance bypass supply is incorporated into the UPS design. It enables the

critical load to be powered from the utility (bypass) supply while the UPS is shut down for routine

maintenance.

9.1.2 Operating Modes

The UPS is designed to operate as an on-line, double-conversion, reverse-transfer system in the

following modes:

Normal Mode

Operating in normal mode, the Liebert NX’s rectifier derives power from a utility AC source and

supplies regulated DC power to the inverter, which regenerates precise AC power to supply the

connected equipment. The rectifier also uses the utility source power to charge the batteries.

Battery Mode

When utility AC power fails, the Liebert NX protects the critical load by instantaneously channeling

battery power to the inverter, which continues supporting the critical load without interruption.

When utility power returns and is within acceptable limits, the Liebert NX automatically shifts back

to Normal mode, with the rectifier powering the critical load.

Bypass Mode

When the Liebert NX is in bypass mode, the load is directly supported by utility power and is without

battery backup protection.

The Liebert NX’s inverter static switch and bypass static switch will shift the load from the inverter

to bypass mode without an interruption in AC power if the inverter is synchronous with the bypass

and any of the following occurs:

• inverter fails

• inverter overload capacity is exceeded

• inverter is manually turned off by the user

NOTE

When the UPS is operating in bypass mode or on maintenance bypass, the connected

equipment is not protected from power failures or surges and sags.

NOTE

If the inverter is asynchronous with the bypass, the static switch will transfer the load from the

inverter to the bypass WITH interruption in AC power to the critical load. This interruption

will be less than 10ms. This interruption time may be altered by modifying the Output transfer

interrupt time setting.

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Emerson Liebert NX User Manual Page 88

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