Liebert 610 Inverter, Output Regulation and Overload Performance, Non-Linear Load Characteristics

Theory of Operation

2.2.4Inverter

The inverter is a solid state device that converts the DC output of the rectifier/charger or the battery to AC power.

Operation

The inverter converts DC power—from either the battery or the rectifier/charger—into three pulse- width-modulated/six-step waveforms. These waveforms are filtered into low-distortion sine wave power. The inverter is controlled by digitally controlled logic. This logic controls the precise synchro- nization, amplitude and frequency of the output voltage based on connected load characteristics.

In addition to the inverter efficiently supplying a regulated AC output from a DC source, the output isolation transformer acts as a second stage of isolation between the critical load bus and the commer- cial source power. The inverter is configured to handle most critical load inrush surges. It maintains output voltage Total Harmonic Distortion (THD) within specifications even when handling non-linear computer loads. Refer to Non-Linear Load Characteristics.

Output Regulation and Overload Performance

The inverter is capable of sustaining full output voltage (±2% of the nominal voltage) for up to 150% overload at the output for as long as 30 seconds without reducing the output voltage. It can also han- dle at least 125% of the rated current for up to 10 minutes and 104% of the rated current continu- ously. If an overload exceeds the system capacity and a bypass source is available, the critical load is transferred to the bypass source and the inverter is disconnected from the load. Refer to 3.5 - Auto- matic Operations.

Non-Linear Load Characteristics

Computers and computer equipment with switch-mode power supplies (SMPS) may generate non-lin- ear currents rich in fifth and seventh harmonics.

The inverter pulse-width-modulated/six-step waveform, coupled with the output filter, provides a nat- ural path for reducing the fifth and seventh harmonic currents produced by the load. The inverter/fil- ter limits the output voltage THD to less than 5% with up to 100% typical electronic data processing (EDP) loads. EDP equipment characteristically includes both non-linear and linear load components.

Unbalanced Load Characteristics

Unbalanced loads are actively regulated. The phase-to-phase voltage balance is maintained to within 2%, even with a 50% load imbalance.

The three-phase, root mean square (RMS) average voltage is also regulated through a separate con- trol circuit using phase-to-phase sensing.

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