Offline frequency UPS

These key points might influence the decision in using an offline system:

• Output waveform is not closely regulated under normal operations

• 2 to 10 ms. load break when transferring the load between inverter and bypass

• Lower capital costs than an online system because of the lower rated components

• Lower running cost as the charger and inverter are not permanently on load

Offline UPS Summary

Offline uninterruptible power supply systems are so-called because the load is fed directly from the raw mains during normal operation, rather than from the inverter output. To that extent, the energy storage components – charger, battery and inverter – are offline as far as the load is concerned, although the charger and battery remain connected to the mains to ensure the battery is always fully charged.

As low voltage is the most common cause of load transfer, an offline UPS system works by immediately connecting the inverter to the critical load if the bypass voltage falls below or exceeds preset limits. During this changeover there is an inevitable break in power to the load of typically 2 to 10 ms. In practice, however, most loads can ride through this period without any problems.

Most offline models include a degree of spike suppression and radio frequency interference filtering in their bypass circuits to ensure that any mains disturbances during acceptable bypass voltage ranges are smoothed through, however this risk is not 100% eliminated.

The advantages of offline UPS systems are that they are cheaper to purchase, and without the charger and inverter being permanently on load, they are more economical and energy efficient.

As the inverter runs from battery power once the load is transferred, the inverter can only run until the battery voltage falls to its end-of-discharge level unless another bypass supply is restored.

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