Several uninterruptible power supply (UPS system) technologies are available, all with various advantages and disadvantages. This article will help guide you in making the right choice for your application.

All ups systems consist of a rectifier to convert normal mains AC power to DC power, an inverter to convert DC power to AC power, and an energy storage medium, usually a lead-acid battery.

An offline UPS provides a basic level of power protection. Some form of surge suppression is usually incorporated, and if the input mains voltage is out of tolerance (too high, too low or has failed), the UPS inverter will start up to power the equipment. If it occurs in the region of 10-20,000 seconds, which is typically unnoticed by most equipment, the main supply will fail. Inverters in offline UPS are almost always square waves.

A line-interactive UPS is similar to an offline UPS but has the added benefit of voltage regulation. It does this without going back to the battery, which saves battery life. Because the line-interactive UPS has an in-line transformer, it also provides built-in filtering, allowing a higher level of power protection. As with the offline type, there is also an interruption during the transition from mains to battery power. On higher-quality line-interactive devices, this interruption can be as low as 2 to 4,000ths of a second. Line Interactive UPS systems are available with square wave and sine wave inverters.

Online double-conversion UPS systems are considered by many to offer the highest level of power protection. The inverter is always on and supplies power to the load. This means that when the UPS returns to battery power, there are no deviations in the output voltage and no interruption. The output waveform of an online UPS system is almost always a sine wave and is usually of very high quality and can enhance the operation of certain equipment. On-line double conversion UPSs also have a bypass circuit so that they can continue to supply power to the load even if the UPS fails or becomes overloaded.

Things to consider when choosing a technology include:

  • load’s power consumption
  • the type of luggage you have
  • how important is the load
  • The environment in which the UPS will be installed
  • required runtime
  • your budget

Most UPSs above this level (for specialized equipment or wired installations) become online double conversion technologies as economies of scale begin to make other technologies less cost-effective. Below this level, however, any technology can be used, but above about 2 KVA, line-interactive systems start to become heavy and unwieldy due to the size of the transformers required.

Both square wave and sine wave products will adequately power this equipment if the load consists of a computer-type power supply. If the load contains motors, transformers, pumps, or other inductive components (input power supplies), square wave systems are not suitable and sine wave systems should be selected.

Loads that are susceptible to mains disturbances, such as analytical equipment or audio applications, should choose a sine wave system. If mains distortion affects the performance of the equipment, an online double conversion should be chosen where a pure sine wave is always present.

If the load is critical to your operation, you should take advantage of online double conversion technology. This provides additional security against any power failure as well as the comfort of a fallback bypass in case the UPS fails. You may want to parallelize the UPS again, and this is only possible with an online double-conversion UPS. (I will cover parallel systems and redundancy in a separate article). If you have a PC looking for a simple battery backup to stop annoying reboots or trips, an offline or line-interactive device will suffice.

Off-line and line-interactive UPS systems are normally quiet in operation and (usually) do not use cooling fans in normal operation. This means it is suitable for placement in an office or home environment. Online double-conversion UPS systems tend not to be suitable for use in office environments as they require forced cooling and can be quite noisy.

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