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Generators can offer facilities and their operations long-term protection in the event of an interruption of service, but they cannot offer protection against many common faults in power systems. Facilities can achieve that level of protection only with an uninterruptible power supply (UPS). While there are several different configurations for UPS, online systems are the most common.
An online UPS has three components: a charger/rectifier, storage batteries, and a power inverter. Incoming alternating current from the utility enters the charger/rectifier, which converts it to direct current. This direct current charges the batteries and supplies power to the inverter, which converts the direct current back to alternating current. In systems that provide power for loads in the event of extended outages, a generator typically is connected to the batteries.
The UPS offers the advantage of supplying power to the loads continuously, no matter what happens to the utility power. But the benefits of a UPS go beyond the ability to continuously supply power. The process of taking alternating current from the utility and converting it to direct current and back to alternating current also eliminates most power disturbances, including noise, transients, and voltage fluctuations.
Managers specifying a UPS to protect facilities and systems need to be certain it is sized properly for the load it is designated to protect. They at least must be sure to size the UPS so it can provide 150-200 percent of the connected load. This spare capacity protects the UPS from additional power loads while the equipment is starting, and it allows room for growth.
Managers also need to properly size the batteries in the UPS to provide the desired runtime in the event of a power loss. For some applications, the UPS only needs to provide power long enough to allow an orderly shutdown of connected equipment.