Uninterruptible power supplies

Before we learn about UPS system first let we check out what is full form of UPS. There are several applications of UPS where even a temporary power failure can cause a great deal of public inconvenience leading to large economic losses. UPS can be used in electrical equipment where unexpected power disruption could cause injuries, serious business, or data losses, and also some many more advantages of the UPS system. Earlier UPS systems were based on an arrangement shown in the figure. 

Working of rotating type UPS system :
  • This scheme is usually called rotating types UPS. This arrangement consists of DC motor driven alternator, the shaft of which is also coupled to a diesel engine. 
  • The UPS system needed is taken from the alternator output terminals. When the main supply fails, the diesel engine is run to take over the load. 
  • The battery bank is able to maintain the alternator speed through the dc motor and the flywheel, thus giving a no break supply to the critical load.
  • Static UPS system is two types, short break UPS and no break UPS. 
Short  break UPS :
  • In a situation where short interruption in supply can be tolerated, the short break UPS showed in the figure. In this system, the main ac supply is rectified to dc.
  • This dc output from the rectifier charges the batteries and is also converted to ac by an inverter. After passing through the filter, AC can be delivered to load in case normally off contacts are closed.
 No break UPS  :
  • When no break UPS supply is required, the static UPS system shown in the figure. In this supply is rectified and the delivered power to maintain the required charge on the batteries. 
When users no break UPS system has the following advantages :
  • The inverter can be used to condition the supply delivered to load
  • Load gets protected from transients in the main ac supply
  • The inverter output frequency can be maintained at the desired value In case of inverter failure is detected, the load is switched on the main ac supply directly by turning on the normally off static switch and opening the normally on a static switch.
The standby batteries in the UPS system are either nickel-cadmium or lead-acid types. NC battery has the following advantages :
  • Their electrolyte is non-corrosive. 
  • Their electrolyte does not emit an explosive gas when charging
  • NC batteries cannot be damaged by overcharging or discharging, these have therefore longer life
  • Cost of  NC  batteries is like, however, two or three times that of lead acid batteries.

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