Maintenance circle

 

Word for the day: UPS

 

This article is intended to give a very simple broad overview of UPS since at this point of time we are limiting the newsletter to a two page publication. If more finite details are required, you can contact us at feedback@maintenancecircle.com

 

Uninterruptible Power Supply – UPS for short – has almost become essential equipment not only for providing critical back up power to computers, and servers but also for sophisticated PLC, CNC controlled industrial machines. UPS is also generally called "battery back-up" system & provides "back-up" or "support" power when the main (utility) power supply stops due to various reasons. Today's advance technology makes it possible for providing back-up power to a simple desktop computer as well as a large manufacturing unit, consisting of various machines. Of course, it proportionately increases the size & cost. UPS is NOT designed to provide a very long time or continuous backup. It is designed only to support the load for certain duration which varies from few minutes to one or two hours. If the utility power is not restored within the stipulated time, the UPS will either abruptly or gradually shuts down the system thus switching off loads and backup batteries will be completely drained.

 

Generally, the backup power is obtained from lead-acid batteries. These batteries are connected in parallel & number of batteries depends on the load to be supported during power failure. Fundamentally, the UPS is rated in VA or KVA or MVA. VA stands for Volt-Ampere and denotes apparent power that the unit has to support during a power failure. Backup time is another parameter that has to be considered while selecting UPS. Longer backup time will increase number of batteries & UPS cost will also increase proportionately. In simple terms, for example, if a computer draws 4 amperes at 215 AC volts, its VA rating will be equal to 4 X 215 = 860 VA. An additional safety factor will be added in selecting UPS to avoid tripping due to starting current. So for this application, a 1000VA or 1 KVA UPS can be selected.

 

Apart from these two major parameters, following points can also be considered while selecting UPS for any application:

 

• Location of UPS – Indoor or Outdoor
  
• UPS with sophisticated diagnostic to handle internal faults & to avoid faulty shutdowns
  
• UPS with on-line monitoring software, LAN connectivity & other sophisticated features
  
• The criticality of load – For instance, a banking computer system needs more reliable UPS with advanced features compared to an UPS chosen for a home computer
  
• Custom built features that could be specific to the load
  
• Starting & surge currents taken by the loads
  

The adjacent block diagram shows basic components of a UPS system. Two major variants of this model will be discussed in further paragraphs. From the block diagram, note that whenever utility power is available, it will be fed to the load directly or indirectly. When the utility power fails due to any reason, UPS control system will switch over instantaneously to its back up power. Usually, the backup power is a combination of 12 or 24 or 48 V DC lead acid batteries connected in parallel. The number of batteries depends on load for which UPS is designed & selected. Now let us take a look at two broad classifications into which UPS is divided. They are called "Off-line" or "Standby" & "On-line" or "Line Interactive" types.

 

Block diagram 2 explains the underlying concept of first type. In this type, the utility power is supplied directly to load without the involvement of UPS control system, in normal mode when utility power is present. Whenever this power is not available, UPS control system immediately change over to backup power within few milliseconds (typically 50 to 100 milli-seconds).

 

The changeover is usually done using a solid state electronic switch & is monitored by the UPS control system. If the backup duration exceeds designed time, it can prompt the user to shutdown loads. If utility power is restored within this time, UPS changes over & concurrently, charges the partially or fully drained batteries. The DC voltage of batteries is converted into AC voltage using a solid state electronic device called "Inverter" with related components. This type of system is very cost effective and has simple electronic circuitry. UPS with this type of concept is usually designed for loads up to 1000 VA for domestic or less critical loads.     Apart from computers, this type is widely used in many manufacturing units for protecting drives, control systems as well. One of the major advantages of this type of system is increased life of battery, since UPS charges them only after it has drained to certain minimum level. Inability to improve the "quality" of utility power when supplying to loads is probably one of its major disadvantages. This type of UPS is economical compared to ONLINE system.

 

Block diagram 3 explains the underlying concept of second type. In this type, UPS is fitted in INLINE to utility power. The utility power is ALWAYS converted to DC using a "rectifier" unit. Then an "inverter" converts the rectified DC back to AC voltage. Here, whenever the utility power fails, UPS can switch over to backup power within ten milli-seconds. But since the utility power is continuously rectified & converted, the UPS is bigger in size due to more components & also the electronic circuitry is very complex. These types of systems are used for loads that demand less interruption time & "clean" voltage. One of the major advantages of "on-line" UPS system is its ability to provide stable & clean output. Since utility power is always rectified & converted, all sub-harmonics, frequency variations & voltage variation to some extent are controlled & output will remain stable. Complex electronic circuitry, increased initial costs are few of its major disadvantages.

 

Even though the UPS technology is sophisticated, following common points, when taken care will avoid major breakdowns & reduce production / data / manpower damages.

 

• If the backup batteries are not sealed maintenance free type, check the
  specific gravity of electrolyte weekly
  
• Record charging voltage & charging current regularly in a log book
  
• Disconnect all batteries & measure voltage values individually.
  If the difference is more than 20%, replace or service the suspect battery
  
• Do not overload the UPS system
  
• Based on UPS installation, maintain ambient working environment
  
• Do not let dust, debris accumulate over UPS cubicles & panels
  
• Regularly monitor the back-up time that the UPS is supposed to provide at various load values.
  This will indirectly indicate the load on UPS & battery condition.
  

DO NOT CONNECT HIGH MOTOR (INDUCTIVE) LOADS TO ANY UPS SYSTEM