What is a UPS ?

Updated: Mar 8, 2021

Power outages are very common in our country. They hit us without notice most of the time. Apart from disrupting our daily routines, they can affect us very badly during critical situations. Sudden power outages can also damage sensitive and expensive electronics or appliances. In view of this, investing in a power back-up device has become an critical investment at homes and offices. There are many solutions to consider - fuel generators, emergency lamps, solar power etc... However, since most homes and offices commonly look for UPS, the focus of this post is restricted to this.


UPS (Uninterrupted Power Supply), simply put, is a device which stores electrical energy in a battery and delivers this stored energy when there is a power outage almost instantaneously to the load. This ensures continuous working of the appliances or devices until either the mains power returns or when the battery charge gets drained out.


The below block diagram explains how a typical UP works.( Online UPS is different which we will discuss in a separate post)

When there is mains power available, the power is supplied to the load directly with or without any regulation. A part of the mains power is simultaneously used to re-charge the battery. When a power outage is sensed, DC power stored in the battery is drawn, through a switching mechanism, by the Inverter, converts it into AC power and supplies to the Load.

Simple block diagram explains how UPS works
simple UPS block diagram


Basic parts of a UPS


Broadly we can consider the UPS system as two logical parts. The Main control unit and the Battery


Main control Unit: It is the heart of your UPS subsystem. It is the one which has a Charger ( Rectifier) an Inverter and a Switching circuit among a whole load of other electronic circuitry.


Charger: This converts the input AC power to DC. It is the one which re-charges the battery when mains power is present. A charger capacity is defined by the manufacturer in terms of its DC voltage ( Vdc) which is usually 12Vdc and its multiples. Eg.12Vdc, 24Vdc, 48Vdc 192Vdc etc. and the maximum current in Amps at which it can charge the battery. The number of batteries required and the maximum capacity of batteries which you need to connect with the main control unit is limited by this. It is important to look for this information when you buy a UPS.

Inverter: It is the one which gets into action to convert the DC power from the batteries to AC power and supply to the appliances. Its capacity determines how much load it can support on battery mode. Its output capacity is in watts. Eg. a 1000VA UPS will provide an output of 800 Watts on battery mode. usually 80% of its VA rating. ( due to its Power factor)

Switching circuit: The switching circuit is the one which detects the power changes and accordingly switches over to the battery mode or mains mode. It can take from 3 ms to 40 ms depending upon the technology. Shorter the switchover time, the better it is to ensure continuous running of most devices and appliances.


Battery: This is the most important part of any power back-up system, It is crucial to invest in the right type and capacity depending upon your application. A battery is specified by its nominal DC voltage ( normally 12Vdc batteries are used in UPS application), Capacity in terms of Ampere-Hours (AH) Eg. 100AH, 150AH, 200AH etc.. and construction type ( Flat Plate, Tubular, Tall tubular etc..). There are various types of batteries. The most commonly used batteries for UPS applications are flooded Lead acid batteries and VRLA batteries ( Normally referred to as SMF batteries) Depending upon the UPS type and charger type, the appropriate battery is to be used. For long backup requirements and harsh power conditions flooded lead acid batteries are used ( Flat Plate or Tubular plate batteries). SMF batteries are normally used in short standby back-up applications. It is not recommended for deep discharge application and will not last long.


Types of UPS


There are three types ( Topology) of UPS : Online UPS , Line Interactive UPS and the Standby UPS. The right one needs to be invested based on the need.


Online UPS are used where sophisticated or business critical equipment or devices need to be protected. They are the ultimate in power protection as they isolate the load completely from incoming power disturbances. They normally are available in capacities of 1KVA and upwards.


Line Interactive UPS are designed to provide Automatic Voltage Regulation (AVR) while on mains power and switchover to battery power in the event of a power outage. They are UPS which are suitable for stand alone computers, entry level servers and small networks. Normally these are available in capacity from 600VA - 5KVA.


Stand-by UPS ( commonly referred in the market as Inverter or Home UPS) are primarily designed to support a variety of appliances and devices with more focus on longer power back-up and minimum power protection. They are normally used in homes and offices to provide back-up to lighting, cooling and other common appliances. They come in capacities of 100VA - 100KVA and beyond.


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