Selection of Various Types of UPS (Part-2)

(2) Line-Inter active UPS:

  • Working Principle of Line Interactive UPS is same as OFF Line/ stand UPS. It connected directly from mains, switching to battery (via the inverter) in mains Power cut condition.
  • The designing of line interactive UPS is same as OFF Line UPS in addition the design Line Interactive generally includes an automatic voltage regulator (AVR) or a tap-changing transformer. This enhances the regulation of voltage by regulating transformer taps as the input voltage differs.
  • The main difference between an off-line and a line-interactive UPS is that a line-interactive UPS in the stand-by mode has active voltage regulation.
  • Voltage regulation is a significant feature when the conditions of a low voltage exist, otherwise the UPS would transfer to battery and then finally to the load. The usage of more common battery can cause early battery failure.
  • It typically uses either a Ferro resonant transformer or a buck-boost transformer. Both helps to reduce the frequency of transfers to battery, slightly improving efficiency and reducing battery wear.
  • Ferro resonant designs also offer power conditioning and tight voltage regulation, as well as an energy store that can maintain uninterrupted power supply output while the inverter switches on.

Circuit Diagram:


Working Function:

  •  Normal Condition:
  • In Normal Power Condition, power supply will continuously provide to Load with some filtering and voltage regulation circuit.
  • During normal operation, the Line Interactive UPS takes utility power and passes it through a transformer with various tap selections on the output. When utility power is high, the Line Interactive UPS selects a tap to lower (buck) the output voltage. Similarly, when the utility voltage is low, the UPS selects a tap to increase (boost) the output voltage.
  • In Normal Condition Battery is charged continuous charge through Battery Charger
  • Battery charger convert AC power to DC Power and this DC Power charged Battery.
  • Power outage Condition:
  • When utility power fails, the device will start its internal inverter Circuit by Mechanical Switch.
  • Mechanically transfer Switch Transfer from utility power to Battery Power, inverter output.
  • This transfer can take as 2 to 4 ms.


  • small Size
  • Low cost
  • High Efficiency (because less power conversion is when AC input is present).
  • Sine Way Output.
  • Battery life is good compared to OFF Line UPS.
  • Voltage regulation is fair (more than OFF Line UPS but Less than ON Line UPS)
  • EMI/RFI/Noise Rejection is good.
  • Change over Time is 2 to 4 Milliseconds.
  • Lower electricity consumption (less costly to operate).
  • Higher reliability (Lower component count and lower operating temperatures).


  • No isolation between main supply and load
  • Higher Heat Output
  • More Expensive
  • Problematic with power factor corrected loads.


  • For small business.
  • IT Racks, Network Switches, Medical Instrument System where data loss is a serious problem.
  • The line interactive UPS may not be the appropriate choice for installations where AC power is unstable or highly distorted, because battery power will be used too often to keep the UPS output within specifications.


  • UPS in the range of 500VA to 5kVA power.

 (3) ON Line UPS/ Double Conversion UPS

  • It is truly uninterrupted power system (UPS) provide continuous power to load in any condition.
  • Online UPS sometimes called “double conversion” UPS. 
  • Today most users with highly-critical loads are choose online UPS .It is used to protect sensitive equipment and data from mains problems at all times with any extra cost.
  • This UPS have no power transfer switches and therefore no transfer time is existed under the mains power failure. Thus this is truly an uninterrupted system.
  • In Online UPS to maintain the charge of the battery, a battery charging unit is continuously powered from the AC mains.
  • Online UPSs are often called ‘double conversion’ types because incoming power is Firstly converted once AC to DC for the battery and then back Secondly Converter DC to AC before reaching the load which is therefore well-insulated from the mains like an electrical firewall between the incoming power and sensitive electronic equipments. It also control of the output voltage and frequency regardless of the input voltage and frequency.
  • The online UPS continuously filters power through the battery before sending it to your computer.
  • By contrast, online UPS systems draw power through the power conditioning and charging components during normal operation, so the load always receives conditioned power rather than raw mains.

 Circuit Diagram:


Working Function:

  • The designing of this UPS is similar to the Standby UPS, excluding that the primary power source is the inverter instead of the AC main.
  • In this UPS design, any cutoff of input AC Supply does not cause triggering of the transfer switch, because the input AC Supply is charging the backup battery source which delivers power to the o/p inverter. So, during failure of input AC Supply, this UPS operation results in no transfer time.
  • The Transfer switch will automatically transfer the load to mains in case of overload or UPS failure.
  • Normal Condition or Power outage Condition:
  • In Normal Power Condition, power supply will continuously feed from the Inverter, providing conditioned, stabilized sinusoidal voltage.
  • Input Power is filter and regularized by RFI Filter circuit then it is feed to Battery charger which is convert AC Power to DC Power. This DC Power is charged Battery continuously.
  • Battery DC power is converted to AC power by Inverter Circuit.


  • The cost is high compare to other type of UPS.
  • It provides isolation between main supply and load.
  • The output is pure Sign wave.
  • 100% Power Conditioning
  • Constant voltage output.
  • Correction of Input Power Factor
  • Zero transfer time
  • The output voltage is free from distortion due to inverter is always ON.
  • It offers the best power protection, covering any and all types of mains disturbances of supply such as blackout, brownouts, spikes etc are absent in the output.
  • Voltage regulation is better
  • Transfer time is practically zero since inverter is always ON.
  • High Reliability, Units can be connected in parallel redundant configuration.
  • This is the best choice, considering such issues as modularity, ability to work from generator, power factor correction, maintenance, hot swapping, fault clearing, supervising, and communicating.


  • More Expensive
  • Lower Efficiency (Due to inverter is always ON).
  • Higher Heat Output
  • Higher battery TCO
  • Higher operating cost (Supplies power is charge Battery Charger and Inverter both).
  • The wattage of the rectifier is increased since it has to supply power to inverter as well as charge battery


  • It the preferred choice for most business applications.
  • Induction motor drives and similar other motor control applications.
  • Medical equipments and Intensive care units.
  • Electronics manufacturers.
  • Data and call centers.
  • TV stations
  • Production-based manufacturers.


  • From 1 KVA up to 5 MVA.

 Comparison of all types of UPS: 

Comparison of all types of UPS

Features OFFLINE Line-Interactive ON Line
Size of UPS Compact Moderate Big
Cost Cheap Cheaper expensive
Circuit Simplicity Simple Simple Complicated
Transfer Time 4 to 10 millisecond 2 to 4 millisecond 0
Efficiency High Moderate Low
Power Consumption Less Less High
Battery Charging Time More More Less
Battery Life Less Less More
Backup Time Short Short More
Surge Protection
Voltage Regulation Low Better Best
Load Protection Low Better Best
Size Up to 2KVA Up to 5KVA 5 to 500KVA
Reliability Low Better Best
Isolation from Mains Not Available Not Available Available
Noise Reduction Good Good Best
Frequency Stability Not Stable Not Stable Always Stable
Voltage Conditioning Low Better Best
Cost/KVA Low Medium High
Inverter always Operating Yes Yes Yes
Application For Domestic Desktops IT Racks ,Switches ,Distributed Server Data Center, Hospital, Banks
Capacity Up to 800VA 800VA to 1500VA 1000VA to 5000VA

Selection of UPS:

 (1) Size of the UPS (VA & Watts)

  • To decide Power Capacity of the required UPS, we should decide which should be protected and its power consumption in Amps, VA, or Watt.

(2) Back-up time

  • Battery Backup time is the time that batteries are able to back-up operation and feed the load upon failure of utility power. Load consumption and size of UPS batteries decide the back-up time.

(3) Type of the UPS you need

  • UPS’s are divided to three main classes.
  • The Off Line (Stand-by) UPS is the simplest and the least expensive.
  • The Line Interactive type, which overcomes the major disadvantages of  the off-line unit  
  • The On-Line UPS, which provides the best power protection.

(4) Cost:

  • For applications where low cost is critical and it does not matter if backup times are short, an OFF Line UPS is proper solution. However it will not provide adequate protection against spikes or sags from the grid.
  • For applications that require complete isolation from any changes in grid power, such as many medical applications then On Line UPS is the best solution.
  • For applications where power losses due to inefficiencies are less of a concern and eliminating the delay from grid power available to back power is paramount, online UPS is the only solution.
  • For typical applications where conditioning Power is required and very short transfer times from grid to backup power are acceptable and daily energy consumption is a concern, Line Interactive is the preferred solution

 (5) Non-Essential or Critical Load

  • For small office where PC loads is less and small network data protection is required, a small single-phase UPS is often an adequate solution.
  • Most single-phase UPSs use off-line or line-interactive topologies.
  • If the equipment to be protected is critical, an online UPS is the best choice.
  • For loads above 10kVA, the most practical solution is a three-phase UPS, which is most normally with true-online topology. Three-phase online UPSs offer the advantage of providing centralized protection using a single UPS.

 (6) Efficiency:

  • Efficiency is mainly affected on UPS design or operating mode.
  • standby and line-interactive UPSs are more energy efficient than ON Line UPSs because there is no power conversion from AC to DC and then back to AC
  • Efficiency is a factor of UPS size. Larger UPS modules typically have higher energy efficiency than smaller ones, because the support power required for control electronics and auxiliary components becomes a smaller portion of the total capacity of the UPS system.
  • For example, a 500 kW UPS module of a given design would typically be more efficient than a 5 kW UPS module of the same design.

Efficiency of UPS

Capacity Size Standby UPS Line-interactive UPS On Line UPS
5 kW 95 % 96  % 91 %
100 kW  98 % 97 % 98 %
500 kW 99% 98 % 99%

(7) Form Factor:

  • Form factor refers to the the outer-shape of the unit. The Form Factor refers outer shape of UPS.
  • Tower: This is smaller and a stand-alone unit, and It is primarily designed for simple home/office setups.
  • Rack mounted: is larger, designed for a standard rack shelf, and is primarily used for more complex commercial operations.

(8) Noise

  • UPS fan noise may or may not be an issue as per your requirement.
  • Smaller UPS does not normally require a fan for cooling, but larger ones often will.
  • If r work requires perfect silence, make sure your UPS is fan-free.

Resolve Power Quality problem by Type of UPS:

Power Quality Problems & solution by UPS

Power Quality


Description Effect Solved by UPS
Temporary Interruption Accidental total loss of utility power  (Seconds to minutes) Equipment shutdown, loss of  data, file , hard disk and operating system Corruption Off-line – Yes

Line-interactive – Yes

On-line – Yes



Accidental total loss of utility power (minutes to Hour) Equipment shutdown, loss of  data, file , hard disk and operating system Corruption Off-line – No

Line-interactive –No

On-line – Yes



Very short planned or

Accidental power loss.

(Milliseconds to seconds)

Computer and network equipment reboots or hangs, loss of work and data, file Off-line – Maybe

Line-interactive – Maybe

On-line – Yes

Sag or Under-Voltage A decrease in utility

voltage Sags  (Milliseconds to a few seconds)

Shrinking display screens,

Computer hangs or reset,

equipment power supply

damage, loss of data, file

Off-line – No

Line-interactive – Yes

On-line – Yes

Swell or Over-Voltage An increase in Utility

Voltage ( Milliseconds to a

few seconds)

Permanent equipment damage, Computer and network equipment reboots or hangs, loss of data Off-line – No

Line-interactive – Yes

On-line – Yes

Transient, Impulse or


A sudden change in

voltage up to several

hundreds to thousands of

volts (Microseconds)

Network Errors, Burned or

damaged equipment, computer and network

equipment reboots or hangs,

loss of work and data, file

Off-line – Yes

Line-interactive – Yes

On-line – Yes, Higher level of protection.

Noise An unwanted electrical

signal of high frequency

from other equipment

Slow LAN, audible noise in

telephone and audio equipment.


Off-line – No

Line-interactive – No

On-line – Yes

Harmonic Distortion An alteration of the pure

sine wave, due to nonlinear


Causes motors, transformers

and wiring to overheat, lowers operating efficiency

Off-line – No

Line-interactive – No

On-line – Yes


Selection of Various Types of UPS (Part-1)


  • Whenever there is a power cut, electricity supply to Computer, Desktop or other critical appliances is cut off and they stop working. However, if we have a backup supply device such as UPS, we can ensure uninterrupted supply of power to appliances to be not bothered with power cuts.
  • Electrical power supply comes from utility companies is not pure it has different Electrical abnormalities like surges, under voltage, Over Voltage, Voltage dips, voltage spikes, Noise and harmonics. These Electrical abnormalities can cause serious damage to Electronics equipments, Data Systems, Computer or Desktop.
  • To decrease the risk of power supply distortion, UPS systems are frequently integrated in electrical networks. Electronic power supply equipment makers can offer consistent, high-quality power flow for various Electrical / Electronic load gear likes continuous industrial processing applications, medical services, emergency gear, telecommunications, & computerized data systems.
  • Today’s UPS systems usually provide some level of power conditioning and protection against fluctuations in voltage from the grid.


  • UPS means uninterrupted power supply.
  • Uninterruptible power supply (UPS) provides uninterrupted power to the equipment. It means switching time from power cut to battery power is very less hence important equipment like computer, desktop is not switch off and we can lose data.
  • A UPS is a complete system that is consisting of many parts that include batteries, a charge controller, circuitry any transfer switch for switching between the mains and back-up battery, and an inverter. An inverter is needed because the battery can only store DC power and we need to convert that back to AC in order to match the appliances connected in the main power line.
  • UPS= Battery charger + Inverter
  • UPS is nothing but inverter with inbuilt battery charger.
  • UPS is used only to backup your system. If we connect desktop computer on inverter. Inverter takes some seconds to give battery power to equipment hence equipment shutdowns for some second in  any power loss condition and we can lose important data of desktop or computer.
  • Inverter is not suitable for computer backup due to the delay in switching.
  • One of more useful functions of UPS is to provide surge protection so connected devices can be protected from line Surge and does not damage. UPS is also capable of conditioning the power from the lines to provide clean and stable power throughout.

Block Diagram of UPS:

  • The block diagram of this UPS is shown as below


  • The mains power comes to the UPS. The AC is converted to DC and this DC is constantly charging the battery. The output of the battery is fed to the Sine wave inverter and it converts DC to AC and this feeds the equipment. Since power out is always drawn from the battery, there is no time lag when mains switches off, it just stops the battery from being charged and the UPS continues to supply power till the battery runs out.
  • Battery Charger (Rectifier ): To convert AC Power (from Power Grid) to DC Power to charge Battery
  • Battery: To provide DC Power.
  • Inverter: To convert DC Power (from Battery) to AC Power (to power load i.e., electrical and electronic equipment.)
  • Controller: To control functions of Rectifier (Charger) and Inverter. (i.e., when to start or stop charging battery, when to start or stop power from battery to load, how fast to change from Grid Power to Battery Power and so on)

 Type of UPS:

  • The UPS is mainly categorized into three types according to their functions. They are as
  1. Offline Standby (where system or data loss is an inconvenience)
  2. Line-interactive (system or data loss is a serious problem).
  3. Online/Double Conversion (system or data loss is unacceptable).

 (1) OFF Line UPS / Standby UPS:

  • Off-line UPS systems are so-called “OFF Line” because load is normally connected directly to the incoming AC mains. When the incoming AC mains fails or fall below a pre-determined level, then the offline UPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery.
  • For switching purpose UPS consists mechanically / Static switches which immediately connect the load on its DC-AC inverter output under the mains power failure condition. During this changeover there is an inevitable break in power to the load of typically 2 to 10 milliseconds. In practice, however, most loads can ride through this period without any problems.
  • The switching process causes a momentary lapse in power which is dangerous for certain highly-sensitive equipment. This is why technically, the standby UPS is not considered a “true UPS”, as it is not truly “uninterruptible”.
  • The typical lapse time 5ms,is well within tolerance for normal desktop computers

 Circuit Diagram:


 Working Function:

  •  Normal Condition:
  • In Normal Power Condition, power supply will continuously provide to Load with some filtering (typically the same as on a surge protection power strip) from the utility.
  • In Normal Condition Battery is charged continuous charge through Battery Charger
  • Battery charger convert AC power to DC Power and this DC Power charged Battery.
  • Power outage Condition:
  • When utility power fails, the device will start its internal inverter.
  • When utility power fails mechanically transfer Switch Transfer from utility power to Battery Power, inverter output.
  • This transfer can take as 25 ms, which may be too long for some Electronics loads.
  • An Offline UPS will transfer to battery backup during Power cut condition.
  • Off-line systems are generally equipped with spike suppressors, aimed to protect the hardware from high voltages on the utility grid.


  • Cheaper than other type of UPS.
  • Small size.
  • High efficiency.
  • More economical
  • More energy efficient.
  • Lower operating temperature (Due to Fewer parts)
  • Simple Internal control Circuit.
  • Lower initial cost (fewer parts) and lower operating cost (Supplies power is charge only Charger).


  • No isolation between main supply and load
  • No Power Conditioning.
  • Slower Transfer Time.
  • Harmonic distortion is high.
  • UPS output is Quasi square wave.
  • The output contains voltage spikes, brownouts, blackouts.
  • Output is not perfectly reliable.
  • A more serious problem of offline systems is that the load is continuously exposed to spikes, transients and any other abnormalities coming from the power line. This creates a risk of loss or damage to sensitive equipment and data. However in many systems this risk is mitigated but does not eliminated, by spike suppression and radio frequency filtering
  • Most of the short term spikes surges and high frequency harmonics are decayed by means of special filters but transient of mains power existence, like over voltages can harm protected data.


  • small offices, personal home computers and other less critical application
  • Computers, printers, scanners etc.
  • Emergency power supplies, EPABX.


  • Up to 800VA.
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