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

FeaturesOFFLINELine-InteractiveON Line
Size of UPSCompactModerateBig
Circuit SimplicitySimpleSimpleComplicated
Transfer Time4 to 10 millisecond2 to 4 millisecond0
Power ConsumptionLessLessHigh
Battery Charging TimeMoreMoreLess
Battery LifeLessLessMore
Backup TimeShortShortMore
Surge Protection
Voltage RegulationLowBetterBest
Load ProtectionLowBetterBest
SizeUp to 2KVAUp to 5KVA5 to 500KVA
Isolation from MainsNot AvailableNot AvailableAvailable
Noise ReductionGoodGoodBest
Frequency StabilityNot StableNot StableAlways Stable
Voltage ConditioningLowBetterBest
Inverter always OperatingYesYesYes
ApplicationFor Domestic DesktopsIT Racks ,Switches ,Distributed ServerData Center, Hospital, Banks
CapacityUp to 800VA800VA to 1500VA1000VA 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

CapacitySize Standby UPSLine-interactive UPSOn Line UPS
5 kW95 %96  %91 %
100 kW 98 %97 %98 %
500 kW99%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


DescriptionEffectSolved by UPS
Temporary InterruptionAccidental total loss of utility power  (Seconds to minutes)Equipment shutdown, loss of  data, file , hard disk and operating system CorruptionOff-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 CorruptionOff-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, fileOff-line – Maybe

Line-interactive – Maybe

On-line – Yes

Sag or Under-VoltageA 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-VoltageAn increase in Utility

Voltage ( Milliseconds to a

few seconds)

Permanent equipment damage, Computer and network equipment reboots or hangs, loss of dataOff-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.

NoiseAn 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 DistortionAn 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