Page 217 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Online UPS converts AC mains into DC before inverting again to AC to power the load with a synthetic sine wave.
A battery connected across the DC link acts as the backup power source.
This gives a supply for the computer that totally isolates the input mains from the load, removing all mains noise
and with no break when the mains fails.
Standby/OFF Line block diagram (Fig 4) : In the off line UPS, the load is connected directly to the mains when
the mains supply is available. When working over voltage/under voltage conditions are detected on the mains, the
off line UPS transfers the load to the inverter. When the line is present, the battery charger charges the battery
and the inverter may either be shut down or will be idling. Thus in an off line UPS, there is a load transfer involved
every time, the mains is interrupted and restored. This transfer is effected by change- over relays or static transfer
switches. In any case there will be a brief period during which the load is not provided with voltage. If the load is
a computer and the transfer time is more than 5ms, then there is a chance that the computer will reboot.
Some modified designs incorporate a limited range of voltage regulation by transformer tapping and a certain
degree of transient protection by using RF filters and MOV’s (Metal Oxide Varistor). Off line UPS is an economical
and simple design and hence it is preferred for small rating, low cost units aimed at individual PC user’s market.
When the load is really a critical one an off line UPS is not acceptable. Usually square wave output off line UPS
are available in market with lower loading capacities.
Advantages of OFF line UPS: High efficiency, small size, low cost.

Disadvantages: There can be change over complaint in offline UPS. Off line very much depends on battery. If
battery fails entire system fails. Sometimes during change- over computer re-boots which causes loss of files.
Another disadvantage is that output voltage will be a varying one. Usually in the range of 200V-240V and hence
not suitable to all electronic gadgets.
Front panel indications and rear panel sockets/switches used in UPS : All UPS systems have

• Fuse/Fuse holder
• Switches
• Sockets
• Panel indicator (LED and Neon lamp)
• Meters (Volt/Ampere)
Fig 5 and 6 shows the front and rear panel controls/sockets.

ON line UPS
In an ON line UPS, the inverter always supplies the load irrespective of whether mains power is available or not.
The load is always left connected to inverter and hence there is no transfer process involved. When the mains
power is present, it is rectified and applied in parallel with the battery. Hence all the supply system transients are
isolated at the battery and the inverter always delivers pure sine wave of constant amplitude to the load.
Fig 7 represents a basic block diagram of an ON Line UPS.
In the block diagram (Fig 7), the mains input is stepped down to a lower level and applied to a thyristor based phase
controlled AC to DC converter, employing firing angle(a) control. The PWM inverter which usually employs pulse
width modulation using triangular/square wave carrier runs in battery mode. The output is filtered and given to
the load. The PWM inverter is switched in the frequency range (50Hz) depending on the power rating and hence
the DC side current drawn by the inverter will contain switching frequency components.
Along with the charging current the second harmonic component of DC side current of the inverter also flows into
the battery. This second harmonic is quite large in value and this represents unnecessary strain on the battery.
This is one of the major disadvantages of this design since it affects the battery life adversely.
When the mains is present the load power flows though the converter, reaches the battery node and from there
flows into the inverter i.e there is double conversion of power. The converter, Inverter and the two level shifting
transformers incur power losses in this process. Hence the efficiency of this design is lower than the Off line design.
In a properly designed control system the battery voltage is measured and compared with a set float voltage. The
error is processed in a proportional controller and the processed error decides the charging current that should
flow into the battery. Charging current will be a constant one for On line UPS.

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CITS : Power - Electrician & Wireman - Lesson 93-100