CHAPTER 1
1. INTRODUCTION
The
term Uninterruptible Power Supplies (UPS) has been applied both to an
uninterruptible power system and to the specific battery-inverter equipment
incorporated into the system. In this chapter, we will use the term UPS for the
specific equipment.
The
UPS was developed in parallel with digital computers and other IT equipment to
provide a reliable uninterruptible electric power source to create a standard
the electric-utility industry could not provide. The computer industry would
not have developed without some type of UPS, which either used engine-generator
sets or static inverters employing power electronic devices. If the concept of
an independent UPS had not been
developed, then all electronic equipment
sensitive to disturbances in source voltage would have had to incorporate
energy storage means—for example, batteries to counteract such disturbances in
the source voltage.
1.1 History
The
silicon-controlled rectifier (SCR) was introduced in 1957. The development of
inverters using SCRs for UPS was expedited by the book, Principles of Inverter
Circuits, by Bedford and Hoft, published in 1964.
An
early publication by Fink, Johnston, and Krings in 1963 described three-phase
static inverters for essential loads up to 800 kVA.
Kusko
and Gilmore described in 1967 a four-module redundant UPS for 500 kVA for the
FAA Air Route Air Traffic Control Centers. The overall MTBF requirement was
100,000 h and the MTTR was 1h. The individual modules were assumed to have an
MTBF of 1100 h. Modern UPS
modules are considerably more reliable.
Fig.1.1
block diagram
The major change in the design of inverters
since their inception for UPSs is the replacement of the silicon controlled
rectifiers (SCRs) as the switch devices by IGBTs. The SCRs require forced
commutation to turnoff the current—that is, to open the switch. The IGBTs are
controlled by the gate voltage. The forced commutation for SCRs required
additional circuits and operations that increased the failure rates and reduced
the reliability of the
inverters.
Furthermore, the IGBTs can be switched much
faster than SCRs, which enabled pulse-width modulation (PWM)—in other words,
the voltage and current waveform shaping to be used.An early concept of UPS
that preceded the SCR is shown in Figure.
It was a rotary machine set consisting of a
diesel engine, clutch, electric motor, flywheel, and generator. When utility
power was available, the motor drove the generator. When utility power failed,
the engine would be started while the flywheel and the generator slowed.
At a certain speed, the clutch was engaged so
the engine could drive the generator to restore and maintain its synchronous
speed. A Motor-Generator,
which is a UPS, but employs batteries to provide
energy when utility power fails.
1.2 Common Power
Problems
There are various common
power problems that UPS
units are used to correct:
1.3 Types of UPS
·
Offline / standby
·
Line-interactive
·
Double-conversion / online
·
Hybrid Topology / Double Conversion
on Demand
·
Ferro-resonant
1.4 CIRCUIT DIAGRAM
Fig: - 1
FIG. 1.2
-“BASIC CIRCUIT DIAGRAM OF MINI UPS SYSTEM”
1.5 LIST OF COMPONENT
Components
|
No. of Components
|
Ratings
|
Transformer
|
1
|
230v AC
Primary to 12-0-12v, 1A Secondary Transformer
|
Transistors
|
2
|
T1 BC548
T2 TIP127
|
ICs
|
2
|
IC 7809
IC 7805
|
Diodes
|
4
|
1N4007
|
Zener
Diodes
|
2
|
ZD1 12.5v,
0.5w
ZD2 12v, 1w
|
Resistors
|
6
|
R1 68Ω, 0.5w
R2=R3 1K
R4 47Ω, 1w
R5=R6 390Ω
|
Capacitors
|
1
|
470µF, 25v
|
Variacs
|
2
|
VR1 10K
VR2 22K
|
Battery
|
1
|
12v 4.5 AH
|
LEDs
|
3
|
1 Red, 2 White
|
Switch
|
1
|
On/Off Switch
|
1.6 Circuit Operation
This circuit provides an uninterrupted power supply
(UPS) to operate 12V, 9V and 5V DC-powered instruments at up to 1A current. The
backup battery
takes up the load without spikes or delay when the mains power
gets interrupted. It can also be used as a workbench power supply that provides
12V, 9V and 5V operating voltages. The circuit immediately disconnects the load
when the battery voltage reduces to 10.5V to prevent deep discharge of the
battery. LED1 indication is provided to show the full charge voltage level of
the battery. Miniature white LEDs (LED2 and LED3) are used as emergency lamps
during power failure at night.
FIG: 1.3 -: “PCB CIRCUIT
DIAGRAM”
1.7 Circuit Explanation
A standard step-down transformer
provides 12V of AC, which is rectified by diodes D1 and D2. Capacitor C1
provides ripple-free DC to charge the battery and to the remaining circuit.
When the mains power is on, diode D3 gets forward biased to charge the battery.
Resistor R1 limits the charging current. Potentiometer VR1 (10k) with
transistor T1 acts as the voltage comparator to indicate the voltage level. VR1
is so adjusted that LED1 is in the ‘off’ mode. When the battery is fully
charged, LED1 glows indicating a full voltage level of 12V.
When the mains power fails, diode D3
gets reverse biased and D4 gets forward biased so that the battery can
automatically take up the load without any delay. When the battery voltage or
input voltage falls below 10.5V, a cut-off circuit is used to prevent deep
discharging of the battery. Resistor R3, zener diode ZD1 (10.5V) and transistor
T2 form the cut-off circuit. When the voltage level is above 10.5V, transistor
T2 conducts and its base becomes negative (as set by R3, VR2 and ZD1). But when
the voltage reduces below 10.5V, the zener diode stops conduction and the base
voltage of transistor T2 becomes positive. It goes into the ‘cut-off’ mode and
prevents the current in the output stage. Preset VR2 (22k) adjusts the voltage
below 0.6V to make T2 work if the voltage is above 10.5V.
When power from the mains is
available, all output voltages—12V, 9V and 5V—are ready to run the load. On the
other hand, when the mains power is down, output voltages can run the load only
when the battery is fully charged (as indicated by LED1). For the partially
charged battery, only 9V and 5V are available. Also, no output is available
when the voltage goes below 10.5V. If battery voltage varies between 10.5V and
13V output at terminal A may also vary between 0.5V and 12V, when the UPS
system is in battery mode.
Outputs at points B and C provide 9V
and 5V, respectively, through regulator ICs (IC1 and IC2), while output A
provides 12V through the zener diode. The emergency lamp uses two ultra-bright
white LEDs (LED2 and LED3) with current limiting resistors R5 and R6. The lamp
can be manually switched ‘on’ and ‘off’ by S1.
The circuit is assembled on a
general- purpose PCB. There is adequate space between the components to avoid
overlapping. heat sinks for transistor T2 and regulator ICs (7809 and 7805) to
dissipate heat are used.
The positive and negative rails should be strong
enough to handle high current. Before
connecting the circuit to the battery and transformer, connect it
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