working of half wave, full wave, pushpull single pahse inverter and 3 phase inverter in 180 mode operation

1. DC-AC INVERTERS
HALF BRIDGE AND FULL
BRIDGE
MODULE-3

2.  A single-phase square wave type voltage source inverter produces square shaped output voltage for a single-
phase load.
 Such inverters have very simple control logic and the power switches need to operate at much lower
frequencies compared to switches in some other types of inverters.
 The first generation inverters, using thyristor switches, were almost invariably square wave inverters because
thyristor switches could be switched on and off only a few hundred times in a second.
 In contrast, the present day switches like IGBTs are much faster and used at switching frequencies of several
kilohertz.
 Single-phase inverters mostly use
1. Half bridge or
2. Full bridge topologies.
3. Push pull
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3.  The input dc voltage (Edc) is constant and the switches are lossless.
 In half bridge topology the input dc voltage is split in two equal parts through an ideal and loss-
less capacitive potential divider.
 The half bridge topology consists of one leg (one pole) of switches whereas the full bridge
topology has two such legs.
 Each leg of the inverter consists of two series connected electronic switches.
 Each of these switches consists of an IGBT type controlled switch.
 These switches are capable of conducting bi-directional current but they need to block only one
polarity of voltage.
 The junction point of the switches in each leg of the inverter serves as one output point for the
load.
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4.  In half bridge topology the single-phase load is
connected between the mid-point of the input dc
supply and the junction point of the two switches
 VO acquires a magnitude of +0.5 Vdc when T1 is on
and the magnitude reverses to -0.5 Vdc when T2 is
turned on.
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6. Mode 1 (0 to T/2):-
 During this mode switch T1 is ON and switch T2 is
OFF From period 0 to T/2.
 Current flowing path during this mode is
 V/2-T1-B-R(Load resistor)-A-V/2.
 Hence the voltage across the load is positive V/2
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8.  Mode 2 (T/2 to T):-
 During this mode switch T1 is OFF and switch T2 is ON From
period T/2 to T.
 Current flowing path during this mode is V/2-A-R(Load
resistor)-B-T2-V/2.
 Hence the voltage across the load is negative V/2
 The main drawback of half bridge inverter is that two DC
voltage source are required.
 By using full bridge inverter we can overcome that drawback
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11.  Switch S1 is on for the time duration 0 ≤ t ≤ T/2
 switch S2 is on for the time duration T/2 ≤ t ≤ T.
 When switch S1 is turned on, the instantaneous voltage across the load is
ν o = Vdc/ 2
When the switch S2 is only turned on, the voltage across the load is
ν o = ̶ Vdc/ 2.
 The switches should neither be simultaneously on nor be simultaneously off.
Simultaneous turn-on of both the switches will amount to short circuit across the
dc bus and will cause the switch currents to rise rapidly
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14.  Mode 1 (0 to T/2):-
 During this mode switch S1 and switch S2 are ON
and switch S3 and switch S4 are OFF From
period 0 to T/2.
 Current flowing path during this mode is Vdc – S1-
P -R(load reistor) – Q – S2 – Vdc.
 Voltage across the load resistor is positive Vdc
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16.  Mode 2 (T/2 to T):-
 During this mode switch S3 and switch S4 are ON and switch
S1 and switch S2 are OFF From period T/2 to T.
 Current flowing path during this mode is Vdc – S3 – Q –
R(load reistor) – P – S4 – Vdc.
 Voltage across the load resistor is negative Vdc.
 Load is resistive hence it does not store any charge. therefore,
feedback diode D1, D2, D3 and D4 are not effective here.
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20.  Computers require continuous power supply.
 Even a small temporary failure in power supply to these appliances may
cause a great deal of public inconvenience.
 The examples such appliances which require continuity in power supply are;
safety monitors, computers, communication systems, hospital intensive care
units etc.
 Early UPS systems are of rotary type which consists of combination of a DC
motor driven alternator run with the help of battery and a diesel engine.
 However such a system is in efficient, uneconomical and bulky one
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21.  The static UPS systems are presently common
and are widely used.
 These don’t have any moving parts and are made
with solid state power electronic switches.
 These are of two types; namely
1. Short break
2. No break UPS systems.
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22.  The short break UPS will have small interruption time of about 4 to 5
ms.
 The block diagram of such a device is shown in below figure.
 Under normal conditions the load is directly connected to the supply
line.
 On the instance of power failure the UPS switches to the battery with the
help of static switch.
 A momentary interruption in supply can be observed in case of lamps
and florescent tubes.
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24.  Main AC supply is rectified and it is delivered to
batteries to maintain charge in them.
 Rectifier also supplies power to inverter
continuously which is then given to AC load
through filter and normally on switch.
 In case of main supply failure, batteries take over
with no break in supply to the critical load.
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26.  Advantages
1. The inverter conditions the supply delivered
tom the load
2. Load get protected from the transients
3. The inverter output frequency can be
maintained at desired value.
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27.  In case of inverter failure, the load is switched on
to the main ac supply by turning on the normally off
switch.
 It takes 4 to 5 ms interruption in the supply.
 After inverter fault is cleared, ups is again restored
to the load through normally on switch.
 The batteries will be fully recharged by this time.
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28.  The batteries used in ups systems are of either nickel-
cadmium(NC) or lead-acid type.
 The advantages of using these batteries are
1. Non corrosive
2. Does not emit any explosive gases while charging
3. They are not damaged by overcharging, or
discharging
4. Longer life
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29.  When three single-phase inverters are connected in parallel a three
phase inverter is formed.
 The gating signal has to be displaced by 1200 with respect to each
other so as achieve three phase balanced voltages.
 A 3-phase output can be achieved from a configuration of six
transistors and six diodes.
 Two type of control signal can be applied to transistors, they are
1. 1800 Conduction mode
2. 1200 conductionmode
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31.  When S1 is switched on, terminal a is connected to the positive terminal
of dc input voltage.
 When S4 is switched on terminal a is brought to negative terminal of the
dc source.
 There are 6 modes of operation in a cycle and the duration of each mode
is 600.
 The conduction sequence of transistors is 123,234,345,456,561,612.
 The gating signals are shifted from each other by 600 to get 3-𝜑
balanced voltages
 In 1800 conduction mode each transistor conducts for 180 degree.
 There should be a phase difference of 1200 between the switching of
each phase in order to get balanced output voltage.
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