Download Modified Seried Inverter

4.1 INTRODUCTION
The category of converters, which converts dc power into ac
power popularly known as the inverters.
• The application areas for the invertors includes the
Uninterrupted Power Supply the ac motor speed
controllers,etc.
•
• The inverters can be classified based on number of factors like:
1. the nature of output waveform (sine, square, quasi square, PWM etc),
2. the power devices being used (thyristor, transistor, MOSFETs, IGBTs),
3. the configuration being used (series, parallel, half bridge, full bridge).
• The size and the cost of the circuit can be reduced to some extent if the
operating frequency is increased but then the inverter grade thyristors
which are special thyristors manufactured to operate at a higher
frequency must be used, which are costly.
4.2 Basic Series Inverters
(Self Commutated Inverter)
The series inverter uses a class A type commutation.
The commutating components L1, C1 are connected in
series to form an under damped tuned circuit.
•
Since the SCRs turn off themselves this circuit is
known as commutated inverters .
•
Fig. (a )Basic series configuration
Operation :
• At instant t0 SCR1 is turned on. Let the initial voltage
capacitor be “vc” with its left plate negative w .r. t. right
plate and the sinusoidal load current starts flowing.
Fig (b) Mode 1 (t0 to t1)
• The
capacitor C1 start charging in the opposite
direction as shown in fig B.The load current eventually
comes to zero at instant t1 and SCR1 comes out of
conduction due to natural commutation.
•The voltage on the capacitor C1 at instant t1 is greater
than V with its left plate positive w.r.t. its right plate.
• As there is no discharge path for the capacitor, this
voltage will be held constant up to instant t2 where
SCR2 is triggered.
Fig ( c ) Mode 2 (t1 to t2)
• At instant t2, SCR2 is turned on and the load voltage and current
both becomes negative.
• The capacitor now discharges resonantly through SCR2, R, L1, as
shown in fig (c)
• At instant t3 the discharge current goes to zero and SCR2 turned
off again due to natural commutation. The voltage on C1 is equal
to vc.
• Off time :During the time interval between t1 and t2 both the
SCRs are in the off state. Load voltage as well as load current are
zero. Therefore this interval is known as off time of the circuit.
Disadvantages:
• Limitation on the maximum operating
frequency
• Distortion in the output wave form
• High rating of commutating components
• The peak amplitude and duration of output
current depends on the load parameters
resulting in poor regulation for the inverter.
• The power flow from the dc source is
intermittent. Therefore, the dc supply must
have a large peak current rating and the
input current contains high percentage of
harmonics.
Modified Series Inverter
The operation can be divided into two modes.
Mode 1:
At the instant when SCR T2 is triggered, the voltage across the
capacitor will be slightly less than (E c + E dc)and the load voltage
and current will be closed to zero. Hence the voltage across the
capacitor minus the load voltage will appear across L2.Since L1 is
closely coupled to L2, the same voltage will appear across L1.
Mode 2:
The voltage across L1 will tend to increase the cathode potential
of SCR T1 more than its anode potential and therefore, SCR T1
will be reverse biased and turn-off. Thus, even if SCR T2 is
turned on before SCR T1 is switched off, it will not result into
short circuiting of the d.c. source. A similar operation will take
place if SCR T1 is triggered before SCR T2 is turned off.
Circuit diagram for Modified series inverter
Waveforms For improved Series Inverter
Circuit Diagram for Basic Parallel Inverter
Basic Parallel Inverter
 A parallel inverter is used to produce a squarewave from a d.c. supply.
 In this inverter, the commutating capacitor comes
in parallel with the load during the operation of the
inverter. Hence it is called as ‘parallel inverter’.
Operation
Mode 1:
 This mode begins when T1 is fired and current
flows through the inductance L and the thyristor
T1.
 When SCR is turned on, a d.c. voltage E dc appears
across half the transformer primary, which means
the total primary voltage is 2 E dc, hence the
capacitor is charged to 2 E dc.
Mode 2:
• This mode begins when thyristor T2 is fired. When T2
is turned on, the commutating capacitor applies
voltage -2 Edc to appear across T1, it will be turn off.
• SCR T2 will now be conducting and the voltage of 2 E
dc will appear across the transformer primary and
commutating capacitor, but with reverse polarity.
Mode 3:
•During mode 3, this SCR is again turned on.
Commutating capacitor applies a voltage -2 E dc to
appear across T2.
•when this reverse voltage is applied for sufficient time
across T2, it will be turned off. If trigger pulses are
applied periodically to alternate thyristors, an
approximately rectangular voltage waveform will be
obtained at transformer output terminals.
Waveforms For Basic Parallel Inverter
Circuit Diagram Parallel Inverter With
Feedback Diodes
Parallel Inverter With Feedback Diodes
The circuit operation can be divided into different
operating modes.
 Thyristor T1 and T2 are the main load carrying
thyristors.
Inductor L and capacitor C are the commutating
components. Diodes D1 and D2 are the feedback
diodes. Which permit the load reactive power to be
fed back to the d.c. supply.

Mode 1 :• During this mode, thyristor T1 is triggered at instant R.
Battery voltage forces the current to the primary
section through path Edc -C-A-T1-L- E dc . Terminal C is
positive with respect to A.
• The flux produced due to this current induces the
voltage in all sections of transformer winding. The
load voltage is nearly equal to Ede and is in such
direction so as to force current into the dot at
terminal P. Due to autotransformer action, voltage
Edc is induced in CE section of primary winding.
Mode 2:

This mode begins with thyristor T2 switched on at instant S.
When T2 is turned on, capacitor C will immediately apply a
reverse voltage of 2 E dc across SCR T1 and turned off. When
SCR T1 is turned off, the capacitor will discharge through SCR
T2, inductor L, diode D1, and a portion of a transformer
primary winding BA.

Thus, the energy stored in the capacitor will be fed back to
the load through the transformer coupling of windings BA
and PQ.

The load current which earlier flowing through SCR T1, will
now flow through CB and diode D1 to negative input
terminal. This can happen only if diode D1 is forward biased
and capacitor discharge current is more than the load
current.
• The current through inductance L will now flow through diode D2, DE and
SCR T2, and the trapped energy in inductor L will be fed back to the load.
D point is now connected to the negative supply terminal, the load
voltage polarity will be reversed.
• Also capacitor C will be charged in the opposite direction slightly more
than supply voltage. Thyristor T2 will stop conducting. Energy is
transferred from the capacitor and inductor to the load.
Mode 3 :
• This mode begins with when load current becomes zero, diode D2 will be
blocked and SCR T2 will have to triggered again at instant U to reverse the
direction of the load current. When thyristor T2 starts conducting, the load
voltage will again become equal to Edc.
Waveforms For parallel Inverter with
Feedback Diode
Single Full Bridge Inverter
Introduction –

A serious drawback of the half bridge
inverter is that, it requires a 3-wire dc
supply. This is overcomed by the
commonly full bridge inverter.
Full-bridge Single-phase Inverter
Construction:
•
It has consist of four thyristor and four
freewheeling diode.
• Two thyristor T1 and T2 must be gated
simultaneously at frequency F=1/T and
thyristor T3and T4 must be gated 180
out of phase frequency can be
controlled by varying the periodic time
T.
Operation• When we apply positive load voltage Edc then
thyristor T1 and T2 conduct.
• When we apply negative voltage –edc then
thyristor T3and T4 is conduct.
• Diode D1to D4 serve to feed the load reactive
power back to the dc supply.
• In place of SCRT1, hear two thyristor T1 and T2
conduct similarly in place of SCR2 thyristor T3 and
T4 conduct and in place of D1 and diodeD1, D2
conduct, where as instead of D2, hear D3and D4
conduct
• The load voltage wave form is fairly rectangular
and is not affected by the nature of the load.
Waveform-
• Advantage :
• No need of an output transformer.
• Efficiency is high.
• The current rating of power device is equal
to the load current.
• Disadvantage :
• Number of four transistors are required
• Costs is high
• Application :
• Used in commutation circuit for bridge
inverter
McMurray Bedford Half–bridge
Inverter
Construction :It is a complementary impulse
commutated inverter.
 This means that if two inductors are
tightly coupled, triggering of one thyristor
, turns off another thyristor .
 Main thyristors T1,T2 .
 Feedback diodes D1,D2.
 two capacitors C1,C2 .
 magnetically coupled inductors L1 and L2.
 inductance L .

Operation
Mode- 1
 Thyristor t1 is triggered, then SCR T1 is turn on, upper
d.c. source load current Il to the load.
 As the load current is constant. Voltage drop across
L1 is negligible. With zero voltage drop across L1 , T1 ,
C1 and across C2 is Edc load.
Mode -2
 When SCR T2 is triggered ,turn off the SCR T1
 voltage across C1 and C2 cannot be change
 equal voltages is induced across L1
 Voltage across thyristor T1 is ET1 = Edc
 Ic1 = Ic2 KCL at node ‘o’ in fig.
Ic + Ic = I + I ;
I = I =Ic
Mode -3

At instant t1 , where capacitor C1 is charged to supply
voltage Edc , i.e. Ic 1 =0 at t 1, Vc 2 =0. Just after t 1, current
( Il + Im/2) through C1 tends to charge it with bottom plate .

At t1 ,the energy stored in inductor L2 is dissipated.

This energy is dissipated at instant t 2, therefore I2
decays to zero & a result SCR T2 is turned off at T2.
Mode -4

When the current IT2 through L2 & T2 has decayed
to zero . the load current IL=ID2 still continues flowing
through the diode D2 as ID2 during (t3-t2 ) interval.
Mode -5
• As soon as iL equal to ID2.tend to reverse , diode
D2 is blocked.
• Thyristor T2 already gated during the interval
(t3-t2)gets turn ON to carry the load current in
the reverse direction .
• The capacitor C1 , now charged to the source
voltage Edc is ready for commutating the main
thyristor T2 .
Waveforms
McMurray Bedford Full-bridge
Inverter
Operation :Mc Murray Bedford full bridge inverter circuit can be
realized by connecting two half bridge inverters.
 for Mode 1, thyristors T1 and T2 are conducting and
load current flows through Edc, T1, L1 load Zl , L2,
and T2.
 Voltage across C1, C2 is zero but capacitors C3, C4
are charged to voltage Edc.
 For initiating communication of T1, T2 thyristors T3,
T4 are triggered.
 This reverse-biases T1, T2 by voltage (-Edc) and
makes them turned-off.

4.3 PULSE –WIDTH MODULATION
Pulse-width modulation technique is a
control within the inverter & is also
known as a variable-duty-cycle
regulation.

 This method of regulation employs
variation of the conduction time per
cycle to alter the rms output voltage of
the inverter. In order to accomplish this
regulation technique.
fig. of pulse –width modulation
0peration:In fig,
SCR1&SCR2 - Two main loads carrying SCRs
SCR3, SCR4 - Two auxiliary SCRs which are of
smaller rating
C1 & C2 - Two separate commutating
capacitors.
When,
SCR1 - ON
Power is delivered to the load at the same time, C1 is
charged to the voltage of the transformer section AB
with a polarity as shown above fig,
SCR1 - OFF
At any desired instant by triggering SCR3
After interval, SCR2 – ON
To deliver power in the negative half-cycle.
C2 charged at the same time by the voltage of transformation
section CD.
SCR2 - OFF by firing SCR4
In this method produces a quasi-square-wave output
as below in fig,
Quasi-square-wave output of an inverter
Single pulse width modulation
There is one pulse per half-cycle, and its width is varied
• The modulation index is:
Ar
M
Ac
• The rms output voltage is:
Vo  Vs


• Advantages:
• Less effect of noise
• synchronization between the transmitter & receiver
is not essential.
.
Disadvantages:
• In order to avoid any wave form distortion, the
bandwidth required for the PWM communications large as
compare to BW of PAM
• Average power transmitted can be as low as 50% of
maximum power
Application:
• voltage regulators.
• class D audio amplifiers ,which are highly effectively.
• The following fig, shows the harmonic reduction profile with
variation of the modulation index M
• The domain harmonic is the third and DF decreases significantly
at a low output voltage.
Multiple pulse modulation
• The harmonic contents can be reduced by using several pulses in
each half cycle of output voltage. This type of modulation is also
known as uniform-pulse – width-modulation
• The number of pulses per half cycle is:
mf
fc
p

2 fo
2
Here, mf = modulation frequency ratio
• The rms output voltage is:
Vo  Vs
p

• The following waveform shows the harmonic reduction against
variation of the modulation index & P=5
Sinusoidal pulse modulation
• Instead of maintaining the width of all pulses the same, the width
of each pulse is varied in proportion to amplitude of a sine wave
• This kind of modulation is known as SPWM.
• The rms output voltage is:
 m 1/ 2
Vo  Vs ( )
m 1 
p
• The DF and LOH are reduced significantly, as shown below:
INTRODUCTION
•
•
•
•
•
Chopper is a static device.
A variable dc voltage is obtained from a constant dc
voltage source.
Also known as dc-to-dc converter.
Widely used for motor control.
Also used in regenerative braking.
Thyristor converter offers greater efficiency,
faster
response, lower maintenance, smaller size and smooth
control
Definition of Chopper
 A chopper is an electronic switch that is used to interrupt
one signal under the control of another.
 In other words we can say that, a chopper is a kind of
switch which allows the power flow in the circuit for a
required duration.
 Thus, the chopper is also known as a d.c. to d.c. converter.
Types of Choppers
 Step-down choppers:
In step down chopper output voltage is
less than input voltage.
 Step-up choppers:
In step up chopper output voltage is more
than input voltage.
Step-up chopper
Definition :
The chopper can be use to produced higher
voltages at the load than the input voltage (i.e., E0 ≥
Edc ). This is called as Step-up chopper
Working
 Case 1 : When the chopper is on the current IL flows
through inductor and it stores the energy during on period.
 Case 2 : When the chopper is on the inductor force to
permit the flow of current through the diode and the load.
Therefore the load voltage becomes E0 = Edc + EL .
E0 = Edc + L. d/dt .IL
 When the inductor current decreases to zero, the polarity
across inductor gets reversed and the process is repeated.
Step-Down chopper
Definition :
When the output voltage is less than the input dc voltage (E0 < Edc )
then it is called as Step-down copper.
i.e., E0 < Edc
Working :
Case 1 : During the ON period of chopper the input
voltage Edc connected to the load and the inductor
stores the energy across it.
Case2 : During the OFF period of chopper the
inductor current and load current because of
freewheeling diode get short circuited through the
load and inductor. Therefore the load voltage is zero
during the off period.
The output voltage E0 is expressed as,
E0 = Edc × (TON / TON +TOFF )
...(1)
Hence,
E0 = Edc (TON / T )
...(2)
Here,
TON +TOFF = T = Chopping Period
TON = ON period of chopper
TOFF = OFF period of chopper
α = TON / T
The ratio of TON / T is called as duty cycle and represented as ‘α’.
E0 = Edc . α
Where,
the chopping frequency control the output voltage.
E0 = Edc . TON .F
Where,
F = 1/T = Chopping frequency
Chopper Configuration
Chopper Configuration
•Figure shows the quadrant based chopper
classification according to the nature of voltage and
current .
•The dc chopper circuit is the combination of all
these quadrants , in which the dc motor has to be
operated as a load.
• In quadrant second and fourth, the direction of
energy flow is reversed and the motor is used as a
generator rather than drive (motor).
• In quadrant first and third , the motor is used as a
drive in clockwise and anticlockwise direction
respectively.
Classification Of Choppers
Choppers are classified as:
 Class A Chopper
 Class B Chopper
 Class C Chopper
 Class D Chopper
 Class E Chopper
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class A Chopper
i0
+
v0
Chopper
V
FWD
L
O v
A 0 V
D

Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
i0
Working
 When chopper is ON, supply voltage V is connected across
the load.
 When chopper is OFF, vO = 0 and the load current
continues to flow in the same direction through the FWD.
 The average values of output voltage and current are
always positive.
 Class A Chopper is a first quadrant chopper .
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
 Class A Chopper is a step-down chopper in which
power always flows form source to load.
 It is used to control the speed of dc motor.
 The output current equations obtained in step down
chopper with R-L load can be used to study the
performance of Class A Chopper.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Waveforms
ig
Thyristor
gate pulse
t
i0
Output current
CH ON
t
FWD Conducts
v0
Output voltage
tON
t
T
Class B Chopper
D
i0
v0
+
R
L v0
V
Chopper
E
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

i0
 When chopper is ON, E drives a current through L
and R in a direction opposite to that shown in figure.
 During the ON period of the chopper, the inductance L
stores energy.
 When Chopper is OFF, diode D conducts, and part of
the energy stored in inductor L is returned to the
supply.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
 Average output voltage is positive.
 Average output current is negative.
 Therefore Class B Chopper operates in second
quadrant.
 In this chopper, power flows from load to source.
 Class B Chopper is used for regenerative braking of dc
motor.
 Class B Chopper is a step-up chopper.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Waveforms
ig
Thyristor
gate pulse
t
i0
tOFF
tON
T
Output current
Imax
Imin
v0
t
D
conducts Chopper
conducts
Output voltage
t
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class C Chopper
CH1
D1
i0
+
v0
R
V
CH2
D2
L v0
Chopper
E
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
i0

 Class C Chopper is a combination of Class A and
Class B Choppers.
 For first quadrant operation, CH1 is ON or D2
conducts.
 For second quadrant operation, CH2 is ON or D1
conducts.
 When CH1 is ON, the load current is positive.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
 The output voltage is equal to ‘V’ & the load receives
power from the source.
 When CH1 is turned OFF, energy stored in
inductance L forces current to flow through the diode
D2 and the output voltage is zero.
 Current continues to flow in positive direction.
 When CH2 is triggered, the voltage E forces current
to flow in opposite direction through L and CH2 .
 The output voltage is zero.
 On turning OFF CH2 , the energy stored in the
inductance drives current through diode D1 and the
supply.
 Output voltage is V, the input current becomes
negative and power flows from load to source.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
 Average output voltage is positive
 Average output current can take both positive and
negative values.
 Choppers CH1 & CH2 should not be turned ON
simultaneously as it would result in short circuiting
the supply.
 Class C Chopper can be used both for dc motor
control and regenerative braking of dc motor.
 Class C Chopper can be used as a step-up or stepdown chopper.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Waveforms
ig1
Gate pulse
of CH1
t
ig2
Gate pulse
of CH2
t
i0
Output current
t
D1
CH1
ON
D2
CH2
ON
D1
CH1
ON
V0
D2
CH2
ON
Output voltage
t
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class D Chopper
v0
CH1
D2
R i0
L
V
+
v0
D1
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
E

CH2
i0
 Class D is a two quadrant chopper.
 When both CH1 and CH2 are triggered simultaneously,
the output voltage vO = V and output current flows
through the load.
 When CH1 and CH2 are turned OFF, the load current
continues to flow in the same direction through load,
D1 and D2 , due to the energy stored in the inductor L.
 Output voltage vO = - V .
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
 Average load voltage is positive if chopper ON
time is more than the OFF time
 Average output voltage becomes negative if tON <
tOFF
 Hence the direction of load current is always
positive but load voltage can be positive or
negative.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
WAVEFORMS
ig1
Gate pulse
of CH1
t
ig2
Gate pulse
of CH2
t
i0
Output current
v0
CH1,CH2
ON
t
D1,D2 Conducting
Output voltage
V
Average v0
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
t
Class E Chopper
CH1
i0
V
+
CH2
CH3
D1
R
L
v0
D2
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
D3
E

CH4
D4
Four Quadrant
Operation
v
0
CH2 - D4 Conducts
D1 - D4 Conducts
CH1 - CH4 ON
CH4 - D2 Conducts
i0
CH3 - CH2 ON
CH2 - D4 Conducts
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
D2 - D3 Conducts
CH4 - D2 Conducts
 Class E is a four quadrant chopper
 When CH1 and CH4 are triggered, output current
iO flows in positive direction through CH1 and CH4,
and with output voltage vO = V.
 This gives the first quadrant operation.
 When both CH1 and CH4 are OFF, the energy
stored in the inductor L drives iO through D2 and
D3 in the same direction, but output voltage vO =
-V.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
 Therefore the chopper operates in the fourth
quadrant.
 When CH2 and CH3 are triggered, the load current
iO flows in opposite direction & output voltage vO = V.
 Since both iO and vO are negative, the chopper
operates in third quadrant.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• When both CH2 and CH3 are OFF, the load
current iO continues to flow in the same
direction D1 and D4 and the output voltage
vO = V.
• Therefore the chopper operates in second
quadrant as vO is positive but iO is negative.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Jones Chopper
Working :
o
Figure shows the basic power circuit of Jones chopper.
This chopper circuit is an example of Class D
commutation. In this circuit, SCR T1 is the main thyristor ,
whereas SCR T2 , capacitor C , D2 , and autotransformer
(T) forms the commutating circuit for the main thyristor
T1 .
o Therefore , the special features of this circuit is the tapped
autotransformer T through a portion of which the load
current flows. Here , L1 and L2 are closely coupled so that
the capacitor always gets sufficient to turn off the main
SCR T1.
 If the main thyristor T1 is on for a long period , then
the motor will reach the maximum steady-state speed
determined by the battery voltage , the motor and the
mechanical load characteristics. If thyristor T1 is off ,
the motor will not rotate. Now , if thyristor T1 is
alternatively on and off in a cyclic manner , the motor
will rotate at some speed between maximum and zero.
 Let us assume that initially capacitor C is charged to a
voltage Edc with the polarity shown in figure . As
shown in fig. , SCR T1 is triggered at time t = t1 ,
current flows through the path CA – T1 – L2 – CB and
capacitor C charges to opposite polarity , i.e. Plate B
positive and plate A negative.
• However , diode D1 represents further oscillation of the
resonating L2 C circuit. Hence capacitor C retains its charge
until SCR T2 is triggered. The capacitor voltage waveform are
drawn at bottom plate B of capacitor.
• Now , at time t = t3 , SCR T2 is triggered. Current flow
through the path CB – T2 – T1 – CA . Therefore, discharge of
capacitor C reverse biases SCR T1 and turns it off. The
capacitor again charges up with plate A positive and SCR T2
turns off because the current through it falls below the
holding current value when capacitor C is recharged.
The cycle repeats when SCR T1 is again triggered. The use of
autotransformer insures that whenever current is delivered
from dc source to the load , a voltage is included in L2 in the
correct polarity for changing the commutating capacitor to a
voltage higher than Edc . Thus , the autotransformer
measurably enhances the reliability of the circuit.
At t5 , the bottom plate (B) of capacitor C reaches a peak
value. Since at t5 , the capacitor is charged to a voltage greater
than Edc , diode D1 is again forward biased . Capacitor C now
discharges to a value lower than Edc . The time duration t3 to
t4 is the circuit turn off time presented to SCR T1.