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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
ENGINEERING
COURSE: POWER ELECTRONICS
BRANCH: EEE
CLASS: III/I Sem
...
NO
...


SEMICONDUCTOR

3 - 32

UNIT II: FIRING AND COMMUTATION

33 - 50

DEVICES
2

CIRCUITS OF SCR
3

UNIT III: SINGLE PHASE HALF

51 - 60

CONTROLLED CONVERTERS
4

UNIT IV: SINGLE PHASE FULLY

61 - 70

CONTROLLED CONVERTERS
5

UNIT V: THREE PHASE LINE

71 – 87

COMMUTATED CONVERTER
6

UNIT VI: AC VOLTAGE CONTROLLERS

88 – 99

& CYCLO CONVERTERS

DEPARTMENT OF EEE – SVECW

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Unit-1
Power Semi Conductor
Devices

DEPARTMENT OF EEE – SVECW

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1
...

Power engineering deals with the static and rotating power equipment for the
generation, transmission and distribution of electric power
...

Power electronics may be defined as the subject of applications of solid state
power semiconductor devices (Thyristors) for the control and conversion of electric
power
...

1
...
Then the other Power devices like metal tank rectifier, grid
controlled vacuum tube rectifier, ignitron, phanotron, thyratron and magnetic amplifier,
were developed & used gradually for power control applications until 1950
...

The second electronic revolution began in the year 1958 with the development of
the commercial grade Thyristor by the General Electric Company (GE)
...
After that many different types of power semiconductor
devices & power conversion techniques have been introduced
...

1
...


1
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DOMESTIC APPLICATIONS
Cooking Equipments, Lighting, Heating, Air Conditioners, Refrigerators &
Freezers, Personal Computers, Entertainment Equipments, UPS
...
Machine tools, arc furnaces, induction
furnaces, lighting control circuits, industrial lasers, induction heating, welding
equipments
...

TELECOMMUNICATIONS
Battery chargers, power supplies (DC and UPS), mobile cell phone battery
chargers
...

UTILITY SYSTEMS
High voltage DC transmission (HVDC), static VAR compensation (SVC),
Alternative energy sources (wind, photovoltaic), fuel cells, energy storage systems,
induced draft fans and boiler feed water pumps
...
5 POWER SEMICONDUCTOR DEVICES
Power Diodes
...

Power MOSFETS
...

Thyristors
Thyristors are a family of p-n-p-n structured power semiconductor switching devices

1
...
The family of thyristor devices include SCR's, Diacs, Triacs,
SCS, SUS, LASCR's and so on
...
7 POWER SEMICONDUCTOR DEVICES USED IN POWER ELECTRONICS
The first thyristor or the SCR was developed in 1957
...

After 1970, various types of power semiconductor devices were developed and became
commercially available
...

Thyristors
...

Power MOSFET's
...

Static Induction Transistors (SIT's)
...

Reverse conducting Thyristors (RCT's)
...

Gate assisted turn-off Thyristors (GATT)
...

MOS-Controlled Thyristors (MCT's)
...
8 POWER DIODES
Power diodes are made of silicon p-n junction with two terminals, anode and
cathode
...
Modern
techniques in diffusion and epitaxial processes permit desired device characteristics
...

1
...
These devices are used a switching devices and are operated in the
saturation region resulting in low on-state voltage drop
...
The transistor remains on so long as the
control signal is present
...
However their
voltage and current ratings are lower than those of thyristors and are therefore used in low
to medium power applications
...
9
...
The modified structure leads to significant
differences in the I-V characteristics and switching behavior between power transistors
and its logic level counterpart
...
9
...

The difference in the two structures is obvious
...
The vertical structure is preferred because it maximizes the cross sectional
area and through which the current in the device is flowing
...

The doping of emitter layer and collector layer is quite large typically 10 19 cm-3
...

The thickness of the drift region determines the breakdown voltage of the
transistor
...


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Practical power transistors have their emitters and bases interleaved as narrow
fingers as shown
...
This multiple emitter layout also reduces parasitic ohmic resistance in the base
current path which reduces power dissipation in the transistor
...
2
1
...
3 STEADY STATE CHARACTERISTICS
Figure 3(a) shows the circuit to obtain the steady state characteristics
...
Fig 3(c)
shows the output characteristics of the transistor which is a plot IC versus VCE
...

The power transistor has steady state characteristics almost similar to signal level
transistors except that the V-I characteristics has a region of quasi saturation as shown by
figure 4
...
3: Characteristics of NPN Transistors

There are four regions clearly shown: Cutoff region, Active region, quasi saturation and
hard saturation
...
Hence no
collector current flows and transistor is off
...
Hence collector current flows depending
upon the load
...
e
...
The BVSUS is the maximum collector to
emitter voltage that can be sustained when BJT is carrying substantial collector current
...

I

B5

IB4
Active region
IB3

I

Primary
breakdown

B2
IB1
IB<0
IB=0
IB=0

0
BV

CEO

vC
E

BV

SUS
BVCBO
Fig
...
Large power dissipation normally leads to primary breakdown
...
This is
explained in detail later
...
9
...
5: Transfer Characteristics

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1
...
From fig
...
6: Transistor Switch
If the base current is increased above I BM ,VBE increases, the collector current
increases and VCE falls belowVBE
...
4 to 0
...
The transistor saturation may
be defined as the point above which any increase in the base current does not increase the
collector current significantly
...
If the collector emitter
voltage is VCE sat the collector current is
VBE increases due to increased base current resulting in increased power loss
...

However the power is increased at a high value of ODF, the transistor may be damaged
I B IBS
may operate in active region, VCE increases resulting in increased power loss
...
11 SWITCHING CHARACTERISTICS
A forward biased p-n junction exhibits two parallel capacitances; a depletion layer
capacitance and a diffusion capacitance
...
Under steady state the capacitances do not play any role
...

1
...
7: Transient Model of BJT
DEPARTMENT OF EEE – SVECW

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Fig
...
As the
voltage VB rises from zero to V1 and the base current rises to IB1, the collector current
does not respond immediately
...
The delay is due to the time required to charge up the BEJ to the
forward bias voltage VBE(0
...
The collector current rises to the steady value of I CS and
this time is called rise time tr
...
As
a result excess minority carrier charge is stored in the base region
...
This extra charge which is
called the saturating charge is proportional to the excess base drive
...

When the input voltage is reversed from V1 to -V2, the reverse current –IB2 helps
to discharge the base
...

Once the extra charge is removed, BEJ charges to the input voltage –V2 and the base
current falls to zero
...


DEPARTMENT OF EEE – SVECW

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Turn-on time ton : The turn-on time can be decreased by increasing the base drive for a
fixed value of collector current
...
However tr increases with increase in IC
...
tf is a function of capacitance and increases with IC
...
t f is less
sensitive to negative base drive
...
IC falls to
10% of its on-state value
...

1
...
11: Thermal Equivalent Circuit of Transistor

1
...

BVSUS : The maximum voltage between the collector and emitter that can be sustained
across the transistor when it is carrying substantial collector current
...

BVCBO : This is the collector to base break down voltage when emitter is open circuited
...
15 BASE DRIVE CONTROL
This is required to optimize the base drive of transistor
...
ton can be reduced by allowing base current peaking during
can be increased to a sufficiently high value to maintain the transistor in quasi-saturation
region
...


DEPARTMENT OF EEE – SVECW

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A typical waveform for base current is shown
...
12: Base Drive Current Waveform
Some common types of optimizing base drive of transistor are
Turn-on Control
...

Proportional Base Control
...
16 TURN-ON CONTROL

Fig
...
The discharging time constant is 2 R2 C1
...
17 TURN-OFF CONTROL
If the input voltage is changed to during turn-off the capacitor voltage VC is added
to V2 as reverse voltage across the transistor
...
As the capacitor C1 discharges, the reverse voltage will be reduced to a steady
state value, V2
...


DEPARTMENT OF EEE – SVECW

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Fig: 14
...
18 PROPORTIONAL BASE CONTROL
This type of control has advantages over the constant drive circuit
...

When switch S1 is turned on a pulse current of short duration would flow through
the base of transistor Q1 and Q1 is turned on into saturation
...
The
transistor would latch on itself and S1 can be turned off
...
The switch S1 can be implemented by a small signal
transistor and additional arrangement is necessary to discharge capacitor C1 and reset the
transformer core during turn-off of the power transistor
...
19 ANTISATURATION CONTROL

Fig: 16: Collector Clamping Circuit
If a transistor is driven hard, the storage time which is proportional to the base
current increases and the switching speed is reduced
...
This can be
accomplished by VCC VCM CE voltage to a pre-determined level and the collector current
clamping
is given by IC

...


This means that the CE voltage is raised above saturation level and there are no excess
carriers in the base and storage time is reduced
...
At the same time, a fast turn-on is accomplished
...

ADVANTAGES OF BJT’S
BJT’s have high switching frequencies since their turn-on and turn-off time are
low
...

BJT has controlled turn-on and turn-off characteristics since base drive control is
possible
...

DEMERITS OF BJT
Drive circuit of BJT is complex
...
It cannot be used in parallel operation due to problems of negative
temperature coefficient
...
20
...
20
...
FET is manufactured by diffusing two
areas of p-type into the n-type semiconductor as shown
...
Since it is similar
to two diodes one is a gate source diode and the other is a gate drain diode
...
2: Structure of FET with biasing
In BJT’s we forward bias the B-E diode but in a JFET, we always reverse bias the
gate-source diode
...
Therefore
IG 0 , therefore Rin
ideal
The term field effect is related to the depletion layers around each p-region as
shown
...
With gate reverse biased, the electrons need to flow from source to
drain, they must pass through the narrow channel between the two depletion layers
...

Therefore JFET acts as a voltage controlled device rather than a current controlled
device
...

JFET CHARACTERISTICS

DEPARTMENT OF EEE – SVECW

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The maximum drain current out of a JFET occurs when VGS
VDS is increased for
0 to a few volts, the current will increase as determined by ohms law
...
If VDS is increased
to a level where the two depletion region would touch a pinch-off will result
...
Between 0 volts and pinch off voltage VP is the ohmic region
...

If negative voltage is applied between gate and source the depletion region similar to
those obtained with VGS
VDS
...

1
...
2 Classification of MOSFET
MOSFET stands for metal oxide semiconductor field effect transistor
...
20
...
A n-channel is formed by diffusing
between source and drain
...
The gate is also connected to a
metal contact surface but remains insulated from the n-channel by the SiO2 layer
...
4: Structure of n-channel depletion type MOSFET
OPERATION
When VGS
V and VDS is applied and current flows from drain to source similar to
JFET
...
Therefore recombination occurs and
will reduce the number of free electrons in the n-channel for conduction
...

For positive values,Vgs , additional electrons from p-substrate will flow into the
channel and establish new carriers which will result in an increase in drain current with positive
gate voltage
...
20
...
20
...
21 ENHANCEMENT TYPE MOSFET
Here current control in an n-channel device is now affected by positive gate to source
voltage rather than the range of negative voltages of JFET’s and depletion type
MOSFET
...
21
...
The
source and drain terminals are connected through metallic contacts to n-doped regions, but the
absence of a channel between the doped n-regions
...


Fig
...
21
...
With VDS set at some positive voltage
and VGS set at 0V, there are two reverse biased p-n junction between the n-doped regions and p
substrate to oppose any significant flow between drain and source
...
However the electrons in the p-substrate will be
attracted to the positive gate and accumulate in the region near the surface of the SiO2 layer
...

The level of VGS that results in significant increase in drain current is called threshold
voltage VT
...
If VGS is constant VDS is increased; the drain current will eventually reach a
saturation level as occurred in JFET
...
21
...
21
...
This MOSFET is turned
‘ON’ when a voltage is applied between gate and source
...
Thus gate has control over the conduction of the MOSFET
...
Hence they operate at very high
frequencies; hence MOSFET’s are preferred in applications such as choppers and inverters
...

The paralleling of MOSFET’s is easier due to their positive temperature coefficient
...
Hence MOSFET’s are used for low power applications
...
21
...
On the other side of n substrate, a metal
layer is deposited to form the drain terminal
...
Further n regions are diffused in the p regions as shown
...

A power MOSFET actually consists of a parallel connection of thousands of basic
MOSFET cells on the same single chip of silicon
...
When gate terminal is made positive with respect to
source, an electric field is established and electrons from n channel in the p regions
...

Power MOSFET conduction is due to majority carriers therefore time delays caused by
removal of recombination of minority carriers is removed
...
The thickness of the
drift region determines the breakdown voltage of MOSFET
...

1
...
6 SWITCHING CHARACTERISTICS
The switching model of MOSFET’s is as shown in the figure 6(a)
...
The switching waveforms are as
shown in figure 7
...
The rise time tr is the gate charging time from this threshold level
to the full gate voltage Vgsp
...
The fall time is the time required for the input capacitance to discharge from
pinch off region to the threshold voltage
...


Fig
...
7: Switching waveforms and times of Power MOSFET
GATE DRIVE
The turn-on time can be reduced by connecting a RC circuit as shown to charge the
capacitance faster
...

1
...
A BJT has higher switching loss bit lower conduction loss
...
But at lower operating
frequencies BJT is superior
...
This makes parallel
operation of MOSFET’s easy
...
A BJT is a negative temperature coefficient, so current shaving
resistors are necessary during parallel operation of BJT’s
...
But BJT exhibits negative temperature coefficient which results in secondary
breakdown
...


1
...
MOSIGT OR IGBT

The metal oxide semiconductor insulated gate transistor or IGBT
combines the advantages of BJT’s and MOSFET’s
...
Further IGBT is free from second
breakdown problem present in BJT
...
23
...
However, the
substrate is now a p layer called the collector
...

This n channel short circuits the n region with n emitter regions
...
Eventually a forward current is established
...
Equivalent circuit is
as shown below
...
The two pnp
and npn is formed as shown
...

Therefore T1 starts conducting
...
Therefore
regenerative action takes place and large number of carriers are injected into the n drift region
...

When gate drive is removed IGBT is turn-off
...
Therefore T1 will turn-off it
T2 turns off
...
If R1 small T1 will not conduct therefore
IGBT’s are different from MOSFET’s since resistance of drift region reduces
when gate drive is applied due to p injecting region
...


1
...
24
...
9: IGBT bias circuit

Static V-I characteristics ( IC versus VCE )
Same as in BJT except control is by VGE
...

Transfer Characteristics ( IC versus VGE )
Identical to that of MOSFET
...


Page 26

APPLICATIONS
Widely used in medium power applications such as DC and AC motor drives,
UPS systems, Power supplies for solenoids, relays and contractors
...
The ratings up to 1200V, 500A
...
It is
very important that the series-connected transistors are turned on and off simultaneously
...
The devices should be matched for gain, transconductance, threshold voltage, on state
voltage, turn-on time, and turn-off time
...

Transistors are connected in parallel if one device cannot handle the load current demand
...
But in practice, it is not always possible to
meet these requirements
...


Fig
...
Current sharing under
dynamic conditions can be accomplished by connecting coupled inductors
...
The result is low impedance path, and
the current is shifted to Q2
...


DEPARTMENT OF EEE – SVECW

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Fig
...
During current sharing, if one BJT carries
more current, its on-state resistance decreases and its current increases further, whereas
MOSFETS have positive temperature coefficient and parallel operation is relatively easy
...
IGBTs require special care to match the
characteristics due to the variations of the temperature coefficients with the collector current
...
25 di dt AND dv dt LIMITATIONS
Transistors require certain turn-on and turn-off times
...


DEPARTMENT OF EEE – SVECW

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During turn-on, the collector rise and the di dt is
di

I

L

I

cs
...
(2)

dt

tf

tf

The conditions di dt and dv dt in equation (1) and (2) are set by the transistor switching
characteristics and must be satisfied during turn on and turn off
...
A typical
switch with di dt and dv dt protection is shown in figure (a), with operating wave forms in
figure (b)
...
The inductor LS , which limits the di dt , is sometimes called series snubber
...
When
transistor Q1 is turned on, the collector current rises and current of diode Dm falls, because
Dm will behave as short circuited
...
26 SCR-Principle of Operation

The SCR is a four-layer, three-junction and a three-terminal device and is shown in fig
...
The
end P-region is the anode, the end N-region is the cathode and the inner P-region is the gate
...
Essentially the device is a switch
...


The thyristor then switches on and current flows and continues to conduct without further gate
signals
...
For switching off or
reverting to the blocking state, there must be no gate signal and the anode current must be
reduced to zero
...

In absence of external bias voltages, the majority carrier in each layer diffuses until there is a
built-in voltage that retards further diffusion
...
These carriers then become
minority carriers and can recombine with majority carriers
...

A voltage bias, as shown in figure, and an external circuit to carry current allow internal currents
which include the following terms:
The current Ix is due to
 Majority carriers (holes) crossing junction J1
 Minority carriers crossing junction J1
 Holes injected at junction J2 diffusing through the N-region and crossing junction J1 and
DEPARTMENT OF EEE – SVECW

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

Minority carriers from junction J2 diffusing through the N-region and crossing junction J1
...

Turning-off Methods of an SCR

As already mentioned in previous blog post, once the SCR is fired, it remains on even when
triggering pulse is removed
...
So SCR cannot be turned off by simply removing the gate
pulse
...

a) Natural Commutation
When the anode current is reduced below the level of the holding current, the SCR turns off
...
Since the anode voltage remains positive with respect to the cathode in a dc circuit, the
anode current can only be reduced by opening the line switch S, increasing the load impedance
RL or shunting part of the load current through a circuit parallel to the SCR, i
...
short-circuiting
the device
...
The voltage reverses every half cycle in an ac circuit, so that an SCR
in the line would be reverse biased every negative cycle and would turn off
...
To create a reverse biased voltage across the SCR, which
is in the line of a dc circuit, capacitors can be used
...

In power electronic applications one advantage of using SCRs is that they are compact
...
There has also been an attempt
to miniaturize capacitors used for forced commutation and for filtering
...
Small sizes of capacitors are at present being achieved by the use of metalized
plastic film or a plastic film and aluminium foil
...
The
current ratings are presently below 10 A and this type will not be considered further
...
1Two Transistor analogy of SCR
The principle of thyristor operation can be explained with the use of its two-transistor model (or
two-transistor analogy)
...
4
...
From this figure,
two-transistor model is obtained by bisecting the two middle layers, along the dotted line, in two
separate halves as shown in Fig
...
15 (b)
...
The circuit representation of the twotransistor model of a thyristor is shown in Fig
...
15 (c)
...


For transistor Q1 in Fig
...
15 (c), emitter current IE = anode current Ia and IC = collector current
IC1
...
(4
...
4)

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where

α2 – common-base current gain of Q2,ICBO2 =common-base leakage current of Q2 and

Ik = emitter current of Q2
...
(4
...
4) is equal to the external circuit
current Iα entering at anode terminal A
...
5)
When gate current is applied, then Ik = Ia + Ig
...
(4
...
With an increase in
emitter current, a builds up rapidly as shown in Fig
...
16
...
6) and forward leakage current
somewhat more than ICBO1 + ICBO2 flows
...
(4
...
Actually, external load limits the
anode current to a safe value after the thyristor begins conduction
...
These 0
...
(4
...

Now a sufficient gate-drive current between gate and cathode of the transistor is applied
...
With the
establishment of emitter current Ik of Q2, current gain α2 of Q2 increases and base current
IB2 causes the existence of collector current IC2 = β2IB2 = β2 Ig
...
Currents IB1 and IC1 lead to the establishment of emitter
current Ia of Q1 and this causes current gain α1 to rise as desired
...
As
amplified collector current IC2 is equal to the base current of Q1 current gain α1 eventually rises
further
...
This regenerative or
positive feedback effect causes α1+ α2to grow towards unity
...

When regeneration has grown sufficiently, gate current can be withdrawn
...
This characteristic of the thyristor makes it suitable for pulse
triggering
...
Under these conditions, thyristor has very low impedance and is in the forward
on-state
...
As a result, the leakage
current at the middle junction J2 of thyristor increases, which is also the collector current of Q2 as
well as Q1 With increase in collector currents IC1 and IC2 due to avalanche effect, the emitter
currents of the two transistors also increase causing α1+ α2 to approach unity
...
The forward-voltage triggering for
turning-on a thyristor may be destructive and should therefore be avoided
...
Let the capacitance of this junction be Cj
...
In case it is assumed that entire forward voltage va appears across reverse biased junction
J2 then charging current across the junction is given by
i = Cj dva /dt
This charging or displacement current across junction J2 is collector currents of Q2 and
Q1 Currents IC2, IC1 will induce emitter current in Q2, Q1 In case rate of rise of anode voltage is
large, the emitter currents will be large and as a result, α1+ α2 will approach unity leading to
eventual switching action of the thyristor
...
This leakage current serves as the collector junction current of the component transistors
Q1 and Q2
...
As a result, (α1+ α2) approaches unity
...

(v) Light triggering : When light is thrown on silicon, the electron-hole pairs increase
...

As stated before, gate-triggering is the most common method for turning-on a thyristor
...

The operational differences between thyristor-family and transistor family of devices may now
be summarised as under :
i) Once a thyristor is turned on by a gate signal, it remains latched in on-state due to internal
regenerative action
...

ii) In order to turn-off a thyristor, a reverse voltage must be applied across its anode-cathode
terminals
...


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2
...
The basic elements of such a triggering circuit are shown in figure
...
If the load line does not pass to the right of the peak point
P, the device cannot turn on
...
Then VE = VBB – IRE RE
So, RE(MAX) = VBB – VE / IRE = VBB – Vp / IP at the peak point
...

RE > = VBB – VV / IV
So, the range of resistor RE is given as
VBB – VP / IP >RE > VBB – VV / IV
The resistor R is chosen small enough so as to ensure that SCR is not turned on by voltage
VR when emitter terminal E is open or IE = 0
The voltage VR = RVBB/R + RBB for open-emitter terminal
...
By varying RE, we can change the time constant RE C and alter the point at which the
UJT fires
...


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2
...
Under such situations the SCRs are required to
be connected in series or in parallel to meet the requirements
...

Like any other electrical equipment, characteristics/properties of two SCRs of same make and
ratings are never same and this leads to certain problems in the circuit
...

2
...
1 Series Connection of an SCR

When the required voltage rating exceeds the SCR voltage rating, a number of SCRs are required
to be connected in series to share the forward and reverse voltage
...

(ii) Difference in recovery characteristics
...
For steady-state conditions, voltage sharing is
achieved by using a resistance or a Zener diode in parallel with each SCR
...
Diodes D1 connected in parallel with resistor Rl, helps in dynamic stabilisation
...
Additionally the R-C circuit can also serve the function of‘snubber circuit‘
...
If ΔQ is the difference in recovery charge of two devices arising out of different recovery
current for different time and ΔV is the permissible difference in blocking voltage
then C1 = ΔQ/ ΔV
...

Since the capacitor C1 can discharge through the SCR during turn-on, there can be excessive
power dissipation, but the switching current from C1 is limited by the resistor R1 This resistance
also serves the purpose of damping out ‘ringing’ which is oscillation of C1 with the circuit
inductance during commutation
...

2
...
2 Parallel Connection of an SCR

When the load current exceeds the SCR current rating, SCRs are connected in parallel to share
the load current
...
The device having lower dynamic resistance will tend to share more current
...
This process is cumulative and
continues till the device gets punctured
...
Arrangement of SCRs in the cubicle also plays
vital role
...
Further the holding currents of the devices should not be so
much different that at reduced load current one of the device gets turned-off because of fall of
current through it blow its holding current value
...

Another point to be considered is the on-state voltage across the device
...
For operation of all
the SCRs connected in parallel at the same temperature, it becomes necessary to use a common
heat sink for their mounting, as illustrated in figure
...
In this circuit the resistors Rx and R2 are chosen so as to cause equal voltage
drop in both arms
...
Firing circuits giving high
rate of rise can be used to reduce mismatch of gate characteristics and delay time
...
This is done to ensure that the devices share current
equally under worst operating conditions
...
Cylindrical construction is perhaps the best from this
point of view
...
Even with all the measures taken, it is preferable to derate the device for
series/parallel operation
...

Normal derating factors are 10 to 15% for parallel connection of SCRs depending upon the
number of devices connected in parallel
...

2
...
However, the amplifying gain of this
regenerative device being in the order of the 108, the SCR cannot be turned OFF via the gate
terminal
...
These methods of turn-off do not refer to those cases where the anode
current is gradually reduced below Holding Current level manually or through a slow process
...


Fig
...
1 Turn-off dynamics of the SCR
In all practical cases, a negative current flows through the device
...
The device can block a forward voltage only after a further tfr, the forward
recovery time has elapsed
...
The external circuit must therefore

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reverse bias the SCR for a time toff > tq
...
This dv/dt is less than the static counterpart
...
Others have chosen different classification rules
...
The faster ones are popularly
known as 'Inverter grade' and the slower ones as 'Converter grade' SCRs
...

Classification of forced commutation methods
The six distinct classes by which the SCR can be turned off are:
Class A
Class B
Class C
Class D

Self commutated by a resonating load
Self commutated by an L-C circuit
C or L-C switched by another load carrying SCR
C or L-C switched by an auxiliary SCR

Class E An external pulse source for commutation Class
F AC line commutation
These examples show the classes as choppers
...

2
...
1 Class A, Self commutated by resonating the load

Fig
...
2 A resonant load commutated SCR and the corresponding waveforms
When the SCR is triggered, anode current flows and charges up C with the dot as positive
...
The current through the SCR builds up and
completes a half cycle
...

The capacitor voltage is at its peak when the SCR turns off and the capacitor discharges into the
resistance in an exponential manner
...

DEPARTMENT OF EEE – SVECW

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2
...
2 Class B, Self commutated by an L-C circuit
The Capacitor C charges up in the dot as positive before a gate pulse is applied to the
SCR
...

The constant load current Iload flows through R - L load
...
A sinusoidal current
flows through the resonant L- C circuit to charge-up C with the dot as negative at the end of the
half cycle
...
The SCR will turn off when the resonant–circuit (reverse) current is just greater than the
load current
...


Fig
...
3 Class B, L-C turn-off
Problem #1
A Class B turn-off circuit commutates an SCR
...

Dimension the commutating components L and C
...

Soln # 1
The commutating capacitor is charged to the supply voltage = 100 V
The peak resonant current is,
i
peak =V C
L

Assuming,

i

peak ~1
...
Iload
C

DEPARTMENT OF EEE – SVECW

L

= (15100)2 = 0
...
This corresponds to 0
...
The capacitor voltage at that instant is 75
volts
...

If the SCR is to commutate at twice this load current, for a rated "Inverter grade' SCR turnoff time of 20 μsecs,
It can be observed that if the peak of the commutating current is just equal to the load
current, the turn-off time would be zero as the capacitor would not be able to impress any
negative voltage on the SCR
...
4
...
One of them may be the main SCR and the other auxiliary
...
The configuration may have four SCRs with the
load across the capacitor, with the integral converter supplied from a current source
...
C then charges up in the polarity shown
...


Fig
...
4 Class C turn-off, SCR switched off by another load-carring SCR

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2
...
4 Class D, L-C or C switched by an auxiliary SCR
Example 1
The circuit shown in Figure 3
...
The auxiliary
SCR would have a resistor in its anode lead of say ten times the load resistance
...
3
...
Class D commutation by a C (or LC) switched by an
Auxiliary SCR
...
As soon as C is charged to the supply voltage, SCRA will turn off
...
This extra voltage would discharge through the diode-inductor-load circuit
...
The charge on C is
reversed and held at that level by the diode D
...
If the load carries a constant current as
in Fig
...
4, the capacitor again charges linearly to the dot as positive
...
The load consists of a clamped
inductive load such that the load current is reasonably constant at 25 amperes
...
Determine whether the SCR will be satisfactorily
commutated
...
The supply voltage is 220 VDC
...

When SCRM is triggered, the 25 Amps load current and the L-C ringing current flows
through it
...
5
...
0568
L 2
...
1
mH

Assuming that the capacitor charges to 70% of its original charge because of losses in the
C- SCRM -L-D network, and it charges linearly when SCRA is again triggered,
Iload
...
7
...
10−6 t
q =1540 / 25 =61
...

The maximum current that can be commutated with the given Capacitor at the 220 V
supply voltage is
Iload =1540 /12 =128
Amps
For the 25 Amps load current the capacitor just enough would have a rating of
C = Iload
...
7
...
12) /154 =1
...
0
μF
If the supply voltage is reduced by a factor K, the required capacitor rating
increases by the same factor K for the same load current
...
4
...
It is capable of
carrying the load current with a small voltage drop compared with the supply voltage
...
To turn SCR1 off
a positive pulse is applied to the cathode of the SCR from an external pulse generator via the
pulse transformer
...
Thus the pulse from the transformer
reverses the voltage across the SCR, and it supplies the reverse recovery current and holds the
voltage negative for the required turn-off time
...
3
...
4
...
With a highly inductive load, the current may remain continuous for some time till the

Fig
...
7 Class F, natural commutation by supply voltage
energy trapped in the load inductance is dissipated
...
The negative polarity of the
voltage appearing across the outgoing SCR turns it off if the voltage persists for the rated turnoff period of the device
...

The rectifier in Fig
...
6 is supplied from an single phase AC supply
...
The converter has an
input inductance Ls arising manly out of the leakage reactance of the supply transformer
...
The triggering angle for the
converter is around 600
...

When the incoming SCRs, Th2 and Th2' are triggered, the current through the incoming
devices cannot rise instantaneously to the load current level
...

This current can be described by:
V sin(ωt −900 )
s

V
s

V cos(ωt)
s

V
s

I sc =

+
cosα =
+
cosα
ωLs
ωLs
ωLs
ωLs
where α the triggering angle and Isc and Vs as shown in Fig
...
6
...
When the current rises in the incoming SCRs, which in the outgoing

DEPARTMENT OF EEE – SVECW

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ones fall such that the total current remains constant at the load current level
...

The reverse biasing voltage of these SCRs must continue till they reach their forward blocking
state
...
It is lowest when α ~ 900
...

The period when both the devices conduct is known as the 'overlap period'
...
If the 'fully-controlled'
converter in Fig
...
7 is used as an inverter with triggering angles > 900, the converter triggering
can be delayed till the 'margin angle' which includes the overlap angle and the turn-off time of
the SCR - both dependent on the supply voltages
...
A changing
voltage impressed on this junction capacitance results in a current, I = C dv/dt
...
This
regenerative action is similar to that which occurs when gate current is injected
...

Since dv/dt turn-on is non-destructive, this phenomenon creates no problem in applications in
which occasional false turn -on does not result in a harmful affect at the load
...
However, at large currents where dv/dt turn-on is accompanied by partial turnon of the device area a high di/dt occurs which then may be destructive
...
One solution to this problem is to reduce the dv/dt imposed by the circuit to a
value less than the critical dv/dt of the SCR being used
...
8 to suppress excessive rate of rise of anode voltage
...
Variations of the basic circuit is also shown
where the section of the network shown replaces the SCR and the R-C basic snubber
...
A technique can be used to
simplify snubber circuit design by the use of nomographs which enable the circuit designer to
select an optimized R-C snubber for a particular set of circuit operating conditions
...
This can be done by selecting an SCR
designed specially for high dv/dt applications, as indicated by the specification sheet
...


DEPARTMENT OF EEE – SVECW

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Fig
...
8 dv/dt supression circuits
Questions
#1

For a Class D turn-off SCR, the load consists of a resistance only
...


Ans: (Hints): The capacitor would now charge in an exponential manner
...

#2

For a Class F converter, will the overlap period rise with the leakage inductance of the
converter? What happens to the output voltage?

Ans: Yes
...
The output voltage
decreases
...
The input
current maximum would be as for a shorted network with the leakage inductance only present
...
Most of the above circuits are also called 'forced commutated' DC-DC chopper
circuits
...
1 Operation and analysis of single phase half controlled converters
Instructional Objectives
On completion the student will be able to
Draw different topologies of single phase half controlled converter
...

Construct the conduction table and thereby draw the waveforms of different system
variables in the continuous conduction mode of operation of the converter
...

Find out an analytical condition for continuous conduction relating the load parameters
with the firing angle
...


3
...

They can supply unidirectional current with both positive and negative voltage polarity
...
However, many of the industrial
application do not utilize the inverter mode operation capability of the fully controlled converter
...
Single phase fully
controlled converters have other disadvantages as well such as relatively poor output voltage
(and current for lightly inductive load) form factor and input power factor
...
The disadvantages of the single phase fully controlled converter are also
related to the same capability
...
1(a)
...
Of course this circuit will not
be able to operate in the inverter mode
...

For that, two of the thyristors of a single phase fully controlled converter has to be replaced by
two diodes as shown in Fig 11
...
The resulting converters are called single phase half
controlled converters
...
As the input voltage passes
through negative going zero crossing D4 comes into conduction commutating D2 in Fig 11
...
1 (c)
...
As far as the input and output behavior of the circuit is concerned the circuits in Fig 11
...
In Fig 11
...
However, in Fig 11
...
In this lesson the operating
principle and characteristics of a single phase half controlled converter will be presented with
reference to the circuit in Fig 11
...

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3
...
1 (b), it can be stated that for any load current to flow one device from
the top group (T1 or T3) and one device from the bottom group must conduct
...
On the other hand T1 D4 and T3 D2 conducts
simultaneously whenever T1 or T3 are on and the output voltage tends to go negative
...
Of course
it is always possible that none of the four devices conduct
...
The operating modes of this converter and the voltage across different devices
during these operating modes are shown in the conduction table of Fig 11
...
This table has been
prepared with reference to Fig 11
...


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It is observed that whenever D2 conducts the voltage across D4 is -vi and whenever D4 conducts
the voltage across D2 is vi
...

Similar conclusions can be drawn regarding the conduction of T1 and T 3
...
Load current flows through T1 and D2
...
If the load current further continuous till T3 is fired current commutates from T1 to T3
...
Otherwise the mode of conduction becomes discontinuous
...
1
Fill in the blanks(s) with the appropriate word(s)
In a half controlled converter two ___________________ of a fully controlled converter
are replaced by two ___________________
...

The input/output waveforms of the two different circuit topologies of a half controlled
converter are ___________________ while the device ratings are
o ___________________
...

A half controlled converter has improved input ___________________ compared to a
fully controlled converter
...


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2
...
11
...
Assume ripple free continuous
output current
...
The ration of the thyristors to the diode RMS current ratings will be
unity for the circuit of Fig 11
...

From the second conduction diagram the thyristors conduct for π - α radians while the diodes
conduct for π + α radians
...

Thyristor RMS current rating =
Diode RMS current rating

1−α / π
1+α / π

in this case
3
...
On the other hand T1 starts conduction when it is fired in the positive half cycle of
the input voltage waveform and continuous conduction till T3 is fired in the negative half cycle
...
11
...


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Referring to Fig 11
...
Output voltage during this period
becomes equal to vi
...
The output voltage
remains clamped to zero till T3 is fired at ωt = π + α
...
Where upon load current again free wheels through T3 and D2 while the load voltage is
clamped to zero
...
Hence

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Clearly in addition to the average component, the output voltage (and current) contains a large
number of harmonic components
...
Magnitude of the harmonic voltages can be found by Fourier series analysis of
the load voltage and is left as an exercise
...

However, the closed form expression of io can be found as explained next
...
14)

However, it will be very difficult to find out the characteristic parameters of ii using equation
11
...
Considerable simplification can
however be obtained if the actual ii waveform is replaced by a quasisquare wave current
waveform with an amplitude of Ioav as shown in Fig 11
...


DEPARTMENT OF EEE – SVECW

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3
...

So far we have discussed the operating characteristics of a single phase half controlled converter
in the continuous conduction mode without identifying the condition required to achieve it
...

Referring to Fig 11
...
Therefore, the load current increases till vo becomes equal to E again at
ωt = π – θ
...
Now if io becomes zero before T3 is
fired at ωt = π + α the conduction becomes discontinuous
...
11
...
Clearly, two
possibilities exist
...
In the second
case io continuous beyond ωt = π but becomes zero before ωt = π + α
...

Fig
...
6 shows the wave forms in these two cases
...
The analysis of the first case is
left as an exercise
...
23)

However IORMS cannot be computed directly from VORMS
...
This will also help to find out an expression for the conduction angle β
...

(iv) At the boundary between continuous and discontinuous conduction the value of the
output current at ωt = α is ___________________
...

(vi)

For the same value of the firing angle the average output voltage of a single phase half
controlled converter is ___________________ in the discontinuous conduction mode
compared to the continuous conduction mode
...


Answer: (i) zero; (ii) π; (iii) higher; (iv) unidirectional
...
A single phase half controlled converter charges a 48v 50Ah battery from a 50v, 50Hz single
phase supply through a 50mH line inductor
...
1Ω
...
Find out whether the conduction will be continuous or discontinuous
at this condition
...

Lesson Summary
Single phase half controlled converters are obtained from fully controlled converters
by replacing two thyristors by two diodes
...
From the operational point
of view these two topologies are identical
...

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For the same firing angle and input voltage the half controlled converter in the
continuous conduction mode gives higher output voltage compared to a fully
controlled converter
...

For the same firing angle and load current the half controlled converter in the
continuous conduction mode has better input power factor compared to a fully
controlled converter
...


Practice Problems and Answers
Q1
...
1(b) fails to turn on at the desired instant
...

Q2
...
At α = 0° the half
controlled converter produces the rated field voltage
...
A single phase half controlled converter supplies a 220V, 1500rpm, 20A dc motor from a
230V 50HZ single phase supply
...
0Ω and
inductance of 50mH
...


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UNIT-IV
SINGLE PHASE FULLY CONTROLLED CONVERTERS

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4
...

Instructional Objectives
On completion the student will be able to
•

Differentiate between the constructional and operation features of uncontrolled and
controlled converters

•

Draw the waveforms and calculate their average and RMS values of different

variables associated with a single phase fully controlled half wave converter
...

•

Identify the mode of operation of the converter (continuous or discontinuous) for a

given load parameters and firing angle
...

•

Explain the operation of the converter in the inverter mode
...
2 Introduction
Single phase uncontrolled rectifiers are extensively used in a number of power electronic
based converters
...
They have, in
general, been proved to be efficient and robust power stages
...
The main among them is their inability to control the output dc voltage / current
magnitude when the input ac voltage and load parameters remain fixed
...
These two disadvantages are the direct consequences of using power diodes in
these converters which can block voltage only in one direction
...

Thyristors are semicontrolled devices which can be turned ON by applying a current pulse at
its gate terminal at a desired instance
...
Therefore, the fully controlled converter continues to exhibit load dependent output
voltage / current waveforms as in the case of their uncontrolled counterpart
...
Working principle of thyristors
based single phase fully controlled converters will be explained first in the case of a single
thyristor halfwave rectifier circuit supplying an R or R-L load
...


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Full bridge is the most popular configuration used with single phase fully controlled rectifiers
...


4
...
3
...
10
...
At t = 0 when the input supply voltage becomes positive the
thyristor T becomes forward biased
...
During the period 0 < t , the thyristor blocks the supply voltage and the
load voltage remains zero as shown in fig 10
...
Consequently, no load current flows
during this interval
...
The
voltage across the thyristor collapses to almost zero and the full supply voltage appears across
the load
...
The load being
purely resistive the load current io is proportional to the load voltage
...
In the process the thyristor undergoes
reverse recovery and starts blocking the negative supply voltage
...
The
same process repeats there after
...
3
...
2 (a) and (b) shows the circuit diagram and the waveforms of a single phase fully
controlled halfwave rectifier supplying a resistive inductive load
...


As in the case of a resistive load, the thyristor T becomes forward biased when the supply
voltage becomes positive at t = 0
...
As the thyristor turns ON at t = the input voltage appears across the load and the load current starts
building up
...
Finally, at t = ( > ) the load current becomes zero and the
thyristor undergoes reverse recovery
...

It is to be noted that the value of depends on the load parameters
...
Since the thyristors
does not conduct over the entire input supply cycle this mode of operation is called the
"discontinuous conduction mode"
...
4 Single phase fully controlled bridge converter

Fig 10
...
It is
one of the most popular converter circuits and is widely used in the speed control of
separately excited dc machines
...

The single phase fully controlled bridge converter is obtained by replacing all the
diode
of the corresponding uncontrolled converter by thyristors
...
From the circuit diagram of Fig 10
...
It can also be argued that neither
T1T3 nor T2T4 can conduct simultaneously
...
Similar argument holds
for
T1 and T2
...
Therefore, the
only possible conduction modes when the current i0 can flow are T1T2 and T3T4
...
This situation will typically
occur when the load current becomes zero in between the firings of T1T2 and T3T4
...
In this mode the load current remains
zero
...


DEPARTMENT OF EEE – SVECW

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Fig 10
...
It is to be noted that whenever T1 and T2 conducts, the
voltage
across T3 and T4 becomes -vi
...
e,
over the negative half cycle of the input supply voltage
...
The voltage across the devices when none of the
thyristors conduct depends on the off state impedance of each device
...
3 (b) assume identical devices
...
If i0 is always greater than zero
then
the converter is said to be operating in the continuous conduction mode
...

However, in the discontinuous conduction mode none of the thyristors conduct over some
portion of the input cycle
...

4
...
Fig 10
...
The firing angle of the converter is
...
As T1T2 are fired at t = they turn on
commutating T3T4 immediately
...
Till this point T1T2
conducts
...
4
...
However, using actual
expression for ii will lead to exceedingly complex calculation
...
This will be justified provided the load is
highly inductive and the ripple on i0 is negligible compared to I0
...
5
...

Evidently the input current displacement factor defined as the cosine of the angle
between input voltage (vi) and the fundamental component of input current (ii1) waveforms
is
cos (lagging)
...
Larger
the ' ' poorer is the power factor
...
Exact composition of the harmonic
currents
can be obtained by Fourier series analysis of ii and is left as an exercise
...

i)

A single phase fully controlled bridge converter can operate either in the _________ or
________ conduction mode
...


iii)

In the continuous conduction mode the output voltage waveform does not depend on the
________ parameters
...


v)

The input displacement factor of a single phase fully controlled bridge converter in the
continuous conduction mode is equal to the cosine of the ________ angle
...


4
...
In figure 10
...
Therefore i0 continues to
decrease till a new pair of thyristor is fired at t = +
...
Obviously then,
at the boundary between continuous and discontinuous conduction the minimum value of i 0
which occurs at t = and t = + will be zero
...
26) we obtain the
condition for continuous conduction as
...
6 shows waveforms of different variables on the boundary between continuous and
discontinuous conduction modes and in the discontinuous conduction mode
...
Therefore, all the analysis of continuous conduction mode applies to
this case as
well
...

During
this interval none of the thyristors conduct
...
6(b)
...
The load current continues to increase till t = -
...
Since the
thyristors cannot conduct current in the reverse direction i0 remains at zero till t = + when
T3 and T4 are fired
...
During this
period v0 attains the value E
...
1 Introduction
The three phase fully controlled bridge converter has been probably the most widely used power
electronic converter in the medium to high power applications
...
The controlled rectifier can provide controllable out put
dc voltage in a single unit instead of a three phase autotransformer and a diode bridge rectifier
...
Control over the output dc voltage is obtained by controlling the conduction interval
of each thyristor
...
Since thyristors can block voltage in both directions it is possible to
reverse the polarity of the output dc voltage and hence feed power back to the ac supply from the
dc side
...
The
thyristors in the converter circuit are commutated with the help of the supply voltage in the
rectifying mode of operation and are known as “Line commutated converter”
...
for commutation and are
referred to as the “Load commutated inverter”
...
Of course the
magnitude of harmonic voltage is lower in three phase converter compared to the single phase
circuit
...
Input current wave shape become rectangular and contain 5th and higher
order odd harmonics
...

The frequency of the harmonic voltage and current can be increased by increasing the pulse
number of the converter which can be achieved by series and parallel connection of basic 6 pulse
converters
...

With the introduction of high power IGBTs the three phase bridge converter has all but been
replaced by dc link voltage source converters in the medium to moderately high power range
...
) the basic
B phase bridge converter block is still used
...

5
...
13
...
It can be argued as in the case of an
uncontrolled converter only one device from these two groups will conduct
...
Now the thyristors are fired in the sequence T1 → T2 → T3 → T4 → T5 → T6 → T1
with 60° interval between each firing
...
This leaves only six possible
conduction mode for the converter in the continuous conduction mode of operation
...
Each conduction mode is of 60° duration and appears in the
sequence mentioned
...
13
...
The phasor diagram of the line
voltages appear in Fig
...
1 (c)
...
For example the thyristor T1 is fired at the end
of T5 T6 conduction interval
...
Therefore T1 is
fired α angle after the positive going zero crossing of vac
...
The phasor diagram of Fig
...
1 (c) also confirms that all the thyristors are fired
in the correct sequence with 60° interval between each firing
...
13
...
13
...
To arrive at the
waveforms it is necessary to draw the conduction diagram which shows the interval of
conduction for each thyristor and can be drawn with the help of the phasor diagram of fig
...
1
(c)
...
Once the conduction diagram is
drawn all other voltage waveforms can be drawn from the line voltage waveforms and from the
conduction table of fig
...
1 (b)
...
It is clear from the waveforms that output voltage and current
waveforms are periodic over one sixth of the input cycle
...
The input current on the other hand contains only odds harmonics of
the input frequency other than the triplex (3rd, 9th etc
...
The next section will analyze
the operation of this converter in more details
...
3Analysis of the converter in the rectifier mode
The output voltage waveform can be written as
The input phase current ia is expressed as
α ≤ ω t ≤ α+ π
3
2π
4π
α+
≤ ω t ≤ α+
3
3
5π
α+
≤ ω t ≤ α+ 2π
3
otherwise

ia = i 0

ia = - i0

ia = i 0
ia = 0

From Fig
...
2 it can be observed that i0 itself has a ripple at a frequency six times the input
frequency
...

However, considerable simplification in the expression of ia can be obtained if i0 is replaced by
its average value I0
...
e, the
load is highly inductive
...

π
, Now if θ ≤ α+

i

then i0 is minimum at ωt = α
...
However discontinuous conduction is rare in these

conversions and will not be discussed any further
...
4 Analysis of the converter in the inverting mode
...
It follows from equation
13
...
This is the rectifier mode of operation of the converter
...
The converter in
that case is said to be operating in the inverter mode
...
13
...
Fig
...
3 shows the circuit connection and wave
forms in the inverting mode of operation where the load current has been assumed to be
continuous and ripple free
...
The
same expressions hold for the dc and harmonic compounds in the output voltage and current
...
8
...
24)

(13
...
13
...
Therefore, power in the ac side flows from the converter to the
source
...
13
...
13
...
For successful
commutation of the outgoing thyristor it is essential that this interval is larger than the turn off
time of the thyristor i
...


Which imposes an upper limit on the value of α
...

Exercise 13
...
The battery bank
has an internal resistance of 0
...
Assuming continuous
conduction find out
...

The range of ac input power factor
...


DEPARTMENT OF EEE – SVECW

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When the battery bank is charged with a constant average charging current of 100 Amps through
a 250 mH lossless inductor
...
9 × VB Nom = 216 volts
and VB Max = 1
...

Since the average charging current is constant at 100 A
...
01 = 265 volts
V0 Min = VB Min + 100 × RB = 216 + 100 × 0
...


2
...

The motor has an armature resistance of 0
...
What should be the transformer turns
ratio such that the converter produces rated motor terminal voltage at 0º firing angle
...
The same converter is now used to brake the motor
regeneratively in the reverse direction
...

Answer: From the given question
3 2
∴ VL = 162
...

Primary side phase voltage =

230

3

V = 132
...
799 =1:1
...

During regenerative braking in the reverse direction the converter operates in the
inverting mode
...
8o
α Max = 180 – β Min = 178
...
2o = - 219
...
89 V
...
02 = 210 V
...
89 ×600 = 656
...

210

5
...
However in very large power applications (such as HV DC transmission
systems) the device ratings become impractically large
...
Therefore several such converters are connected in series parallel
combination in order to increase the voltage / current rating of the resulting converter
...


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Fig
...
4(a) schematically represents series connection of two six pulse converters where as Fig
...
4(b) can be considered to be a parallel connection
...
In both these figures CONV – I and
CONV – II have identical construction and are also fired at the same firing angle α
...
Then one can write

Now if cos 3Kφ = 0 for some K then the corresponding harmonic disappear from the fourier
series expression of v0
...

This phase difference can be obtained by the arrangement shown in Fig
...
4(c)
...

Similarly it can be shown that the input side line current iABC have harmonic frequency of the
form
11ω, 13ω, 23ω, 25ω, 35ω, 37ω, …………
...

In a similar manner more number of 3 phase 6 pulse converters can be connected in series /
parallel and the φ angle can be adjusted to obtain 18 and 24 pulse converters
...
However,
some applications such as a four quadrant dc motor drive require this capability from the dc
source
...
13
...
In this figure converter -I supplies positive
load current while converter-II supplies negative load current
...
Thus the two converters taken together can operate in all four
quadrants and is capable of supplying a four quadrant dc motor drive
...


DEPARTMENT OF EEE – SVECW

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DEPARTMENT OF EEE – SVECW

Page 82

Obviously since converter-I and converter-II are connected in antiparallel they must produce the
same dc voltage
...
30)

Although Equations 13
...
Therefore to avoid a direct short
circuit between two different supply lines the two converters must never be gated
simultaneously
...
Gate pulses to
converter-II are blocked at that time
...
Thus there is no circulating current flowing through
the converters and therefore it is called the non-circulating current type dual converter
...
To overcome this problem an
interphase reactor may be incorporated between the two converters
...
The resulting converter
is called the circulating current type dual converter
...
4 Gate Drive circuit for three phase fully controlled converter
Several schemes exist to generate gate drive pulses for single phase or three phase converters
...
This can be achieved as follows
...
32
...
13
...

The phasor diagram of the phase shift circuit is shown in Fig
...
6(b)
...
The firing pulse is generated at the point when
these two waveforms are equal
...

The output of the phase shift network is called carrier waveform
...
However the phase shift network here
consists of a three phase signal transformer with special connections as shown in Fig
...
7
...
The primary windings are connected in delta while the secondary windings are connected
in zigzag
...
13
...

Therefore, to implement inverse cosine the carrier wave for T2 must lead vbc by 90º
...
13
...

The same figure also shows the zigzag connection for other phase
...
13
...
The phase shift network will not be required in this case
...
3
1
...

ii) Constituent six pulse converters of a 12 pulse converter have _________ firing angles
...

iv) The input supply to a 12 pulse converter can be obtained through a _________
connected transformer
...

vi) In a dual converter if one converter is fired at an angle ‘α’ the other has to be fired at
_________
...

viii) In a circulating current type dual converter an __________ is used between the
converters to limit the circulating current
...

x) In a three phase fully controlled converter the carrier waves for firing pulse generation
are obtained using three ___________ connected single phase transformers
...


3
...

Answer: With delta-double star connection of the signal transformers the carrier wave forms will
be in phase with the line voltage waveforms
...
Hence inverse casine control law cannot be implemented
...


•

A three phase fully controlled converter can operate either as a rectifier or as an inverter
...

•

The input current of a three phase fully controlled converter contains only odd harmonics
other than tripler harmonics
...
α
being the firing angle
...


•

In the inverting mode the firing angle should be less than 180º for safe commutation of the
thyristors
...


•

In higher pulse number converters all component converters are fired at the same firing
angle while their input supplies are phase shifted from one another by a predetermined
angle
...


•

Dual converters can be of circulating and non circulating current type
...
In a three phase fully controlled converter, a three phase delta/zig-zag
connected signal transformer is used to generate the required carrier waves for this
purpose
...
1Cycloconverters
In Figs
...
2 and 3
...
In the first case there
are two three-phase midpoint controlled rectifiers connected back to back
...
Both are used for three-phase to
three-phase conversion
...
3
...
1 Classification of frequency converters
...
2 Cycloconverter scheme with three-phase midpoint controlled rectifier
...
3 Cycloconverter scheme with three-phase bridge controlled rectifier
...
The output voltage Va and current ia have Va1 and ia1 fundamental
components with φ1 phase displacement and numerous harmonics
...
The firing angles are
αP and αN for the p and n converters, respectively
...
During this period the other converter is blocked
...

It is possible to operate without blocking the converters
...
However, additional inductances are
necessary to limit the circulating currents between two converters since the instantaneous voltages
of the two converters differ from one another
...
4
converters
...
time for cycloconverter with three-phase bridge

Nine
independent
bidirectional
switches
a
R

(60
HZ)

Load
(variable

S
b

voltage
and
variable
frequenc
y)

T
c

FIGURE 3
...


DEPARTMENT OF EEE – SVECW

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The phase control of the p and n converters is modulated by a sine or trapezoidal wave
...
During every cycle of the output
voltage both of the converters must work as rectifiers and inverters
...
If the frequency reaches the well-defined value the current harmonics
become unacceptable
...
3
...
3
...

The cycloconverter is usually used for three-phase, high-power, low-speed synchronous motor
drives and rarely employed for induction motor drives
...
2 AC VOLTAGE CONTROLLERS

6
...
Portions of the supply sinusoid appear at the load while the semiconductor switches
block the remaining portions
...


Fig 26
...
(a) Back-to -back
SCR; (b) One SCR in (a) replaced by a four-diode full wave diode bridge; (c) A
bi-directionally conducting TRIAC; (d) The SCR in (b) replaced by a transistor
...
1 (a), (b) and (c) perform quite similarly
...
The TRIAC based converter
may be considered as the basic topology
...
However, dv dtre −applied their ratings being poor, they tend
to turn-on in the opposite direction just subsequent to their turn-off with an inductive load
...
The TRIAC is common
only at the low power ranges
...

A transistorised AC-AC regulator is a PWM regulator similar to the DC-DC converters
...
Consequently, only controlled freewheeling devices can be used
...
2 Operation with resistive loads
Fig
...
2 illustrates the operation of the PAC converter with a resistive load
...
The current follows the voltage wave shape in each
half and extinguishes itself at the zero crossings of the supply voltage
...
There is no scope for conduction
overlap of the devices
...
In the diode-SCR topology, two diodes are forward biased in each half
...
It is thus
always forward biased
...


DEPARTMENT OF EEE – SVECW

Page 93

Fig
...
2 Operation of a Phase Angle Controlled AC-AC converter with a resistive load

Fig
...
3 The rms output voltage and the most important harmonics versus triggering
angle α
...
This is one of the main reasons why such controllers are today not
acceptable
...
2 is half wave symmetric
...
The controller in Fig
...
4 ensures this for the TRIAC based
circuit
...
This ensures elimination of DC and even components in the output voltage
...
26
...

For the SCR based controllers, identical comparators for the two halves of the AC supply, which
generates pulses for the two SCRs ensures DC and even harmonic free operation
...
However, in
resistance heating type of application all harmonics are of no consequence
...

6
...
Fig
...
2 shows the supply
voltage and the non-sinusoidal load current
...
Cosφ1 is also called the 'Displacement
Factor'
...
This
power factor is inspite of the actual load being resistive! The reactive power is drawn also y the
trigger-angle dependent harmonics
...
26
...
26
...
The rms
load voltage can also be similarly obtained by integrating between α and π and the result can be
combined with Eq
...
5 to give
power
factor = per −unit
rms
load − current
per −unit

=
=

B

load

power

p
...


1

Fig
...
5 Variation of various performance parameters with triggering angle

DEPARTMENT OF EEE – SVECW

Page 95

6
...
5
...
It quenches not at the zero crossing of the applied voltage as with the
resistive load but after that instant
...
A single-pulse trigger for the TRIAC 26
...
The devices would fail to conduct when they are intended to,
as they do not have the supply voltage forward biasing them when the trigger pulse arrives
...
A train of pulses is required here
...

The load current waveform is further explained in Fig
...
6
...
The first is the steady state component of the load current, iss and the second, itr is
the transient component
...
26
...
7 Load current for a single phase AC-AC converter with a R_L load
...


DEPARTMENT OF EEE – SVECW

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With an inductance in the load the distinguishing feature of the load current is that it must
always start from zero
...
This current restricted to the half periods of conduction is called
the 'steady-state component' of load current iss
...
This condition sets the
initial value of the transient component to that of the steady state at the instant that the
SCR/TRIAC is triggered
...
26
...

When a device is in conduction, the load current is governed by the equation
L
i

di dt + Ri =v s

load

2V [ sin (ω t −φ)+ sin (α −φ)e − RL (α ω −t ) ]
Z
Since at t = 0, iload = 0 and supply voltage vs = √2Vsinωt the solution is of the form
=

The instant when the load current extinguishes is called the extinction angle β
...
The load current I that case is perfectly sinusoidal
...
5 AC-AC Chopper

Fig
...
8 A complete Transitorised AC-AC chopper topology of the version shown in
Fig
...
1 and the corresponding load voltage and current waveforms for an inductive
load
...
5 Duty Ratio chopping
...
1 has to be augmented with two additional controlled
devices clamping the load as indicated in Fig
...
7
...
These devices which are mostly transistors of the same variety as used for
the chopper are necessary to clamp the voltages generated by the switching-off of the current
carrying inductors in the load while the input capacitor takes care of the line inductances
...
Mostly switching frequency harmonics are present in both the waveforms
...
5 PAC as a static switch
Both single phase and three phase PACs are often used as static switches for applications like
switching on of highly inductive loads without transients or for regulating output AC voltages by
switching in tapings of a transformer
...
Sequence control can
be two or multiple phase depending upon the application
...
26
...
The outer TRIACs connected to thwe higher voltage leads of
the input transformer are triffered at the desired angle α, to realize the required load voltage
...
This device continues conduction into the next half of the supply voltage till the
load current falls to zero
...
TR1 can be however triggered by a single pulse
...
26
...
At what trigger angle will the operation be free from
transients?
A1

For the transformer load

φL ≈ 90ο

Therefore for transient free operation α = 90ο
Q2 For the load described in Q1, the PAC is triggered by a single pulse at α = 60ο
...


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A2 Since α < φL, the load current should have been continuous
...
For this load which can be
considered to be highly inductive β ≈ 360ο, say ≈ 360ο
...
The anti-parallel SCR is triggered at α = 60ο corresponding to a β ≈ 180 + 60 = 240ο when
it is still reverse biased
...
The load thus sees only a unipolar current
...
A2
...


Fig
...
A2 The load current waveform and its steady-state and transient components
when a highly inductive load is switched using single narrow trigger pulses

---