Industrial Electronics – Question Question Bank with Answers
Unit – Unit – 1 1 Power Semiconductor devices PART- A (2 MARKS) QUESTIONS & ANSWERS 1. Define Reverse recovery time of POWER DIODE? The reverse recovery time is defined as the time between the instant forward diode current becomes zero and the instant reverse recovery current decays to 25 % of its reverse peak value. 2. Compare power MOSFET with BJT. a. Power MOSFET has lower lower switching losses losses but BJT has has higher switching switching losses. losses. b. Power MOSFET has more more on state resistance and conduction losses but but BJT has lower losses c. MOSFET is voltage controlled device whereas BJT is current controlled device. d. MOSFET has positive temperature coefficient device whereas BJT is current controlled device. 3. Applications of IGBT Medium power applications such as dc & ac motor drives, UPS systems power supplies and drive for solenoids, relays and contractors. 4. Different methods to turn on the thyristors ? GATE TRIGGERING , dv/dt triggering, Temperature triggering and Light triggering. 5. Define Latching Current. The minimum value of anode current current which it it must maintain conduction when gate signal is removed.
attain during turn-on turn-o n process to
6. What is forced Commutation? In some thyristor thyristor circuit, the input voltage voltage is dc and the forward forward current of the thyristor is forced to zero by an additional circuitry commutation circuit to turn-off the thyristor. This technique is called forced commutation . 7. Define snubber circuit A subber circuit circuit consists of a series combination of resistance resistance Rs and Cs in parallel with the thyristor. It is mainly used for dv/dt protection. 8. Define circuit turn off time Circuit turn off time is is defined as the tine between the instant anode current current beco mes zero and the instant instant reverse voltage due to practical practical circuit reaches zero. 9. What is meant by secondary breakdown ? The secondary breakdown is destructive phenomenon, results from the current flow to a small portion of the base producing localised hotspots.. If the energy in these hot spots is sufficient, the excessive excessive localized heating heating ay damage damage the transistor. Thus seccondary breakdown is caused by a localised thermal runway, resulting from high current concentrations. 10. What are the advantages of TRIAC? Triac can be triggered with +ve / -ve polarity voltages.
A Triac needs a single fuse protection, prot ection, which also simplifies the construction. A Triac needs a single heat sink of slightly larger size whereas antiparallel thyristor pair needs two heat sinks. Part B 1. Explain the Construction & Working Principle of Power MOSFET The Power MOSFET technology has mostly reached maturity and is the most popular device for SMPS, lighting ballast type of application where high switching frequencies are desired but operating voltages are low. Being a voltage fed, majority carrier device (resistive behaviour) behaviour) with a typically rectangular Safe Operating Area, it can be conveniently utilized. Utilising shared manufacturing processes, comparative costs of MOSFETs are attractive. For low frequency applications, where the currents drawn by the equivalent capacitances across its terminals are small, it can also be driven directly by integrated circuits. These capacitances are the main hindrance to operating the MOSFETS at speeds of several MHz. The resistive characteristics of its main terminals permit easy paralleling externally also. At high current low voltage applications the MOSFET offers best conduction voltage specifications as the R specification is current rating dependent. However, the inferior features of the DS(ON)
inherent anti-parallel diode and its higher conduction losses at power frequencies and voltage levels restrict its wider application.
As mentioned in the introduction section, Power MOSFET is a device that evolved from MOS integrated circuit technology. The first attempts to develop high voltage MOSFETs were by redesigning lateral MOSFET to increase their voltage blocking capacity. The resulting technology technology was called lateral double deffused MOS (DMOS). However However it was soon s oon realized that much larger breakdown voltage and current ratings could be achieved by resorting to a vertically oriented structure. Since then, vertical DMOS (VDMOS) structure has been adapted by virtually all manufacturers of Power MOSFET. A power MOSFET using VDMOS technology has vertically oriented three layer structure of alternating p type and n type semiconductors as shown in Fig 6.2 (a) which is the schematic representation of a single MOSFET cell structure. A large number of such cells are connected in parallel (as +
shown in Fig 6.2 (b)) to form a complete device. The two n end layers labeled “Source” and “Drain” are heavily doped to approximately the same level. The Th e p type middle layer is termed the body (or substrate) and has moderate doping level (2 to 3 orders of magnitude lower than +
-
n regions on both sides). The n drain drift region has the lowest doping density. Thickness of this region determines the breakdown voltage of the device. The gate terminal is placed -
over the n and p type regions of the cell structure and is insulated from the semiconductor body be a thin layer of silicon s ilicon dioxide (also called the gate oxide). The source and the drain region of all cells on a wafer are connected to the same metallic contacts to form the Source and the Drain terminals of the complete device. Similarly all gate terminals are also connected together. The source is constructed of many (thousands) small polygon shaped areas that are surrounded by the gate regions. The geometric shape of the source regions, to same extent, influences the ON state resistance of the MOSFET.
Operating principle of a MOSFET At first glance it would appear that there is no path for any current to flow between the source and the drain terminals since at least one of the p n junctions (source – (source – body body and body-Drain) will be reverse biased for either polarity of the applied voltage between the source and the drain. There is no possibility of current injection from the gate terminal either since the gate oxide is a very good insulator. However, application of a positive voltage at the gate terminal with respect to the source will covert the silicon surface beneath the gate oxide into an n type layer or “channel”, thus connecting the Source to the Drain as explained next. The gate region of a MOSFET which is composed of the gate metallization, the gate (silicon) oxide layer and the p-body silicon forms a high quality capacitor. When a small voltage is application to this capacitor structure with gate terminal positive with respect to the source (note that body and source are shorted) a depletion region forms at the interface between the SiO and the silicon as shown in Fig 6.4 (a). 2
The positive charge induced on the gate metallization repels the majority hole carriers from the interface region between the gate oxide and the p type body. This exposes the negatively charged acceptors and a depletion region is created. Further increase in V causes the depletion layer to grow in thickness. At the same time the electric GS
field at the oxide-silicon interface gets larger and begins to attract free electrons as shown in Fig 6.4 (b). The immediate source of electron is electron-hole generation by thermal ionization. The holes are repelled into the semiconductor bulk ahead of the depletion region. The extra holes are neutralized by electrons from the source. As V increases further the density of free electrons at the interface becomes equal to the free hole GS
density in the bulk of the body region beyond the depletion layer. The layer of free electrons at the interface is called the inversion layer and is shown in Fig 6.4 (c). The inversion layer has all the properties of an n type semiconductor and is a conductive path or “channel” between the drain and the source which permits flow of current between the drain and the source. Since current conduction in this device takes place through an n- type “channel” created by the electric field due to gate source voltage it is called “Enhancement type n -channel MOSFET”. The value of V at which the inversion layer is considered to have formed is is called the “Gate GS
– Source Source threshold voltage V
GS
(th)”. As V
GS
is increased beyond V (th) the inversion layer GS
gets some what thicker and more conductive, since the density of free electrons increases
further with increase in V . The inversion layer screens the depletion layer adjacent to it GS
from increasing V . The depletion layer thickness now remains constant. GS
2. Explain the Construction & Working Principle of Power TRIAC. Discuss its four modes of operations Fig. 4.12 (a) and (b) show the circuit symbol and schematic cross section of a triac respective. As the Triac can conduct in both the directions the terms “anode” and “cathode” are not used for Triacs. The three terminals are marked as MT (Main Terminal 1), MT 1
2
(Main Terminal 2) and the gate by G. As shown in Fig 4.12 (b) the gate terminal is near MT
1
and is connected to both N and P regions by metallic contact. Similarly MT is connected to 3
2
1
N and P regions while MT is connected to N and P regions. 2
2
2
4
1
Since a Triac is a bidirectional device and can have its terminals at various combinations of positive and negative voltages, there are four possible electrode potential combinations as given below 1. MT positive positive with respect to MT , G positive with respect to MT 2
1
1
2. MT positive positive with respect to MT , G negative with respect to MT 2
1
1
3. MT negative with respect to MT , G negative with respect to MT 2
1
4. MT negative with respect to MT , G positive with respect to MT 2
1
1
1
The triggering sensitivity is highest with the combinations 1 and 3 and are generally used. However, for bidirectional control and uniforms gate trigger mode sometimes trigger modes 2 and 3 are used. Trigger mode 4 is usually averded. Fig 4.13 (a) and (b) explain the conduction mechanism of a triac in trigger modes 1 & 3 respectively.
In trigger mode-1 the gate current flows mainly through the P N junction junction like an ordinary thyristor. 2
2
When the gate current has injected sufficient charge into P layer the triac starts conducting through 2
the P N P N layers like an ordinary thyristor. 1
1
2
2
In the trigger mode-3 the gate current I forward biases the P P junction and a nd a large number of g
2
3
electrons are introduced in the P region by N . Finally the structure P N P N turns on completely. 2
3
2
1
1
4
From a functional point of view a triac is similar to two thyristors connected in anti parallel. st
rd
Therefore, it is expected that the V-I characteristics of Triac in the 1 and 3 quadrant of the V-I plane will w ill be similar to the forward characteristics characteristics of a thyristors. As shown in Fig. 4.14, with no signal to the gate the triac will block both half cycle of the applied ac voltage provided its peak value is lower than the break over voltage (V ) of the device. However, the turning on of the triac can be BO
controlled by applying the gate trigger pulse at the desired instance. Mode-1 triggering is used in the first quadrant where as Mode-3 triggering is used in the third quadrant. As such, most of the thyristor characteristics apply to the triac (ie, latching and holding current). However, in a triac the two conducting paths (from MT to MT or from MT to MT ) interact with each other in the structure of 1
2
1
1
the triac. Therefore, the voltage, current and frequency ratings of triacs are considerably lower than thyristors. At present triacs with voltage and current ratings of 1200V and 300A (rms) are available. Triacs also have a larger on state voltage drop compared to a thyristor. Manufacturers usually specify characteristics characteristics curves relating r elating rms device current and maximum maximum allowable case temperature as shown in Fig 4.15. Curves relating the device dissipation and RMS on state current are also provided for different conduction angles.
3. Explain the Construction & Working Principle of IGBT
The introduction of Power MOSFET was originally regarded as a major threat to the power bipolar transistor. However, initial claims of infinite infinite current gain for the power MOSFETs were diluted by the need to design the gate drive circuit capable of supplying the charging and discharging current of the device input capacitance. This is especially true in high frequency circuits where the power MOSFET is particularly valuable due to its inherently high switching speed. On the other hand, MOSFETs have a higher on state resistance per unit area and consequently higher higher on state loss. This is particularly true for higher voltage devices (greater than about 500 volts) which restricted the use of MOSFETs to low voltage high frequency circuits (eg. SMPS).
Constructional Constructional Features of an IGBT Vertical cross section of a n channel IGBT cell is shown in Fig 7.1. Although p channel IGBTs are possible n channel devices are more common and will be the one discussed in this lesson.
The i-v characteristics of an n channel IGBT is shown in Fig 7.4 (a). They appear qualitatively similar to those of a logic level BJT except that the controlling parameter is not a base current but the gate-emitter voltage. When the gate emitter voltage is below the threshold voltage only a very small leakage current flows though the device while the collector – emitter voltage almost equals the supply voltage (point C in Fig 7.4(a)). The device, under this condition is said to be operating in the cut off region. The maximum forward voltage the device can withstand in this mode (marked V in Fig CES
7.4 (a)) is determined by the avalanche break down voltage of the body – drain p-n junction. junction. Unlike a BJT, however, this break down voltage is independent of the collector current as shown in Fig 7.4(a). IGBTs of Non-punch through design can block a maximum reverse voltage (V ) RM
equal to V
CES
in the cut off mode. However, for Punch Through IGBTs V
RM
is negligible (only a
few tens of volts) due the presence of the heavily doped n+ drain buffer layer. layer. As the gate emitter voltage increases beyond the threshold voltage the IGBT enters into the active region of operation. In this mode, the collector current i is determined by the transfer c
characteristics of the device as shown in Fig 7.4(b). This characteristic is qualitatively similar to
that of a power MOSFET and is reasonably linear over most of the collector current range. The ratio of i to (V – v ) is called the forward transconductance (g ) of the device and is an c
gE
gE(th)
fs
important parameter in the gate drive circuit design. The collector emitter voltage, on the other hand, is determined by the external load line ABC as shown in Fig 7.4(a). The switching waveforms of an IGBT is, in many respects, similar to that of a Power MOSFET. This is expected, since the input stage of an IGBT is a MOSFET as shown in Fig 7.5(b). Also in a modern IGBT a major portion of the total device current flows through the MOSFET. Therefore, the switching voltage and current waveforms exhibit a strong similarity with those of a MOSFET. However, the output p-n-p transistor does have a significant effect on the switching characteristics of the device, particularly during turn off. Another important difference is in the gate drive requirement. To avoid dynamic latch up, (to be discussed later) the gate emitter voltage of an IGBT is maintained at a negative value when the device is off.
4. Explain the Construction & Wor Working king Principle Principle of SCR
-
As shown in Fig 4.1 (b) the primary crystal is of lightly doped n type on either side of which two p type layers with doping levels higher by two orders of magnitude are grown. As in the case of power -
diodes and transistors depletion layer spreads mainly into the lightly doped n region. The thickness of this layer is therefore determined by the required blocking voltage of the device. However, due to conductivity modulation by carriers from the heavily doped p regions on both side during ON +
condition the “ON state” voltage drop is less. The outer n layers are formed with doping levels +
higher then both the p type layers. The top p layer acls as the “Anode” terminal while the bottom n layers acts as the “Cathode”. The “Gate” terminal connections are made to the bottom p layer. As it will be shown later, that for better switching performance it is required to maximize the peripheral contact area of the gate and the cathode c athode regions. Therefore, the cathode regions are finely distributed between gate contacts of the p type layer. An “Involute” structure for both the gate and the cathode regions is a preferred design structure. The circuit symbol in the left hand side inset defines the polarity conventions of the variables used in this figure. With ig = 0, V has to increase up to forward break over voltage V before significant anode AK
BRF
current starts flowing. However, at V
BRF
forward break over takes place and the voltage across the
thyristor drops to V (holding voltage). Beyond this point voltage across the thyristor (V H
) remains
AK
almost constant at V (1-1.5v) while the anode current is determined by the external load. H
The magnitude of gate current has a very strong effect on the value of the break over voltage as shown in the figure. The right hand side figure in the inset shows a typical plot of the forward break over voltage (V ) as a function of the gate current (I ) BRF
g
After “Turn ON” the thyristor is no more affected by the gate current. Hence, any current pulse (of required magnitude) which is longer than the minimum needed for “Turn ON” is sufficient to effect control. The minimum gate pulse width is decided by the external circuit and should be long enough to allow the anode current to rise above the latching current (I ) level. L
The left hand side of Fig 4.3 shows the reverse i-v characteristics of the thyristor. Once the thyristor is ON the only way to turn it OFF is by bringing the thyristor current below holding current (I ). The H
gate terminal has no control over the turn OFF process. In ac circuits with resistive load this happens automatically dur ing ing negative zero crossing of the supply voltage. This is called “natural commutation” or “line commutation”. However, in dc circuits some arrangement has to be made to ensure this condition. This process pr ocess is called “forced “ forced commutation.” During reverse blocking if i = 0 then only reverse saturation current (I ) flows until the reverse g
voltage reaches reverse break down voltage (V
s
). At this point current starts rising sharply. Large
BRR
reverse voltage and current generates excessive heat and destroys the device. If i > 0 during reverse g
bias condition the reverse saturation current rises as explained explained in the t he previous section. s ection. This can be avoided by removing the gate current while the thyristor is reverse biased.
Switching Characteristics of a Thyristor During Turn on and Turn off process a thyristor is subjected to different voltages across it and different currents through it. The time variations of the voltage across a thyristor and the current through it during Turn on and Turn off constitute the switching characteristics of a thyristor.
Turn on Switching Characteristics A forward biased thyristor is turned on by applying a positive gate voltage between the gate and cathode as shown in Fig 4.10.
Fig 4.10 shows the waveforms of the gate current (i ), anode current (i ) and anode cathode voltage g
A
(V ) in an expanded time scale during Turn on. The reference circuit and the associated waveforms AK
are shown in the inset. The total switching period being much smaller compared to the cycle time, i
A
and V before and after switching switching will appear appear flat. AK
As shown in Fig 4.10 there is a transition time “t
ON
” from forward off state to forward on state. This
transition time is called the thyristor turn of time and can be divided into three separate intervals namely, (i) delay time (t ) (ii) rise time (t ) and (iii) spread time (t ). These times are shown in Fig d
r
p
4.10 for a resistive load.
Turn off Switching Characteristics Once the thyristor is on, and its anode current is above the latching current level the gate loses control. It can be turned off only by reducing the anode current below holding current. The turn off time t of a thyristor is defined as the time between the instant anode current becomes zero and the q
instant the thyristor regains forward blocking capability. If forward voltage is applied across the device during this period the thyristor turns on again. During turn off time, excess minority carriers from all the four layers of the thyristor must be removed. Accordingly t is divided in to two intervals, the reverse recovery time (t ) and the gate q
rr
recovery time (t ). Fig 4.11 shows the variation of anode current and anode cathode voltage with qr
time during turn off operation on an expanded scale.
5. Discuss the types of Power Power DIODES. Mention its rating, applications As mention in the introduction Power Diodes of largest power rating are required to conduct several kilo amps of current in the forward direction with very little power loss while blocking several kilo volts in the reverse direction. Large blocking voltage requires wide depletion layer in order to restrict the maximum electric field strength below the “impact ionization” level. Space charge density in the depletion layer should also be low in order to yield a wide depletion layer for a given maximum Electric fields strength. These two requirements will be satisfied in a lightly doped p-n junction diode of sufficient width to accommodate accommodate the required depletion layer. -
To arrive at the structure shown in Fig 2.3 (c) a lightly doped n epitaxial layer of specified width (depending on the required break down voltage) and donor atom density (N ) is dD
-3
+
grown on a heavily doped n substrate (N
dK
donor atoms.Cm ) which acts as the cathode. -3
Finally the p-n junction is formed by defusing a heavily doped doped (N
aA
region into the epitaxial layer. This p type region acts as the anode. -3
Impurity atom densities in the heavily doped cathode (N 19
approximately of the same order of magnitude (10
dk
+
acceptor atoms.Cm ) p -3
.Cm ) and anode (N .Cm ) are aA
-3
Cm ) while that of the epitaxial layer 14
(also called the drift region) is lower by several orders of magnitude (N
dD
≈ 10
-3
Cm ). In a
low power diode this drift region is absent. The Implication of introducing this drift region in a power diode is explained next.
In the previous section it was shown how the introduction of a lightly doped drift region in the p-n structure of a diode boosts its blocking voltage capacity. It may appear that this lightly doped drift region will offer high resistance during forward conduction. However, the effective resistance of this region in the ON state is much less than the apparent ohmic resistance calculated on the basis of the geometric size and the thermal equilibrium carrier densities. This is due to substantial injection of +
+
excess carriers from both the p and the n regions in the drift region as explained next. +
-
As the metallurgical p n junction becomes forward biased there will be injection of excess p type -
carrier into the n side. At low level of injections (i.e δ << n ) all excess p type carriers recombine -
p
no
with n type carriers in the n drift region. However at high level of injection (i.e large forward current -
+
density) the excess p type carrier density distribution reaches the n n junction and attracts electron +
-
+
from the n cathode. This leads to electron injection into the drift region across the n n junction with carrier densities densities δ = δ . This mechanism is called “double injection” n
p
Power Diodes take finite time to make transition from reverse bias to forward bias condition (switch ON) and vice versa (sw itch OFF). Behavior of the diode current and voltage during these switching periods are important due to the following reasons. • Severe over voltage / over current may be caused by a diode switching at different points in the circuit using the diode. • Voltage and current exist simultaneously during switching operation of a diode. Therefore, every switching of the diode is associated with some energy loss. At high switching frequency this may contribute significantly to the overall power loss in the diode. It is observed that the forward diode voltage during turn ON may transiently reach a significantly higher value V compared to the steady slate voltage drop at the steady current I . fr
F
In some power converter circuits (e.g voltage source inverter) where a free wheeling diode is used across an asymmetrical blocking power switch (i.e GTO) this transient over voltage may be high enough to destroy the main power switch. V (called forward recovery voltage) is given as a function of the forward di/dt in the fr
manufacturer’s data sheet. Typical values lie within the range of 10-30V. 10 -30V. Forward recovery time (t ) is typically within 10 us. fr
6. Discuss the turn on methods of SCR Fig 4.10 shows the waveforms of the gate current (i ), anode current (i ) and anode cathode g
voltage (V
A
) in an expanded time scale during Turn on. The reference circuit and the
AK
associated waveforms are shown in the inset. The total switching period being much smaller compared to the cycle time, i and V before and after switching switching will appear appear flat. A
AK
As shown in Fig 4.10 there is a transition time “t
” from forward off state to forward on state.
ON
This transition time is called the thyristor turn of time and can be divided into three separate intervals namely, (i) delay time (t ) (ii) rise time (t ) and (iii) spread time (t ). These times are d
r
p
shown in Fig 4.10 for a resistive load. Delay time (t ): After switching on the gate current the thyristor will start to conduct over the d
portion of the cathode which is closest c losest to the gate. g ate. This conducting conducting area starts s tarts spreading at a finite speed until the entire cathode region becomes conductive. Time taken by this process constitute the turn on delay time of a thyristor. It is measured from the instant of application of the gate current to
the instant when the anode current rises to 10% of its final value (or V
AK
falls to 90% of its initial
value). Typical value valu e of “t ” is a few micro seconds. d
Rise time (tr): For a resistive load, “rise time” is the time taken by the anode current to rise from 10% of its final value to 90% of its final value. At the same time the voltage V falls from 90% of AK
its initial value to 10% of its initial value. However, current rise and voltage fall characteristics are strongly influenced by the type of the load. For inductive load the voltage falls faster than the current. While for a capacitive load V falls rapidly in the beginning. However, as the current increases, rate AK
of change of anode voltage substantially decreases. If the anode current rises too fast it tends to remain confined in a small area. This can give rise to local “hot spots” and damage the device. Therefore, it is necessary to limit the rate of rise of the ON state current Adidt by using an inductor in series with the device. Usual values of maximum allowable Adidt is in the range of 20-200 20-200 A/μs. Spread time (tp): It is the time taken by the anode current to rise from 90% of its final value to 100%. During this time conduction spreads over the entire cross section of the cathode of the thyristor. The spreading interval depends on the area of the cathode and on the gate structure of the thyristor.
⎛⎜⎝⎠
Forward voltage trigerring Gate trigerring LASCR dv/dt trigerring di/dt trigerring
*************************
Industrial Electronics – Question Question Bank with Answers Unit – 2 2 Converters PART- A (2 MARKS) QUESTIONS & ANSWERS 1. What is the function of freewheeling diode? When there is no input voltage to main the continuous current flow in the load, a reverse biased diode connected parallel to the load conducts and allows allows the load current to flow continuously, this diode is called freewheeling diode and the concept is called freewheeling. It prevents the output voltage Vo from becoming negative. 2. Advantages of Freewheeling Diodes? Input power factor is improved Load current waveform is improved and thus t hus the load performance is better. 3. What is meant by continuous current operation of thyristor converter ? When a freewheeling diode is connected across the output, load current continuous flows through the load. Whenever the load load voltage tends to go negative, freewheeling freewheeling diode starts to conduct. As a result load current is transferred from thyristor to freewheeling diode. This is called continuous current operation operat ion of thyristor converter. 4. What is meant by commutation angle / overlap angle ? The commutation period, when outgoing outgoing and incoming incoming thyristors are conducting, is is also known as the overlap period. The angular period period both devices share share conduction is known known as the commutation angle / overlap angle. 5. What is meant by ac voltage controller? AC voltage controller controller is a device device which convertes fixed fixed alternating voltage into a variable voltage without change in supply frequency. 6. What are the applications of AC voltage controller ?
Domestic and industrial heating Lighting Control Speed control of single phase & 3phase 3p hase Motors Transformer tag changing 7. What are the advantages & disadvantages of AC voltage controller ? Advantages High efficiency Flexibility Flexibility in control co ntrol Less Maintenance Disadvantage Introduction fo fo Harmonics in the supply supply current and load load voltage waveform particularly at low output voltage levels.
8. What is meant by cyclo converter? It converts converts input input power at one frequency frequency to outpur power at a different different frequency with one stage conversion. Cyclo converter is also known as frequency changer. 9. what are the applications of Cyclo converter ? Induction heating Speed control of high power ac drives Static VAR generation ower supply in air craft and ship boards. 10. List the applications of converters a. Electro-chemical and electro metallurgical plants b. Electroplating and electro printing c. Steel rolling mills, paper mills, textile mills, printing press d. Electric Traction e. HVDC transmission f. Portable hand tool drives g. UPS Part B 1. Explain the working of 3 phase full converter for firing angle 60 or 30 or 90 or 120 degrees draw the relevant output wave forms in graph sheet. A three phase fully controlled converter is obtained by replacing all the six diodes of an uncontrolled uncontrolled converter by six thyristors t hyristors as shown in Fig. 13.1 (a)
For any current to flow in the load at least one device from the top group (T , T , T ) and one from 1
3
5
the bottom group (T , T , T ) must conduct. It can be argued as in the case of an uncontrolled 2
4
6
converter only one device from these two groups will conduct. Then from symmetry consideration it can be argued that each thyristor conducts for 120° of the input cycle. Now the thyristors are fired in the sequence T → T → T → T → T → T → T with 60° 1
2
3
4
5
6
1
interval between each firing. Therefore thyristors on the same phase leg are fired at an interval of 180° and hence can not conduct simultaneously. This leaves only six possible conduction mode for the converter in the continuous conduction mode of operation. These are T T , T T , T T T T , 1
2
2
3
3
4,
4
5
T T , T T . Each conduction mode is of 60° duration and appears in the sequence mentioned. The 5
6
6
1
conduction table table of Fig. F ig. 13.1 (b) shows voltage across different devices devices and the dc output voltage for each conduction interval. The phasor diagram of the line voltages appear in Fig. 13.1 (c). Each of these line voltages can be associated with the firing of a thyristor with the help of the conduction table-1. For example the thyristor T is fired at the end of T T conduction interval. During this period 1
5
6
the voltage across T was v . Therefore T is fired α angle after the positive going zero crossing of 1
ac
1
v . Similar observation can be made about other thyristors. The phasor diagram of Fig. 13.1 (c) also ac
confirms that all the thyristors are fired in the correct sequence with 60° interval between each firing. Fig. 13.2 shows the waveforms of different variables (shown in Fig. 13.1 (a)). 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. 13.1 (c). If the converter firing angle is α each thyristor is fired “α” angle after the positive going zero crossing of the line voltage with which it‟s firing is associated. 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. 13.1 (b). Similarly line currents can be drawn from the output current and the conduction diagram. It is clear from the waveforms that output voltage and current waveforms are periodic over one sixth of the input cycle. Therefore this converter is also called the “six pulse” converter. The input current on rd
th
the other hand contains only odds harmonics of the input frequency other than the triplex (3 , 9 etc.) harmonics. The next section will analyze the operation of this converter in more details.
Write the sequence and draw the output in graph graph sheet - refer class notes
2. Explain the principle of phase angle control
Fig 10.3 (a) shows the circuit diagram of a single phase fully controlled bridge converter. It is one of the most popular converter circuits and is widely used in the speed control of separately excited dc machines. Indeed, the R – L – E load shown in this figure may represent the electrical equivalent circuit of a separately excited dc motor. The single phase fully controlled bridge converter is obtained by replacing all the diode of the corresponding uncontrolled converter by thyristors. Thyristors T and T are fired together while T 1
2
3
and T are fired 180º after T and T . From the circuit diagram of Fig 10.3(a) it is clear that for any 4
1
2
load current to flow at least one thyristor from the top group (T , T ) and one thyristor from the 1
3
bottom group (T , T ) must conduct. It can also be argued that neither T T nor T T can conduct 2
4
1
3
2
4
simultaneously. For example whenever T and T are in the forward blocking state and a gate pulse is 3
4
applied to them, they turn ON and at the same time a negative voltage is applied across T and T 1
2
commutating them immediately. Similar argument holds for T and T . 1
2
For the same reason T T or T T can not conduct simultaneously. Therefore, the only 1
4
2
3
possible conduction modes when the current i can flow are T T and T T . Of coarse it is possible 0
1
2
3
4
that at a given moment none of the thyristors conduct. This situation will typically occur when the load current becomes zero in between the firings of T T and T T . Once the load current becomes 1
2
3
4
zero all thyristors remain off. In this mode the t he load current remains zero. Consequently the converter is said to be operating in the discontinuous conduction mode. Fig 10.3(b) shows the voltage across different devices and the dc output voltage during each of these conduction modes. It is to be noted that whenever T and T conducts, the voltage across T 1
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3
and T becomes – becomes – v . Therefore T and T can be fired only when v is negative i.e, over the negative 4
i
3
4
i
half cycle of the input supply voltage. Similarly T and T can be fired only over the positive half 1
2
cycle of the input supply. The voltage across the devices when none of the thyristors conduct depends on the off state impedance of each device. The values listed in Fig 10.3 (b) assume identical devices. Under normal operating condition of the converter the load current may or may not remain zero over some interval of the input voltage cycle. If i is always greater than zero then the converter is said to 0
be operating operating in the continuous continuous conduction mode. In this mode mode of operation of the converter converter T T and 1
T T conducts for alternate half cycle of the input supply. 3
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3. Explain the operation of dual converters with neat sketch
Although Equations 13.30 ensures that the dc voltages produced by these converters are equal the output voltages do not match on an instantaneous basis. Therefore to avoid a direct short circuit between two different supply lines the two converters must never be gated simultaneously. Converter-I receives gate pulses when the load current is positive. Gate pulses to converter-II are blocked at that time. For negative load current converter-II
thyristors are fired while converter-I gate pulses are blocked. Thus there is no circulating current flowing through the converters and therefore it is called the non-circulating current type dual converter. It requires precise sensing of the zero crossing of the output current which may pose a problem particularly at light load due to possible discontinuous conduction. To overcome this problem an interphase reactor may be incorporated between the two converters. With the interphase reactor in place both the converters can be gated simultaneously with α = π – α – α . The resulting converter is called the circulating current type 2
1
dual converter.
4. Explain the operation of 3 phase bridge converter and derive the output voltage.
5. Explain the principle of operation of Single phase c ycloconvert ycloconverters ers The basic principle of operation of a cyclo-converter is explained with reference to an equivalent circuit shown in Fig. 29.1. Each two-quadrant converter (phase-controlled) is represented as an alternating voltage source, which corresponds to the fundamental voltage component obtained at its output terminals. The diodes connected in series with each voltage source, show the unidirectional conduction of each converter, whose output voltage can be either positive or negative, being a two-quadrant one, but the direction of current is in the direction as shown in the circuit, as only thyristors − unidirectional switching devices, are used in the two converters. Normally, the ripple content in the output voltage is neglected. neg lected. The control principle used in an ideal cyclo-converter is to continuously modulate the firing angles of the individual converters, so that each produces the same sinusoidal (ac) voltage at its output terminals. Thus, the voltages of the two generators (Fig. 29.1) have the same amplitude, frequency and phase, and the voltage of the cyclo-converter is equal to the voltage of either of these generators. It is possible for the mean power to flow either „to‟ or „from‟ the output terminals, and the cyclo-converter cyclo -converter is inherently capable of operation with loads of any phase angle − inductive or capacitive. Because of the uni -directional current carrying property of the individual converters, it is inherent that the positive half-cycle of load current must always be carried by the positive converter, and the negative half-cycle by the negative converter, converter, regardless of the phase of the current with respect to the voltage. voltage. This means that each two-quadrant converter operates both in its rectifying (converting) and in its inverting region during the period of its associated half-cycle of current. The output voltage and current waveforms, illustrating the operation of an ideal cyclo-converter circuit with loads of various displacement angles, are shown in Fig. 29.2. The displacement angle
of the load (current) is (Fig. 29.2a). In this case, each converter carries the load current only, when it operates in its rectifying region, and it remains idle throughout the whole period in which its terminal voltage is in the inverting region of operation. In Fig. 29.2b, the displacement angle of the load is lagging. During the first period of each half-cycle of load current, the associated converter operates in its rectifying region, and delivers power to the load. During the latter period in the half-cycle, the associated converter operates in its inverting region, and under this condition, the load is regenerating power back into the cyclo-converter output terminals, and hence, into the ac system at the input side. These two are illustrative cases only. Any other case, say capacitive load, with the displacement angle as leading, the operation changes with inverting region in the first period of the half-cycle as per displacement angle, and the latter period operating in rectifying region. This is not shown in Fig. 29.2, which can be studied from a standard text book.
6. Explain the operation of single phase AC vo ltage controller with R load
The regulators in Fig 26.1 (a), (b) and (c) perform quite similarly. They are called Phase Angle Controlled (PAC) AC-AC converters or AC-AC choppers. The TRIAC based converter may be considered as the basic topology. Being bi-directionally conducting devices, they act on both polarities of the the applied voltage. voltage. However, reapplieddvdt− their ratings being poor, they tend to tur n-on in the opposite direction just subsequent to their turn-off with an inductive load. The 'Alternistor' was developed with improved features but was not popular. The TRIAC is common only at the low power ranges. The (a) and (b) options options are improvements improvements on (c) mostly regarding current handling and turn-off-able current rating.
A transistorised AC-AC regulator is a PWM regulator similar to the DC-DC converters. It also requires a freewheeling path across the inductive load, which has also got to be bi-directional. Consequently, only controlled freewheeling devices can be used.
Unit Test 3 – Industrial Industrial Electronics – Electronics – Question Question Bank 2 Marks 1. What is meant meant by a dc chopper ? A dc chopper is a high speed static switch used to obtain variable dc voltage from a constant dc voltage. It is also known as dc to dc converter. A chopper can be consider as dc equivalent to an dc transformer with continuously variable turns ratio. Like a transformer it can be used to step up / down a dc voltage source. 2. What is meant meant by step down and step up chopper? The average output voltage Vo is less than the input voltage Vs, ie Vo < Vs this method of chopper is called step step down chopper / buck converter. Average output voltage Vo is greater than input voltage Vs ie. Vo > Vs, this this chopper is called called step up chopper/ boost converter. converter. 3. What is meant by PWM control in dc chopper? In this control method the the on time time Ton is is varied but chopping frequency frequency f is kept constant. The width of the pulse is varied and this type of control is known as PWM. 4. What is TRC and CLC in terms of chopper?
5. Applications of series inverter. The thyristorised series inverters produces an approzimately sinusoidal sinusoidal waveform at a high output frequency, ranging from 200 Hz to 100 KHz. It is is commonly used for fixed fixed output applications such as Ultrasonic generators, Induction Induction heating, Sonar transmitter, Fluorescent Lighting.
6. What are the types of PWM control? Single pulse width modulations Multiple pulse width modulations Sinusoidal pulse width modulations Modified Sinusoidal pulse width modulations 7. Compare CSI and VSI VSI In VSI input voltage is maintained constant. The output voltages does not depend on the load The Magnitude of output current and its waveform depends upon the nature of the load impedance. IT requires feedback diodes. Commutation is complex. CSI Input Current is constant but adjustable. The amplitude of output current does not depend on the load. The magnitude of output voltage voltage and its waveform depends upon the nature of the load impedance. It does not require any feedback diode. Commutation circuit is simple…….. Contains only capacitors. 8. List the applications of Inverter? a. Variable speed ac motor drives. b. Induction Heating c. Aircraft power supplies d. Domestic power supplies e. UPS 9. What are the disadvantages of harmonics present in the inverter system? Harmonics currents will lead to excessive heating in the induction motors. motors. This will reduce the load carrying capacity of the motor 10. What are the methods of voltage control in Inverters? External control of ac output voltage External control of dc input voltage
Part B 1. Explain the working of 3 phase full bridge inverter for 180 conduction and draw the relevant output waveforms in the graph sheet.
Cu rrent source Inverter. 2. Explain the principle and operation of Current
3. Explain the working of step up and step down DC chopper with TRC and CLC control.
4. Explain the different configurations of chopper. Refer class notes for derivation and diagram – Book Book – page page no. 229
5. a. b. c. d.
Explain the voltage control of Inverters using PWM techniques Single-pulse-width modulation Multi-pulse-width modulation Sinusoidal pulse-width modulation. Modified sinusoidal pulse-width modulation
Single Pulse-Width Modulation
There is one pulse per half-cycle, and its width w idth is varied.
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The dominant harmonic is the third. DF increases significantly at a low output voltage.
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The dominant harmonic is the third. DF increases significantly at a low output voltage.
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Multiple-Pulse-Width Modulation
The harmonic content can be reduced by using u sing several pulses in each half-cycle of output voltage.
This type of modulation is also known as uniform-pulse-width modulation (UPWM). 15
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The following figure shows the harmonic profile against the variation of modulation index, and p=5.
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Sinusoidal Pulse-Width 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.
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The rms output voltage is: p
V o V s ( m 1
m
)1 / 2
The DF and LOH are reduced significantly, as shown below.
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Modified Sinusoidal Pulse-Width Modulation
This utilizes a different method of modulation.
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The harmonic profile is shown below.
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6. Explain the working of 3 phase ph ase full bridge inverter for 120 conduction and draw the relevant output waveforms in the graph sheet.
7. Explain the working of AC chopper
Unit IV – DC DC and AC DRIVES
1. What are drives and electrical drives? Motion control is required in large number of industrial and domestic applications like transportation systems, rolling mills, paper machines, and textile mills. Machine tools, fans, pumps, robots washing machines etc. Systems employed for motion control are called drives d rives and the prime movers such as diesel or petrol engines, gas or steam turbines, steam engines, hydraulic motors and electric motors, for supplying mechanical energy for motion control and drives employing electric motors are known a s electrical drives. 2. What are the advantage and disadvantages of D.C. drives? The advantages of D.C. drives are, a. Adjustable speed b. Good speed regulation c. Frequent starting, braking and reversing. The disadvantage of D.C. drives is the presence of a mechanical commutator, which limits the maximum power rating and the speed. 3. Give some applications of D.C. drives. The applications of D.C. drives are, a. Rolling mills b. Paper mills c. Mine winders d. Hoists e. Machine tools f. Traction g. Printing presses h. Excavators i. Textile mils j. Cranes. 4. What is braking? Mention its types. The motor works as a generator developing a negative torque, which opposes the motion, is called barking. It is is of three types. They are, a. Regenerative braking. b. Dynamic or rheostat braking. c. Plugging or reverse voltage braking. 5. What are the three types of speed control? The three types of speed control as, a. Armature voltage control b. Field flux control c. Armature resistance control. 6. What is called continuous and discontinuous conduction? A D.C. motor is fed from a phase-controlled converter the current in the armature may flow in discrete discrete pulses is called discontinuous conduction. A D.C. motor is fed from a phase controlled converter the current in the armature may flow continuously with an average value superimposed on by a ripple is called continuous conduction. 7. Give the applications of induction motors drives. Although variable speed induction motor drives are generally expensive than D.C. drives, they are used in a number of applications such as fans, blowers, mill run-out tables, cranes, conveyors, traction etc., because of the advantages of induction motors. Other applications applications involved are underground and underwater installations, and explosive and dirty environments. 8.
Mention the advantages of a converter fed indication motor over a line fed motor. A converter fed induction motor has the following advantages over line fed motor. a. Smooth start up is guaranteed by variable frequency starting from a low value. b. Soft starting and acceleration at constant current a nd torque are possible.
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c. The network is no longer subjected to a high switching surge current as with the direct switch On of cage induction motor, and as such, special starting equipment can be omitted even at high ratings. d. High moments of inertia can be accelerated without need to over dimension the motor. e. The converter acts as a decoupling device. Therefore feedback from the motor to the point of short circuit does not take place, when line short circuits occur. The short circuit rating on the basis of which the switchgear has to be over dimensioned is therefore low, permitting a saving to be made. 9. How is the speed control by variation of slip frequency obtained? Speed control by variation of slip frequency is obtained by the following ways. a. Stator voltage control using a three-phase voltage controller. b. Rotor resistance control using a chopper controlled resistance in the rotor circuit. c. Using a converter cascade in the rotor circuit to recover slip energy. d. Using a cycloconverter in the rotor circuit. 10. What is meant by V/f control? When the frequency is reduced, the input voltage must be reduced proportionally so as to maintain constant flux. Otherwise the core will get saturated resulting in excessive iron loss and magnetizing current. This type of induction motor behavior is similar to the working of dc series motors. 11. What is slip controlled drive? When the slip slip is used as a controlled quantity to maintain the flux constant in the motor the drive is called slip conrolled drive. By making the slip negative (i.e., decreasing the output frequency of the inverter) The machine may be made to operate as a generator and the energy of the rotating parts fed back to the mains by an additional line side converter or dissipated in a resistance for dynamic barking. By keeping the slip frequency constant, braking at constant torque and current can be achieved. Thus braking is also fast. 12. How is the D.C. dynamic braking is obtained? D.C. dynamic barking is obtained when the stator of an induction motor running at a speed is connected to a D.C. supply. D.C. current flowing though the stator produces a stationary magnetic field. field. Motion of rotor in this field induces voltage in the rotor winding. Machine, therefore, works as a generator. generator. Generated energy is dissipated in the rotor circuit resistance, thus giving dynamic barking. 13. What is meant by regenerative braking? Regenerative braking occurs when the motor speed exceeds the synchronous speed. In this case,the induction motor would run as the induction generator generator is converting the mechanical power into electrical power, which is delivered back to the electrical system. This method of braking is known as regenerative braking. 14. Why the drive motor must be slightly over dimensioned in slip energy recovery scheme? The losses in the the circuits cause a slight reduction in efficiency. This affiance is further affected by additional losses due to the the non-sinusoidal nature of the rotor current. Therefore the drive motor must be slightly slightly over dimensioned. The motor used must have a rating rating 20% higher than the required power. 15. How is super synchronous speed achieved? Super synchronous speed can be achieved if the power is fed to the rotor from A.C. mains. This can be made possible by replacing the converter cascade by a cycloconverter. cycloconverter. A cycloconverter allows power flow in either direction making the static sherbiuss drive operate at both sub and super synchronous speeds.
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Part B 1. Explain the operation of a single phase fully controlled converter fed separately excited DC
motor with neat waveforms and derive the speed torque characteristics.
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2. Deduce an expression relating speed and torque of a single phase full converter fed separately excited DC motor drive operating in the continuous current mode and discontinuous modes .same answer of the previous question 3. Explain the motoring and braking operation of three phase fully controlled rectifier control of DC separately excited motor with aid of diagrams and waveforms. Also obtain the expression for motor terminal voltage and speed.
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4. Explain in detail the working of a multi quadrant control of chopper fed DC series motor
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5. With necessary diagram, explain the theoretical principles of stator voltage con trol
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6. Derive an expression for the torque of an inverter fed three phase induction motor when it is operated with V/F control. Show that the maximum torque remains unaltered in this scheme.
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Low speed.
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7. Explain static rotor resistance control in closed loop speed control
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Unit V – Applications Applications of Inverters. Part A 1. What are the advantages of / Protection available in On-line UPS ? It can protect the critical load against surges, line line noise, spikes line noise, frequency frequency and voltage variation, variation, brownout outages. All All these protections are not available in the off-line UPS System. 2. Define Redundancy. It means the use of more power conditioners that it is required for the critical loads. In that case, it one of the conditioners fails it can be isolated and the remaining conditioners can serve the critical loads without any disturbance. 3. What are the applications of UPS ? Communication systems, Medical Equipments, Process Industires – continuous Monitoring of processes, Personal Computers.
4. What are the functions of a power po wer supply?
5. What are the limitations of linear regulator?
6.
What do you mean by SMPS?
7.
What is Induction Heating? Induction heating is the process of heating an electrically conducting object (usually a metal) by by electromagnetic induction, induction, where where eddy currents (also called Foucault currents) are generated within the metal and resistance leads to Joule heating of the metal. An induction heater (for any process) consists of an electromagnet, electromagnet, through which a highfrequency alternating current (AC) is passed. Heat may also be generated by magnetic hysteresis losses in materials that have significant significant relative permeability. The permeability. The frequency of AC used depends on the object size, material type, coupling (between the work coil and the object to be heated) and the penetration depth.
8.
Define Dielectric Heating. Dielectric heating, also known as electronic heating, RF heating, high-frequency high-frequency heating is the process in which a high-frequency alternating electric field, or radio or radio wave or microwave or microwave electromagnetic radiation heats a dielectric a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation within the dielectric. At lower frequencies in conductive fluids, other mechanisms such as as ion-drag are more important in generating thermal energy.
9. What are the applications of Electronic timer? Electronic timers are essentially quartz clocks with special electronics, and can achieve higher precision than mechanical timers. Electronic timers have digital electronics, but may have an analog an analog or digital digital display. Integrated display. Integrated circuits have made digital made digital logic so inexpensive that an electronic timer is now less expensive than many mechanical and electromechanical timers. Individual timers are implemented as a simple single-chip computer system, system, similar to a watch and usually using the same, mass-produced, mass-produced, technology.Many timers are now implemented in software. software. Modern controllers use a programmable programmable logic controller rather than a box full of electromechanical parts. The logic is usually designed as if it were relays, using a special computer language called ladder logic. In logic. In PLCs, timers are usually simulated by the software built into the controller. Each timer is just an entry in a table maintained by the software.
10.
What are the types Digital counters? Asynchronous (ripple) counter – changing state bits are used as clocks to subsequent state flip-flops Synchronous counter – all state bits change under control co ntrol of a single clock Decade counter – counts through ten states per stage Up/down counter – counts both up and down, under command of a control input Ring counter – formed by a shift register with feedback connection in a ring Johnson counter – a twisted ring ring counter Cascaded counter
Part B 1. Explain the operation of SMPS in detail.
2. Explain the working of a linear voltage regulator with its basic block diagram.
3. Explain the operation of Off line and On line UPS in detail.
4. Explain the operation of Induction heating and dielectric heating.
5. Explain the applications of electronics timers
6. Write notes on Digital Counters
