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Industrial Electronics Question Bank with Answers

Unit 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 switching losses but BJT has higher switching losses.

b. Power MOSFET has more on state resistance and conduction losses but BJT haslower 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 anddrive 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 which it must attain during turn-on process to

maintain conduction when gate signal is removed.

6. What is forced Commutation?

In some thyristor circuit, the input voltage is dc and the forward current of the thyristoris forced to zero by an additional circuitry commutation circuit to turn-off the thyristor.This technique is called forced commutation .

7. Define snubber circuitA subber circuit consists of a series combination of resistance Rs and Cs in parallel with

the thyristor. It is mainly used for dv/dt protection.

8. Define circuit turn off timeCircuit turn off time is defined as the tine between the instant anode current becomeszero and the instant reverse voltage due to 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 issufficient, the excessive localized heating ay damage the transistor. Thus seccondary

breakdown is caused by a localised thermal runway, resulting from high currentconcentrations.

10. What are the advantages of TRIAC?Triac can be triggered with +ve / -ve polarity voltages.


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A Triac needs a single fuse protection, which also simplifies the construction.

A Triac needs a single heat sink of slightly larger size whereas antiparallel thyristor pairneeds two heat sinks.

Part B1. Explain the Construction & Working Principle of Power MOSFET

The Power MOSFET technology has mostly reached maturity and is the most popular devicefor 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) with a typically rectangular Safe Operating Area, it can be conveniently utilized.Utilising shared manufacturing processes, comparative costs of MOSFETs are attractive. Forlow frequency applications, where the currents drawn by the equivalent capacitances acrossits terminals are small, it can also be driven directly by integrated circuits. These capacitancesare the main hindrance to operating the MOSFETS at speeds of several MHz. The resistivecharacteristics of its main terminals permit easy paralleling externally also. At high currentlow voltage applications the MOSFET offers best conduction voltage specifications as theR

DS(ON)specification is current rating dependent. However, the inferior features of the

inherent anti-parallel diode and its higher conduction losses at power frequencies and voltagelevels restrict its wider application.


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As mentioned in the introduction section, Power MOSFET is a device that evolved fromMOS integrated circuit technology. The first attempts to develop high voltage MOSFETswere by redesigning lateral MOSFET to increase their voltage blocking capacity. Theresulting technology was called lateral double deffused MOS (DMOS). However it was soonrealized that much larger breakdown voltage and current ratings could be achieved byresorting to a vertically oriented structure. Since then, vertical DMOS (VDMOS) structurehas been adapted by virtually all manufacturers of Power MOSFET. A power MOSFETusing VDMOS technology has vertically oriented three layer structure of alternating p typeand n type semiconductors as shown in Fig 6.2 (a) which is the schematic representation of asingle 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 andDrain are heavily doped to approximately the same level. Th e p type middle layer is termedthe 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 dioxide (also called the gate oxide). The source and the drainregion of all cells on a wafer are connected to the same metallic contacts to form the Sourceand the Drain terminals of the complete device. Similarly all gate terminals are alsoconnected together. The source is constructed of many (thousands) small polygon shapedareas that are surrounded by the gate regions. The geometric shape of the source regions, tosame extent, influences the ON state resistance of the MOSFET.

Operating principle of a MOSFETAt 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 body and body-Drain)will be reverse biased for either polarity of the applied voltage between the source and thedrain. There is no possibility of current injection from the gate terminal either since the gateoxide is a very good insulator. However, application of a positive voltage at the gate terminalwith respect to the source will covert the silicon surface beneath the gate oxide into an n typelayer or channel, thus connecting the Source to the Drain as explained next. The gateregion of a MOSFET which is composed of the gate metallization, the gate (silicon) oxidelayer and the p-body silicon forms a high quality capacitor. When a small voltage isapplication 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 2 and the silicon as shown in Fig 6.4 (a).


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The positive charge induced on the gate metallization repels the majority hole carriers from theinterface region between the gate oxide and the p type body. This exposes the negativelycharged acceptors and a depletion region is created.

Further increase in VGS

causes the depletion layer to grow in thickness. At the same time the electric

field at the oxide-silicon interface gets larger and begins to attract free electrons as shown inFig 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 extraholes are neutralized by electrons from the source.

As VGS

increases further the density of free electrons at the interface becomes equal to the free hole

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 allthe properties of an n type semiconductor and is a conductive path or channel between thedrain and the source which permits flow of current between the drain and the source. Sincecurrent conduction in this device takes place through an n- type channel created by theelectric field due to gate source voltage it is called Enhancement type n -channel MOSFET.

The value of VGS

at which the inversion layer is considered to have formed i s called the Gate

Source threshold voltage VGS

(th). As VGS

is increased beyond VGS

(th) the inversion layer

gets some what thicker and more conductive, since the density of free electrons increases


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further with increase in VGS

. The inversion layer screens the depletion layer adjacent to it

from increasing VGS

. The depletion layer thickness now remains constant.

2. Explain the Construction & Working Principle of Power TRIAC. Discuss its fourmodes of operationsFig. 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 cathodeare not used for Triacs. The three terminals are marked as MT1

(Main Terminal 1), MT2

(Main Terminal 2) and the gate by G. As shown in Fig 4.12 (b) the gate terminal is near MT1

and is connected to both N3

and P2

regions by metallic contact. Similarly MT1

is connected to

N2

and P2

regions while MT2

is connected to N4

and P1

regions.

Since a Triac is a bidirectional device and can have its terminals at various combinations of positiveand negative voltages, there are four possible electrode potential combinations as given below

1. MT2 positive with respect to MT

1, G positive with respect to MT

1

2. MT2 positive with respect to MT

1, G negative with respect to MT

1

3. MT2

negative with respect to MT1

, G negative with respect to MT1

4. MT2

negative with respect to MT1, G positive with respect to MT

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 triacin trigger modes 1 & 3 respectively.


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In trigger mode-1 the gate current flows mainly through the P2 N

2 junction like an ordinary thyristor.

When the gate current has injected sufficient charge into P2

layer the triac starts conducting through

the P1 N

1P

2 N

2layers like an ordinary thyristor.

In the trigger mode-3 the gate current Ig

forward biases the P2

P3 junction and a large number of

electrons are introduced in the P2

region by N3. Finally the structure P

2 N

1P

1 N

4turns on completely.


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From a functional point of view a triac is similar to two thyristors connected in anti parallel.

Therefore, it is expected that the V-I characteristics of Triac in the 1st

and 3rd

quadrant of the V-I plane will be similar to the forward characteristics of a thyristors. As shown in Fig. 4.14, with nosignal to the gate the triac will block both half cycle of the applied ac voltage provided its peak valueis lower than the break over voltage (V

BO) of the device. However, the turning on of the triac can be

controlled by applying the gate trigger pulse at the desired instance. Mode-1 triggering is used in thefirst quadrant where as Mode-3 triggering is used in the third quadrant. As such, most of the thyristorcharacteristics apply to the triac (ie, latching and holding current). However, in a triac the twoconducting paths (from MT 1 to MT 2 or from MT 1 to MT 1) interact with each other in the structure ofthe triac. Therefore, the voltage, current and frequency ratings of triacs are considerably lower thanthyristors. 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 specifycharacteristics curves relating rms device current and maximum allowable case temperature as shownin Fig 4.15. Curves relating the device dissipation and RMS on state current are also provided fordifferent 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 current gain for the power MOSFETs werediluted by the need to design the gate drive circuit capable of supplying the charging anddischarging current of the device input capacitance. This is especially true in high frequencycircuits where the power MOSFET is particularly valuable due to its inherently high switchingspeed. On the other hand, MOSFETs have a higher on state resistance per unit area andconsequently higher on state loss. This is particularly true for higher voltage devices (greater thanabout 500 volts) which restricted the use of MOSFETs to low voltage high frequency circuits (eg.SMPS).

Constructional Features of an IGBTVertical 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.


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The i-v characteristics of an n channel IGBT is shown in Fig 7.4 (a). They appear qualitativelysimilar to those of a logic level BJT except that the controlling parameter is not a base current butthe gate-emitter voltage.When the gate emitter voltage is below the threshold voltage only a very small leakage currentflows 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 CES in Fig7.4 (a)) is determined by the avalanche break down voltage of the body drain p-n junction.Unlike a BJT, however, this break down voltage is independent of the collector current as shownin Fig 7.4(a). IGBTs of Non-punch through design can block a maximum reverse voltage (V

RM)

equal to VCES

in the cut off mode. However, for Punch Through IGBTs VRM

is negligible (only a

few tens of volts) due the presence of the heavily doped n+ drain buffer layer.As the gate emitter voltage increases beyond the threshold voltage the IGBT enters into the activeregion of operation. In this mode, the collector current i

cis determined by the transfer

characteristics of the device as shown in Fig 7.4(b). This characteristic is qualitatively similar to


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that of a power MOSFET and is reasonably linear over most of the collector current range. Theratio of i

cto (V

gE v

gE(th)) is called the forward transconductance (g

fs) of the device and is an

important parameter in the gate drive circuit design. The collector emitter voltage, on the otherhand, 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 ina 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 ofa MOSFET. However, the output p-n-p transistor does have a significant effect on the switchingcharacteristics of the device, particularly during turn off. Another important difference is in thegate drive requirement. To avoid dynamic latch up, (to be discussed later) the gate emittervoltage of an IGBT is maintained at a negative value when the device is off.

4. Explain the Construction & Working Principle of SCR


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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 toconductivity 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 regions. Therefore, the cathode regions are finelydistributed between gate contacts of the p type layer. An Involute structure for both the gate andthe cathode regions is a preferred design structure.

The circuit symbol in the left hand side inset defines the polarity conventions of the variables used inthis figure.With ig = 0, V

AKhas to increase up to forward break over voltage V

BRF before significant anode

current starts flowing. However, at VBRF

forward break over takes place and the voltage across the

thyristor drops to VH

(holding voltage). Beyond this point voltage across the thyristor (VAK

) remains

almost constant at V H (1-1.5v) while the anode current is determined by the external load.The magnitude of gate current has a very strong effect on the value of the break over voltage asshown in the figure. The right hand side figure in the inset shows a typical plot of the forward breakover voltage (V

BRF) as a function of the gate current (I

g)

After Turn ON the thyristor is no more affected by the gate current. Hence, any current pulse (ofrequired magnitude) which is longer than the minimum needed for Turn ON is sufficient to effectcontrol. The minimum gate pulse width is decided by the external circuit and should be long enoughto allow the anode current to rise above the latching current (I

L) level.


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The left hand side of Fig 4.3 shows the reverse i-v characteristics of the thyristor. Once the thyristoris ON the only way to turn it OFF is by bringing the thyristor current below holding current (I

H). The

gate terminal has no control over the turn OFF process. In ac circuits with resistive load this happensautomatically dur ing negative zero crossing of the supply voltage. This is called naturalcommutation or line commutation. However, in dc circuits some arrangement has to be made toensure this condition. This process is called forced commutation.During reverse blocking if i

g= 0 then only reverse saturation current (I

s) flows until the reverse

voltage reaches reverse break down voltage (V BRR ). At this point current starts rising sharply. Largereverse voltage and current generates excessive heat and destroys the device. If i

g> 0 during reverse

bias condition the reverse saturation current rises as explained in the previous section. This can beavoided by removing the gate current while the thyristor is reverse biased.

Switching Characteristics of a ThyristorDuring Turn on and Turn off process a thyristor is subjected to different voltages across it anddifferent currents through it. The time variations of the voltage across a thyristor and the currentthrough it during Turn on and Turn off constitute the switching characteristics of a thyristor.

Turn on Switching CharacteristicsA forward biased thyristor is turned on by applying a positive gate voltage between the gate andcathode as shown in Fig 4.10.


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Fig 4.10 shows the waveforms of the gate current (i

g), anode current (i

A) and anode cathode voltage

(VAK

) in an expanded time scale during Turn on. The reference circuit and the associated waveforms

are shown in the inset. The total switching period being much smaller compared to the cycle time, iA

and VAK

before and after switching will appear flat.

As shown in Fig 4.10 there is a transition time tON

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 intervalsnamely, (i) delay time (t

d) (ii) rise time (t

r ) and (iii) spread time (t

p). These times are shown in Fig

4.10 for a resistive load.

Turn off Switching CharacteristicsOnce the thyristor is on, and its anode current is above the latching current level the gate losescontrol. It can be turned off only by reducing the anode current below holding current. The turn offtime t

qof a thyristor is defined as the time between the instant anode current becomes zero and the

instant the thyristor regains forward blocking capability. If forward voltage is applied across thedevice during this period the thyristor turns on again.During turn off time, excess minority carriers from all the four layers of the thyristor must beremoved. Accordingly t

qis divided in to two intervals, the reverse recovery time (t

rr ) and the gate

recovery time (tqr

). Fig 4.11 shows the variation of anode current and anode cathode voltage with

time during turn off operation on an expanded scale.


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5. Discuss the types of Power DIODES. Mention its rating, applications

As mention in the introduction Power Diodes of largest power rating are required to conductseveral 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 widedepletion layer in order to restrict the maximum electric field strength below the impactionization 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 tworequirements will be satisfied in a lightly doped p-n junction diode of sufficient width toaccommodate 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 (NdD

) is

grown on a heavily doped n+

substrate (NdK

donor atoms.Cm-3

) which acts as the cathode.

Finally the p-n junction is formed by defusing a heavily doped (NaA

acceptor atoms.Cm-3

) p+

region into the epitaxial layer. This p type region acts as the anode.

Impurity atom densities in the heavily doped cathode (Ndk

.Cm-3

) and anode (NaA

.Cm-3

) are

approximately of the same order of magnitude (1019

Cm-3

) while that of the epitaxial layer

(also called the drift region) is lower by several orders of magnitude (NdD

1014

Cm-3

). In a

low power diode this drift region is absent. The Implication of introducing this drift region ina power diode is explained next.


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In the previous section it was shown how the introduction of a lightly doped drift region in the p-nstructure of a diode boosts its blocking voltage capacity. It may appear that this lightly doped driftregion will offer high resistance during forward conduction. However, the effective resistance of thisregion in the ON state is much less than the apparent ohmic resistance calculated on the basis of thegeometric 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

p


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Power Diodes take finite time to make transition from reverse bias to forward bias condition (switchON) and vice versa (switch OFF).Behavior of the diode current and voltage during these switching periods are important due to thefollowing reasons.

Severe over voltage / over current may be caused by a diode switching at different points in thecircuit 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 switchingfrequency 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 significantlyhigher value V

frcompared to the steady slate voltage drop at the steady current I

F.

In some power converter circuits (e.g voltage source inverter) where a free wheeling diode is usedacross an asymmetrical blocking power switch (i.e GTO) this transient over voltage may be highenough to destroy the main power switch.

Vfr

(called forward recovery voltage) is given as a function of the forward di/dt in the

manufacturers data sheet. Typical values lie within the range of 10 -30V. Forward recoverytime (t

fr ) is typically within 10 us.


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6. Discuss the turn on methods of SCR

Fig 4.10 shows the waveforms of the gate current (ig), anode current (i

A) and anode cathode

voltage (VAK

) in an expanded time scale during Turn on. The reference circuit and the

associated waveforms are shown in the inset. The total switching period being much smallercompared to the cycle time, i

Aand V

AK before and after switching will appear flat.

As shown in Fig 4.10 there is a transition time tON

from forward off state to forward on state.

This transition time is called the thyristor turn of time and can be divided into three separateintervals namely, (i) delay time (t

d) (ii) rise time (t

r ) and (iii) spread time (t

p). These times are

shown in Fig 4.10 for a resistive load.Delay time (t

d): After switching on the gate current the thyristor will start to conduct over the

portion of the cathode which is closest to the gate. This conducting area starts spreading at a finitespeed until the entire cathode region becomes conductive. Time taken by this process constitute theturn on delay time of a thyristor. It is measured from the instant of application of the gate current to


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the instant when the anode current rises to 10% of its final value (or VAK

falls to 90% of its initial

value). Typical valu e of td is a few micro seconds.

Rise time (tr): For a resistive load, rise time is the time taken by the anode current to rise from10% of its final value to 90% of its final value. At the same time the voltage V

AKfalls from 90% of

its initial value to 10% of its initial value. However, current rise and voltage fall characteristics arestrongly influenced by the type of the load. For inductive load the voltage falls faster than the current.While for a capacitive load V

AKfalls rapidly in the beginning. However, as the current increases, rate

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 tolocal hot spots and damage the device. Therefore, it is necessary to limit the rate of rise of the ONstate current Adidt by using an inductor in series with the device. Usual values of maximumallowable Adidt is in the range of 20- 200 A/s.Spread time (tp): It is the time taken by the anode current to rise from 90% of its final value to100%. During this time conduction spreads over the entire cross section of the cathode of thethyristor. The spreading interval depends on the area of the cathode and on the gate structure of thethyristor.

Forward voltage trigerringGate trigerringLASCRdv/dt trigerringdi/dt trigerring

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Industrial Electronics Question Bank with Answers

Unit 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 the load current to flowcontinuously, 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 the load performance is better.

3.

What is meant by continuous current operation of thyristor converter ? When a free wheeling diode is connected across the output, load current continuous flowsthrough the load. Whenever the load voltage tends to go negative, freewheeling diode startsto conduct. As a result load current is transferred from thyristor to freewheeling diode. Thisis called continuous current operation of thyristor converter.

4. What is meant by commutation angle / overlap angle ?The commutation period, when outgoing and incoming thyristors are conducting, is alsoknown as the overlap period. The angular period both devices share conduction is known asthe commutation angle / overlap angle.

5. What is meant by ac voltage controller?AC voltage controller is a device which convertes fixed alternating voltage into a variablevoltage without change in supply frequency.

6. What are the applications of AC voltage controller ?

Domestic and industrial heatingLighting ControlSpeed control of single phase & 3phase MotorsTransformer tag changing

7. What are the advantages & disadvantages of AC voltage controller ?AdvantagesHigh efficiencyFlexibility in controlLess MaintenanceDisadvantageIntroduction fo Harmonics in the supply current and load voltage waveform particularly atlow output voltage levels .


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8. What is meant by cyclo converter?It converts input power at one frequency to outpur power at a different frequency with onestage 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 drivesStatic VAR generationower supply in air craft and ship boards.

10. List the applications of convertersa. Electro-chemical and electro metallurgical plants

b. Electroplating and electro printingc. Steel rolling mills, paper mills, textile mills, printing pressd. Electric Tractione. HVDC transmissionf. Portable hand tool drives

g.

UPS

Part B1. 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 anuncontrolled converter by six thyristors as shown in Fig. 13.1 (a)

For any current to flow in the load at least one device from the top group (T1, T

3, T

5) and one from

the bottom group (T2, T

4, T

6) must conduct. It can be argued as in the case of an uncontrolled

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 inputcycle. Now the thyristors are fired in the sequence T

1 T

2 T

3 T

4 T

5 T

6 T

1with 60


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interval between each firing. Therefore thyristors on the same phase leg are fired at an interval of180 and hence can not conduct simultaneously. This leaves only six possible conduction mode forthe converter in the continuous conduction mode of operation. These are T

1T

2, T

2T

3, T

3T

4,T

4T

5,

T5T

6, T

6T

1. Each conduction mode is of 60 duration and appears in the sequence mentioned. The

conduction table of Fig. 13.1 (b) shows voltage across different devices and the dc output voltage foreach conduction interval. The phasor diagram of the line voltages appear in Fig. 13.1 (c). Each ofthese line voltages can be associated with the firing of a thyristor with the help of the conductiontable-1. For example the thyristor T

1is fired at the end of T

5T

6conduction interval. During this period

the voltage across T1

was vac

. Therefore T1

is fired angle after the positive going zero crossing of

vac

. Similar observation can be made about other thyristors. The phasor diagram of Fig. 13.1 (c) also

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 thewaveforms it is necessary to draw the conduction diagram which shows the interval of conduction foreach thyristor and can be drawn with the help of the phasor diagram of fig. 13.1 (c). If the converterfiring angle is each thyristor is fired angle after the positive going zero crossing of the linevoltage with which its firing is associated. Once the conduction diagram is drawn all other voltagewaveforms 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 isclear from the waveforms that output voltage and current waveforms are periodic over one sixth ofthe input cycle. Therefore this converter is also called the six pulse converter. The input current on

the other hand contains only odds harmonics of the input frequency other than the triplex (3rd

, 9th

etc.)harmonics. The next section will analyze the operation of this converter in more details.


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Write the sequence and draw the output in graph sheet - refer class notes


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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 ofthe most popular converter circuits and is widely used in the speed control of separately excited dcmachines. Indeed, the R L E load shown in this figure may represent the electrical equivalent circuitof a separately excited dc motor.The single phase fully controlled bridge converter is obtained by replacing all the diode of thecorresponding uncontrolled converter by thyristors. Thyristors T

1and T

2are fired together while T

3

and T4

are fired 180 after T1

and T2. From the circuit diagram of Fig 10.3(a) it is clear that for any

load current to flow at least one thyristor from the top group (T1, T

3) and one thyristor from the

bottom group (T2, T

4) must conduct. It can also be argued that neither T

1T

3nor T

2T

4can conduct

simultaneously. For example whenever T3

and T4

are in the forward blocking state and a gate pulse is

applied to them, they turn ON and at the same time a negative voltage is applied across T1

and T2

commutating them immediately. Similar argument holds for T1

and T2.

For the same reason T1T

4or T

2T

3can not conduct simultaneously. Therefore, the only

possible conduction modes when the current i0

can flow are T1T

2and T

3T

4. Of coarse it is possible

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 1T2 and T 3T4. Once the load current becomeszero all thyristors remain off. In this mode the load current remains zero. Consequently the converteris 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 eachof these conduction modes. It is to be noted that whenever T

1and T

2conducts, the voltage across T

3

and T4 becomes v

i. Therefore T

3and T

4can be fired only when v

iis negative i.e, over the negative

half cycle of the input supply voltage. Similarly T1

and T2

can be fired only over the positive half

cycle of the input supply. The voltage across the devices when none of the thyristors conductdepends on the off state impedance of each device. The values listed in Fig 10.3 (b) assume identicaldevices.Under normal operating condition of the converter the load current may or may not remain zero oversome interval of the input voltage cycle. If i

0is always greater than zero then the converter is said to

be operating in the continuous conduction mode. In this mode of operation of the converter T1T

2and

T3T

4conducts for alternate half cycle of the input supply.


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3. Explain the operation of dual converters with neat sketch


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Although Equations 13.30 ensures that the dc voltages produced by these converters areequal the output voltages do not match on an instantaneous basis. Therefore to avoid a directshort circuit between two different supply lines the two converters must never be gatedsimultaneously. 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


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thyristors are fired while converter-I gate pulses are blocked. Thus there is no circulatingcurrent flowing through the converters and therefore it is called the non-circulating currenttype dual converter. It requires precise sensing of the zero crossing of the output currentwhich may pose a problem particularly at light load due to possible discontinuousconduction. To overcome this problem an interphase reactor may be incorporated betweenthe two converters. With the interphase reactor in place both the converters can be gatedsimultaneously with

2=

1. The resulting converter is called the circulating current type

dual converter.

4. Explain the operation of 3 phase bridge converter and derive the output voltage.


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5. Explain the principle of operation of Single phase cycloconverters

The basic principle of operation of a cyclo-converter is explained with reference to an equivalentcircuit shown in Fig. 29.1. Each two-quadrant converter (phase-controlled) is represented as analternating voltage source, which corresponds to the fundamental voltage component obtained atits output terminals. The diodes connected in series with each voltage source, show theunidirectional conduction of each converter, whose output voltage can be either positive ornegative, being a two-quadrant one, but the direction of current is in the direction as shown in thecircuit, as only thyristors unidirectional switching devices, are used in the two converters.

Normally, the ripple content in the output voltage is neglected. The control principle used in anideal cyclo-converter is to continuously modulate the firing angles of the individual converters, sothat each produces the same sinusoidal (ac) voltage at its output terminals. Thus, the voltages of thetwo generators (Fig. 29.1) have the same amplitude, frequency and phase, and the voltage of thecyclo-converter is equal to the voltage of either of these generators. It is possible for the mean powerto flow either to or from the output terminals, and the cyclo -converter is inherently capable of

operation with loads of any phase angle inductive or capacitive. Because of the uni -directionalcurrent carrying property of the individual converters, it is inherent that the positive half-cycle ofload current must always be carried by the positive converter, and the negative half-cycle by thenegative converter, regardless of the phase of the current with respect to the voltage. This means thateach two-quadrant converter operates both in its rectifying (converting) and in its inverting regionduring the period of its associated half-cycle of current.The output voltage and current waveforms, illustrating the operation of an ideal cyclo-convertercircuit with loads of various displacement angles, are shown in Fig. 29.2. The displacement angle


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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 whichits terminal voltage is in the inverting region of operation. In Fig. 29.2b, the displacement angleof the load is lagging. During the first period of each half-cycle of load current, the associatedconverter 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 thecyclo-converter output terminals, and hence, into the ac system at the input side. These two areillustrative 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 displacementangle, 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.


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6. Explain the operation of single phase AC voltage controller with R load


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The regulators in Fig 26.1 (a), (b) and (c) perform quite similarly. They are called Phase AngleControlled (PAC) AC-AC converters or AC-AC choppers. The TRIAC based converter may beconsidered as the basic topology. Being bi-directionally conducting devices, they act on both

polarities of the applied voltage. However, reapplied dvdt their ratings being poor, they tend to turn-on inthe opposite direction just subsequent to their turn-off with an inductive load. The 'Alternistor' wasdeveloped with improved features but was not popular. The TRIAC is common only at the low

power ranges. The (a) and (b) options are improvements on (c) mostly regarding current handlingand turn-off-able current rating.


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A transistorised AC-AC regulator is a PWM regulator similar to the DC-DC converters. It alsorequires a freewheeling path across the inductive load, which has also got to be bi-directional.Consequently, only controlled freewheeling devices can be used.


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Unit Test 3 Industrial Electronics Question Bank

2 Marks1. What is 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 equivalentto an dc transformer with continuously variable turns ratio. Like a transformer it can be usedto step up / down a dc voltage source.

2. What is 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 down chopper / buck converter. Average output voltage Vo is greaterthan input voltage Vs ie. Vo > Vs, this chopper is called step up chopper/ boost converter.

3. What is meant by PWM control in dc chopper?In this control method the on time Ton is varied but chopping 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 waveform at a highoutput frequency, ranging from 200 Hz to 100 KHz. It is commonly used for fixed outputapplications such as Ultrasonic generators, Induction heating, Sonar transmitter, FluorescentLighting.


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6. What are the types of PWM control?Single pulse width modulationsMultiple pulse width modulationsSinusoidal pulse width modulationsModified Sinusoidal pulse width modulations

7.

Compare CSI and VSIVSIIn VSI input voltage is maintained constant.The output voltages does not depend on the loadThe Magnitude of output current and its waveform depends upon the nature of the loadimpedance.IT requires feedback diodes.Commutation is complex.CSIInput Current is constant but adjustable.The amplitude of output current does not depend on the load.

The magnitude of output voltage and its waveform depends upon the nature of the loadimpedance.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 Heatingc. Aircraft power suppliesd. Domestic power suppliese. UPS

9. What are the disadvantages of harmonics present in the inverter system?Harmonics currents will lead to excessive heating in the induction motors. This will reducethe load carrying capacity of the motor

10. What are the methods of voltage control in Inverters?External control of ac output voltageExternal control of dc input voltage


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Part B1. Explain the working of 3 phase full bridge inverter for 180 conduction and draw the

relevant output waveforms in the graph sheet.


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2. Explain the principle and operation of Current source Inverter.


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4. Explain the different configurations of chopper.Refer class notes for derivation and diagram Book page no. 229


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5. Explain the voltage control of Inverters using PWM techniquesa. Single-pulse-width modulation

b. Multi-pulse-width modulationc. Sinusoidal pulse-width modulation.d. Modified sinusoidal pulse-width modulation


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Single Pulse-Width Modulation

There is one pulse per half-cycle, and its width is varied.

12

The dominant harmonic is the third.

DF increases significantly at a low output voltage. 1


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The dominant harmonic is the third.

DF increases significantly at a low output voltage. 14


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Multiple-Pulse-Width Modulation

The harmonic content can be reduced by usingseveral pulses in each half-cycle of output

voltage.

This type of modulation is also known asuniform-pulse-width modulation (UPWM).

15

16


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The following figure shows the harmonic profileagainst the variation of modulation index, andp=5.

18

Sinusoidal Pulse-WidthModulation

Instead of maintaining the width of all pulses

the same, the width of each pulse is varied inproportion to amplitude of a sine wave.

This kind of modulation is known as SPWM.

19


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20

The rms output voltage is:

The DF and LOH are reduced significantly, asshown below.

2/1

1

)( p

m

m so V V

21


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Modified Sinusoidal Pulse-WidthModulation

This utilizes a different method of modulation.

23

The harmonic profile is shown below.

24

6. Explain the working of 3 phase full bridge inverter for 120 conduction and draw the

relevant output waveforms in the graph sheet.


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7. Explain the working of AC chopper


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1

Unit IV DC and AC DRIVES1. What are drives and electrical drives?

Motion control is required in large number of industrial and domestic applications liketransportation systems, rolling mills, paper machines, and textile mills. Machine tools, fans,

pumps, robots washing machines etc. Systems employed for motion control are called drivesand 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 controland drives employing electric motors are known as 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 regulationc. Frequent starting, braking and reversing.

The disadvantage of D.C. drives is the presence of a mechanical commutator, whichlimits 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 millsc. Mine winders d. Hoists

e. Machine tools f. Tractiong. Printing presses h. Excavatorsi. 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 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 controlc. 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 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 continuousconduction.

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 applicationsinvolved 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 and torque are possible.


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2

c. The network is no longer subjected to a high switching surge current as with thedirect 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 shortcircuit rating on the basis of which the switchgear has to be over dimensioned istherefore 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 lossand magnetizing current. This type of induction motor behavior is similar to the workingof dc series motors.

11. What is slip controlled drive?When the 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 theoutput frequency of the inverter) The machine may be made to operate as a generator and theenergy of the rotating parts fed back to the mains by an additional line side converter ordissipated 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 aspeed is connected to a D.C. supply. D.C. current flowing though the stator produces astationary magnetic field. Motion of rotor in this field induces voltage in the rotor winding.

Machine, therefore, works as a generator. Generated energy is dissipated in the rotor circuitresistance, 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 is converting themechanical power into electrical power, which is delivered back to the electrical system. Thismethod of braking is known as regenerative braking.

14. Why the drive motor must be slightly over dimensioned in slip energy recoveryscheme?

The losses in the circuits cause a slight reduction in efficiency. This affiance is furtheraffected by additional losses due to the non-sinusoidal nature of the rotor current. Therefore

the drive motor must be slightly over dimensioned. The motor used must have a 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. Acycloconverter allows power flow in either direction making the static sherbiuss driveoperate at both sub and super synchronous speeds.


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3

Part B1. 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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11

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 control


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21

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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22

Low speed.


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23

7. Explain static rotor resistance control in closed loop speed control


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Unit V Applications of Inverters.

Part A1. What are the advantages of / Protection available in On-line UPS ?

It can protect the critical load against surges, line noise, spikes line noise, frequency andvoltage variation, brownout outages. All these protections are not available in the off-lineUPS System.

2. Define Redundancy.It means the use of more power conditioners that it is required for the critical loads. Inthat case, it one of the conditioners fails it can be isolated and the remaining conditionerscan serve the critical loads without any disturbance.

3. What are the applications of UPS ?Communication systems, Medical Equipments, Process Industires continuous Monitoringof processes, Personal Computers.

4. What are the functions of a power supply?

5. What are the limitations of linear regulator?


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6. What do you mean by SMPS?

7. What is Induction Heating?Induction heating is the process of heating an electrically conducting object (usually ametal) by electromagnetic induction, where eddy currents (also called Foucault currents)are generated within the metal and resistance leads to Joule heating of the metal. Aninduction heater (for any process) consists of an electromagnet, through which a high-frequency alternating current (AC) is passed. Heat may also be generated by magnetichysteresis losses in materials that have significant relative permeability. The frequency of

AC used depends on the object size, material type, coupling (between the work coil and theobject to be heated) and the penetration depth.

8. Define Dielectric Heating.Dielectric heating , also known as electronic heating , RF heating , high-frequency heating isthe process in which a high-frequency alternating electric field, or radio wave or microwaveelectromagnetic radiation heats a dielectric material. At higher frequencies, this heating iscaused by molecular dipole rotation within the dielectric. At lower frequencies inconductive fluids, other mechanisms such as ion-drag are more important in generatingthermal energy.

9. What are the applications of Electronic timer?Electronic timers are essentially quartz clocks with special electronics, and can achievehigher precision than mechanical timers. Electronic timers have digital electronics, but mayhave an analog or digital display. Integrated circuits have made digital logic so inexpensivethat an electronic timer is now less expensive than many mechanical and electromechanicaltimers. Individual timers are implemented as a simple single-chip computer system, similarto a watch and usually using the same, mass-produced, technology.Many timers are nowimplemented in software. Modern controllers use a programmable logic controller ratherthan 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 PLCs, timers are usually simulated

by the software built into the controller. Each timer is just an entry in a table maintained bythe software .

10. What are the types Digital counters?

Asynchronous (ripple) counter

changing state bits are used as clocks to subsequentstate flip-flops Synchronous counter all state bits change under control 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 counter Cascaded counter
http://en.wikipedia.org/wiki/Heatinghttp://en.wikipedia.org/wiki/Metalhttp://en.wikipedia.org/wiki/Electromagnetic_inductionhttp://en.wikipedia.org/wiki/Eddy_currenthttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Induction_heaterhttp://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Hysteresishttp://en.wikipedia.org/wiki/Permeability_%28electromagnetism%29http://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/w/index.php?title=Molecular_dipole&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Ion-drag&action=edit&redlink=1http://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/wiki/Quartz_clockhttp://en.wikipedia.org/wiki/Analogue_electronicshttp://en.wikipedia.org/w/index.php?title=Digital_device&action=edit&redlink=1http://en.wikipedia.org/wiki/Integrated_circuithttp://en.wikipedia.org/wiki/Digital_logichttp://en.wikipedia.org/wiki/Embedded_systemhttp://en.wikipedia.org/wiki/Watchhttp://en.wikipedia.org/wiki/Mass_productionhttp://en.wikipedia.org/wiki/Embedded_systemhttp://en.wikipedia.org/wiki/Programmable_logic_controllerhttp://en.wikipedia.org/wiki/Ladder_logichttp://en.wikipedia.org/wiki/Shift_registerhttp://en.wikipedia.org/wiki/Shift_registerhttp://en.wikipedia.org/wiki/Shift_registerhttp://en.wikipedia.org/wiki/Shift_registerhttp://en.wikipedia.org/wiki/Ladder_logichttp://en.wikipedia.org/wiki/Programmable_logic_controllerhttp://en.wikipedia.org/wiki/Embedded_systemhttp://en.wikipedia.org/wiki/Mass_productionhttp://en.wikipedia.org/wiki/Watchhttp://en.wikipedia.org/wiki/Embedded_systemhttp://en.wikipedia.org/wiki/Digital_logichttp://en.wikipedia.org/wiki/Integrated_circuithttp://en.wikipedia.org/w/index.php?title=Digital_device&action=edit&redlink=1http://en.wikipedia.org/wiki/Analogue_electronicshttp://en.wikipedia.org/wiki/Quartz_clockhttp://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/w/index.php?title=Ion-drag&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Molecular_dipole&action=edit&redlink=1http://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Permeability_%28electromagnetism%29http://en.wikipedia.org/wiki/Hysteresishttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Induction_heaterhttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Eddy_currenthttp://en.wikipedia.org/wiki/Electromagnetic_inductionhttp://en.wikipedia.org/wiki/Metalhttp://en.wikipedia.org/wiki/Heating


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2. Explain the working of a linear voltage regulator with its basic block diagram.


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3. Explain the operation of Off line and On line UPS in detail.


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4. Explain the operation of Induction heating and dielectric heating.


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5. Explain the applications of electronics timers


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6. Write notes on Digital Counters

ei2301 ie notes full

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