Synchronous motors Advantages and Disadvantages

Synchronous motors Advantages Disadvantages

In industries induction motors are employed mostly because of less cost, rugged construction, good starting torques and very less maintenance. Synchronous motors are rarely used in industries for drive applications. They are generally used as power factor correction device. In industries they are employed to improve the power factor of the system. Some of the advantages and disadvantages related to synchronous motors are explained below: 

Advantage or Merits:

• One of the major advantage of using synchronous motor is the ability to control the power factor. An over excited synchronous motor can have leading power factor and can be operated in parallel to induction motors and other lagging power factor loads thereby improving the system power factor. 
• In synchronous motor the speed remains constant irrespective of the loads. This characteristics helps in industrial drives where constant speed is required irrespective of the load it is driving. It also useful when the motor is required to drive another alternator to supply at a different frequency as in frequency changes
• Synchronous motors can be constructed with wider air gaps than induction motors which makes these motors mechanically more stable
• In synchronous motors electro-magnetic power varies linearly with the voltage
• Synchronous motors usually operate with higher efficiencies ( more than 90%) especially in low speed and unity power factor applications compared to induction motors

  Disadvantages or Demerits:

• Synchronous motors requires dc excitation which must be supplied from external sources
• Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing
• The cost per kW output is generally higher than that of induction motors
• These motors cannot be used for variable speed applications as there is no possibility of speed adjustment unless the incoming supply frequency is adjusted (Variable Frequency Drives)
• Synchronous motors cannot be started on load. Its starting torque is zero
• These motors have tendency to hunt
• When loading on the synchronous motor increases beyond its capability, the synchronism between rotor and stator rotating magnetic field is lost and motor comes to halt 
• Collector rings and brushes are required resulting in increase in maintenance
• Synchronous motors cannot be useful for applications requiring frequent starting or high starting torques required

Engineering Services (IES) Questions

Engineering Services (IES) Questions and Answers:Machines 2

Indian Engineering Services Objective Questions and Answers

1) Match the Following   ( IES- 2003)

      List I                                        List II 

A. Transformer                           1. Slip Test

B. DC Motor                               2. Blocked Rotor Test

C. Alternator                               3. Sumpner's Test

D. Induction Motor                      4. Swinburne's Test

Codes:

         A          B          C         D

a)      3           4          1         2

b)      4           3          2         1

c)      3           4          2         1

d)      4           3          1         2

2) Possible three to three phase transformer connection for parallel operation is: ( IES- 2002)

a) Delta-Star to Delta-Star

b) Delta-Delta to Delta-Star

c) Star-Star to Delta-Star

d) Delta-Star to Star-Delta

3) Stepper Motors are widely used for  ( IES- 2002)

a) Very high power requirement

b) Very high speed operation

c) Very low speed operation

d) Control system applications

4) Armature torque of a d.c motor is a function of which of the following factors ( IES- 2003)

1. Speed

2. Field Flux

3. Armature Current

4. Residual Magnetism

Select the correct answer

a) 2 and 3

b) 1 and 4

c) 3 and 4

d) 1 and 2

5) The dummy coils in the DC machine are useful to

a) Increase the efficiency of the machine

b) Improve the commutation

c) Reduce the armature reaction

d) Maintain mechanical balance

6) The synchronous reactance is the ( IES- 2003)

a) Reactance due to armature reaction of the machine

b) Reactance due to the leakage flux

c) Combined reactance due to leakage flux and armature reaction

d) Reactance either due to armature reaction or leakage flux

7) The crawling in the induction motor is due to  ( IES- 2003)

a) Improper design of the stator laminations

b) Low voltage supply

c) Improper design of rotor laminations

d) Harmonics developed in the motor

8) In an Induction motor, when the number of stator slots is equal to an integral multiple of rotor slots  ( IES- 2003)

a) There must be a discontinuity in the torque slip charcteristics

b) A high starting torque will be available

c) The maximum torque will be high

d) The machine may fail to start

Answers:

(1) a (2) a (3) d (4) a

(5) d (6) c (7) d (8) d

Engineering Services (IES) Objective Questions

Engineering Services (IES) Objective Questions Answers:Machines3

Indian Engineering Services (IES) Objective Questions and Answers:

1) A self excited d.c shunt generator, driven by its prime mover at the rated speed fails to build up the voltage across its terminals at no load. What reason can be assigned for this? (IES 2006) 

a) The initial shunt field mmf does not assist the residual magnetism

b) The field circuit resistance is higher than the critical resistance

c) One of the inter-pole connection is removed

d) Brush axis slightly shift from the geometrical neutral axis of the machine

2) Wave winding is employed in a dc machine of: (IES 2006)

a) High current and low voltage rating

b) Low current and high voltage rating

c) High current and high voltage rating

d) Low current and low voltage rating

3) The resultant flux density in the air gap of a synchronous generator is the lowest during: (IES 2006)

a) Open circuit

b) Solid short circuit

c) Full load

d) Half load

4) If the load of an Induction motor is increased from no load to full load, its slip and the power factor will respectively (IES 2006)

a) decrease, decrease

b) decrease, increase

c) increase, decrease

d) increase, increase

5) A single phase Induction motor is running at N rpm. Its synchronous speed is Ns. If its slip with respect to forward field is 's', what is the slip with respect to the backward field is: (IES 2006)

a) s

b) -s

c) (1-s)

d) (2-s)

6)  Match the following: (IES 2003)

List I                                                List II 

A. DC Motor                        1. Circle Diagram

B. DC Generator                  2. V-Curves

C. Alternator                        3. Open circuit characteristics

D. Induction Motor                4.Speed-Torque characteristics

Codes:

         A          B          C         D

a)      4           3          1         2

b)      3           4          2         1

c)      4           3          2         1

d)      3           4          1         2

7)  A smaller air gap in a polyphase induction motor helps to: (IES 2004)

a) reduces the chances of crawling

b) Increases the starting torque

c) reduces the chances of cogging

d) reduce the magnetising current

8) If the supply voltage of the induction motor is reduced by 10%. By what percentage approximately will the maximum torque decreases? (IES 2004)

a) 5%

b) 10%

c) 20%

d) 40% 

 Answers:

(1) b (2) b (3) b (4) d

(5) d (6) c (7) d (8) c

Encyclopedia of Language and Linguistics - 14 Volume Set

Encyclopedia of Language and Linguistics - 14 Volume Set

The first edition of ELL (1993, Ron Asher, Editor) was hailed as "the field's standard reference work for a generation". Now the all-new second edition matches ELL's comprehensiveness and high quality, expanded for a new generation, while being the first encyclopedia to really exploit the multimedia potential of linguistics.

* The most authoritative, up-to-date, comprehensive, and international reference source in its field
* An entirely new work, with new editors, new authors, new topics and newly commissioned articles with a handful of classic articles
* The first Encyclopedia to exploit the multimedia potential of linguistics through the online edition
* Ground-breaking and International in scope and approach
* Alphabetically arranged with extensive cross-referencing
* Available in print and online, priced separately. The online version will include updates as subjects develop

ELL2 includes:
* c. 7,500,000 words
* c. 11,000 pages
* c. 3,000 articles
* c. 1,500 figures: 130 halftones and 150 colour
* Supplementary audio, video and text files online
* c. 3,500 glossary definitions
* c. 39,000 references
* Extensive list of commonly used abbreviations 
* List of languages of the world (including information on no. of speakers, language
family, etc.)
* Approximately 700 biographical entries (now includes contemporary linguists)
* 200 language maps in print and online

• The first Encyclopedia to exploit the multimedia potential of linguistics
• Ground-breaking in scope - wider than any predecessor
• An invaluable resource for researchers, academics, students and professionals in the fields of: linguistics, anthropology, education, psychology, language acquisition, language pathology, cognitive science, sociology, the law, the media, medicine & computer science.
• The most authoritative, up-to-date, comprehensive, and international reference source in its field

CONTENTS

Section headings (section editors):
Phonetics (John H. Esling, University of Victoria, Canada).
Phonology (Richard Wiese, University of Marburg, Germany).
Morphology (Laurie Bauer, Victoria University of Wellington, New Zealand). 
Syntax (Jim P. Blevins, University of Cambridge, UK). 
Typology & Universals (Bernd Heine, University of Cologne, Germany). 
Historical & Comparative Linguistics (Mark Hale, Concordia University, Canada). 
Sign Language (Bencie Woll, City University, England).
Foundations of Linguistics (Billy Clark, Middlesex University, UK). 
Semantics - grammatical (Osten Dahl, Stockholm University, Sweden). 
Semantics (logical & lexical) (Keith Allan, Monash University, Australia). 
Pragmatics (Jacob Mey, University of Southern Denmark, Denmark).
Lexicography (Patrick Hanks, Akademie der Wissenschaften, Germany). 
Philosophy & Language (Rob Stainton, University of Western Ontario, Canada; and Alex Barber, Open University, England). 
Translation (Kirsten Malmkjaer, Middlesex University, England). 
Text Analysis & Stylistics (Catherine Emmott, University of Glasgow, Scotland). 
Spoken Discourse (Rosanna Sornicola, Universita di Napoli Federico II, Italy). 
Linguistic Anthropology (Michael Silverstein, University of Chicago, USA).
Variation & Language (Miriam Meyerhoff, University of Edinburgh, Scotland). 
Society & Language (Raj Mesthrie, University of Cape Town, South Africa). 
Education & Language (Bernard Spolsky, Bar-Ilan University, Israel). 
Applied Linguistics (Margie Berns, Purdue University, USA). 
Law & Language (John Gibbons, Hong Kong Baptist University, Hong Kong; and Dennis Kurzon, Haifa University, Israel). 
Semiotics (Marcel Danesi, University of Toronto, Canada). 
Media & Language (Sue McKay, University of Queensland, Australia). 
Politics & Language (Ruth Wodak, University of Lancaster, UK).
Religion & Language (Erik Fudge, University of Reading, UK). 
Medicine & Language (Francoise Salager-Meyer, Merida, Venezuela). 
Psycholinguistics (Anne Anderson, University of Glasgow, Scotland). 
Animal Communication (Marc Naguib, Universitat Bielefeld, Germany). 
Language Acquisition (Elena Lieven, Max Planck Institute for Evolutionary Anthropology, Germany). 
Cognitive Science (Jon Oberlander, University of Edinburgh, Scotland). 
Language Pathology & Neurolinguistics (Harry A. Whitaker, Northern Michigan University, USA). 
Natural Language Processing, Machine Translation and Computational Corpus Linguistics (Graeme Hirst, University of Toronto, Canada). 
Speech Technology (Jennifer Lai, IBM Research, New York, USA).
Computational Linguistics (Allan Ramsay, UMIST, Manchester, UK).
Languages of the World (Sarah Ogilvie, Oxford English Dictionary, UK). 
Countries & Languages (Lutz Marten, School of Oriental and African Studies, London). 
Writing Systems (Peter T. Daniels, Bronx, New York, USA). 
Biographies (Kurt Jankowsky, Georgetown University, USA). 
History of Linguistics (Andrew Linn, The University of Sheffield, UK). 
Glossary (Philip Durkin, Oxford English Dictionary, UK and Kathryn Allan, University of Salford, UK).

BASICS ON MOSFET.

MOSFET:

MOSFET stands for metal oxidesemiconductor field effect transistor. It is capable of voltage gain and signal power gain. The MOSFET is the core of integrated circuit designed as thousands of these can be fabricated in a single chip because of its very small size. Every modern electronic system consists of VLST technology and without MOSFET, large scale integration is impossible.

It is a four terminals device. The drain and source terminals are connected to the heavily doped regions. The gate terminal is connected top on the oxide layer and the substrate or body terminal is connected to the intrinsic semiconductor.

MOSFET has four terminals which is already stated above, they are gate, source drain and substrate or body. MOS capacity present in the device is the main part. The conduction and valance bands are position relative to the Fermi level at the surface is a function of MOS capacitor voltage. The metal of the gate terminal and the sc acts the parallel and the oxide layer acts as insulator of the state MOS capacitor. Between the drain and source terminal inversion layer is formed and due to the flow of carriers in it, the electric current flows in MOSFET the inversion layer is properties are controlled by gate voltage. Thus it is a voltage controlled device.

Two basic types of MOSFET are n channel and p channel MOSFETs. In n channel MOSFET iselectric current is due to the flow of electrons in inversion layer and in p channel electric current is due to the flow of holes.

Working Principle of MOSFET

The working principle of MOSFET depends up on the MOS capacitor. The MOS capacitor is the main part. The semiconductor surface at below the oxide layer and between the drain and source terminal can be inverted from p-type to n-type by applying a positive or negative gate voltages respectively. When we apply positive gate voltage the holes present beneath the oxide layer experience repulsive force and the holes are pushed downward with the substrate. The depletion region is populated by the bound negative charges, which are associated with the acceptor atoms. The positive voltage also attracts electrons from the n+ source and drain regions in to the channel. The electron reach channel is formed. Now, if avoltage is applied between the source and the drain, electric current flows freely between the source and drain gate voltage controls the electrons concentration the channel. Instead of positive if apply negative voltage a hole channel will be formed beneath the oxide layer.

Now, the controlling of source to gate voltage is responsible for the conduction of electric current between source and the drain. If the gate voltage exceeds a given value, called the three voltage only then the conduction begins.

The electric current equation of MOSFET in triode region is -


Where, un = Mobility of the electrons
Cox = Capacitance of the oxide layer
W = Width of the gate area
L = Length of the channel
VGS = Gate to Source voltage
VTH = Threshold voltage
VDS = Drain to Source voltage.

P-Channel MOSFET

MOSFET which has p - channel region between source any gate is known as p - channel MOSFET. It is a four terminal devices, the terminals are gate, drain, source and substrate or body. The drain and source are heavily doped p+ region and the substrate is in n-type. Theelectric current flows due to the flow of positively charged holes that’s why it is known as p-channel MOSFET. When we apply negative gate voltage, the electrons present beneath the oxide layer, experiences repulsive force and they are pushed downward in to the substrate, the depletion region is populated by the bound positive charges which are associated with the donor atoms. The negative gate voltage also attracts holes from p+ source and drain region in to the channel region. Thus hole which channel is formed now if a voltage between the source and the drain is applied electric current flows. The gate voltage controls the hole concentration of the channel. The diagram of p- channel enhancement and depletion MOSFET are given below.

p - channel MOSFET - Enhancement Mode

p - channel MOSFET - Depletion Mode

N-Channel MOSFET

MOSFET having n-channel region between source and drain is known as n-channel MOSFET. It is a four terminal device, the terminals are gate, drain and source and substrate or body. The drain and source are heavily doped n+ region and the substrate is p-type. The electric current flows due to flow of the negatively charged electrons, that’s why it is known as n- channel MOSFET. When we apply the positive gate voltage the holes present beneath the oxide layer experiences repulsive force and the holes are pushed downwards in to the bound negative charges which are associated with the acceptor atoms. The positive gatevoltage also attracts electrons from n+ source and drain region in to the channel thus an electron reach channel is formed, now if a voltage is applied between the source and drain. The gate voltage controls the electron concentration in the channel n-channel MOSFET is preferred over p-channel MOSFET as the mobility of electrons are higher than holes. The diagrams of enhancements mode and depletion mode are given below.

n - channel MOSFET - Enhancement Mode

Applications of Cyclo Converter

What are the applications of cyclo converter?

A cycloconverter or a cycloinverter converts an AC waveform, such as the mains supply, to another AC waveform of a lower frequency, synthesizing the output waveform from segments of the AC supply without an intermediate direct-current link. Cycloverters are used in very large variable frequency drives, with ratings of several megawatts. They are used in the induction heating, and in high power applications. A cycloconverter is a type of power controlled in which an alternating voltage at supply frequency is converted directly to an alternating voltage at load frequency without any intermediate d.c stage. A cycloconverter is to controlled through the timing of its firing pulses, so that it produces analternating output voltage. By controlling the frequency and depth of phase modulation of the firing angles of the converters, it is possible to control the frequency and amplitude of the output voltage.

Thus, a cycloconverter has the facility for continuous and independent control over both its output frequency and voltage. This frequency is normally less than 1/3 of the input frequency. The quality of output voltage wave and its harmonic distortion also impose the restriction on this frequency. The distortion is very low at low output frequency. Cycloconverter eliminates the use of flywheel because the presence of flywheel in machineincreases torsional vibration and fatigue in the component of power transmission system. There are presently two main applications for the cycloconverter. In first application area, the Cycloconverter is used as a variable frequency variable speed drives for AC machines. The input of the cycloconverter is connected to a power supply with fixed frequency and the machine to be driven is connected to the outputof the cycloconverter. In the second application area, in contrast, the cycloconverter is used to provide constant frequency power output from avariable frequency power source. Due to the power capability of the devices and the upper frequency limitation of the output, it is possible to use the thyristor line-commutated cycloconverters to control low speed but very large horsepower motors.

Triggering of Thyristor

Today, the world is witnessing energy crises. This necessitates the efficient utilization of electrical energy. Power electronics helps in accomplishing this task of efficient energy usage. thyristoris an important family of devices in power electronic system. SCR (Silicon Control Rectifier) is the important device in the thyristor family. As the SCR is used more widely hence SCR is known as thyristor.

Applications of power electronics deals with the flow of electronic power. In order to achieve better efficiency the semiconductor devices used in power electronic system are operated as switches. One of the semiconductor device used in a power electronic system is thyristor. Few of the other devices used as switches are diodes, bipolar junction transistors(BJTs) , metal oxide semiconductor field effect transistor (MOSFET), insulated gate bipolar transistor (IGBT), gate turn off thyristor(GTOs).

The term thyristor is a general name for a family of semiconductor device. Thyristor families consist of large number of switching devices.

 A thyristor is a solid state power semiconductor device. It has four alternating layer and three junctions J1, J2, J3 of N and P type semiconductor material. A thyristor has three terminals. Namely anode, cathode and gate. Thyristor acts as a bistable switch, conducts when its anode is made positive with respect to cathode and gate signal (between gate terminal and cathode terminal) is applied.

Triggering means turning ON of a device from its off state. Turning ON of a thyristor refers to thyristor triggering. Thyristor is turned on by increasing the anode electric currentflowing through it. The increase in anode electric current can be achieved by many ways.

1. Voltage Thyristor Triggering: - Here the applied forward voltage is gradually increased beyond a pt.known as forward break over voltage VBO and gate is kept open. This method is not preferred because during turn on of thyristor, it is associated with large voltage and large electric current which results in huge power loss and device may be damaged.

2. Thermal Thyristor Triggering: - If the temperature of the thyristor is high, it results in increase in the electron-hole pairs. Which in turn increase the leakage electric current α1 & α2 to raise. The regenerative action tends to increase (α1 + α2) to units and the thyristor may be turned on. This type turn on is not preferred as it may result in thermal turn away and hence it is avoided.

3. Light Thyristor Triggering: - These rays of light are allowed to strike the junctions of thethyristor. This results in increase in number of electron-hole pair and thyristor may be turned on. The light activated SCRs (LASER) are triggered by using this method.

4. dv/dt triggering: - If the rate of rise of anode to cathode voltage is high , the chargingelectric current through the capacitive junction is high enough to turn on the thyristor. A high value of charging electric current may destroy the thyristor hence the device must be protected against high dv/dt.

5. Gate triggering: - This method of thyristor triggering is widely employed because of ease C8 control over the thyristor gate triggering of thyristor allows us to turn of the thyristorwhenever we wish. Here we apply a gate signal to the thyristor. Forward biased thyristor will turn on when gate signal is applied to it. Once the thyristor starts conducting, the gate loses its control over the device and the thyristor continues to conduct. This is because of regenerative action that takes place within the thyristor when gate signal is applied.

When the thyristor is forward biased, and a gate signal is injected by applying positive gatevoltage is applied between gate and cathode terminals, then the thyristor is turned on.

Fig. shows the waveform of anodeelectric current after the application of gate signal. ton is the turn on delay time. The turn on delay time is the time interval between the application of gate signal and conduction of thyristor. The turn on delay time ton is defined as the time interval between 10% of steady state gate electric current 0.1Ig and 90% of steady state thyristor on state electric current 0.9IT.ton is the sum of delay time td and rise time tr. The delay time td is defined as the time interval between 10% of steady state gate electric current(0.1 Ig) and 10% of on state thyristorelectric current (0.1IT). The rise time tr is defined as the time taken by thethyristor anode electric current from 10% of thyristor on state electric current(0.1IT) to 90% of on state thyristor electric current (0.9IT).

While designing gate thyristor triggering circuit following points should be kept in mind.

1. When thyristor is turned on the gate signal should be removed immediately. A continuous application of gate signal even after the triggering on and thyristor would increase the power loss in the gate junction.

2. No gate signal should be applied when thyristor is reversed biased; otherwise thyristor
3. The pulse width of the gate signal should le longer than the time required for the anodeelectric current to rise to the holding electric current value IH.

Thyristor can not be turned off by applied negative gate signal. To stop the conduction of thethyristor we have to bring the anode electric current flowing through the thyristor to a level below holding electric current level. Holding electric current may be defined as the minimum anode electric current required to maintain the thyristor in the on state without gate signal below which the thyristor stops conduction.

If we want to turn on the thyristor, the electric current flowing through the thyristor must be greater than latching electric current of the thyristor. Latching electric current is the minimum anode electric current required to maintain the thyristor in the on state with at gate signal. Here we should note that even the thyristor anode electric current falls below latching electric current (once it is turned on and gate signal is removed) thyristor does not stop conduction. But if it falls below holding electric current (Latching electric current is more than holding current) then thyristor turn off.

CHARACTERISTICS OF THYRISTOR

CHARACTERISTICS OF THYRISTOR

V-I Characteristics of a Thyristor

• Reverse Blocking Mode 

• Forward Blocking Mode

• Forward Conduction Mode

A thyristor is a four layer 3 junction p-n-p-n semiconductor device consisting of at least three p-n junctions, functioning as an electrical switch for high power operations. It has three basic terminals, namely the anode, cathode and the gate mounted on the semiconductor layers of the device. The symbolic diagram and the basic circuit diagram for determining the characteristics of thyristor is shown in the figure below,

V-I Characteristics of a Thyristor

From the circuit diagram above we can see the anode and cathode are connected to the supply voltage through the load. Another secondary supply Es is applied between the gate and the cathode terminal which supplies for the positive gate electric current when the switch S is closed.

On giving the supply we get the requiredV-I characteristics of a thyristor show in the figure below for anode to cathode voltage Vaand anode electric current Ia as we can see from the circuit diagram. A detailed study of the characteristics reveal that the thyristor has three basic modes of operation, namely the reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. Which are discussed in great details below, to understand the overall characteristics of a thyristor.

Reverse Blocking Mode of Thyristor

Initially for the reverse blocking mode of the thyristor, the cathode is made positive with respect to anode by supplying voltage E and the gate to cathode supply voltage Es is detached initially by keeping switch S open. For understanding this mode we should look into the fourth quadrant where the thyristor is reverse biased. 

Here Junctions J1 and J3 are reverse biased whereas the junction J2 is forward biased. The behavior of the thyristor here is similar to that of two diodes are connected in series with reverse voltage applied across them. As a result only a small leakage electric current of the order of a few μAmps flows. This is the reverse blocking mode or the off-state, of the thyristor. If the reverse voltage is now increased, then at a particular voltage, known as the critical breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse electric current increases rapidly. A large electric current associated with VBR gives rise to more losses in the SCR, which results in heating. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reversevoltage applied across a thyristor is less than VBR, the device offers very high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as open circuit.

Forward Blocking Mode

Now considering the anode is positive with respect to the cathode, with gate kept in open condition. The thyristor is now said to be forward biased as shown the figure below.

As we can see the junctions J1 and J3arenow forward biased but junction J2goes into reverse biased condition. In this particular mode, a small current, called forward leakage electric current is allowed to flow initially as shown in the diagram for characteristics of thyristor. Now, if we keep on increasing the forward biased anode to cathode voltage.

In this particular mode, the thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward break over voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending beyond a certain point, then the reverse biased junction J2 will have an avalanche breakdown at a voltage called forward break over voltage VB0 of the thyristor. But, if we keep the forward voltage less than VBO, we can see from the characteristics of thyristor, that the device offers a high impedance. Thus even here the thyristor operates as an open switch during the forward blocking mode.

Forward Conduction Mode

When the anode to cathode forward voltage is increased, with gate circuit open, the reverse junction J2 will have an avalanche breakdown at forward break over voltage VBO leading to thyristor turn on. Once the thyristor is turned on we can see from the diagram for characteristics of thyristor, that the point M at once shifts toward N and then anywhere between N and K. Here NK represents the forward conduction mode of the thyristor. In this mode of operation, the thyristor conducts maximum electric current with minimum voltagedrop, this is known as the forward conduction forward conduction or the turn on mode of the thyristor.