European Politics Essay Example | Topics and Well Written Essays - 2250 words

European Politics - Essay Example The constitution of the European countries have explicitly protected the interests of the minority communities; and encouraged their participation in social, economic and political fronts. Childre (2003) observed that the constitution of the European countries supports democratic practices, and firmly encourages that the democratic notions shall be adopted and implemented on the basis of its merit. The European society is cosmopolitan society, but the majority of the aboriginals are reluctant to consider this as fact. The contemporary political theorists considered cosmopolitanism as "citizenship of the world, which is a critique of ordinary theories of political obligation, with their tendency to focus on our duties to fellow citizens, not to people elsewhere", as discussed by Patrick (2005). The consequence of the cosmopolitanism is expected to be "single world government with corresponding global citizenship"; this was evaluated by Patrick (2005). Surprisingly such aspirations have not discussed by the serious circles. Ulrich (2006) proposed that the modified and renewed version of the cosmopolitanism includes "everyone in the world in a single global web of mutual obligations". However the reservations and criticism mounted against cosmopolitanism is relevant to the negligence of the "obligations of reciprocity"; there has been consensus on the fact that the society has "obligations to give benefits in return for benefits received". The issue commonly observed by the society at large is relev ant to the psychological possibility based upon the desirability claims, "the elimination of a special motivating attachment to fellow-citizens is not possible, but the elimination of special motivating attachments to fellow-citizens is expected to develop "certain desirable form of political life impossible". In this context, the cosmopolitan has revealed two broad options i.e. "the viability of politics as usual depends not upon certain beliefs that fellow-citizens deserve more of one's service, but upon commitments to the polity itself", therefore if the possibility of the strictly cosmopolitan is possible then Patrick (2005) believes that "a commitment to a universal set of principles embodied in a particular political constitution and a particular set of political institutions" is engaged. The practicability of such desirable politics has the potential to disarm the anti-cosmopolitan; Ulrich (2006) believes that this has provided that the denial of the form of political life is practical, and for this purposes the "moral commitments run over into a discussion of political theory". Literature Review Wayne (2001) believed that within the European society we still find traces of non-compliance against democratic principle of equality and justice. Racism and xenophobia has limited and discouraged the participation of the minority or under-privileged communities into political and social affairs. The European society which has been staunch supporter and enthusiast towards the proactive participation of the minority communities into public and private affairs; the current political state of some European countries indicates pessimistic outlook, where the interests and rights of the minority communities have been violated or ignored. The European society is multi-linguistic, multi-ethnic, multi-cultural and multi-religious society. The penetration of the Africans, Arabs

Advancements to allow fast HPLC Essay Example | Topics and Well Written Essays - 3750 words

Advancements to allow fast HPLC - Essay Example One of the major domains of chemistry that promises lots of advancements due to extensive research is Analytical Chemistry. Analytical Chemistry is a branch of chemistry that deals with the analysis of the chemical nature of matter. Analysis of chemical composition of matter is the supporting pillar to the development of any new product or structure. The collective term used for the various laboratory procedures available to separate matter into its constituent compositions to analyze their chemical nature is known as Chromatography. High Performance Liquid Chromatography (HPLC) is a type of Column Chromatography, which is used to separate a mixture into its constituent components with the use of chemical interactions. A series of chemical interactions are used in between the substance of subject and the chromatography column.Although the concept of Liquid Chromatography dates back to 1940s, the path breaking research, in the field of HPLC is being carried out on a large scale even t oday. More samples of matter are being analyzed for their chemical behavior than ever before in the recent times. Chemists, researchers and pharmaceutical companies need to perform chromatography on large quantities of samples. The call of the day is to speed up the process of HPLC while at the same time not forsake its accuracy. The solution is to speed up the process of HPLC by a magnitude. This process known as Fast HPLC is a boon to all people and organizations concerned. The most common principle behind Fast HPLC is rapid analysis using short columns. The particles filled in the columns are also short. Various developments are taking place in the HPLC technology to aid Fast HPLC and in all cases it is observed that Fast HPLC operate nearly five times faster while at the same time not sacrificing performance, reliability and simplicity. This report aims to enumerate the various developments that are being carried out in the field of HPLC that aid Fast HPLC. The HPLC process is described with relevant terms. Several research works being carried out are explained with the intended purpose of the research. Particular topics covered in this report include advancements in monolithic columns, small particle columns and high pressure and high temperature modes and relevant developments in other methods. The report aims at understanding the advantages brought about by these advancements in the field of HPLC. Specific concentration of the report is the developments in the field of columns used in HPLC. An analysis is made as to how, advances in the columns speeds up the process of chromatography. The applications of these developments are enumerated and an analysis is provided as to how the modern developments have changed the way the process is conducted. [1][2][3] Etc. The Process The performance of HPLC is affected by a number of factors. The most important among the factors are 'Column Efficiency', 'Performance of Mobile Phase', 'Performance of Stationary Phase', 'Injectors', 'Pumps', 'Detectors', 'Columns and Column Packing' and 'Automation'. Advancements in several of these factors have led to the development of Fast HPLC. Advancements: Fast HPLC Monolithic columns: A method popularly used as a separation media for liquid chromatography has been that of monolithic stationary phases and columns. These have not taken much time to become popular however some of their features have yet to be clearly studied. Researchers comment that chromatographic behavior of these columns should be studied along with their physico-chemical and structural properties to attain progress in their design and production. Recently a technique known as fast high performance LC is becoming popular for laboratory. Here shorter columns that have higher flow rates are used thereby analysis time is

Problems and Risks with Information Security in the modern world Research Paper

Problems and Risks with Information Security in the modern world - Research Paper Example This essay discusses that increasing and tougher market competition, globalisation, rapid developments in technology, higher demands from customers and other similar factors are some of the changes that companies are confronted with as they try to create their niche in the global market. In response, companies have redefined and re-shaped their strategies in order to equip them with the necessary tools and mechanisms that will enable the organisation to address the challenges brought by change. One of the most important challenges that organisations have to deal with is the rapid evolution and changes in information technology. Information is one of the primary assets organisations. It is being exchanged among organisations, employees, customers, management, partners and other stakeholders of the company. Meanwhile, developments in communication and computer technologies have paved for faster exchanges of information through the use cellular phones, internet, and other technological devices that allow people to be connected anytime, anywhere. In this regard, information technology has become an integral part and tool of organisations as firms respond to the challenges of the market and cope with the increasing demands of the customers. However, as information becomes more valuable to firms and organisational processes become more dependent on information systems (IS), IS security has become one of the major issues that modern enterprises have to tackle. (Belsis, Kokolakis & Kiountouzis 2005). Threats, identity theft, fraud, viruses, data integrity, data protection and other similar concerns have been part of the many issues that IS security have to deal with as the proliferation of the use of computers continue in the Information age. In this regard, this research will focus on the problems and risks in information security in general and in the banking sector in particular. Objectives of the Study The study intends to examine the nature of information security . It also seeks to determine the various problems and risks that information security is dealing with. Likewise, the study aims to identify some of the theories that are proposed in order to address the concerns and issues pertinent to information security. In addition, it aims to understand information security from the viewpoint of the banking sector. Finally, the study seeks to test information that will identify further concerns in information security in the banking sector. Research Questions In view of the integral role that information security in organisations, the research will be addressing two main questions and these are, first,

Teacher Tenure in California Research Paper Example | Topics and Well Written Essays - 1250 words

Teacher Tenure in California - Research Paper Example he teachers become eligible for the tenure they gain security towards their position and can only be dismissed if the school authority is able to put forward a just cause. The state law even proposed that the tenured teachers may be dismissed if they are giving unsatisfactory performance (Jederberg 43). However, as per the policy if the performance of students is low in the state it is not considered to be a condition for dismissal of teachers which in turn has resulted into more number of students exhibiting low performance. The dismissal conditions as highlighted by the court decision states that the school authority needs to give a prior notice to the employee before initiating any form of proceedings regarding dismissal due to unsatisfactory performance of a teacher. The teacher tenure policy majorly focused towards the well being of students and to retain those teachers who performed well and discard those who are not able to meet the standards of the school policy. The ones who supported the policy stated that the policy majorly aimed towards development and growth of students so that they are able to perform well in their career path. The other end of this provision was that it provided the flexibility to the school authority to focus more on performing teachers and reduce the cost by eliminating those teachers who did not perform well. The notice that would be given to the teachers before the dismissal must also include some of the behavioral instances that need to be communicated three months prior to filing charges. The major purpose of the policy was not to hide the underperforming teachers behind the tenure and to facilitate more of academic achievement of the students. There are almost 6 million students who are enrolled in the public s chools of California and thus the law was to support these students so that they are provided with an appropriate guidance. The tenure is always a security provision for teachers but this policy was aligned more towards

Southwest vs. Jetblue Airlines Essay Example | Topics and Well Written Essays - 1000 words

Southwest vs. Jetblue Airlines - Essay Example Southwest Airlines was first incorporated as Air Southwest Company on 15, March 1967. Its mission is to provide air transport to passengers within Texas State. However, Southwest Airline is currently operating in six states. It is serving over seventy cities, which enables the airline to make about three thousand three hundred flights daily. The porter’s five forces of the Airline industry with reference to Southwest Airline The first factor is the new entrant’s factor. Air industry is critical to economic growth and development. It is critical in any economy. Therefore, it is an industry perceived by many as highly lucrative. However, huge amount of capital is required to start air transport business and industry is riddled with high level risks. The industry also experiences complicated licensing, certification and regulations procedures, which makes it hard to penetrate. Furthermore, increasing cost of fuel makes it difficult for many people to venture it. The second factor is threats of substitutes. The substitutes to air transport are road, rail and water. Southwest Airline faces competition from road because most (80%) of its customers are domestic customers. If road transport improves, the airline may lose bulk of its domestic customers. Southwest Airline has been lobbying against the introduction of high speed rail transport services in the State of Texas. This is to prevent competition from substitutive high speed train services. Third factor to consider is the bargaining power of customers. Customers do not have a strong bargaining power in the air industry. As a result, they do not have a collective bargaining power to influence air prices. The fourth factor is the bargaining power of suppliers. Suppliers of aircrafts and spares are influential to prices making it difficult for operators to bargain. This is due to the standards on aircrafts and its spares that FAA imposes on operators. The operators must purchase genuine parts that are mostly expensive. The suppliers of fuel are few; therefore, they largely influence the prices of fuel. Airline operators do not have powers over the fuel suppliers and are left to develop ways of improving fuel efficiency. For example, Southwest Airlines introduced use of high pressure water to clean the engine, an exercise that improves fuel efficiency by 1.9 percent. The company has also managed to contain its employees through strong team coordination, open communication and promotion of work-life balance. Furthermore, the employees are also supported by top management. This has made employees proud of the company. Fifth factor to consider is the level of rivalry within the industry. This is among the real threats that can bring the industry down. Southwest Airline faces direct competition from JetBlue and AirTran Holdings, which are also low cost carriers. To minimize competition and increase its revenue base, the company acquired Muse Air in 1985, Morris Air in 1993, ATA Airlin es in 2008 and Air Tran Airways in 2011. Acquisitions enabled Southwest Airline to absorb key routes in the Pacific Norwest and acquire operating certificate as well as landing slots in LaGuardia Airport. However, the company failed to acquire Frontier Airlines thus lost Denver market. Strategic valuation method The strategic valuation method that is useful in valuing the company is the shareholder Value Analysis. Shareholder value analysis refers to financial analysis, which estimates the

Human Element Essay Example for Free

Human Element Essay This study is related to an important topic that influence the evolution of both public and private institutions via improving human resource management and, hence improving the human resource itself, which occupies a vital and outstanding position in the comprehensive development strategies. Also, because of the role that effective human resource plays in planning and implementing development. Therefore, human element is considered an investment in the bright future of the institutions. Human resource is an innovative element that is described as an interactive rather than a rigid element, which is able to coordinate, maintain a balance between the various constituent of production and take the responsibility for the results of operations. On the other hand, external circumstances, such as the economic, political, social cultural and environmental factors, affect human element more than any other element. The researcher has selected Giad Automobiles Manufacturing Co. LTD, as a case study to introduce the influence of human resource development on employees’ performance. The results have been determined based on the analysis of the employees’ responses to the questionnaire that surveyed the employees’ community in the company. The questionnaire has tested the following hypotheses: 1- There is a relationship between the development of human resource management and the development of the working human element. 2- There is a relationship between the development of the human element and the development and improvement of performance in the institution, which leads to an increase in the production and, hence achieving the objectives. The researcher has applied several methodologies, such as case study methodology, descriptive methodology, and statistical analysis. The researcher has confirmed the validity of all the above hypotheses, in addition to the research outcomes that have led the researcher to conclude the study with some suggestions and recommendations.

Control Of A Switched Reluctance Motor Engineering Essay

Control Of A Switched Reluctance Motor Engineering Essay The main objective of this chapter is to spot the light on the practical performance of some form of control on an actual switched reluctance motor in preference to merely theoretical method including simulation and modelling. Moreover, the design of the controller involves reducing the noise. The crucial motivation for this is to enhance new expertise, sensibly regarding circuit design and construction the use of a Programmable interface controller (PIC). A switched reluctance motor (SRM) has been appeared since the nineteenth century, but the renewal of this motor has been to discover a high-power switched device [13]. The earlier motor was facing a controversial issue in their control where the power electronic switching was very expensive and has many drawbacks. The Switched Reluctance Motor (SRM) is an electromagnetic, rotary machine in which torque is produced by the tendency of its movable part to move to a position where the inductance of the excited winding is maximized [12]. Furthermore, (SRM) has been proposed for variable speed applications [13]. In general a salient-pole synchronous machine without field excitation or permanent magnet is called a reluctance machine. From the general definition (SRM) is classified as a synchronous machine, but has different construction. These differences are both stator and rotor have salient poles. However, the stator has wound field coils but the rotor has no coils or permanent magnet. Moreover, the stator has higher number of poles or (tooth) than the rotor. While each pole in the rotor is excited by the opposite pole in the stator due to a sequence of current pulses which produce magnetic field on each stator poles. The Switched Reluctance Motor (SRM) can be more capable for variable speed than AC and DC motor. Since it is simple, robust, has a high efficiency and high power density [3, 10, 12]. Figure 1.1: two- two poles SRM [12] Over the past 30 years the power electronics growth has made the exploit of the characteristics of reluctance machines sufficiently well. Consequently, several successful products are now manufactured. The rotation of the motor depends on the angle between the rotor poles and the stator poles where the current is switched on or off to each poles in the stator where the position of the rotor is very significant. For the motoring principle example, two- two poles (SRM) where stator has two poles with two wound field coils connected in series and two poles in the rotor without magnets or windings and free to rotate as it shown in figure 1.1. If the rotor poles edge is started to be aligned, the current is switched on. Consequently, the inductance is increased and the torque in the rotor is occurred to pull the rotor in clockwise direction. The direction of the rotation is addressed by the current. The current is switched off; when the rotor poles are fully aligned with the stator poles that make the rotor free to move. However, if the current is not switched off the torque at the fully aligned is become negative that produces an attraction between the poles where the negative torque and positive torque cancel each other [12]. The ideal current waveform is therefore a series of pulses synchronized with the rising inductance intervals. The cycle of torque production associated with one current pulse is called a stroke.[12] As a result, to increase the resolution more teeth can be add to the stator and the rotor. From the instantaneous electromagnet torque equation which was derived by T.J.E.Miller(2001) [12] Te = i2/2 dL/dÃŽÂ ¸ that shows the torque is proportional to the square of the current. Therefore, the current is always unipolar. However, at the end of each stoke the voltage must be reversed to return the flux linkage to zero. The power switching IGBT, GTO and MOSFET or any other switching devices can be used in the power electronic topologies [3]. These switches are used to open and close the exiting power in the stator winding. 1.5. Applications of SRM Since the SRM has a significant characteristic operation and design which shows many advantages and makes them suitable for various applications. R.Krishnan (2001) [10] shows the wide range of applications. For example, plotter drive, air-handler motor drive, hand fork lift/ pallet truck motor drive, door actuator and washers and dryers machine, those applications are low power applications where the drives are less than 3hp. Moreover, the medium power applications range is less than 300kw such as industrial general purpose drive, train air conditioning drive and mining drive. However, the SRM did not find much attention by the manufacturers in this range of power. The high power drives are still under study for drives up to 1000hp for fan and pump applications where the converter is very competitive. The high speed applications such as screw rotary compressor drives, centrifuge for medical applications, and aerospace applications the SRM is a perfect option for them due to the small size of the rotor and high power density. Some efforts of study are in Mild Hybrid Vehicles that is reported by (Watterson, P.A. ; et al) [17] in 2008. 1.6 Research area Since the SRM has appeared and has disadvantages such as reduction of acoustic noise and torque ripple. According to the noise and vibration sources can be classified into four main categories: magnetic, mechanical, aerodynamic, and electronic. The classification was done by R.Krishnan (2001) [10] as it shows in figure 1.2. Most researches are based on the design of the motor or in the controller to improve the motor performers and reduce the noise. Iqbal Husain (1994) [8] discusses the effect of the high ripple toque which causes vibration and acoustic noise. The case was in driving SRM for low speed and high performance applications by a new PWM strategy current control. The method was based on optimum profiling of the phase current during an extended overlapping conduction period of two phases. The result shows a smooth operation with minimum torque pulsations by controlling the current profiling. Moreover, other studies were in magnetic radial force as M. N. Anwar (2000) [1] show s the lower noise is occurred when the dominant mode frequencies is high. As a result the research has experimented a proposed design considerations to a 4-phase, 8/6 (1-repetition) and a 3-phase, 12/8 (2-repetition) 1.0 kW SRM with low acoustic noise requirement. The results of the proposal design shows that the noise level has been reduced; however, a 3-phase SRM is noisier than a 4-phase SRM. There are other research was on reducing the noise by Adding extra winding to the phase winding to reduce magnetic stress during commutations with Two stage power converter. Adding extra winding to the phase winding in 6/4 SRM with Two stage power converter shows a significant improvement in the efficiency and reduce in the vibration and noise [16]. The electronic sources of noise occur by reason of the harmonics of voltage and current. The significant research is reported by (Lecointe J.P; et al) in (2004) [11]. The research was in adding an auxiliary winding to the model of a BDFRM (brus hless doubly-fed reluctance machine) to shows that the additional winding has a benefit in reducing the noise in SRMs where a specially derived current is injected to create a suppression force. The BDFRM is used to analyse the equivalent circuit where the equivalent circuit of the model is similar to SRM. However, the process still needs to be complete using two current sources. Figure 1.2: sources of noise in electrical machines [10] Advantages and disadvantages of 3 basic power electronic circuits There are many power electronic topologies that is used for SRM depends on the machine phase configurations. R. Krishnan (2001) [10], figure 1.3, has classified the power converters for switched reluctance machines. The simple three topologies are: Figure 1.3: Classification of power converters for SRM [10] 1.7.1. Single-Switch-per-Phase Circuits (Split dc supply converter) The basic operation of this topology, figure 1.4, is to split the reverse dc power supply into two capacitors. The phase voltage is half the dc voltage when the switch is on and negative half voltage when the switch is off. Consequently, this configuration is a disadvantage. Moreover, the maximum speed and the output power are half the rated while the current and the voltage are half. This topology is only used for low-cost application due to the high power losses. However, the advantages of this topology are: simple and less cost. [3, 10] Figure 1.4: Single-Switch-per-Phase Circuits (Split dc supply converter) [10] 1.7.2. Single-Switch-per-Phase Circuits (Bifilar type) The basic operation of this topology uses a bifilar winding, figure 1.5, with the motor winding to regenerate the stored energy to the supply that the phase winding is connected in series with the switch, and the diode is connected in series with the bifilar winding. The reflected energy is an advantage for this topology in some applications, where the transistor voltage is much higher than the supply voltage. However, the cost increases as the extra winding is added to the motor and the complex design of the power electronic. Moreover, the power density of the motor reduces because of the bifilar winding. [3,10] Figure 1.5: Single-Switch-per-Phase Circuits (Bifilar type) [10] 1.7.3. Two-switch/phase Asymmetric Bridge Converter From the half bridge topology that is illustrated in figure 1.6. if both Q1 and Q2 are turned on the apply voltage at the winding is equal to the supply voltage. However, if both Q1 and Q2 are turned off the apply voltage at the winding is equal to a negative supply voltage where D1 and D2 to avoid the freewheel currents. The advantages of this topology that it can give a negative voltage to reduce the torque ripple refer to noise for a high performance SRM drive system and it can be used for generator or motor operations. The disadvantages more control is needed, more fault in the switching [3, 10, 12]. Figure 1.6: Two Switched / Phase Asymmetric Bridge Converter [12] 1.8 Speed and Position Controller The switched-reluctance motor is basic control system form of a torque-controlled drive as compared in performance to a D.C motor (separately-excited). Commonly, controlled speed or even position is the most requirements. The SR motors speed increases if torque is still produced. As the D.C. drive the torque of SR motor controller is included within a speed regulating loop. The rotor position sensor, which is the encoder, is used as a feedback to derive the speed of the motor readily as it shown in figure 1.7. From the figure it can be seen that an additional feedback is introduced to achieve the position control of the SRM [3]. Figure 1. 7: basic speed and position controller for SRM [3] Aims and Objectives The aims of the project are to design, build and test a power electronic circuit for Switched Reluctance motor (SRM) using Bifilar type. The objectives are as following: Understand the operation of two- two poles SRM to realise the behavior of the motor and their characteristic. We can know from their characteristic how to make the motor spin and the control techniques that is used for them. Select and Design Power Electronic Circuit can help to active a high efficiency, low noise and low cost. Test Power Electronic Circuit with Resistive Load: to find the advantages and disadvantages of the circuit when the resistive load is applied to the motor. Design, Build and Test current limiting circuit where the current limitation can achieve low torque rebel. From the low torque rebel the noise can be decrease. Rotate motor using direct feedback where the position of the rotor is important to detect the moment of switching on and off. Study and understand PIC 18F 46K20. Program PIC to control SR Motor by designing a simulation using the MATLAB or other programmes. Chapter 2 Choosing power electronic component 2. Overview Over the time motor control has a significant growth in the industrial control where digital signal processors together with external hardware and appropriate software are used widely. In order to control the motor the switching device should switch on and off according to the position of the motor rotor and the apply current to the stator winding. The aim of this chapter is to highlight the details of the components that were used in the controller design. 2.1 Technical operation of Simple SRM The simple SRM has 2 poles in the stator and 2 poles in the rotor, figure 1 which the stator iron core has square shape with two winding in the upper side opposite the poles. Both stator and rotor are salient poles where the free spinning rotor is placed between the stator poles. In order to spin the rotor, stator poles are energised by the winding when the rotor poles are unaligned with stator poles. The inductance increased and the torque is produced as the rotor approaches the aligned. At this point the stator winding is demagnetised allows the rotor free to spin. The time of the energised and de-energised of the motor windings depend on the rotor speed. Figure 2.1: (a) Simple SRM 2/2 poles with bifilar winding[12] 2.3 Winding connections The motor has a bifilar winding to build up the current in the stator quickly, figure 1, which facilitates the higher torque and magnetic flux densities occurring. As a result the stored energy regenerated to the supply that the phase winding is connected in series with the switch device. This leaded to three achievable connections, figure 2, for bifilar winding. Figure 2.2: Possible winding connections The first connection shows the parallel connection that has a resistance connected in parallel to the motor windings to reduce the power losses when the switch is off. This connection has the main disadvantage due to the high loss. The second connection shows series connection that two switching devises are used which increase the cost of the design. However, this topology is the most common one for SRM according to reduce the switching heat losses, protections and their control flexibility that is handling the phase current and the machines acoustic noise. The third connection is the most cost effective and easier to control for the simple SRM figure 2. Moreover, it is offering the zero voltage supply as a reference for all power switching that make it suitable for low- voltage systems. Simply this configuration operates as it shows in figure 2.3, When the current of phase-A is turned off by removing the base drive signal to T1, the induced EMF in the winding is of such polarity that D1 is forward biased. This leads to the circulation of current through D1, the bifilar secondary winding, and the source, thus transferring energy from the machine winding to the source. The various timing waveforms of the circuit are shown in Figure 2.3. During current turn-off, the applied voltage across the bifilar secondary winding is equal to the dc link voltage. The voltage reflected into the main winding is dependent upon the turns ratio of the windings. Considering the turns ratio between the main winding in series with the power switch and the aux iliary winding in series with the diode as a, the voltage across the power switch is VT 1 = Vdc + aVdc =(1 + a)Vdc This shows that the voltage across T1 can be very much greater than the source voltage. One switch per phase comes with a voltage penalty on the switch. The volt ampere (VA) capability of the switch will not be very different for one switch compared to two switches per phase circuit. [10] Figure 2.3: (a) Converter for an SRM with bifilar windings; (b) operational waveforms of bifilar converter. [10] Thus, Figure 4 shows the primary plan for designing the current limiting circuit for the third connection of SRM and each block are explained next. Figure 2.4 basic configurations for current limiting 2.4. Power switching component Nowadays semiconductor and electronic switching devices offer a significant performance to control the electrical motors. Moreover, their size and cost make them more convenient to use in the motor control. The electronic devices has a significant role to control SRM where the switching on and off for the voltage is the major role to control the SRM with high performance. As a result, the main electronic devices that can be used are field effect transistors. There are two types of the field-effect transistor J-FET and MOSFET (see the appendix figure 3) [4]. The MOSFET has taken a widely role as a best choice to the simple SRM for many reasons that is discussed later in this chapter. 2.4.1 N-channel power MOSFET (STP36NF06) Simply the MOSFET is a metal-oxide-semiconductor field effect transistor where the MSFET has a significant characteristic [2]. The characteristic of N-channel MOSFET shows a significant use in switching. The structures of the MOSFET are shown in figure 5 where the MOSFET consist of Si P material substrate, two N material substrate that one is the source and the other is drain, and between them there is an insulator Sio2 and at the top metal conductor which is known as a gate. The length (L) below the gate and between the source and the drain is known as a channel. Fundamentally, the MOSFET output current is proportional to the charge in the semiconductor material by the control electrode[15]. Figure 5 MSFET structures [6] The MOSFET is used as a switching device to provide a high speed switching. The significant advantages of the field-effect transistor are: Higher operation temperature. Lower switching leakage. High input impedance. Low noise. Lower power dissipation during switching. In addition, The N-channel MOSFET has advantages such as ease to use for high frequency switching and simpler to control where the MOSFET gate drive do not require a continuous current to switch ON. When the MOSFET is needed to switch ON positive threshold voltage is needed to apply to the gate drive and Zero current to completely switch off. Moreover, the N-channel MOSFET has higher density integration and possible in rapid reduction in capacitances. [2]. MOSFET (STP36NF06) gate requires drive properly the later components are used to limit the current and the gate signal for the MOSFET. The gate drive is a power amplifier that accepts a low power input from a controller devices and generate the appropriate high current gate drive for a power MOSFET[4]. 2.5. Bipolar junction Transistor Transistors are considered one of the main elements of semiconductors that have been discovered in modern times. Transistors are used in the amplifiers of electrical signals and electronic switching, which have helped a number of factors such as small size, ease in manufacturing, cost effective and consume less power to extend significantly. Figure 2.6 (a) transistor structures (b) npn and pnp structures 2.5.1. ZTX 653 NPN transistor The NPN transistor is a type of bipolar transistors known as a medium power transistor. The NPN ZTX 653 transistor has interesting features that make it suitable for the push pull topology and amplifier topology (pre totem pole connections). The features of NPN ZTX 653 transistor are [6] * 100 Volt VCEO * 2 Amp continuous current * Low saturation voltage * Ptot=1 Watt 5.5.2. ZTX 753 PNP transistor The PNP transistor is another type of bipolar transistors that is used in the push pull topology. The PNP ZTX 753 transistor is suitable to employ in the push pull topology (Totem Pole) according to their features. Their features are [6] * 100 Volt VCEO * 2 Amp continuous current * Low saturation voltage * Ptot=1 Watt Figure 2.7: npn and pnp characters 2.5.3 Gate drivers (Totem Pole) This type of connection is known as a discrete drivers or push pull drives which is in common use today. Figure 2.8 shows the complimentary arrangement pair bipolar NPN and PNP emitter configuration. Moreover, the N-channel and P-channel MOSFT can be employing for totem pole connection. However, the N-channel and P-channel MOSFT driver suffers from shoot through current, caused by the threshold voltage overlap during ON and OFF transitions, resulting in increased drive power requirements [9]. Figure 2.8: push pull configuration The bipolar Totem Pole, figure 2.8, is non-inverting and offers no voltage gain to improve the pre-driver rise or fall times. It does provide current gain to reduce the driver impedance to speed the charge and discharge of the device capacitances. Once the input capacitances are charged and the power device has been switched, the driver does not require holding current. It has medium speed and does not perform well at higher conversion frequencies [9]. This circuit uses a complementary pair of transistors similar betas and power rating one is an npn power Darlington, and the other is a pnp power Darlington. When a high voltage (e.g., +15 V) is applied to the input, the upper transistor (npn) conducts, allowing current to pass from the positive supply through the motor and into ground. If a low voltage (0 V) is applied to the input, the lower transistor (pnp) conducts, allowing current to pass through the MOSFET gate from ground into the negative supply terminal.[14] 2.5.4. Common emitter amplifier (inverter) This configuration is a simple electronic circuit that is used to boost and invert the low signal. Figure 2.9 shows the circuit that contain a high resistance value with NPN transistor. The low signal is switch on and off the transistor where the resistance is limiting the current that apply to the transistor. This topology is used to boost and invert the output of signal processing unit to appropriate the signal that can switch the MOSFET as it will be discussed in the following point. Figure 2.9 common emitter configuration 2.6. Current control and limiting The winding current is controlled inorder to be not more than 8A. When the MOSFET is switched ON without any current limiting circuit, the current shoots up without any control. As a result, the technique for current limiting is arranged as follows When the winding current is less than 8A the MOSFET switches ON. When the winding current is exceeds 8A the MOSFET switches OFF. Therefore for appropriate current control, square wave signal is applied to the gate of the MOSFET. The frequency of input signal at the gate of the MOSFET is take into account to make the MOSFET switch properly with regard to the above current control arrangement. In order to do that feedback from the source of the MOSFET is carried to non-inverting terminal of the comparator. Figure 10 shows the operational waveform of the MOSFET. It can be seen that, the MOSFET needs to switch off when the voltage at inverting input of the comparator reaches 0.8V and switch on when the voltage is zero. Moreover, the reference voltage is 0.8 volt from the voltage divider law when a 0.1ÃŽÂ © resistance is connected to the source of the MOSFET. The 555 timer can offer the control of MOSFETs switching frequency. Figure 2.10: the expected waveforms from the design. 2.6.1 Timer NE555P The 555 timer IC is an incredibly useful precision timer that can act as either a timer or an oscillator. In timer mode is known as monostable mode the 555 simply acts as a one-shot timer; when a trigger voltage is applied to its trigger lead, the chips output goes from low to high for a duration set by an external RC circuit. In oscillator mode is known as astable mode the 555 acts as a rectangular-wave generator whose output waveform (low duration, high duration, frequency, etc.) can be adjusted by means of two external RC charge/discharge circuits.[14] In this project the monstable circuit is highlighted due to it has one stable state. Figure 2.11 (a) monostable configuration of timer 555 (b) monostable operations[14] In the monostable configuration, figure 2.11(before a trigger pulse is applied) the 555s output is low, while the discharge transistor in on, shorting pin 7 to ground and keeping C discharged. Also, pin 2 is normally held high by the 10-k pull-up resistor. Now, when a negative-going trigger pulse (less than 1à ¢Ã‚ Ã¢â‚¬Å¾3VCC) is applied to pin 2, comparator 2 is forced high, which sets the flip-flops Q_ to low, making the output high (due to the inverting buffer), while turning off the discharge transistor. This allows C to charge up via R1 from 0 V toward VCC. However, when the voltage across the capacitor reaches 2à ¢Ã‚ Ã¢â‚¬Å¾3VCC, comparator 1s output goes high, resetting the flip-flop and making the output low, while turning on the discharge transistor, allowing the capacitor to quickly discharge toward 0 V. The output will be held in this stable state (low) until another trigger is applied [14]. According to the operation of the monstable the pulse width can be modified by changing the value of R1 and C. T= 1.11 R1C Thus, to obtain capable functioning of the circuit the variable resistance R1 with a maximum value 100KÃŽÂ © and the threshold capacitance C value is 100nf are chosen. The timer input is the output of the comparator where the two values of the voltage are compared to make the output of the timer based on these two values. The two voltages are one is 5V and the other is the feedback from the MOSFETs source. 2.6.2. (LM393) Comparator The comparator is used to compare the voltage between the source voltage and the feedback from the MOSFET. This is important to control the current by triggering the timer to limit the current as it is mentioned before. LM393 has several advantages for timer input which are Higher accuracy in comparators. High voltage range (2.0V to 36V) Biasing with lower input current. Lower input offset current  ±5 nA Figure 2.12 LM393 connections to the circuit In order to establish the reference voltage variable resistance is required in the voltage dividers law. The operation of the comparator is simply as follows If IN+ > IN- the comparator output = V8. If IN+ From the previous operation it can be seen that the comparator send the signal to trigger the input of the timer and the timer acts upon the applied signal therefore the MOSFET switches on and off with regard to the timer output. However, the output of the timer switches on the MOSFET when the current is high which will be disastrous to the motor. To avoid this not to happen, a design of logic gates are incorporated in the current limiting topology of the SRM. 2.6. 3. Logic gate design (SN74LS00N) Logic gate is designed according to the output of the timer which gives the positive pulses when the current exceeds to 8A and no pulse when the current is less than 8A. With the purpose of switch (totem pole) the gate of the MOSFET turns on and off at required instants of time using an external Drive Signal. Figure 2.13 shows the technique of the combination between the Drive Signal and the timer output to control the MOSFET. SN74LS00N has four NAND gates that can be used to reduce the number of ICs in PCB connection. The logic gate is connected to the timer output which will be inverted with a common emitter configuration to switch on the MOSFET when the timer output is low (Itimer Figure 2.13: (a) SN74LS00N chip (b) logic circuit connection (c) NAND gate logic signal. The MOSFET behaviour that is need according to the timer output is shown in table 1. To do that the NOT gate, OR gate and AND gate are needed before the pre totem pole connections (common ammeter topology). Current condition Timer(A) Drive Signal(B) Output of the logic gate (C) MOSFET gate signal (Output after totem pole) I > 8A MOSFET must be off 1 0 1 0 1 1 1 0 I MOSFET can be on 0 0 1 0 0 1 0 1 Table 2.1 digital signal behaviour Boolean algebra From the logic gate output we can get the function: (1) This function can be simplified (2) Figure 2.14 (a) logic gate from equation 2 (b) table shows the equavelant NAND gates [7](c) the final simblified gates using the table (b) 2.7 Curant limiting circuit simulation Curent limiting circuit ,figure 2.15 , is the simulation circuit and the combination of each part that discussed previously. Figure 2.15: current limiting circuit