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12:51 PM

  Control Systems by Norman S. Nise (1992, Hardcover) free download PDF



**Highly regarded for its practical case studies and accessible writing, Norman Nise's Control Systems Engineering . It takes a practical approach, presenting clear and complete explanations. Real world examples demonstrate the analysis and design process, while helpful skill assessment exercises, numerous in-chapter examples, review questions and problems reinforce key concepts.
. In addition, "What If" experiments help expand an engineers knowledge and skills. Tutorials are also included on the latest versions of MATLAB, the Control System Toolbox, Simulink, the Symbolic Math Toolbox, and MATLABs graphical user interface (GUI) tools. A new progressive problem, a solar energy parabolic trough collector, is featured at the end of each chapter. Ten new simulated control lab experiments now complement the online resources that accompany the text. This edition also includes Hardware Interface Laboratory experiments for use on the MyDAQ platform from National Instruments. A tutorial for MyDAQ is included as** Appendix.d





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4:56 AM

 

P-N junction diode

In a N-type 

material the electron is called the majority carrier and the hole of the minority career.

In a P-type

 material the hole is the majority career and the electron is the minority career. The N- and P- type materials represent the basic building blocks semiconductor devices.

The semiconductor diode is simply bringing these materials together constructed from the same base-Ge or Si. At instant the two materials are joined the electrons and hole in the region of the junction will combine resulting in a lake of carriers in the region near the junction. This region of uncovered positive and negative ions is called the depletion region due to the depletion of carriers in this region.

Construction and type of P-N junction Diode

The most extensively used elements in the manufacture of junction diodes are germanium and silicon although some other materials are also assuming importance in recent years. 

A p-n junction diode known as a semiconductor and crystal diode consist of a P-N junction, formed either in germanium or silicon crystal. The diode has two terminals namely anode and cathode.The anode refers to the P-type region and cathode refers to the N-type region as shown in figure below.

electrical/electronic engineering
n and p-type



The arrow head shown in the circuit symbol, points the direction of currents flow, when it is forward biased. It is the same direction in which the movement of holes takes place.The commercially available diodes, usually have some nations to identify the p and n terminals or leads. The standard notation consist of type numbers preceded by IN, such as IN 240 and IN 1250. Here 240 and 1250 correspond to colour bands. In some diodes, the schematic symbol of a diode is painted or colour are marked on the body.

electronics engineering
Diode p-n junction
The commercially available diodes, usually have some notations to identify the p and n terminals and lead. The standard notation consists of type numbers preceded by IN, such as IN 240 and IN 1250. Here 240 and 1250 correspond to colour bands. In some diodes, the schematic symbol of a diode is painted or the colour dots are marked on the body

Potential Barrier and Biasing

A P-N junction diode which consist of P- and N- type semiconductors formed together to make a P-N junction is the place dividing the two zones is known as junction.

Potential barrier
As a result of diffusion some electrons and holes migrate across the junction there by forming a depletion layer on either side of the junction by neutralisation of holes in the p-regional and of free electrons in the N- region. This diffusions of holes and electrons across the junction continues till a potential barrier is developed in the depletion layer which then prevents further diffusion. By the application of an external voltage this potential barrier is either increased or decreased.

The barrier voltage of a P-N junction depends upon three factors namely density,electronic charge and temperature. For a given P-N junction, the first two factors are constant, thus making the value of Vb dependent only on temperature. It has been observed that for both germanium and silicon the value of Vb decrease by 2m Vl degrees celsius.Mathematically, the decrease in barrier voltage is -0.002* delta t, where delta t is the increase in temperature in degrees celsius. 

Forward biasing
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Forward biasing


The junction is said to be biased in the forward direction when then positive battery terminal is connected to p-type region and the negative battery terminal to the n-type. This arrangement permits the flow of current across the P-N junction. The holes are repelled by the positive battery terminals and electron by the negative battery terminal with the result that both hole and electrons will be driven towards the junction where they will recombine. Hence as long as the battery voltage is applied large current flows. In other words, the forward bias lower the potential barrier across the depletion layer thereby allowing more current to flow across the junction.

Reverse biasing

electronics engineering
Reverse biasing

The junction is said to be reversed biased also called Zener diode when battery connections to the battery are reversed holes are attracted by the negative battery terminal and electrons by the positive battery terminal so that both holes and electrons move away from the junction. Since there is no recombination of electron-hole pairs, diode current is negligible and the junction has high resistance. Reverse biasing increases the potential barrier at the junction, thereby allowing very little current to flow through the junction.

V-I Characteristics of a P-N junction Diode

The V-I voltage-ampere characteristic of a typical p-n junction diode with respect to break-down voltage.
electronics engineering

              V-I Characteristics of a P-N junction Diode


  • For typical junction concentration the current densities at a temperature of 300K, forward voltage ranges between 0.2 and 0.3 in germanium and between 0.5 and 0.75 volt in silicon.
  • The reverse current s related to minority carrier concentration,which depends upon temperature and the energy of the material.
  • Reverse current increases exponentially with temperature. It is a limiting factor in the high-temperature junction of semiconductor junction device.
  • The high-frequency response of a semiconductor diode may be seriously limited by charge stored in the depletion region. This charge gives a capacitive effect since it changes with voltage the value of the stored charge is that of the ionized impurity atoms in the depletion regions on either side of the junction.
  • The width of the depletion region increase with higher doping. The result is lower capacitance, as in the case of a parallel-plate capacitor with wider spacing between plates.
  • The maximum reverse voltage of a P-N junction is limited by the field in the depletion region. The field accelerates carriers, which may gain enough energy to create new hole-electron pairs by colliding with atoms of the lattice structure. Each of these carriers may also create a hole-electron pair. As reverse voltage is increased, as avalanche breakdown point is reached at which this multiplicative action causes the current to increase abruptly.
  • Avalanche breakdown voltage is higher in lightly doped regions, since depletion region is wider, making the terminal electric field smaller for any given voltage.

4:52 AM

 

DC machine-types of dc motor

/ Construction of Dc machine


  •        A d.c machine consist of main two parts
  1. Stationary part- It is designed mainly for producing a magnetic flux.
  1. Rotating part- It is called the armature, where mechanical energy is converted into electrical such as electrical generator.
electrical skill
dc machine

The stationary and rotating parts are separated from each other by an air gap

  • The stationary part of dc machine consist of main poles, designed to create the magnetic flux, commutating poles interposed between the main poles and designed to ensure sparkless operation of the brushes at the commutator in very small machines with a lack of space commutating poles are not used and frame/yoke.
  • The armature is a cylindrical body rotating in the space between the poles and comprising a slotted armature core, a winding inserted in the armature core slots, a commutator, and brush gear.
The frame is the stationary part of a machine to which are fixed the main and commentating poles and by means of which the machine is bolted its bed plate. The right shape portion which serves as the part for the main and commutating pole fluxes is called yoke.

//** Cast iron used to be the material for the frame/yoke in early machines but now it has been replaced by cast steel. This is because cast iron is saturated by an flux density of about 0.8 wb/m square while saturation which cast steel is at about 1.5 wb/m square. Thus the cross-section of a cast iron frame is about twice that of a cast steel frame for the same value of magnetic flux. Hence, if it is necessary to reduce the weight of machine, cast steel is used. Another disadvantage with the use of cast iron is that its mechanical and magnetic properties are uncertain due to the present of below holes in the casting. Lately, rolled steel yokes have been developed with the improvements in the welding techniques. The advantage of fabricated yokes are that three are no pattern changes and magnetic and mechanical properties of the frame are absolutely consistent.


  • //It may be advantageous to use cast iron for frames but for medium and large sizes usually rolled steel is used.

/If the armature diameter does not exceed 35 to 45 cm, then in addition to the poles, end shield or frame-heads which carry the bearings are also attached to the frame. When the armature diameter exceed 1 m, it is common practice to use pedestal-type bearings, mounted separately, on the machine bed plate outside the frame.

*The end shield bearings, and sometimes the pedestal bearings, are of ball or rolled type.However, more frequently plane pedestal bearing are used.In machines with large diameter armatures a brush-holder yoke is frequently fixed to the frame.

/Field poles


**** Formerly the poles were cast integral with the yoke. This practice is still being followed for small machines. But in present day machines it is usual to use either a completely laminated pole,. or solid steel poles with laminated pole shoes.

*** Laminated construction is necessary because of the pulsations of field strength that result when the notched armature rotor magnetic structure passes the pole shoe. Variations in field strength result in internal eddy currents being generated in a magnetic structure.. they eddy current cause losses, they may be largely prevented by having laminated magnetic structures. Laminated structure allow magnetic flux to pass along the length of the lamination, but do not allow electric eddy currents to pass across the structure from one lamination to another. The under end of the laminated pole is curved to fit very closely into the main frame..
electrical skill
      dc machine yoke/body part
*** Different method are used for attaching poles to the yoke.In case of big size,smaller size the back of poles is drilled and trapped to receive pole bolt.In larger sizes, a circular or a rectangular pole bar is fitted to the pole. This pole bar is drilled and tapped and the bolts passing through laminations screw the tapped bar.

4:49 AM

 ** Three-phase induction motor **

 *** An induction motor is simply an electric transformer whose magnetic circuit is separated by an air gap into two relatively movable portions, one carrying the primary and the secondary winding. Alternating current supplied to the primary winding from an electric power system includes an opposing currents in the secondary winding, when latter is short-circuited or closed through an external impedance. Relative motion between the primary and secondary structure is produced by the electromagnetic forces corresponding to the power thus transferred across the air gap by induction..

electrical engineering
induction-motor


*** The essential feature which distinguishes the induction machine from another types of electric motor is that the secondary currents are created solely by induction,as a transformer instead of being supplied by a D.C. exciter or other external power source, as in synchronous and D.C. machines.

Advantages

Three-phase induction motor is the most commonly used motor in industrial applications because of the advantage listed below.
  1. Simple design
  2. Rugged construction
  3. Reliable operation
  4. Low initial cost
  5. Easy operation and simple maintenance
  6. High efficiency 
  7. Simple control gear for starting and speed control.
Application

Induction motors are available with torque characteristics suitable for a wide variety of loads.
  1. The standard motor has a starting torque of about 120 to 150 percent of full-load torque.such motors are suitable for most applications.
  2. For starting loads such as small refrigerating machines or plunger pumps operating against full pressure or belt conveyors, high torque motors with a starting torque of twice normal full-load torque, or more, are used.
  3. For driving machines that use large flywheels to carry peak loads, such as punch presses and shears, a high-torque motor with a slip at full-load up to 10 percent is available.The high slippermits enough charge in speed to make possible to obtain any value of starting torque up to the maximum breakdown torque. 
  4. Bye the use of o wound-rotor with suitable controlled and external resistance connected in series with the rotor winding, it is possible to obtain any value of starting torque up to the maximum breakdown torque. Such motors are well adapted constant-speed drives for loads that have large friction loads to overcome starting.

Classification of A.C. motors

1. According to the type of current.
  • Single-phase
  • Three-phase
2. According to speed.
  • Constant speed 
  • Variable speed
  • Adjustable speed 
3. According to principle of operation
  1. Synchronous motor
  • Plain
  • super
  4. According to structural features
  • Open
  • Enclosed
  • Semi-enclosed
  • Ventilated
  • Pipe-ventilated
  • Riveted frame eye
  • Splash proof
  • Totally enclosed fan-cooled
  • Explosion proof
  • Water proof
Constructional Details


The stator 


The stator frame consist of a symmetrical and substantial casting, having feet cast integral with it. The stator core, consisting of high grade, low loss electrical sheet-steel stampings, is assembled in the frame hydraulic pressure. The thickness of stampings laminations is usually from 0.35 to 0.6 mm. The stator laminations are punched in one piece for small induction motor. In induction machine of large size the stator core is assembled from a large number of segmental laminations.

electrical engineering
The stator
****The stator winding are given the utmost care to make them mechanically and electrically sound, so as to ensure long life and high efficiency. After the winding is in position it is thoroughly dried out whilst still hot and is completely immersed in a high grade synthetic resin varnish. It is the acid, alkali, moisture and oil proof. 

For small motor working at ordinary voltages, single layer much winding is used. For medium size machines double layer lap winding with diamond shaped coils is used. Single layer concentric windings are used for large motors working at high voltages. 

Frames of electrical machines house the starter core. Frames of small and medium sizes of induction motors have hollow cylindrical from and that of large motors have the shape of a circular box. In small induction motors, having a frame diameter of up to about 150 cm, the frame also supports the end shields. The frame should be strong and rigid as rigidity is very important in the case of induction motors of large dimension. This is because of the length of the air gap is very small and if the frame is not rigid, it would create an irregular air gap around the machine resulting in production of unbalanced magnetic pull.

The Rotor
electrical skill
The rotor


The rotors are of two types
1. Squirrel-cage
The squirrel-cage rotor is made up of stampings which are keyed directly to the shaft. The slots are partially closed and the winding consists of embedded copper bars t which the short-circuited rings are brazed. The squirrel cage rotor is so robust that it is almost indestructible. The gear majority of present day induction motors are manufactured with squirrel cage rotor a common practice being to employ winding of cast aluminium is forced in under pressure, to forms bars,end rings and cooling fans as die cast rotor and has become very popular as there are no joints and thus there is no possibility of high contact resistance.


2. Wound rotor
The wound rotor has also slotted stampings and the windings are formed wound. The wound rotor construction is employed for induction motors requiring speed control or extremely high value of starting torque. The wound rotor has completely insulated copper windings very much like the stator windings. The windings can be connected in star or delta and the three ends are brought out at the three slip rings. 
The current is collected from this slip rings with carbon brushes from which it is led to the resistance for starting purpose. When the motor is running , the slip rings are short-circuited by means of a collar, which is pushed along the shaft and connects all the slip rings together on the inside. Usually the brushes are provided with a device for lifting them from the slip rings when the motor has started up, thus reducing the wear and the frictional losses.

The number of slots in the rotor should never be equal to the number of slots in the stator If they are, there would be a variation of reluctance of the magnetic path from maximum, when teeth are opposite slots, to minimum when teeth are opposite teeth. The resulting flux pulsations would have a high frequency, since the periodic time would be the interval period for a tooth to occupy similar positions opposite two successive teeth. This will not only cause extra ions loss but the rotor will tend to lock with the stator if at the time of starting teeth are opposite teeth.The best plan is to make the number of the stator and the rotor teeth prime to each other.

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