Universal Motors

Generally, the electric motors are opearted either in DC Power or AC Power. But for some specific applications , it is desirable to introduce a motor that operates on either ac or dc supply. The word ‘Universal’ signifies that something which is compaible with versatile inputs. We have build small series motors upto ½ KW rating which operates on single phase ac supply as well as on dc . Such motors are called universal motors . An universal motor is a specifically designed series wound motor, that operates at approximately the same speed and output on either ac or dc voltage. In case of universal motor, the speed of rotation is slightly lesser when opearting in AC. Becuase, the reactance voltage drop is present on ac but not on dc. So, the motor speed is somewhat lower for same load in ac operation than dc . This takes place especially at high loads . Most universal motors are designed to operate at speeds exceeding 3500 rpm

Construction Of Universal Motor

Basically , an universal motor is similar to a dc series motor in construction. However it is constructed with few series field turns, laminated armature and field circuits, low reluctance magnetic path, increased armature conductors and commutator segments. The frames of universal motor are usually made up of alumunium , rolled steel and cast iron. The commutation on ac is much poorer than on dc , due to current induced in the short circuited armature coils .If we use wide brushes then the short circuited current is excessive and motor starting torque is reduced . Brushes used are high resistance carbon ones so as to aid commutation . Compensating windings which are used in most large rating motors to improve commutation are not used in universal motors . In this motor armature current is quite small which cannot cause any commutation problems .

Working principle of universal motor

Now let us discuss the operation of this kind of motor in brief . In series circuit, same amount of current flows throgh all components. Similarly in a series wound motor, the same current flows through field windings and armature winding both. In an universal motor, both windings connected in series with each other. When the motor is supplied from ac or dc supply, magnetic fields are developed around the armature winding and field winding. They  reacts on each other to produce an unidirectional torque forcefully. In some other words, the interaction in between series magnetic field and armature field causes to develop a torque and this torque leads to rotate the shaft. However a series motor which is specifically designed for dc operation suffers from following drawbacks when it is used on single phase ac supply :-

• Its efficiency is low due to hysteresis and eddy current losses .
• The p.f is low due to large reactance of the field and armature windings .
• The sparking at the brushes is excessive.

Characteristics Of Universal Motor

The torque – speed ( τ – N) characteristics of an universal motor is quite similar to that of a series wound dc motor . It has high starting torque at low speed and low starting torque at high speed. In small series motors losses are large at no-load to restrict the speed to a definite value. A centrifugal switch is placed on the motor shaft. The tension of the springs of the switch is adjusted so that the switch opens at a pre determined speed . An external resistor R is placed in series with armature to reduce the speed. When speed drops due to increase of load ,the switch contacts close thereby shorting the resistor R thus raising the speed . Universal motors are high speed , small size motors as compared to other motors of same output . Here the full load power factor is high (0.9).

Speed control of universal motor

Speed control of universal motor is best obtained by solid-state devices. Since the speed of these motors is not limited by the supply frequency and may be as high as 20,000 rpm , they are most suitable for applications requiring high speeds . The factors that determine the speed for any dc motor are the same as those for ac series or universal motors i.e flux and generated voltage . Generated voltage change is rarely employed in speed control method. Instead line voltage is varied .This has been accomplished by means of tapped resistor , rheostat in series with the line.

Another method is by using a tapped field , thereby reducing the flux and hence raising the speed . This can be achieved by any one of the methods that follow :

• By using field poles wound in various sections with wires of different size and bringing out the taps from each section .

• By using tapped nichrome wires coils wound over a single field pole. In this method torque decreases with increase in speed .

Applications Of Universal Motor

Universal motors finds its applications in various devices . These are :

• The very small power output rating universal motors, which usually does not exceed 5 to10 watts are employed in equipments such as sewing machines , fans , portable hand tools, hair dryers , motion picture projectors and electric shavers.

• The higher rating (5-500 W) universal motor are used in vacuum cleaners, food mixers, blenders, cameras and calculating machines .

• This type of machine is used in table fans , polishers , portable drills and other kitchen appliances .a

Corona Effects

The presence of corona can reduce the reliability of a system by degrading insulation.
While corona is a low energy process, over long periods of time, it can substantially
degrade insulators, causing a system to fail due to dielectric breakdown. The effects of
corona are cumulative and permanent, and failure can occur without warning. Corona
causes:

• Light
• Ultraviolet radiation
• Sound (hissing, or cracking as caused by explosive gas expansions)
• Ozone
• Nitric and various other acids
• Salts, sometimes seen as white powder deposits
• Other chemicals, depending on the insulator material
• Mechanical erosion of surfaces by ion bombardment
• Heat (although generally very little, and primarily in the insulator)
• Carbon deposits, thereby creating a path for severe arcing
• It also causes degradation of insulators.

crona is accomplained by blue light & hizzing sound which leads to losses in it

Prevention of crona

Corona can be avoided by minimizing the voltage stress and electric field gradient. This is
accomplished by using utilizing good high voltage design practices, i.e., maximizing the
distance between conductors that have large voltage differentials, using conductors with Page 4
large radii, and avoiding parts that have sharp points or sharp edges. Corona inception
voltage can sometimes be increased by using a surface treatment, such as a semiconductor
layer, high voltage putty or corona dope.

Also, use a good, homogeneous insulator. Void free solids, such as properly prepared
silicone and epoxy potting materials work well. If you are limited to using air as your
insulator, then you are left with geometry as the critical parameter.

Finally, ensure that steps are taken to reduce or eliminate unwanted voltage transients,
which can cause corona to start.

Crona

 A corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized. The discharge will occur when the strength (potential gradient) of the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. It is often seen as a bluish (or other color) glow in the air 

For corona effect to occur effectively, two factors here are of prime importance as mentioned below:-

1) Alternating potential difference must be supplied across the line.
2) The spacing of the conductors, must be large enough compared to the line diameter.

Corona Effect in Transmission Line

When an alternating current is made to flow across two conductors of the transmission line whose spacing is large compared to their diameters, then air surrounding the conductors (composed of ions) is subjected to di-electric stress. At low values of supply end voltage, nothing really occurs as the stress is too less to ionize the air outside. But when the potential difference is made to increase beyond some threshold value of around 30 kV known as the critical disruptive voltage, then the field strength increases and then the air surrounding it experiences stress high enough to be dissociated into ions making the atmosphere conducting. This results in electric discharge around the conductors due to the flow of these ions, giving rise to a faint luminescent glow, along with the hissing sound accompanied by the liberation of ozone, which is readily identified due to its characteristic odor. This phenomena of electrical discharge occurring in transmission line for high values of voltage is known as the corona effect in power system. If the voltage across the lines is still increased the glow becomes more and more intense along with hissing noise, inducing very high power loss into the system which must be accounted for

Protection relays

A relay is an electrically operated switch.A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.

Pickup level of actuating signal: The value of actuating quantity (voltage or current) which is on threshold above which the relay initiates to be operated.
If the value of actuating quantity is increased, the electro magnetic effect of the relay coil is increased and above a certain level of actuating quantity the moving mechanism of the relay just starts to move.

Terms related to relay:-

Reset level: The value of electric current or voltage below which a relay opens its contacts and comes in original position.

Operating time of relay -Just after exceeding pickup level of actuating quantity the moving mechanism (for example rotating disc) of relay starts moving and it ultimately close the relay contacts at the end of its journey. The time which elapses between the instant when actuating quantity exceeds the pickup value to the instant when the relay contacts close.

Reset time of relay – The time which elapses between the instant when the actuating quantity becomes less than the reset value to the instant when the relay contacts returns to its normal position.

Reach of relay – A distance relay operates whenever the distance seen by the relay is less than the pre-specified impedance. The actuating impedance in the relay is the function of distance in a distance protection relay. This impedance or corresponding distance is called reach of the relay.

SL	LINES TO BE PROTECTED	RELAYS TO BE USED
1	400 KV Transmission Line	Main-I: Non switched or Numerical Distance Scheme Main-II: Non switched or Numerical Distance Scheme
2	220 KV Transmission Line	Main-I : Non switched distance scheme (Fed from Bus PTs) Main-II: Switched distance scheme (Fed from line CVTs) With a changeover facility from bus PT to line CVT and vice-versa.
3	132 KV Transmission Line	Main Protection : Switched distance scheme (fed from bus PT). Backup Protection: 3 Nos. directional IDMT O/L Relays and 1 No. Directional IDMT E/L relay.
4	33 KV lines	Non-directional IDMT 3 O/L and 1 E/L relays.
5	11 KV lines	Non-directional IDMT 2 O/L and 1 E/L relays.

Input/outputs pins,ports circuits of 8051 microcontroller

One of the main feature of micro-controller 8051 is the built in input & output ports that connects the micro-controller to the outside world. To make it commercially viable it should incorporate a many functions as were technically & economically feasible. The micro-controller has 40 pins & it’s design success depends upon the flexibility in the use of pins.

It has 24 such pins which can be used to serve one of two entirely different function yielding a total configuration of 64 pins. The function that a pin performs depends on what is physically connected to it & what software commands are used to program the pins. Both these factors are under the control of 8051 programmer.

Given this flexibility, 8051 can simply be applied as a single component I/O only or can be expanded to include memory, parallel ports & serial data communication.

Each port has D-type output latch for each pin. SFR’s (Special Function Registers)  for each port is made up of these 8 output latches, which can be addressed at the SFR address for that port.

For example:- 8 latches for port 0 are addressed at location 80 H

There are 2 different paths that read the latch & pin data which is done by using two different buffers. The upper buffer is enable when latch data is to be read & lower buffer is enable when pin data is to read. The status of each latch can be read from  latch buffer while an input buffer is connected to each pin so that pin status can be read independently.

Port 1 pins do not have alternate functions where as all other ports i.e.  0,2,3 are  programmable ports. The programmable ports have different alternate functions. The configuration between the output latch & the port pins determines the function nature of a particular port pin. 

FERRANTI EFFECT

 The receiving end voltage often increases beyond the sending end voltage, leading to a phenomena known as Ferranti effect in power system.

Why Ferranti Effect occurs in a Transmission Line

A long transmission line can be considered to composed a considerably high amount of capacitance and inductance distributed across the entire length of the line. Ferranti Effect occurs when current drawn by the distributed capacitance of the line itself is greater than the current associated with the load at the receiving end of the line( during light or no load). This capacitor charging current leads to voltage drop across the line inductance of the transmission system which is in phase with the sending end voltages. This voltage drop keeps on increasing additively as we move towards the load end of the line and subsequently the receiving end voltage tends to get larger than applied voltage leading to the phenomena called Ferranti effect in power system. It is illustrated with the help of a phasor diagram below.

Now for analysis of Ferranti effect let us consider the phasor diagrame shown above.

Here Vr is considered to be the reference phasor, represented by OA.

Ferranti Effect

This is represented by the phasor OC.

Now in case of a long transmission line, it has been practically observed that the line electrical resistance is negligibly small compared to the line reactance, hence we can assume the length of the phasor Ic R = 0, we can consider the rise in the voltage is only due to OA - OC = reactive drop in the line.

Now if we consider c0 and L0 are the values of capacitance and inductance per km of the transmission line, where l is the length of the line.


Since, in case of a long transmission line, the capacitance is distributed throughout its length, the average current flowing is,

Thus the rise in voltage due to line inductance is given by,

From the above equation it is absolutely evident, that the rise in voltage at the receiving end is directly proportional to the square of the line length, and hence in case of a long transmission line it keeps increasing with length and even goes beyond the applied sending end voltage at times, leading to the phenomena called Ferranti effect in power system.

HRC FUSE

In electrical system fuse acts as protection device and depending on application different type of fuse is to select. Out of these different type of fuses HRC is also one of the type and it stands for " High Rupturing Capacity".This type of fuses normally used where some delay is acceptable for protecting the system. 

The enclosure of HRC fuse is either of glass or some other chemical compound. This enclosure is fully air tight to avoid the effect of atmosphere on the fuse materials. The ceramic enclosure having metal end cap at both heads, to which fusible silver wire is welded. The space within the enclosure, surrounding the fuse wire or fuse element is completely packed with a filling powder. This type of fuse is reliable and has inverse time characteristic, that means if the fault current is high then rupture time is less and if fault current is not so high then rupture time is long.

Operation of HRC Fuse

When the over rated current flows through the fuse element of high rupturing capacity fuse the element is melted and vapourized. The filling powder is of such a quantity that the chemical reaction between the silver vapor and the filling powder forms a high electrical resistance substance which very much help in quenching the arc.

Introduction to electric drives

Whenever the term electric motor or generator is used, we tend to think that the speed of rotation of these machines are totally controlled only by the applied voltage and frequency of the source current. But the speed of rotation of an electrical machine can be controlled precisely also by implementing the concept of drive. The main advantage of this concept is, the motion control is easily optimized with the help of drive. In very simple words, the systems which controls the motion of the electrical machines, are known as electrical drives. A typical drive system is assembled with a electric motor (may be several) and a sophisticated control system that controls the rotation of the motor shaft. Now a days, this control can be be done easily with the help of software. So, the controlling becomes more and more accurate and this concept of drive also provides the ease of use. This drive system is widely used in large number of industrial and domestic applications like factories, transportation systems, textile mills, fans, pumps, motors, robots etc. Drives are employed as prime movers for diesel or petrol engines, gas or steam turbines, hydraulic motors and electric motors.

Now coming to the history of electrical drives, this was first designed in Russia in the year 1838 by B.S.Iakobi, when he tested a DC electric motor supplied from a storage battery and propelled a boat. Even though the industrial adaptation occurred after many years as around 1870. Today almost everywhere the application of electric drives is seen.

The very basic block diagram an electric drives is shown below. The load in the figure represents various types of equipments which consists of electric motor, like fans, pumps, washing machines etc.

Distribution System

A distribution system is a system which come into play after transmission & is used to distributing the electrical power.
 It mainly consist of three components :-
1.Feeder
2.Distributor
3.Supply mains

Feeder:-

It is used to connect the transmission system to the distributor or we can say that from sub-station to distributor.it has supply station at one side & distributor to other side.It hah no tappings on it.

Distributor:-

It is pesent on one end of Feeder.It has a number of tapping on it to which supply mains wires are connected.

Supply mains:-

It the medium through which user gets the supply.

                                                       

Difference b/w Fuse & Circuit breaker

 Fuses are inexpensive and can be purchased from any hardware store. They also tend to react very quickly to overloading, which means that they can offer more protection to sensitive electronic devices. This quick reaction can be a disadvantage, however, if the circuit is prone to surges that regularly cause fuses to blow.

Fuses must always be replaced once they are blown, which can be challenging in a darkened room or if the appropriate replacement is not immediately available. Another issue is that a do-it-yourselfer can mistakenly select a fuse that has a voltage or current rating that is too high for his needs, which can result in an overheated circuit. In addition, there may be exposed electrical connections in a fuse box, which can pose a danger to someone who does not follow the proper safety precautions.

Circuit breakers have many advantages, not the least of which is how quickly they can be reset. It is usually clear which switch has tripped, and it can be easily reset in most cases. For the average homeowner, it is also safer because there is no question about choosing the right fuse rating and all of the electrical connections are hidden in a breaker box.

A drawback to using a circuit breaker is that it is usually more expensive to install and repair. A circuit breaker also typically does not react as quickly as a fuse to surges in power, meaning that it is possible that electronics connected to the circuit could be damaged by "let-through" energy. It also is more sensitive to vibration and movement, which can cause a switch to trip for reasons unrelated to an electricity overload.

A fuse and circuit breaker are not interchangeable for all power applications. For example, a fuse cannot be used in situations that require a GFCI. Electricians are best qualified to determine whether a fuse or circuit breaker system is better for a particular electrical installation or upgra

Double Pressure Dead Type SF6

Double Pressure Dead Type SF6

This circuit breaker is the modification fo the simple SF6 circuit breaker which is discuss in previous blog but how lets see!!!

See the diagram shown above ,

 In this C.B(Circuit Breaker) the air (SF6) is circulated from the compressor to the high pressure auxillary tank though a inlet that is provided as shown at a rate of 14kg/m2  from which it is then passed to the main SF6 tank  axially & then to the arc chamber where arc is formed .So when this SF6 gas passes from the arc chamber it has a cooling effect on the arc by taking the heat of arc along with itself & resulting in the quenching of arc .This is a fast process of  quenching the arc as the dielectric medium  provided by the gas is more .This air is now goes to low pressure tank through which it again goes to compressor  through an outlet at a rate of 2kg/m2 .Now the SF6 can  again be re-circulated for another arc extinguishing operation.

ADVANTAGES:-
This system requires less maintenance & is more self- dependent & is reliable too!!

Since whole the apparatus is closed there is very minor chances of corrosion of the apparatus . ALSO it can use the same amount of SF6 again & again for the arc extinguishing process thus reducing the cost of operation!!!

Next time I will talk on HRC FUSES & ITS OPERATION during an arcing time!!!   

Puffer action in SF6 Circuit Breaker

                                           
Sf6 is a air circuit breaker since sf6 gas is used in it for extinguishing the arc.
The quenching of arc in SF6 circuit breaker takes place due to the puffer phenomenon taking place inside this circuit breaker

PUFFER PHENOMENON:-

Puffer means quick & short blast of gas on the arc

In SF6 c.b blast os SF6 gas is made on the arc throgh puffer action & the arc gets quenched

As shown in fig above,the fixed & moving contacts are kept in insulating nozzle section.The whole assembley i.e insulating nozzle,puffer action cylinder with moving contact ,a fixed piston is provided to it through the other end of moving contact.The chamber is filled with SF6 gas. Under normal working condition,the contacts are closed but on the of the fault the contacts open so moving contacts move right & SF6 gas move left side.

An arc is struck b/w the moving contact & fixed contact. The travel of movable cylinder causes increase in pressure on sf6 gas & this high pressure SF6 gas travels towards arc i.e it puffs over the arc or in simple we can say it blast over the arc

so when this puffer action is completed the arc gets extinquihed.
This type of c.b is called simple SF6 c.b. but it has limitation of  refilling the SF6 which is has by disconnecting  from its circuit for this we introduced Double pressure dead type SF6.
i will discuss this in next blog definetaly!!!!!

NON-LINEAR CONTROL SYSTEM

The system in which elements of  do not follows superimposition principal & if we draw a curve b/w two quantaties for this system will be non-linear   i.e not a straight line.

There are two types of system considered in a non-linear control system:-
1.Analog system
2.Digital system

Analog system:- In this system analog signals are produced & the signals are continuous with respect to time or we can say that there are continuous time signal

Digital system:-In this system digital signal are produced & these signals are non-continuos with respest to time or we can say that these are discrete time signals


                                        


In study we often study digital system

To study this system we have 2 techniques:-
1.Convential Tranfer Function Approch
2.State Variable Approach

1.Convential Tranfer Function Approch:-In this approach we dont consider the initial conditions of the system


2.State Variable Approach:-In this approach we always consider the intial conditions of the system.

Mostly we use state variable approach is more accurate&precise.
I will discuss about State Variable Approach in my next blog. 

Control system

Control system is define as a system which consists of several elements(components) that can be conntolled to give the desire output.

Types of control system:-

1.Linear control system
2.Non-Linear contol system

Difference between linear control system & non-linear control system:-

L.C.S:- The elements of this follows superimposition principal & if we draw a curve b/w two quantaties for this system will be linear i.e a straight line

N.L.C.S:-The elements of this  do not follows superimposition principal & if we draw a curve b/w two quantaties for this system will be non-linear   i.e not a straight line.

( the superposition principle also known as superposition property, states that, for all linear systems, the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually. )

CIRCUIT BREAKER

A circuit breaker is an automatically-operated electrical circuit designed to protect an electrical circuit from damage caused by overload or short circuit. A circuit breaker is a switch device which is used for making and breaking and electrical circuit, once or repeatedly, several times, during normal as well as abnormal operating conditions.

FUNCTIONS
1.Circuit breakers are used to protect an electrical circuit from overloading or short-circuiting. This prevents damage to equipment and wiring, as well as reducing the risk of fire.
2.Closing, opening and carrying full load rated current of the circuit for a prolonged period without any excessive temperature rise.
3.To carry the full load current at all times.
4.Breaking the heavy currents in case of short circuits.
5.Carrying current of short circuit magnitude until the fault is cleared by a circuit breaker nearest to the fault if it is used as back up protection.
6.Withstanding the effect of arcing at its contact terminal and thermal and electromagnetic stresses due to flow of heavy current during fault.
7.To open and close the circuit on no loads.

BASIC PRINCIPAL OF CIRCUIT BREAKER
All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker.The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.
Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.
The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by the erosion due to interrupting the arc. Miniature and molded case circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.
When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms. Different techniques are used to extinguish the arc including:
•Lengthening of the arc
•Intensive cooling
•Division into partial arcs
•Zero point quenching
•Connecting capacitors in parallel with contacts in DC circuits
Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit

ARC PHENOMENON IN CIRCUIT BREAKER
When a loose connection (a gap) is made in the faulted circuit so loose that the current flow is non-continuous, it is called an arcing or arc fault. An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. The arc consists of a column of ionized gas having molecules which have lost one more electrons. The electrons being negatively charged are attracted towards the positive contact with high velocity and on the way they detach more electrons by impact. The positive ions are attracted towards the negative contact but they comprise almost the entire weight of the atom, they move relatively slow.
Initiation of arc
To initiate the arc, when the fault current occurs and the electrodes separate, the electron from the cathode must be emitted.

The emission of the arc takes place due to the following reasons.

1. When the contacts are separated than the area and pressure between the contacts is decreased and the resistances will increases but the fault current is same. So due to the high current the high potential gradient is formed which can dislodge the electrons from the contacts.

2. When the contact separates the contact area is also decreases. Due to the high current flowing, the current density will increase. Due to the high current densities the temperature will increase resulting in the thermionic emission.

Maintenance of arc When the electron are emitted from the cathode, they collide with other neutral electron and other particles with great velocities and make them also the negatively charge and swifts them along with itself toward the cathode. The process is known as ionizations. The ionized ions further ionize other particle and create the avalanche of the electron to reach to the anode. The process of ionization is furthered achieved by the following reasons:

•The high temperature caused by the high current densities when the contact separates.
•The high voltage gradient formed when the contact separates.
•Due to increase in the mean free path when the contact separates.
Arc extinction-The phenomenon of the arc formation is inevitable when the contacts separated during the faults.

The main reasons to form the arc are
1. The high voltage gradient between the contacts
The arc formed by the high gradient can be reduces by making the high separation between the contacts but this is not possible for the high voltage system because the separation need very large.
2. The ionization of neutral particles.
Also the formation of ions is proportional to number of electron per cubic centimeters, so to avoid this we need to have the high diameter which is again not practical.
Following methods are use for the arc extinguishing.

A. High resistance method

The high resistance method uses the process to increase the effective resistance of the arc with the time so that the current is reduced to such a value that heat produce by it is not sufficient to produce the arc. During this the heat is dissipated inside the circuit breaker the circuit breaker should be designed to withstand such large quantities of energy.
The resistance of the arc can be increased by the following ways.
• Cooling of arc The arc resistance can be increased by adding the neutral or cold air between the contacts.

• Increasing the length of arc The resistance of the arc can be increased by increasing the mean length between the contacts. This decreases the voltage gradient of the contact and the arc phenomenon can be reduces. But this process is not practical because this increases the length of the contacts for the high voltage system.

• Reducing the cross section area The cross section of the arc can be reduced by having the small contact surface area or letting the arc pass through the small hole to reduce the arc. This process can help to reduce the voltage necessary to maintain the arc.

• Splitting the arc This is the best method of increasing the resistance of the arc. The arcs so formed are spitted into the small channels to reduce the effect of it. The provision of splitter is designed in the circuit breaker and the formed arc is passed through it to form the series of arc into the splitter. This increases the mean length of the arc and the cooling is better. B. Low current or the current zero interruption. This method is applicable for only the ac supply because in this we get have the zero current in each half cycle. In this process when the current reaches the zero value it has the minimum effect and the fresh air is supplied to turn down the arc. This method is widely used in the modern circuit breaker. This phenomenon is explained by the given theories.

• Energy balance or Cassie theory: This theory states that if the rate of heat dissipation between the contacts is greater than the rate at which heat is generated, the arc will be extinguish, otherwise it will restrike. During the faults the high heat is produced due to the higher voltage gradient or the high current densities between the contacts. Thus if the heat generated could be removed by cooling, lengthening or by arc splitter at a higher rate than the generation of the arc, then the arc will be extinguish.

• Recovery rate or Slepian’s theory: This theory states that is the rate at which the ions and the electrons combine to form or are replaced by the neutral molecules i.e. the rate at which the gap recovers its dielectrics strength is faster then the rate at which voltage stress rises, the arc will be extinguished: if otherwise the arc may be interrupted for a brief period but it again restrike. So in this process when the current is at zero value, the fresh air is entered to neutral the electrons. For this the following process are applied:

a) Lengthening the gap The dielectrics strength and the length between the contacts are proportional to each other. Lengthening the contact gap can be the obvious process. The permissible arc length is limited; however, by other considerations e.g. arc energy and possibility of transient voltages due to the current chopping.
 b) Increasing the pressure in the vicinity of the arc By increasing the pressure the density of the particle constituting the discharge also increases. The increased density of particle causes higher rate of deionization and thus the dielectric strength of the medium between the contacts is increased.

c) Cooling If the particle is allowed to cool the natural combination of ionized particles will take place more rapidly resulting increase in dielectric strength of the medium. Cooling by conduction to adjacent parts e.g. baffles or by the use of gas such as hydrogen that has as high diffusion and great absorption rate is, therefore, effective. d) Blast effect By blowing a stream of air through the arc ionized particles between the contacts are swept away and replaced by unionized particles. These unionized particles increased the dielectrics strength of the, medium considerably.

RESTRIKING VOLTAGE IN CIRCUIT BREAKER

•In ac circuit breakers, the current interruption takes place invariably at the natural zeroes of current wave.
•At current zero, a high frequency transient voltage appears across the breaker contacts and is caused by the rapid distribution of energy between the magnetic and electric fields associated with the plant and transmission line of the power system.
•This transient voltage is known as the restriking voltage. This voltage appearing across the breaker contacts at the moment of final current zero has a profound influence on the arc extinction process.
•Under the influence of this voltage the arc tries to restrike and hence it is named as restriking voltage. After current zero, the arc gets extinguished if the rate of rise of restriking voltage between the contacts is less than the rate at which dielectric strength of the medium between the contacts recovered.
•Thus, restriking voltage may be defined as the resultant transient voltage which appears across the breaker contacts at the instant of arc extinction.