ELECTRIC CURRENT

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An electric current can be broadly defined as charged particles in motion - a flow of charge. The most obvious effect of a current would seem to be the transfer of charge from one place to another. So it is, when a charged body is discharged by touching it with an earthed wire, or when lightning strikes a tree. However, it is important to realise that most currents flow in closed loops or circuits with no net transfer of charge. When there is a steady current the distribution of charge around a circuit remains constant and every part of the circuit remains substantially neutral.

The effects of electric current 

You can't see anything move in an electric current but currents do have a number of extremely important effects, which can be broadly classified as shown in the first column of table 3.1. This list is by no means exhaustive. Some of the topics are discussed in later chapters. In this chapter we are concerned in general terms with the charge, mass and energy transferred by a current

The measurement of electric current

Electric current is measured by means of a device which responds quantitatively to one or other of the effects of the current. Because current is a flow of charge, the device usually has to be connected so that the current goes through it.

Each system has advantages and disadvantages. The choice is determined by the exigencies of the situation. For example a clip-on ammeter does not require the interruption of the current, but it is bulky and not very sensitive. By contrast a digital ammeter responds quickly. Moving coil meters have the advantage of simplicity and long-term stability.

Movement of charge

Any movement of charge constitutes an electric current. The charge carriers could be electrons in a vacuum, electrons in a metal, 'holes' in a semiconductor or ions in a solution. The charge may move in free space, through a conductor or on a conveyor belt. An obvious manifestation of charge movement is an electric spark. At each spark some charge is transferred. Together a sequence of sparks makes up an intermittent current in which each spark contributes a current pulse. If the spark gap is narrowed so that the sparks become more frequent, they tend to merge to make a continuous current. 

Definition and unit of current 

The value of a current in a wire at any point is defined to be equal to the rate of flow of net positive charge past that point. The direction of the current is defined to be the direction of flow of positive charge.

            If the charge carriers are actually negative then the direction of the current is opposite to the flow direction of the particles. Thus a current of one ampere to the right could be either positive charge flowing to the right at 1 coulomb per second or negative charge flowing to the left at 1 coulomb per second, or some combination of flows in both directions such 0.5 C.s-1 each way.

The SI unit of electric current is the ampere (symbol A), equal to one coulomb per second. Because current is easier to measure than charge, the physical standards have been established using current as the base quantity, so the coulomb is defined as an ampere-second (A.s). The ampere is defined in terms of magnetic effects 

Current 

When there is an electric current in a circuit, energy is generally being transferred from a source to a load: energy is transferred from a battery to a lamp or from a nerve cell to measuring electrodes. In such circuits, the current is associated with the energy transfer.

SPEED CONTROL OF DC SERIES MOTOR

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Speed Control of Series Motors

1. Flux Control Method: 

: Variations in the flux of a series motor can be brought about in any one of the following ways:

 (a) Field Diverters:

 The series winding are shunted by a variable resistance known as field diverter. Any desired amount of current can be passed through the diverter by adjusting its resistance. Hence the flux can be decreased and consequently, the speed of the motor increased.

 (b) Armature Diverter:

A diverter across the armature can be used for giving speeds lower than the normal speed. For a given constant load torque, if Ia is reduced due to armature diverter, the  must increase (∵TaIa ) This results in an increase in current taken from the supply(which increases the flux and a fall in speed (N  I/ )). The variation in speed can be controlled by varying the diverter resistance.

(c) Trapped Field Control Field: This method is often used in electric traction. The number of series filed turns in the circuit can be changed. With full field, the motor runs at its minimum speed which can be raised in steps by cutting out some of the series turns.

 (d) Paralleling Field coils: this method used for fan motors, several speeds can be obtained by regrouping the field coils. It is seen that for a4-pole motor, three speeds can be obtained easily.

 Variable Resistance in Series with Motor

By increasing the resistance in series with the armature the voltage applied across the armature terminals can be decreased. With reduced voltage across the armature, the speed is reduced. However, it will be noted that since full motor current passes through this resistance, there is a considerable loss of power in it.

LIGHT EMITTING DIODE (LED)

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To explain the theory and the underlying principle behind the functioning of an LED

The first known report of a light-emitting solid-state diode was made in 1907 by

the British experimenter H. J. Round. In the mid 1920s, Russian Oleg Vladimirovich Losev independently created the first LED, although his research was ignored at that time.

• In 1955, Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys.

• Experimenters at Texas Instruments, Bob Biard and Gary Pittman, found in 1961 that gallium arsenide gave off infrared radiation when electric current was applied. Biard & Pittman received the patent for the infrared light-emitting diode.

• In 1962, Nick Holonyak Jr., of the General Electric Company and later with the University of Illinois at Urbana-Champaign, developed the first practical visible spectrum LED. He is seen as the "father of the light-emitting diode".

• In 1972, M. George Craford, Holonyak's former graduate student, invented the first yellow LED and 10x brighter red and red-orange LEDs.

• Shuji  Nakamura of Nichia Corporation of Japan demonstrated the first high brightness blue LED based on In GaN. The 2006 Millennium Technology Prize was awarded to Nakamura for his invention.

THEORY

A Light emitting diode (LED) is essentially a pn junction diode. When carriers are injected across a forward-biased junction, it emits incoherent light. Most of the commercial LEDs are realized using a highly doped n and a p Junction.

To understand the principle, let’s consider an unbiased pn+ junction band . The depletion region extends mainly into the p-side. There is a potential barrier from Ec on the n-side to the Ec on the p-side, called the built-in voltage, V0. This potential barrier prevents the excess free electrons on the n+ side from diffusing into the p side.

When a Voltage V is applied across the junction, the built-in potential is reduced from V0 to V0 – V. This allows the electrons from the n+ side to get injected into the p-side. Since electrons are the minority carriers in the p-side, this process is called minority carrier injection. But the hole injection from the p side to n+ side is very less and so the current is primarily due to the flow of electrons into the p-side. Appendix 1) results in spontaneous emission of photons (light). This effect is called injection electroluminescence. These photons should be allowed to escape from the device without being reabsorbed.

The recombination can be classified into the following two kinds

• Direct recombination

• Indirect recombination

Direct Recombination:

In direct band gap materials, the minimum energy of the conduction band lies directly above the maximum energy of the valence band in momentum space energy 

. In this material, free electrons at the bottom of the conduction band can recombine directly with free holes at the top of the valence band, as the momentum of the two particles is the same. This transition from conduction band to valence band involves photon emission (takes care of the principle of energy conservation). This is known as direct recombination. Direct recombination occurs spontaneously. GaAs is an example of a direct band-gap material.

Indirect Recombination

In the indirect band gap materials, the minimum energy in the conduction band is shifted by a k-vector relative to the valence band. The k-vector difference represents a difference in momentum. Due to this difference in momentum, the probability of direct electronhole recombination is less. In these materials, additional dopants(impurities) are added which form very shallow donor states. These donor states capture the free electrons locally; provides the necessary momentum shift for recombination. These donor states serve as the recombination centers. This is called Indirect (non-radiative) Recombination.

 E-k plot of an indirect band gap material and an example of how Nitrogen serves as a recombination center in GaAsP. In this case it creates a donor state, when SiC is doped with Al, it recombination takes place through an acceptor level. The indirect recombination should satisfy both conservation energy, and momentum. Thus besides a photon emission, phononemission or absorption has to take place. GaP is an example of an indirect band-gap material.

The wavelength of the light emitted, and hence the color, depends on the band gap energy of the materials forming the p-n junction. The emitted photon energy is approximately equal to the band gap energy of the semiconductor. The following equation relates the wavelength and the energy band gap.

LED Materials

An important class of commercial LEDs that cover the visible spectrum are the III-V(see Appendix 5). ternary alloys based on alloying GaAs and GaP which are denoted by GaAs1- yPy.  InGaAlP is an example of a quarternary (four element) III-V alloy with a direct band gap. The LEDs realized using two differently doped semiconductors that are the same material is called a homo junction. When they are realized using different band gap materials they are called a hetero structure device hetero structure LED is brighter than a homo Junction LED.

LED Structure

The LED structure plays a crucial role in emitting light from the LED surface. The LEDs are structured to ensure most of the recombinations takes place on the surface by the following two ways.

• By increasing the doping concentration of the substrate, so that additional free minority charge carriers electrons move to the top, recombine and emit light at the surface.

• By increasing the diffusion length L = √ Dτ, where D is the diffusion coefficient and τ is the carrier life time. But when increased beyond a critical length there is a chance of re-absorption of the photons into the device. The LED has to be structured so that the photons generated from the device are emitted without being reabsorbed. One solution is to make the p layer on the top thin, enough to create a depletion layer. Following picture shows the layered structure. 

There aredifferent ways to structure the dome for efficient emitting LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

VACUUM CIRCUIT BREAKER( V C B )

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                                  A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. For higher voltage vacuum technology has been developed but not commercially viable. The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called vacuum interrupter. The vacuum interrupter consists of a steel arc chamber in the centre symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar.

The material used for current carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material to make VCB contacts. Vacuum interrupter technology was first introduced in the year of 1960. But still it is a developing technology. As time goes on, the size of the vacuum interrupter is being reducing from its early 1960’s size due to different technical developments in this field of engineering. The contact geometry is also improving with time, from butt contact of early days it gradually changes to spiral shape, cup shape and axial magnetic field contact. The vacuum circuit breaker is today recognized as most reliable current interruption technology for medium voltage switchgear. It requires minimum maintenance compared to other circuit breaker technologies.

Advantages of Vacuum Circuit Breaker or VCB

Service life of vacuum circuit breaker is much longer than other types of circuit breakers. There is no chance of fire hazard as oil circuit breaker. It is much environment friendly than SF6 Circuit breaker. Beside of that contraction of VCB is much user friendly. Replacement of vacuum interrupter (VI) is much convenient

Operation of Vacuum Circuit Breaker

The main aim of any circuit breaker is to quench arc during current zero crossing, by establishing high dielectric strength in between the contacts so that reestablishment of arc after current zero becomes impossible. The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. This high dielectric strength makes it possible to quench a vacuum arc within very small contact gap. For short contact gap, low contact mass and no compression of medium the drive energy required in vacuum circuit breaker is minimum. When two face to face contact areas are just being separated to each other, they do not be separated instantly, contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. Although this happens in a fraction of micro second but it is the fact. At this instant of de-touching of contacts in a vacuum, the current through the contacts concentrated on that last contact point on the contact surface and makes a hot spot. As it is vacuum, the metal on the contact surface is easily vaporized due to that hot spot and create a conducting media for arc path. Then the arc will be initiated and continued until the next current zero.

MINIATURE CIRCUIT BREAKER (MCB)

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MCB

Nowadays we use more commonly miniature circuit breaker or MCB in low voltage electrical network instead of fuse.

The MCB has some advantages compared to fuse.

1.It automatically switches off the electrical circuit during abnormal condition of the network means in over load condition as well as faulty condition. The fuse does not sense but miniature circuit breaker does it in more reliable way. MCB is much more sensitive to over current than fuse.

2.Another advantage is, as the switch operating knob comes at its off position during tripping, the faulty zone of the electrical circuit can easily be identified. But in case of fuse, fuse wire should be checked by opening fuse grip or cutout from fuse base, for confirming the blow of fuse wire.

3.Quick restoration of supply can not be possible in case of fuse as because fuses have to be rewirable or replaced for restoring the supply. But in the case of MCB, quick restoration is possible by just switching on operation.

4.Handling MCB is more electrically safe than fuse.

Because of to many advantages of MCB over fuse units, in modern low voltage electrical network, miniature circuit breaker is mostly used instead of backdated fuse unit.Only one disadvantage of MCB over fuse is that this system is more costlier than fuse unit system.

Working Principle Miniature Circuit Breaker

There are two arrangement of operation of miniature circuit breaker. One due to thermal effect of over current and other due to electromagnetic effect of over current. The thermal operation of miniature circuit breaker is achieved with a bimetallic strip whenever continuous over current flows through MCB, the bimetallic strip is heated and deflects by bending. This deflection of bimetallic strip releases mechanical latch. As this mechanical latch is attached with operating mechanism, it causes to open the miniature circuit breaker contacts. But during short circuit condition, sudden rising of current, causes electromechanical displacement of plunger associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing immediate release of latchmechanism consequently open the circuit breaker contacts. This was a simple explanation of miniature circuit breaker working principle.

Miniature Circuit Breaker Construction 

The trip unit is the main part, responsible for proper working of miniature circuit breaker. Two main types of trip mechanism are provided in MCB. A bimetal provides protectionagainst over load current and an electromagnet provides protection against short-circuit current. 

Operation of Miniature Circuit Breaker

There are three mechanisms provided in a single miniature circuit breaker to make it switched off. If we carefully observe the picture beside, we will find there are mainly one bi - metallic strip, one trip coil and one hand operated on - off lever. Electric current carrying path of a miniature circuit breaker shown in the picture is like follows. First left hand side power terminal - then bimetallic strip - then current coil or trip coil - then moving contact - then fixed contact and - lastly right had side power terminal. All are arranged in series.

If circuit is overloaded for long time, the bi - metallic strip becomes over heated and deformed. This deformation of bi metallic strip causes, displacement of latch point. The moving contact of the MCB is so arranged by means of spring pressure, with this latch point, that a little displacement of latch causes, release of spring and makes the moving contact to move for opening the MCB. The current coil or trip coil is placed such a manner, that during short circuit fault the mmf of that coil causes its plunger to hit the same latch point and make the latch to be displaced. Hence the MCB will open in same manner. Again when operating lever of the miniature circuit breaker is operated by hand, that means when we make the MCB at off position manually, the same latch point is displaced as a result moving contact separated from fixed contact in same manner. So, whatever may be the operating mechanism, that means, may be due to deformation of bi - metallic strip, due to increased mmf of trip coil or may due to manual operation, actually the same latch point is displaced and same deformed spring is released, which ultimately responsible for movement of the moving contact. When the the moving contact separated from fixed contact, there may be a high chance of arc. This arc then goes up through the arc runner and enters into arc splitters and is finally quenched. When we switch on an MCB, we actually reset the displaced operating latch to its previous on position and make the MCB ready for another switch off or trip operation.

CURRENT TRANSFORMER(C T)

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CURRENT TRANSFORMER BASIC 

A current transformer is a transformer, which produces in its secondary winding a current, which is proportional to the current flowing in its primary winding. The secondary current is usually smaller in magnitude than the primary current. The principal function of a CT is to produce a proportional current at a level of magnitude, which is suitable for the operation of measuring or protective devices such as indicating or recording instruments and relays. The rated secondary current is commonly 5A or 1A, though lower currents such as 0.5A are not uncommon. It flows in the rated secondary load, usually called the burden, when the rated primary current flows in the primary winding. The primary winding can consist merely of the primary current conductor passing once through an aperture in the current transformer core or it may consist of two or more turns wound on the core together with the secondary winding. These are two basic CT types. The first is commonly called a “ring” type CT as the core is usually annular, but in some cases it may be square or rectangular in shape. The second is usually known as a “wound primary” type CT

WORKING OF CT

The primary and secondary currents are expressed as a ratio such as100/5. With a 100/5 ratio CT, 100A flowing in the primary winding will result in 5A flowing in the secondary winding, provided the correct rated burden is connected to the secondary winding. Similarly, for lesser primary currents, the secondary currents are proportionately lower. It should be noted that a 100/5 CT would not fulfil the function of a 20/1 or a 10/0.5 CT as the ratio expresses the current rating of the CT, not merely the ratio of the primary to the secondary currents.

The extent to which the secondary current magnitude differs from the calculated value expected by virtue of the CT ratio is defined by the [accuracy] “Class” of the CT. The greater the number used to define the class, the greater the permissible “current error” [the deviation in the secondary current from the calculated value]. Except for the least accurate classes, the accuracy class also defines the permissible phase angle displacement between primary and secondary currents. This latter point is important with measuring instruments influenced both by magnitude of current and by the phase angle difference between the supply voltage and the load current, such as kWh meters, wattmeter’s, var meters and power factor meters.

BURDEN RATING

Common burden ratings are 2.5, 5, 10, 15 and 30VA. Currenttransformers are usually either “measuring” or “protective” types, these descriptions being indicative of their functions. The principal requirements of a measuring CT are that, for primary currents up to 120% or125% of the rated current, its secondary current is proportional to its primary current to a degree of accuracy as defined by its “Class” and, in the case of the more accurate types, that a specified maximum phase angle displacement is not exceeded.

A desirable characteristic of a measuring CT is that it should “saturate” when the primary current exceeds the percentage of rated current specified as the upper limit to which the accuracy provisions apply. This means that at these higher levels of primary current the secondary current is les than proportionate. The effect of this is to reduce the extent to which any measuring device connected to the CT secondary is subjected to current overload.

On the other hand the reverse is required of the protective type CT, the principal purpose of which is to provide a secondary current proportional to the primary current when it is several, or many, times the rated primary current. The measure of this characteristic is known as the “Accuracy Limit Factor” (A.L.F.). A protection type CT with an A.L.F. of 10 will produce a proportional current in the secondary winding [subject to the allowable current error] with primary currents up to a maximum of 10 times the rated current.

Preferred primary and secondary current ratings [and therefore ratios], classes, burdens and accuracy limit factors are defined in BS3938 and other comparable national standards, together with other minimum performance requirements, physical construction requirements, etc.

It should be remembered when using a CT that where there are two or more devices to be operated by the secondary winding, they must be connected in series across the winding. This is exactly the opposite of the method used to connect two or more loads to be supplied by a voltage or power transformer where the devices are paralleled across the secondary winding.

With a CT, an increase in the burden will result in an increase in the CT secondary output voltage. This is automatic and necessary to maintain the current to the correct magnitude. Conversely, a reduction in the burden will result in a reduction in the CT secondary output voltage.

This rise in secondary voltage output with an increase in burden means that, theoretically, with infinite burden as is the case with the secondary load open circuit, an infinitely high voltage appears across the secondary terminals. For practical reasons this voltage is not infinitely high, but can be high enough to cause a breakdown in the insulation between primary and secondary windings or between either or both windings and the core. For this reason, primary current should never be allowed to flow with no load or with a high resistance load connected across the secondary winding.

CTs should be specified as follows:

RATIO: input / output current ratio

VA: total burden including pilot wires.

CLASS: Accuracy required for operation

DIMENSIONS: maximum & minimum limits

Metering CTs

In general, the following applies:

CLASS

• 0.1 or 0.2 for precision measurements

• 0.5 for high grade kilowatt hour meters for commercial grade kilowatt hour meters

• 3 for general industrial measurements

• 3 or 5 for approximate measurements

Protection CTs

In addition to the general specification required for CT design, protection CT’s require an Accuracy Limit Factor (ALF). This is the multiple of rated current up to which the CT will operate while complying with the accuracy class requirements.

In general the following applies:

• Instantaneous overcurrent relays & trip coils - 2.5VA Class 10P5

• Thermal inverse time relays - 7.5VA Class 10P10

• Low consumption Relay - 2.5VA Class 10P10

• Inverse definite min. time relays (IDMT) overcurrent - 15VA Class10P10/15

• IDMT Earth fault relays with approximate time grading - 15VA Class10P10

• IDMT Earth fault relays with phase fault stability or accurate time grading required - 15VA Class 5P10

STAR DELTA STARTER

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The star delta starting is a very common type of starter and extensively used, compared to the other types of the starters. This method used reduced supply voltage in starting. of a 3phase induction motor with a star – delta starter. The method achieved low starting current by first connecting the stator winding in star configuration, and then after the motor reaches a certain speed, throw switch changes the winding arrangements from star to delta configuration. By connecting the stator windings, first in star and then in delta, the line current drawn by the motor at starting is reduced to one-third as compared to starting current with the windings connected in delta. At the time of starting when the stator windings are start connected

Working Principal of Star-Delta Starter:

This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three. After a period of time the winding are reconfigured as delta and the motor runs normally.

Star/Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.

Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW). The Star/Delta (or Wye/Delta) starter is one of the lowest cost electromechanical reduced voltage starters that can be applied.

The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line.

There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating

Control Circuit of Star-Delta Starter

The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (KT) circuit.

When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC.

When Star Auxiliary Contactor (1)( which is placed on Main Contactor coil circuit )became NO to NC it’s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s  Main and Auxiliary Contactor Change its Position from NO To NC. This sequence happens in a friction of time.

After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected in parallel across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coil activated which eventually maintains the control circuit active even after releasing the ON push button switch.

When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact KT (3) become NC to NO.

Once the time delay is reached its specified Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit will change its position from NC to NO and at the Same Time  Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from NO To NC so Delta coil energized and  Delta Main Contactor becomes NO To NC. Now Motor terminal connection change from star to delta connection.

A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. Thus, the delta contactor coil cannot be active when the star contactor coil is active, and similarly, the star contactor coil cannot also be active while the delta contactor coil is active.

The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switch break the control circuit and the motor when necessary. The thermal overload contact is a protective device which automatically opens the STOP Control circuit in case when motor overload current is detected by the thermal overload relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor is detected by the thermal overload relay.

At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power and control circuits can be arranged to this in one of two ways – open transition or closed transition.