IIT-Madras invented indigenous way of low voltage, brown-out dc power distribution

IIT-Madras has developed an indigenous way of power distribution in which houses will be able to receive an uninterrupted 24 X 7 electricity. The project includes distribution of low voltage, brown-out mode of power distribution in which Alternating Current is also converted into Direct Current before supplying electricity to houses. Here, brown-out means is that say 10 percent of power is transmitted. Instead of 100 percent power cut you can have 90 percent power cut and meet the demand for a few critical devices in every home like fans, lights, TVs and cellphones. Under this project each house will receive 100 watts of DC power, which is sufficient to run 1 or 2 fans along-with 2 to 3 tube-lights and mobile charger.

Apart from this, some features are that houses covered under this project will also receive power through a separate line supplying Alternating Current to meet rest of their needs. Both lines will be separate and houses will get power supply through separate energy meters. Also, value of direct current is so low so that electricity boards will not be needing to cut power to do repairs.

The pilot project has been completed in which around 200 houses in Madurantakam have been chosen to implement and study the project. 

Coulomb's Law

       Coulomb is a unit of charge (q), named after a French physicist - Charles Augustin de Coulomb. When two charged particles are placed near each other, they exert force on each other. The intensity and type of force (attractive or repulsive) depend on placement of particles & type of charge (positive or negative) on particles, respectively. This law, therefore, explains behaviour of two charges or charged particles and gives the relationship of force acting between them when they are placed under the electrostatic force field of each other. There are two statements under Coulomb's law ;

Coulomb's 1st Law

First Law of Coulomb ;  

       This law says that same kind of charges repel each other whereas opposite kind of charges attract each other i.e., positive charge creates repulsive force or push force on positive charge and negative charge also creates repulsive force or push force on negative charge whereas positive charge attracts negative charge and vice versa.

Coulomb's 2nd Law

Second Law of coulomb ;

       This law says that the intensity of force acting between two charges is directly proportional to the product of individual charges, indirectly proportional to the square of distance between them and also depends on the medium.

Let,

                F    =    Force in Newton
                q₁  =    Charge on one particle in Coulombs
                q₂  =    Charge on another particle in Coulombs

=>

       where, K is constant of proportionality or coulomb’s constant, which depends on surrounding medium.

       where, Ɛ0  is permittivity of vaccum or air whose value is 8.854 X 10 ^-12 Farad / meter.

               &, Ɛm  is the relative permittivity of the medium containing charges with respect to free space.

Resistors, Resistivity, Color Coding of Resistors

     Resistor is an electronic or electrical component that opposes the flow of current in a circuit. Such an oppose by resistors to flow of current is known as resistance. Practically all materials offer some resistance to flow of current.

     Resistors are most common components used in electronic circuits. Usually resistors have two leads which are connected in series with other components in the circuit to limit flow of current through components connected in circuit.

     Resistance of a Resistor is measured in "Ohm". The symbol used to represent Ohm is a Greek letter "Ω" (Omega). Symbol used to represent the Resistor is "R" and figure representation of Resistor is given below ;

Symbol of Resistor

Another way of representation symbol of Resistor

RESISTIVITY 

     Resistivity or Specific Resistance of a substance is defined as the resistance of a unit long wire having a unit cross-section area, which is kept at 20⁰ C. Symbol used to represent the resistivity is a Greek letter "ρ" (Rho). SI unit of measurement of Electrical Resistivity or Specific Resistance of a substance is "Ω-m".

IMPORTANT ; As the resistance of a substance is a function of size, shape and environmental conditions of the substance, thus Resistivity becomes a very important term because it gives a means of comparing resistance of various substances and help us determining best conductors among others.

RELATIONSHIP BETWEEN ELECTRICAL RESISTANCE AND ELECTRICAL RESISTIVITY OF A SUBSTANCE 

Let us assume a piece of substance having ;

1. Resistance = R  Ω
2. Resistivity = ρ  Ω-meters,
3. Length = L meters, &
4. Cross-sectional Area = A square meters

 {because resistance is proportional to length and

   inversely proportional to cross-sectional area}

FACTORS DETERMINING RESISTANCE 

     From the equation between electrical resistance & electrical resistivity of a substance explained above, now we can say that the factors determining resistance are resistivity (ρ), shape i.e. length & cross-sectional area. Also, environmental conditions i.e., temperature affects resistance. All these factors have been explained below ;

Resistivity ( ρ ) ;
     We know that resistivity or specific resistance of a substance is the resistance of a unit long wire having a unit cross-sectional area at 20⁰ C and resistivity is proportional to resistance. So higher the resistivity will be, the higher will be the resistance of that substance and vice versa.


Length ;
     Length is also proportional to the resistance. Therefore, a wire with a longer length say 15 meters will have higher resistance to electric current flow than a wire of not so long length say 10 meters, if other conditions like resistivity, cross-sectional area and temperature are kept unchanged. This is because the resistive path increases with the length.


Cross-sectional Area ;
     Cross-sectional Area is inversely proportional to the resistance. Therefore, a wire with a bigger cross-sectional area will have less resistance than a wire having comparatively smaller cross-sectional area, if other conditions like resistivity, length & temperature are kept unchanged. As we know that current is the flow of electrons through a conductor when we apply electro-motive force or voltage across a conductor. This means, number of loosely bounded electrons will be more if cross-sectional area is increased and conductor allows current flow with ease and thus resistance experienced by flow of current will also decrease.


Temperature ;
     Different materials have different properties and also change in temperature affects their resistivity differently. Resistance of some substances increase with increase in temperature and vice versa. Such substances are said to have positive temperature coefficient. Resistance of some other substances decrease with increase in temperature and vice versa. Such substances are said to have negative temperature coefficient.

CLASSIFICATION OF RESISTORS   

Classification based on applications ;

• Fixed Value Resistors 
• Variable Resistors

Classification based on constructional features ;

• Carbon composite filled resistors
• Wire-wound resistors
• Deposited film resistors

Classification based on input signal sensing principle

• Light Dependent Resistors (LDRs)
• Thermistors

All these resistors classified above have been explained below ;

CARBON COMPOSITE FILLED RESISTORS 

     These are fixed value resistors and most commonly used resistors. In these resistors, resistive path or resistance to current flow is obtained with the help of mixture of composite materials including fine carbon particles which are conductive in nature and fine particles of other suitable non-conductive materials which are used to bind mixture and hold them tightly together. This tightly bounded carbon composite looks like a cylindrical in shape which is protected inside ceramic coating and two connecting metallic leads, one at each end are joined to make connections in circuits.

     Desired resistance value is obtained by increasing or decreasing carbon contents in the mixture.

WIRE-WOUND RESISTORS 

     These resistors essentially consists of a length of wire of specific resistance (alloy of various suitable metals), which is wrapped around a core of non-conductive materials usually ceramic core. Wire is wrapped around in such a way that it runs from one end to another so that two connecting leads are obtained from core ends i.e., one connecting lead is available at each end. Also resistive wire is wrapped spirally around core in such a way that it does not make contact radially at any point throughout its length. Wire wound resistors are especially used where high wattage resistors are required. These resistors are available in various shapes and sizes. Wire-wound resistors are also made available as fixed value resistors as well as variable resistors. Examples of variable wire-wound resistors are Rheostats & Wire-wound potentiometers.

     Desired resistance value is obtained by selecting alloy metal for resistive wire to make resistive path and also by increasing or decreasing length of wire. Alloy metals used for wire and its cross-sectional area also determine wattage rating of resistors.

DEPOSITED FILM RESISTORS  

     These resistors, as name suggests, are made by depositing layer of resistive material, which may contain carbon film of metallic film, onto a core of some non-conductive material. Rest of its construction is similar to that carbon composite filled resistors.

RHEOSTATS RESISTORS 

     These are wire-wound type of resistors. These are variable resistors. Rheostats are analog devices. Various resistance values are obtained by a slider. A slider is a simple sliding mechanism which slides along the length of the core on which wire is wrapped with the help of shafts. Shafts are fixed parallel to core to facilitate movement of slider. Tip of the slider consists of a conductive material or a piece of metal which remains in contact with the wire wrapped around core. One output lead is connected to slider and another lead is taken from one of the extreme ends of resistive wire. As slider moves towards the end from where out put lead is taken out, length of resistive path is decreased and hence the resistance also decreases. Similarly, when slider moves away, length of resistive path increases which increases the resistance. In this way, resistance value is increased or decreased by moving slider along the core.

POTENTIOMETERS 

     These are variable resistors. Potentiometers may be made to take analog readings as well as readings in discrete steps. Various resistance values are obtained by a slider arm. One end of the slider arm is fixed for any eccentric or linear movements and only allowed to rotate at its axis. Another end remains in contact with the circular resistive path made on flat surface. Resistive path of potentiometers can be made by depositing layers of resistive materials onto a surface of non-conductive material or it can be made using wire by wrapping around a core like Rheostats. The only different between Rheostat and wire wound potentiometer is that core is not straight like a bar. In wire-wound potentiometers core is bend in such a way that it makes a circular path for slider. Slider can be made to move in discrete steps or it can be made to move continuously in a circular path.

LIGHT DEPENDENT RESISTORS (LDRs) 

     These are variable resistors. Resistance of LDRs varies with variation in light striking it. When light intensity striking LDR falls, its resistance increase and vice versa. LDRs are used in camera to switch ON flash in automatic mode while capturing pictures in low lights and also they are used to control street lights for automatic switching ON and OFF to save power consumption. LDRs also called Photo-resistors.

THERMISTORS  

     These are variable resistors. Resistance of Thermistors varies with variation in temperature of the atmosphere surrounding it. Both positive temperature coefficient (PTC) and negative temperature coefficient (NTC) substances are used in construction of Thermistors. Resistance of PTC Thermistors increases with increase in temperature and vice versa. On the other hand, resistance of NTC Thermistors decreases with increase in temperature and vice versa. Thermistors can be used to protect circuits from over current and they can be used in appliances like electrical geysers, electrical hotplates, etc., to protect overheating and damage to man and machine.

COLOR CODING OF RESISTORS

Following table is used to know the value of color bands of resistors ;

Following method is used to calculate resistance value from the colour bands of resistors using colour coding table given above ;

For example, we take resistor given below and calculate its value using colour bands ;

1. First band of resistor is Yellow ; Value of Yellow is 4,
2. Second band of resistor is Violet ; Value of Violet is 7,
3. Third band of resistor is Black ; Value of Black is 1,
4. Fourth band of resistor is Gold ; Value of Gold is 5%,
5. Write down value of first band i.e., 4,
6. Write down value of second band i.e., 7,
7. From first and second bands, we obtained digit 47,
8. Third band is multiplier band and value of third band is 1,
9. After multiplication, we obtained figure 47,
10. Calculate 5% of figure obtained from first, second & third bands i.e., 2.35,
11. We obtained value of above resistor as 47 Ω  +/- 2.35 Ω.

Let's take one more example of resistor given below and calculate its value using colour bands ;

1. First band of resistor is Red ; Value of Red is 2,
2. Second band of resistor is Black ; Value of Black is 0,
3. Third band of resistor is Brown ; Value of Brown is 10,
4. Fourth band of resistor is Gold ; Value of Gold is 5%,
5. Write down value of first band i.e., 2,
6. Write down value of second band i.e., 0,
7. From first and second bands, we obtained digit 20,
8. Third band is multiplier band and value of third band is 10,
9. After multiplication, we obtained figure 200,
10. Calculate 5% of figure obtained from first, second & third bands i.e., 10,
11. We obtained value of above resistor as  200 Ω  +/- 10 Ω.

SELECTION OF RESISTORS

Selection criterion of Resistors consists of three most important factors given below ;

1. Resistance value,
2. Wattage Rating, &
3. Tolerance / Precision

RESISTORS IN SERIES

     In circuits of resistors connected in series, voltage drops across each resistor and this voltage drop depends upon value of resistors. For example ;

Vtotal = V1 + V2                                                   .... (i)

As per Ohm's law ;

I = V/R

Or, V = I . R                                                 .... (ii)

Therefore, putting values of equation (ii) & in equation (i), we  get ;

Itotal . Rtotal = I1 . R1  + I2 . R2                .... (iii)

Or, Rtotal = R1 + R2                                       .... (iv)

                                                                                        (because Itotal = I1 = I2, as value of current remains

                                                                         same because there is only one path for flow of current)

R_total = R₁ + R₂ + R₃ + ........... + R_n          (universal form of equation (iv) for 'n' number of 

                                                                         resistors in series)

RESISTORS IN PARALLEL

     When resistors are connected in parallel with each other, current then have more than one path to flow through circuit as we can see in figure below ;

Therefore, 

Itotal = I1 + I2                                               .... (i)

As per Ohm's law ;

I = V/R                                                         .... (ii)

Therefore, putting value of equation (ii) in equation (i), we get equation (iii),

Vtotal /Rtotal = V1/R1 + V2/R2                   .... (iii)

Or, 1/Rtotal = 1/R1 + 1/R2                               .... (iv) 

                                                                                        (because Vtotal = V1 = V2, as value of voltage remains same)

Or Rtotal = (R1 . R2) / (R1 + R2)

When all the resistors connected in parallel are of same value, then total resistance will be ;

Rtotal = Resistor Value of One Resistor / Number of Resistors

RESISTORS CONNECTED IN SERIES-PARALLEL COMBINATIONS 

     Complex circuits have resistors connected in series and as well as parallel. Those circuits are reduced to simplify by first calculating resistance of two or more resistors either connected in series or parallel. For example, we take below circuit and calculate resistance ;

     In above problem, it can be observed from figure that resistors R2 & R3 are connected in parallel. First we will reduce circuit by calculating resistance value of these two resistors as given below ;

     We know that formula for calculating resistance of resistors connected in parallel is ;

R4 = (R2 . R3) / (R2 + R3)                        

We get R4 and circuit can now be drawn to simplest form as given below ;

    Now, it can be observed from above figure that resistors R1 & R4 are connected in series and there resistance can be calculated as given below ;

Therefore, Rtotal = R1 + R4

CURRENT TRANSFORMER, POTENTIAL TRANSFORMER OR CT, PT

Current Transformer

    Current transformers and Potential transformers are widely used in industries to measure alternating currents and voltages, respectively, of high magnitudes and to operate and control protection devices. Current transformers and Potential transformers, like other transformers, have two windings i.e., Primary and Secondary windings. The output of these transformers, i.e., secondary winding remain connected to other instruments like energy meters and other protection devices. Current transformers and Potential transformers reduce high value alternating currents and voltages, respectively, flowing in main transmission line or supply system to very low values in proportions specified. Thus, these transformers provide economic, accurate, easy, simple and safe way of handling supply currents and voltages to get monitored and controlled. The magnitude of outputs of these transformers depends upon the ratio of the transformer. These transformers are also known as Instrument Transformers. Current transformer and potential transformer are also commonly called CT and PT respectively.

CONSTRUCTION OF CURRENT TRANSFORMERS ;

     Current transformers have primary windings, secondary windings and a core of magnetic material like normal transformers. Function of current transformers has to reduce transmission or supply current to low values in specific proportions to be sensed by other display, metering and protective instruments. For this reason, primary windings of current transformers have very few turns and some CTs have only one turn whereas, secondary windings of current transformers have large number of turns. Conductors of primary windings of CTs are of thicker gauges or heavy wires of higher diameters to handle high transmission currents. Primary windings are connected in Series of transmission lines. Some current transformers do not have primary windings instead a transmission line or supply line is passed through a hole of magnetic core wrapped over with conductors of secondary windings. These transformers are available in many shapes and sizes.

CONSTRUCTION OF POTENTIAL TRANSFORMERS ;

        Construction of Potential transformers is similar to Current Transformers, as discussed above, except the number of turns in secondary winding are much less than primary winding just like any step down transformer.

Types of Instrument Transformers ;

                

     Some of the common types of Instrument Transformers are Wire-wound and Ring or Toroidal type transformer. Rectangle CTs & Split-core CTs are also available, so that CTs can be fixed without removing or opening connections. Standard output of Instrument CTs is 1 Amp or 5 Amp and standard output of Instrument PTs are 110 V or 220 V. CTs can be of Oil-immersed as well.

                

      Important characteristics of Instrument transformers, which should be taken into consideration while selecting Instrument transformers are ; i) Input & Output ratio or Transformer Ratio, ii) Standard Load, iii) Accuracy, iv) Rated Voltage, etc.

    Some of the Applications of Instrument transformers are ; i) Metering Inputs, ii) Monitoring Loading and Un-loading of power transformers, heavy motors, etc., iii) Protection against over-loading of various equipment like power transformers, to operate protective relays, circuit breakers and switch gears, iv) Earth fault or leakage protection, etc.

     Current Transformer and Potential Transformer together can be used to measure Power with the help of Watt Meter.

      

SECONDARY OF CURRENT TRANSFORMERS CANNOT BE LEFT OPEN OR KEPT SHORT CIRCUITED ;

     Secondary terminals of Current transformers should not be left open because of high induced e.m.f. in secondary windings. Current transformers work as Step-up transformers which increase input e.m.f. as primary windings have only one or two turns of conductors whereas secondary windings have many hundreds of turns of conductor wrapped around magnetic core thereby magnify e.m.f. or voltage to many times which can be dangerous and source of accident. So secondary terminals of these transformers are kept short circuited when not connected with helping instruments.

     In the end, instrument transformers prove to be very useful in industries in sensing current flow and e.m.f. between two or more terminals of high magnitudes very efficiently, economically, safely and easily. 

     

Few other posts seeking your attention are ;

Coulomb's Law

Star Delta Starter Theory

     Electrical motors have been used in industries for quite a long period of time to convert electrical energy into mechanical energy. Three phase induction motors, also called asynchronous motors, are most extensively used motors in industries because of certain advantages like self starting, robust design, simple construction, less maintenance, efficient and comparatively low cost, though there is a problem of peak starting current associated with these motors. Peak starting current can be up to 5 to 7 times of full load current (flc) and sometimes it may become as high as 10 times of flc. However, the problem of peak starting current remains only for few seconds till the motor attains its speed, this problem may become severe especially with the motors above 10 HP. To deal with the problem of peak starting current or inrush current associated with three phase induction motors, many different starters having different mechanism and principle of operation are used. Star-delta starters are used for motors ranges from 5 hp or 3.5 kW. Star delta starters first configure windings of 3 phase motors in star thereby reduce voltage across each winding and then after few seconds these starters configure windings in delta and motors start run at full load voltage without any difficulty.

 Introduction to Star-Delta Concept 

     The magnitude of voltage induced in rotor conductors depends upon the magnetic flux linking with the rotor conductors and the Slip. Slip is the difference between the synchronous speed of the rotating magnetic flux produced from voltage applied to stator windings and the actual rotational speed of the rotor. At the moment of starting, Slip is maximum and decreases as the motor gains speed. Emf, therefore, induced in rotor conductors is high at starting as it is proportional to the Slip. Also magnitude of e.m.f. induced in rotor conductors will become high at the moment of starting, if full rated voltage i.e., line voltage is applied directly to the motor as the strength of the magnetic flux linking with rotor depends upon the voltage applied to the motor.
     Because the impedance of rotor windings being short circuited is very low and voltage induced is very high across windings, the motor current becomes many times of full load current or flc during starting. This high current drawn by the motor may burn motor windings and / or may cause unwanted disturbances in the voltage supply regulation and hence affects other loads adversely connected to the same supply.


   Three phase induction motors are ,therefore, started through appropriate star-delta starters because star-delta starters reduce voltage or start motors at reduced voltage by first configuring windings in star connections to overcome problem of high current surge at starting.

 Working Principle of Star Delta Starter 

     There are three states of Star-Delta starters, a). Star Connected State,  b). Open State, and c). Delta Connected State. During starting time Main and Star Contactors remain closed and complete Circuit. In Star Connected State, voltage applied is reduced to 1/√3 of the Line Voltage across each winding. As and when motor attains good rotational speed, say about 90% of full r.p.m. after few seconds, timer connected in starter disconnects Star Contactor first and then connects Delta Contactor. Between these two, Star connected and Delta connected states, circuit becomes open and motor neither remains in Star nor in Delta State. This is called open transition switching. In Delta connected state voltage applied to windings is equal to Line Voltage.

 Items Required to Make Star Delta Starter 

1. Three Contactors ( One Main Contactor, One Star Contactor and One Delta Contactor ),
2. Over Load Relay ( or OLR )
3. Timer,
4. Fuse Switch Unit ( or FSU ),
5. 2 Pole MCB,
6. Fuse,
7. Start Push Button ( NO )
8. Stop Push Button ( NC )

 Working of Star Delta Starter 

     Referring to the Control Circuit of Star Delta Starter shown here, lets understand working ;

• Switching "Start PB" completes Star circuit by switching ON main contactor "K1" and Star contactor "K2" through Timer "T1, NC". Now motor keeps on running in Star configuration.
• Once circuit is completed and main contactor "K1" is activated, one of its "NO" contact point, connected parallel to "Start PB", becomes close and keeps circuit complete by providing a continuous holding even after Push button is released.
• It should remember that the motor remains in Star state configuration till "Timer T1's NC" remains close. After a time delay is reached, Timer T1's NC becomes open, thereby de-activating Star contactor. Immediately after that Timer T1's NO becomes close, thereby activating Delta contactor "K3" to complete Delta circuit. In the time between switching from star state to delta state, which is normally few mili-seconds, circuit becomes open and motor neither remains in Star nor Delta state. Now motor keeps on running normally at full load voltage. To stop motor, "Stop PB" is pressed and circuit becomes open as all contactors are de-activated.
• Both contactors, star & delta contactors, are also electrically isolated from being accidentally operational by providing one another's NC in series with each other. For example, "K3, NC in series with "K2" contactor and "K2, NC" in series with "K3" contactor. Also Over Load protection is provided through OLR.

 Advantages of Star Delta Starters 

1. Simple in Design, &
2. Comparatively cheaper.

 Dis-Advantages of Star Delta Starters 

1. Open transition switching,
2. Once components are selected and circuit is designed, modifications in design are not possible, &
3. Less Torque at the time of starting than during delta configuration operation.

Few other posts seeking your attention are ;

1.    Near Field Communication technology ( NFC )