interview question explained-8.1

Hi folks.
Here we are going to continue our discussion about differences in metering CT and protective CT.

Let us talk about the magnetising curve first.

 This is typical magnetisation characteristics of a ferromagnetic material. Imagine it as leg of the person sitting on chair. The first point is similar to his ankle and second point is similar to his knee. The important point to note here is the region between these 2 points is linear. So we get output proportional to input (true reflection). Below ankle point and above knee point is not possible (due to magnetic inertia and due to saturation respectively, if you want to know).

For a metering CT, we want linear reflection of input from 20% to 120% of the load current. So this region should fall between the 2 markers we have set. Also when fault occurs, The currents values rise to very large extent (even 20 times the rated value). Under these conditions, CT should saturate and protect the measuring devices connected across it. So measuring CT is operated just below the knee point. The core material used is not of superior quality as it is supposed to get saturated at 120% of load.

For protective CT, we want linear reflection of the fault current. If we do not get linear reflection of normal current, not an issue. So 1.5 to almost 20 (or even higher) times of the rated secondary current should fall in the linear region. So protective CT is operated near ankle point. The core material used is of superior quality as we do not want saturation even the 50 times the rated secondary current of CT.

Thank you for your time. Please feel free to leave comments about your views. You can mail me your doubts at dnachiketa1010@gmail.com, I will be very glad to post about them.

interview question explained-8

Hi folks.
Here we are going to talk about a very important practical aspect of power system engineering.

Q. Why secondary of the Current transformer is never kept open or should not be opened?

Let us first see what is current transformer. It has same working principle (self and mutual induction) as that of voltage transformer. I want to measure the AC current flowing through a conductor. For that purpose, I put a circular magnetic core around it. I wrap a wire on that core. Now the rate of change of flux in the core in responsible for the induced EMF in the wrapped wire. That flux will be produced by the straight conductor carrying current. So the current flowing through the wrapped wire is giving me the reflection of the current flowing through the straight conductor. This is nothing but current transformer.

To have the transformer action, the flux produced by primary (straight conductor) should be opposed by the flux produced by the secondary (wrapped conductor). If we open the secondary of the Voltage transformer (traditional transformer, you know from 12th), the primary current is very small (no load current). So the flux produced in the core is very small and will not cause any undesirable effects.

For a current transformer, The current in primary is not depending on the current in secondary. It is depending on the load connected across it. So when secondary is opened, The opposing flux produced by secondary is vanished. So the only flux present in core is flux due to primary and it is very high. This large flux induces extremely high voltages across open secondary. This voltages may rise to several KV that is enough to damage any equipment, or the insulation or in worst cases to kill the operator.

You can also understand this as CT always tries to maintain constant secondary current. When there is some resistance across its secondary, (typical relay coil resistance is 0.5 Ohm) Small voltage is enough to make flow rated current. When the secondary is opened, To make flow the same current through high air resistance, the CT develops high voltages across it.

So there is spacial care taken provide a shorting switches across the secondary of the transformer. Many relay constructions have a spring operated shorting switch that keeps secondary of CT shorted when the body of relay is removed form its plastic casing. Please keep this thing in mind. Just for information, The voltage developed by a protective CT should be greater then the voltage developed by metering CT. We will soon talk about Difference between these two as it is interesting to see how hardware construction changes to satisfy peculiar purpose.

Thank you for your time and please feel free to leave comments about your views. You can also mail me at dnachiketa1010@gmail.com

Transformers-2

Hi folks.
Here we are going to talk about the tertiary winding of the transformer.  To start with, lets revise the facts about tertiary winding and then we will understand why it is so.

1.Tertiary winding is always connected in delta.
2. It is never used on the transformer where any of the windings (primary or secondary) are connected on delta.
3. It is never used on the transformer with 3 limbed core (implies used on transformer with 5 limbed core or 3 single phase type construction).

To understand 1st and 2nd point,

• Due to non linearity of the core, we can’t generate the sinusoidal flux in the core by applying sinusoidal voltage.
•  Without sinusoidal flux in the core, the secondary induced EMF will not be sinusoidal. This will make transformer unsuitable to supply single phase loads on secondary.
• To make flux in the core sinusoidal, we need to apply EMF that has 3rd harmonic components in it.
• Star windings don’t help the 3rd harmonic currents (co-phasal nature). But delta winding does. 
• So if any of the windings are connected in delta, 3rd harmonic voltage will be available. 
• This voltage will induce sinusoidal flux in the core and secondary EMF will be sinusoidal which is most desirable.

To understand 3rd point,

• Like 3rd harmonic flux, 3rd harmonic currents are also co-phasal in nature. They need closed path within the phases.
• If you observe, the extra 2 limbs in the 5 limbed core provide that path. So 3rd harmonic flux exists and affects the induced secondary EMF.
• If the core is 3-limbed, the 3rd harmonic flux has no closed path within the phases. So it cant flow and affect the output.

Even though we are connecting source the gives output at fundamental frequency only, from where are we getting the 3rd harmonic EMF in delta is very interesting concept. We will soon talk about it.  Also if we don’t connect the tertiary winding in the star-star 5 limbed core, problem of floating neutral will happen. You can search for above on your own.

Thank you for your time and please feel free to comment about your doubts and suggestions. You can mail me at dnachiketa1010@gmail.com about your areas of interest.

interview question explained-7

Hi folks.

There are some areas in our branch where, if we apply basic concepts, we end up with mined blowing results. Let us talk about one of such concepts.So here comes one of such concepts.

Q. there is a delta-star connected power transformer. A L-G fault occurs on the secondary of the transformer. What will be the fault sensed on the primary side?

Answer: For sure it’s not L-G. Let us see what it is.

As we all know, L-G fault is associated with 3 symmetric components which are positive, negative and zero. These 3 components pass through star winding of the transformer and enter delta. In delta winding, the zero sequence components circulates and only positive and negative get chance to pass. So the device connected on primary will sense a fault that relates positive and negative components that is L-L fault.  So L-G fault on the primary of the transformer will be sensed as L-L fault on the secondary of the transformer.

The Basics:  Why did the zero sequence components pass through star winding but were blocked by delta winding?
Straight forward answer is, they are in the same phase (phase difference is zero).  If you carefully observe the 3 phase wave form, when 1 phase is at its maximum, the rest are at half of the negative maximum. It means one phase in incoming, and rest 2 combining acting as a return path. For zero sequence, as there is no phase difference, all the maxima will be happening at the same time instance. So they fail to flow through star connections (and line conductors). Yet in delta connection, they get a closed path in the phases (draw diagram of star and delta, you will get it). So they circulate within the phases and cause serious overheating issues.

There are some cases where zero sequence currents escape delta. Imagine a 3-phase arc furnace. You can’t assure that arc will be taking place between 3 electrodes all the time. If arc takes place between only 2 electrodes, there will be supply imbalance enabling zero sequence currents to come out of delta.

Thank you for your time and please feel free to comment on your views. You can mail me your doubts and suggestions on dnachiketa1010@gmail.com 

Behavior of inductor and capacitor with varying supply frequency

Hi folks.
For last two days I was in the clutches of the HV project and didn't get time to post. Yesterday a friend of mine asked me to post about working of the inductor and capacitor at very high and very low frequency. Thank you Sumit for this opportunity and helping me to make this whole thing a 2 way communication.:)

Well, Let us talk about ideal inductor and capacitor first. 
The ideal inductor behaves as short circuit at lower frequencies and open circuit at very high frequencies. It is obvious as the inductive reactance is directly proportional to the frequency of supply.  You can understand this as inductance is the property due to which element is able to oppose the change in current flowing through it. 
The ideal capacitor behaves as open circuit at lower frequencies and short circuit at very high frequencies. The capacitive reactance is inversely proportional to the frequency of supply. Let us go into the basics of this phenomenon.

Imagine that you have 2 metal plates separated by a dielectric. Say capacitor is charging from 0V. When 1st +ve charge comes on plate 1 it induces equal and opposite charge on plate 2. When 2nd +ve charge comes, it is opposed by the 1st charge and the charge induced by this 2nd will be opposed by charge already present on plate 2. Now 3rd charge will be opposed by 2 charges. So if you carefully observe, 

1. Capacitor is opposing unidirectional flow of charges. (This explains it is open for DC).
2.Capacitor tends to saturate for unidirectional flow of charges which is what we call charging of capacitor.
3.The first charge did not face any opposition which explains an uncharged capacitor is SC at t=0+.

The practical inductor has inter-turn capacitance that acts as SC and creates problems at high frequency operation.  It exists due to the potential gradient between the windings and air as dielectric. The Aryton windings are used to reduce this (using loosely wound turns- increase d and reduce capacitance).
The practical capacitor has inter-turn inductance due to the circular arrangement of the metal foil and dielectric. This inductance will keep quiet at lower frequencies but take over the capacitive properties at higher frequencies.

So there are complex construction constraints for power capacitors and inductors. In short, Low frequency gives power in hands of capacitor and high frequency gives power in the hands of inductor. In the middle somewhere, our poor champ resistor lies, where resonance takes place. We will talk about it soon.

Thank you for your time and please feel free to leave comments about your views or extra information.  You can mail me at dnachiketa1010@gmail.com

Inductors demystified.

Hi folks.
Let us talk about the dual of capacitor - inductor. Transmission line inductance plays a very vital role in terms of stability of the system.

Suppose you have a simple series circuit of a inductor, a switch, a DC source and a bulb. What happens when you close the switch, the bulb goes on burning brightly and then attains maxima. When you open the switch, the bulb burns very brightly and then quickly goes out.

The reason for this strange behavior is the inductor. When current first starts flowing in the coil, the coil wants to build up a magnetic field. While the field is building, the coil inhibits the flow of current. Once the field is built, current can flow normally through the wire. When the switch gets opened, the magnetic field around the coil keeps current flowing in the coil until the field collapses (and energy gets dumped in bulb). This current keeps the bulb lit for a period of time even though the switch is open (mechanically). In other words, an inductor can store energy in its magnetic field, and an inductor tends to resist any change in the amount of current flowing through it.
(The application of above concept can be seen in the power electronic circuits where inductor forces current through the reverse biased switches and so they fail to turn off.)

Henries
The capacity of an inductor is controlled by four factors:
The number of coils - More coils means more inductance.
The material that the coils are wrapped around (the core)
The cross-sectional area of the coil - More area means more inductance.
The length of the coil - A short coil means narrower (or overlapping) coils, which means more inductance.
Putting iron in the core of an inductor gives it much more inductance than air or any non-magnetic core would.
The standard unit of inductance is the henry. The equation for calculating the number of henries in an inductor is:
H = (4 * Pi * #Turns * #Turns * coil Area * mu) / (coil Length * 10,000,000)
The area and length of the coil are in meters. The term mu is the permeability of the core. Air has a permeability of 1, while steel might have a permeability of 2,000.

Inductor Application: Traffic Light Sensors
We know inductance depends on the core material used. The sensor constantly tests the inductance of the loop in the road, and when the inductance rises (due to metal parts of car)  it knows there is a car waiting.

You cannot afford to forget this:
(We will go quite parallel to capacitor if you notice.)
The current flowing through an inductor can never change instantaneously. It is a current stiff element.


(CAUTION-  It is important to understand this as 2 pure voltage stiff elements and 2 pure current stiff elements should never be connected. Look for yourself in the VSI and CSI design constraints or Can you imagine what will happen if pure capacitor is connected across voltage source?)

If the rate of change of current is high, inductor will generate a voltage impulse (high magnitude for short duration).
If we draw graph of voltage across inductor and time, positive area (positive volt-Sec) and negative area (Negative volt-Sec) should be equal for 1 cycle.  This is called as volt-Sec balance theory. If you notice carefully enough, it is nothing but flux balance (Faraday's law).
Whatever may be the current through inductor, the energy stored by inductor from starting time to time instant ‘t’ is equal to the energy stored by the capacitor at time instant ‘t’. (again a very classic concept!)
Inductor connected DC source charges linearly (and not exponentially(RL circuit) – very common misconception) with slope Vdc/L.

Thank you for your time and please feel free to leave comments.
you can mail me your opinions on dnachiketa1010@gmail.com

power electronics-1

Hi folks.
Here we are going to talk about one of the 3 pillars of electrical engineering – Power electronics (other two being electrical machines and power systems). 
Semiconductor switches play a very vital role in any power electronic circuit. Let us talk about them first.

There are switches we use in home (the mechanical ones) that we operate conveniently with hands. We can’t use them in electrical circuits where we need high precision and speed of operation (this where relays fail). Functionality would be same if you compare.
What do we exactly want from a switch?
It should have high break down voltage (voltage it can sustain without damaging when reverse biased).
It should have low conduction loss (ON state voltage drop across it should be minimum).
It should have fast transition from On to Off and vice versa to achieve higher switching frequency.

We have some switches that use lightly doped semiconductor on either side of the PN junction to achieve high break down strength (lesser the impurity, more the force needed to throw them across barrier). But lightly doped materials have higher resistance and higher ON state voltage drop (lesser number of charges available to flow). Let us call these devices as GROUP 1(G1).

So overcome this issue we (the electrical engineers) invented a phenomenon called conductivity modulation. (Understand it as some process and for those who have got extra interest- it a phenomenon where p and n- junction is there and hole injection takes place in n- causing voltage peak in turn on characteristics). So due to this process turn loss reduces. Turn of process is carried out by injecting charges and Turn off process is carried out by removal of charge. But this takes time and reduces the speed of operation. Let us call them GROUP 2 (G2).

G1 have no conductivity modulation and hence they can operate at higher switching frequency than G2. 
G1 have very high losses if operated for high power applications and hence most commonly used for low power applications. G2 have low power loss and hence used for high power applications.
G1 are majority carrier device and G2 are minority carrier devices (due to conductivity modulation). 

G1 devices = MOSFET
G2 devices = POWER DIODE, SCR, GTO (Notice the presence of p and n- junction as well as voltage peak in turn ON characteristics of these devices).

Comparing, MOSFETs are used for high frequency (in MHz) and low power applications. They have high ON state loss and are majority carrier devices. SCRs are used for high power (current rating in KA) and low frequency (less than 250 Hz). They have low ON state loss and are minority carrier devices.

Thank you for your time and please feel free to leave comments about your views. You can mail your doubts at dnachiketa1010@gmail.com

Capacitors demystified

Hi folks.
We all need to deal with capacitors every now and then. Whenever we get unexpected voltage boost ( say ferranti effect), 99% of times capacitance is responsible. To determine voltage rating of equipments and so many other reasons, study of capacitor is inevitable.

In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric. You can easily make a capacitor from two pieces of aluminium foil and a piece of paper. It won't be a particularly good capacitor in terms of its storage capacity (leaking charge issues), but it will work.
In theory, the dielectric can be any non-conductive substance. However, for practical applications, specific materials are used that best suit the capacitor's function. Mica, ceramic, cellulose, porcelain, Mylar, Teflon and even air are some of the non-conductive materials used. The dielectric dictates what kind of capacitor it is and for what it is best suited. NASA uses glass capacitors to power up the space shuttle's circuitry and helps deploy space probes.

Air - Often used in radio tuning circuits
Mylar - Most commonly used for timer circuits like clocks, alarms and counters
Glass - Good for high voltage applications
Ceramic - Used for high frequency purposes like antennas, X-ray and MRI machines
Oil- Used in fan and long-time rating devices
Paper and electrolytic- many electronic circuits
Super capacitor - Powers electric and hybrid cars
(How super capacitors work-)

Even nature shows the capacitor at work in the form of lightning. One plate is the cloud, the other plate is the ground and the lightning is the charge releasing between these two "plates" due to dielectric breakdown.

Farad
A capacitor's storage potential, or capacitance, is measured in units called farads. A 1-farad capacitor can store one coulomb of charge at 1 volt.

To get some idea storing ability of capacitor, think about this:
A standard alkaline AA battery holds about 2.8 amp-hours.
That means that a AA battery can produce 2.8 amps for an hour at 1.5 volts (about 4.2 watt-hours - a AA battery can light a 4-watt bulb for a little more than an hour).
Let's call it 1 volt to make the maths easier. To store one AA battery's energy in a capacitor, you would need 3,600 * 2.8 = 10,080 farads to hold it, because an amp-hour is 3,600 amp-seconds.
If it takes something the size of a can of tuna to hold a farad, then 10,080 farads is going to take up a LOT more space than a single AA battery! Obviously, it's impractical to use capacitors to store any significant amount of power unless you do it at a high voltage.

Applications
 Electronic flash on a camera uses a capacitor - the battery charges up the flash's capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly. This can make a large, charged capacitor extremely dangerous - flash units and TVs have warnings about opening them up for this reason. (Capacitor holds and stores atmospheric charges. So terminals of power capacitors are always shorted through a resistor and never kept open for safety). They contain big capacitors that can, potentially, kill you with the charge they contain.
Big lasers use this technique as well to get very bright, instantaneous flashes.
Capacitors can also eliminate ripples and filtering, coupling. If a line carrying DC voltage has ripples or spikes in it, a big capacitor can even out the voltage by absorbing the peaks and filling in the valleys.
A capacitor can block DC voltage. If you hook a small capacitor to a battery, then no current will flow between the poles of the battery once the capacitor charges. However, any alternating current (AC) signal flows through a capacitor is allowed. That's because the capacitor will charge and discharge as the alternating current fluctuates, making it appear that the alternating current is flowing.

Capacitive touch screens
One of the more futuristic applications of capacitors is the capacitive touch screen. These are glass screens that have a very thin, transparent metallic coating. A built-in electrode pattern charges the screen so when touched; a current is drawn to the finger and creates a voltage drop. This exact location of the voltage drop is picked up by a controller and transmitted to a computer. These touch screens are commonly found in interactive devices and smart Phones.

History of the Capacitor
The invention of the capacitor varies somewhat depending on who you ask. There are records that indicate a German scientist named Ewald Georg von Kleist invented the capacitor in November 1745. Several months later Pieter van Musschenbroek, a Dutch professor at the University of Leyden came up with a very similar device in the form of the Leyden jar, which is typically credited as the first capacitor. Since Kleist didn't have detailed records and notes, or the notoriety of his Dutch counterpart, he's often overlooked as a contributor to the capacitor's evolution. However, over the years, both have been given equal credit as it was established that their research was independent of each other and merely a scientific coincidence.

The Leyden jar was a very simple device. It consisted of a glass jar, half filled with water and lined inside and out with metal foil. The glass acted as the dielectric, although it was thought for a time that water was the key ingredient. There was usually a metal wire or chain driven through a cork in the top of the jar. The chain was then hooked to something that would deliver a charge, most likely a hand-cranked static generator. Once delivered, the jar would hold two equal but opposite charges in equilibrium until they were connected with a wire, producing a slight spark or shock.

Benjamin Franklin worked with the Leyden jar in his experiments with electricity and soon found that a flat piece of glass worked as well as the jar model, prompting him to develop the flat capacitor, or Franklin square (Google it). Years later, English chemist Michael Faraday (only cool guy with 2 units named after him and you know them) would pioneer the first practical applications for the capacitor in trying to store unused electrons from his experiments. As a result of Faraday's achievements in the field of electricity, the unit of measurement for capacitors, or capacitance, became known as the farad.

You cannot afford to forget this:
The voltage across a capacitor can never change instantaneously. It is a voltage stiff element.
If the rate of change of voltage is high, capacitor will generate a current impulse (high magnitude for short duration).
If we draw graph of current through capacitor and time, positive area (positive Amp-Sec) and negative area (Negative Amp-Sec) should be equal for 1 cycle.  This is called as Amp-Sec balance theory. If you notice carefully enough, it is nothing but charge balance.
Whatever may be the voltage applied across capacitor, the energy stored by capacitor from starting time to time instant ‘t’ is equal to the energy stored by the capacitor at time instant ‘t’. (Very classic concept!)
Capacitor connected DC source charges linearly (and not exponentially(RC circuit) – very common misconception) with slope Idc/C.

Thank you for your time. Please feel free to comment about your views. You can mail me your doubts at dnachiketa1010@gmail.com

interview question explained-6

hi folks.
Let us talk about a question that will make us think - OMG! I never thought this way..!

Q. why all the electrical machines are based on magnetic field interacting with current and not electric field interacting with current/charged body? is it not possible or make or is there any other reason?

Answer - folks, it is possible to make machines that work on the principle of electrostatics. Few examples would be corona motor or Whimshrust generator we talked about. We can get useful energy output in the form of electrical energy (from 2nd). About 70% of the electrical energy we generate is converted back to mechanical so corona motor will be helpful there. So it is possible. Then why don't we replace the induction motor with corona motor?

The answer my friends lies in the values of epsilon and mu for air. Epsilon is permittivity of the air and indicates the ability of air to carry electric flux lines and is of the order 10^-12. Mu is permeability of air, indicating ability of air to carry magnetic flux and is of the order of 10^-7.  So ability of air to carry magnetic energy is far more (about 100000 times) than the ability to carry electric energy (flux).

The torque output produced by any machine is directly proportional to flux present in the air gap. So power output and energy efficiency of the machines based on magnetic fields will be high. So we go for those only.  The machines based on electric fields have specialized applications in various fields.

Thank you for your time and please feel free to comment about any extra information or doubts.
You can also mail me your doubts and suggestions about blog on dnachiketa1010@gmail.com

Interview question explained-5

Hi folks.
One of frequently asked interview question is-
Q. Why the symbol chosen for resistance is a zigzag line?

Answer- If same question were asked for inductor, you can answer it by inductor is basically a coiled conductor or for capacitor, It is two conducting plates separated by a dielectric- so we show plates.

Well, in network theory, we show straight line for a conductor with zero resistance. It basically indicates that current faces no problem for passing through it. To show the extra efforts put by current (that appear in the form of voltage drop), why not to increase the distance traveled by the current inside a resistor? So the resistor has a symbol where it indicates the opposition to the flow of current (as its basic definition says). 

Thank you for your time. Please feel free to comment about your views and doubts. 

Update about online courses

Hi folks.
Many of you might be aware of NPTEL website that has video (along with transcripts) and audio lectures for almost every engineering stream.
NPTEL (National Programme on Technology Enhanced Learning) is a joint initiative of the IITs and IISc. Through this initiative, they offer online courses and certification in various topics. 

ABOUT ONLINE COURSES-
Online courses are Free for all. 
There is Certification exam at the end For a nominal fee.
(THIS CERTIFICATION WILL BE EXTREMELY HELPFUL FOR YOU IN TERMS OF CAMPUS AND PLACEMENT)

HOW TO APPLY- 
Go to- https://onlinecourses.nptel.ac.in/explorer
Click on a course name to see more details about it.
This site uses your Google account for authentication. You will need to log in to register for any course.
After registration you will be able to see the course contents as and when they are made available by the course instructors.
Every course has weekly online assignments and video lectures. You can ask doubts to your course mentors directly. 
The certification exam is not mandatory.  You can learn the part you want and forget about exam!
The courses now available are -
1. Basic Electrical Circuits (very imp for us)
2. Introduction to programming in C (out of interest or for people wishing to work in software)

Thank you for your time and please feel free to comment about more useful websites.

Faraday Cage demystified.

Hi folks.

Electricity is the lifeblood of many aspects of our world. Without volts and amps, many of our technological innovations would fail to exist. Even our bodies wouldn't function without an electrical charge zipping through our cells and neurons. But what electricity gives, electricity can take away. Although this form of energy is vital to so much of our lives, it's one of those things that are only good in the right amounts. Too much electricity can electrocute people. Likewise, it can kill our modern electronics and machines.

So what saves linemen working on 765 KV line? What saves us from the high energy radiations inside the microwave oven?

Thanks to Michael Faraday, the brilliant 19th-century scientist, and one of his namesake inventions, the Faraday cage, we humans have developed plenty of ways to control electricity and make it safer for our computers, cars and other inventions and for us, too. Faraday cages shield their contents from static electric fields. An electric field is a force field surrounding a charged particle, such as an electron or proton.

Electromagnetic radiation is all around us. It's in visible and ultraviolet light, in the microwaves that cook our food and even in the FM and AM radio waves that pump music through our radios. But sometimes, this radiation is undesirable and downright disruptive. That's where Faraday cages come in.

It is nothing but a net woven from metal wires and object kept inside is protected from the field present outside or vice versa (microwave ovens case). The gap between the wires should be less than the wavelength of the radiation to be blocked. As a Faraday cage distributes that charge or radiation around the cage's exterior, it cancels out electric charges or radiation within the cage's interior. In short, a Faraday cage is a hollow conductor, in which the charge remains on the external surface of the cage. It works best when grounded as it can direct all induced currents to ground.

A lot of buildings act as Faraday cages too, if only by accident. With their plaster or concrete walls strewn with metal rebar or wire mesh, they often cut down wireless Internet networks and cell phone signals (radio waves have wavelengths in few meters that is more than the distance between the iron strands).But the shielding effect most often benefits humankind. Microwave ovens reverse the effect, trapping waves within a cage and quickly cooking your food. Screened (faraday cage used as coating) TV cables help to maintain a crisp, clear image by reducing interference. (See for yourself in case you find a waste piece of one!)

Power utility linemen often wear specially made suits that exploit the Faraday cage concept. Within these suits, the linemen can work on high-voltage power lines with a much-reduced risk of electrocution.

Governments can protect vital telecommunications equipment from lightning strikes and other electromagnetic interference by building Faraday cages around them.  Also you'll find Faraday cages in the form of MRI (magnetic resonance scanning) rooms. MRI scans rely on powerful magnetic fields to create medically useful scans of the human body. MRI rooms must be shielded to prevent stray electromagnetic fields from affecting a patient's diagnostic images.

All modern armed forces depend on electronics for communications and weapons systems, but there's a catch --these systems are vulnerable to high EMPs (electromagnetic pulses). Electromagnetic bomb (E-bomb) is used to create these pulses and destroy the communication networks of the enemy. To safeguard critical systems, militaries sometimes use faraday cage shielded bunkers and vehicles.

We will talk more about making your own faraday cage and electromagnetic bomb in coming articles. Thank you for your time and please feel free to post about your areas of interest.

WAR OF CURRENTS: some history lessons for an electrical engineer...

Hi folks.

When you flip a switch and a lamp illuminates the room, you probably don't give much thought to how it works or to the people who made it all possible. If you were forced to thank the genius behind the lamp, you might name Thomas Alva Edison, the inventor of the incandescent light bulb. But just as influential perhaps more so was an incredible visionary named Nikola Tesla.

Tesla arrived in the United States in 1884, at the age of 28, and by 1887 had filed for a series of patents (7 patents in 3 years!) that described everything necessary to generate electricity using alternating current, or AC. To understand the significance of these inventions, you have to understand what the field of electrical generation was like at the end of the 19th century. It was a war of currents -- with Tesla acting as one general and Edison acting as the opposing general.

Edison unveiled his electric incandescent lamp to the public in January 1880. Soon thereafter, his newly devised power system was installed in the First District of New York City. When Edison flipped the switch during a public demonstration of the system in 1881, electric lights twinkled on and created demand for this brand-new technology. Although Edison's early installations called for underground wiring, demand was so great that parts of the city received their electricity on exposed wires hung from wooden crossbeams. By 1885, avoiding electrical hazards had become an everyday part of city life; so much so that Brooklyn named its baseball team the Dodgers because its residents commonly dodged shocks from electrically powered trolley tracks (really!).

Edison was a (extremely) staunch supporter of DC, but it had limitations. The biggest was the fact that DC was difficult to transmit economically over long distances. Edison knew that alternating current didn't have this limitation, yet he didn't think AC a feasible solution for commercial power systems. Elihu Thomson, one of the principals of Thomson-Houston and a competitor of Edison, believed otherwise. In 1885, Thomson sketched a basic AC system that relied on high-voltage transmission lines to carry power far from where it was generated. Thomson's sketch also indicated the need for a technology to step down the voltage at the point of use. Known as a transformer, this technology would not be fully developed for commercial use until Westinghouse Electric Company did so in 1886.

Even with the development of the transformer and several successful tests of AC power systems, there was an important missing link- How to use that transmitted power to get useful work? That link was the AC motor.

While Edison toiled to commercialize his electric lamp, Tesla worked through a problem that had intrigued him since he was a student at the Joanneum Polytechnic School in Graz, Austria. While a student there, Tesla saw a demonstration of a Gramme dynamo. A dynamo is a generator that uses a commutator - contacts mounted on the machine's shaft - to produce direct current instead of alternating current. Tesla mentioned to his instructor that it might be possible to do away with the commutator, which sparked horribly as the dynamo operated. This suggestion brought ridicule from his teacher, but it captured Tesla's imagination.

In 1881, Tesla had an inspired idea: What if one were to change the magnetic field in the stator of a dynamo instead of altering the magnetic poles of the rotor? This was a revolutionary concept that turned conventional concept on its head.

Let us see why it was so amazing. We all know for generation of EMF we need a conductor, magnetic field and relative motion. Before Tesla, all machines were working like stationary field and rotating conductor. Tesla was the first one to imagine rotating field and stationary conductors. This is highly impossible with DC currents. So Tesla started to search for currents that change direction.   In a traditional dynamo, the stationary stator provides a constant magnetic field, while a set of rotating windings the rotor turns within that field. Tesla saw that if this arrangement were reversed, the commutator could be eliminated.

Of course, bringing this idea to reality would take years of work. Tesla began in 1882 while employed at Continental Edison Company in Paris. During the day, he would install incandescent lighting systems based on Edison's DC power system. In his spare time, he would experiment with AC motor designs. This went on for two years, until Tesla transferred to the Edison Machine Works in New York City. By some accounts, Tesla described his ideas about AC to the famed American inventor, but Edison showed no interest. Instead, he had Tesla make improvements to existing DC generation plants. Tesla did so, only to be disappointed when Edison failed to pay him properly. Tesla quit, and the paths of the two men diverged permanently.

(After this, they both criticized each other publicly and when they received Nobel Prize, They refused to receive it together and the end result was no-one got Nobel Prize. Can you believe that?)

Tesla received financial backing from Charles Peck, an attorney, and Alfred S. Brown, a superintendent at Western Union. Peck and Brown helped Tesla establish a laboratory just a few blocks away from Edison's lab in Manhattan, and encouraged the young engineer to perfect his AC motor. Tesla did just that, building what would become known as a polyphase induction motor. The term polyphase refers to a motor based on multiple alternating currents, not just one. The term induction refers to the process whereby the rotating stator magnets induce current flow in the rotor. Tesla's original motor was a two-phase version that featured a stator with two pairs of magnets, one pair for each of two phases of AC.

In 1887, Tesla filed for seven U.S. patents describing a complete AC system based on his induction motor and including generators, transformers, transmission lines and lighting. A few months later, Tesla delivered a lecture about his revolutionary new system to the American Institute of Electrical Engineers. The lecture caused a sensation and, despite an anti-AC campaign initiated by Edison, convinced many experts that an AC power system was more than just feasible - it was far superior to DC.To bring a good idea to market, it takes some clout. In this case, the clout came from an inventor who made a fortune in the railroad industry.

George Westinghouse, whose own electric company was struggling to work out the details of a successful AC power system, heard about Tesla's 1888 lecture and immediately was intrigued. When Peck and Brown approached Westinghouse about commercializing Tesla's inventions, the entrepreneur responsible for the railroad air brake made a generous offer. He agreed to pay $25,000 in cash, as well as $50,000 in notes and a small royalty for each horsepower of electricity originating from the motor.

Westinghouse carried Tesla's inventions back to Pittsburgh, Penn., where he hoped to use the technology to power the city's streetcars. Tesla followed, and as an employee of the Westinghouse Electric Company, consulted on the implementation. The project didn't proceed smoothly, and Tesla frequently battled with Westinghouse engineers. Eventually, however, everyone pulled together to come up with just the right formula: an AC system based on three-phase, 60-cycle current. Today, almost all power companies in the United States and Canada supply 60-cycle current, which means the AC completes 60 changes of direction in one second. This is known as the frequency of the system.

By the early 1890s, Edison and the supporters of DC felt genuinely threatened. They continued to make claims that AC was dangerous and pointed to a disastrous electrocution attempt in 1890 as evidence. But they suffered a severe blow in 1893, when Westinghouse won the bid to illuminate the Chicago World's Fair. His competition was General Electric (GE), the company formed by the merger between Edison General Electric and Thomson-Houston. GE was the leading torchbearer for DC-based power (and still is one of the most awesome companies for us). Westinghouse won the bid on cost, but when President Grover Cleveland flipped a switch to light 100,000 incandescent lamps across the fairgrounds, very few doubted the superiority of AC power.

Westinghouse mollified many remaining doubters in 1895 by designing a hydroelectric plant at Niagara Falls that incorporated all of the advances made in AC. At first, the plant only supplied power to Buffalo, New York. But it wasn't long before power was being transmitted to New York City. (The generators still working have Tesla’s name inscribed in their name plate.)

 In fact, it can be said that Tesla's AC motor and poly-phase AC system won the war of currents because they form the basis of all modern power generation and distribution. However, direct current --Edison's baby --didn't disappear completely. It still operates automobile electrical systems, locomotives and some types of motors and long-distance transmission.

These were some history lessons you should know if you call yourself an electrical engineer. Do feel proud to carry on this legacy and thank you for your time. 

Transformers -1

Hi folks.
I think we didn't talk much about the heart of the power system itself - transformers.

The core of the transformer is a very important component of it and governs the operating character of the transformer.Let us talk something about the materials used for core. As we always say, It is not made up of silicon steel (popularly known as electrical steel). It is made up of something called- CRGO steel (Cold Rolled Grain Oriented Steel).

Well it is basically silicon steel but treated specially we can say. Addition of silicon to iron is important as it significantly increases the electrical resistivity of the steel, which decreases the induced eddy currents and narrows the hysteresis loop (area under B-H curve is proportional to the losses) of the material, thus lowering the core loss. However, the grain structure hardens and embrittles the metal, which adversely affects the workability of the material.
So this Si-steel has BCC structure - Body centered cubic structure (you can find this in some standard chemistry book or Google it). This crystal when subjected to magnetic fields can undergo magnetization in 3 ways (directions)-
1. Along the shorted edge of the cube
2. Along the face diagonal of the cube
3. Along the through diagonal of the cube

Studies show that shorter the length of axis of magnetization, lower is the reluctance offered. In the last articles, we have seen, to reduce magnetizing current permeability should be very high and that implies reluctance offered should be very low.
So we take special efforts to magnetize every crystal (almost) along the shortest axis that is the edge of the cube.  This texture is developed by a series of careful working and annealing operations. Annealing, in metallurgy and materials science, is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and to make it more workable. It involves heating a material to above its critical temperature, maintaining a suitable temperature, and then cooling.

The question remains is why don't we use this superior materials for all the electrical machines such as induction machines or synchronous machines?
The reason is if we see the flow of flux in these machines, it is not unidirectional and uniform in sense like transformer. So the reluctance offered in one direction will reduce and other direction might increase and this will create a harmful imbalance.
If you are interested, search for amorphous core transformers that are still superior compared to these. 

Thank you for your time and please feel free to leave comments about your areas of interest. I will surely try to post about them.

Pacemakers demystified.

Hi folks.

I was showing my blog to a friend of mine and when she read this name, she said - what is this pacemaker? So lets talk something about that.

Let me also tell you the reason for including it in blogs name. It is the best example of how much electricity is close to you- inside you, It can not be seen and felt but we are people controlling it so precisely that life of the people is at stake. 

We're all born with a 'pacemaker'. It's called the sinoatrial node (SA node), a small area at the top of the right atrium (upper chamber) of the heart. The SA node automatically generates an electrical signal that causes the upper chambers of the heart to contract. This signal begins in the SA node and travels to the atrioventricular node (AV node, which is also the SA node's back-up). The AV node acts slows down the electrical signal as it moves on to the lower chambers of the heart, while the atria contracts. With each contraction blood is pumped through the body.

In some people, the heart's electrical system -- called the cardiac conduction system -- misfires. A pacemaker is often able to correct and regulate the problem.

Unfortunately, the AV node is a poor substitute for the SA node and is usually only able to cause about 40 heartbeats per minute. In this situation an implantable permanent pacemaker can make a world of difference.

A pacemaker is an electronic device used to prevent a heart from beating abnormally. It's a generator made up of a battery and computer circuitry housed in a metal casing. The casing is implanted under the skin in the upper chest or shoulder region. Pacemaker wires are threaded through the veins of the shoulder and guided into the heart with the help of X-rays. Once the wires are positioned in the heart they are hooked up to the generator.

The first permanent pacemaker implant happened in 1958. Today there are three basic types of pacemakers:

• Single-chamber pacemakers
• Dual-chamber pacemakers.
• Bi-ventricular pacemakers

There are two main types of pacemaker programming: demand and rate-responsive. A demand pacemaker is designed to sense when the heart needs assistance by measuring each heartbeat and firing when the heartbeat becomes too slow or misses a beat. Rate-responsive pacemakers adjust heart rates depending on the patient's level of activity. They measure the SA node rate but also breathing, blood temperature and other factors.

Today's permanent pacemakers last at least 6 to 10 years depending on how frequently the device has to work. Every time a pacemaker is triggered it drains its battery. Occasionally a temporary pacemaker is used, usually during a patient's recovery from a heart attack or during an emergency situation to immediately speed up a slow heart rate.

So I really hope you liked knowing about the pacemakers. How pacemakers were invented is also a nice story. Do search for it. Thank you for your time and please free to comment about your interests.

Interview questions explained-4

Hi folks.
A very famous and confusing question asked is-

what is the resistance of the ground- high or low and what is resistivity of the ground- high or low?

Answer = The resistance of the ground is very low and is of the order of 0.5 Ohms and less than 1 ohm. So resistance of the ground is very less. If we measure the resistivity of ground that highly depend on the type of soil, we find it in the order of 400-2000 OhmM. the conductors we use have resistivity in 10^-6 OhmM. The area of cross section of earth as conductor is the reason for very high resistivity and yet low resistance.

this low resistance help us to provide return path for fault currents and avoid the problems of arcing grounds. Thank you for your time and please feel free to leave comment.

Interview question explained-3

Hi folks.
Lets talk something about transformers. (if you don't think of Optimus prime or Auto-bots after listening to this word and instead imagine a pole mounted dump metal box with oil stains on it, You are a true electrical engineer).

Q 1) Magnetization current in the transformer is desirable or not?
Q 2) Why transformer makes noise while working?

Answer 1- If we think logically, transformer draws magnetizing current to magnetize the core or to produce flux in the core. We must have flux in the core to have transformer action. So the magnetizing current is desirable. But it is absolutely FALSE #logical thinking is not always right!
Lets us see why. The answer to the above question lies in the ideal transformer concept. Ideal transformer has infinite core permeability. It implies that it needs zero external efforts to set up flux in the core. In case of practical transformer, as permeability is finite, we need to put in some energy to align the dipoles and to produce flux. That extra energy we give in the form of magnetizing current. So ideally magnetizing current should be zero. Magnetizing current under saturation condition leads to high no load current and worst power factor. So even though flux in the core is desirable, Magnetizing current is not.

Answer 2- Magnetostriction. We all know that there is dipole alignment process going on inside core when we magnetize it or demagnetize it. When the core is magnetized, to accommodate for the aligned dipoles, it expands. Similarly, for demagnetization, due to reverse process, it contracts. So the core undergoes contraction and expansion twice in one cycle of supply frequency (draw a sine wave and you will get it). So we get 2 compression and rarefaction (of resultant sound wave) in the 1 cycle of supply voltage. For 50 Hz supply, we get 100 Hz as the frequency of noise and that falls in our audio range (20-20,000 Hz). So we hear noise, that is often called magnetic hum. To avoid this, the laminations in the core are tightly stacked.
For the electronic transformers the same noise is produced but as they work on very high frequency, We are unable to hear that.  

Interview question explained-2

Hi folks.
A very tricky interview question we are going to discuss now.
Q. Resistance of a conductor depends on the frequency of the supply. True or false?

As soon as we hear this, we say false. Why? because we don't find any angular frequncy term introduced in R which is there in Xc or Xl.
But the answer my friends is TRUE.
Let us see why.
 
Have you heard that the AC resistance of the conductor is more than its DC resistance?
The reason for this is Skin effect. As the frequency of the supply increases, the current surprisingly tends to flow in the skin instead of body of the conductor.This is due the flux linkages in the conductor due to AC current carried by itself. You can understand it something like, An straight conductor when carries AC has inductance even though it is not in the coil form as we always imagine. For 50 Hz the value of resistance is 1.6 times that of DC resistance. We take this 1.6 as an approximation constant to account for skin effect. So as the frequency increases, the tendency of current to flow through skin increases and the effective cross section of the conductor decreases. Effective cross section means the cross section of conductor through which current is actually flowing. (It will be ring like structure if you imagine it and current concentrated in the outer parts).
In the basic resistance formula, we find that Area of cross section is inversely proportional to resistance. So The resistance of conductor increases.

If this is so, Why do we take Resistance constant in Electrical circuit analysis or day to day life?
Answer to 1st Question is ECA is an ideal subject where we assume that F parameter has no effect on R and in day to day life we ignore the skin effect which enables us to say firmly- R IS INDEPENDENT OF FREQUENCY..!

Skin effect is the base of design of ACSR conductors that we see on distribution poles. It is  helpful there. As we know current is actually flowing in outer parts, We can use aluminum outside (high conductivity and low mechanical strength) and we use steel inside (low conductivity and high mechanical strength). As a result we get conductor that has good conductivity as well as mechanical strength at low cost.

So next time when you will keep resistance constant under changing frequency, just keep in mind that you are ignoring skin effect. Thank you for you time and please feel free to comment or mail me about your areas of interest. It will be my genuine pleasure to post about them.

Some standard books to refer

Hi folks.
We are always worried about what book to refer and what not to, as we find very large number of XYZ publications available in market.
Here are some standard book that must be used. Beyond these the sky is the limit but these are few that should form the conceptual base for further studies.

(REF- books are suggested by expert faculties at ACE Hyderabad)

• Network Theory : Engineering Circuit Analysis by Hayt & Kemmerly
  (may be one by Ravish Singh is good to start with)
• Analog Devices : Electronic Devices and Circuit Theory by Boylestad
• Digital Circuits : Digital logic Design by Morris Mano
  (Jain & Jain is also good)
• Control Systems : Control Systems by Nagrath & Gopal
  (Norman Nise would also work)
• Electrical Machines: Electrical Machinery by PS Bimbhra
  (Nagrath kothari is option B)
• Power Electronics : Power Electronics by UA Bakshi
  (may be PS Bhimbara if you can find in library)
• Power Systems: Power System Engineering by Nagrath & Kothari
• Signals and Systems: Signals and Systems by Oppenheim Wilsky       
• Engineering Mathematics: Advanced Engineering Mathematics by Kreyszig
• For Objective Problems: GATE for EE by RK Kanodia
  (objective questions by GK Publications and and by Handa is also good)
  
  For all the remaining subjects xerox is the best choice!
  thank you for your time.

Whimshurst influence generator

hi folks.
I am currently working on whimshurst influence generator as my HV engineering project. Its an awesome handy gadget that you too can build generating voltages of the order of few KVs. The voltages are high enough to cause breakdown of the air at NTP.

you can check out an MIT video and an another page explaining its working.

http://www.youtube.com/watch?v=Zilvl9tS0Og
http://www.coe.ufrj.br/~acmq/whyhow.html

I will surely give  a demo of that in coming club service after I am done with it. If you too are interested then lets build it together..!

 There are few precautions you need to take to handle this gadget. So I suggest you to go through that first. All in all, it is a very fascinating hobby project.
After all, You are not a true electrical engineer if you don't get to play with high voltages..!

Please check out the above link and also search for corona motor (a motor that works on atmospheric charge and guess what, you can drive it using your own whimshrust generator).

Thank you for your time. and please feel free to comment about any more gadget that you are aware about.

Importance of fundamentals

Courtesy : http://electrical-mentor.blogspot.in/p/blog-page_6.html

(it is a blog written by one of the faculties of ACE academy who did M. Tech. in power and control from IIT Kanpur)

Dear Students,
The intention in writing this article is, so many students are asking me in the following way:

“Is it compulsory to read standard text books for every subject to get <100 GATE rank? Otherwise these ACE material and online tests are enough to get that rank”

One thing I would like to say that fundamentals in any subject are like foundation for any building. I can consider these types of questions like “Is it strong foundation required to construct nice building”. Answer is obviously yes. If you construct your building without strong foundation, you cannot celebrate house warming ceremony and you cannot stay in that building.

Getting good rank without fundamentals is exactly similar situation. Getting good rank should not be goal, whatever the education you are having during your student life, it should give confidence to face all the challenges in your life. It can be professional life or personal life. You may feel boring these types of lectures when you are in student, but this is the fact. You will realize this as you are getting old from the student stage

Even in our power electronics also, out of all the components everyone is interested in fundamental component only as it is useful component. There will be no more counter argument for this statement

Until you prepare objective books and mugging up the formulas & bits, you cannot answer new questions and challenges, Always try to learn the procedure, concepts and the approach

For the corporate companies, what you are solving is not an important matter but how you are solving is important. What is the approach you are demonstrating is important and how differently you are reaching the solution will really make differentiation from others

Love the subject, Learn fundamentals and enjoy the life.

Induction machine-1

Hi folks.

Let us talk about most common misconception that we people have.  If I say, I have a 6 pole induction machine what does it mean?

1      I have a machine with 6 poles per phase and total no of poles after energizing the winding will be 6*3 phases = 18

2    I have 6 poles in total. So it must be 2 poles per phase and 2*3= 6 (total poles)

Well. Actually both the above answers are wrong.

When I say a 6 pole induction machines it by default means that there are 6 poles in the machine per phase. But the total no of poles in the machine is not 18. It is 6 only. 

When we excite the winding of this machine, we get 6 poles in the space around rotor symmetrically distributed in space. And they belong to different phases at different instant of time. So there are 6 poles in the space in total. But while stating the rating of the machine we state it as 6 poles ‘per phase’ only.

I know it’s a bit confusing but if you draw 3 winding at 120 degree spatially displaced and pass 3 phases then it is quite clear that the 2 poles are resultant of the individual magnetic field produced. The total no of poles is 2 per phase as we have already stated and one of the poles (that are rotating, say N) belongs to the phase carrying positive maximum and other pole (say S) is the resultant of the remaining two phases that are on half of the negative maximum.

Thank you for your time and please feel free to  leave comment about any doubts and suggestions.