INDUCTOR

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.
This post is taken from learnwithtesla.blogspot.com

CAPACITOR

Hello 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 equipment's 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.

This is taken from learnwithtesla.blogspot.com