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

TRANSFORMER -2

Hello folks,

Let us now understand what is this magnetic humming in transformer in detail. It is also a well-known interview question.

Q. What is Magnetic humming in transformer? How can it be minimized?

The   process of bunching of all the laminations is called core staggering. If laminations are not properly staggered, there is possibility of thin air gap between laminations. It increases reluctance of magnetic path, thereby increasing the excitation current in transformer. So transformer makes more noise which is magnetic humming.

The basic reason behind the production of noise in transformer is Magnetostiction phenomena. It is the tendency of any magnetic material due to which slight changes in the dimensions of magnetic material take place whenever it is subjected to alternating nature of magnetic force. These dimensional changes in laminations when the core is subjected to alternating mmf is (10^-8) cm. When thickness of all laminations increase, transformer core area also increases. Core is subjected to continuous expansion and compression due to magnetostiction and hence there is mechanical stress on core. Now core starts vibrating due to mechanical stresses on transformer. This noise is in audible frequency range (20Hz-20KHz), which is called magnetic humming.

Consider the sinusoidally varying flux. In first cycle, there is increment and decrement in flux. When there is strengthening of flux (0-45) , thickness of lamination increases slightly, whereas there is no change in core dimensions from (45-90). The same process repeats for negative half cycle of sine wave. So in 1 cycle of flux, 2 times dimensional changes are taking place in core. Therefore, frequency of noise is double than supply frequency. If supply frequency = 50 Hz, then noise frequency = 100 Hz. Noise level depends on the flux density in the core.

In solid core, there is no humming sound as there is no noise produced. We cannot get noise in transformer with DC excitation because here we have steady mmf, so there is no magnetization-demagnetization (there is only magnetization).  There is no noise from transformer in electronics because they operate at higher frequencies (MHz) which is not in the audible range. (Here noise is present but not audible).

We get more noise in Power transformer (in Substations) as compared to distribution transformer (normal usage) because power transformers have higher flux densities. We cannot curtail noise problems but we can reduce noise level by proper core staggering.

Thank you for your time.  

POWER ELECTRONICS - 3

Hello folks,

Today we will discuss about Zeta converter (this is last DC-DC converter we will look up to)

A Zeta converter is a fourth-order DC-DC converter that capable of amplifying and reducing the input voltage levels without inverting the polarities as it includes two capacitors and two inductors as dynamic storage elements. Compared with other converters in the same class, such as Cuk and SEPIC converters, the Zeta converter has received the least attention.

A zeta converter, with regard to energy input, can be seen as buck-boost-buck converter and with regard to the output; it can be seen as boost-buck-boost converter.

Capacitor C1 will be in parallel with C2, so C1 is charged to the output voltage, VOUT, during steady-state CCM. When SW is off, the voltage across L1 must be VOUT since it is in parallel with C2. Since C2 is charged to VOUT, the voltage across Q1 when Q1 is off is VS + VOUT; therefore the voltage across L1a is –VOUT relative to the drain of Q1. When Q1 is on, capacitor C1, charged to VOUT, is connected in series with L1; so the voltage across L1 is +VS, and diode D1 sees VS + VOUT. When SW is on, energy from the input supply is being stored in L1 and C1. L1 also provides IOUT. When SW turns off, L1’s current continues to flow from current provided by C1, and L1 again provides IOUT.

Modes of Operation (shown in figure)

Mode-1:- The first mode is obtained when the switch is ON (closed) and instantaneously, the diode D is OFF. During this period, the current through the inductor L1 and L2 are drawn from the voltage source Vs. This mode is the charging mode.

Mode-2:- The second mode of operation starts when the switch is OFF and the diode D is ON position. This stage or mode of operation is known as the discharging mode since all the energy stored in L2 is now transferred to the load R.

The operational duty ratio is same as that of Sepic converter. At equilibrium, L1 average current equals Iin and L2 average currents equal Iout,  since there is no DC current through the flying capacitor C1 in the circuit. Also there is no DC voltage across either inductor. Therefore, C1 sees ground potential at its left side and VOUT at its right side, resulting in DC voltage across C1 being equal to VOUT.

These are few of the building blocks of advanced DC-DC Converters. Still there is a lot to be explored in the world of Converters and Inverter. You can study them as per your interest

Thank you for your time.

POWER ELECTRONICS - 2

Hello folks,

As we were discussing about the Basic Building Blocks of Advanced DC-DC Converters, today we will discuss about SEPIC converter and analyze it.
SEPIC Converters:

The 2nd Basic Building Blocks is Sepic Converter. Both Cúk and buck-boost converter operation cause large amounts of electrical stress on the components, this can result in device failure or overheating. SEPIC converters solve both of these problems.

The SEPIC officially stands for “Single-Ended Primary Inductance Converter. Thus the SEPIC is also basically a BOOST-BUCK converter akin to the CUK converter. (The Boost stage comes first followed by the Buck stage and it is also I-V-I converter). Its output voltage is having same polarity as that of input voltage.

It has become popular in recent years in battery-powered systems as most battery operated circuits require dc-dc conversion to maintain full operation. In most circumstances that require stepping up and down the input voltage, SEPIC converters are worth the price of the extra inductor and capacitor for the efficiency and stable operation they provide.

                                                                Fig 1 :  Basic Sepic Converter

  

         Fig 2: Sepic Converter SW ON- OFF circuit diagram

One benefit of the SEPIC converter is that the input ripple current in the input capacitor is continuous. This reduces the amount of input capacitance necessary for low-ripple voltage, which reduces EMI (Electro Magnetic Interference). SEPIC converter maintains a fixed output voltage regardless of whether the input voltage is above, equal or below the output voltage.

Operation of converter into two modes: (Shown in fig)

Mode-1:- When the pulse is high/the switch is on, inductor L1 is charged by the input voltage Vin and inductor L2 is charged by capacitor C1. The diode D is off and the output is maintained by capacitor C. The fact that both L1 and L2 are disconnected from the load when the switch is on leads to complex control characteristics.

L1 charges, C1 discharges, L2 charges, C discharges 

Mode-2:- When the pulse is low/the switch is off, the inductors outputs through the diode to the load and the capacitors are charged. When the power switch is turned off, the first inductor L1 charges the capacitor C1 and also provides current to the load, as shown in Fig. The second inductor L2 is also connected to the load during this time.

L1 discharges, C1 charges, L2 discharges, C charges 

Thus, the converter is in “buck” mode for D < 0.5, and in “boost” mode for D > 0.5. SEPIC converters make it possible to efficiently convert a DC voltage to either a lower or higher voltage.

Thank you for your time. Let me know your area of interest on which we can have a two-way communication.

POWER ELECTRONICS - 1

Hello Folks,

To understand any complex DC-DC Converter in Power Electronics, knowledge of basic converters is obligatory. AC-AC conversion can be easily done with a transformer; however dc-dc conversion is not as simple. Diodes, voltage bridges and voltage regulators are found to be inefficient for this. The most efficient method of regulating voltage through a circuit is with a dc-dc converter.

The dc-dc converters can be viewed as dc transformers that deliver to the load a dc voltage or current at a different level than the input source. This dc transformation is performed by electronic switching means, not by electromagnetic means such as in conventional transformers. The output voltages of dc-dc converters range from one volt for special VLSI circuits to tens of kilovolts in X-ray lamps.
DC-DC power converters are employed in a variety of applications, including power supplies for personal computers, office equipment, spacecraft power systems, laptop computers, and telecommunications equipment, as well as dc motor drives.

The three basic types of DC-DC converter circuits are buck, boost and Buck-Boost. The Buck converter may consequently be seen as a Voltage to Current converter, the Boost as a Current to Voltage converter and the Buck-Boost as a Voltage-Current-Voltage Converter.

The very 1st Basic building block of advanced DC-DC Converter is Cuk Converter. Cúk converter is used as a Current-Voltage-Current converter. Cúk converter is actually the cascade combination of a boost and a buck converter. It provides an output voltage that is less than or greater than the input voltage & the output voltage polarity is opposite to that of input voltage.

Many years ago, Dr. Cúk invented the integrated magnetic concept called DC-transformer, where the sum of  DC fluxes created by currents in the winding of the input inductor L1 and transformer T is equal to DC flux created by the current in the output inductor L2 winding. Hence the DC fluxes are opposing each other and thus result in a mutual cancellation of the Dc fluxes.

It combines the characteristic low input current ripple of the boost converter with the low output current ripple of the buck converter. The buck, boost and Buck-Boost converters all transfer energy between input and output using inductor and analysis is based of voltage balance across the inductor. The CÚK converter uses capacitive energy transfer and analysis is based on current balance of the capacitor.

The ideal switch (and ideal components) circuit diagram for the Cuk converter with BJT NPN transistor (Self-commuted device) used as a switch is shown in Fig 1

                                       Fig 1: Cúk Converter with BJT used as a switch

                   Fig 2a: Cúk converter with switch closed  

Fig 2b: Cúk converter with switch open

The input circuit in the Cuk converter is, clearly, a Boost converter and the output circuit is seen to be a Buck converter. The Cuk converter requires two (dependent) switches, two inductors L1 and L2, and two capacitors.

In fig 1, the capacitor C1 acts as a primary means to store and transfer the power from input to output. As a result, the input current is continuous (unlike buck-boost converter). The voltage vc1 is always greater than either input or output voltage. Due to the inductor on the output stage, the Cúk converter can provide a better output current characteristic. The average output to input relations are similar to that of a Buck-Boost converter circuit. The output voltage is controlled by controlling the switch-duty cycle. It can be used for step up and step down of voltage by varying duty ratio in the equation,

                                                     Vo/Vin = D/ (1-D)

Operation of converter into two modes:

Mode-1:- When BJT switch is turned on, the current through L1 rises and at the same time the voltage of C1 reverse biases diode D hence turning it off. The capacitor C1 discharges its energy to the circuit C1-C-load-L2. (Fig. 2a)

Mode-2:- When BJT switch is turned off, the capacitor will start to charge from input supply Vin and the energy stored in the inductor is transferred to the load. The capacitor C1 is the medium for transferring energy from source to load. (Fig. 2b)

The circuits have low switching losses and high efficiency. Cúk converter provides capacitive isolation which protects against switch failure (unlike the buck topology). With Cúk converter energy is transferred when switch opens and also when switch is closed (This doesn’t exists in case of buck and boost converter). It uses L-C filter, so peak-peak ripple current of inductors are less compared to Buck-Boost converter.

We will shortly discuss the other Advanced DC-DC Converters. Thank you for your time.

FLUORESCENT TUBE

Hello Folks,

Even though fluorescent lights all around us, this devices is a total mystery to most people. We will just have a look what is going on inside these white tubes? We will analyze how fluorescent lamps emit such a bright glow without getting scalding hot like an ordinary light.

The general design of a simple fluorescent lamp consists of a sealed glass tube. The tube contains a small bit of mercury and a gas (usually argon) kept under very low pressure. The tube also contains a phosphor powder, coated along the inside of the glass. The tube has two electrodes, one at each end, which are wired to an electrical circuit. The electrical circuit, which includes a starter and ballast, is hooked up to an alternating current (AC) supply.

When you turn on the tube light, current flows through the electrical circuit to the electrodes. When an AC voltage is applied to a tube light fixture, the voltage passes through the choke, the starter, and the filaments of the tube. There is a considerable voltage across the electrodes (approximately 1000V), so electrons will migrate through the gas from one end of the tube to the other. This energy changes some of the mercury in the tube from a liquid to a gas. As electrons and charged atoms move through the tube, some of them will collide with the gaseous mercury atoms. These collisions excite the atoms, bumping electrons up to higher energy levels. When the electrons return to their original energy level, they release light photons. As electrons return to their original energy level, they begin to release light. However, the light they emit is ultraviolet, and not visible to the naked eye. This is why the tube was coated with phosphorous. When exposed to the ultraviolet light, the particles emit a white light which we can see. Once the conduction of electrons between the electrodes is complete, no more heating of the filaments is required and whole system works at a much lower current. The entire fluorescent lamp system depends on an electrical current flowing through the gas in the glass tube. The figure explains how an atom emits electron and how exactly is the current flow and working of fluorescent tube.

The starter is basically a time delay switch. Its job is to let the current flow through to the electrodes at each end of the tube, causing the filaments to heat up and create a cloud of electrons inside the tube. The starter then opens after a second or two. The voltage across the tube allows a stream of electrons to flow across the tube and ionize the mercury vapor. Without the starter, a steady stream of electrons is never created between the two filaments, and the lamp flickers.

The ballast works mainly as a regulator. They consume, transform, and control electrical power for various types of electric-discharge lamps, providing the necessary circuit conditions for starting and operating them.

In a fluorescent lamp, the voltage must be regulated because the current in the gas discharge causes resistance to decrease in the tube. The AC voltage will cause the current to climb on its own. If this current isn’t controlled, it can cause the blow out of various components.

Today the most popular fluorescent lamp design is “rapid start” lamp (without Starter).

Concluding the discussion, we can say that the basic principle is: an electric current stimulates mercury atoms, which causes them to release ultraviolet photons. These Photons in turn stimulate a phosphor, which emits visible light photons.

Thank you for your time. Do comment about your area of interest.