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.

TRANSFORMER -1

Hello folks,

Let us talk something about the soul of the power system – Transformers.
The core of the transformer is a very important component of transformer and governs the operating character of it. As we always say, it is not made up of silicon steel (electrical steel). It is made up of material called - CRGO steel (Cold Rolled Grain Oriented Steel). CRGO is silicon steel but treated specially.

Features of Si Steel are Ferromagnetic material, High permeability to the flow of flux (magnetization is easy), Low hysteresis coefficient and so hysteresis loss is low.

If core is made up of pure steel, it has good magnetic properties but it has more conductivity. Strength of eddy current inside core depends on conductivity of steel. There exists a large circulating current (eddy current) as conductivity of core is more. About 4-5% silica is added to pure steel, to reduce eddy current loss. Conductivity of steel is reduced without affecting its magnetic strength. Yet eddy current is not reduced to that extended which is needed. So we go for CRGO.

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 (Google it to know about it). This crystal, when subjected to magnetic field, 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

Shorter the length of axis of magnetization, lower is the reluctance offered. To reduce magnetizing current, permeability should be very high and 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. It involves heating a material to above its critical temperature, maintaining a suitable temperature, and then cooling. Transformer core is laminated to reduce eddy current loss.

Order of flux density of core in its increasing order is

Cast iron  >  Cast steel  >  Mild steel  >  Silicon Steel >  CRGO

Thank you for your time and do comment about your areas of interest. We can have a two-way discussion on that topic.

BATTERY BASICS AND ITS RATINGS

Hello folks,

Let us talk about some real world voltage sources. The most commonly used voltage source is the battery, which we all must have frequently used in many of the electrical equipment and gadgets. The battery is the DC voltage source which means voltage is 1) constant 2) Independent of load current 3) Constant with respect to time. There exist many diverse types and versions of battery. In practice, battery has 2 terminals, one is positive terminal and other is negative terminal. [Fig. 1]

Inside this ideal battery, there exists a chemical process. Consider a container that contains two electrodes i.e. positive electrode (anode) and negative electrode (cathode). This container is filled with an electrolyte. Among all the batteries, the most commonly used is Lead Acid Battery that is used in almost all the automobiles. In this battery, the anode is Lead Oxide (PbO2), cathode is Lead (Pb) and dilute sulphuric acid (dil. H2SO4) is used as an electrolyte. The electrons get generated at the cathode, flow to the anode, then through external conductor they flow towards external circuit and then again back to the cathode. In this process, water is generated in the lead acid battery and it dilutes the battery and hence the charge goes down. So repeatedly the charge goes down and battery gets discharges. We need to recharge the battery to its original capacity i.e. the concentration is brought back and now battery is in the charged state. [Fig. 2]

                               Fig 1. Battery Symbol       Fig.2 Practical Battery

In practical battery, the electrolyte is in gel form. On any practical battery we can see name plate ratings. For e.g. Let’s consider the 2 most important ratings in case of battery

i)                Terminal voltage (Nominal DC voltage) across 2 terminals Vt  and

ii)              Ampere hours (Ah) rating

Let Vt = 12 V         Ah = 7

7 Ah means we can draw 7A for 1 hour. But it’s impossible in actual practice. So it implies that each battery has maximum discharge current capability. Let that battery has 10 cells and now we can draw 7/10=0.7A for 10 hours. This is 7 Ah.

Energy capacity of battery = 12 V * 7 Ah = 84 Wh

Energy stored in the battery is called battery capacity which is measured either in Wh (Watt-hours), kWh (kilo Watt-hours), or Ah (Ampere-Hours). The most common measure of battery capacity is Ah, defined as number of hours for which a battery can provide a current equal to the discharge rate at nominal voltage of the battery.

The charging/discharging rates affect the rated battery capacity. If battery is being discharged very quickly (discharge current is high), then the amount of energy that can be extracted from the battery is reduced and the battery capacity is lower. Alternately, if the battery is discharged at a very slow rate using a low current, more energy can be extracted from a battery and the battery capacity is higher.   

PHYSICAL INTERPRETATION OF RLC

Hello folks,

A simple question based on which, people can differentiate an Electrical Engineer from a Non-Electrical person is a quiet logical question. To answer this question one need to have the basic knowledge regarding RLC components without whom Electrical Engineering is incomplete. This is also a famous interview question.

And the question is: WHAT IS PHYSICAL INTERPRETATION OF RLC?                                      

[i.e. How can you explain the basic elements of Electrical Engineering (R,L,C) to a non-electrical guy?]

There are numerous ways to answer this question as per your imagination and understand of RLC. But here I would like to share one co-relation relating to our daily life. Everyone is free to define their own analogy for RLC components.     

Resistor, Capacitor and Inductor are the most primary components in Electrical technology.

Resistors are the components the dissipate energy (usually in the form of heat) and this energy cannot be recovered as it has no memory. As the symbol shows, the resistor can be related to speed breakers on the road because of which the speed of a vehicle is reduced. The zigzag shape of symbol of resistor indicates the same.

Capacitor is an element that can store potential energy, while energy is transferred from load to source, in the Potential form. It can store voltage as the memory and hence, voltage across capacitor is a state variable. To understand how it stores energy, let us consider a simple analogy of real world – Water Purifiers in our home. It has tap at the bottom and a pipe at top to fill the purifier. Let us assume that water is flowing continuously through the pipe into the purifier and tap at bottom is also open to facilitate continuous outflow of water. The inflow and outflow of the water can be so adjusted that there is some specified level of water built up and this is the buffer water. So the water that is getting stored in the purifier is like stored potential energy. This is how we can relate capacitor to water purifier. The input flow of water is somewhat like input flow of charges in the capacitor. This is called charging of capacitor. The flow of water outside purifier is the flow of charges outside capacitor and this is discharging of energy in capacitor (discharge of water outside purifier). The capacity of water tank is analogous to the capacitance value of the capacitor. The height of the water that is being stored in the water tank is the potential of the energy and it is the voltage potential of the capacitor. So the capacitor can be undoubtedly perfectly related to water purifier.

Inductor is an element that can store kinetic energy and here flow of current through inductor is a state variable. To understand inductor let us consider a flowing mass of water and the rotating turbine in specific direction. So here the rotation of the blades of turbine is by virtue of the fact that energy is stored in the flowing water body and wheel rotates when the water is flown onto it. So the flowing water mass has the kinetic energy stored in it by the virtue of it being in motion. It is that energy which inductor stores equivalently in case of electric circuits and the water flowing is the current. So the energy stored by the virtue of current flowing is the kinetic energy which is stored in the inductor.

Thank you for your time and feel free to share topics of your area of interest.

ELECTRICAL ENGINEERING

ELECTRICAL ENGINEERING

Hello folks,

Let us start our discussion with the basics of electrical engineering. Have you ever investigated about wall outlet, which is the most common voltage source that we can find everywhere?

In olden days, we used to find 2 holes in the wall outlet but now-a-days we can see 3 holes in the wall outlet in our home. These 3 terminals that are present in every socket are Live, Neutral and Earth. The load is connected between live and the neutral. The Earth terminal is mandatorily connected to the chassis of equipment because earth is always at absolute zero potential and possibility of shock can be discarded. After connecting Earth to chassis of the equipment, even if you touch the chassis you won’t get shock.

Earth pin is having more thickness as compared to other 2 pins in 3-pin wall outlet. As per the basic formula for resistance, if we increase area of cross section then resistance will decrease. So this will provide least resistance path for the leakage currents and we, the operators will be safe.

When we are putting plug in socket, the first pin to come in contact with metal inside socket is Earth pin. So before giving supply to the device we ensure that all the leakage charge is grounded. This is reason why the length of Earth Pin is larger than 2 other Pins.

If you measure voltage between Live and Neutral, you will get a pure sinusoidal wave shape. In India, maximum voltage is 325 V (230*1.414), frequency 50 Hz and time period 20mS.

The color codes of wires in a U.S. 110-volt wall outlet are :

• Copper, Green, or green with yellow stripes – Earth Pin
• White – Neutral Pin
• Black, red, or any other color – Phase Pin

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