Power Electronics – Switching Devices A power electronic switching device is a combination of active switchable power semiconductor drivers that have been integrated into one. The main characteristics of the switch are determined by internal correlation of functions and interactions of its integrated system. The figure given below shows how a power electronic switch system works. The external circuit of the above diagram is usually held at a high potential relative to the control unit. Inductive transmitters are used to support the required potential difference between the two interfaces. Power switching devices are normally selected based on the rating at which they handle power, that is, the product of their current and voltage rating instead of their power dissipation rate. Consequently, the major attractive feature in a power electronic switch is its capability to dissipate low or almost no power. As a result, the electronic switch able to achieve a low and continuous surge of power. Learning working make money
Category: power Electronics
Discuss Power Electronics Power Electronics refers to an interdisciplinary subject within electrical engineering that deals with the design, control and conversion of power in its electric form. A system that converts electric energy to an electric load through a control circuit is known as a Power Electronic System. The purpose of this tutorial is to introduce and explain the main concepts in Power Electronics, which include Power Semi-Conductor Devices, Phase-Controlled Converters, DC to DC Converter, Inverters and AC to AC Converters. Learning working make money
Reactive Power Control of Converters In high voltage direct current (HVDC) converters, the stations are line commutated. This implies that the initial current of the valve can only be delayed in reference a zero value of the converter bus voltage in AC form. Consequently, for better control of voltage, the converter bus is connected to a reactive power source. Reactive power sources are used to vary capacitors in static systems. The response of the reactive power system is dictated by voltage control in dynamic conditions. When operating unstable AC systems, problems tend to arise because of unstable voltage and overvoltage surges. A better coordination of reactive power sources is required to simplify the control of the firing angles. As a result, this feature of the reactive power converter is increasingly being applied in modern converters using HVDC. Reactive Power Control in Steady State The equations expressing reactive power as a function of active power are given in terms of unit quantities. Base converter voltage is given by − $$V_{db}=3sqrt{frac{2}{pi }}times V_{L}$$ Where VL = Line to line voltage (on winding side) Base DC Current (Idb ) = Rated DC Current (Idr) Base DC Power (Pdc) = nb × Vdb × Idb, where nb = number of bridges in series BaseBase AC voltage (Vb) = (Va) Base AC Power = Base DC Power $$sqrt{frac{18}{pi }}times V_{a}times I_{db}times n_{b}$$ Learning working make money
Power Electronics – Quick Guide Power Electronics – Introduction Power Electronics refers to the process of controlling the flow of current and voltage and converting it to a form that is suitable for user loads. The most desirable power electronic system is one whose efficiency and reliability is 100%. Take a look at the following block diagram. It shows the components of a Power Electronic system and how they are interlinked. A power electronic system converts electrical energy from one form to another and ensures the following is achieved − Maximum efficiency Maximum reliability Maximum availability Minimum cost Least weight Small size Applications of Power Electronics are classified into two types − Static Applications and Drive Applications. Static Applications This utilizes moving and/or rotating mechanical parts such as welding, heating, cooling, and electro-plating and DC power. DC Power Supply Drive Applications Drive applications have rotating parts such as motors. Examples include compressors, pumps, conveyer belts and air conditioning systems. Air Conditioning System Power electronics is extensively used in air conditioners to control elements such as compressors. A schematic diagram that shows how power electronics is used in air conditioners is shown below. Power Electronics – Switching Devices A power electronic switching device is a combination of active switchable power semiconductor drivers that have been integrated into one. The main characteristics of the switch are determined by internal correlation of functions and interactions of its integrated system. The figure given below shows how a power electronic switch system works. The external circuit of the above diagram is usually held at a high potential relative to the control unit. Inductive transmitters are used to support the required potential difference between the two interfaces. Power switching devices are normally selected based on the rating at which they handle power, that is, the product of their current and voltage rating instead of their power dissipation rate. Consequently, the major attractive feature in a power electronic switch is its capability to dissipate low or almost no power. As a result, the electronic switch able to achieve a low and continuous surge of power. Power Electronics – Linear Circuit Elements Linear circuit elements refer to the components in an electrical circuit that exhibit a linear relationship between the current input and the voltage output. Examples of elements with linear circuits include − Resistors Capacitors Inductors Transformers To get a better understanding of linear circuit elements, an analysis of resistor elements is necessary. Resistors A resistor is a device in which the flow of an electric current is restricted resulting in an energy conversion. For example, when electricity flows through a light bulb, the electricity is converted into a different form of energy such as heat and/or light. The resistance of an element is measured in ohms (Ω). The measure of resistance in a given circuit is given by − $$R=rho frac{L}{A}$$ Where R − resistance; ρ − resistivity; L − length of wire; and A − cross-sectional area of wire Symbol of Various Resistors Resistor A variable resistor A potentiometer Capacitors A capacitor refers to an electrical device that has two conducting materials (also known as plates) separated by an insulator known as a dielectric. It uses electric field to store electric energy. The electric field is developed when the capacitor is connected to a battery, thus making positive electric charges accumulate on one plate and negative electric charges on the other plate. When energy is stored in the electrical field of a capacitor, the process is called charging, and when energy is removed, the process is called discharging. The level of electrical energy stored in a capacitor is called capacitance and is measured in farads (F). One farad is the same as one coulomb per unit volt given by 1 C/V. The difference between a capacitor and a battery is that a capacitor stores electrical energy while a battery stores chemical energy and releases the energy at a slow rate. Symbol of Various Capacitors The various symbols of a capacitor are given in the table below. Fixed Capacitor Variable Capacitor Polarized Capacitor Inductors Inductors are electronic devices that use magnetic field to store electric energy. The simplest form of an inductor is a coil or a wire in loop form where the inductance is directly proportional to the number of loops in the wire. In addition, the inductance depends on the type of material in the wire and the radius of the loop. Given a certain number of turns and radius size, only the air core can result in the least inductance. The dielectric materials, which serve the same purpose as air include wood, glass, and plastic. These materials help in the process of winding the inductor. The shape of the windings (donut shape) as well as ferromagnetic substances, for example, iron increase the total inductance. The amount of energy that an inductor can store is known as inductance. It is measured in Henry (H). Symbol of Various Inductors Fixed inductor Variable inductor Transformers This refers to a device that alters energy from one level to another through a process known as electromagnetic induction. It is usually used to raise or lower AC voltages in applications utilizing electric power. When the current on the primary side of the transformer is varied, a varied magnetic flux is created on its core, which spreads out to the secondary windings of the transformer in form of magnetic fields. The operation principle of a transformer relies on Faraday’s law of electromagnetic induction. The law states that the rate of change of the flux linking with respect to time is directly related to the EMF induced in a conductor. A transformer has three main parts − Primary winding Magnetic core Secondary winding Symbol of a Transformer Additional Devices Electromagnetic Devices The concept of electromagnetism is widely used in technology and it is applied in motors, generators and electric bells. For example, in a doorbell, the electromagnetic component attracts a clapper that hits the bell and causes it to ring.
Power Electronics – Matrix Converters A matrix converter is defined as a converter with a single stage of conversion. It utilizes bidirectional controlled switch to achieve automatic conversion of power from AC to AC. It provides an alternative to PWM voltage rectifier (double sided). Matrix converters are characterized by sinusoidal waveforms that show the input and output switching frequencies. The bidirectional switches make it possible to have a controllable power factor input. In addition, the lack of DC links ensures it has a compact design. The downside to matrix converters is that they lack bilateral switches that are fully controlled and able to operate at high frequencies. Its voltage ratio that is output to input voltage is limited. There are three methods of matrix converter control − Space vector modulation Pulse width modulation Venturi – analysis of function transfer The Matrix Converter Circuit The diagram given below shows a single-phase matrix converter. It contains four bi-directional switches with each switch having the ability to conduct in both forward blocking and reverse voltage. Space Vector Modulation (SVM) SVM refers to a method of algorithm used to control the PWM. It creates AC waveforms that drive AC motors at various speeds. In the case of a three-phase inverter having DC supply power, its three main legs at the output are connected to a 3-phase motor. The switches are under control to ensure that no two switches in the same leg are ON at the same time. Simultaneous ON states could result in the DC supply shorting. This leads to eight switching vectors where two are zero and six are active vectors for switching. Learning working make money
Power Electronics – Cycloconverters A cycloconverter refers to a frequency changer that can to change AC power from one frequency to AC power at another frequency. This process is known as AC-AC conversion. It is mainly used in electric traction, AC motors having variable speed and induction heating. A cycloconverter can achieve frequency conversion in one stage and ensures that voltage and the frequencies are controllable. In addition, the need to use commutation circuits is not necessary because it utilizes natural commutation. Power transfer within a cycloconverter occurs in two directions (bidirectional). A major problem with cycloconverters is that when it is operating at small currents, there are inefficiencies created with firing delay. Furthermore, operations are only smooth at frequencies that are not equal half frequency input values. This is true because a cycloconverter is an AC- AC converter that is phase controlled. Therefore, for it to give the required AC output voltage, it has to do a selection of the voltage input segments by applying line (natural) commutation. This explains why the output frequency is lower than the frequency input. Harmonics in a cycloconverter are mainly affected by methods of control, overlap effect, the number of pulses in a given cycle, operation mode and mode of conduction. There are two types of cycloconverters− Step Up cycloconverter − These types use natural commutation and give an output at higher frequency than that of the input. Step Down cycloconverter − This type uses forced commutation and results in an output with a frequency lower than that of the input. Cycloconverters are further classified into three categories − Single phase to single-phase − This type of cycloconverter has two full wave converters connected back to back. If one converter is operating the other one is disabled, no current passes through it. Three-phase to single-phase − This cycloconverter operates in four quadrants that is (+V, +I) and (−V, −I) being the rectification modes and (+V, −I) and (−V, +I) being the inversion modes. Three-phase to three-phase − This type of cycloconverter is majorly used in AC machine systems that are operating on three phase induction and synchronous machines. Learning working make money
Power Electronics – Types of Inverters An inverter refers to a power electronic device that converts power in DC form to AC form at the required frequency and voltage output. Inverters are classified into two main categories − Voltage Source Inverter (VSI) − The voltage source inverter has stiff DC source voltage that is the DC voltage has limited or zero impedance at the inverter input terminals. Current Source Inverter (CSI) − A current source inverter is supplied with a variable current from a DC source that has high impedance. The resulting current waves are not influenced by the load. Single Phase Inverter There are two types of single phase inverters − full bridge inverter and half bridge inverter. Half Bridge Inverter This type of inverter is the basic building block of a full bridge inverter. It contains two switches and each of its capacitors has a voltage output equal to $frac{V_{dc}}{2}$. In addition, the switches complement each other, that is, if one is switched ON the other one goes OFF. Full Bridge Inverter This inverter circuit converts DC to AC. It achieves this by closing and opening the switches in the right sequence. It has four different operating states which are based on which switches are closed. Three Phase Inverter A three-phase inverter converts a DC input into a three-phase AC output. Its three arms are normally delayed by an angle of 120° so as to generate a three-phase AC supply. The inverter switches each has a ratio of 50% and switching occurs after every T/6 of the time T (60° angle interval). The switches S1 and S4, the switches S2 and S5 and switches S3 and S6 complement each other. The figure below shows a circuit for a three phase inverter. It is nothing but three single phase inverters put across the same DC source. The pole voltages in a three phase inverter are equal to the pole voltages in single phase half bridge inverter. The two types of inverters above have two modes of conduction − 180° mode of conduction and 120° mode of conduction. 180° mode of conduction In this mode of conduction, every device is in conduction state for 180° where they are switched ON at 60° intervals. The terminals A, B and C are the output terminals of the bridge that are connected to the three-phase delta or star connection of the load. The operation of a balanced star connected load is explained in the diagram below. For the period 0° − 60° the points S1, S5 and S6 are in conduction mode. The terminals A and C of the load are connected to the source at its positive point. The terminal B is connected to the source at its negative point. In addition, resistances R/2 is between the neutral and the positive end while resistance R is between the neutral and the negative terminal. The load voltages are gives as follows; VAN = V/3, VBN = −2V/3, VCN = V/3 The line voltages are given as follows; VAB = VAN − VBN = V, VBC = VBN − VCN = −V, VCA = VCN − VAN = 0 Waveforms for 180° mode of conduction 120° mode of conduction In this mode of conduction, each electronic device is in a conduction state for 120°. It is most suitable for a delta connection in a load because it results in a six-step type of waveform across any of its phases. Therefore, at any instant only two devices are conducting because each device conducts at only 120°. The terminal A on the load is connected to the positive end while the terminal B is connected to the negative end of the source. The terminal C on the load is in a condition called floating state. Furthermore, the phase voltages are equal to the load voltages as shown below. Phase voltages = Line voltages VAB = V VBC = −V/2 VCA = −V/2 Waveforms for 120° mode of conduction Learning working make money
Power Electronics – Introduction Power Electronics refers to the process of controlling the flow of current and voltage and converting it to a form that is suitable for user loads. The most desirable power electronic system is one whose efficiency and reliability is 100%. Take a look at the following block diagram. It shows the components of a Power Electronic system and how they are interlinked. A power electronic system converts electrical energy from one form to another and ensures the following is achieved − Maximum efficiency Maximum reliability Maximum availability Minimum cost Least weight Small size Applications of Power Electronics are classified into two types − Static Applications and Drive Applications. Static Applications This utilizes moving and/or rotating mechanical parts such as welding, heating, cooling, and electro-plating and DC power. DC Power Supply Drive Applications Drive applications have rotating parts such as motors. Examples include compressors, pumps, conveyer belts and air conditioning systems. Air Conditioning System Power electronics is extensively used in air conditioners to control elements such as compressors. A schematic diagram that shows how power electronics is used in air conditioners is shown below. Learning working make money
Power Electronics – Inverters Solved Example A single phase half bridge inverter has a resistance of 2.5Ω and input DC voltage of 50V. Calculate the following − Solution − a. The RMS voltage occurring at the fundamental frequency $E_{1RMS}=0.9times 50V=45V$ b. The power Output RMS output voltage $E_{ORMS}=E=50V$ Output power $=E^{2}/R=left ( 50right )^{2}/2.5=1000W$ c. Peak current and average current Peak current $I_{p}=E_{0}/R=50/2.5=20A$ Average current$=I_{p}/2=20/2=10A$ d. Harmonic RMS voltage $E_{n}=left { left ( E_{ORMS} right )^{2}-left ( E_{1RMS} right )^{2} right }^{0.5}=left [ 50^{2} -45^{2}right ]^{0.5}=21.8V$ e. Total harmonic distortion $E_{n}/E_{1RMS}=21.8/45=0.48times 100%=48%$ Learning working make money
Power Electronics – Dual Converters Dual converters are mainly found in variable speed drives (VFDs). In a dual converter, two converters are linked together back to back. The operation of a dual converter is explained using the diagram below. It is assumed that − A dual converter is an ideal one (gives pure DC output) at its terminals. Each two-quadrant converter is a controlled DC source in series with a diode. Diodes D1 and D2 show the unidirectional flow of current. Considering a dual converter operating without circulating current, the AC current is barred from flowing by controlled firing pulses. This ensures that the converter carrying the load current conducts while the other converter is blocked. This means that a reactor between the converters is not needed. Battery Charger A battery charger also known as a recharger utilizes electric current to store energy in a secondary cell. The charging process is determined by the type and size of the battery. Different types of batteries have different tolerance levels to overcharging. The recharging process may be achieved by connecting it to a constant voltage or constant current source. Charging Rate (C) The charging rate is defined as the rate of charging or discharging a battery and is equal to the battery capacity in one hour. A battery charger is specified in terms of its charging rate C. For example, a battery charger with a rating of C/10 would give a charging capacity in 10 hours while one rated 3C would charge a battery in 20 minutes. Types of Battery Chargers There are many types of battery chargers. In this tutorial, we will consider the five main types. Simple chargers − Operates by supplying a constant DC power source into the battery being charged. Fast chargers − Uses control circuitry to charger the battery rapidly and in the process prevent the battery cells from damage. Inductive chargers − Uses electromagnetic induction to charge the battery. Intelligent chargers − Used to charge a battery that contains a chip that communicates with the smart charger. Motion powered charger − Makes use of human motion to charge a battery. A magnet placed between two springs is moved up and down by human motion thus charging the battery. Learning working make money