Category: power Electronics

Single Phase AC Voltage Controllers A single-phase AC controller (voltage controller) is used to vary the value of the alternating voltage after it has been applied to a load circuit. A thyristor is also placed between the load and the constant source of AC voltage. The root mean square alternating voltage is regulated by changing the thyristor triggering angle. In the case of phase control, the thyristors are employed as switches to establish a connection from the AC input supply to the load circuit during each input cycle. For every positive input voltage, chopping occurs and voltage is reduced. Circuit Diagram with a Resistive Load During half part of the cycle, the thyristor switch is turned ON to enable the voltage input to appear across the load. This is followed by the OFF state during the last half cycle so as to disconnect the load from the source voltage. When the triggering angle α is controlled, the RMS value of the voltage on the load is also controlled. The triggering angle α is therefore, defined as the value of ωt at which the thyristor switches ON. Multistage Sequence Control of AC Converter When two or more sequence control stages are connected, it is possible to have an improvement in power factor and further reduction in THD (total harmonic distortion). An n-stage sequence control converter has n windings in the transformer secondary part with each rated es/n (the source voltage). When two AC converters are placed parallel to each other, the zero sequence way is created. A little difference between the two converters causes a great zero sequence in circulating current. The diagram below shows the parallel system of a converter. The direction of the current is anti-clockwise with respect to that of the voltage system. Learning working make money

DC to DC Converters Solved Example A step up chopper has an input voltage of 150V. The voltage output needed is 450V. Given, that the thyristor has a conducting time of 150μseconds. Calculate the chopping frequency. Solution − The chopping frequency (f) $f=frac{1}{T}$ Where T – Chopping time period = $T_{ON}+T_{OFF}$ Given − $V_{S}=150V$$V_{0}=450V$ $T_{ON}=150mu sec$ $V_{0}=V_{Sleft ( frac{T}{T-T_{ON}} right )}$ $450=150frac{T}{T-150^{-6}}$ $T=225mu sec$ Therfore, $f=frac{1}{225ast 10^{-6}}=4.44KHz$ The new voltage output, on condition that the operation is at constant frequency after the halving the pulse width. Halving the pulse width gives − $$T_{ON}=frac{150times 10^{-6}}{2}=75mu sec$$ The frequency is constant thus, $$f=4.44KHz$$ $$T=frac{1}{f}=150mu sec$$ The voltage output is given by − $$V_{0}=V_{S}left ( frac{T}{T-T_{ON}} right )=150times left ( frac{150times 10^{-6}}{left ( 150-75 right )times 10^{-6}} right )=300Volts$$ Learning working make money

Power Electronics – Pulse Converters Phase Controlled Converter A phase controlled converter converts AC to DC energy (line commutated). In other words, it is used in the conversion of fixed-frequency and fixed-voltage AC power into variable DC voltage output. It is expressed as Fixed Input − Voltage, frequency and AC power Variable output − DC voltage output The AC input voltage that goes into a converter is normally at fixed RMS (root mean square) and fixed frequency. The inclusion of phase-controlled thyristors in the converter ensures that a variable DC output voltage is obtained. This is made possible by altering the phase angle at which the thyristors are triggered. As a result, a pulsating waveform of the load current is obtained. During the input supply half cycle, the thyristor is in forward bias and is switched ON via the application of sufficient gate pulse (trigger). Current starts to flow once the thyristor has been switched ON, that is, at a point ωt=α to point ωt=β. The moment the load current drops to zero, the thyristor switches OFF as a result of line (natural) commutation. There are a number of power converters that utilize natural commutation. These include − AC to DC converters AC to AC converters AC voltage controllers Cycloconverters The above power converters will be explained in the next chapters in this tutorial. 2- Pulse Converter A 2-phase pulse converter, also known as a level 2 pulse width modulator (PWM) generator, is used to generate pulses for pulse width modulation converters that are carrier based. It does this by utilizing level-two topology. This block controls switching devices for control purposes like IGBTs and FETs that exist in three types of converters namely − 1 arm (single phase half bridge) 2 arms (single phase full bridge) 3 arms ( three phase bridge) The reference input signal in a 2-pulse converter is compared to a carrier. If the reference input signal is more than the carrier, the pulse equals to 1 for the upper device and 0 for the lower device. In order to control a device with a single-phase full bridge (2 arms), it is necessary to apply unipolar or bipolar pulse width modulation. In unipolar modulation each of the two arms is independently controlled. A second reference input signal is generated internally through a shift in initial reference point by 180° When the bipolar PWM is applied, the state of the lower switching device in the second single phase full bridge is similar to the upper switch in the first single phase full bridge device. Using a unipolar modulation leads to smooth AC waveforms while the bipolar modulation results in less varying voltage. 3-Pulse Converter Consider a three-phase 3-pulse converter, where each of the thyristor is in conduction mode during the third of the supply cycle. The earliest time a thyristor is triggered into conduction is at 30° in reference to the phase voltage. Its operation is explained using three thyristors and three diodes. When the thyristors T1, T2 and T3 are replaced by diodes D1, D2 and D3, conduction will begin at angle 30° in respect to the phase voltages uan, ubn and ucn respectively. Therefore, the firing angle α is measured initially at 30° in reference to the phase voltage corresponding to it. The current can only flow in one direction through the thyristor, which is similar to inverter mode of functioning where power flows from the DC side to the AC side. In addition, the voltage in the thyristors is controlled by controlling the firing angle. This is achieved when α = 0(possible in a rectifier). Thus, the 3-pulse converter acts as an inverter and a rectifier. 6-Pulse Converter The figure below shows a six-pulse bridge controlled converter connected to a three-phase source. In this converter, the number of pulses is twice that of phases, that is p = 2m. Using the same converter configuration, it is possible to combine two bridges of the six-pulse to obtain a twelve or more pulses converter. When commutation is not available, two diodes will conduct at any particular time. Furthermore, to obtain a voltage drop across the load, two diodes must be at positioned at opposite legs of the bridge. For example, diodes 3 and 6 cannot be ON at the same time. Therefore, the voltage drop across the DC load is a combination of line voltage VL from the three-phase source. It is important to note that more the number of pulses, the greater the utilization of the converter. In addition, the fewer the number of pulses the lesser the utilization of the converter. Learning working make money

Power Electronics – Performance Parameters It is important to determine the performance parameters for different converters whose topologies can be single phase or multi-phase. Assumptions The devices used are ideal, that is, they do not have any losses The devices have resistive loads DC Voltage on Load $$V_{DC}=frac{1}{T} int_{0}^{T}V_{L}left ( t right )dt$$ RMS Voltage on Load $$V_{L}=sqrt{frac{1}{T}}int_{0}^{T}V_{L}^{2}left ( t right )dt$$ Form Factor $$FF=frac{V_{L}}{V_{DC}}$$ Ripple Factor $$RF=frac{sqrt{V_{L}^{2}-V_{DC}^{2}}}{V_{DC}}=sqrt{FF^{2}-1}$$ Efficiency(Rectification Factor) $$eta =frac{P_{DC}}{P_{L}+P_{D}}$$ Where the above are defined as − $P_{DC}=V_{DC}times I_{DC}$ $P_{L}=V_{L}times I_{L}$ $P_{D}=R_{D}times I_{L}^{2}$($P_{D}$ is the rectifier losses and $R_{D}$ the resistance) $$eta =frac{V_{DC}I_{DC}}{left ( V_{L}I_{L} right )+left ( R_{D}I_{L}^{2} right )}=frac{V_{DC}^{2}}{V_{L}^{2}}times frac{1}{1+frac{R_{D}}{R_{L}}}$$ But $R_{D}=0$ Therefore, $$eta =left ( frac{V_{DC}}{V_{L}} right )^{2}=left ( frac{1}{FF}right )^{2}$$ Transformer Utilization Factor $$TUF=frac{P_{DC}}{VA :Rating :of :the :Transformer }=frac{P_{DC}}{frac{VA_{p}+VA_{s}}{2}}$$ VAp and VAs are the primary and secondary power ratings of the transformer. Learning working make money

Power Electronics – MOSFET Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a type of transistor used to switch electronic signals. It has four terminals namely; source (S), Drain (D), Gate (G) and Body (B).The MOSFET’s body is normally connected to the terminal of the source(S), which results in three-terminal device similar to other field effect transistors (FET). Since these two main terminals are usually interconnected via short circuit, only three terminals are visible in electrical diagrams. It is the most common device in circuits that are both digital and analogue. Compared to the regular transistor, a MOSFET needs low current (less than one mill-ampere) to switch ON. At the same time, it delivers a high current load of more than 50 Amperes. Operation of a MOSFET MOSFET has a thin layer of silicon dioxide, which acts as the plate of a capacitor. The isolation of the controlling gate raises the resistance of the MOSFET to extremely high levels (almost infinite). The gate terminal is barred from the primary current pathway; thus, no current leaks into the gate. MOSFETs exist in two main forms − Depletion state − This requires the gate-source voltage (VGB) to switch the component OFF. When the gate is at zero (VGB) the device is usually ON, therefore, it functions as a load resistor for given logic circuits. For loading devices with N-type depletion, 3V is the threshold voltage where the device is switched OFF by switching the gate at negative 3V. Enhancement state − The gate-source voltage (VGB) is required in this state to switch the component ON. When the gate is at zero (VGB) the device is usually OFF and can be switched ON by ensuring the gate voltage is higher than the source voltage. Symbol and Basic Construction Where, D − Drain; G − Gate; S − Source; and Sub − Substrate Learning working make money

Power Semiconductor Devices Solved Ex A (BJT) emits a current of 1mA, and has emitter efficiency of 0.99. The base transport factor is 0.994 and a depletion layer recombination factor is 0.997. For the BJT calculate the following − The transport factor The rewritten transport factor is given by − $$alpha =gamma _{E}times alpha _{T}times delta _{r}$$ Substituting the values, we get $$alpha =0.99times 0.994times 0.997=0.981$$ The current gain Current gain is given by − $$beta =frac{I_{C}}{I_{B}}=frac{alpha }{1-alpha }$$ Substituting the values, we get $$beta =frac{0.981}{1-0.981}=51.6$$ The collector current $$I_{C}=alpha times I_{E}=0.981times 1=0.981mA$$ The base current $$I_{B}=I_{E}-I_{C}=1-0.981=19mu A$$ Learning working make money

Power Electronics – IGBT The insulated gate bipolar transistor (IGBT) is a semiconductor device with three terminals and is used mainly as an electronic switch. It is characterized by fast switching and high efficiency, which makes it a necessary component in modern appliances such as lamp ballasts, electric cars and variable frequency drives (VFDs). Its ability to turn on and off, rapidly, makes it applicable in amplifiers to process complex wave-patterns with pulse width modulation. IGBT combines the characteristics of MOSFETs and BJTs to attain high current and low saturation voltage capacity respectively. It integrates an isolated gate using FET (Field effect transistor) to obtain a control input. IGBT Symbol The amplification of an IGBT is computed by the ratio of its output signal to its input signal. In conventional BJTs, the degree of gain (β) is equal to the ratio of its output current to the input current. IGBT has a very low value of ON state resistance (RON) than a MOSFET. This implies that the voltage drop (I2R) across the bipolar for a particular switching operation is very low. The forward blocking action of the IGBT is similar to that of a MOSFET. When an IGBT is used as controlled switch in a static state, its current and voltage ratings equal to that of BJT. On the contrary, the isolated gate in IGBT makes it easier to drive BJT charges and hence less power is required. IGBT is switched ON or OFF based on whether its gate terminal has been activated or deactivated. A constant positive potential difference across the gate and the emitter maintains the IGBT in the ON state. When the input signal is removed, the IGBT is turned OFF. IGBT Principle of Operation IGBT requires only a small voltage to maintain conduction in the device unlike in BJT. The IGBT is a unidirectional device, that is, it can only switch ON in the forward direction. This means current flows from the collector to the emitter unlike in MOSFETs, which are bi-directional. Applications of IGBT The IGBT is used in medium to ultra-high power applications, for example traction motor. In large IGBT, it is possible to handle high current in the range of hundred amperes and blocking voltages of up to 6kv. IGBTs are also used in power electronic devices such as converters, inverters and other appliances where the need for solid state switching is necessary. Bipolars are available with high current and voltage. However, their switching speeds are low. On the contrary, MOSFETs have high switching speeds although they are expensive. Learning working make money

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. Controllers Controllers are devices that receive electronic signals transferred from a measured variable in a process and compare the value obtained with a set point of control. It utilizes digital algorithms to correlate and compare functions. Sensors Sensors are used to determine current, which constantly varies to provide feedback for purposes of control. Sensing current makes it possible to achieve a smooth and accurate converter function. Current sensors are critical in converters such that the information in parallel or multiphase converters is easily shared. Filters Electronic filters are also used to carry out processing of signals to remove undesired frequencies. They are analog circuits and exist in either active or passive state. Learning working make money

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

Power Electronics – TRIAC The acronym TRIAC stands for Triode for Alternating Current. A TRIAC is a semiconductor device with three terminals that control the flow of current, thus the name Triac. Unlike SCR, TRIAC is bi-directional while SCR is bi-directional. It is ideal for operation utilizing AC power for switching purposes since it can control current flow for both halves in an alternating current cycle. This explained clearly in the diagram below. TRIAC Symbol The circuit diagram for a TRIAC is shown below. It resembles two thyristors placed back to back. TRIAC Structure The TRIAC Structure is regarded as a DIAC having an extra gate contact incorporated to ensure device control. Similar to other power devices, the TRIAC is manufactured from silicon. Consequently, the process of fabricating the silicon leads to the production of cheaper devices. As indicated below, the TRIAC has a six areas namely; four N-type regions and two P-type regions. TRIAC Operation The operation of the TRIAC is based on the thyristor. It facilitates the switching function in AC electrical components and systems. They are widely used in light dimmers because they allow both halves of the AC cycle to be utilized. As a result, this makes them more efficient in power usage. As much as it is possible to use thyristors to function as TRIAC, it is not cost efficient for operations that require low power. It is possible to view a TRIAC in terms of two thyristors. TRIACs are normally used in applications that do not require very high power because they exhibit non-symmetrical switching in their operation. This is disadvantageous for applications utilizing high power as it causes electromagnetic interference. As a result, TRIACs are used in motor controls, light residential light dimmers and small electric fans to control speed. Learning working make money