Category: power Electronics

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

Power Electronics – Useful Resources The following resources contain additional information on Power Electronics. Please use them to get more in-depth knowledge on this. Useful Video Courses 252 Lectures 35.5 hours 10 Lectures 41 mins 315 Lectures 8.5 hours 14 Lectures 29 mins 132 Lectures 13 hours 128 Lectures 10.5 hours Learning working make money

Power Electronics – Pulse Width Modulation PWM is a technique that is used to reduce the overall harmonic distortion (THD) in a load current. It uses a pulse wave in rectangular/square form that results in a variable average waveform value f(t), after its pulse width has been modulated. The time period for modulation is given by T. Therefore, waveform average value is given by $$bar{y}=frac{1}{T}int_{0}^{T}fleft ( t right )dt$$ Sinusoidal Pulse Width Modulation In a simple source voltage inverter, the switches can be turned ON and OFF as needed. During each cycle, the switch is turned on or off once. This results in a square waveform. However, if the switch is turned on for a number of times, a harmonic profile that is improved waveform is obtained. The sinusoidal PWM waveform is obtained by comparing the desired modulated waveform with a triangular waveform of high frequency. Regardless of whether the voltage of the signal is smaller or larger than that of the carrier waveform, the resulting output voltage of the DC bus is either negative or positive. The sinusoidal amplitude is given as Am and that of the carrier triangle is give as Ac. For sinusoidal PWM, the modulating index m is given by Am/Ac. Modified Sinusoidal Waveform PWM A modified sinusoidal PWM waveform is used for power control and optimization of the power factor. The main concept is to shift current delayed on the grid to the voltage grid by modifying the PWM converter. Consequently, there is an improvement in the efficiency of power as well as optimization in power factor. Multiple PWM The multiple PWM has numerous outputs that are not the same in value but the time period over which they are produced is constant for all outputs. Inverters with PWM are able to operate at high voltage output. The waveform below is a sinusoidal wave produced by a multiple PWM Voltage and Harmonic Control A periodic waveform that has frequency, which is a multiple integral of the fundamental power with frequency of 60Hz is known as a harmonic. Total harmonic distortion (THD) on the other hand refers to the total contribution of all the harmonic current frequencies. Harmonics are characterized by the pulse that represent the number of rectifiers used in a given circuit. It is calculated as follows − $$h=left ( ntimes P right )+1 quad or quad -1$$ Where n − is an integer 1, 2, 3, 4….n P − Number of rectifiers It is summarized in the table below − Harmonic Frequency 1st 60 Hz 2nd 120 Hz 3rd 180Hz 4th 240Hz 5th . . 49th 300Hz . . 2940Hz Harmonics have an impact on the voltage and current output and can be reduced using isolation transformers, line reactors, redesign of power systems and harmonic filters. Series Resonant Inverter A resonant inverter is an electrical inverter whose operation is based on oscillation of resonant current. Here, the switching device and the resonanting component are connected in series to each other. As a result of the natural features of the circuit, the current passing through the switching device drops to zero. This type of inverter yields a sinusoidal waveform at very high frequencies in the range of 20kHz-100kHz. It is therefore, most suitable for applications that demand a fixed output such as induction heating and flourescent lighting. It is usually small in size because its switching frequency is high. A resonant inverter has numerous configurations and thus it is categorized into two groups − Those with unidirectional switches Those with bidirectional switches Learning working make money

Power Electronics – Control Methods In a converter, there are two basic methods of control used to vary the output voltage. These are − Time ratio control Current limit control Time Ratio Control In time ratio control, a constant k given by $frac{T_{ON}}{T}$ is varied. The constant k is called duty ratio. Time ratio control can be achieved in two ways − Constant Frequency In this control method, the frequency (f = 1/T0N) is kept constant while the ON time T is varied. This is referred to as pulse width modulation (PWM). Variable Frequency In variable frequency technique, the frequency (f = 1/T) is varied while the ON time T is kept constant. This is referred to as the frequency modulation control. Current Limit Control In a DC to DC converter, the value of the current varies between the maximum as well as the minimum level for continuous voltage. In this technique, the chopper (switch in a DC to DC converter) is switched ON and then OFF to ensure that current is kept constant between the upper and lower limits. When the current goes beyond the maximum point, the chopper goes OFF. While the switch is at its OFF state, current freewheels via the diode and drops in an exponential manner. The chopper is switched ON when the current reaches the minimum level. This method can be used either when the ON time T is constant or when the frequency (f=1/T). Learning working make money

Phase Controlled Converters Solved Example A separately excited DC motor has the following parameters: 220V, 100A and 1450 rpm. Its armature has a resistance of 0.1 Ω. In addition, it is supplied from a 3 phase fullycontrolled converter connected to a 3-phase AC source with a frequency of 50 Hz and inductive reactance of 0.5 Ω and 50Hz. At α = 0, the motor operation is at rated torque and speed. Assume the motor brakes re-generatively using the reverse direction at its rated speed. Calculate the maximum current under which commutation is not affected. Solution − We know that, $$V_{db}=3sqrt{frac{2}{pi }}times V_{L}-frac{3}{pi }times R_{b}times I_{db}$$ Substituting the values, we get, $220=3sqrt{frac{2}{pi }}times V_{L}-frac{3}{pi }times 0.5times 100$ Therefore, $V_{L}=198V$ Voltage at rated speed = $220-left ( 100times 0.1 right )=210V$ At the rated speed, the regenerative braking in the reverse direction, $=3sqrt{frac{2}{pi }}times 198cos alpha -left ( frac{3}{pi }times 0.5+0.1right )times I_{db}=-210V$ But $cos alpha -cos left ( mu +alpha right )=frac{sqrt{2}}{198}times 0.5I_{db}$ For commutation to fail, the following limiting condition should be satisfied. $mu +alpha approx 180^{circ}$ Therefore, $quad cos alpha =frac{I_{db}}{198sqrt{2}}-1$ Also, $frac{3}{pi }I_{db}-frac{3sqrt{2}}{pi }times 198-left ( frac{3}{pi }times 0.5+0.1 right )I_{db}=-210$ This gives, $quad 0.3771I_{db}=57.4$ Therefore, $quad I_{db}=152.2A$ Learning working make money

Power Electronics – Resonant Switching Resonant switch converters refers to converters that have inductor and capacitor (L-C) networks and whose current and voltage waveforms vary in a sinusoidal manner during each period of switching. There are various resonant switch converters − Resonant DC to DC converters DC to AC inverters Resonant AC inverters to DC converters In this tutorial, we will focus on Resonant DC to DC converters Resonant DC to DC Converters The concept of switch mode power supply (SMPS) is explained below using a DC to DC converter. The load is given a constant voltage supply (VOUT) that is obtained from a primary source of voltage supply VIN. The value of VOUT is regulated by varying resistor in series (RS) or the current source connected in shunt (IS). By controlling VOUT through varying IS and ensuring RS is kept constant, a considerable amount of power is lost in the converter. Switched Mode Power Supply (SMPS) An SMPS (switched mode power supply) refers to an electronic device that uses a switching regulator for the purpose of converting electrical power in an efficient manner. SMPS takes power from the main power lines and transfers it to a load. For example, a computer while ensuring the voltage and current characteristics are converted. The difference between an SMPS and a linear supply of power is that the former keeps switching ON and OFF during low dissipation and uses less time during high dissipation regions. This ensures less energy is wasted. Actually, an SMPS does not dissipate any power. The size of an SMPS is smaller and very light, compared to a normal linear supply power device of the same size and shape. The figure below shows the circuit diagram for an SMPS. When the switching frequency is varied, the stored energy can be varied for each cycle and hence the voltage output is varied. The waveforms below are for a half bridge converter also known as a push-pull. It is used in applications utilizing high power. The input voltage is halved as indicated in the waveform. Learning working make money

Effect of Source Inductance The analysis of most converters is usually simplified under ideal conditions (no source impedance). However, this assumption is not justified since source impedance is normally inductive with a negligible resistive element. Source inductance has a significant impact on the converter performance because its presence alters the output voltage of the converter. As a result, the output voltage reduces as the load current reduces. In addition, the input current and output voltage waveforms change significantly. Source inductance effect on a converter is analyzed in the following two ways. Effect on Single Phase Assuming that the converter operates in conduction mode and the ripple from the load current is negligible, the open circuit voltage becomes equal to average DC output at a firing angle of α.The diagram below shows a fully controlled converter with source in single phase. The thyristors T3 and T4 are assumed to be in conduction mode when t = 0. On the other hand, T1 and T2 fire when ωt = α Where − Vi = input voltage Ii = input current Vo = output voltage Io = output voltage When there is no source inductance, commutation will occur at T3 and T4. Immediately thyristors T1 and T2 are switched ON. This will lead the input polarity to change instantaneously. In the presence of source inductance, change of polarity and commutation does not occur instantaneously. Thus, T3 and T4 do not commutate as soon as T1 and T2 are switched ON. At some interval, all the four thyristors will be conducting. This conducting interval is called the overlap interval (μ). The overlap during commutation reduces the DC output voltage and the angle of extinction γ resulting in failed commutation when αis close to 180°. This is shown by the waveform below. Effect on Three Phase Just like the single-phase converter, there are no instantaneous commutations due to the presence of the source inductances. Taking the source inductances into consideration, the effects (qualitative) on the converter performance is the same as in a single phase converter. This is shown in the diagram below. Learning working make money

Silicon Controlled Rectifier A silicon controlled rectifier or semiconductor-controlled rectifier is a four-layer solidstate current-controlling device. The name “silicon controlled rectifier” is General Electric”s trade name for a type of thyristor. SCRs are mainly used in electronic devices that require control of high voltage and power. This makes them applicable in medium and high AC power operations such as motor control function. An SCR conducts when a gate pulse is applied to it, just like a diode. It has four layers of semiconductors that form two structures namely; NPNP or PNPN. In addition, it has three junctions labeled as J1, J2 and J3 and three terminals(anode, cathode and a gate). An SCR is diagramatically represented as shown below. The anode connects to the P-type, cathode to the N-type and the gate to the P-type as shown below. In an SCR, the intrinsic semiconductor is silicon to which the required dopants are infused. However, doping a PNPN junction is dependent on the SCR application. Modes of Operation in SCR OFF state (forward blocking mode) − Here the anode is assigned a positive voltage, the gate is assigned a zero voltage (disconnected) and the cathode is assigned a negative voltage. As a result, Junctions J1 and J3 are in forward bias while J2 is in reverse bias. J2 reaches its breakdown avalanche value and starts to conduct. Below this value, the resistance of J1 is significantly high and is thus said to be in the off state. ON state (conducting mode) − An SCR is brought to this state either by increasing the potential difference between the anode and cathode above the avalanche voltage or by applying a positive signal at the gate. Immediately the SCR starts to conduct, gate voltage is no longer needed to maintain the ON state and is, therefore, switched off by − Decreasing the current flow through it to the lowest value called holding current Using a transistor placed across the junction. Reverse blocking − This compensates the drop in forward voltage. This is due to the fact that a low doped region in P1 is needed. It is important to note that the voltage ratings of forward and reverse blocking are equal. Learning working make money