AC/DC Converters – Rectifiers. Rectification. AC/DC converters serve as rectifiers. They convert ac to dc in a number of industrial, domestic, agricultural, and other applications. Rectifiers are used as stand- alone units feeding single and multiple dc loads and as input stages of ac systems because of their virtually unlimited output power and fine controllability. Their speed of response is usually adequate to handle electromechanical transients occurring in motor drives and power suppliers. AC/DC line- commutated converters or, as they also called, converters with natural commutation or passive rectifiers, are the most usual choice for applications, where a single- phase and three- phase supply is available. How Do Ac To Dc Power Converters WorkforceAC/DC are an Australian rock band, formed in 1973 by brothers Malcolm and Angus Young. A hard rock/blues rock band, they have also been considered a heavy metal band. FAQ's About Power Converters/Chargers NEW! Interactive Video. Are you purchasing a Progressive Dynamics product on the internet? Why do Recreational Vehicles need. Marine ac and dc electrical systems seminar by dennis krug and john chadwick. Current Logic introduce modulated technique for high voltage input high power dc-dc converters. ![]() This is due to simplicity of the circuits requiring a minimum number of active and passive components. Thyristors are the main line- commutated power switches. The term “line- commutated” describes the type of commutation, i. To turn on a thyristor, an injection of a current pulse into its gate is required. In low- power applications, vehicle, medicine, and household devices, where there is no ac supply or where reactive current and harmonics caused by a line commutation would be unreachable, it is accepted to employ forced commutatedconverters having a more complex circuitry and sometimes involving higher losses. A special situation exists also with dc and ac loads, where the response of a line- commutated converter may be insufficient to cope with the stringent dynamic and energy efficiency demands and where an additional converter supplied by a dc link and operated with a high switching frequency is necessary. For these purposes active rectifiers are developed. The basic rectifier topologies are given in the circuit diagrams of Fig. The systems built on diodes are called uncontrolled rectifiers, and those built on thyristors and transistors are known as controlled rectifiers because their dc output can be changed. The rectification processes are quite varied; accordingly, there are different types of rectifying circuits- midpoint (M) and bridge (B) rectifiers- single- phase (M1, M2, B2) and three- phase (M3, B6) rectifiers- half- wave (1 pulse per supply period) and full- wave (2, 3, 6 pulses) rectifiers. Rectifier data. Rectifiers differ by the shape of dc signal, ripples, and efficiency that depends on rms, average, and amplitude values of voltage, current, and power. Their power range is very wide, from milliwatts to megawatts. Low- power devices operate usually from a single- phase supply while high- power rectifiers are mainly used in a three- phase configuration. In the table below, the main data of different non- controlled rectifier circuits with a resistive load are given. The apparent rms power Ps, phase voltage Us, and current Is supply the rectifier directly, through chokes, or via a transformer. In the last case Us = U2, Ps = P2. ![]() The average rectified load power Pd, voltage Ud, and current Id are the pulsating dc signals with 1, 2, 3, or 6 pulses per supply period T. Parameters k. U, k. ![]() I, k. P, k. R, and k. F are called voltage, smoothed current, power, reverse, and forward circuit factors. The ripple factor kr of the output waveform is usually determined by the rectified peak- to- peak ripple voltage swing Ur (see Fig. The peak inverse voltage UR and forward current IF of each rectifier device depends on the circuit type. The power factor of a rectifier defines the ratio of the real average active power Pd to the apparent power Ps, which is the product of the rms voltage and the rms current, therefore . For the given voltage, the rms current drawn is. Is = Pd/Uscos. The current flows through the utility distribution and transmission lines, transformers and so on exciting the losses in their resistances. If the losses grow, possibly to the point of overloading, harmonic currents may cause resonance conditions in transmission lines. The supply voltage waveform will also be distorted, adversely affecting other linear loads, if a significant portion of the load supplied by the consumer absorbs power by mean of distorted currents. This is the reason why consumers prefer unity power factor that draws power at the minimum value of the rms current. Commonly used simple half- wave single- phase rectifier circuits (M1) are represented in Fig. The resulting output voltage and current waveforms of the half- wave diode rectifier are shown in Fig. ![]() During the positive alternation of the ac sinusoidal wave, the anode of the diode VD is positive and the cathode is negative, the diode conducts since it is forward- biased. The positive alternation of the ac will then appear across the load motor M. During the negative alternation of ac cycle, the anode becomes negative while the cathode is positive. The diode is reverse- biased by this voltage and practically no current will flow through the load. Therefore, no voltage appears across the load. Such type of the converter is called a half- wave rectifier because the negative half cycles have been clipped off. Since the load voltage has mainly a positive half cycle, the load current is direct and discontinuous, meaning that it flows in only one direction and has breaks. Thus, the rectifier is capable of operating in the first quadrant of the current- voltage plane, as Fig. In the case of resistive load, Ud is always positive and ac line current (the same as the rectified current Id) has the half- sine shape and flows in phase with voltage. The loads like motors have the properties of the resistive- inductive load; hence, Ud goes negative shortly, and ac line current (the same as Id) is out of phase with the voltage. When inductance aspires to infinity, the current waveform distorts and becomes almost rectangle. The two- thyristor single- phase full- wave rectifier (midpoint rectifier or M2) is shown in Fig. It produces a rectified rippled output voltage and current for each alternation of the ac input, as shows Fig. Inductive load causes current overlap designated by the overlap angle y. The rectifier utilizes a center- tapped transformer that transfers alternating source voltage to the diode rectifier circuit. The anodes of each device VS1 and VS2 are connected to the opposite ends of the transformer secondary winding. The cathodes are then joined together to form a common positive output. The load motor M is connected between the common cathode point and the center- tap connector of the transformer. The transformer, two diodes or thyristors, and the load form a complete path for the current. By adjusting the thyristor firing angle . Thus, the rectifier is capable of operating in two quadrants of the current- voltage plane: the first quadrant is the rectifying mode of operation and the fourth quadrant supports the inverting mode of the operation. Therefore, it is a two- quadrant converter (Fig. As long as the converter cannot produce the negative output current, the motor control signals and braking modes should be limited. In the inverting mode, it is more convenient to express the firing angle in terms of the angle of advance from the end limit of the interval available for successful commutation than as a delay a from the beginning of the interval. This angle of advance shown in Fig. R. 1, d, shows. Using four devices instead of two, this design eliminates the need for the center tap. During the performance of a bridge rectifier, two diodes are forward- biased in each alternation of the ac input voltage. When the positive alternation occurs, devices VS2 and VS3 are forward- biased, while VS1 and VS4 are reverse- biased. The conduction path follows from the ac source, through VS2, the load, then through VS3, and back to the source. This causes the same alternation to appear across the load. During the negative alternation, the current flows from the source through VS1, via the load, then through VS4, and back to the supply line. A range of board-mount AC front-end modules is available for the design of a full AC-DC power system. Global, autoranging input ranges are available from 85 to. This causes the second alternation to appear across the load in the same direction as the first alternation. This means that voltage developed across the load is the same for each alternation. As a result, both alternations of the input appear as the output across the load and pulsating current flows via the dc output. The timing diagrams and the current- voltage plane of the circuit are the same as for the full- wave rectifier. Replace VS2 and VS4 by the pair of capacitors, named C1 and C2. In this half- bridge topology, a voltage doubling is obtained. VD1 charges C1 during the positive alternation of the supply voltage and. VD2 charges C2 during the negative alternation. On the output of the rectifier, the sum of both voltages is presented. It is preferable to use a three- phase supply source, except at a fractional kilowatt, if such supply is available. L 1, L2, and L3 of the three- phase source are connected to the anodes of thyristors VS1, VS2, and VS3 via the transformer. The load motor M is connected between the cathodes of the thyristors and the neutral of the transformer. When the phase L 1 is at its positive value, maximum conduction occurs through VS1 since it is forward- biased. No conduction occurs through VS1 during the negative alternation of the phase L 1. Other thyristors operate in a similar manner, conducting during the positive ac input alternation and not conducting during the associated negative ac alternation. The rectifying diagram of a thyristor three- phase three- diode circuit is shown in Fig. In the case of resistive- inductive load, the current continues through the diode or thyristor after the voltage has changed its sign. For that reason, the thyristor does not close at the zero- voltage instant, but remains open. By adjusting the thyristors firing angle, the negative voltage may be obtained, thus it is a two- quadrant converter again. A commonly used six- pulse counterpart of this rectifier circuit is represented in Fig. This three- phase bridge rectifier (B6) uses six diodes or thyristors.
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