AC-AC Voltage Controllers PDF

Chapter 4 AC-AC voltage Controllers Phase-controlled: single-phase and three-phase ac-ac controllers 2/15/2021 Chapter 4 -AC/AC controllers 1  AC Controllers AC controllers are power electronic c...

Chapter 4 AC-AC voltage Controllers Phase-controlled: single-phase and three-phase ac-ac controllers 2/15/2021 Chapter 4 -AC/AC controllers 1  AC Controllers AC controllers are power electronic converters supplied from a fixed ac source and produce voltage of adjustable output voltage. The fundamental output frequency equal to the input frequency. Single-phase AC controllers are supplied from a single-phase ac voltage source and have a single-phase output. In three-phase controllers, both the input and the output are of the three-phase type. AC voltage controllers are based on power switches that conduct current in both directions. However, as most switching devices are unidirectional, bidirectional conduction is achieved by connecting two antiparallel switches.. 2/15/2021 Chapter 4 -AC/AC controllers 2  Single-phase AC Controllers T1 ii = io io A triac may be used instead of the two antiparallel connected SCRs shown. T2 SOURCE vi vo LOAD Assuming RL load, when one of the SCRs is conducting, the other SCR is reverse biased. Always the input current, ii, equals the output current, io. With either SCR conducting, and the output voltage, vo, 𝑖 𝑖 = 𝑖𝑜 𝑣𝑜 = 𝑣𝑖 equals the input voltage, vi. 2/15/2021 Chapter 4 -AC/AC controllers 3  Output Voltage and Current. When T1 is forward biased and fired at 𝛼f, the conducted current initially increases then drops to zero, as tending to follow the voltage by the phase angle (). When the input voltage is negative, T2 is forward biased and ready for firing. The firing occurs at 𝜔t=𝛼f+𝜋. The waveform of current through T2 is a mirror image of that through T1, so both the output current and the voltage have the half- wave symmetry and no dc component. 2/15/2021 Chapter 4 -AC/AC controllers 4  Waveforms of output voltage and current in a single-phase AC voltage controller (𝜑=30) 𝛼𝑓 = 45° 𝛼𝑓 = 135° The minimum feasible firing angle equals the load angle. When the firing angle increases, the conduction angle, 𝛽, decreases and the current pulses become smaller and shorter. Consequently, the rms value of the current and output voltage decreases. 2/15/2021 Chapter 4 -AC/AC controllers 5  Operation with 𝛼𝑓 < ∅ When fired with single (one) triggering pulse, the AC controller acts as a half-wave rectifier which is no wanted! For this reason, multipulse gate signals are used in all phase-controlled ac voltage controllers. With multipulse triggering, 𝑣𝑜 = 𝑣𝑖 when 𝛼𝑓 < ∅ which means no output control occurs and therefore it is not feasible to vary  in the range 0 < 𝛼𝑓 < ∅ 2/15/2021 Chapter 4 -AC/AC controllers 6  AC Controller Analysis The rms value of the output voltage can be found as 1 𝛼𝑒 2 𝑉𝑜 = ‫׬‬ 𝑉𝑖,𝑝 sin 𝜔𝑡 𝑑𝜔𝑡 𝜋 𝛼𝑓 1 1 𝑉𝑜 = 𝑉𝑖 𝛼 − 𝛼𝑓 − [sin 2𝛼𝑒 − sin 2𝛼𝑓 ] 𝜋 𝑒 2 where 𝛼e is the extinction angle. The extinction angle depends on the firing angle, 𝛼f, and the load angle, (𝜑) and found as the angle at which the current reaches zero. The load current, io(𝜔t), for ( 𝛼f < 𝜔t ) 2 2/15/2021 Chapter 4 -AC/AC controllers 10  A single-phase ac voltage controller is used to control lighting. i- Find the Example 1 reduction in power consumed by the lighting when the firing angle is 60. ii- What firing angle would result in a 50% reduction of that power? Solution: i- The incandescent lighting constitutes a resistive load. Consequently, the ratio of rms values of the output and input voltages, that is, the magnitude control ratio, 𝜋 1 𝜋 1 𝜋 𝑉𝑜, 𝜑=0 = 𝑉𝑖 𝜋 − + sin 2 = 0.897𝑉𝑖 3 𝜋 3 2 3 With a constant resistance, the power is proportional to the squared rms value of voltage, so the resultant power constitutes 0.8972 = 0.805 of the full power. Thus, the consumed power is reduced by 19.5%. ii- The magnitude control ratio for the 50% power reduction is: 𝑀 = 0.5 = 1/ 2 which is achieved as alpha =/2 2/15/2021 Chapter 4 -AC/AC controllers 11  A single-phase ac voltage controller supplies a load consisting of a 2.5-Ω Example 2 resistance and a 6-mH inductance. The controller is fed from a 120-V, 60-Hz line, and multipulse gate signals are used to activate controller’s triacs. i- Calculate the rms output voltage for firing angles of 45◦, 90◦, and 120◦. 2/15/2021 Chapter 4 -AC/AC controllers 12  For the single-phase voltage controller, the source is 120 V rms at Example3 60 Hz, and the load is a series RL combination with R =20 and L =50 mH. The delay angle f is 90°. Determine (a) an expression for load current for the first half-period, (b) the rms load current, (c) the rms SCR current, (d) the average SCR current, (e) the power delivered to the load, and (f) the power factor. 2/15/2021 Chapter 4 -AC/AC controllers 13 2/15/2021 Chapter 4 -AC/AC controllers 14