EE 423 POWER ELECTRONICS EXPERIMENT #3 CONTROLLED RECTIFIERS Rectifiers convert single phase or multiphase alternating current to direct current. To obtain controlled output voltages, phase control thyristors are used instead of diodes. The output voltage of thyristor rectifiers is varied by controlling the delay or firing angle of thyristors. A phase-control thyristor is turned on by applying a short pulse to its gate and turned off due to natural or line commutation; and in case of a highly inductive load, it is turned off by firing another thyristor of the rectifier during the negative half-cycle of input voltage. These phase-controlled rectifiers are simple and less expensive; and the efficiency of these rectifiers is, in general, above 95%. Since these rectifiers convert from ac to dc, these controlled rectifiers are also called ac-dc converters and are used extensively in industrial applications, especially in variable-speed drives, ranging from fractional horsepower to megawatt power level. SINGLE PHASE CONTROLLED RECTIFIERS PURPOSE To analyze a single phase controlled rectifier with RL load. EQUIPMENT AC power supply (Function generator; max 10V, 100 mA), Oscilloscope, board and other circuit elements (SCR, resistor, capacitor, diode etc.) PROCEDURE Single Phase Half-Wave Controlled Rectifier 1- Set up the circuit shown in the figure (without FWD – Free wheeling diode) 2- By varying α between 0< α <180, Observe change in load current and voltage, and relation with angle beta 3- For any value of α (α≠0) draw the waveforms of load current and voltage on the same axes. 4- Indicate important points on your graphs. 5- Calculate average value of the load voltage you draw. 6- Repeat the items above with FWD connected. Calculate the value of average DC load voltage which you draw as a function of α and record it in your report. BT169 CH1 L=200mH 1N4001 VS CH2 220kΩ 10Ω 150nF A FWD R=330Ω 1N4001 GND Single phase half wave controlled rectifier circuit VL Vm IL α π 2π 2π+ α β Waveforms for single phase half wave rectifier with RL load (FWD is not connected) Single Phase Full-Wave Controlled Rectifier Set up the following circuit and repeat the same procedure. BT169 D1 VS CH1 D3 L=200mH 1N4001 CH2 D2 D4 220kΩ 10Ω 150nF 1N4001 R=1.5kΩ FWD GND D1,D2,D3,D4: 1N4001