Power Electronics Single Phase Controlled Rectifiers Dr. Firas Obeidat 1 Table of contents • Single Phase Controlled Half Wave Rectifier- Resistive Load 1 • Single Phase Controlled Half Wave Rectifier- RL Load 2 •Single Phase Full Wave Rectifier- Resistive Load 3 • Single Phase Full Wave Rectifier- RL Load, Discontinuous 4 Current • Single Phase Full Wave Rectifier- RL Load, Continuous Current 5 • Single Phase Full Wave Rectifier- RL Load, L>>R 6 • Single-Phase Bridge Half-Controlled Rectifier 6 2 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Half Wave Rectifier Resistive Load A way to control the output of a half- wave rectifier is to use an SCR1 instead of a diode. Two conditions must be met before the SCR can conduct: 1. The SCR must be forward-biased (vSCR > 0). 2. A current must be applied to the gate of the SCR. The SCR will not begin to conduct as soon as the source becomes positive. Conduction is delayed until a gate current is applied, which is the basis for using the SCR as a means of control. Once the SCR is conducting, the gate current can be removed and the SCR remains on until the current goes to zero. 3 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Half Wave Rectifier Resistive Load If a gate signal is applied to the SCR at 휔t=α, where α is the delay (firing or triggering) angle. The average (dc) voltage across the load resistor and the average (dc) current are Vdc= 푉푑푐 푉푚 퐼 = = (1 + 푐표푠α) 푑푐 푅 2π푅 The rms voltage across the resistor and the rms current are computed from 푉푚 1 푠푖푛2α 푉푟푚푠 푉푚 1 푠푖푛2α 푉 = (π − α + ) 퐼푟푚푠 = = (π − α + ) 푟푚푠 2 π 2 푅 2푅 π 2 2 푉푟푚푠 The power absorbed by the resistor is 푃 = 푎푐 푅 4 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Half Wave Rectifier Resistive Load Example: The single-phase half wave rectifier has a purely resistive load of R and the delay angle is α=π/2, determine: 푉푑푐, 퐼푑푐, 푉푟푚푠, 퐼푟푚푠. 푉 π 푉 = 푚 1 + 푐표푠 = 0.1592푉 푑푐 2π 2 푚 푉 π 푉 퐼 = 푚 1 + 푐표푠 = 0.1592 푚 푑푐 2π푅 2 푅 π 푉푚 1 π sin⁡(2 ) 푉 = (π − + 2 ) = 0.3536푉 푟푚푠 2 π 2 2 푚 π 푉푚 1 π sin⁡(2 ) 푉푚 퐼 = (π − + 2 ) = 0.3536 푟푚푠 2푅 π 2 2 푅 5 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Half Wave Rectifier Resistive Load Example: Design a circuit to produce an average voltage of 40V across a 100Ω load resistor from a 120Vrms 60-Hz ac source. Determine the power absorbed by the resistance and the power factor. Vdc= so Vrms 푉푟푚푠 75.6 푆 = 푉 퐼 = 120 × 0.756 = 90.72⁡푉퐴 퐼 = = = 0.756퐴 푠,푟푚푠 푟푚푠 푟푚푠 푅 100 푃푅 57.1 푝푓 = = = 0.629 푆 90.72 6 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Half Wave Rectifier RL Load The current is the sum of the forced and natural responses. The constant A is determined from the initial condition 휔t=α i(α)=0: Substituting for A and simplifying, The extinction angle 훽 is defined as the angle at which the current returns to zero, as in the case of the uncontrolled rectifier. When 휔t=훽 7 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Half Wave Rectifier RL Load The above equation must be solved numerically for 훽. The angle (훽-α) is called the conduction angle 훶. The average (dc) output voltage is Vdc= The average (dc) output voltage is 푉푚 퐼 = 퐼 = (푐표푠α − 푐표푠훽) 퐼푑푐 = or 푑푐 표 2휋푅 The rms current is computed from Or it can be written as 훽 2 1 푉푚 1 1 푉 = (푉 푠푖푛휔푡)2⁡푑휔푡 = (훽 − 훼 − 푠푖푛2훽 + 푠푖푛2훼) 푟푚푠 2휋 푚 4휋 2 2 훼 2 푉푟푚푠 푉푟푚푠 1 푉푚 1 1 퐼푟푚푠 = = = (훽 − 훼 − 푠푖푛2훽 + 푠푖푛2훼) 푍 푅2 + (휔퐿)2 푅2 + (휔퐿)2 4휋 2 2 8 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Half Wave Rectifier RL Load Example: For the circuit of controlled half-wave rectifier with RL Load, the o source is 120Vrms at 60 Hz, R=20Ω, L=0.04H, and the delay angle is 45 . Determine (a) an expression for i(휔t), (b) the rms current, (c) the power absorbed by the load, and (d) the power factor. (a) (b) (c) (c) 9 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Full Wave Rectifier The first figure shows a fully controlled bridge rectifier, which uses four thyristors to control the average load voltage. Thyristors T1 and T2 must be fired simultaneously during the positive half wave of the source voltage vs to allow conduction of current. To ensure simultaneous firing, thyristors T1 and T2 use the same firing signal. Alternatively, thyristors T3 and T4 must be fired simultaneously during the negative half wave of the source voltage. For the center-tapped transformer rectifier, T1 is forward-biased when vs is positive, and T2 is forward-biased when vs is negative, but each will not conduct until it receives a gate signal. The delay angle is the angle interval between the forward biasing of the SCR and the gate signal application. If the delay angle is zero, the rectifiers behave exactly as uncontrolled rectifiers with diodes. 10 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Full Wave Rectifier Resistive Load The average component of the output voltage and current waveforms are determined from Vdc= 푉푑푐 푉푚 퐼 = = (1 + 푐표푠훼) 푑푐 푅 휋푅 The rms component of the output voltage and current waveforms are determined from 휋 1 1 훼 sin⁡(2훼) 푉 = 푉 푠푖푛휔푡 2푑휔푡 = 푉 − + 푟푚푠 휋 푚 푚 2 2휋 4휋 훼 The rms current in 푉푟푚푠 푉푚 1 훼 sin⁡(2훼) 퐼푟푚푠 = = − + the source is 푅 푅 2 2휋 4휋 the same as The power delivered to the load is the rms current in 2 푝 = 퐼푟푚푠 푅 the load. 11 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Full Wave Rectifier Resistive Load Example: The full-wave controlled bridge rectifier has an ac input of 120Vrms at 60 Hz and a 20Ω load resistor. The delay angle is 40o. Determine the average current in the load, the power absorbed by the load, and the source voltamperes. Vdc= 푉푑푐 95.6 퐼 = = = 4.77퐴 푑푐 푅 20 The rms current in the source is also 5.80 A, and the apparent power of the source is 12 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Full Wave Rectifier RL Load, Discontinuous Current Load current for a controlled full-wave rectifier with an RL load (fig. a) can be either continuous or discontinuous. For discontinuous current 1- at 휔t=0 with zero load current, SCRs T1 and T2 in the bridge rectifier will be forward-biased and T3 and T4 will be reverse-biased as the source voltage becomes positive. 2- Gate signals are applied to T1 and T2 at 휔t=α, turning T1 and T2 on. With T1 and T2 on, the load voltage is equal to the source voltage. The output current can be given as 13 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Full Wave Rectifier RL Load, Discontinuous Current The above current function becomes zero at 휔t=훽. If , the current remains at zero until 휔t=π+α when gate signals are applied to T3 and T4 which are then forward-biased and begin to conduct. This mode of operation is called discontinuous current as shown in fig. b. 훽<π+α → Discontinuous current Analysis of the controlled full-wave rectifier operating in the discontinuous current mode is identical to that of the controlled half-wave rectifier except that the period for the output current is π rather than 2π rad. 14 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Full Wave Rectifier RL Load, Discontinuous Current The average (dc) output voltage is 훽 1 푉푚 푉 = 푉 푠푖푛휔푡⁡푑푡휔푡 = (푐표푠훼 − 푐표푠훽) 푑푐 휋 푚 휋 훼 The average (dc) output current is 푉푑푐 푉푚 퐼 = = (푐표푠α − 푐표푠훽) 퐼푑푐 = or 푑푐 푅 휋푅 The rms voltage is computed from 훽 2 1 푉푚 1 1 푉 = (푉 푠푖푛휔푡)2⁡푑휔푡 = (훽 − α − 풔풊풏ퟐ훽 + 풔풊풏ퟐα) 푟푚푠 휋 푚 2휋 2 2 α The rms current is computed from Or it can be written as 2 푉푟푚푠 푉푟푚푠 1 푉푚 1 1 퐼푟푚푠 = = = (훽 − α − 풔풊풏ퟐ훽 + 풔풊풏ퟐα) 푍 푅2 + (휔퐿)2 푅2 + (휔퐿)2 2휋 2 2 15 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Full Wave Rectifier RL Load, Discontinuous Current Example: A controlled full-wave bridge rectifier has a source of 120Vrms at 60Hz, R=10Ω, L=20mH, and α=60o. Determine (a) an expression for load current, (b) the average load current, and (c) the power absorbed by the load. (a) (b) 푉푚 169.7 퐼 = = 푐표푠α − 푐표푠훽 = 푐표푠60 − 푐표푠216 = 7.07퐴 푑푐 휋푅 휋10 (c) 2 1 푉푚 1 1 퐼푟푚푠 = (훽 − α − 풔풊풏ퟐ훽 + 풔풊풏ퟐα) 푅2 + (휔퐿)2 2휋 2 2 1 169.72 1 1 퐼 = (3.78 − 1.047 − 풔풊풏(ퟐ × 216) + 풔풊풏(ퟐ × 60)) = 8.8⁡A 푟푚푠 12.5 2휋 2 2 (d) 2 2 푝 = 퐼푟푚푠 푅 = 8.8 × 10 = 774.4⁡W 16 Dr. Firas Obeidat Faculty of Engineering Philadelphia University Single Phase Controlled Full Wave Rectifier RL Load, Continuous Current If the load current is still positive at 휔t=π+α when gate signals are applied to T3 and T4 in the above analysis, T3 and T4 are turned ON and T1 and T2 are forced OFF.