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MODULE -2 Single-Phase Controlled Rectifiers 1 INTRODUCTION TO CONTROLLED RECTIFIERS Controlled rectifiers are line commutated ac to dc power converters which are used to convert a fixed voltage, fixed frequency ac power supply into variable dc output voltage. Type of input: Fixed voltage, fixed frequency ac power supply. Type of output: Variable dc output voltage 2 APPLICATIONS OF PHASE CONTROLLED RECTIFIERS ☞DC motor control in steel mills, paper and textile mills employing dc motor drives. ☞AC fed traction system using dc traction motor. ☞Electro-chemical and electro-metallurgical processes. ☞Magnet power supplies. ☞Reactor controls. ☞Portable hand tool drives. ☞Variable speed industrial drives. ☞Battery charges. ☞High voltage DC transmission. ☞Uninterruptible power supply systems (UPS). 3 DIFFERENT TYPES OF SINGLE PHASE CONTROLLED RECTIFIERS Single Phase Controlled rectifiers are further subdivided into different types ☞Half wave controlled rectifier which uses a single thyristor device (which provides output control only in one half cycle of input ac supply, and it provides low dc output). ☞Full wave Controlled rectifiers (which provide higher dc output) ☞Full wave controlled rectifier using a center tapped transformer (which requires two thyristors). ☞Full wave bridge Controlled rectifiers (which do not require a center tapped transformer) ☞Single phase semi-converter (half controlled bridge converter, using two SCR’s and two diodes, to provide single quadrant operation). ☞ Single Phase Full Converter (fully controlled bridge converter which requires four SCR’s, to provide two quadrant operations). 4 THREE PHASECONTROLLED RECTIFIERS ARE OF DIFFERENT TYPES ☞Three phase half wave controlled rectifiers ☞Three phase full wave controlled rectifiers ☞Semi converter (half controlled bridge converter). ☞Full converter (fully controlled bridge converter). 5 INTRODUCTION Diodes of rectifier circuits are replaced by thyristors. In thyristor based rectifiers, output voltage can be controlled. So they are termed as controlled rectifiers. Controlled rectifiers produce variable DC output, whose magnitude is varied by Phase control. Phase Control DC output from rectifier is controlled by controlling duration of the conduction period by varying the point at which gate signal is applied to SCR. Main drawback of phase control is Radio Frequency Interference (RFI) 6 Controlled rectifiers are of two types, 1- Fully Controlled rectifiers DC current is unidirectional, but DC voltage has either polarity. With one polarity, flow of power is from AC source to DC load---Rectification. With the reversal of DC voltage by the load, flow of power is from DC load to AC source---Inversion. 2- Half controlled rectifiers Half of SCRs are replaced by diodes. DC output current and voltage are unidirectional. i.e., flow of power is from AC source to DC load. 7 HALF-WAVE CONTROLLED RECTIFIERS 8 With Resistive Load 9 CALCULATION OF VOUT(AVG) VO dc Vdc 1 2 2 v O .d t ; 0 vO Vm sin t for t to VO dc Vdc VO dc 1 2 1 Vm sin t .d t 2 V m sin t .d t 10 VO dc VO dc VO dc VO dc Vm sin t.d t 2 Vm 2 cos t Vm cos cos ; cos 1 2 Vm 1 cos ; Vm 2VS 2 11 Power Electronics 12 12 Power Electronics CONTROL CHARACTERISTIC VO(dc) Vdm 13 0.6Vdm 0.2 Vdm 0 60 120 180 Trigger angle in degrees 13 voltage with respect to Vdm , the Vdc Vn Vdm 14 Normalized output voltage Power Electronics Normalizing the dc output Vm 1 cos 2 Vm Vdc 1 Vn 1 cos Vdcn Vdm 2 14 With an Inductive (RL) Load 15 With Inductive Load and Freewheeling Diode 16 FULL-WAVE CONTROLLED CENTERTAP RECTIFIERS 17 With Resistive Load 18 With an Inductive (RL) Load 19 Control Characteristics for center-tap rectifier 20 With Freewheeling Diode 21 EXAMPLE 6.4 Explain with the help of waveforms the operation of a full-wave center-tap rectifier with RL load for the following firing angles: (a) 0° (b) 45° (c) 90° (d) 135° (e) 180° Assume highly Inductive Load 22 Voltage and current waveforms for α=0° During positive-half cycle of source voltage, SCR1 is forward biased and SCR2 is reverse biased. During negative halfcycle, SCR2 is forward biased and SCR1 is reverse biased. In either case voltage across the load is Vs. Output is similar to uncontrolled rectifier. Each SCR conducts for 180° and supplies current to the load for this period 23 Voltage and current waveforms for α=45° Average DC output voltage decreases. If SCR1 is triggered at 45°, SCR2 will conduct upto that point, even though the source voltage is zero, due to highly inductive nature of load. When SCR1 is turned on, SCR2 is turned off. Current to the load is supplied by SCR1 and SCR2, each conducting for 180° 24 Voltage and current waveforms for α=90° Average DC voltage is zero, so there is no transfer of power from AC source to DC load. Each SCR remains in conduction for 180° As firing angle is increased from 0 to 90°, the power supplied to the DC load decreases, becoming zero at α=90° 25 Voltage and current waveforms for α=135° Average DC voltage is negative. Load current still flows in each SCR for 180° in its original direction. Load voltage has changed polarity. Power now flows from DC load to AC source . Circuit acts like an inverter. 26 Voltage and current waveforms for α=180° Average output DC voltage is at its maximum negative value. SCRs remain in conduction for 180° 27 EXAMPLE 6.5 Show direction of power flow and operating mode (rectifying or inversion) of center-tap rectifier circuit with following firing angles: A) α > 0° B) α < 90° C) α > 90° D) α < 180° 28 SOLUTION For firing angle in the range 0° < α < 90° 1. Average output voltage is positive. 2. Converter operates in the rectifying mode. 3. Power to the load is positive 4. Power flow is from AC source to the DC load. For firing angle in the range 90° < α < 180° 1. Average output voltage is negative 2. Converter operates in inversion mode 3. Power to the load is negative 4. Power flow is from DC load to AC source 29 ASSIGNMENT # 2 In example 6.4, draw waveforms for voltage across thyristor 2. i.e., VSCR2 30 FULL-WAVE CONTROLLED BRIDGE RECTIFIER 31 With Resistive Load 32 With an Inductive (RL) Load 33 For L >>> R 34 1 VO dc Vdc vO .d t ; 2 0 The o/p voltage waveform consists of two o/p pulses during the input supply time period of 0 to 2 radians. Hence the Average or dc o/p voltage can be calculated as 35 2 Power Electronics The average dc output voltage can be determined by using the expression 35 VO dc VO dc 36 VO dc 2 Vdc Vm sin t.d t 2 2Vm Vdc cos t 2 2Vm Vdc cos Power Electronics 36 Power Electronics 37 By plotting VO(dc) versus , we obtain the control characteristic of a single phase full wave fully controlled bridge converter (single phase full converter) for constant & continuous load current operation. 37 Power Electronics To plot the control characteristic of a Single Phase Full Converter for constant & continuous load current operation. We use the equation for the average/ dc output voltage VO dc Vdc 2Vm cos 38 38 Power Electronics 39 39 Vdm Power Electronics Vdc Vdm cos VO(dc) 0.6Vdm 0.2 Vdm 0 -0.2Vdm 30 60 90 120 150 180 -0.6 Vdm -Vdm Trigger angle in degrees 40 40 Power Electronics 41 During the period from t = to the input voltage vS and the input current iS are both positive and the power flows from the supply to the load. The converter is said to be operated in the rectification mode Controlled Rectifier Operation for 0 < < 900 41 Power Electronics 42 During the period from t = to (+), the input voltage vS is negative and the input current iS is positive and the output power becomes negative and there will be reverse power flow from the load circuit to the supply. The converter is said to be operated in the inversion mode. Line Commutated Inverter Operation for 900 < < 1800 42 Controlled Rectifier Operation 43 0< < 900 Power Electronics TWO QUADRANT OPERATION OF A SINGLE PHASE FULL CONVERTER 900< <1800 Line Commutated Inverter Operation 43 Control characteristics for bridge rectifier 44 With RL load and freewheeling diode 45 HALF-CONTROLLED OR SEMICONTROLLED BRIDGE RECTIFIERS 46 In fully-controlled rectifier, only rectification can be obtained by connecting a freewheeling diode across the output terminals of the rectifier. Another method of obtaining rectification in bridge rectifiers is replacing half of the SCRs with diodes. These circuits are called semicontrolled bridge rectifiers. 47 Full-wave semicontrolled bridge rectifier circuit 48 Semicontrolled bridge rectifier with FWD 49 DUAL CONVERTER 50 Exercise Problems 6.1, 6.5, 6.6, 6.9, 6.12, 6.21, 6.22, 6.24 Also give analysis of waveforms in each case of all above exercise problems