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PHASE CONTROLLED RECTIFIERS Phase Controlled Rectifier Page 1 3.1 Single phase half wave controlled rectifier with resistivel oad Circuit Diagram & Waveforms : OPERATION : For Positive half cycle : In a positive half cycle of supply voltage, i.e. when the anode of SCR is positive with respect to cathode and SCR is fired, power is delivered to the load. For Negative half cycle : In the négative half cycle the SCR is reverse bised and blocks the line voltage, and no curent can flow th rough the load. Phase Controlled Rectifier Page 2 3.2 Single phase full wave Controller rectifier with Resistiveload Circit Diagram& Waveforms : OPERATION : Single phase half wave rectifires are used in practice, but the voltage and current relations developed for the circuits are quite useful in understanding the operation of more practical full wave or bridge circuit. Two phase half wave or single phase full wave circuits with transformer midpoint connections are generally used for rectifires of low ratings. Phase Controlled Rectifier Page 3 OPERATION : For positive half cycle : In the positive half cycle of the supply ,i.e. when the terminal ‘a’ of the transformer is positive with respect to the terminal ‘b’,if the SCR1 is fired at an angle α,current would flow from terminal ‘a’ to SCR1,the node resistance RL and back to the centre tap of the transformer. This current continues to flow up to the angle Π when the line voltage reverses its polarity and SCR1 is turned off. For negative half cycle : In the negative half cycle, SCR2 is fired at angle Π+α.current flows through SCR2 the load resistance and back to centre tap of the transformer. This current continues till 2Π when SCR2 is turned off. Thus across the load there are two pulses of current in the same direction. when SCR1 is turned on at the angle α,the voltage across it is negligibly small, and about 1.0v would be maintain across it up to the angle Π till the line voltage reverse and it is turned off. Phase Controlled Rectifier Page 4 3.3 Fullwavecontrolledrectifier with RL load (without freewheeling diode) Circuit Diagram : OPERATION : It consist of single phase fullwave or two phase halfwave controlled rectifier with RL load. In the halfwave rectifier that the presence of inductance in the load does not allow the load current to be zero at the point of supply voltage reversal. Phase Controlled Rectifier Page 5 Waveforms : For positive half cycle : At the positive half cycle if SCR1 is fired at an angle α,the load current starts to flow from α and continues beyond П. The load current may be in the form of discontinuous pulses or continuous depending on the firing angle α. For negative half cycle : In the negative half cycle of the supply voltage, the conduction is taken over by SCR2, so it is turn ON. The reverse voltage that turns OFF the conducting SCR1 is line voltage itself, this type of commutation is known as line commutation. Due to line commutation SCR1 turn OFF. The important difference between RL load and purely resistive load is that, in the RL load, some portion of load voltage is negative (from П to β). Phase Controlled Rectifier Page 6 3.4 Effect of freewheeling diode A flyback diode is a diode used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed. When power to inductive loads such as coils and inductors is turned off, there is a sharp voltage spike. The direction of this voltage is opposite to the applied voltage in accordance with Lenz’s Law. When a current flows through the coil of a relay, the coil gets electromagnetically charged. The energy is stored in the magnetic field around the coil. When the power supply to the coil is interrupted and the current in the coil tends to decrease, the magnetic field discharges causing a surge in voltage. The voltage, thus induced, can jump across the contacts of relays connected to the coils. The sparks and arcing produced can affect the life of the contacts. The voltage spikes can also damage electronic components like transistors which may be driving the relay coils . Freewheel diodes are connected in reverse bias vis-à-vis the supply voltage. Hence, when the voltage spike appears in the opposite direction, they are short-circuited through the diode. The voltage spike is thus short-circuited across the coil. This protects the connected circuits. Phase Controlled Rectifier Page 7 The inductor has property to store energy .In ac current, during positive high cycle the inductor stores energy with opposite polarity. At negative half cycle the inductor deenergies ,so there must be reverse current which will make SCR on .This may cause reverse damage to circuit .To avoid energy dissipation freewheeling diode is used. To cut off the negative portion of the instantaneous output voltage & smooth the output current ripple free, a freewheeling diode is used. Phase Controlled Rectifier Page 8 3.5 SINGLE PHASE FULL CNTROLLED RECTIFIRE WITH RL LODE & FREEWILING DIODE Circuit Diagram & Waveforms: Phase Controlled Rectifier Page 9 OPERATION: CASE 1 – During positive half cycle of secondary winding is negative with respect to B .In this case SCR1 is in forward blocking stage and SCR 2 becomes off and current flows through SCR1 A-SCR1-RL –Mand some charge that is electromagnetic energy is red due to RL load. So we use freewiling diode across the RL load .so that the diode uses the electromagnetic energy &freewheeling diode is in forward biased condition. CASE2 During negative half cycle of input AC signal terminal A of secondary winding is positive with respect to B. In this case SCR1 is reverse blocking state i.e off &SCR2 is in the forward blocking stage and current flows through SCR2BSCR2-RL-M. Phase Controlled Rectifier Page 10 3.6 SINGLE PHASE HALF CONTROLLED BRIDGE RECTIFIER WITH RESISTIVE LOAD:- For symmetrical operation:CIRCUIT DETAIL:The fig. shows circuit diagram of single phase half controlled bridge rectifier with resistive load. In this circuit, a single gate pulse can be used for triggering either of SCR. The SCR which is in forward blocking state is only triggered at instant of firing. Circuit Diagram: Phase Controlled Rectifier Page 11 CASE 1: During positive half cycle L is positive with respect to N.. Then SCR T1 is in forward blocking state and diode D2 is ON. When gate pulse is applied at SCR T1 then current flow through path L-T1-RL-D2-N. CASE 2: During negative half cycle, N is positive with respect to L. Then SCR T2 is in forward blocking state and diode D1 is conducting. When gate pulse is applied to SCR T2 then current starts to flow through path N-T2-RL-D1-L. Waveforms: Phase Controlled Rectifier Page 12 3.7 3 phase half controlled bridge converter (basic construction): The 3 phase half controlled bridge converter is as shown in the fig. (1)in this there are all 6 devices,3 SCR’s and the remaining 3 diodes. The load is connected between the common cathode point of the thyristors and the common anode of the diodes. The bridge circuit in fig. shows that the neutral point N of the input 3 phase ac supply is not connected anywhere in the ckt. Circuit Diagram: The half wave converter can be redrawn as shown in fig(2). The SCR’s S1,S2& S3 form one half wave control rectifier and the diodes D1,D2 &D3 form another rectifier ckt. Phase Controlled Rectifier Page 13 In this way half wave converter is equivalent to addition of two half wave ckts. The load is fed via the upper 3 phase half wave rectifier & the return path for the current is provided via the lower half wave rectifier to one of the 3 supply lines. No neutral is required. At a time two devices conduct simultaneously one from each half wave rectifier. One SCR & one diode will conduct simultaneously. The load vtg therefore will be a line vtg. & not the phase vtg. Which line vtg. Appears across the load is decided by which pair of devices conduct at the given instant of time. Phase Controlled Rectifier Page 14 3.7.4 3hase half converter with purely resistive load With a purely resistive load (R) the load current will be in phase with the load vtg. The instantaneous load current will be equal to the instantaneous load vtg divided by the resistance R. Naturally the shape of the load current waveform will be same as that of the load vtg. Depending on the value of firing angle “α”, this ckt can operate in two different modes of operation i.e., continuous & discontinuous. Circuit Diagram: Phase Controlled Rectifier Page 15 Important Points: 6 PULSE OUTPUT: the load vtg waveform contains 6 pulses per cycle of the input vtg. As shown in fig(4). The ripple frequency of the output vtg waveform is therefore 6 times the supply frequency i.e,300Hz. CONDUCTION ANGLE: each device SCR or diode conduct for 2π⁄3 radians or 120⁰. NO. FREEWHEELING: as load is resistive, it cannot store energy. Therfore freewheeling action doesn’t take place in any of two modes of operation with resistive load. Phase Controlled Rectifier Page 16 3.8 Operation of 3 phase fully control rectifier with purely resistive load: The voltage & current waveforms through purely resistive load are as shown in fig. The load voltage waveform is same as that with the load inductive load. The load current however will not be ripple free. Instead the instantaneous load current is equal to the instantaneous load voltage divided by the load resistance R. Therefore the shape of the load current waveform is same as that of the load voltage and it is in phase with the load voltage. The sequence in which the diodes are turned on & off also remains same as that with the inductive load. The outgoing diodes are turned off due to line communication. Each diode conducts for a duration of 120°or 2 π/3 radians and the current through each diode during its conduction period is equal to the instantaneous load current during that period. Circuit Diagram: Phase Controlled Rectifier Page 17 Result: the three phase bridge rectifier & fullwave circuit have the advantages of higher average output voltage ,lower rippel factor,higher rippel frequency,better rectification efficiency & better transformer utilization factor. the condution angel for diode is 120°or 2π⁄3 radians for half wave & bridge but 60° for full wave circuit. The peak inverse voltage also remains same i.evm(line) or √3vm(ph) or √6vs(rms) for all the three configuration. Phase Controlled Rectifier Page 18