UNIT 4 CONTROLLED RECTIFIERS

1. UNIT 4 CONTROLLED RECTIFIERS

2. Topic details Introduction Principles of phase controlled converter operation 1ф fully controlled converters Dual converters 1ф semi converters

3. INTRODUCTION Controlled rectifiers are basically AC to DC converters. The power transmitted to the load is controlled by controlling the triggering angle of the devices. The DC voltage is not pure , as from a battery, but contains ripples superimposed on the DC component. Controlled rectifiers are also called as ac-dc converters. main supply →AC voltage controller → load ↑…………………….↓

4. Line commutated AC to DC converters The inputs are fixed voltage and fixed frequency ac supply The outputs are variable dc output voltage The two types of commutation are Natural and AC line commutation Different types of Line Commutated Converters AC to DC Converters (Phase controlled rectifiers) AC to AC converters (AC voltage controllers) AC to AC converters (Cyclo converters) at low output frequency APPLICATIONS 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.

5. Resistive load

6. At t = 0 when the input supply voltage becomes positive the thyristor T becomes forward biased. unlike a diode, it does not turn ON till a gate pulse is applied at t = . During the period 0 < t , the thyristor blocks the supply voltage and the load voltage remains zero Consequently, no load current flows during this interval. As soon as a gate pulse is applied to the thyristor at t =  it turns ON. The voltage across the thyristor collapses to almost zero and the full supply voltage appears across the load. From this point onwards the load voltage follows the supply voltage. The load being purely resistive the load current io is proportional to the load voltage. At t =  as the supply voltage passes through the negative going zero crossing the load voltage and hence the load current becomes zero and tries to reverse direction. In the process the thyristor undergoes reverse recovery and starts blocking the negative supply voltage. Therefore, the load voltage and the load current remains clamped at zero till the thyristor is fired again at t = 2 + . The same process repeats

7. SINGLE PHASE HALF WAVE CONTROLLED RECTIFIER WITH AN RL LOADThe thyristor is forward biased during the positive half cycle of input supply. Let us assume that is triggered at , by applying a suitable gate trigger pulse to during the positive half cycle of input supply.The output voltage across the load follows the input supply voltage when is ON.The load current flows through the thyristor and through the load in the downward direction. This load current pulse flowing through can be considered as the positive current pulse. Due to the inductance in the load, the load current flowing through would not fall to zero at , when the input supply voltage starts to become negative.A phase shift appears between the load voltage and the load current waveforms, due to the load inductance.

9. In the case of a resistive load, the thyristor T becomes forward biased when the supply voltage becomes positive at ωt = 0. However, it does not start conduction until a gate pulse is applied at ωt = α. As the thyristor turns ON at ωt = α the input voltage appears across the load and the load current starts building up. However, unlike a resistive load, the load current does not become zero at ωt = π, instead it continues to flow through the thyristor and the negative supply voltage appears across the load forcing the load current to decrease. Finally, at ωt = β (β > π) the load current becomes zero and the thyristor undergoes reverse recovery. From this point onwards the thyristor starts blocking the supply voltage and the load voltage remains zero until the thyristor is turned ON again in the next cycle. It is to be noted that the value of β depends on the load parameters. Therefore, unlike the resistive load the average and RMS output voltage depends on the load parameters. Since the thyristors does not conduct over the entire input supply cycle this mode of operation is called the “discontinuous conduction mode”.

10. TO DERIVE AN EXPRESSION FOR AVERAGE (DC) LOAD VOLTAGE Considering sinusoidal input supply voltage we can write the expression for the supply voltage as

11. AVERAGE DC LOAD CURRENT:

12. SINGLE PHASE HALF WAVE CONTROLLED RECTIFIER WITH RL LOAD AND FREE WHEELING DIODEWith a RL load it was observed that the average output voltage reduces. This disadvantage can be overcome by connecting a diode across the load as shown in figure. The diode is called as a Free Wheeling Diode (FWD).

13. SYMBOL

14. At , ωt=∏ the source voltage Vs falls to zero and as Vs becomes negative, the freewheeling diode is forward biased. The stored energy in the inductance maintains the load current flow through R, L, and the FWD. Also, as soon as the FWD is forward biased ,at ωt=∏ , the SCR becomes reverse biased, the current through it becomes zero and the SCR turns off. During the period , ωt=∏ to β load current flows through FWD (free wheeling load current) and decreases exponentially towards zero at ωt= β . Also during this free wheeling time period the load is shorted by the conducting FWD and the load voltage is almost zero, if the forward voltage drop across the conducting FWD is neglected. Thus there is no negative region in the load voltage wave form. This improves the average output voltage.

15. SINGLE PHASE SEMI CONVERTER

16. It can be stated that for any load current to flow (T1 or T3) must conduct at one time and similarly (D2 or D4) must conduct along with T1 or T3. However, T1 T3 or D2 D4 cannot conduct simultaneously. On the other hand T1 D4 and T3 D2 conducts simultaneously whenever T1 or T3 are on and the output voltage tends to go negative. Therefore, there are four operating modes of this converter when current flows through the load. Of course it is always possible that none of the four devices conduct. The load current during such periods will be zero.

17. whenever D2 conducts the voltage across D4 is -vi and when D4 conducts the voltage across D2 is vi. Since diodes can block only negative voltage it can be concluded that D2 and D4 conducts in the positive and the negative half cycle of the input supply respectively. when T1 is fired in the positive half cycle of the input voltage. Load current flows through T1 and D2. If at the negative going zero crossing of the input voltage load current is still positive it commutates from D2 to D4 and the load voltage becomes zero. If the load current further continuous till T3 is fired current commutates from T1 to T3. This mode of conduction when the load current always remains above zero is called the continuous conduction mode. Otherwise the mode of conduction becomes discontinuous mode of operation. Conducts in both the directionIt has a single gateUsed for heat control, light control and motor controlTurned off by line commutation of AC supplyNot suitable for inductive loads

18. WORKING WITH INDUCTIVE LOAD VI-CHARACTERISTICS SYMBOL

19. With inductive load three modes are possible: Continuous load current Discontinuous load current Continuous and ripple free current for large inductive loads. CONTINUOUIS CURRENT MODE: T1 D2 starts conduction at ωt = α. Output voltage during this period becomes equal to vi. At ωt = π as vi tends to go negative D4 is forward biased and the load current commutates from D2 to D4 and freewheels through D4 and T1. The output voltage remains clamped to zero till T3 is fired at ωt = π + α. The T3 D4 conduction mode continues uptoωt = 2π.

20. DISCONTINUOUIS CURRENT MODE

21. The current through the load becomes zero for some duration, it is called discontinuous mode of operation. when T1 is fired at ωt = α the output voltage (instantaneous value) is larger than the back emf. Therefore, the load current increases till Vo becomes equal to E again at ωt = π – θ. There,the load current starts decreasing. If Io becomes zero before T3 is fired at ωt = π + α the conduction becomes discontinuous. So clearly the condition for continuous conduction will be i = α ≥ 0 If the condition violated , then it becomes discontinuous mode of operation.

22. SINGLE PHASE FULL CONVERTER WITH R LOAD It contains 4 SCRs namely Q1, Q2, Q3 andQ4. The conduction af all these SCRs are controlled ,hence it is called full converter.

24. WORKING WITH INDUCTIVE LOAD

27. Here the load current flows against the supply voltage. The energy stored in the load inductance is supplied partially to the main supply and to the load itself. Hence this is called feedback operation. The output voltage is negative from ∏ to ∏+ α, since supply voltage is negative But, the load current keeps on reducing At ∏+ α,SCRs T3 and T4 are triggered, the load current starts increasing. The load current remains continuous in the load. The similar operation repeats.

29. DUAL CONVERTERS Dual Converter is an Electronic Device or Circuit made by the combination of two bridges. One of them works as Rectifier(Converts A.C. to D.C.) i.e α<90 and other bridge works as Inverter(converts D.C. into A.C.) i.e α>90 . Thus an electronic circuit or device in which two processes take place at same time, is known as Dual Converter.

31. The instatenuous values of Vo1 andVo2 are not same. Hence a small amount of circu;lating current flows between the two converters. This current does not flows through the load and is limited by the circulating current reactor Lr.