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UNIT III DC Choppers Introduction • Chopper is a static device. • A variable dc voltage is obtained from a constant dc voltage source. • Also known as dc-to-dc converter. • Widely used for motor control. • Also used in regenerative braking. • Thyristor converter offers greater efficiency, faster response, lower maintenance, smaller size and smooth EE2301-POWER ELECTRONICS control. Choppers are of Two Types Step-down choppers. Step-up choppers. In step down chopper output voltage is less than input voltage. In step up chopper output voltage is more than input voltage. EE2301-POWER ELECTRONICS Principle Of Step-down Chopper Chopper i0 V + V0 R EE2301-POWER ELECTRONICS • A step-down chopper with resistive load. • The thyristor in the circuit acts as a switch. • When thyristor is ON, supply voltage appears across the load • When thyristor is OFF, the voltage across the load will be zero. EE2301-POWER ELECTRONICS v0 V Vdc t tOFF tON i0 V/R Idc t T EE2301-POWER ELECTRONICS Vdc Average value of output or load voltage. I dc Average value of output or load current. tON Time interval for which SCR conducts. tOFF Time interval for which SCR is OFF. T tON tOFF Period of switching or chopping period. 1 f Freq. of chopper switching or chopping freq. T EE2301-POWER ELECTRONICS Average Output Voltage tON Vdc V tON tOFF tON Vdc V V .d T tON but t d duty cycle EE2301-POWER ELECTRONICS Average Output Current Vdc I dc R V tON V I dc d R T R RMS value of output voltage 1 VO T tON v dt 2 o 0 EE2301-POWER ELECTRONICS But during tON , vo V Therefore RMS output voltage 1 VO T tON V dt 2 0 2 tON V VO tON .V T T VO d .V EE2301-POWER ELECTRONICS Output power PO VO I O VO IO But R Output power 2 O V PO R 2 dV PO R EE2301-POWER ELECTRONICS Effective input resistance of chopper V Ri I dc R Ri d The output voltage can be varied by varying the duty cycle. EE2301-POWER ELECTRONICS Methods Of Control • The output dc voltage can be varied by the following methods. – Pulse width modulation control or constant frequency operation. – Variable frequency control. EE2301-POWER ELECTRONICS Pulse Width Modulation • tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant. • Output voltage is varied by varying the ON time tON EE2301-POWER ELECTRONICS V0 V tON tOFF t T V0 V t tON tOFF EE2301-POWER ELECTRONICS Variable Frequency Control • Chopping frequency ‘f’ is varied keeping either tON or tOFF constant. • To obtain full output voltage range, frequency has to be varied over a wide range. • This method produces harmonics in the output and for large tOFF load current may become discontinuous EE2301-POWER ELECTRONICS v0 V tON tOFF t T v0 V tON tOFF t T EE2301-POWER ELECTRONICS Step-down Chopper With R-L Load Chopper i0 + R V FWD L E EE2301-POWER ELECTRONICS V0 • When chopper is ON, supply is connected across load. • Current flows from supply to load. • When chopper is OFF, load current continues to flow in the same direction through FWD due to energy stored in inductor ‘L’. EE2301-POWER ELECTRONICS • Load current can be continuous or discontinuous depending on the values of ‘L’ and duty cycle ‘d’ • For a continuous current operation, load current varies between two limits Imax and Imin • When current becomes equal to Imax the chopper is turned-off and it is turned-on when current reduces to Imin. EE2301-POWER ELECTRONICS v0 Output voltage V tON i0 tOFF t T Imax Output current Imin Continuous current i0 t Output current Discontinuous current t EE2301-POWER ELECTRONICS Principle Of Step-up Chopper I L + D + C V Chopper L O A D VO EE2301-POWER ELECTRONICS • Step-up chopper is used to obtain a load voltage higher than the input voltage V. • The values of L and C are chosen depending upon the requirement of output voltage and current. • When the chopper is ON, the inductor L is connected across the supply. • The inductor current ‘I’ rises and the inductor stores energy during the ON time of the chopper, tON. EE2301-POWER ELECTRONICS • When the chopper is off, the inductor current I is forced to flow through the diode D and load for a period, tOFF. • The current tends to decrease resulting in reversing the polarity of induced EMF in L. • Therefore voltage across load is given by dI VO V L i.e., VO V dt EE2301-POWER ELECTRONICS • A large capacitor ‘C’ connected across the load, will provide a continuous output voltage . • Diode D prevents any current flow from capacitor to the source. • Step up choppers are used for regenerative braking of dc motors. EE2301-POWER ELECTRONICS Expression For Output Voltage Assume the average inductor current to be I during ON and OFF time of Chopper. When Chopper is ON Voltage across inductor L V Therefore energy stored in inductor = V .I .tON Where tON ON period of chopper. EE2301-POWER ELECTRONICS When Chopper is OFF (energy is supplied by inductor to load) Voltage across L VO V Energy supplied by inductor L VO V ItOFF where tOFF OFF period of Chopper. Neglecting losses, energy stored in inductor L = energy supplied by inductor L EE2301-POWER ELECTRONICS VItON VO V VO VO V tON ItOFF tOFF tOFF T V T tON Where T = Chopping period or period of switching. EE2301-POWER ELECTRONICS T tON tOFF 1 VO V tON 1 T 1 VO V 1 d tON Where d duty cyle T For variation of duty cycle ' d ' in the range of 0 d 1 the output voltage VO will vary in the range V VO EE2301-POWER ELECTRONICS Performance Parameters • The thyristor requires a certain minimum time to turn ON and turn OFF. • Duty cycle d can be varied only between a min. & max. value, limiting the min. and max. value of the output voltage. • Ripple in the load current depends inversely on the chopping frequency, f. • To reduce the load ripple current, frequency should be as high as possible. EE2301-POWER ELECTRONICS Problem • A Chopper circuit is operating on TRC at a frequency of 2 kHz on a 460 V supply. If the load voltage is 350 volts, calculate the conduction period of the thyristor in each cycle. V 460 V, Vdc = 350 V, Chopping period Output voltage T f = 2 kHz 1 f 1 T 0.5 m sec 3 2 10 tON Vdc V T EE2301-POWER ELECTRONICS Conduction period of thyristor T Vdc tON V 0.5 10 3 350 tON 460 tON 0.38 msec EE2301-POWER ELECTRONICS Problem • Input to the step up chopper is 200 V. The output required is 600 V. If the conducting time of thyristor is 200 sec. Compute – Chopping frequency, – If the pulse width is halved for constant frequency of operation, find the new output voltage. EE2301-POWER ELECTRONICS tON 200 s, V 200 V , Vdc 600V T Vdc V T tON T 600 200 6 T 200 10 Solving for T T 300 s Chopping frequency f 1 T 1 f 3.33 KHz 6 300 10 Pulse width is halved tON 200 10 6 100 s EE2301-POWER ELECTRONICS 2 Frequency is constant f 3.33KHz 1 T 300 s f T Output voltage = V T tON 300 10 6 200 300 100 10 6 EE2301-POWER ELECTRONICS 300 Volts Problem • A dc chopper has a resistive load of 20 and input voltage VS = 220V. When chopper is ON, its voltage drop is 1.5 volts and chopping frequency is 10 kHz. If the duty cycle is 80%, determine the average output voltage and the chopper on time. VS 220V , R 20, f 10 kHz tON 0.80 T Vch = Voltage drop across chopper = 1.5 volts d Average output voltage Vdc Vdc tON VS Vch T 0.80 220 1.5 174.8 Volts EE2301-POWER ELECTRONICS Chopper ON time, tON dT Chopping period, 1 T f 1 3 T 0.1 10 secs 100 μsecs 3 10 10 Chopper ON time, tON dT tON 0.80 0.1 10 3 tON 0.08 10 3 80 μsecs EE2301-POWER ELECTRONICS Problem • In a dc chopper, the average load current is 30 Amps, chopping frequency is 250 Hz, supply voltage is 110 volts. Calculate the ON and OFF periods of the chopper if the load resistance is 2 ohms. I dc 30 Amps, f Chopping period, 250 Hz , T V 110 V , R 2 1 1 4 10 3 4 msecs f 250 Vdc & Vdc dV R dV I dc R I R 30 2 d dc 0.545 V 110 I dc EE2301-POWER ELECTRONICS Chopper ON period, tON dT 0.545 4 10 3 2.18 msecs Chopper OFF period, tOFF T tON tOFF 4 10 3 2.18 10 3 tOFF 1.82 10 3 1.82 msec EE2301-POWER ELECTRONICS • A dc chopper in figure has a resistive load of R = 10 and input voltage of V = 200 V. When chopper is ON, its voltage drop is 2 V and the chopping frequency is 1 kHz. If the duty cycle is 60%, determine – Average output voltage – RMS value of output voltage – Effective input resistance of chopper – Chopper efficiency. EE2301-POWER ELECTRONICS Chopper i0 + R V v0 V 200 V , R 10, Chopper voltage drop Vch 2V d 0.60, f 1 kHz. Average output voltage Vdc d V Vch Vdc 0.60 200 2 118.8 Volts RMS value of output voltage VO VO V Vch 0.6 200 2 153.37 d EE2301-POWER ELECTRONICS Volts Effective input resistance of chopper is V IS Ri V I dc Vdc 118.8 11.88 Amps R 10 V V 200 16.83 IS I dc 11.88 I dc Ri Output power is PO dT 1 T PO d 2 v0 1 dt R T 0 V Vch R dT 0 V Vch R 2 dt 2 0.6 200 2 2 PO 10 2352.24 watts Input power, 1 Pi T PO 1 T dT ViO dt 0 dT 0 V V Vch R dt EE2301-POWER ELECTRONICS Vch PO R 0.6 200 200 2 PO 2376 watts 10 Chopper efficiency, dV V PO 100 Pi 2352.24 100 99% 2376 EE2301-POWER ELECTRONICS