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DC Chopper Control Techniques

Need of Chopper Control schemes To control the operation of Chopper, we need to control average output voltage of chopper circuit The average output voltage is varied or controlled by periodic opening and closing of switches used in chopper circuit. Chopper Control schemes Time Ratio Control Constant frequency system/Pulse width modulation control Variable frequency system/Frequency modulation control 1.By varying on period of switch keeping chopping frequency ,f constant. Constant frequency system/Pulse width modulation control Switches are turned on at constant chopping frequency and on time of switch is varied to vary average output voltage. The total period T of one cycle of output waveform is constant i.e. f=1/T is constant. The variation of on period results in pulse width control, therefore this method is called as PWM control Features of Constant frequency system/Pulse width modulation control Low ripple content as compared to variable frequency method Small size of filter required Preferred for faster response Applications DC motor speed control (2) By varying frequency f and keeping on period of switch constant. Variable frequency system/Frequency modulation control The on period of switch TON is kept constant and average output voltage is varied by varying chopping frequency f In PWM scheme low values of output voltage is obtained by reducing TON but in practice TON cannot be reduced below a minimum value Tmin Therefore, duty ratio, D is reduced by increasing total period T, keeping Ton constant at Tmin. Duty ratio can be varied by varying T with either TON or TOFF constant to get controlled V0avg As frequency is varied , this method is called Frequency modulation Features of Variable frequency system/Frequency modulation control At low frequencies, the current may become discontinues causing high harmonic content. As this method gives harmonics at unpredictable frequencies, filter design is difficult System cannot be optimized due to variable frequency Control range of frequencies is limited Operation at high frequencies is accompanied with losses in component System become sluggish at low frequencies Current Limit Control In this technique load current I0 is allowed to vary between predetermined max and min limits. Since the chopper operates between Imax and Imin with Imin ≠0, the output current is continues If I0 tends to increase beyond maximum limit, thyristor is turned OFF and if I0 tends to fall below minimum limit the thyristor is turned ON The switching frequency is determined by difference of Imax and Imin If Imax - Imin is large ⇒ fS will be less and ripple in output current ∆I = Imax - Imin will be more. If Imax - Imin is small ⇒ fS will be more and ripple in output current ∆I = Imax - Imin will be less. In this scheme chopper is turned On when I0 is equal to preset min value Imin and chopper is kept ON till I0 increases to another preset value Imax. The chopper frequency and pulse width are dependent on load parameters. Advantages of Current Limit Control The ripple in load current ∆I = Imax - Imin can be regulated to small preset value for wide range of loads Applications DC motor control and SMPS #DCChopperControlTechniques #TimeRatioControl #CurrentLimitControl #PWMControl #FrequencyModulationControl