Presented by:
BHOGARAJU RAVALI

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A NEW METHOD FOR POWER QUALITY IMPROVEMENT IN CLASSICAL AC/AC VOLTAGE CONTROLLER USING PWM TECHNIQUE
INTRODUCTION
AC voltage controllers, allow controlling the output voltage only, while the output frequency is the same as the input frequency.
AC voltage controllers are widely used to obtain variable AC voltage from fixed AC source. Triacs or thyristors are usually employed as the power control elements of such controllers.
Advantages include simplicity and ability of controlling large power economically
The switching mode power conversion gives high efficiency, but the main disadvantage is that harmonics are generated at both the supply and load sides due to the nonlinearity of switches.
To cope with the harmonic pollution many alternative methods are proposed in the past.
One of the well-known methods is the PWM control technique.
The advantages of PWM technique to be gained include nearly sinusoidal input-output current/voltage waveforms, better input power factor, better transient response, elimination of the low order harmonics and consequently, smaller input/output filter parameters.
PROBLEM METHODOLOGY
The main factor for harmonic producing in traditional AC voltage controllers is the discontinuity of the load voltage.
In the proposed method, the discontinuity of the load voltage has been minimized by using PWM technique in the discontinuous section in which the load voltage is zero.
The schematic of power circuit used in the proposed PWM-AC voltage controller is given in Figure
The load is connected in series with the power source.
The series switch S1 is used to connect or disconnect the load terminals to the supply, that is, they regulate the power delivered to the load.
The parallel switch S2 provides a freewheeling path for the load current to discharge its stored energy when the series switch is turned off.
The switches S1 and S2 are fully controlled power switches capable of conducting current in both directions (bi-directional).
POWER SEMI CONDUCTOR DEVICES
Thyristor and Triac
Gate turn off thyristor (GTO)
Power transistor
Power MOSFET
Insulated gate bipolar transistor
PRACTICAL CIRCUIT IMPLEMENTED
The below figure shows the practical circuit implemented
 In the circuit, we are practically using four power devices.
 With resistive loads, there is no need of S3 and S4 power devices.
 In the positive half cycle S1 is triggered as required and output, i.e., ac chopped waveform is obtained.
 During the negative half cycle, S2 is triggered as required and output is obtained.
 As with resistive load, as there is no freewheeling action required, the S3 and S4 devices can be simply neglected.
 In the case of inductive loads, the inductive current need to be freewheeled during the off period of the main switches S1 and S2.
 If, it is not ac, only simple diodes can be used as free wheeling diodes. But, in the present case as diodes cannot be used, the power devices are used here also.
 During the positive half cycle, if AC output is required S1 is turned ON, and during the OFF period S4 is triggered to provide the free wheeling action. Similarly, during the negative half cycle S2 is triggered for an output, and S3 is triggered during the S2 OFF time to provide freewheeling action.
OPERATION OF AC CHOPPERS
 An ac voltage regulator is used as one of the power electronics systems to control an output ac voltage for power ranges from few watts (as in the light dimmers) up to fractions of megawatts (as in starting systems for large induction motors).
 Phase-angle control of thyristors was traditionally used in this type of regulators.
 There are some disadvantages associated with these such as high lower order harmonic content in o/p resulting in low pf (esp. at large firing angles) and load voltage waveform is determined by the load phase angle that also affects control range in terms of firing angle.
 The performance of the regulator can be improved if it is designed to operate as a chopper.
In this case input supply voltage is chopped into segments and output voltage level is decided by the ratio between ON/OFF periods.
Chopper mode of operation can be realized by using two ac switches one connected in series and the other in parallel with the load as shown in Fig below
PROPOSED PWM AC CHOPPER
The power circuit of a PWM ac chopper is composed of two pairs of inverse parallel power transistors connected one in series, and one in parallel with the load.
The series connected transistors regulate the power delivered to the load, and the parallel ones provide the freewheeling path to discharge the stored energy when the series ones are turned off.
The fundamental voltage can be adjusted according to the required duty cycle, i.e., the ratio of on-time ton to a modulation period T,. The harmonics are suppressed in proportion to the difference between the frequencies of the ac line voltage and the modulation
This form of PWM control produces an approximately sinusoidal load current for inductive loads even without filters
MAIN CONNECTION DIAGRAM
COMPONENTS USED
Isolated transformer
Pulse transformer
Zero crossing detector
Crystal oscillator
Diodes
Voltage regulators
Opto couplers
Micro controller
RESULT
In the present project an AC chopper has been designed and developed and is working satisfactorily, and the resulted waveform can be observed on the CRO
The output power can also be controlled by adopting a pwm technique, varying duty cycle i.e on or off ratio of the cycle. Even this option is achieved and is demonstrated satisfactorily.
The output waveform obtained as observed on the CRO is shown
Result observed on the CRO:
CONCLUSION
 By using PWM-AC chopper, the harmonics can be effectively eliminated and the o/p waveform obtained is pure sinusoidal even after chopping and this can be made use in applications like speed control of small induction motors of fractional hp rating..