Single phase sinusoidal input to non-sinusoidal output Cycloconverter

Content

Section I: Introduction

Section II: Proposed diagram with reduce number of Thyristors

Section III: Theory

Section IV: Simulation in MATLAB Simulink:

Section V: Conclusion

Section VI: References

Section I: Introduction

Cycloconveters are the arrangement of power electronics which is used to convert Alternating Current (AC) at one frequency to AC at another frequency. The power circuit diagram of single phase bridge type cycloconverter is the basic circuit diagram to explain concept of cycloconverters. Fig. 1 shows the power circuit diagram ^{[1] [2]}. There are two, P and N, converters for carrying positive and negative half cycle output signals.

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Fig. 1 Power circuit diagram of single phase step down bridge type cycloconverter.

Section II: Proposed diagram with reduce number of Thyristors

Fig. 2 shows circuit diagram of modified circuit diagram of Fig. 1. In this circuit diagram there are two thyristor T1 and T2 for P converter and T3 and T4 for N converter. For the sake of clearness in operation, operating principle of modified diagram can be explained more specifically.

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Fig. 2 Proposed Diagram of Modified Cycloconverter with a Shunt Capacitor

A shunt capacitor is connected to resistive load (R). Suppose we want 220v(RMs) 50HZ(time period=20ms ) to 25HZ (time period=40ms ) cycloconverter. At first, T1 and T2 are ON at , which opens valve of T1 and T2 for the flow of the first half cycle of AC input. Upto capacitor is charged with maximum input voltage [illustration not visible in this excerpt] Now for 15ms (i.e remaining time period of first half of output waveform) capacitor discharges with suitable time for discharging (further clarification is done in Section III). After 20ms T3 and T4 are ON for negative half cycle of output. Same process of charging and discharging of capacitor takes place for negative cycle of output. Fig. 3 shows output waveform for this specified case.

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Fig. 3 Output of 50Hz to 25Hz Cycloconverter

Section III: Theory

A shunt capacitor is connected with R-load. In first half cycle of output, when Thyristors T1 and T2 are ON at 0ms, capacitor is charged upto time period [illustration not visible in this excerpt] . Now, we choose value of capacitor such that it discharges thorough load resistor for [illustration not visible in this excerpt] , until voltage across capacitor (V_c) becomes [illustration not visible in this excerpt] just before zero crossing of first half cycle of output voltage. We can also select a capacitor based on the value of V_c when it becomes say [illustration not visible in this excerpt](i.e.V_c at T_D, discharging time constant of capacitor) at zero crossing of output voltage.

The voltage across capacitor when it discharges is given by ^[3]:

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We know capacitor discharging time, [illustration not visible in this excerpt]

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After mathematical reduction we get as,

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During negative half cycle when Thyristors T3 and T4 are ON at [illustration not visible in this excerpt], the charging and discharging of capacitor continues but with reversal appearance than that of positive half cycle. In this way desired frequency of output voltage with non-sinusoidal waveform is obtained.

Specified Case:

Let us take an instance that input frequency to be 50HZ for rms input voltage of 220V where maximum input voltage [illustration not visible in this excerpt]and resistive load is[illustration not visible in this excerpt]. A cycloconverter with output frequency, [illustration not visible in this excerpt], etc. can be obtain by choosing a suitable capacitor connected in shunt with the resistive load.

1. For [illustration not visible in this excerpt] is charged upto with maximum voltage[illustration not visible in this excerpt] Now we have to select a capacitor that discharges upto [illustration not visible in this excerpt]. Let, output voltage value just before zero crossing of first half cycle of output waveform (i.e. at [illustration not visible in this excerpt]) be [illustration not visible in this excerpt], which is output voltage at discharging time of capacitor, T_D. Now, value of capacitor can be calculated as

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Simulation, with above input parameters, is done in Simulink Environment of MATLAB and output voltage waveform is shown in Fig. 4.[illustration not visible in this excerpt]

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Fig. 5 Block Diagram of Modified Single Phase Cycloconverter

Similarly, if just before zero crossing of output voltage we want output voltage across capacitor to be then for different values of , values of capacitor are calculated by using equation(1) and shown in Table I.

Table: I

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For several values of, simulation result shows various output voltage waveform:[illustration not visible in this excerpt]

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2. For [illustration not visible in this excerpt] to[illustration not visible in this excerpt], [illustration not visible in this excerpt]and[illustration not visible in this excerpt]. In this case capacitor charges upto [illustration not visible in this excerpt]and discharge for[illustration not visible in this excerpt]. Setting all parameter same, different values of capacitance of shunt capacitor can be calculated for different values of[illustration not visible in this excerpt] as shown in Table II.

Table II

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Section IV: Simulation in MATLAB Simulink:

Fig. 5 shows simulation block diagram made in MATLAB.

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Fig. 5 Block Diagram of Modified Single Phase Cycloconverter

The block diagram Modified Single Phase Cycloconverter in figure is a subsystem created using P and N converter which is shown in Fig. 6

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Fig. 6 P and N conveter of Modified Single Phase Cycloconverter

Simulation file for 50Hz to 25Hz, x=60 and C=0.0294, can be downloaded from link below:

https://drive.google.com/open?id=0By-me65CUlVyVkgydFJRaHJ2VlE

Various parameter like input voltage, value of capacitor C and gate pulses for thyristors can be changed to get the desired output frequency.

Section V: Conclusion

By reducing a number of thyristor in both P and N converters and choosing a suitable shunt capacitor at load terminal, a modified step down cycloconveter converts sinusoidal input at one frequency(F_m) to non-sinusoidal output another frequency (F_o), with [illustration not visible in this excerpt]=2,4,6

Section VI: References

[...]

^[1] Rashid, Muhammad H. Power Electronics Handbook. San Diego: Academic, 2001. Print.

^[2]Pelly, B. R. Thyristor Phase-controlled Converters and Cycloconverters: Operation, Control, and Performance. New York: Wiley-Interscience, 1971. Print

^[3] "Charging and Discharging a Capacitor." Charging and Discharging a Capacitor. N.p., n.d. Web. 10 Feb.2016.<http://macao.communications.museum/eng/exhibition/secondfloor/MoreInfo/2_3_5_ChargingCapacitor.html>.