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Cycloconverters
- Anoop MathewHOD
Department of Electronics and Communication Engineering
VedaVyasa Institute of Technology, Kerala 673632
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CycloconvertersØ AC power at one frequency is converted directly to an AC
power at another frequency without any intermediate DC stage.
Ø The output of cycloconverter provides ac power at a lower frequency than input.
Ø Bidirectional power-flow is possible.
Ø Able to operate with loads of any power factor.
Ø Generally, output frequency is lower than source frequency.
Ø input power factor is worse if the fabricated output voltage is decreased.
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Cycloconverters – Equivalent Ckt.
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Working Principle
Ø Fundamental wave is generated by the two, 2-quadrent phase converters.
Ø Diode for unidirectional conduction.Ø Ripple content is neglected.Ø Positive current is carried by positive
converter and negative by negative.
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Cycloconverter – General Characteristics.
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1-Φ to 1-Φ Cycloconverter (Bridge)
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Description
Ø 2 bridges are in back-to-back configuration.Ø “circulating-current free mode of operation”Ø 2 bridges should not conduct simultaneously –
cause short circuit.
Only one converter conducts at a timeP-bridgeà Load current in positive half of output.N-bridge à Load current in negative half of output.
“circulating-current free mode
of operation”
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Description (Contd ...)
Non-circulating-current mode of operation
Ø No circulating current.Ø Complex control strategy.Ø To simplify, a circulating current has to be developed
by a current limiting reactor. (it is connected between the positive and the negative converters like dual-converter)
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1Φè1Φ cycloconverter – R-load
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1Φè1Φ cycloconverter – R-load
Ø Output current is zero as input voltage is zero.
Ø Diagonal switches in each converter is in OFF state in half cycles alternatively.
Ø Natural commutation.
Ø Operation is discontinuous.
Ø The first 3 half cycles are contributed by bridge-1 and next 3 by bridge-2 in order to make the output frequency = (1/3) x input frequency.
Ø 3 half cycles by B1 + 3 half cycles by B2 = one cycle of output wave.
Ø For lower o/p frequency, no. of cycles/bridge should be increased.
non-circulatingcurrent mode
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Depending on the firing angle and load power factor.
1Φè1Φ cycloconverter – RL-load
Continuous Load current mode Discontinuous Load current Mode
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Discontinuous Load current Mode
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Discontinuous Load current Mode
Ø L is low.Ø Non-circulating mode of operation takes place.Ø From the plot, o/p frequency = (¼) input frequencyØ Due to the load inductance, an elapsed period of current flow will occur even after the input voltage is reversed.Ø The pattern of firing angle -> first decreasing and then increasing.Ø Ripple frequency = 100 HzØ The ripple in load current being filtered by the inductance present in the load.
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Continuous Load current Mode
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Continuous Load current Mode
Ø Higher inductance is in load side.Ø Non-circulating mode of operation is used. (only one of the bridges -#1 (positive), or #2 (negative), conducts at a time)Ø Non-circulating mode of operation takes place.Ø From the plot, o/p frequency = (¼) input frequencyØ Due to the load inductance an elapsed period of current flow will occur even after the input voltage is reversed.Ø The pattern of firing angle -first decreasing and then increasing.Ø Ripple frequency = 100 Hz
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Advantages of Cycloconverter Ø Direct frequency conversion without any
intermediate stage.Ø Natural commutation – Compact system.Ø Power transfer in either directions.Ø Operates in any p.f.Ø Capable of regeneration over the complete speed
range, down to standstill.Ø Commutation failure leads only to blown-off of
individual fuses only.
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Advantages of Cycloconverter (Contd ...)
Ø Delivers a high quality sinusoidal waveform at low output frequencies -since it is fabricated from a large number of segments of the supply waveform.
Ø This is often preferable for very low speed applications.
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Disadvantages of Cycloconverter
Ø Large number of thyristors are required in a cyclo-converter.
Ø Complex control circuitry.Ø The output frequency is limited to one-
third of the input frequency. (For reasonable power output and efficiency)
Ø The power factor is low, particularly at reduced output voltages.
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Applications
Ø Speed control of drives.Ø Variable input frequency, constant
frequency output. (constant frequency power supplies)
Ø Induction heating. (Low frequency 3-phase to high frequency 1-phase AC)
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References
[1] A. Maamoun, “DEVELOPMENT OF CYCLOCONVERTERS”, IEEE Trans. 2003.
[2] Ned Mohan et al, “Power Electronics”, Weilypublications 2005
[3] M.D. Singh & K.B Khanjandani, “Power Electronics”,TMH 2006
[4] Cyril W. Lander “ Power Electronics”, McGraw Hill International Editions 2007