1. cycloconverters

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Cycloconverters

- Anoop MathewHOD

Department of Electronics and Communication Engineering

VedaVyasa Institute of Technology, Kerala 673632


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.


Cycloconverters – Equivalent Ckt.


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.


Cycloconverter – General Characteristics.


1-Φ to 1-Φ Cycloconverter (Bridge)


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”


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)


1Φè1Φ cycloconverter – R-load


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


Depending on the firing angle and load power factor.

1Φè1Φ cycloconverter – RL-load

Continuous Load current mode Discontinuous Load current Mode


Discontinuous Load current Mode


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.


Continuous Load current Mode


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


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.


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.


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.


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)


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


Thank You

Questions ???

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1. cycloconverters

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