Unit-5

Miscellaneous Converters &

Control Schemes

EE-321 N

Lecture-14

Cosine Control Scheme

Introduction

• A power electronic converter is useful only when its output voltage is controllable

• There are different techniques available for controlling the output of different converters like time ratio control for DC choppers, PWM control for inverters etc.

• For phase controlled rectifiers the technique is control of switching angle

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Contd...

• Moreover, the previously described techniques (Unit-2) are not suitable if the converter has more than one thyristor (which is generally the case)

• The schemes employed for switching angle control are:

1. Cosine wave crossing control

2. Ramp comparator control

3. Digital firing scheme

4. Equidistant pulse firing scheme

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1. Cosine Control

• This method employs a cosine wave obtained from the input supply thru an integrator.

• The cosine wave is compared against a dc reference signal and correspondingly trigger pulses are obtained

• The basic principle is similar to that of a dual converter in which output voltage ratio is equal to cosine of the firing angle

• Based on this the rectifier can be thought of as a amplifier with linear transfer characteristic

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Block Diagram

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Waveforms

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Operation

• The input supply is synchronized and stepped down through a synchronizing t/f to obtain v1

• Then it is fed to integrator to obtain the cosine signal v2

• This cosine wave is compared against the dc control voltage which is limited to ±Ecm

• v3 is the o/p of comparator 1 which is obtained when Ec is higher than v2

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Operation...contd

• Similarly, output v4 is obtained from comparator 2.

• The signals v3 & v4 make the clock pulse generators to produce output pulses which in turn set the respective F/Fs

• The o/ps vi & vj of the F/Fs can be used to trigger SCRs in a single phase converter

• In practice, F/F outputs are ANDed with high frequency carrier wave (3-5 kHz) to obtain pulse train

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Relation between Output Voltage & Control Voltage

From the waveforms it can be observed that firing angle is governed by the intersection of cosine wave and control signal & can be expressed as:

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2m c

1 c

2m

cos

cos

V E

E

V

Contd...

The output of a single phase full converter is given as:

Which can be written as

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m0

2cos

VV

1 cm m0 c c c

2m 2m

2 2cos cos

EV VV E k E

V V

Contd...

This shows that cosine control scheme provides a linear transfer characteristic which improves the closed loop response of the converter

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Remark 1

• This scheme is sensitive to line supply variations. For 3 ph converters adjustments should be made to equalize all the line voltages otherwise firing angles will be unequal

• But this scheme has a “self-regulating” property because any change in the input voltage will lead to corresponding change in the cosine wave leading to mean dc voltage being constant

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Remark 2

• The size of the control scheme circuit increases as the number of phases/pulses increases because firing pulse is produced individually for each thyristor

• This can be overcome by using a cosine time multiplexing instead of individual timing control

• Cosine control scheme also applies to dual and cycloconverters

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Further Reading

1. Dubey, G. K., et. al. , “Thyristorized Power Controllers”, New-Age International, 1986.

2. Pelley, B. R., “Thyristor Phase-Controlled Converters & Cycloconverters”. Wiley-Interscience, 1971.

3. Arora, O. P., “Power Electronics Laboratory: Theory, Practice & Organization ”, Narosa Publishing House, 2007.

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