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Single-Phase Dual

Converter In previous section, we have seen that the

single-phase full converters with inductive

loads allow only two-quadrant operation.

If two of these converters are connected

back to back, both the output voltage and

the output current can be reversed.

This system will provide four-quadrant

operation and it is called a dual converter.

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Dual converters are used in high-power

variable-speed drives.

If1 and 2 are the delay angles of

converters 1 and 2, the correspondingaverage output voltages will be Vdc1 and Vdc2.

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The delay angles are controlled such that

one converter operates as a rectifier and theother converter operates as an inverter.

However, both converters produce the sameaverage output voltage.

Therefore,

5

22

11

cos2

cos2

m

dc

m

dc

VV

VV

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One converter is rectifying and the other one isinverting, therefore:

Since the instantaneous output voltages of thetwo converters are out of phase, there will be an

instantaneous voltage difference between thetwo converters.

6

12

112

21

)cos(coscos

dcdc

VV

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This will result in a circulating current between

the two converters. The dual converters can be operated with or

without a circulating current.

In case of operation without the circulatingcurrent, only one converter operates at a timeand carries the load current.

The other converter is completely blocked byinhibiting gate pulses.

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Principles of Three-Phase Half-

Wave Converters

Three-phase converters provide higher

average output voltage, and the frequency of

the ripples on the output voltage is highercompared to that of single-phase converters.

Therefore, the filtering requirements forsmoothing out the load current and load

voltage are simpler.

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If the phase voltage is , then theaverage output voltage will be:

The rms value of the output voltage will be:

10

tVv man sin

cos

2

33)(sin

2

3 6/5

6/

m

mdc

VtdtVV

2/1

2/1

6/5

6/

22

)2cos8

3

6

1(3

)(sin23

mrms

mrms

VV

tdtVV

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Three-Phase Full

Converters Three-phase converters are extensively

used in industrial applications up to 120 kWlevel, where two-quadrant operation isrequired.

This circuit is known as a three-phasebridge converter.

The thyristors are fired at an interval of 60degrees.

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The frequency of the output ripple

voltage is 6fs.

The following figure shows a full

converter circuit with a highly inductive

load.

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The line-to-neutral voltages are:

Then the line-to-line voltages are:

14

)3

2

(sin

)3

2(sin

sin

tVv

tVv

tVv

mcn

mbn

man

)6

5(sin3

)2

(sin3

)6

(sin3

tVvvv

tVvvv

tVvvv

mancnca

mcnbnbc

mbnanab

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The average output voltage is found from:

The rms value of the output voltage is:

15

cos33

)(3 2/

6/

m

abdc

VtdvV

2/1

2/12/

6/

2

)2cos4

33

2

1(3

)(3

mrms

abrms

VV

tdvV

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A three-phase bridge gives a six-pulse output

voltage.

For high -power applications such as high-voltage dc transmission, a 12 pulse output isgenerally required to reduce the output ripplesand to increase the ripple frequencies.

Two six-pulse bridges can be combined either inseries or in parallel to produce a 12-pulse

output.

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