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UNIT V AC voltage controller and cycloconverter www.Vidyarthiplus.com www.Vidyarthiplus.com

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UNIT V

AC voltage controller and

cycloconverter

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4.1.1 Single-phase AC voltage controller

R u 1 u o

i o

VT 1

VT 2

u

O

u 1

u o

i o

VT

ω t

O ω t

O ω t

O ω t

The phase shift range

(operation range of phase

delay angle): 0 ≤≤≤≤αααα≤≤≤≤ ππππ

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RMS value of output voltage

RMS value of output current

RMS value of thyristor current

Power factor of the circuit

( ) ( )π

απα

πωω

π

π

α

−+== ∫ 2sin

2

1dsin2

11

2

1o UttUU (4-1)

(4-2) R

UI o

o =

(4-3) ( ) )2

2sin1(

2

1sin2

2

1 1

2

1

π

α

π

αω

ω

π

π

α+−=

= ∫ R

Utd

R

tUIT

(4-4) π

απα

πλ

−+==== 2sin

2

1

1

o

o1

oo

U

U

IU

IU

S

P

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Inductive (Inductor- resistor) load , operation principle

R

L

u 1 u o

i o

VT 1

VT 2

O

u 1

u o

i o

u VT

O

O

ω t O

u G1

u G2

O

O

ω t

ω t

ω t

ω t

ω t

The phase shift range:

φ ≤≤≤≤α ≤≤≤≤ ππππ

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4.2 Other AC controllers

4.2.1 Integral cycle control—AC power controller

Circuit topologies are the same as AC voltage controllers.

Only the control method is different.

Load voltage and current are both sinusoidal when thyristors are conducting.

R u 1 u o

i o

VT 1

VT 2

π

M

Line period

Control period

= M *Line period

= 2 π

4 π

M O

Conduction

angle =

2 π N

M

3 π

M

2 π

M

u o

u 1 u o , i o

ω t

U 1 2

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4.3 Thyristor cycloconverters

4.3.1 Single- phase thyristor-cycloconverter

• Circuit configuration and operation principle

P N

Z

ωt ο

uo ap= 2

π Output

voltage

ap=0

Average

output voltage

ap=

2

π

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• Single- phase thyristor-cycloconverter

Modes of operation

t

t

t

t

t

O

O

O

O

O

u o , i o

u o i o

t 1 t

2 t 3

t 4 t 5

u o u P

u N

u o

i P

i N

u P u N u o

i o i N i P

blocking P

N

Rectifi

cation

Inver

sion

blocking Rectifi

cation

Inver

sion

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Typical waveforms

1

O

O

2

3 4

5

6

u o

i o

ω t

ω t

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• Modulation methods for firing delay angle

Calculation method

– For the rectifier circuit

ωt

ωt

αcosd0o Uu =

tUu oomo sinω=

ttU

Uoo

d0

om sinsincos ωγωα ==

)sin(cos o

1tωγα −=

(4-15)

–For the cycloconverter

output

(4-16)

–Equating (4- 15) and (4-16)

–therefore (4-17)

(4-18)

Principle of cosine

wave-crossing method

u2 u3 u4 u5 u6 u1

ap3 ap4

uo

us2 us3 us4 us5 us6 us1

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Output voltage ratio

(Modulation factor)

)10(0

≤≤= γγd

om

U

U

π

2 2 π π ωο t 3 π

2

0

30

60

90

120

150

Output voltage phase angle

α / ( º) γ=0

γ=0.1 0.2

0.3

0.8 0.9 1.0

0.1 0.2 0.3

0.8

0.9

1.0

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4.3.2 Three- phase thyristor-cyclo converter

• The configuration with common input line

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• The configuration with star-connected output

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Typical waveforms

200 t / ms

Output voltage

Input current with

Single-phase output

Input current with

3-phase output

200 t / ms

200 t / ms

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• Input and output characteristics

The maximum output frequency and the harmonics in the output voltage are the same as in single-phase circuit. Input power factor is a little higher than single-phase circuit. Harmonics in the input current is a little lower thanthe single- phase circuit due to the cancellation of some harmonics among the 3 phases.

To improve the input power factor:

–Use DC bias or 3k order component bias on each of the 3 output phase voltages

• Features and applications

Features:

–Direct frequency conversion—high efficiency

–Bidirectional energy flow, easy to realize 4- quadrant operation

–Very complicated—too many power semiconductor devices

–Low output frequency

–Low input power factor and bad input current waveform

Applications:

–High power low speed AC motor drive

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4.4 Matrix converter

• Circuit configuration

input

output

a) b)

a b c

u

v

w

S 1

1

S 1

2

S 1

3

S 2

1

S 2

2

S 2

3

S 3

1

S 3

2

S 3

3

S ij

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• Usable input voltage

a) b) c)

a) Single-phase input

voltage

b) Use 3 phase voltages

to construct output

voltage

c) Use 3 line-line voltages

to construct output

voltage

Um

U1m

Um 1

2

√3

2 U1m

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• Features

Direct frequency conversion—high efficiency can realize good input and output waveforms, low harmonics, and nearly unity displacement factor

Bidirectional energy flow, easy to realize 4- quadrant operation

Output frequency is not limited by input frequency

No need for bulk capacitor (as compared to indirect frequency converter)

Very complicated—too many power semiconductor devices

Output voltage magnitude is a little lower as compared to indirect frequency converter.

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