Unit5

UNIT V AC voltage controller and

cycloconverter

www.Vidyarthiplus.com



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

t

O t

O t

O t

The phase shift range (operation range of phase delay angle):

0




Resistive load, quantitative analysis

RMS value of output voltage

RMS value of output current

RMS value of thyristor current

Power factor of the circuit

( ) ( )pi

pi

pi

pi

pi

+== 2sin21dsin21 1

2

1o UttUU (4-1)

(4-2) RUI oo =

(4-3) ( ) )2

2sin1(21sin2

21 1

2

1

pi

pi

pi

pi

+=

= R

Utd

RtUIT

(4-4) pi

pi

pi +==== 2sin

21

1

o

o1

oo

UU

IUIU

SP




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

t

O

u G1

u G2

O

O

t

t

t

t

t

The phase shift range:




4.2 Other AC controllers

4.2.1 Integral cycle controlAC 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

pi M

Line period

Control period

= M *Line period

= 2 pi

4

pi

M

O

Conduction angle =

2

pi N M

3

pi M

2

pi

M

u o

u

1 u o , i o

t

U 1 2




4.3 Thyristor cycloconverters

4.3.1 Single- phase thyristor-cycloconverter Circuit configuration and operation principle

P N

Z

t

uo ap= 2 Output

voltage

ap=0 Average

output voltage

ap= 2




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




Typical waveforms

1

O

O

2

3 4 5

6

u o

i o

t

t




Modulation methods for firing delay angle Calculation method For the rectifier circuit

t

t

cosd0o Uu =

tUu oomo sin=

ttUU

oo

d0

om sinsincos ==

)sin(cos o1 t =

(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




Output voltage ratio

(Modulation factor)

)10(0

= d

om

UU

pi 2

2 pi pi t 3 pi 2

0 30

60 90

120

150

Output voltage phase angle

/ ( ) =0 =0.1 0.2

0.3 0.8 0.9 1.0

0.1 0.2 0.3

0.8 0.9 1.0




4.3.2 Three- phase thyristor-cyclo converter

The configuration with common input line




The configuration with star-connected output




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




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 conversionhigh efficiency Bidirectional energy flow, easy to realize 4- quadrant operation Very complicatedtoo many power semiconductor devices Low output frequency Low input power factor and bad input current waveform Applications: High power low speed AC motor drive




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




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




Features

Direct frequency conversionhigh 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 complicatedtoo many power semiconductor devices Output voltage magnitude is a little lower as compared to indirect

frequency converter.



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