Power Electronics

Chapter 4 AC to AC Converters ( AC Controllers and Frequency Converters )

Power Electronics

Classification of AC to AC converters
Same frequency variable magnitude AC power Variable frequency AC power

AC power

AC controllers

Frequency converters (Cycloconverters)

AC to AC converters
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Power Electronics

Classification of AC controllers
Phase control: AC voltage controller (Delay angle control) Integral cycle control: AC power controller

AC controller
PWM control: AC chopper (Chopping control) On/off switch: electronic AC switch PWM: Pulse Width Modulation
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Classification of frequency converters

Frequency converter (Cycloconverter)
Phase control: thyristor cycloconverter (Delay angle control) PWM control: matrix converter (Chopping control)

Cycloconverter is sometimes referred to

in a broader senseany ordinary AC to AC converter in a narrower sensethyristor cycloconverter

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Outline
4.1 AC voltage controllers 4.2 Other AC controllers 4.3 Thyristor cycloconverters 4.4 Matrix converters

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4.1 AC voltage controllers

4.1.1 Single-phase AC voltage controller 4.1.2 Three-phase AC voltage controller Applications Lighting control Soft-start of asynchronous motors Adjustable speed drive of asynchronous motors Reactive power control

4.1.1 Single-phase AC voltage controller

Power Electronics

Resistive load
VT1 io VT2

u1 O uo

u1

uo

O io O u VT

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

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Resistive load, quantitative analysis

RMS value of output voltage
Uo =

(
1

2U1 sin t d( t ) = U1
2

1 sin 2 + 2

(4-1)

RMS value of output current

Io = Uo R

(4-2)

RMS value of thyristor current

1 2U1 sin t d ( t ) = U1 IT = R R 2

sin 2 1 (1 + ) (4-3) 2 2

Power factor of the circuit

P UoIo Uo = = = = S U1 I o U1

1 sin 2 + 2

(4-4)

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Inductive (Inductor-resistor) load, operation principle

VT1 io u1 VT2 uo R
u1 O uG1 O uG2 O uo O io 0.6

t t t

The phase shift range:

O uVT O

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Inductive load, quantitative analysis

Differential equation
di L o + Rio = 2U 1 sin t dt (4-5) io t = = 0
/( )
180 140 100 60
90 = 75 60 45 30 15 0

Solution

(4-6)

20 0 20 60 100 ) /( 140 180

Considering io=0 when t=+ We have

sin( + ) = sin( ) e

tg

(4-7)

The RMS value of output voltage, output current, and thyristor current can then be calculated.
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Inductive load, when <

The circuit can still work.
u1

The load current will be continuous just like the thyristors are short-circuit, and the thyristors can no longer control the magnitude of output voltage. The start-up transient will be the same as the transient when a RL load is connected to an AC source at t = ( < ).

O iG1 O

t t t t

iG2

O io

iT1 +

iT2

Start-up transient

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Harmonic analysis

Power Electronics

There is no DC component and even order harmonics in the current.

The current waveform is halfwave symmetric.

100 80 In/I*/% 60 40 20 3 5 7
Fundamental

The higher the number of harmonic ordinate, the lower the harmonic content.

= 90 is when harmonics is the

most severe. The situation for the inductive load is similar to that for the resistive load except that the corresponding harmonic content is lower and is even lower as is increasing. 0 60

120 /( )

180

Current harmonics for the resistive load

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4.1.2 Three-phase AC voltage controller Classification of three-phase circuits

Y connection

Line-controlled connection

Branch-controlled connection

Neutral-point-controlled connection 13

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3-phase 3-wire Y connection AC voltage controller

ia ua

VT 1
a

U a0'

VT 3
n u
b

VT 4
b n'

VT 5
u

VT 6
c

VT 2

For a time instant, there are 2 possible conduction states:

Each phase has a thyristor conducting. Load voltages are the same as the source voltages. There are only 2 thyristors conducting, each from a phase. The load voltages of the two conducting phases are half of the corresponding line to line voltage, while the load voltage of the other phase is 0.

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3-phase 3-wire Y connection AC voltage controller

Resistive load, 0 < 60
VT VT VT u ab 2 u ao' 0
5 6 1

VT 4 VT 3 VT
2

VT 1 VT 6 VT
5

ua

u ac 2

3
1

2 t 3 t

4 3

5 3

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Power Electronics

3-phase 3-wire Y connection AC voltage controller

Resistive load, 60 < 90
VT u VT
6

VT VT
2

VT VT
4

VT
ab

VT

2 u
ao'

u
a

ac

4 3

5 3 2

3 t

2 3 t
2

t
3

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Power Electronics

3-phase 3-wire Y connection AC voltage controller

Resistive load, 90 < 150
VT
5

VT

VT

VT VT
1

VT

VT

VT

VT VT u 4
ab

VT VT u 6u a
ac

VT

VT

VT

VT

2
ao'

5 3

2 3

4 3

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Power Electronics

3-phase 3-wire branch-controlled connection AC voltage controller

The operation principle is the same as 3 independent singlephase AC voltage controllers. ApplicationThyristor-controlled reactor (TCR)
To control the effective current flowing through the reactor by controlling delay angle, therefore control the reactive power absorbed by the reactor.
ua a

ia

b ub

c uc

a)

b)

c)

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Power Electronics

4.2 Other AC controllers

4.2.1 Integral cycle controlAC power controller 4.2.2 Electronic AC switch 4.2.3 Chopping controlAC chopper

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4.2.1 Integral cycle control AC power controller

uo VT1
Conduction 2N = M angle

io u1
VT2

2 U1 O M

2 M

uo,io 3 M 4 M

u1

uo

Line period Control period =M *Line period =2

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.
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Power Electronics

Spectrum of the current in AC power controller

There is NO harmonics in the ordinary sense.
In/I0m

0.6 0.5 0.4 0.3 0.2 0.1 0 2 4 6 8 10 12 14

Harmonic order as to control frequency

There is harmonics as to the control frequency. As to the line frequency, these components become fractional harmonics.

Harmonic order as to line frequency

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4.2.2 Electronic AC switch

Circuit topologies are the same as AC voltage controllers. But the back-to-back thyristors are just used like a switch to turn the equipment on or off. ApplicationThyristor-switched capacitor (TSC)

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TSC waveforms when the capacitor is switched in/out

us
uVT iC us
1

t t VT1 VT2 t1 t2 t t

uC C

uC uVT1 iC

VT1 VT2

The voltage across the thyristor must be nearly zero when switching in the capacitor, and the current of the thyristor must be zero when switching out the capacitor.
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Power Electronics

TSC with the electronic switch realized by a thyristor and an anti-parallel diode

The capacitor voltage will be always charged up to the peak of source voltage. The response to switching-out command could be a little slower (maximum delay is one line-cycle).
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4.2.3 Chopping controlAC chopper

Principle of chopping control
The mean output voltage over one switching cycle is proportional to the duty cycle in that period. This is also called Pulse Width Modulation (PWM).

Advantages
Much better output waveforms, much lower harmonics For resistive load, the displacement factor is always 1.

Waveforms when the load is pure resistor

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AC chopper Modes of operation

u o >0, io>0: u o >0, io<0: u o <0, io>0: u o <0, io<0:

V1 charging, V3 freewheeling V4 charging, V2 freewheeling V3 charging, V1 freewheeling V2 charging, V4 freewheeling

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Power Electronics

4.3 Thyristor cycloconverters (Thyristor AC to AC frequency converter)

Another namedirect frequency converter (as compared to AC-DC-AC frequency converter which is discussed in Chapter 8) Can be classified into single-phase and threephase according to the number of phases at output 4.3.1 Single-phase thyristor-cycloconverter 4.3.2 Three-phase thyristor-cycloconverter

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4.3.1 Single-phase thyristor-cycloconverter

Circuit configuration and operation principle
P uo Z N

uo

P= 2

Output voltage

P=0

Average output voltage

P=
2

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Power Electronics

Single-phase thyristor-cycloconverter
Modes of operation
uo,io uo io t2 uo t3 t4 t5 t

O t1 uP

iP uP uo

io

iN uN

O uN

O iP O iN O P
Rectifi Inver cation sion

uo

t Blocking
Rectifi Inver cation sion

N Blocking

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Power Electronics

Single-phase thyristor-cycloconverter
Typical waveforms
uo

io

O 1 2 3 4 5 6

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Modulation methods for firing delay angle

Calculation method
For the rectifier circuit u o = U d0 cos (4-15) For the cycloconverter output uo = U om sin o t (4-16)
Equating (4-15) and (4-16) U cos = om sin o t = sin o t U d0 (4-17) Therefore
us2 u2 u3 u4 u5 u6 u1

t P3
us3

P4
us4 us5 uo us6 us1

= cos 1 ( sin o t ) (4-18)

Cosine wave-crossing method

Principle of cosine wave-crossing method

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Calculated results for firing delay angle

180

/()

Output voltage ratio (Modulation factor)

U om = (0 r 1) U d0

150 120 90 60 30 0 2

=0
0.2 0.3 0.8 0.9 1.0

1.0 0.9 0.8 0.3 0.2 0.1

= 0.1

Output voltage phase angle

3 2

0t

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Input and output characteristics

Maximum output frequency: 1/3 or 1/2 of the input frequency if using 6pulse rectifiers Input power factor Harmonics in the output voltage and input current are very complicated, and both related to input frequency and output frequency.
Input displacement factor

0.8 0.6 0.4 0.2

=1 .0

0. 8 0 .6 0 .4 0. 2

0 0 0.2 0.4 0.6 0.8 1.0 0.8 0.6 0.4 0.2 0

Load power factor Load power factor (lagging) (leading)

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Power Electronics

4.3.2 Three-phase thyristor-cycloconverter

The configuration with common input line

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Three-phase thyristor-cycloconverter
The configuration with star-connected output

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Power Electronics

Three-phase thyristor-cycloconverter
Typical waveforms
Output voltage

200 t/ms

Input current with Single-phase output

200 t/ms

Input current with 3-phase output

200 t/ms

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Power Electronics

Input and output characteristics

The maximum output frequency and the harmonics in the output voltage are the same as in singlephase circuit. Input power factor is a little higher than singlephase circuit. Harmonics in the input current is a little lower than the 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
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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

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4.4 Matrix converter

Circuit configuration
Input

Output

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Power Electronics

Matrix converter
Usable input voltage

U1m Um
1 2 Um

3 2

U1m

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

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Power Electronics

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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