ac voltage controllers-01

8/6/2019 AC Voltage Controllers-01

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08/12/2010 ELCT704 Power Electronics 1

Single phase AC VoltageControllers

Mostafa Soliman

Part no. 1


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Outline

Introduction.

On-off (or Integral Cycle) control.

Phase angle control:

Single phase half-wave.

Single phase full-wave with R load.

Single phase full-wave with R-L load.


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AC Voltage Controllers

AC voltage controllers convert the input AC signal withfixedmagnitude and frequency to AC signal with adjustable magnitudeand frequency.

ACVoltageController

V0(RMS)

fS

Variable ACRMS O/P Voltage

ACInput

Voltagefs

Vs

fs

Some applications:

Industrial heating.

Light controls.

Speed control of polyphase (3 phase) Induction motors.


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AC voltage controllers could besingle phase orthree phase and each

one of them can be subdivided intohalf wave orfull wave. In this chapter we are going to studysingle phase half wave andfull

wave AC voltage controllers with no change in the output AC signalfrequency, i.e.adjustable magnitude only.

AC voltage controllers with adjustable magnitude and frequency arecalledcycloconverters and they are beyond the scope of this course.


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Control of AC voltage controllers can be achieved by:

On-off control:

Where thyristor switches connect the load to the AC source forfew

cycles (integr number of cycles) and disconnect it from the AC sourcefor anotherfew cycles (integral number of cycles). Also this type ofcontrol is called Integral Cycle control.

Phase-angle control:

Where thyristor switches connect the load to the AC source for aportion of each cycle of input voltage.


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On-off AC voltage controllers

The figure shows a single phase full wave AC voltage controller with two thyristors

connected back-to-back. T1 is turned ON at the beginning of the +ve half cycles, 0, 2, 4, for n cycles.

T2 is turned OFF at the beginning of the -ve half cycles, , 3, 5, for n cycles.

Vs

Vo

io

ig1

ig2

t

t

t

Gate pulse of T1

Gate pulse of T2


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)(2

)(2

sin)(2

2

1

2

1

)(2)(2)(

1

22

2

0

222

2

0

22

2

0

22

0

2

)2(

0

22

mn

nVV

mn

nVV

tdtV

mn

nV

mn

ntdVV

tdVmn

ntdV

mn

ntdV

mnV

m

o

m

o

mo

so

ss

n

so

The output voltage cycle repeats every

n+m.

For n cycles: Vo = Vs

For m cycles: Vo = 0.

The RMS load, or output, voltage can be

found as:

)()(

cycleduty

RMSsRMSo VKV

mn

nK


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mmmrmssrmso

rmso

mmrmsT

mrmsT

mmavT

mavT

mmpeakT

IK

R

VK

R

VK

R

VK

R

VI

IKImn

nI

tdtImn

nI

IKImn

nI

tdtImn

nI

R

VII

222CurrentRMS(load)outputThe

22

)sin()(2

CurrentThy.RMSThe

)(

sin)(2

CurrentThy.averageThe

CurrentThy.peakThe

)()(

)(

)(

0

2

)(

)(

0

)(

)(

0 2 3 t

Im

nmiT

Waveform of Thyristor Current

Thyristor ratings:


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Power factor:

mn

nK

V

V

V

RIPF

II

VI

VI

VI

RIPF

PF

rmss

rmso

rmss

rmso

rmssrmso

rmssrmss

rmsormso

rmssrmss

rmso

)(

)(

)(

)(

)()(

)()(

)()(

)()(

2

)(

But,

)(

AmperVoltSupplyInput

PowerLoadOutput

The PF is very poor at low value of duty cycle K, i.e. when n is few

number of cycles


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Phase-angle control AC voltage controllers Single phase half-wave AC controllers:

The power flow is controlled during the positive half cycle only by controlling T1.

D1 conducts for the whole negative half cycle.

The output voltage and current are asymmetric and contain dc component.

(Unidirectional controllers)

(Unidirectional controllers)


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D1 conducts for the whole negative half cycle.

The output voltage and current are asymmetric and contain dc component.


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)22sin2(

21

)0()02()2

2sin()0(

2

1

)2

2sin()

2

2sin(

2

)()2cos1()()2cos1(4

)()sin()()sin(2

1

22

22

2

22

rmso

rmso

rms

o

m

mmo

VV

VV

tt

tt

VV

tdttdtV

tdtVtdtVV

The output RMS voltage is

2then0if rmsorms

VVV


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)1(cos2

2

)1(cos2

)11(cos1(2

)cos2cos(coscos(2

)cos()cos(2

)(sin)(sin2

1

2

2

rmsm

dc

mdc

mdc

m

mmdc

VV

V

VV

VV

ttV

tdtVtdtVV

The output average (DC) voltage is

rms

dc

VV

20then0if

)2

2sin2(

2

1

AmperVoltinput

rms

oso

srms

ooout

V

VPFII

IV

IVPPF


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Phase-angle control AC voltage controllers Single phase full-wave AC controllers, R-load:

The power flow is controlled during both the positive half cycle, by controlling T1,and the negative half cycle, by controlling T2.

The output voltage and current are symmetric and contain no dc component.

(bidirectional controllers)

(bidirectional controllers)


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Phase-angle control AC voltage controllers Single phase full-wave AC controllers, R-load :

Thy. T1 conducts only in the +ve half cycle at

t =

. Thy. T2 conducts only in the ve half cycle at

t = +.

Output average voltage and current are zero.


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Phase-angle control AC voltage controllers Single phase full-wave AC controllers, R-load :

Different configurations

2 thyristors and 2 diodes are used

1 thyristor and 4 diodes are used


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Phase-angle control AC voltage controllers Single phase full-wave AC controllers:

)2

2sin(

1

)2

2sin222(

2

1

)

2

2sin()02()

2

2sin()0(

2

1

)2

)22sin(()02()

2

2sin()0(

2

1

)22sin()

22sin(

2

)()2cos1()()2cos1(4

)()sin()()sin(2

1

22

22

2

22

rmso

rmso

rmso

rmso

rms

o

m

mmo

VV

VV

VV

VV

ttttVV

tdttdtV

tdtVtdtVV

The output RMS voltage is

0then0if orms VV


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VO(RMS)

Trigger angle in degrees

0 60 120 180

VS

0.2 VS

0.6VS

Control Characteristic


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2

2sin

2

1

)2

2sin()0(

2

1

)2

2sin(

2

)()2cos1(4

)()sin

(2

1

)(

)(

2

2

)(

2

2

2

)(

R

VI

R

VI

tt

R

VI

tdtR

V

tdR

tVI

rmsrmsThy

rmsrmsThy

rmsrmsThy

m

mrmsThy

The thyristor current falls to zero every half cycle

due to the natural commutation of resistive loads. The thyristor RMS current is:


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1cos2

2

1cos2

coscos2

cos2

)(sin2

)()sin

(2

1

)(

)(

)(

)(

)(

R

VI

RVI

R

VI

tR

VI

tdtR

V

tdR

tVI

rmsdcThy

mdcThy

mdcThy

mdcThy

m

mdcThy

Thyristor dc (or average) current is:

ac voltage controllers-01

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