Introduction of Three Phase Half-Full Wave Converter

HOW-TO GUIDE

Introduction of Three Phase Half/Full Wave Converter

THREE PHASE HALF/FULL CONVERTER

INTRODUCTION Single phase half controlled bridge converters & fully controlled bridge converters are used extensively in industrial applications up to about 15kW of output power. The single phase controlled rectifiers provide a maximum dc output of V dc

2V m max

The output ripple frequency is equal to the twice the ac supply frequency. The single phase full wave controlled rectifiers provide two output pulses during every input supply cycle and hence are referred to as two pulse converters. Three phase converters are 3-phase controlled rectifiers which are used to convert ac input power supply into dc output power across the load. Features of 3-phase controlled rectifiers are Operate from 3 phase ac supply voltage.

Join the Technical Community Today! http://www.pantechsolutions.net

They provide higher dc output voltage and higher dc output power. Higher output voltage ripple frequency. Filtering requirements are simplified for smoothing out load voltage and load current

Three phase controlled rectifiers are extensively used in high power variable speed industrial dc drives.

3-PHASE HALF WAVE CONVERTER Three single phase half-wave converters are connected together to form a three phase half-wave converter as shown in the figure.



THREE PHASE SUPPLY VOLTAGE EQUATIONS

We define three line neutral voltages (3 phase voltages) as follows

v RN

van

Vm sin

t ; V m

vYN

vbn

Vm sin

t

vYN

vbn

Vm sin

t

Max. Phase Voltage

2

3

0 120

Vector diagram of 3-phase supply volta es

2 3

v BN

vcn

Vm sin

t

v BN

vcn

Vm sin

t

0 120

v BN

vcn

Vm sin

t

0 240


The 3-phase half wave converter combines three single phase half wave controlled rectifiers in one single circuit feeding a common load. The thyristor the supply phase windings

'a

n'

T 1 in

acts as one half wave

controlled rectifier. The second thyristor supply phase winding

'b

n'

T 2

in series with the

acts as the second half wave

controlled rectifier. The third thyristor supply phase winding

series with one of

' c n ' acts

T 3

in series with the

as the third half wave controlled

rectifier.

The 3-phase input supply is applied through the star connected supply transformer as shown in the figure. The common neutral point of the supply is connected to one end of the load while the other end of the load connected to the common cathode point. When the thyristor phase voltage

T 1 is

van appears

triggered at

t

6

300

, the

across the load when T 1 conducts. The


load current flows through the supply phase winding

'a

n'

and

through thyristor T 1 as long as T 1 conducts. When thyristor

T 2

is triggered at

t

5 6

1500

,

T 1

becomes reverse biased and turns-off. The load current flows through the thyristor 'b

n'

. When

T 2

T 2

and through the supply phase winding

conducts the phase voltage

load until the thyristor

T 3 is

When the thyristor

T 3 is

triggered at T 2

appears across the load when

T 3 conducts.

T 1

across the

triggered .

is reversed biased and hence

When

vbn appears

t

3 2

2700

,

turns-off. The phase voltage

T 2

vcn

is triggered again at the beginning of the next

input cycle the thyristor naturally as soon as

T 1

T 3

turns off as it is reverse biased

is triggered. The figure shows the 3-

phase input supply voltages, the output voltage which appears across the load, and the load current assuming a constant and ripple free load current for a highly inductive load and the current through the thyristor T 1 .


For a purely resistive load where the load inductance ‘L = 0’ and the trigger angle

6

, the load current appears as

discontinuous load current and each thyristor is naturally commutated when the polarity of the corresponding phase supply voltage reverses. The frequency of output ripple frequency for a 3-phase half wave converter is

3 f S ,

where

f S

is

the input supply frequency. The 3-phase half wave converter is not normally used in practical converter systems because of the disadvantage that the supply current waveforms contain dc components (i.e., the supply current waveforms have an average or dc value). TO DERIVE AN EXPRESSION FOR THE AVERAGE OUTPUT VOLTAGE OF A 3-PHASE HALF WAVE CONVERTER FOR CONTINUOUS LOAD CURRENT The reference phase voltage is v

RN

angle

van

Vm sin

t

. The trigger

is measured from the cross over points of the 3-phase

supply voltage waveforms. When the phase supply voltage


van

begins its positive half cycle at appears at

t

6

The trigger angle

for the thyristor

the cross over point at

t

300 .

300 to 1500 ,

t

the first cross over point

300 .

radians

during the period

0,

t

The thyristor

T 1 is T 1 is

measured from forward biased

when the phase supply voltage

van

has higher amplitude than the other phase supply voltages. Hence T 1 can be triggered between 300 to 1500 . When the thyristor T 1

is triggered at a trigger angle , the average or dc output

voltage for continuous load current is calculated using the equation 5

Vdc

3

6

vO .d

2

t

6

Output voltage

vO

van

Vm sin

t

for

t

300

to 1500

5

Vdc

3

6

Vm sin t.d

2

t

6

As the output load voltage waveform has three output pulses during the input cycle of

2

radians


5 6

3V m

Vdc

sin

2

t.d

t

6

5 6

3V m

Vdc

cos

2

t 6

3V m

V dc

cos

2

5 6

cos

6

Note from the trigonometric relationship cos

V dc

V dc

V dc

A

B

3V m 2

3V m 2 3V m 2

Note:

cos A.cos B sin A.sin B

cos

5 6

cos

sin

cos 1500 cos

sin 1800

300

6

sin

sin 150 0 sin

cos 1800 30 0 cos

cos 1800

5

300

cos

6

.cos

sin

cos 30 0 . cos

sin 180 0 30 0 sin

6

sin

sin 30 0 sin

cos 30 0 .cos

cos 300

sin 300


sin 30 0 sin

Therefore 3V m

V dc

V dc

V dc

V dc

V dc

3V m 2 3V m 2

2

3

2

2

cos 30 0 .cos

sin 30 0 sin

cos

3 3V m

3 cos

2

2

sin 30 0 sin

2cos 300 cos

3Vm

3V Lm

cos 300 cos

2

cos

cos

Where V Lm

3V m

Max. line to line supply voltage

for a 3-phase star connected

transformer. The maximum average or dc output voltage is obtained at a delay angle Vdc

max

=0

V dm

and is given by 3 3

V m

2


Where V m Is

the peak phase voltage.

And the normalized average output voltage is

Vdcn

V n

V dc

cos

V dm

TO DERIVE AN EXPRESSION FOR THE RMS VALUE OF THE OUTPUT VOLTAGE OF A 3-PHASE HALF WAVE CONVERTER FOR CONTINUOUS LOAD CURRENT The rms value of output voltage is found by using the equation 1 5

VO RMS

3

2

6 2

Vm

2

sin 2

t.d

t

6

and we obtain 1

VO

RMS

3V m

1 6

3 8

2

cos 2

Three phase half wave controlled rectifier output voltage waveforms for different trigger angles with RL load


Van

V bn

Vcn

V0 =30 0

0

30

0

60

0

90

0

120

0

150

0

180

0

210

Van

0

240

0

270

0

300

0

330

0

360

V bn

0

390

t

0

420

Vcn

V0 0

0

0

=60

0

30

0

60

0

90

0

120

0

150

0

180

0

210

0

240

0

270

0

300

0

330

0

360

V bn

Van

0

390

t

0

420

Vcn

V0 =90 0

0

30

0

60

0

90

0

120

0

150

0

180

0

210

0

240

0

270

0

300

0

330

0

360

0

390

0

0

420

t

Three phase half wave controlled rectifier output voltage waveforms for different trigger angles with R load


V bn

Van

Vcn

=0 Vs 0

0

30

60

0

0

90

120

0

0

150

0

180

0

210

0

240

0

270

0

300

0

330

0

360

V bn

Van

0

390

t

0

420

Vcn 0

=15 V0

0

0

30

60

0

0

90

120

0

0

150

0

180

0

210

0

240

0

270

0

300

0

330

0

360

V bn

Van

0

390

t

0

420

Vcn 0

=30 V0

0 0

30

60

0

0

90

120

0

0

150

0

180

0

210

0

240

0

270

0

300

0

330

0

360

V bn

Van

0

390

t

0

420

Vcn 0

=60 V0

0

0

30

60

0

0

90

120

0

0

150

0

180

0

210

0

240

0

270

0

300

0

330

0

360

0

390

0

420

t


TO DERIVE AN EXPRESSION FOR THE AVERAGE OR DC OUTPUT VOLTAGE OF A 3 PHASE HALF WAVE CONVERTER WITH RESISTIVE LOAD OR RL LOAD WITH FWD. In the case of a three-phase half wave controlled rectifier with resistive load, the thyristor T 1

conducts up to

voltage

van decreases

to zero at

zero and the thyristor t

300

to 1800

triggered at

t

300

and

radians. When the phase supply

1800

t

T 1 is

T 1

t

, the load current falls to

turns off. Thus

T 1

conducts from

.

Hence the average dc output voltage for a 3-pulse converter (3-phase half wave controlled rectifier) is calculated by using the equation

Vdc

vO

3

1800

vO .d

2

30

van

Vm sin

t

0

t;

for

t

300 to 1800


Vdc

Vdc

Vdc

V dc

3 2

1800

Vm sin

t .d

t

300

3V m 2

1800

sin

t.d

t

0

30

3V m 2

1800

cos

t 300

3V m 2

cos1800

Since

cos1800

We get

V dc

3V m 2

cos

30 0

1,

1 cos

300


THREE PHASE FULL CONVERTER Three phase full converter is a fully controlled bridge controlled rectifier using six thyristors connected in the form of a full wave bridge configuration. All the six thyristors are controlled switches which are turned on at a appropriate times by applying suitable gate trigger signals. The three phase full converter is extensively used in industrial power applications upto about 120kW output power level, where two quadrant operations is required. The figure shows a three phase full converter with highly inductive load. This circuit is also known as three phase full wave bridge or as a six pulse converter. The thyristors are triggered at an interval of

3

radians

(i.e. at an interval of 600 ). The frequency of output ripple voltage is

6 f S and

the filtering requirement is less than that of three

phase semi and half wave converters.


At

t

6

, thyristor

T 6

is already conducting when the

thyristor T 1 is turned on by applying the gating signal to the gate of T 6

T 1 .

During the time period

t

6

to

2

, thyristors

T 1 and

conduct together and the line to line supply voltage

vab

appears across the load. At

t

2

biased

, the thyristor

immediately

commutation.

During

thyristor

T 2

voltage

T 1 and

and the

T 6

T 2

is triggered and

turns time

off

period

due t

2

T 6

is reverse

to

natural to

5 6

,

conduct together and the line to line supply

vac appears

across the load.


The thyristors are numbered in the circuit diagram corresponding to the order in which they are triggered. The trigger sequence (firing sequence) of the thyristors is 12, 23, 34, 45, 56, 61, 12, 23, and so on. The figure shows the waveforms of three phase input supply voltages, output voltage, the thyristor current through

T 1 and

T 4

, the supply current through

the line ‘a’.

We define three line neutral voltages (3 phase voltages) as follows v RN

van

Vm sin

vYN

vbn

Vm sin

t

v BN

vcn

Vm sin

t

Where

V m the

;

t

Vm

2 3 2 3

Max. Phase Voltage

Vm sin

t

0 120

Vm sin

t

1200

Vm sin

t

0 240

peak phase voltage of a star (Y) is connected

source. The corresponding line-to-line voltages are v RY

vab

van

v bn

3V m sin

t

6


vYB

vbc

vbn

v cn

3V m sin

t

v BR

vca

vcn

v an

3V m sin

t

2

2


T6

T1

T2

T3

T4

T5

T6

T1

T2

iG1

t 0

iG2

0

(30 + ) 0 60

0

(360 +30 + ) t 0

iG3

60

t 0

iG4

60

t 0

iG5

60

t 0

iG6

60

t

Gating (Control) Signals of 3-phase full converter To derive an expression for the average output voltage of three phase full converter with highly inductive load assuming continuous and constant load current The output load voltage consists of 6 voltage pulses over a period of 2

radians, hence the average output voltage is

calculated as

VO dc

Vdc

6

2

vO .d t

2

;

6


vO

3Vm sin

vab

3

Vdc

t

6

2

3Vm sin

t

.d t

6

6

3 3Vm

V dc

Where

3V mL

cos

VmL

3V m

cos

Max. line-to-line supply voltage

The maximum average dc output voltage is obtained for a delay angle

= 0, Vdc

3 3Vm

V dm

max

3V mL

The normalized average dc output voltage is Vdcn

V n

V dc V dm

cos

The rms value of the output voltage is found from 1 2

VO rms

6

2 2

vO .d

2

t

6


1 2

VO rms

6

2 2

vab .d

2

t

6

1 2

VO rms

3

2

3Vm2 sin 2

2

t

6

.d

t

6

1

VO

rms

3V m

1 2

3 3 4

2

cos 2


Did you enjoy the read? Pantech

solutions

creates

information

packed technical documents like this one every month. And our website is a rich and trusted resource used by a vibrant online community of more than 1, 00,000 members from organization of all shapes and sizes.


What do we sell? Our

products

range

from

Various

Microcontroller development boards, DSP Boards, FPGA/CPLD boards, Communication Kits, Power electronics, Basic electronics, Robotics, Sensors, Electronic components and much more . Our goal is to make finding the parts and information you need easier and affordable so you can create awesome projects and training from Basic to Cutting edge technology.


Introduction of Three Phase Half-Full Wave Converter

Recommend Documents