Three Phase Controlled Rectifiers

Power Electronics by Prof. M. Madhusudhan Rao 11

3 Phase Controlled Rectifiers

• Operate from 3 phase ac supply voltage.• They provide higher dc output voltage.• Higher dc output power.• Higher output voltage ripple frequency.• Filtering requirements are simplified for

smoothing out load voltage and load current.

• Extensively used in high power variable speed industrial dc drives.

• Three single phase half-wave converters can be connected together to form a three phase half-wave converter.

3-Phase Half Wave Converter(3-Pulse Converter)

with RL Load

Continuous & ConstantLoad Current Operation

Vector Diagram of 3 Phase Supply Voltages

1 2 00

1 2 00 RN AN

v vv vv v

3 Phase Supply Voltage Equations

We deifine three line to neutral voltages (3 phase voltages) as follows

sin ; Max. Phase Voltage

sin 120

sin 240

RN an m

YN bn m

BN cn m

v v V tV

v v V t

van vbn vcn van

Constant Load Current

Each thyristor conducts for 2/3 (1200)

To Derive an Expression for the

Average Output Voltage of a 3-Phase Half Wave Converter

with RL Loadfor Continuous Load Current

3065 1506

7 2706

2Each thytistor conducts for 120 or radians3

T is triggered at t

If the reference phase voltage is sin , the average or dc output

voltage for continuous load current is calculated using the equation

3 sin .2

RN an m

v v V t

V V t d t

3 sin .2

3 cos2

3 5cos cos2 6 6

VV t d t

Note from the trigonometric relationshipcos cos .cos sin .sin

5 5cos cos sin sin6 63

cos 150 cos sin 150 sin32 cos 30

s .cos sin sin6 6

B A B A B

0sin 30 sin

0 0 0 0

Note: cos 1

cos 180 30 cos sin 180 30 sin32 cos 30 .cos sin 30 sin

cos 30 cos sin 30 sin32 cos 30 .cos sin 30 s

80 30 cos 30

sin 180 30 sin 30

03 2cos 30 cos23 32 cos2 2

3 3 33 cos cos2 23 cos2

Where 3 Max. line to line supply voltage

The maximum average or dc output voltage is obtained at a delay angle 0 and is given by

Where is the peak phase voltage.And the normalized average output voltage is

ddcn n

The rms value of output voltage is found by using the equation

3 sin .2

and we obtain

1 33 cos 26 8

mO RMS

V V t d t

3 Phase Half Wave Controlled Rectifier Output Voltage

Waveforms For RL Load at

Different Trigger Angles

12 0 0

18 0 0

21 0 0

24 0 0

27 0 0

30 0 0

33 0 0

36 0 0

42 0 0

= 30 0

= 60 0

01 2 0

01 8 0

03 9 0

= 90 0

V bn V cn

3 Phase Half Wave Controlled Rectifier With

R Load and

RL Load with FWD

3 Phase Half Wave Controlled Rectifier Output Voltage

Waveforms For R Load or RL Load with FWD

atDifferent Trigger Angles

1 20 0

1 50 0

2 10 0

2 40 0

30 0 0

33 0 0

3 60 0

4 20 0

= 1 5 0

V b n V c n

V a n V b n V c n

1 20 0

1 50 0

33 0 0

3 90 0

= 3 0 0

V a n V b n V c n

= 6 0 0

V a n V b n V c n

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

30 180 ;

5 1506

150 300 ;

sin 120

O an m

O bn m

T is triggered at t

T conducts from to

v v V t

T is triggered at t

T conducts from to

v v V t

7 2706

270 420 ;

sin 240

sin 120

O cn m

T is triggered at t

T conducts from to

v v V t

sin ; for 30 to 180

3 sin .2

3sin .

O an m

V v d t

v v V t t

V V t d t

VV t d t

3 cos2

3 cos180 cos 302

cos180 1, we get3 1 cos 302

Three Phase Semiconverters• 3 Phase semiconverters are used in Industrial dc

drive applications upto 120kW power output.• Single quadrant operation is possible.• Power factor decreases as the delay angle increases.• Power factor is better than that of 3 phase half wave

converter.

3 Phase Half Controlled Bridge Converter

(Semi Converter)with Highly Inductive Load &

Continuous Ripple free Load Current

Wave forms of 3 Phase Semiconverter for

3 phase semiconverter output ripple frequency of output voltage is 3

The delay angle can be varied from 0 to During the period

30 2107 , thyristor T is forward biased

If thyristor is triggered at ,6

& conduct together and the line to line voltage appears across the load.

7At , becomes negative & FWD conducts.6

The load current contin

ues to flow through FWD ; and are turned off.

If FWD is not used the would continue to

conduct until the thyristor is triggered at 5 , and Free wheeling action would 6

be accomplished through & .

If the delay angle , e3

ach thyristor conducts

2for and the FWD does not conduct. 3 mD

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

2sin sin 12032sin sin 1203

sin 240

RN an m m

YN bn m m

BN cn m m

v v V t V

v v V t V t

is the peak phase voltage of a wye-connected source.m

3 sin653 sin6

3 sin2

3 sin6

RB ac an cn m

YR ba bn an m

BY cb cn bn m

RY ab an bn m

v v v v V t

Wave forms of 3 Phase Semiconverter for

To derive an Expression for the

Average Output Voltage of 3 Phase Semiconverter

for > / 3 and Discontinuous Output Voltage

For and discontinuous output voltage:3

the Average output voltage is found from

3 3 sin2 6

V v d t

V V t d t

3 3 1 cos2

3 1 cos2

3 Max. value of line-to-line supply voltageThe maximum average output voltage that occurs at a delay angle of 0 is

The normalized average output voltage is

0.5 1 cos

The rms output voltage is found from

acO rms

V v d t

3 3 sin2 6

3 sin 234 2

mO rms

V V t d t

Average or DC Output Voltage of a

3-Phase Semiconverter for / 3,

and Continuous Output Voltage

For , and continuous output voltage3

3 . .2

3 3 1 cos2

dc ab ac

V v d t v d t

15 262

0.5 1 cos

RMS value of o/p voltage is calculated by usingthe equation

3 . .2

3 23 3 cos4 3

ab acO rms

mO rms

V v d t v d t

Three Phase Full Converter

• 3 Phase Fully Controlled Full Wave Bridge Converter.

• Known as a 6-pulse converter.• Used in industrial applications up to 120kW

output power.• Two quadrant operation is possible.

• The thyristors are triggered at an interval of / 3.

• The frequency of output ripple voltage is 6fS.

• T1 is triggered at t = (/6 + ), T6 is already conducting when T1 is turned ON.

• During the interval (/6 + ) to (/2 + ), T1 and T6 conduct together & the output load voltage is equal to vab = (van – vbn)

• T2 is triggered at t = (/2 + ), T6 turns off naturally as it is reverse biased as soon as T2 is triggered.

• During the interval (/2 + ) to (5/6 + ), T1 and T2 conduct together & the output load voltage vO = vac = (van – vcn)

• Thyristors are numbered in the order in which they are triggered.

• The thyristor triggering sequence is 12, 23, 34, 45, 56, 61, 12, 23, 34, ………

sin 240

RN an m m

YN bn m m

BN cn m m

v v V t V

v v V t V t

is the peak phase voltage of a wye-connected source.m

The corresponding line-to-line supply voltages are

3 sin6

3 sin2

RY ab an bn m

YB bc bn cn m

BR ca cn an m

v v v v V t

To Derive An Expression For The Average Output Voltage Of

3-phase Full Converter With Highly Inductive Load Assuming Continuous And

Constant Load Current

6 . ; 2

3 sin6

dc OO dc

O ab m

V V v d t

v v V t

The output load voltage consists of 6 voltage pulses over a period of 2 radians, Hence the average output voltage is calculated as

3 3 sin .6

3 3 3cos cos

Where V 3 Max. line-to-line supply voThe maximum average dc output voltage is obtained for a delay angle

m mLdc

m mdmdc

V V t d t

V VV V

The normalized average dc output voltage is

The rms value of the output voltage is found from

dcdcn n

OO rms

V v d t

3 3 sin .2 6

1 3 33 cos 22 4

abO rms

mO rms

V v d t

V V t d t

Three Phase Dual Converters

• For four quadrant operation in many industrial variable speed dc drives , 3 phase dual converters are used.

• Used for applications up to 2 mega watt output power level.

• Dual converter consists of two 3 phase full converters which are connected in parallel & in opposite directions across a common load.

Outputs of Converters 1 & 2

• During the interval (/6 + 1) to (/2 + 1), the line to line voltage vab appears across the output of converter 1 and vbc appears across the output of converter 2

sin 240

RN an m

YN bn m m

BN cn m m

v v V tV

v v V t V t

The corresponding line-to-line supply voltages are

3 sin6

3 sin2

RY ab an bn m

YB bc bn cn m

BR ca cn an m

v v v v V t

• If vO1 and vO2 are the output voltages of converters 1 and 2 respectively, the instantaneous voltage across the current limiting inductor during the interval (/6 + 1) t (/2 + 1) is given by

To obtain an Expression for the Circulating Current

3 sin sin6 2

3 cos6

The circulating current can be calculated byusing the equation

r O O ab bc

v v v v v

v V t t

1 3 cos .6

3sin sin

i t v d tL

i t V t d tL

Vi t t

Four Quadrant OperationConv. 2 Inverting2 > 900

Conv. 2 Rectifying2 < 900

Conv. 1 Rectifying1 < 900

Conv. 1 Inverting1 > 900

• There are two different modes of operation. Circulating current free

(non circulating) mode of operation Circulating current mode of operation

Non Circulating Current Mode Of Operation

• In this mode of operation only one converter is switched on at a time

• When the converter 1 is switched on,For 1 < 900 the converter 1 operates in the Rectification modeVdc is positive, Idc is positive and hence the average load power Pdc is positive.

• Power flows from ac source to the load

• When the converter 1 is on,For 1 > 900 the converter 1 operates in the Inversion modeVdc is negative, Idc is positive and the average load power Pdc is negative.

• Power flows from load circuit to ac source.

• When the converter 2 is switched on,For 2 < 900 the converter 2 operates in the Rectification modeVdc is negative, Idc is negative and the average load power Pdc is positive.

• The output load voltage & load current reverse when converter 2 is on.

• Power flows from ac source to the load

• When the converter 2 is switched on,For 2 > 900 the converter 2 operates in the Inversion modeVdc is positive, Idc is negative and the average load power Pdc is negative.

• Power flows from load to the ac source.• Energy is supplied from the load circuit to the

ac supply.

Circulating Current Mode Of Operation

• Both the converters are switched on at the same time.

• One converter operates in the rectification mode while the other operates in the inversion mode.

• Trigger angles 1 & 2 are adjusted such that (1 + 2) = 1800

• When 1 < 900, converter 1 operates as a controlled rectifier. 2 is made greater than 900 and converter 2 operates as an Inverter.

• Vdc is positive & Idc is positive and Pdc is positive.

• When 2 < 900, converter 2 operates as a controlled rectifier. 1 is made greater than 900 and converter 1 operates as an Inverter.

• Vdc is negative & Idc is negative and Pdc is positive.

Three Phase Controlled Rectifiers

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