microsoft powerpoint - chapter 2_partb [compatibility mode]

8/12/2019 Microsoft PowerPoint - Chapter 2_PartB [Compatibility Mode]

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SCHMY-MMU, Preliminary Meeting 23rdAugust 2002

5

Chapter 2 Part B

Line-Commutated Diode Rectifiers

Uncontrolled Rectifier (Natural Commutating Rectifier)

Controlled Rectifier (Phase Controlled Rectifiers)

Diode

V

t

Constant DC VoltageAC Power Source

Thyristor/SCR

V

t

Variable DC VoltageAC Power Source

AGAIN


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How a Thyristor Works ?

1. ANODE gets more positive voltage

Than Cathode

2. Gate Pulse

PN N P

A K

G

Single-phase, half-wave controlled rectifiers

with pure resistive load,

+

vS(t)

+ vthy(t)

R

i(t)

~

+

vR(t)

+

vo(t)

wt

vS

vS

vthy

vthy

vR

voi

i

0

0

wt

2

)cos1(22

+= VVo


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vL(t)

+

L

+

vS(t)

+ vthy(t)

R

i(t)

~

+

vR(t)

+

vo(t)

iG

Area A

Area B

wt

vS

vS

vthy

vthy

vRvo

vR

vo

i

i

0

0

wtp

2

(2)


with Inductive load,

)cos(cos2

2

+=

VV

o

Simulation

vL(t)+

L+

vS(t)

+ vthy(t) i(t)

~

+

V1(t)

+

vo(t)

iG

Area A

Area B

wt

vS

vS

vthy

vthy

vo

vo

i

i

0

0

wtp

2

1

V1

V1

vo

(3)


with Internal DC Voltage,


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Role of free-wheeling diode

To avoid negative voltage when inductive load involved.

vL(t)

+

L

+

vS(t)

+ vthy(t)

R

i(t)

~

+

vR(t)

+

vo(t)

iG

DArea A Area B

wt

vS

vS

vthyvthy

vo

vR

vo

i0

0

wtp

2

D on

LR discharge

purely natural response

Must area A = area B

During < wt < :

vL + vR= voDuring < wt < 2+:

vL

+ vR

= 0

i > 0 (exp decay)

i(t) is expressed in two

separate equations.


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Half Controlled Rectifier with R Load

0 2+

ig

v

Vo

Io

Average Load/Output Voltage

Average Load/Output Current Firing/trigger angle

Note

When= 0, The output is same as uncontrolled rectifier.

V = rms value

Half Controlled Rectifier with RL Load

0 2+

ig

v

Vo

IoAverage Load/Output Voltage

Average Load/Output Current

Firing/trigger angle

2+

Current decay to zero


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Full Controlled Rectifier with R Load



Firing/trigger angle

If = 0 ,The output will be same as a

Full Uncontrolled Rectifier that uses DIODE.

Full Controlled Rectifier with DC Motor Load



V

V

Io

Io

Vo

I

I

Vo

S1, S2 S3, S4S1, S2

For 0 < < 90

Ea = armature back emf.

+


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Full Controlled Rectifier with DC Motor Load



V

V

Io

Io

Vo

I

I

Vo

S1, S2 S3, S4

S1, S2

For 90 < < 180

Ea = armature back emf.

+

V2

VoFor fully controlled rectifier,

The DC Motor operates

in two modes.

1. Rectification

[As Motoring]

2. Inversion

[As Regenerative Braking]

V0 = positive

Ea = Positive

Io= positive

Power Flow (+ve) from input

AC to DC machine

V0 = positive

Ea = Positive

Io= positive


AC to DC machine

V0 = negative

Ea = negative

Io= positive

Power Flow (-ve) from

DC machine to AC supply

V0 = negative

Ea = negative

Io= positive



Converter Output Characteristics for continuous load current


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Review on Four Quadrants Operation

< 90

> 90

Review on Four Quadrants Operation Lift Application

< 90

> 90


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What do you mean with

Continues Load Current ?

What do you mean with

Continues Load Current ?

When you have an inductive load as in DC motor,

the load current is subject to the ratio of

Constant)Time(R

Lwhere =

R

L

Larger the ratio will produce almost a constant current,

On the other hand,

small ratio will produce ripple current

Lets look at an example next slide

Vo

Io

Vo

Io

Continuous

Load Current

Discontinuous

Load Current

Vo

Io

Continues and Discontinuous Load Current/Conduction


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Three-phase half-wave

Controlled Rectifier circuit with R load

Vphase

Vo

30

150


VAN= rms value of voltage phase A voltage

Vphase

Vo

Io

IS1

IS4

IA

S1 S3 S5IA

IO

S4 S6 S2

VAN VBN VCN

Vab Vac Vbc Vba Vca Vcb

Three-phase full-wave Controlled

Rectifier circuit with R load



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V63

VoFor fully controlled rectifier,

The DC Motor operates

in two modes.

1. Rectification

[As Motoring]

2. Inversion

[As Regenerative Braking]

V0 = positive

Ea = Positive

Io= positive


AC to DC machine

V0 = positive

Ea = Positive

Io= positive


AC to DC machine

V0 = negative

Ea = negativeIo= positive



V0 = negative

Ea = negativeIo= positive



Converter Output Characteristics for continuous load current

(Full Converter)

DC Motor Drives

Direct Current

Supply VoltageElectric Motor

Systems used formotion control

AC DC (Rectifier) when Input is AC

DCDC (DC Chopper) when Input is DC


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Control Rectifier Fed DC Drives

Control in either direction

Allow motor control in Q1 & Q4

Control in one direction ONLY

Allow motor control in Q1 ONLY

For Low power application, 10kW, Three phase rectifiers are used.


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Single phase Fully Controlled Rectifier Control of

DC Separately Excited Motor

tVv ms= sin

aa

a

aaa

IKT

KE

RIEV

=

=

+=

Basic equation applicable

to all DC Motors

armatureofspeed

poleperflux

constantmotor

currentarmature

motorbydeveloptorque

emfback

voltagearmature

=

=

=

=

=

=

=

a

a

a

K

I

T

E

V

A 200 V, 875 rpm, 150 A, separately excited dc motor

has an armature resistance of 0.06 . It is fed from a single phase

fully-controlled rectifier with an ac source voltage of 220 V, 50 Hz.

Assuming continuous conduction, calculate the following:

Firing angle for rated torque at 750 rpm.

Motor speed at = 160 and rated torque.

Exercise


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Part B of Chapter 2

microsoft powerpoint - chapter 2_partb [compatibility mode]

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