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Unit -2

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Lecture-1

Introduction to three phaseconverters

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3

3 Phase Controlled Rectifiers

Operate from 3 phase ac supply voltage.

They provide higher dc output voltage

as compared to 1-Phase controeedrectifier.

Higher dc output power.

Higher output voltage ripple frequency.

Filtering requirements are simplified forsmoothing out load voltage and loadcurrent.

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4

Extensively used in high power variable

speed industrial dc drives.

In three phase drives, the armature current

is mostly continuous and therefore the

motor performance is better compared to

that of single phase drive.

Contd..

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Three-phase Half wave Converter

Drives

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Contd

This type of converter is employed for

motors in the typical range of 10-50 HP.

This drive is impractical for most industrial

applications because supply currents

would contain D.C components.

With a three phase half wave converter

the armature circuit gives armature voltage

as

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Contd..

Where Vm is the peak of the phase voltage

of a star connected three phase A.C supply

If

then

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Lecture-2

Three - phase semi controlled

rectifier control of D.C.

separately excited motor

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Three - phase semi controlled rectifier

control of D.C. separately excited motor

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Contd

This is One quadrant drive i. e Only

1st quadrant operation is possible .

It is limited to application in the rangeof 15-50 HP

The field converter should also be a

single phase or three phase semi-converter

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Voltage across Load and current

through load for different firing angle

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Contd..

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Contd

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Contd..

In 3- phase semi converter, SCRs are

gated at interval of 1200 in a proper

sequence

At large firing angles the motor current

may be continuous or discontinuous,

depending on the current demand and

speed

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Contd..

A 3- phase semi converter has the

unique feature of working as a six

pulse converter for and as athree pulse converter for

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Torque-speed characteristics

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Contd..

If the motor current is continuous, then themotor armature voltage is given by

S d T Ch t i ti f

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Speed-Torque Characteristics for

continuous conduction mode for

different firing angle

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Lecture-3

Three phase fully controlled

rectifier control of d.c.

separately excited motor

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Three phase fully controlled rectifier

control of D.C. separately excited

motor

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21

Out put Voltage and current

waveform

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22

Contd

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23

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)

Contd

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24

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,

Contd

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Torque-speed characteristics

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Contd.. If the motor current is continuous, then the

motor armature voltage is given by

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TWO QUADRANT OPERATION OF A

3- PHASE FULL CONVERTER

Speed Torque Characteristics for

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continuous conduction mode for


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Lecture-5

Three - phase semi controlled rectifiercontrol of D.C. series motor

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Three - phase semi controlled

rectifier control of D.C. series motor

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Voltage across Load and current

through load for different firing angle

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Contd..

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Contd..

In 3- phase semi converter, SCRs are

gated at interval of 1200 in a proper

sequence

At large firing angles the motor current

may be continuous or discontinuous,

depending on the current demand and

speed

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Contd..

A 3- phase semi converter has the

unique feature of working as a six

pulse converter for and as athree pulse converter for

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Contd..

If the motor current is continuous, then themotor armature voltage is given by

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Variation of Output voltage with

firing angle

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Lecture-6

Speed torque Characteristics

For different values of firing

angle

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For different values of firing angle

In case of series motor field circuit is

connected in series with the armature and

motor terminal voltage is controlled by

semi converter or fully controlledconverter.

Series motors are particularly suited for

applications that require high startingtorque such as crane, hoists, elevators,

vehicles etc..

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Contd Inherently, series motor can provide

essentially constant power output andtherefore particularly suitable for tractiondrives

Speed control is very difficult with theseries motor because any change in theload current will immediately be reflected in

a speed change and hence , all speedcontrol systems will use separately excitedsystem.

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Contd

For series motor total resistanceR = (Ra +Rf ) which includes both seriesfield winding resistance and armature

winding resistance. Similarly, L = ( La + Lf) which includes

both series field winding inductance andarmature winding inductance .

Back emf is expressed by the equation,

e = Ka n .. (1)

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Contd..

The flux has two components , One

component a which is produced by the

armature current flowing through the

series field winding. The other componentres is due to the residual magnetism.

res is very small and can be assumed

constant. Therefore, = a + res (2)

C td

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Contd.

If linearity is assumed ,

a = Kf ia .(3)

Substituting eq (2) and eq (3) in eq (1), we ge

te = Ka ( Kfia+ res) n

= Ka Kfian + Ka res n --------------- (4) Average back emf is given by

E = Kaf Ia N + Kres N ---------- (5)

The torque developed by the motor is given byt= Ka ia --------------------- (6)

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Contd..

If the flux res is neglected, then fromeq(2), eq(3), and from eq(6)

t= Kaf ia2 --------------------(7)

Torque is therefore, developed in thesame direction for either direction ofcurrent .Hence, speed reversal in theseries motor can be achieved by reversing

either field winding or the armatureterminals but not both.

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Contd..

The average developed torque is

T = Kaf Ia2 ------------------ (8)

The voltage equation of the armaturecircuit is given by

va = R . ia + La . (dia / dt ) + e ------- (9)

The average armature voltage is given byVa = Ra . Ia + E ----------------- (10)

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Contd

Replacing the average value of back emf

(E) from eq(5)

Va = Ra . Ia + Kaf Ia N + Kres N ---- (11)

Now the speed from above equation

becomes

C td

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Contd

Neglecting the ripple in the motor current

T = Kaf Ia2 ------------- (13)

Therefore , From eq (11) Substituting the value of Ia in Eq(8)

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Contd..

Substituting the value of Ia

in Eq(12)

S d t Ch t i ti

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For different values of firing angle

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Lecture-7

Three - phase Fully controlledrectifier control of D.C. series

motor

Three - phase Fully controlled

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Three phase Fully controlled

rectifier control of D.C. series motor

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51

Out put Voltage and current

waveform

C td

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52

Contd

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53

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)

Contd

C td

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54

T2 is triggered at t = (/2 + ), T6 turns

off naturally as it is reverse biased as soonas T2 is triggered.

During the interval (/2 + ) to (5/6 + ),

T1 and T2 conduct together & the outputload 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,

Contd

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Contd.. If the motor current is continuous, then the

motor armature voltage is given by


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3- PHASE FULL CONVERTER

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Speed-Torque Characteristics

We know, for series motor

Replacing the value of Va for fully controlled converter


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Speed Torque Characteristics forcontinuous conduction mode for


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Lecture-8

Problems

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Problems

The speed of a 150 HP, 650V, 1750 rpm,separately excited D.C motor is controlled by 3-phase, full converter. The converter is operatingfrom a 3-phase, 460v, 50Hz supply. The ratedarmature current of the motor is 170A. The

motor parameters are Ra= 0.099 Ohms, La= 0.73mH, and Ka = 0.33 V/ rpm. Neglect the lossesin converter system. Determine

(a) No load speeds at firing angle = 00 and 300 .Assume that at no-load, the armature current is10% of the rated current and is continuous.

C td

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Contd..

(b) The firing angle to obtain rated speed of

1750 RPM at rated motor current. Also

compute the supply power factor.

(c) The speed regulation or the firing angle

obtained in part (b)

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