EE192221 DC Machines Ep1 FP

8/12/2019 EE192221 DC Machines Ep1 FP

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3 DC MACHINES

PART I : BASIC PRINCIPLES

192221 ELECTRICAL MACHINES I

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Contents

- Electromagnetic conversion

- Basic structure of electric machines

- Evolution of DC machines

- Armature windings

- Armature voltage

- Developed (Electromagnetic) torque

- Magnetization (Saturation) curve- Armature reaction

- Classification of DC machines

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Electromagnetic Conversion

GENERATOR

MOTOR

Electricalsystem

Mechanicalsystem

ELECTRICMACHINEvoltage

current(e,i)

torque

speed(T,n)

Energy flow

Coupling

Magnetic fields(,B,H)

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x x

x x

x xx x

x x

x x

x xx x

v

v

FB

F= electromagnetic force on an electric chargeq = electric charge (Coulomb)

v = speed (m/s)B = magnetic flux density (Tesla)

sinF qvB

( )F q v B

x x

x x

x xx x

F

q

Electromagnetic Force :

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x x

x x

x xx x

x x

x x

x xx x

+

-

vl

e

v

eB

Motional Voltage :

When a conductor moves in a magnetic field, voltage isinduced in the conductor.

sine vBl

( ).e v B l

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x x

x x

x xx x

x x

x x

x xx x

Fl

i

F

iB


When a current-carrying conductor is placed in a magnetic

field, the conductor experiences a mechanical force.

sinF iBl

( )F i l B

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v

e

B

F

i

B

Right Hand Rule(for Generator)

Left Hand Rule(for Motor)

Motion(v) Voltage(e)Current(i) Motion(f)

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Cylindrical Machine(uniform air gap)

Salient Pole Machine(nonuniform air gap)

Basic Structure of Electric Machines

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STATOR

The part of the machine does not move and normally is the

outer frame of the machine.

ROTOR

The part of the machine is free to move and normally is the

inner part of the machine.

ARMATURE WINDING

The winding in which voltage is induced.

FIELD WINDINGThe winding through which a current is passed to produce

the primary source of flux in the machine.

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x x

Field windings FieldwindingsArmature windings

DC MACHINE

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Synchronous MachineField windings

(DC current)

Armature windings(AC current)

Rotor Speed =s

AC MACHINE

Induction MachineArmature /Field windings

(AC current)

Rotor Speed >s Generator

Rotor Speed


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Construction of DC machine

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Evolution of DC Machines

b1b2

s1

s2

b1

b2s1

s2

1 2

ab s1 b1cd s2 b2

s1, s2 : slip ring

b1, b2 : carbon brush

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v

e

B

inde

v

v

B

B+

a

b+

d

c

Beab

B

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Right Hand Rule


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+

a

b

+

d

c

B

Vt

eab

b1

b2s1

s2

1 2

b c

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c1, c2 : commutator

b1, b2 : carbon brushb1

b2

c1 c2

b1

b2

ab c1 b1 or b2cd c2 b2 or b1

b1. x

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+

a

+

d

B

Vt

eab

1 2

b c

b1

b2

c1 c2

. x

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2 l 4 l


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2 poles, 4 slots

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21

V lt t t t


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Voltage at commutators

A 2-pole, 4-turn machine

A 2-pole, 6-turn machine

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abe

12e12,avgE

Single turn machine

Multi-turn machine (six-turn)

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3601 pole pitch = 180

o

o mdedp

=2

ed md

p

electrical degrees mechanical degrees

Electrical degree VS Mechanical degree

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Armature Windings

turn coil winding

There are 2 kinds of interconnection :

(1) Lap winding(2) Wave winding

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progressivewinding retrogressivewinding

LAP WINDING

WAVE WINDING

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progressive winding retrogressive winding

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(1) Lap winding

4 poles

16 coils

16 slots

16 com.4 brushes

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U ll d i di (f l i di )


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+ +

Unrolled winding (for lap winding)

a b c d e f g h i j k l m n o p a

1-1 2-2 3-3 4-4 5-5 6-6 7-7 8-8 9-9 10-1011-1112-1213-1314-1415-1516-16

++

1 2

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g o

ck

h p

d l

6

3

15

2

7

10

14

11

7 14 815

4 1 111230

E i l il i (f l i di )


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Equivalent coil representation (for lap winding)

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(2) Wave winding

4 poles

9 coils18 slots

9 com.

2 brushes

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Unrolled winding (for wave winding)


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+ +

Unrolled winding (for wave winding)

a f b g c h d i e a

1-1 6-6 2-2 7-7 3-3 8-8 4-4 9-9 5-5

+ + 33

E i l t il t ti (f i di )


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Equivalent coil representation (for wave winding)

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Lap winding Wave winding

Coil (C) even number odd/even number Slot (Nslot) C 2C

Commutator

(Ncom) CBrush (NB) p 2m

Coil pitch2/P

Commutatorpitch*

Current path

2( 1)c

Cy

p

cy m

a mp 2a m

p = the number of poles, C= the number of coils

m = 1 for simplex winding* + for progressive winding, - retrogressive winding35

A two pole lab wound dc machine


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A two-pole lab wound dc machine

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A six-pole lab wound dc machine


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A six pole lab wound dc machine

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Armature Voltage


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Armature Voltage

2 ( ) 2 ( )t m

e B lv B l r

te

2 ( )t me B l r ( )

2

pB

rl

m

vv

+

a

b +

d

c

eab( )B

( )B

( )B

ecdrr

l

N

S

38

mpe

Average induced voltage (per turn) :


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mt

pe

ta

NeE

a

a m a m

NpE K

a

2a

Np ZpK

a a

Average induced voltage (per turn) :

Armature voltage :

Machine constant :(Armature constant)

N= the number of turns of armature windinga = the number of current paths

Z= the number of conductors in slots

p = the number of poles

aE

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Developed (Electromagnetic) Torque


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Developed (Electromagnetic) Torque

ac c

If Bli Bl

a

c cT f r

2

a ac

pIT Bl r

a a


ae a a

N pIT K I

a

Torque (on each conductors) :

Average Torque :

Electromagnetic Torque : 2 ceT NT

aI

ci

fc

BB

fc

r

. x

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Magnetization (Saturation) Curve


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Magnetization (Saturation) Curve

y

y

p pg gr

p

p

' '

p

c g

F

R

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rE

aE

fI

rE

aE

fI

1 2n n

1n

2n

a a m aE K E

p f f f pF N I I F

constantn

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EX 1 A dc machine has the rating of 25kW, 250V, 1200 RPM.


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g , ,

Draw the magnetization curve of the dc machine at 1200 RPM

and 1000 RPM.The magnetization curve data at 1200 RPM are provided in thetable below :

If 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.0 1.2 1.4Ea,n1 5 80 136 173 200 219 232 242 250 260 265 270

Ea,n2 4.2 67 113 144 167 183 193 202 208 217 221 225

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The magnetization curve of the dc machine in ex.1t 1200 RPM d 1000 RPM


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at 1200 RPM and 1000 RPM

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Armature Reaction


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Armature reaction- The demagnetizing effects due to fluxes produced by thearmature winding.

- They are cause of field weakening and neutral plane

shifting.

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Neutral plane

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Neutral Plane Shifting


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-e+

-e+

-

e+

-e+

Shifted Neutral plane

SCI SCI

Arc & spark in brushes

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Field Weakening


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g

No load

No load

a aF F

a a

aE

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Effects of Armature Reaction on Pole mmf


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Solutions to the Problem of Armature Reaction


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(1) Interpoles or Commutating poles

- Reduce the effects of armature reaction near commutators.

(2) Compensating windings

- Reduce the effects of armature reaction near main poles.

Solutions to the Problem of Armature Reaction

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(1) Interpoles or commutating poles


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a

int

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(2) Compensating winding


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Cancellation of AR-mmf and Compensating mmf


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Classification of DC Machines


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(1) Separately excited dc machine

(2) Self-excited dc machine

Shunt dc machine Series dc machine

Short Shunt

Long Shunt 55

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EE192221 DC Machines Ep1 FP