1 Alternator Reference (1) B.L.Thereja; Chapter: 37 (2) Rosenblatt; Chapter: 19.

1

Alternator

Reference (1) B.L.Thereja; Chapter: 37

(2) Rosenblatt; Chapter: 19

2

Physical Shape of Alternator

3

Rotor and Stator of Alternator

4

Working principle

In case of alternator (AC generator), armature is stationary and the field is rotating. When DC voltage is applied to the field then a constant magnetic flux is produced. At the presence of this constant magnetic flux, rotor is rotated through the prime over. When field (rotor) rotates, armature conductors are cut by the magnetic flux and hence an emf is induced in it. Since, the magnetic poles are alternately N & S, therefore an alternating emf is produced in the stator conductor.

5

Short-Pitch or Fractional-Pitch Winding

In most alternators, opposite conductors of a coil are less than one pole pitch i.e less than spanning over 1800.

This type of winding is known as short-pitch or fractional-pitch winding

B. L. Thereja; Art:37.11; page:1309

6

Advantages & Disadvantages of short-Pitch Winding

Advantages:They save copper of end connectionThey improve the wave-form of generated emf i.e generated emf can be made to approximate to a sine wave more easily and the distorting harmonics can be reduced or totally eliminated.Due to the elimination of high frequency harmonics, eddy current and hysteresis losses are reduced, thereby increasing the efficiency.

7

Disadvantages

The disadvantages of using short-pitch winding is that, the total voltage around the coils is somewhat reduced. Because the voltage induced in two sides of short-pitched coil are slightly out of phase. Therefore, the resultant vector sum is less than their arithmetic sum


8

Pitch FactorQ. What is pitch-factor? Derive the equation of pitch-factor. Ans: The pitch or coil span factor is defined as the ratio of actual coil voltage to the coil voltage for a full-pitch coil.Hence, pitch factor, KP = Vector sum of induced emf per coil/ Arithmetic sum of induced emf per coilLet, Es be the induced emf in each side of the coil.

Now consider the following figures:B. L. Thereja; Art:37.11; page:1309

9

1E 2E sE

sE

sE sE

sE2

If the coils are full-pitch, then the total induced emf in the coil according to Fig-1 is , E1=2ES

If the coil is short-pitched by an angle, then according to Fig-2 the resultant induce emf is

=

= =

Which is always less than 1.

cos2 2222 sss EEEE cos12 2 sE

2cos2.2 2

sE 2cos2

sE

2cos

22

cos2

1

2

s

s

p E

E

E

EK

Fig-1 Fig-2

10

Distributed Winding In each phase of an alternator if the coils are not concentrated or bunched in one slot, but are distributed in a number of slots, then such type of winding is called distributed winding.

Q. What is distributed winding and distribution factor?


11

Distribution or Breadth or Spread or Winding factor

The ratio of actual phase voltage to that which would exist for a concentrated winding is known as distribution factor. It is also known as “Breadth factor” or “Spread factor” or “Winding factor”. It is denoted by Kd

So, distribution factor, Kd = e.m.f with distributed winding / e.m.f with concentrated winding


12

Derivation of Distribution factor

E

Dc

A

B

m

r

sErE

2

Let, be the angular displacement between slots.

= 1800 / No. of slots = 1800 /nLet, m= No. of slots/phase/pole

= phase spread angleIf be the induced emf in one

slot,then total voltage induced in one polar group i.e arithmetic

sum=mNow, from OAM,

o

msE

sE

r

AMSin

2

2

rSinAM

22

rSinE sAMEAB s 2

N

2

m

M

r

B.L.Thereja; Art:37.12; Page:1311-1312; Fig:37.21

13

Derivation of Distribution factor So, arithmetic sum=

Now, from ONE,

So, vector sum,

So, distribution factor, Kd =

When is very small then, So, Kd

r

NEmSin

2

2

mrSinNE

22

mrSinE r

22

22.

rmSinrSinmmE s

NEEAE r 2

22

22

rmSin

mrSin

2

2

mSin

mSin

2

2

m

mSin

22 Sin

14

Equation of induced emf Let,

Z = No. of conductors or coil sides in series/phase

= 2T [ where T= No. of coils or turns per phase] P = No. of poles; N = Rotor Speed in r.p.m F = Frequency of the induced emf in HZ φ = Flux per pole in wb Kd = Distribution Factor

Kc = pitch or span factor

K f = Form factor=1.11

2

2

mSin

mSin

2cos


15

Equation of Induced e.m.fIn one revolution of the rotor each stator conductor is cut by a flux of φp. So, dφ= φp & dt = s. So, average induced

emf per conductor = Again,

average induced emf per conductor =

If there are z conductors in series/phase, then average emf/phase and R.M.S value of emf/phase= volt. If the coils are short-pitched & distributed, then actual induce emf = volt. If the alternator is star-connected then the line voltage is times the phase voltage.

N

60

p

fN

120

fp

fp

dt

d 2

120

60

fTfz 42 fT411.1

TfKK dc 44.4Tf44.4

3


6060NP

N

p

dt

d

16

Effect of Harmonics on Pitch & Distribution Factor

2

3cos

2

5cos

2cos

cK

Pitch Factor

--For Fundamental

--For 3rd Harmonics

--For 5th Harmonics2

32

3

mSin

mSin

2

2

mSin

mSinKd

Distribution Factor

Math: B.L. Thereja; Example:37.8,37.13

--For fundamental

--For 5th Harmonics

--For 3rd Harmonics

252

5

mSin

mSin

B. L. Thereja; Art:37.14; page:1313-1314

17

Alternator Characteristics

E

g

TVLOAd

aR aI

aX

0)( Taaaag VjXIRIE

= Generated Voltage

= Terminal Voltage

= Armature current

= Armature Resistance

= Armature reactance

TV

aR

aX

aI

gE

Rosenblatt Art:19.9, Fig:19.13; page:381

Taaag VjXRIE )(

1818

Effect of Power factor on Generated Voltage

aaXI

TVaaRI

For Lagging power Factor


sinTV

cosTV

aI

gE

22 sincos aaTaaTg XIVRIVE

1919

aaXI

aaRI

TVaI

gE

Generated Voltage For Unity Power Factor


22aaaaTg XIRIVE

2020

aaXI

aaRI

TV

aI

gE

Generated Voltage For Leading Power Factor


Rosenblatt; Math: Example:19.9, 19.10

22 sincos aaTaaTg XIVRIVE

cosTV

sinTV

1 Alternator Reference (1) B.L.Thereja; Chapter: 37 (2) Rosenblatt; Chapter: 19.

Documents

Transcript of 1 Alternator Reference (1) B.L.Thereja; Chapter: 37 (2) Rosenblatt; Chapter: 19.