Chapter 4 Ee 342 Ac Fundamentals

7/27/2019 Chapter 4 Ee 342 Ac Fundamentals

1/22

Chapter 4 (P#230)

AC MachineryFundamentals

Pole

DC excitation

windingFan

Slip

rings


2/22

AC Generator (Field Fix Armature Rotating)

Will it work asAC Generatorif theArmature is

fixed and Field is rotating ?


3/22


4/22

4.1 A simple Loop in a uniform Magnetic Field (230)

(Voltage Induced in a simple rotating Loop (Page 231 ))

eind = ZBVLsin single conductor (Z=1)

eind = 2 BVLsin loop (2 conductors Z=2)

eind = maxsin t [4-11 ]Z

S

The voltage generated in the loop is a sinusoidal whose

magnitude is equal to the product of the flux inside the machine

and the speed rotating of the machine


5/22

4.1 A simple Loop in a uniform Magnetic Field (230)

(Torque Induced in a current-carrying Loop (Page 234 ))

F = ZBILsin single conductor (Z=1)

F = 2 BILsin loop (2 conductors Z=2)

ind = k Bloopx BS [4-20 ]

The torque induced in the loop is proportional to the strength of

the loops magnetic field, the strength of the external magnetic

field, and the sine of the angle between them.

r

N

S

F

F

X

X

Bloop

BS


6/22

4.2 (page 238) Rotating Magnetic Field

(Page 242 - The relationship between Electric Frequency and the

speed of Magnetic Field Rotation)

).(Pn

f me

344

120

nm = speed of magnetic field in rev/min

fe = electric frequency in Hz (Hertz cycle/sec)

P = Number of poles (North south are counted as 2-poles)

How the stator magnetic field be made to rotate?

The fundamental principle of ac machine operation is that:

IF a 3-phase set of currents, each of equal magnitude and

differing in phase by 1200, flows in a 3-phase winding, then it willproduce a rotating magnetic field of constant magnitude (1.5

times the peak value).


7/22

The three phase winding consists of three separate windings

spaced 1200 electrical apart around the surface of the machine

(Page 244)Reversing the Direction of Magnetic Field Rotation

If the current in any two of the three coils is swapped, the direction

of magnetic fields rotation will be reversed. This means that it is

possible to reverse the direction of rotation of an ac motor just byswitching the connections on any two of three coils.
http://en.wikipedia.org/wiki/File:3-phase_flow.gif


8/22

Page 241 Proof of the Rotating Magnetic Field

Next Few Slides Will Demonstrate The

Revolving Magnetic Field in The Stator Windings


9/22

ALTERNATOR (AC Generator or Synchronous Generators)

The elementary 3-phase 2-pole synchronous generator has a

stator equipped with 3 coils displaced 120o from each other;

although shown as concentrated, they actually are distributed.

When the rotor is excited with dc and rotated, the resultant

field will also rotate so that sinusoidal voltages are generated

in the 3 stator phases, displaced 120o in time and having a

frequency directly related to rotor speed.


10/22

Page 241 Proof of the Rotating Magnetic Field

A -C B -A C

0o 360o

-C

-B

CB

A

-A

-B

Stator winding


11/22

Phase A

magnetic axis

corresponding coil


12/22

Phase Bmagnetic axis

corresponding coil


13/22

Phase C

magnetic axis

corresponding coil


14/22


15/22

in

out

P

P)(Efficiency

in

lossesin

P

PP

lossesout

out

PP

P

in

losses

P

P 1

1. Electrical Losses or Copper Losses (I2R)

2. Core Losses=Hysteresis+Eddy Current Losses (Proportional to Speed & )

3. Mechanical Losses (Proportional to speed)

4. Stray Load Losses 1% of the Full Load rated Capacity (100 W 1 W)

Armature (Stator) Losses Field (Rotor) Losses ( Fixed IF )

AAA RIPlossArmature23 FFF RIP)Rotor(lossField

2


16/22

Pout=VIAcos=3VLILcos

Stray

Losses

1% of Rating

Mechanical

Losses

= Friction

+ WindageLosses

Core

Losses

= EddyCurrent

+ HystersisLosses

I2R Losses= Armature

+ Field

Losses

Mechanical

in= appm

Pout = Pin - Losses

indm

Pconv


17/22

StrayLosses

1% of Rating

Mechanical

Losses

= Friction

+ Windage

Losses

Core

Losses

= EddyCurrent

+ Hystersis

Losses

I2R Losses

= Armature

+ FieldLosses

Pout=VIAcos=3 VLILcos

Pout = Pin - Losses

indm

Pconv

Mechanical

Pout= indm


18/22

AC Generator are compared by their

VOLTAGE

POWER RATINGS

EFFCIENCIES

VOLTAGE REGULATIONS

AC Motors are compared by their:

SPEED

POWER RATINGS

EFFCIENCIES

SPEED REGULATIONS

FL

FLNLgulationReSpeed

FL

FLNL

V

VVgulationReVoltage

Assume the Motors are run

from a constant source

(VT), unless voltage change

is specified

Assume the Generators are are

run from a constant Speed

source called Prime Mover,

unless speed change is specified


19/22

END OF CHAPTER 4

NEXT CHAPTER 5

Synchronous Generators


20/22

http://www.ece.umn.edu/users/riaz/animations/spacevectors.html

http://www.coolmagnetman.com/magacmot.htm

M t G t


21/22

Motor Generator

RA

EA

RA

EA

IA= IL

+

-

+

-

+

-

+

-

IF

VTRA

EA

RA

EA

IA= IL

+

-

+

-

IF

LVT

Separately

&

shunt

M/R & G/R

)R+R(IE=V SAAAT

RA

EA

RA

EA

+

-

+

-

IA= IL=IF

VTVT

RA

EA

RA

EA

IA= IL=IF

RL

2

AI'K=

EA=K

VT=EAIARA

EA=K

T=KIA


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RA

EA

RA

EA

RA

EA

RA

EA

RA

EA

RA

+-

+

-

+

-

RA

EA

RA

EA

RA

EA

IA= IL

+

-

+

-

+

-

+

-

IF

T

IAIL
http://en.wikipedia.org/wiki/File:3-phase_flow.gif

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Chapter 4 Ee 342 Ac Fundamentals