Electrical Machine II -lecture#1_introduction

EEE 214: Electrical Machines-II

Generation Scenario
 95% of total generation is obtained from
Synchronous Generator or Alternator.
 The generation voltages range from 10 kV to 20 kV.
 The size of the large generators ranges from 10 MW
to 2500 MW.
 In Bangladesh most of generators are gas-turbine
and steam-turbine generators.
 At present, a large number of small generators (in
the order of 10 MW) have been installed to meet the
emergency need.

Synchronous Generator
 Synchronous generators or alternators are synchronous
machines used to convert mechanical power to ac electrical
power.
 In a synchronous generator, a dc current is applied to the
rotor winding, which produces a rotor magnetic field.
 The rotor of the generator is then turned by a prime mover,
producing a rotating magnetic field within the machine.
 The rotating magnetic field induces a three-phase set of
voltages within the stator windings of the generator.

Salient and Non-salient Pole Rotor
 The rotor of a synchronous generator is essentially a large
electromagnet.
 The magnetic poles on the rotor can be of either salient or
non-salient construction.
 The term salient means “protruding” or “sticking out”.
 A salient pole is a magnetic pole that sticks out from the
surface of the rotor.
 On the other hand, non-salient pole is a magnetic pole
constructed flush with the surface of the rotor.

Stator and Rotor of a Syn. Gen.
Stator
Complete
Generator
Salient
Pole
Rotor
Non-
salient
Pole
Rotor

Construction of synchronous
machines
The rotor of a synchronous machine is a large electromagnet. The magnetic poles
can be either salient (sticking out of rotor surface) or non-salient construction.
Non-salient-pole rotor: usually two- and four-pole
rotors.
Salient-pole rotor: four
and more poles.
Rotors are made laminated to reduce eddy current losses.

machines
Salient pole with
field windings
Salient pole without
field windings –
observe laminations
A synchronous rotor with 8 salient poles

Construction of Synchronous
Machines
Two common approaches are used to supply a DC current to the field
circuits on the rotating rotor:
1. Supply the DC power from an
external DC source to the rotor by
means of slip rings and brushes;
2. Supply the DC power from a
special DC power source mounted
directly on the shaft of the
machine.
• Slip rings are metal rings encircling the shaft of a machine but insulated
from it.
• DC rotor winding is connected to each of the two slip rings on the
machine’s shaft.
• Graphite-like carbon brushes connected to DC terminals ride on each
slip ring supplying DC voltage to field windings

machines
Slip rings
Brush

machines
 Slip rings and brushes have certain disadvantages:
 increased friction and wear (therefore, needed maintenance),
 brush voltage drop can introduce significant power losses.
 On large generators and motors, brushless exciters are used.
 A brushless exciter is a small AC generator whose field circuits are
mounted on stator and armature circuits are mounted on the rotor.
 The exciter generator’s 3-phase output is rectified to DC by a 3-
phase rectifier (on rotor) and fed into the main DC field circuit.
 It is possible to adjust the field current on the main machine by
controlling the small DC field current of the exciter generator
(located on the stator).
 Since no mechanical contact occurs between the rotor and the
stator, exciters of this type require much less maintenance.

machines
A brushless
exciter: a low 3-
phase current is
rectified and used
to supply the field
circuit of the
exciter (located on
the stator).
The output of the
exciter’s armature
circuit (on the
rotor) is rectified
and used as the
field current of the
main machine.

machines
 To make completely
independent of any
external power
source, a small pilot
exciter is often
added to the circuit.
 It is an AC
generator with a
permanent magnet
mounted on the
rotor shaft
 And a 3-phase
winding on the
stator producing the
power for the field
circuit of the
exciter.

Voltage Induced in a Synchronous
Generator
 Rotor flux distribution - sinusoidal
α=wt
α
B
SinBB m
Position of
maximum
flux
Position of
minimum
flux
t=0

Generator
a
b
c
d
wm
Stator Coil
Stationary
Segment ab
The flux from the rotor
directly cuts the segment
“ab” at 900 and the quantity
B x v is in the direction of l,
So, eab = (v x B).l
= vBl
lSinBv m ][ 
Segment bc
For segment bc, the
quantity B x v is
perpendicular to l,
So, ebc=0
Rotor rotating

Generator
a
b
c
d
wm
Stator Coil
Segment dc
The flux from the rotor
directly cuts the segment
“cd” at 900 and the quantity
B x v is in the direction of l,
So, ecd = (v x B).l
= vBl
lSinBv m )]180([ 0
 
Segment ad
For segment ad, the
quantity B x v is
perpendicular to l,
So, ead=0

Voltage induced in a Synchronous
Generator
tSin
Sin
SinAB
SinBrl
SinBlre
sorvSince
SinlBv
SinlvBSinlBv
lSinBvlSinBv
eeeee
m
m
m
m
mind
m
mm
mm
addcbcabind



















)2(
2
,,
2
)(
0)]180([0][ 0
l
2r

Voltage Induced when multiple
conductor in a slot
 If there are Nc number of conductors (turns) in a
single slot and if the conductors are connected in
series then the induced voltage will be,
tSinNe cmind 

Voltage Induced for ‘P’ no. of Poles
me
ecmeind
P
polesofpairsPFor
tSinNe


)2/(
""


For ‘P’ number of pairs of poles. We can generalize the
equation of induced voltage as,

Rotation speed of synchronous
generator
 By the definition, synchronous generators produce electricity
whose frequency is synchronized with the mechanical
rotational speed.
120
m
e
n P
f 
Where fe is the electrical frequency, Hz;
nm is mechanical speed of magnetic field (speed for synchronous
machine), rpm;
P is the number of poles.
 Steam turbines are most efficient when rotating at high speed;
therefore, to generate 50 Hz, they are usually rotating at 3000
rpm and turn 2-pole generators.
 Water turbines are most efficient when rotating at low speeds
(200-300 rpm); so, they usually turn generators with many poles.

Voltage Induced in 3-phase machines
 If three set of coils are placed at 1200 apart, then the
expression for generated voltage is given by,
)240(
)120(
0
0



tSinEe
tSinEe
tSinEe
mc
mb
ma




An Exercise
 The following information is known about a simple
two-pole generator. The peak flux density of the
rotor magnetic field is 0.2T, and the mechanical rate
of rotation is 3000 rpm. The stator diameter of the
stator is 0.5 m, and its coil length is 0.3 m, and there
are 19 turns per coil. The machine is Y-connected.
(a) What is the three phase voltages of the generator as
a function of time?
(b) What is the rms phase voltage of this generator?
(c) What is the rms terminal voltage of this generator?

References
1. Stephen J. Chapman - Electrical Machinery Fundamental, 4th
Edition, McGrawHill Higher Education, 2005
2. A. E. Fitzgerald, Charles Kingsley, Jr. and Stephen D. Umans –
Electrical Machinery, 6th Edition, McGrawHill Higher Education,
2003
3. V. K. Mehta & Rohit Metha- Principles of Electrical Machines,
2nd Edition, S. Chand Publication, 2009.
4. Lecture Slides

Electrical Machine II -lecture#1_introduction

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