Transcript D.C. GENERATOR

D.C. GENERATOR
Instructor: Dr. ABDAL-RAZAK SH.
Materials covered from Chapter 2: Direct Current
Generators
 Working principle of a generator
 Practical DC generators
 Energy conversion in a DC generator
 Armature reaction in a DC generator
 Classification of DC generators
 Equivalent circuit of DC generators
 No-load characteristics of DC generators
 Load-characteristics and voltage regulation of DC
generators
DC machines
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Dc machines are either motors or
generators.
DC motors applications
DC motor applications
Generating an AC Voltage
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Fig. shows an elementary ac generator
composed of a coil that revolves at a
constant rpm between the N, S poles of
a permanent magnet
The rotation is due to an external
driving force, such as a motor, called
prime mover
The coil is connected to two slip rings
mounted on the shaft
The slip rings are connected to an
external electric load by means of two
stationary brushes x and y
As the coil rotates, a voltage is induced
according Electromagnetic principle
This voltage appears across the load via
slip ring and brush
The amount of flux cut by the
conductors depends on their position in
between the poles
The polarity voltage across the load
changes as the conductors move from N
pole to S pole
Direct-current Generator
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How can we get constant polarity
voltage?
In order to get a constant polarity
voltage, a commutator is used
A commutator in its simplest form
is composed of a slip ring that is
cut in half, with each segment
insulated from the other as well as
from the shaft
Now the voltage across the load is
still fluctuating but always has
same polarity
Practical DC Generators
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In a practical generator, there are a
number of conductors
This conductors are housed in the slot
of the armature
The terminals of the conductor are
connected to the terminals of other
conductors such that the connected
conductors create an winding in the
armature. Sometime armature
conductors are also called armature
coils
Usually there are two ways to connect
the terminals
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–
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One is called lap winding
The other is wave winding
The front terminals of the conductors
are connected to the commutator’s
segment
Elements
of
Armature
Winding
Elements of an armature windings
in DC Machines
A turn – two conductors connected to an end by an end connector
A coil – several turns connected in series
A winding – several coils connected in series
The angle between centers of adjacent poles is 180o (electrical)
180o electrical
= 90o mech
360o electrical
= 180o mech
N
S
S
N
The angle between centers of adjacent poles is 180o (electrical)
If coil sides are placed 180o electrical apart, the coil is said to be full-pitch
N
a
a
180oelec
S
b
N
S
b
The most common ways of connecting coils for armature windings:
Lap winding
Wave winding
Ends of the coils are connected to the commutator bars
In DC machines most of the coils are full-pitch.
Commutator pitch(Yc):Is the number of commutator segment spanned by each coil
Of the winding is denoted by (Yc)
Pole-pitch: Is the distance measured in terms of number of armature slots
(or armature conductors) per pole
𝑁𝑜.𝑜𝑓 𝑠𝑙𝑜𝑡𝑠
𝑁𝑜.𝑜𝑓 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟𝑠
Pole pitch=𝑁𝑜.𝑜𝑓 𝑝𝑜𝑙𝑒𝑠
or Pole pitch= 𝑁𝑜.𝑜𝑓 𝑝𝑜𝑙𝑒𝑠
Coil span or coil pitch(Ys): Is the distance measured in terms of number of armature slots
(or armature conductors) spanned by a coil.
FUTHER ARMATYRE
WINDING TERMINOLOGY
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BACK PITCH(YB): Is the distance measured in in terms of armature
conductors between two sides of a coil at the back of the armature.
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FRONT PITCH(YF): Is the distance measured in in terms of
armature conductors between two sides attached to any one commutator segment.
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RESULTANT PITCH(YR): Is the distance measured in
terms of armature conductors between the beginning of one coil and the beginning of the
next coil to which it is connected.
Progressive winding: YB >YF ; YC = +1
Retrogressive winding: YF >YB ; YC = -1
RELATIONS BETWEEN PITCHES FOR
SIMPLEXLAP WINDING
1)𝑌𝐵 = 𝑌𝐹 ± 2
𝑌𝐵 = 𝑌𝐹 + 2
𝑌𝐵 = 𝑌𝐹 − 2
𝑓𝑜𝑟 𝑝𝑟𝑜𝑔𝑟𝑒𝑠𝑠𝑖𝑣𝑒 𝑤𝑖𝑛𝑑𝑖𝑛𝑔
𝑓𝑜𝑟 𝑟𝑒𝑡𝑟𝑜𝑝𝑟𝑜𝑔𝑟𝑒𝑠𝑠𝑖𝑣𝑒 𝑤𝑖𝑛𝑑𝑖𝑛𝑔
2) Both YB and YF should be nearly equal to pole pitch.
3)Average pitch =
𝑌𝐵 +𝑌𝐹
2
. It equal to pole pitch(=Z/P)
4) Commutator pitch, YC = ±1
5) The resultant pitch (YR) is even being the arithmetical
difference of two odd number
6) Pole pitch = Z/P
YB =
𝑍
+1,
𝑃
YF=
𝑍
-1
𝑃
for progressive winding
FURTHER ARMATYRE
WINDING TERMINOLOGY
Lap winding
Commutator bar
• One coil between adjacent commutator bars
• 1/p of total coils are connected in series
• No. of poles  no. of brushes  no. of parallel paths
Wave winding
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p/2 coils in series between adjacent commutator bars
½ of all coils between brushes
Regardless of no. of poles, there are always 2 parallel path
The distance between end coils (commutator pitch) is 2(NC1)/p
where NC is the no. of commutator bars
SIMPLEX WAVE WINDING
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1) Both pitches YB and YF are odd and are of the same
sign
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2) Average
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3) YC=YA=
𝑍±2
pitch, YA=
𝑃
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑜𝑚𝑚𝑢𝑡𝑎𝑡𝑜𝑟 𝑠𝑒𝑔𝑚𝑒𝑛𝑡±1
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑖𝑟 𝑜𝑓 𝑝𝑜𝑙𝑒𝑠
Value of the Induced Voltage:
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The voltage induced in a dc
generator having a lap
winding is given by the
equation
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Eg=ZnΦ/60 Volt
Where
Eg=voltage between the
brushes [V]
Z=Total number of conductors
on the armature
Φ=Flux per pole [Wb]
Generator Under Load: The Energy conversion
Process
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When a load (e.g., a resistor) is connected between twoterminals of a DC generator, some fundamental flux
and current relationships take place
The current delivered to the load also flows through all
the armature conductors.
The current in the conductors under N pole will have
opposite direction of the current in the conductors
under S pole
Since these current carrying conductors are in a
magnetic field, there will be force on conductors
according to Lorentz’s law.
The individual forces F on different conductors all act
clockwise
In effect they produce torque that acts opposite to the
direction in which the generator is driven by a driver
e.g., a turbine.
Therefore, the prime mover or driver tends to slow
down and need more torque to keep the generator going
on in order to overcome this opposing torque. This is
conversion process helps to convert mechanical energy
to electrical energy
Excitation and Classification of DC
Generators
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Permanent Magnet Generator-PM is used to create magnetic fieldDifficult to have very strong PM and magnetic strength decays with
time, usually used for low power and high efficient Gen.
Excited Generator-Instead of PM, field coil is used to create the
magnetic field
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Self Excited-Current in the field coil comes from the generator itself
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Shunt-The field coil is connected in parallel with the armature
Series-The field coil is connected in series with the armature
Compound-It has two field coils; one is connected in series with armature and
the other is connected in parallel with the armature
Differential Compound-In this case the series and shunt fields are connected
as such the magnetic flux from these field coils cancel each other.
Separately excited-Current in the field coil come from different source
other than the generator itself
Equivalent Circuits of DC Generators:
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A voltage source with a value of E0
which is determined by the rotational
speed, number of conductors and flux
per pole.
Armature conductors or coils,
commutators and Carbon brush are
made of finite resistive element, there
is also losses in these elements.
Therefore, armature and its
conductors are represented by a
resistor Ra
The magnetic field for excited
generator is obtained from a field coil.
The coil has a resistance, Rf
Do not confused with winding and
inductance in case of DC machines!
Equivalent Circuits of DC Generators:
Series Generator
Shunt Generator
Text Book has used Voltage
source to show generator.
Compound Generator
LOSSES IN A D.C. MACHINE
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1- COPPERLOSSES
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2- IRON LOSSES
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ARMATURE Cu LOSS
SHUNT FIELD Cu LOSS
SERIES FIELD Cu LOSS
HYSTERESIS LOSS
EDDY CURRENT LOSS
3- MECHANICAL LOSSES
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FRICTION
WINDAGE
Hysteresis Loss
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Energy is spent when current changes its polarity every
half cycle. Power loss due to Hysteresis is given by the
equation:
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Ph= 𝐾ℎ V f 𝐵𝑛 𝑚𝑎𝑥
watts
kh - is a constant relating to the loop area (Steinmetz coefficient)
f - Frequency of magnetic reversals in Hz (NP/120)
n = 1.5 to 2.5 depending on the material
Bmax= Maximum flux density
V = volume of the material.
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Eddy Current Loss
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Again, is a loss in the magnetic core, that is in the iron
structure, caused by the induced voltages in the iron core.
Iron cores are laminated to reduce this loss. Laminated
cores offer a much higher resistance for the flow of eddy
currents in the core.
Pe= 𝑘𝑒(𝐵𝑚𝑎𝑥. 𝑡𝑓)2 *V watts
ke-constant that depends on the material( electrical
resistance)
t -thickness of laminations.
CONSTANT AND VARIABLE
LOSSES
1- COSTANT LOSSES
a- iron losses
b- mechanical losses
c- shunt field loss
2- VARIABLE LOSSES
a- copper loss in armature winding
b- copper loss in series field winding
POWER STAGES
CONDITION FOR MAXIMUM
EFFICIENCY
Armature Reaction
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In a loaded generator, the current
flowing in the armature coils also
creates a magnetic field or flux
which interacts with the poles’
magnetic field.
In fact the poles’ fields are
distorted and weakened by the
armature flux
This m.m.f. in the armature is
called as armature reaction
End effects
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Sparking in the brush
Resultant poles’ flux reduces and
voltage generated by the
generator decrease!
Armature Reaction -- Generator
Neutral Plane shifts in
the direction of rotation
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DEMAGNETISING AND CROSSMAGNETISING CONDUCTOR
CALCULATING OF DEMAGNETISNG
AMPERE-TURNS PERPOLE(ATd/POLE)
Let
Z=total number of armature conductors
I=current in each armature conductor
= Ia/2……..for simplex wave winding
= Ia/p…… for simplex lap winding
θm= forward lead in mechanical degrees
Total demagnetising armature conductors= conductor in angles AOC and BOD
4𝜃𝑚
×Z
360
1 4𝜃𝑚
𝑡𝑢𝑟𝑛𝑠 =
2 360
=
∴ 𝑇𝑜𝑡𝑎𝑙 𝑑𝑒𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑠𝑖𝑛𝑔 𝑎𝑚𝑝𝑒𝑟𝑒 −
× Z × 𝐼=
These demagnetising ampere turns are due to pair of poles
∴ 𝑑𝑒𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑠𝑖𝑛𝑔 𝑎𝑚𝑝𝑒𝑟𝑒 − 𝑡𝑢𝑟𝑛𝑠/𝑝𝑜𝑙𝑒 =
i.e.,
ATd/pole =
𝜃𝑚
360
×𝑍𝐼
𝜃𝑚
×
360
𝑍𝐼
2𝜃𝑚
360
×𝑍𝐼
CROSS-MAGNETISING AMPERE-TURNS
PER POLE(ATC/POLE)
Total armature reaction ampere-turns per pole =
𝑍/2
𝑃
×𝐼 =
Demagnetising ampere-turns per pole is given by ;
ATd/pole =
𝜃𝑚
360
×𝑍𝐼
∴ 𝐶𝑟𝑜𝑠𝑠 − 𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑠𝑖𝑛𝑔 𝑎𝑚𝑝𝑒𝑟𝑒 − 𝑡𝑢𝑟𝑛𝑠/𝑝𝑜𝑙𝑒 are
ATc/pole =
𝑍
2𝑃
×
=𝑍𝐼
𝜃𝑚
𝐼×𝑍
360
1
𝜃𝑚
−
2𝑃
360
𝐼
𝑍
2𝑃
×𝐼
COMPENSATING WINDING
Zc = No. of compensating conductors/pole face
Za = No. of active armature conductors
Ia = total armature current
Ia/A = current in each armature conductor
Zc Ia = Za Ia/A
Zc = Za/A
AT/pole for COMPENSATING
WINDING
No. of armature conductors/pole = Z/P
No. of armature turns/pole = Z/2P
No. of armature turns under pole face =
𝑍
2𝑃
AT/pole required for compensating winding =
𝑍𝐼
2𝑃
= Armature AT/pole×
×
×
𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒
𝑝𝑜𝑙𝑒 𝑝𝑖𝑡𝑐ℎ
𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒
𝑝𝑜𝑙𝑒 𝑝𝑖𝑡𝑐ℎ
𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒
𝑝𝑜𝑙𝑒 𝑝𝑖𝑡𝑐ℎ
Effect of Armature Inductance on Commutation
– No Load
Consider Coil #2
Induced Voltage
changes from
clockwise
to counter-clockwise
as the coil passes
from under the South
pole, through the
neutral plane, to under
the North pole
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Coil #2 Under the South Pole
Clockwise voltage
induced in coils under
the South pole
Counter-clockwise
voltage induced in
coils under the North
pole
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Coil #2 in the Neutral Plane
Coil #2 is shorted
out by the brush
No voltage is
induced
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Coil #2 Under the North Pole
Counter-clockwise
voltage induced in
the coil
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Under “Loaded” Conditions
Coils #2 and #3
supply current to the
load through the
commutator bar #3
and the brush
to LOAD
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Coil #2 in the Neutral Plane
Current in Coil #2
wants to go to zero, but
cannot change
instantaneously!
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Coil #2 Under the North Pole
As soon as commutator
bar #3 slides off the
brush, the current in coil
#2 is forced to zero
A large emf is induced
between commutator
bar #3 and the brush,
resulting in an “arc”
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Ideal Commutation
For Ideal Commutation, need the coil current in phase
with the coil voltage (no delay)
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METHODS OF IMPROVING
COMMUTATION
Resistance commutation
 E.M.F. commutation
By brush shifting
By using interpoles or compoles
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Interpoles – Generator Action
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Install narrow poles in
the Neutral plane
Only the coil
undergoing
commutation is affected
A neutralizing voltage
forces the reversal of
current in each coil as it
moves through the
region
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No-load operation and saturation curve:
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Voltage building up in a self excited
DC shunt generator
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Initially no voltage, no currents in the
field
However, there is a residual flux in
the field-very important !
This generates a voltage, this voltage
causes current flow in the field coil
If the flux due to the current in the
field is in the same direction, the total
field flux increases-very important!
The induced voltage then increases
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How much voltage will build up?
Infinite amount
Determined by the field resistance
as shown the graph
Controlling Voltage of Shunt Generator
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Recall E0=ZnΦ/60 Volt
In order to change E0, we can
change
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No. of conductors, Z, it is not
possible
rotational speed, n which may
not be feasible
flux per pole. This is usually used
in practice. A rheostat is
connected in series with the field
coil. If the value of the rheostat is
increased, field current decreases
and consequently the induced
voltage E0 decreases.
Voltage Regulation of a DC Shunt Generator:
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Terminal voltage of a self excited
shunt generator decreases as the
load current increases due to
armature reaction and armature
resistance drop. This is called as
the voltage regulation. This is bad
for most electrical loads. In a gold
mine, you are driving motor and
bulb from a generator.
Voltage regulation is defined as
(noload voltage-full load
voltage)/full load voltage
Compound Generators:
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In order to prevent compound generator is
developed to prevent voltage regulation
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The series field’s flux add with shunt field flux
If the load current increases, the shunt field
current decreases and thus shunt field flux
But the series field current increases and thus
the series field flux
Resultant flux remains almost same if number
of turn in series coil is selected properly to
compensate for voltage due to armature
resistance drop
Overcompound Generator-If the series coils
has comparatively higher number of turns
Differential compound generators-Series
field’s flux works in opposite direction of
shunt field flux. It is good for limiting short
circuit current as terminal voltage falls very
quickly in case of short circuit. Welding
applications
Load Characteristics of DC generators
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For differential compound terminal
voltage falls abruptly
For shunt terminal voltage falls quickly
For separate excitation it falls but as not
sharp as the shunt one
For compound, it falls slowly, better
voltage regulation and good for constant
voltage application
For over compounded, it increases
Generator Specifications
Generator Spec.
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Power (how much it can deliver)
Voltage (terminal voltage)
Exciting current (shunt field current)
Temperature rise
Speed
Type (series, shunt, compound..)
Class (for insulation)