Electrical Machines - I

EE8301 – ELECTRICAL MACHINES - I
Unit – IV – DC GENERATORS
By
Mr. D. Karthik Prabhu,
Assistant Professor,
Department of Electrical and Electronics Engineering.
Email: karthikprabhu@ritrjpm.ac.in
Cell: 9750588231 1

UNIT IV DC GENERATORS
Construction and components of DC Machine – Principle of
operation - Lap and wave windings-EMF equations– circuit model
– armature reaction –methods of excitation-commutation and inter
poles - compensating winding –characteristics of DC generators.

LECTURE 5 OF 40
APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS
DC MACHINES
+ +
+
FIELD
POLES
N
S
FIELD
WINDING
ARMATURE
CONDUCTORS
ARMATURE
YOKE
BRUSH
MAIN FIELD AXIS
BRUSH
AXIS + _
+
+
+
+
.
.
.
.
+
+

Constructional details of DC generator

LECTURE 5 OF 40
DC MACHINES
N
S

Te
A B
_
+ 









S N
( GENERATOR )
ELECTRICAL
LOAD
Tm

LECTURE 5 OF 40
DC MACHINES
N
S

Te
A B
_
+ 








S N
TL
( MOTOR )
v
DC
SUPPLY

MAIN CONSTRUCTIONAL FEATURES
LECTURE 7 OF 40
DC MACHINES
3. FIELD or EXCITING COILS
1. BODY OR MAGNETIC FRAME OR YOKE
2. POLE CORE AND POLE SHOES
4. ARMATURE CORE
5. ARMATURE WINDING
6. COMMUTATOR

LECTURE 7 OF 40
DC MACHINES
9. BEARINGS
7. BRUSHES
8. END HOUSINGS
10. SHAFT

LECTURE 7 OF 40
DC MACHINES
Body / Yoke
Field Winding
Shaft
Commutator
Armature
Pulley
Brush
Brush
holder
Field Core
Bearing
Click here to see photograph
End Housing

LECTURE 7 OF 40
DC MACHINES
+
-
YOKE
ARMATURE
COMMUTATOR
SHAFT
BRUSH
FIELD POLE
& COIL

LECTURE 7 OF 40
DC MACHINES
The outer cylindrical
frame to which main
poles and inter poles are
fixed and by means of
the machine is fixed to
the foundation is called
YOKE.
1. MAGNETIC FRAME or YOKE :

LECTURE 7 OF 40
DC MACHINES
It serves two purposes:
a) It provides
mechanical protection
to the inner parts of the
machines.

LECTURE 7 OF 40
DC MACHINES
b) It provides a low
reluctance path for the
magnetic flux.
The yoke is made of
cast iron for smaller …

LECTURE 7 OF 40
DC MACHINES
machines and cast steel
or fabricated rolled
steel for larger
machines.

LECTURE 7 OF 40
DC MACHINES
The pole core and pole
shoes are fixed to the
yoke by bolts. They
serves the following
purpose :
a) They support
the field or exciting
coils.
2. POLE CORE AND POLE SHOES :
POLE CORE

LECTURE 7 OF 40
DC MACHINES
b) They distribute the
magnetic flux on the
armature periphery
more uniformly.

LECTURE 7 OF 40
DC MACHINES
c) The pole shoes have
larger X- section, so,
the reluctance of the
magnetic path is
reduced. The pole core

LECTURE 7 OF 40
DC MACHINES
and pole shoes are
made of laminated
steel assembled by
riveting together under
hydraulic pressure.

LECTURE 7 OF 40
DC MACHINES
Field coils or exciting
coils are used to
magnetise the pole core.
Enameled copper wire is
used for the construction
of these coils.When direct
3. FIELD or EXCITING COILS :

LECTURE 7 OF 40
DC MACHINES
current is passed through
these coils/ winding, it
sets up the magnetic field
which magnetise the pole
core to the reqd. flux.
3. FIELD or EXCITING COILS :

LECTURE 7 OF 40
DC MACHINES
Armature is a rotating part of the DC
machine, reversal of flux takes place,
so hysteresis losses are produced.
To minimise this loss, silicon steel is
used for the construction.
4. ARMATURE CORE:

LECTURE 7 OF 40
DC MACHINES
The rotating armature cuts the main
magnetic field , therefore an e.m.f is
induced in the armature core. This
e.m.f circulates eddy currents in the
core which results in eddy current loss
in it.
4. ARMATURE CORE:

LECTURE 7 OF 40
DC MACHINES
The armature core is laminated to reduce
the eddy current loss.
Armature core serves the following
purposes:
a) It houses the conductors in the slots.
b) It provides an easy path for magnetic
flux
4. ARMATURE CORE:

LECTURE 7 OF 40
DC MACHINES
The no. of
conductors in form
of coils placed in
the slots of the
armature and
suitably inter
connected are
called winding .
5. ARMATURE WINDING ;
ARMATURE WINDING

LECTURE 7 OF 40
DC MACHINES
This is the
armature winding
where conversion
of power takes
place i.e. in case
of generator ,

LECTURE 7 OF 40
DC MACHINES
mechanical power
is converted into
electrical power
and in case of a
motor, electrical

LECTURE 7 OF 40
DC MACHINES
power is
converted into
mechanical
power.

Commutator Pitch:
The Commutator pitch is the number of
Commutator segments spanned by each coil
of the winding.
Pole Pitch:
It is the distance measured in terms of
number of armature slots (or armature
conductors) per pole.
Coil Span (or) Coil Pitch:
It is the distance measured in terms of the
number of armature slots ( or armature
conductors) spanned by a coil.

Full Pitched Coil:
If the coil span or coil pitch is equal to pole
pitch, it is called full-pitched coil.
Fractional Pitched Coil:
If the coil span or coil pitch is less than the
pole pitch, then it is called fractional
pitched coil.

LECTURE 7 OF 40
DC MACHINES
Depending upon the types of inter
connection. of coils , the winding can
be classified into two types;
i) Lap Winding;
The conductors/coils are
connected in such a way that no of
parallel paths are equal to no. of poles.

LECTURE 7 OF 40
DC MACHINES
If machine has ‘P’ no. of poles and ‘Z’
no. of conductors, then there will be ‘P’
no. of parallel paths.And each path will
have ‘Z/P’ no of conductors in series.
Also the no. of brushes are equal to no.
of parallel paths.

LECTURE 7 OF 40
DC MACHINES
Out of which half of the brushes will be
positive and remaining will be negative.
ii) Wave Winding;
The conductors are so connected
that they are divided into two parallel
paths only , irrespective of the no. of
poles. If machines has ‘Z’ no. of …

LECTURE 7 OF 40
DC MACHINES
conductors, there will be only two
parallel paths and each will be having
‘Z/2’ no. of conductors connected in
series with only two brushes.
Click here to study detailed contents of winding

LECTURE 7 OF 40
DC MACHINES
6. COMMUTATOR
It is the most
important part of
a DC machine and
serves the
following purpose
:- i) It connects …

LECTURE 7 OF 40
DC MACHINES
6. COMMUTATOR
the rotating
armature
conductors to the
stationary external
circuit through the
brushes.

LECTURE 7 OF 40
DC MACHINES
ii) It converts
altering current
induced in the
armature
conductors into
unidirectional …..
COMMUTATOR
6. COMMUTATOR

LECTURE 7 OF 40
DC MACHINES
6. COMMUTATOR
current in the
external load
circuit in
generating action
and it converts
alternating torque
into unidirectional COMMUTATOR

LECTURE 7 OF 40
DC MACHINES
6. COMMUTATOR COPPER SEGMENT
RISER
END RING
ADJUSTING NUT
METAL SLEEVESHAFT
MICA INSULATION

LECTURE 7 OF 40
DC MACHINES
6. COMMUTATOR
torque produced in the armature in
motoring action.
The commutator is of cylindrical
shape and is made of wedge shaped
hard drawn copper segments. The
segments are insulated from each ….

LECTURE 7 OF 40
DC MACHINES
6. COMMUTATOR
other by a thin sheet of mica. The
segments are held together by means
of two V-shaped rings that fit into the
V-grooves cut into the segments.
Each armature coil is connected to
the commutator segment through
riser.

LECTURE 7 OF 40
DC MACHINES
7. BRUSHES
Brushes are made of high grade carbon.
They form the connecting link between
armature winding and the external
circuit. The brushes are held in
particular position around the
commutator by brush holders.

LECTURE 7 OF 40
DC MACHINES
8. END HOUSINGS
They are attached
to the ends of
main frame and
support bearing .
The front housing
supports ….. END HOUSING

LECTURE 7 OF 40
DC MACHINES
8. END HOUSINGS
the bearing and
the brush
assembly whereas
rear housing
supports the
bearing only.
END HOUSING

LECTURE 7 OF 40
DC MACHINES
9. BEARINGS
The function of the bearing is to reduce
friction between the rotating and
stationary parts of the machines.These
are fitted in the end housings.
Generally, high carbon steel is used for
the construction of the bearings.

LECTURE 7 OF 40
DC MACHINES
10. SHAFT
The function of
shaft is to transfer
mechanical power
to the machine or
from the machine .
SHAFT

LECTURE 7 OF 40
DC MACHINES
Shaft is made of
mild steel with
maximum breaking
strength. All the
rotating parts like
SHAFT
10. SHAFT

LECTURE 7 OF 40
DC MACHINES
armature core,
commutator,
cooling fan etc. are
keyed to the shaft.
SHAFT
10. SHAFT
See Also

Lap winding Wave winding
Generated emf does not depend
on the number of poles
Generated emf depends upon
the number of poles
Number of parallel paths is
equal to number of poles
Number of parallel paths is
equal to always two
Number of brushes is equal to
number of poles
Number of brushes is equal to 2
No need of dummy coils Need of dummy coils
This winding is used to low
voltage and high current
machines
This winding is used to high
voltage and low current
machines
Lap winding requires equalizer
rings
Not required

Types of Armature winding
Lap winding
In this case, if connection is started from conductor in slot 1 then
connections overlap each other as winding proceeds, till starting point is reached
again.
Wave winding
In this type of connection, winding always travels ahead avoiding
overlapping. It travel like a progressive wave hence called wave winding. To get
an idea of wave winding a part of armature winding in wave fashion

Principle of Operation of DC Generator
• Basic principle of a dc generator is Faraday’s
law of electromagnetic induction i.e.,
whenever a conductor is moved in a magnetic
field, dynamically induced emf is produced in
that conductor.

B
Q
LOAD
A
B
AA
P

MAGNETIC FIELD
0o

B
Q
LOAD
A
B
A
P

MAGNETIC FIELD
_+
e
30o
t

B
Q
LOAD
A
B
A
P

MAGNETIC FIELD
+ _
e
60o
t

Q
LOAD
A
B
A

MAGNETIC FIELD
B P
+ _
e
90o
t

Q
LOAD
BA
A
A P

MAGNETIC FIELD
B
+ _
e
120o
t

Q
LOAD
A
B
A
A
P

MAGNETIC FIELD
B
+ _
e
150o
t

A
Q
LOAD
A
B
AB
P

MAGNETIC FIELD
+
e
180o
t

A
Q
LOAD
A
B
B
P

MAGNETIC FIELD
+ _
e
210o
t

A
Q
LOAD
B
A
B
P

MAGNETIC FIELD
+ _
e
240o
t

Q
LOAD
B
A
B

MAGNETIC FIELD
A P
+ _
e
270o
t

B
Q
LOAD
AB
AA
P

MAGNETIC FIELD
+ _
e
300o
t

B
Q
LOAD
B
A
AA
P

MAGNETIC FIELD
+ _
e
330o
t

B
Q
LOAD
B
A
AA
P

MAGNETIC FIELD
e
360o
t

E M F induced in a DC Generator
• let Ø be the flux per pole in webers
• let P be the number of poles
• let Z be the total number of conductors in the
armature
• All the Z conductors are not connected in
series. They are divided into groups and let A be
the number of parallel paths into which these
conductors are grouped.

• Each parallel path will have Z/A conductors in
series
• Let N be the speed of rotation in revolution
per minute (rpm)
• Consider one conductor on the periphery of
the armature. As this conductor makes one
complete revolution, it cuts PØ webers.
• As the speed is N rpm, the time taken for one
revolution is 60/N sec.
• Since the emf induced in the conductor is
equal to rate of change of flux cut.

• e α dØ/dt
= (PØ)/60/N
e = PNØ/60 volts
Since there are Z/A conductors in series in each
parallel path the emf induced
E g = (NPØ/60) (Z/A) volts
E g = PØZN/60A volts
• The armature conductors are generally connected
in two different ways, viz, lap winding and wave
winding. For lap wound armature A=P. In wave
wound machine, A = 2,always

Types of DC Generators
According to their methods of field
excitation, DC Generators are classified into
two types.
• Separately excited DC generator
• Self-excited DC generator

Separately excited DC generator

• I a = I L
• Ra = Resistance of the armature winding
• Terminal Voltage V = E g-Ia R a – V brush
• V brush = voltage drop at the contact of the brush
• Generally V brush is neglected because of very
low value
• Generally emf E g = V+ I a R a + V brush
• Electric power developed = E gIa
• Power delivered to load = VI a

Self-excited DC Generators
• Series generator
• Shunt generator
• Compound generator

• I a = I L = I se
• Generated emf E g = V+ I a R a + I se R se + V brush
Where,
V = terminal voltage in volts
Ia R a = voltage drop in the armature
Ia R se = voltage drop in the series field winding resistance
V brush = brush drop
• Terminal voltage V = E g-Ia R a - I a R se – V brush
• Power developed in the armature = E gIa
• Power delivered to load = VI a orV I L

• Terminal voltage V = E g-Ia R a
• Shunt field current Ish =V/ R sh
• Armature current I a = I L + I sh
• Power developed by armature = E gIa
• Power delivered to load = V I L

Compound Generator
• Long shunt compound generator
• Short shunt compound generator

Long shunt compound generator
• Series field current I se = I a= I L + I sh
• Shunt field current Ish = V / R sh
• Generated emf E g = V + I a (R a + R se) + V brush
• Terminal voltage V = E g – I a(R a+ R se) – V brush
• Power developed in armature = E g I a
• Power delivered to load = V I L

Short shunt compound generator

Short shunt compound generator
• Series field current = I se =I L
• Load current = I L
• Armature current I a= I sh + I se
• Generated emf E g = V + I a R a + I se R se + V brush
• Voltage across shunt field winding = Ish R sh
• I sh R sh = E g – I a R a– V brush
= V + I a R a + I se R se + V brush – I a R a–V brush
= V + I se R se

• Shunt field current Ish = (V+ I se Rse)/ Rsh
• Terminal voltage V = Eg –I a R a- I se Rse - V brush
• Power developed in armature = Eg I a
• Power delivered to load = V IL

Characteristics of DC Generator
There are three types of Characteristics.
1. Open circuit Characteristics (OCC) or
magnetisation Characteristics (E g Vs If)
2. Internal Characteristics or total
Characteristics (E g Vs Ia )
3. External Characteristics or voltage regulated
Characteristics (V Vs I L )

Separately Excited DC generator
Characteristics

Separately Excited DC generator
Characteristics
Internal and external Characteristics:

DC Shunt generator Characteristics

DC Shunt generator Characteristics
• Internal and external Characteristics

DC Series generator Characteristics

DC Compound Generator
Characteristics

Causes of failure to excite self excited Generator
1. Absence of residual magnetism due to ageing.
2. Generator is driven in opposite direction
3. Field resistance is more than critical resistance.
4. Wrong field winding connections. Due to this flux gets produced in
opposite direction to residual flux. So residual flux cancels the main
flux

Applications of DC Generators
• Shunt generators are used for supplying nearly
constant loads. They are used for battery
charging, for supplying the fields of synchronous
machines and separately excited DC machines
• Since the output voltage of a series generator
increases with load, series generators are ideal
for use as boosters for adding voltage to the
transmission line and to compensate for the line
drop.
• Compound generators maintain better voltage
regulation and hence find use where constancy of
voltage is required.

Applications of Various Generator
Separately excited generators:
Electro-plating, electro-refining of materials etc.
Shunt generators:
Battery charging and ordinary lighting purposes.
Series generators:
Boosters on DC feeders, Welding generator, arc lamps.
Cumulatively compound generators:
domestic lighting purposes
Differentially compound generators:
Electric arc welding.

Losses in a DC machine
• The losses in a dc machine is divided into three
classes.
1. Copper losses
2. Iron losses
3. Mechanical losses

Copper losses
1. Armature cu loss = I2
aRa
2. Shunt field cu loss = I2
shRsh
3. Series field cu loss = I2
seRse

Iron and Core losses
1. Hysteresis loss
2. Eddy current loss

Mechanical Losses
1. Friction loss e.g. bearing losses, brush friction
2. Windage loss i.e. air friction of rotating
armature

Constant and Variable losses
1. Constant losses:
(a) iron losses
(b) mechanical losses
(c) shunt field losses
2. Variable losses:
(a) cu losses in armature winding
(b) cu losses in series field winding
Total losses = constant losses + variable losses

Condition for maximum efficiency

Armature Reaction
In a dc generator, the purpose of field winding is to
produce magnetic field (called main flux) whereas
the purpose of armature winding is to carry
armature current. Although the armature winding is
not provided for the purpose of producing a
magnetic field, nevertheless the current in the
armature winding will also produce magnetic flux
(called armature flux). The armature flux distorts
and weakens the main flux posing problems for the
proper operation of the dc generator. The action of
armature flux on the main flux is called armature
reaction

Geometrical and Magnetic Neutral axes

Explanation of Armature Reaction

Conclusions
• With brushes located along G.N.A (i.e. θ = 0),
there is no demagnetising component of
armature reaction (Fd = 0). There is only distorting
or cross magnetising effect of armature reaction.
• With brushes shifted form G.N.A., armature
reaction will have both demagnetising and
distorting effects. This relative magnitude depend
on the amount of shift. This shift is directly
proportional to the armature curent.

Conclusions
• The demagnetising component of armature
reaction weakens the main flux. On the other
hand, the distorting component of armature
reaction distorts the main flux.
• The demagnetising effect leads to reduced
generated voltage while cross magnetesing effect
leads to sparking at the brushes.

Demagnetising and cross magnetising conductors

Calculation of demagnetising ampere-turns per pole

Calculation of cross magnetising ampere turns per pole

AT/Pole for compensating winding

Commutation
• The reversal of current in a coil as the coil
passes the brush axis is called commutation

Methods of Improving Commutation
• Resistance commutation
• E.M.F. commutation

E.M.F. Commutation
• The reversing voltage may be produced in the
following two ways.
1. By brush shifting
2. By using interpoles or compoles

E.M.F. Commutation
By brush shifting :
This method suffers from following drawbacks
• The reactance voltage depends upon armature
current. Therefore, the brush shift will depend on
the magnitude of armature current which keeps
on changing. This necessitates frequent shifting of
brushes.
• The grater the armature current, the greater must
be the forward lead for a generator. This increases
the demagnetizing effect of armature reaction
and further weakens the main field.

E.M.F. Commutation
Interpoles or compoles:

E.M.F. Commutation
• Interpoles or compoles:

Parallel operation of DC Generator
Conditions for Parallel Operation Of DC Generator
The terminal voltage must be the same.
The polarities of the generator must be identical.
The prime movers driving the armature of the generators must have similar and
stable rotational characteristics.
Necessity of parallel operation:
Continuity of service
Maintenance and repair
Efficiency
Increase In Plant Capacity

Electrical Machines - I

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Electrical Machines - I

Similar to Electrical Machines - I (20)

More from karthik prabhu

More from karthik prabhu (12)

Recently uploaded

Recently uploaded (20)

Electrical Machines - I