DC Generators

Electric Generators
• In the first practical electric generators, called
dynamos, the AC was converted into DC with a
commutator, a set of rotating switch contacts on
the armature shaft.
• The initial electromagnetic generator (Faraday
disk) was invented by British scientist namely
Michael Faraday in the year 1831.
• The first type was a generator of direct current
(DC) electricity.
• The second type was a generator of alternating
current (AC) electricity.

• A DC generator is an electrical device used for
generating electrical energy.
• The main function of this device is to change
mechanical energy into electrical energy.
• There are several types of mechanical energy
sources available such as hand cranks, internal
combustion engines, water turbines, gas and
steam turbines.

CONSTRUCTION OF A DC
MACHINE
PRACTICAL DC MACHINE SECTIONAL VIEW OF DC MACHINE

YOKE
-Acts as outer frame of the machine
- Mechanical support
-Low reluctance path for magnetic flux produced by field winding
-High Permeability
-For Small machines yoke is made of Cast iron
-For Large Machines yoke is made of Cast Steel (Rolled steel)
Large DC machine Small DC machine

POLE CORES AND POLE SHOES
Pole core (Pole body):
Carry the field
Act as electromagnet
Fitted to yoke through bolts or welding
Pole shoe:
Reduce the reluctance of magnetic path
Pole shoes serve two purposes
(i) they support field coils and
(ii) spread out the flux in air gap uniformly.

POLE COILS
•Also known as field coils/ magnetizing coils
•made of copper
•Provide excitation
•Field coils are wound and placed on each pole and are connected in series
•They are wound in such a way that, when energized, they form alternate
North and South poles.

ARMATURE CORE
• Armature core is the rotor of the machine.
• It is cylindrical in shape with slots to carry armature winding.
• The armature is built up of thin laminated circular steel disks
for reducing eddy current losses.
• It may be provided with air ducts for the axial air flow for
cooling purposes.
• Armature is keyed to the shaft.

ARMATURE WINDING
•Winding is made of Copper (or) Aluminum
•Conductors are insulated and placed in the slots of the armature
core.
• The armature winding is the heart of the DC Machine.
•Armature winding is a place where conversion of power takes
place.

ARMATURE WINDING
• On the basis of connections the armature windings are
classified into two types:
– Lap Winding
– Wave Winding.

LAP WINDING and WAVE
WINDING
• The windings may be a lap winding or wave winding.
• The difference between the two consists in the arrangement of
the end of the connections at the front of the armature.
LAP WINDING WAVE WINDING

LAP WINDING
• In the lap winding the finish of each coil is connected
to the start of the next coil so that winding or
commutator pitch is unity.
• The lap winding may be progressive (it progresses in
the direction in which the coils are wound) or
retrogressive (it progresses in the opposite direction).
• Equalizing connections are necessary.
Advantages:
• This winding is necessarily required for large current
application because it has more parallel paths.
• It is suitable for low voltage and high current
generators.

WAVE WINDING
• In the wave winding the finish of each coil is
connected to the start of another coil well away from
the first coil.
• Equalizing connections are not necessary.
Advantages:
• For a given number of poles and armature
conductors, wave winding gives more EMF than the
lap winding.
• Suitable for small generators(500-600V).
• Wave winding is used for high voltage, low current
machines.

COMMUTATOR
•Physical connection to the armature winding is made through a commutator-brush
arrangement.
•The function of a commutator:
 in dc generator -to collect the current generated in armature conductors.
in dc motor -helps in providing current to the armature conductors.
•A commutator consists of a set of copper segments which are insulated from each
other.
•The number of segments is equal to the number of armature coils.
•Each segment is connected to an armature coil and the commutator is keyed to the
shaft.

BEARINGS AND BRUSHES
Brushes
•Made from carbon or graphite.
•They rest on commutator segments and slide on the segments when the
commutator rotates keeping the physical contact to collect or supply the current.
Shaft and bearings
•Made of mild steel with a maximum breaking strength.
•Used to transfer mechanical power from or to the machine.
•The rotating parts like armature core, commutator, cooling fans, etc. are keyed to
the shaft.

Rotor of a dc machine
https://www.youtube.com/watch?v=d_LOXUEFA-o
DC Machine Construction

*All the generators works on a principle of dynamically induced e.m.f.
Faraday's laws of electromagnetic induction
– the relationship between electric circuit and magnetic field.
– basic working principle of the most of the electric motor, generators, transformers, inductors
etc.
Faraday's first law:
Whenever a conductor is placed in a varying magnetic field an EMF gets
induced across the conductor and if the conductor is a closed circuit then induced
current flows through it.
Working Principle of DC
Generator

Faraday's second law of electromagnetic induction states that, the
magnitude of induced emf is equal to the rate of change of flux linkages
with the coil. The flux linkages is the product of number of turns and the
flux associated with the coil.
Phenomenon of Mutual Induction
Alternating current flowing in a coil produces alternating magnetic
field around it. When two or more coils are magnetically linked to each
other, then an alternating current flowing through one coil causes an
induced emf across the other linked coils. This phenomenon is called as
mutual induction.

Flemings Right Hand rule
As per Faraday's law of electromagnetic
induction, whenever a conductor moves
inside a magnetic field, there will be an
induced current in it. If this conductor
gets forcefully moved inside
the magnetic field, there will be a
relation between the direction of applied
force, magnetic field and the current.
This relation among these three
directions is determined by Fleming's
Right Hand Rule.
This rule states "Hold out the right hand
with the first finger, second finger and
thumb at right angle to each other. If
forefinger represents the direction of the
line of force, the thumb points in the
direction of motion or applied force,
then second finger points in the direction
of the induced current.

Working principle of DC Generator
 Let us consider a single turn coil ABCD
rotated on a shaft in a clockwise direction.
The coil is placed in a uniform magnetic
field, between a north pole(N) and south
pole(S). The blue color lines in the below
figure represent the magnetic flux lines.
 The coil sides AB and CD are connected to
the slip rings a and b respectively. From
the slip rings, the current is taken out and
given to the load through brushes 1 and 2.
 When coil sides AB and CD are moving
parallel to the magnetic field, the coil sides
do not cut the flux. Instead, they move
parallel to the flux. Hence, no emf is
induced in the coil.

• At this angle, the coil sides AB and CD are
perpendicular to the magnetic field. Now, the flux
linked with the coil is minimum but the rate of
change of flux is maximum. Hence, maximum emf
is induced in the coil at position 2.
• As the coil rotates further from θ = 90 to θ = 180,
the rate of change of flux linkage reduces steadily,
til position 4 is reached. Here, again the coil sides
become parallel to the magnetic flux lines. The
rate of change of flux linkage will be minimum. So
no emf is induced when it reaches position 4.
• Thus during the first half rotation of the coil from θ
= 0 to θ = 180, no emf is induced at position 0,
then increases and becomes maximum at position
2, then decreases and no emf is induced at
position 4.

• In the next half rotation of the coil, that is, from θ = 180 to
θ = 360, the variation in the emf induced is similar to that
of the first half rotation. No emf is induced at position 4,
then increases and becomes maximum at position 6, then
decreases, and no emf is induced at position 8.
• But the direction of current induced gets reversed in the
second half rotation. The path of current flow is from
DCFGBA. It is just the reverse of the current flow during the
first half rotation of the coil.
• If this rotation of the coil is continued, the change in emf is
repeated and becomes alternately positive and negative.
Such an emf is called as alternating emf. It has both positive
and negative half-cycles and is called a bidirectional output.

Working of DC Generator with split
rings or commutator
• Split rings are made up of conducting cylinder which is cut into two
segments insulated from each other by a thin layer of mica or some
insulating materials. The split rings are also called as a commutator.

• During the first half rotation of the coil, the current flows from
ABFGCD. The brush 1 will get in contact with the commutator
segment E and brush 2 will be in contact with the segment H. So the
current flows from F to G in the load.
• During the second half rotation of the coil, the current gets reversed
and the path of flow is DCFGBA. The brush 1 will get in contact with
the commutator segment H and brush 2 will be in contact with the
segment E. So the current flows from F to G in the load.
• For each half rotation of the coil, the commutator segments change their
position. The current through the load flows in the same direction, from F
to G. Thus the current thus produced is unidirectional but not continuous
like a pure DC current.

EMF EQUATION OF A
GENERATOR
Let  = flux/pole in Weber
Z =Total number of armature conductors
Z=No. of slot × No. of conductors/slot
P= No. of generator poles
A =No. of parallel paths in armature
N= Armature rotation in revolutions per minute (r. p. m)
E= e.m.f induced in any parallel path in armature
Eg= e.m.f generated in any one of the parallel paths
Average e.m.f generated/conductor = d volt
dt
Now, flux cut/conductor in one revolution d = P wb

EMF EQUATION OF A
GENERATOR
Time for one revolution , dt= 60 /N sec
According to Faraday’s Law of electro magnetic induction
E.M.F generated/conductor = d= PN volts
dt 60
No. of conductors (in series) in one parallel path= Z / A
E.M.F generated/path= PN × Z Volts
60 A
Generated E.M.F, Eg= Z N × P Volts
60 A
For i) Wave winding A = 2
ii) Lap winding A = P

Problems
1.4 pole wave wound armature has 720 conductor
and is related 1000 rev/min. If the useful flux is 20
mWb, calculated the generated voltage.
Generated E.M.F, Eg= Z N × P Volts
60 A
Eg=480 V

Problems
• An 8 pole lap connected has armature 4 slot
with 12 conductor an generate of voltage 500
V determine the speed at which running if the
flux per pole is 50 mWb.
Generated E.M.F, Eg= Z N × P Volts
60 A
• N = 1250 r.p.m

EXCITATION:
• The application of energy to something.
• DC generators are classified into two main categories based on
the method of excitation.
(i) Separately excited,
(ii) Self-excited.
Types of Generators
1. Separately excited:
In this type, field coils are energized from an independent
external DC source.
METHODS OF EXCITATION

TYPES OF GENERATOR
2. Self excited: (Voltage building process)
• In this type, field coils are energized from the current produced
by the generator itself.
•Initial emf generation is due to residual magnetism in field poles.
•The generated emf causes a part of current to flow in the field
coils, thus strengthening the field flux and thereby increasing emf
generation.
•Self excited dc generators can further be divided into three types -
(a) Series wound - field winding in series with armature winding
(b) Shunt wound - field winding in parallel with armature
winding
(c) Compound wound - combination of series and shunt winding

Voltage drop in the armature = Ia × Ra
(R/sub>a is the armature resistance)
Let, Ia = IL = I (say)
Then, voltage across the load, V = IRa
Power generated, Pg = Eg×I
E=V t+ I a Ra + V brush
Power delivered to the external load, PL = V×I.
SEPERATELY EXCITED DC GENERATOR

Ia = Isc = IL
E=V t+ I a Ra + I a Rse + V brush
Power generated, Pg = EgI
Power delivered to the load, PL = VIL
Shunt field current, Ish = V/Rsh
E=V t+ I a Ra + V brush
Power generated, Pg= EgIa
Power delivered to the load, PL = VIL
SELF EXCITED DC GENERATOR- SHUNT GENERATOR
SELF EXCITED DC GENERATOR- SERIES GENERATOR

Series field current, Ise = IL
Shunt field current, Ish = (V+Ise Rse)/Rsh
Armature current, Ia = Ish + IL
E=V t+ I a Ra + I L R se + V brush
Power generated, Pg = Eg×Ia
Power delivered to the load, PL=V×IL
Shunt field current, Ish=V/Rsh
Armature current, Ia= Ise= IL+Ish
Series field current, Ise= IL+Ish
E=V t+ I a Ra + I a Rse + V brush
Power generated, Pg= EgIa
Power delivered to the load, PL=V×IL
LONG SHUNT DC COMPOUND GENERATOR
SHORT SHUNT DC COMPOUND GENERATOR

•In a compound wound generator, the shunt field is stronger than
the series field.
•When the series field assists the shunt field, generator is said to
be commutatively compound wound.
•On the other hand if series field opposes the shunt field, the
generator is said to be differentially compound wound.
CUMULATIVE
COMPOUND MOTOR
DIFFERENTIAL
COMPOUND MOTOR

CHARACTERISTICS OF DC
GENERATOR
• No load saturation or Open Circuit
Characteristic (O.C.C.) (E0/If) or
Magnetization characteristics
• Internal or Total Characteristic (E/Ia)
• External Characteristic (V/IL)

Separately excited DC Generator
Open circuit characteristics

1. Open circuit characteristic
It is also known as magnetic characteristic or no-load saturation
characteristic.
This characteristic shows the relation between generated emf at no load (E0)
and the field current (If) at a given fixed speed.
Same for both the DC generators.

2. Internal Characteristic
• This characteristic shows the relation between the on-load generated emf
(Eg) and the armature current (Ia).
• The on-load generated emf Eg is always less than E0 due to the armature
reaction.
• Therefore, internal characteristic curve lies below the O.C.C.
3. External Characteristic
• An external characteristic curve shows the relation between terminal
voltage (V) and the load current (IL).

EXTERNAL CHARACTERISTICS
SEPERATELY EXCITED DC GENERATOR

Self excited DC Generator
1. Open circuit characteristic
It is also known as magnetic characteristic or no-load saturation
characteristic.
This characteristic shows the relation between generated emf at no load (E0)
and the field current (If) at a given fixed speed.
Same for both the DC generators.

Self excited DC Generator
2. Internal Characteristic
• This characteristic shows the relation between the on-load generated emf
(Eg) and the armature current (Ia).
• The on-load generated emf Eg is always less than E0 due to the armature
reaction.
• Therefore, internal characteristic curve lies below the O.C.C.
3. External Characteristic
An external characteristic curve shows the relation between terminal
voltage (V) and the load current (IL).

INTERNAL and EXTERNAL
CHARACTERISTICS
SHUNT DC GENERATOR

CHARACTERISTICS
SERIES DC GENERATOR

CHARACTERISTICS
COMPOUND DC GENERTOR

Voltage Build up in DC Shunt
Generator

What is Critical Field Resistance of DC
Shunt Generator ?
• It is defined as the amount of field resistance
required to generate emf in the armature
winding.
(or)
• The critical field resistance is defined as the field
resistance of the generator which holds the rated
voltage of it.
(or)
• It is the value of field resistance beyond which
the generator fails to build up the voltage if there
is a further increase in the field resistance.

Significance of Critical Field Resistance
The total field circuit resistance must be equal to or less
than the critical field resistance value.

What is Critical Speed of DC Shunt
Generator ?
• Critical speed is defined as the speed at
which the given shunt field resistance is
equal to the critical resistance. It is the
speed at which the shunt generator just
fails to build up its voltage without any
external resistance in the field circuit. It is
denoted by Nc.

Significance of Critical Speed

Problem 1
A shunt generator delivers 450 A at 230 V and the resistance of the
shunt field and armature are 50 Ω and 0.03 Ω respectively. Calculate
the generated e.m.f?

Problem 2
• A long shunt compound generator delivers a load current
of50 A at 500 V, and the resistances of armature, series
and shunt fields are 0.05 ohm, 0.03 ohm, and 250 ohms
respectively. Calculate the generated emf, and armature
current. Allow 1.0 V per brush for contact drop.

• A long shunt compound generator delivers a load current
of 50 A at 500 V, and the resistances of armature, series
and shunt fields are 0.05 ohm, 0.03 ohm, and 250 ohms
respectively. Calculate the generated emf, and armature
current. Allow 1.0 V per brush for contact drop.

Problem 3
• The armature of a 4–pole, lap-wound shunt generator has
120slots with 4 conductors per slot. The flux per pole is
0.05 Wb. The armature resistance is 0.05 Ω and shunt
field resistance is 50 Ω. Find the speed of machine, when
supplying 450 A at a terminal voltage of 250 V.

• The armature of a 4–pole, lap-wound shunt generator has
120slots with 4 conductors per slot. The flux per pole is
0.05 Wb. The armature resistance is 0.05 Ω and shunt
field resistance is 50 Ω. Find the speed of machine, when
supplying 450 A at a terminal voltage of 250 V.

• https://www.electricaldeck.com/2021/03/criti
cal-field-resistance-critical-speed-of-dc-shunt-
generator.html

APPLICATIONS OF DC
GENERATORS
SEPARATELY EXCITED DC GENERATORS
• Used in laboratories for testing as they have a wide range of voltage output.
• Used as a supply source of DC motors.
SHUNT WOUND GENERATORS
• Used for lighting purposes.
• Used to charge the battery.
• Providing excitation to the alternators.
SERIES WOUND GENERATORS
• Used in DC locomotives for regenerative braking for providing field
excitation current.
• Used as a booster in distribution networks.
COMPOUND WOUND GENERATOR
• Over compounded cumulative generators are used in lighting and heavy
power supply.
• Flat compounded generators are used in offices, hotels, homes, schools, etc.
• Differentially compounded generators are mainly used for arc welding
purpose.

DC Generators

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