Unit 3:- D.

C Generator

 Construction and operating principle of D.C generator:-

• A DC Generator is an electrical device which converts mechanical energy
into electrical energy. It mainly consists of three main parts, i.e. magnetic
field system, armature and commutator and brush gear. The other parts of a
DC Generator are magnetic frame and yoke, pole core and pole shoes, field
or exciting coils, armature core and windings, brushes, end housings,
bearings and shafts.
• The diagram of the main parts
of a 4 pole DC Generator or DC
Machine is shown below:

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 Magnetic Field System of DC Generator
• The Magnetic Field System is the stationary or fixed part of the machine. It
produces the main magnetic flux. The magnetic field system consists of
Mainframe or Yoke, Pole core and Pole shoes and Field or Exciting coils.
These various parts of DC Generator are described below in detail.
 Magnetic Frame and Yoke
• The outer hollow cylindrical frame to which main poles and inter-poles are
fixed and by means of which the machine is fixed to the foundation is
known as Yoke. It is made of cast steel or rolled steel for the large
machines and for the smaller size machine the yoke is generally made of
cast iron.
• The two main purposes of the yoke are as follows:-
1. It supports the pole cores and provides mechanical protection to the
inner parts of the machines.
2. It provides a low reluctance path for the magnetic flux.

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 Pole Core and Pole Shoes
• The Pole Core and Pole Shoes are fixed to the magnetic frame or yoke by
bolts. Since the poles, project inwards they are called salient poles. Each
pole core has a curved surface. Usually, the pole core and shoes are made
of thin cast steel or wrought iron laminations which are riveted together
under hydraulic pressure. The poles are laminated to reduce the Eddy
Current loss.
• The poles core serves the following purposes given below:
1. It supports the field or exciting coils.
2. They spread out the magnetic flux over the armature periphery more
uniformly.
3. It increases the cross-sectional area of the magnetic circuit, as a result,
the reluctance of the magnetic path is reduced.

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 Field or Exciting Coils
• Each pole core has one or more field coils (windings) placed over it to
produce a magnetic field. The enamelled copper wire is used for the
construction of field or exciting coils. The coils are wound on the former
and then placed around the pole core.
• When direct current passes through the field winding, it magnetizes the
poles, which in turns produces the flux. The field coils of all the poles are
connected in series in such a way that when current flows through them,
the adjacent poles attain opposite polarity.
 Armature of DC Generator
• The rotating part of the DC machine or a DC Generator is called the
Armature. The armature consists of a shaft upon which a laminated
cylinder, called Amature Core is placed.
 Armature Core
• The armature core of DC Generator is cylindrical in shape and keyed to the
rotating shaft

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• At the outer periphery of the armature has grooves or slots which
accommodate the armature winding as shown in the figure below:

• The armature core of a DC generator or machine serves the following

purposes.

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• It houses the conductors in the slots.
It provides an easy path for the magnetic flux.
• As the armature is a rotating part of the DC Generator or machine, the
reversal of flux takes place in the core, hence hysteresis losses are
produced. The silicon steel material is used for the construction of the
core to reduce the hysteresis losses.
 Armature Winding:-
• The insulated conductors are placed in the slots of the armature core. The
conductors are wedged, and bands of steel wire wound around the core
and are suitably connected. This arrangement of conductors is called
Armature Winding. The armature winding is the heart of the DC Machine.
• the windings are classified into two types named as Lap Winding and
Wave Winding.
• Lap Winding:-In lap winding, the conductors are connected in such a way
that the number of parallel paths is equal to the number of poles. Thus, if
a machine has P poles and Z armature conductors, then there will be P
parallel paths, each path will have Z/P conductors connected in series.

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 Wave Winding:-
• In wave winding, the conductors are so connected that they are divided
into two parallel paths irrespective of the number of poles of the machine.
Thus, if the machine has Z armature conductors, there will be only two
parallel paths each having Z/2 conductors in series. In this case number of
brushes is equal to two, i.e. number of parallel paths.
 Commutator in DC Generator:-
• The commutator, which rotates with the armature, is cylindrical in shape
and is made from a number of wedge-shaped hard drawn copper bars or
segments insulated from each other and from the shaft. The segments
form a ring around the shaft of the armature. Each commutator segment
is connected to the ends of the armature coils.
• It is the most important part of a DC machine and serves the following
purposes.
1. It connects the rotating armature conductors to the stationary external
circuit through brushes.
2. It converts the induced alternating current in the armature conductor
into the unidirectional current in the external load circuit in DC Generator
action, whereas it converts the alternating torque into unidirectional
(continuous) torque produced in the armature in motor action.
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 Brushes
• Carbon brushes are placed or mounted on the commutator and with the
help of two or more carbon brushes current is collected from the
armature winding. Each brush is supported in a metal box called a brush
box or brush holder. The brushes are pressed upon the commutator and
form the connecting link between the armature winding and the external
circuit.
 Bearings
• The ball or roller bearings are fitted in the end housings. The function of
the bearings is to reduce friction between the rotating and stationary
parts of the machine. Mostly high carbon steel is used for the construction
of bearings as it is a very hard material.
 Shaft
• The shaft is made of mild steel with a maximum breaking strength. The
shaft is 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.

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 Operating principle of D.C Generator
• When a conductor moves in a magnetic field, an emf is induced across the
conductor. This is the only basis on which each and every rotating electric
generator works (such as portable generators).

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• According to Faraday’s law of electromagnetic induction, when a
conductor links with a changing flux, it will have an induced emf across it.
• The value of induced emf across the conductor depends on the rate of
change of flux linkage with the conductor. The direction of the induced
emf in the conductor can be determined by Fleming’s Right Hand Rule.
This rule says that on your right hand if you stretch your thumb, first finger
and second finger perpendicular to each other, and if you align your right-
hand thumb along the direction of motion of the conductor in the
magnetic field, and first finger along the direction of magnetic field, then
you second finger indicates the direction of emf in the conductor.

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 E.M.F equation of D.C generator:-
• As the armature rotates, a voltage is generated in its coils. In the case of a
generator, the emf of rotation is called the Generated emf or Armature
emf and is denoted as Er = Eg. In the case of a motor, the emf of rotation is
known as Back emf or Counter emf and represented as Er = Eb.
• The expression for emf is same for both the operations, i.e., for Generator
as well as for Motor.
• Let,
P – number of poles of the machine
ϕ – Flux per pole in Weber.
Z – Total number of armature conductors.
N – Speed of armature in revolution per minute (r.p.m).
A – number of parallel paths in the armature winding.

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• In one revolution of the armature, the flux cut by one conductor is given
as:

• Time taken to complete one revolution is given as:

• Therefore, the average induced e.m.f in one conductor will be:

• Putting the value of (t) from Equation (2) in the equation (3) we will get

The number of conductors connected in series in each parallel path = Z/A.

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• Therefore, the average induced e.m.f across each parallel path or the
armature terminals is given by the equation shown below:

• Where n is the speed in revolution per second (r.p.s) and given as:

• Thus, it is clear that the induced emf is directly proportional to the speed
and flux per pole. The polarity of induced emf depends upon the direction
of the magnetic field and the direction of rotation. If either of the two is
reversed the polarity changes, but if two are reversed the polarity remains
unchanged.

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 Types of D.C generator.
• DC Generator is classified according to the methods of their field
excitation.
• By excitation, the DC Generators are classified as Separately excited DC
Generators and Self-excited DC Generators.
• The self-excited DC Generators are further classified as Shunt wound DC
generators; Series wound DC generators and Compound wound DC
generators.
• The Compound Wound DC generators are further divided as long shunt-
wound DC generators, and short shunt-wound DC generators.
 Separately Excited DC Generator:-
• A DC generator whose field winding or coil is energised by a separate or
external DC source is called a separately excited DC Generator. The flux
produced by the poles depends upon the field current with the
unsaturated region of magnetic material of the poles. i.e. flux is directly
proportional to the field current. But in the saturated region, the flux
remains constant.

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• The figure of self-excited DC Generator is shown below:

Here,
• Ia = IL where Ia is the armature current and IL is the line current.
• Terminal voltage is given as:

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• If the contact brush drop is known, then the equation (1) is written as:

• The power developed is given by the equation shown below:-

• Power output is given by equation (4) shown above.

 Self Excited DC Generator:-
• Self-excited DC Generator is a device, in which the current to the field
winding is supplied by the generator itself. In self-excited DC generator,
the field coils may be connected in parallel with the armature in the
series, or it may be connected partly in series and partly in parallel with
the armature windings.

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• The self-excited DC Generator is further classified as
 Shunt Wound Generator:-
• In a shunt-wound generator, the field winding is connected across the
armature winding forming a parallel or shunt circuit. Therefore, the full
terminal voltage is applied across it. A very small field current Ish, flows
through it because this winding has many turns of fine wire having very
high resistance Rsh of the order of 100 ohms.
• The connection diagram of shunt-wound generator is shown below:

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• The shunt field current is given as:
•
• Where Rsh is the shunt field winding resistance.
• The current field Ish is practically constant at all loads. Therefore, the DC
shunt machine is considered to be a constant flux machine.
• Armature current is given as:

• Terminal voltage is given by the equation shown below:

• If the brush contact drop is included, the equation of the terminal voltage
becomes

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 Series Wound Generator:-
• A series-wound generator the field coils are connected in series with the
armature winding. The series field winding carries the armature current.
• The series field winding consists of a few turns of wire of thick wire of
larger cross-sectional area and having low resistance usually of the order
of less than 1 ohm because the armature current has a very large value
• Its convectional diagram is shown below:

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• Series field current is given as:

• Rse is known as the series field winding resistance.

• Terminal voltage is given as:

• If the brush contact drop is included, the terminal voltage equation is

written as:

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• The flux developed by the series field winding is directly proportional to
the current flowing through it. But it is only true before magnetic
saturation after the saturation flux becomes constant even if the current
flowing through it is increased.
 Compound Wound Generator:-
• In a compound-wound generator, there are two field windings. One is
connected in series, and another is connected in parallel with the
armature windings. There are two types of compound-wound generator.
1. Long shunt compound-wound generator
2. Short shunt compound-wound generator
1. Long Shunt Compound Wound Generator:-
• In a long shunt-wound generator, the shunt field winding is parallel with
both armature and series field winding. The connection diagram of the
long shunt-wound generator is shown below:

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• The shunt field current is given as:

• Series field current is given as:

• Terminal voltage is given as:

• If the brush contact drop is included, the terminal voltage equation is

written as:

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 Short Shunt Compound Wound Generator
• In a Short Shunt Compound Wound Generator, the shunt field winding is
connected in parallel with the armature winding only. The connection
diagram of a short shunt-wound generator is shown below.

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• Series field current is given as:

• The shunt field current is given as:

• Terminal voltage is given as:

• If the brush contact drop is included, the terminal voltage equation is
written as:

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 Armature reaction and commutation:-
• The armature reaction simply shows the effect of armature field on the
main field. In other words, the armature reaction represents the impact of
the armature flux on the main field flux. The armature field is produced by
the armature conductors when current flows through them. And the main
field is produced by the magnetic poles.
• The armature flux causes two effects on the main field flux.
1. The armature reaction distorted the main field flux.
2. It reduces the magnitude of the main field flux.
 Commutation:-
• The currents induced in the armature conductors of a DC generator are
alternating in nature. The change from a generated alternating current to
the direct current applied involves the process of Commutation. When the
conductors of the armature are under the north pole, the current which is
induced flows in one direction. While the current flows in the opposite
direction when they are under the south pole.

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• The term Commutation means the change that takes place in a winding
element during the period of a short circuit by a brush.
 Characteristics of D.C generator:-
• The characteristic of the DC generators explains the relations between the
loads, excitation and terminals voltage through the graph. Following are
the three important characteristics of a DC Generator.
 Magnetization Characteristic:-
• This characteristic gives the variation of generating voltage or no-load
voltage with field current at a constant speed. It is also called no-load or
open circuit characteristic.
 Internal Characteristic:-
• Internal characteristic of DC Generator plots the curve between the
generated voltage and load current.
 External Characteristics (Load Characteristics)
• External or load characteristics give the relation between the terminal
voltage and load current at a constant speed.

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 Characteristics of Separately Excited DC Generator :
• Magnetization(no-load) Characteristic:-
• It shows the relation between no-load generated emf Eo and the field
current at a given speed. It is also known as magnetization characteristics
or open-circuit characteristics (O.C.C.).
• No-load emf,

• Where,
K = constant = ZNP/60
Eo ∝ field current
φ ∝ field current
• It is clear from the above expression that when the field current is
increased by varying the potential divider from zero, the flux φ and hence
Eo increases.

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• The readings of Eo and field current should be tabulated and the relation
between the two should be plotted. The flux increases until
the poles become saturated, after that a greater increase in field current is
required to produce a given φ and hence Eo.

• That is why the upper portion of the curve bends over as shown. The
O.C.C. is useful in finding the required voltage, Eo of the generator at any
particular conditions such as no-load and speed.
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 Load or external characteristics:-
• Now the graph between V and load current should be plotted by curve ab.
This is known as load or external characteristic curve.
• It can be noticed from the graph that the voltage decreases slowly with an
increase in load current. While taking the readings the generator speed
should be maintained constant by varying the prime mover speed.
• The above discussion is as shown in fig below.

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 Internal characteristics:-
• The relation between the e.m.f. actually induced, E and the armature
current Ia gives the Internal characteristics.
• The voltage recorded at zero load current is Eo. It is parallel to abscissa (X-
axis) and is shown by dotted lines in the fig of load characteristics because
on no-load current is zero. Now determine Ia Ra drop at every reading and
add to the load curve. Thus the curve obtained is ac and is known as
internal characteristics.
 Characteristics of Self-excited DC Generators :
 Characteristics of DC Shunt Generator :
• In a dc shunt generator, the field winding is connected across (parallel) the
armature terminals and the armature current Ia produced is the sum of
field current Ish and load current IL.
 No-load Characteristics :
• No-load or open circuit characteristics of a shunt generator are obtained
by plotting ammeter reading (field current) i.e. Ish, versus voltmeter
reading E (generated voltage).
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• Under the no-load condition, the generator is said to be open-circuited and
rotating at a constant speed. From the EMF equation of dc generator, generated
voltage E is directly proportional to flux φ. As φ is directly proportional to field
current Ish, an increase in Ish also increases E.

• Also if there is no field current Ish at starting, we can notice some voltage E
in the armature. This is mainly due to magnetism present in the field
poles, known as residual magnetism. That is why the graph starts from
point A but not from origin 0.
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• Now as the field current Ish, flux φ, increases with an increase in voltage E.
But, after reaching certain current Ish the field gets saturated and
therefore φ and E remain constant as shown in the above figure.
 Load Characteristics :
• When a shunt generator is loaded, voltage builds up, then its terminal
voltage drops due to armature resistance and armature reaction. The
terminal voltage V and load current IL can be measured, the relation
between these V and IL can be drawn like (a-b), known as the external
characteristic curve.

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• If armature resistance drop components are added to the curve (a-b), we
get the internal characteristic curve (a-c), as shown.
• If armature reaction drop components are added to this curve ac we get
curve or line (a-d), which can be called no-load characteristics, but on no-
load the IL = 0. Therefore it is imaginary and hence shown with a dotted
line.
• If it is increased beyond the rated value, the terminal voltage decreases
rapidly due to increased armature reaction, represented by the curve (b-
e). It is known as drooping characteristics.
 Characteristics of DC Series Generator :
• In a dc series generator the field winding, armature, and load are
connected in series with each other i.e., IL = Ise = Ia.
 No-load Characteristics :
• The no-load characteristics of a dc series generator are similar to the no-
load characteristics of a shunt generator. But, at no-load condition, the
generator is running at a constant speed due to series connection no field
current flows as the load current IL, is zero. Hence it is difficult to obtain
no-load characteristics.
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• To draw the O.C.C. of a series generator the field winding should be given
a separate d.c. supply. The diagram below shows the O.C.C. of a separately
excited series generator.

• Now if the field current IL, increases the voltage E will also build up
proportionally. Here also the curve starts from point A due to residual
magnetism.

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 Load Characteristics :
• When the series generator is loaded it starts drawing the current. Here,
we know that IL = Ise = Ia. So as the load current IL increases e.m.f. induced
E also increases. The curve a-c between IL and E shows internal
characteristics. This curve lies below the curve of O.C.C. due to the
armature reaction effect.

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• The external characteristics are shown with a curve, it lies below the
internal characteristics due to a drop in armature and field windings. From
the curves obtained we notice that an increase in IL, increases E.
Therefore, the series generator characteristics are also called Raising
Characteristics.
 Characteristics of DC Compound Generator :
• A compound generator or compound wound generators is a combination
of both shunt and series wound generators. One winding is connected in
series and the other is connected in parallel with the armature.
 Load Characteristics :
• A shunt generator can be made to supply a constant voltage by connecting
a few turns in series with the armature terminals. This arrangement is
known as a compound generator. There are two types of connections for
connecting a series and shunt windings in a compound generator.
• Now, in a compound generator when load current IL increases. The current
Ise in series turns also increases which in turn produces more flux and
thereby generating more e.m.f. But due to increase an in armature current
Ia, there will be an increase in voltage drop due to armature resistance and
armature reaction.
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• If the series field turns are such as to produce e.m.f. equal to drop due to
armature resistance and armature reaction, then the generator is called
flat compounded or level compounded.

• If the series field turns produce e.m.f. more or less than the drop due to
armature resistance and reaction, then the generator is called over
compounded or under compounded respectively.
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 Applications of D.C generator:-
• The applications of the various types of DC Generators are as follows:-
 Separately Excited DC Generators:-
• Separately excited DC Generators are 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:-
• DC shunt-wound generators are used for lighting purposes.
• Used to charge the battery.
• Providing excitation to the alternators.
 Series Wound Generators:-
• DC series wound generators are used in DC locomotives for regenerative
braking for providing field excitation current.
• Used as a booster in distribution networks.
• Over compounded cumulative generators are used in lighting and heavy
power supply.
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• Flat compounded generators are used in offices, hotels, homes, schools,
etc.
• Differentially compounded generators are mainly used for arc welding
purpose.
This is all about applications of DC machines.

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