Basic Construction And Working Of A

DC Generator.

DC Generator
• A dc generator is an electrical machine which converts mechanical
energy into direct current electricity. This energy conversion is
based on the principle of production of dynamically induced emf.
Construction Of A DC Machine:
• Note: Theoretically, a DC generator can be used as a DC motor
without any constructional changes and vice versa is also possible.
Therefore, a DC generator or a DC motor can be broadly termed as
a DC machine. These basic constructional details are also valid for
both DC Generator and DC motor. Hence, let's call this point
as construction of a DC machine instead of just 'construction of a dc
generator'.
• The above figure shows constructional details of a simple 4-pole DC
machine. A DC machine consists of two basic parts; stator and rotor.
Basic constructional parts of a DC machine are described below.
• Yoke: The outer frame of a dc machine is called as yoke. It is made up
of cast iron or steel. It not only provides mechanical strength to the
whole assembly but also carries the magnetic flux produced by the
field winding.
• Poles and pole shoes: Poles are joined to the yoke with the help of
bolts or welding. They carry field winding and pole shoes are fastened
to them. Pole shoes serve two purposes; (i) they support field coils and
(ii) spread out the flux in air gap uniformly.
• Field winding: They are usually made of copper. Field coils are
former 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 a dc 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 (fixed) to the shaft.
• Armature winding: It is usually a former wound copper coil which
rests in armature slots. The armature conductors are insulated from
each other and also from the armature core. Armature winding can be
wound by one of the two methods; lap winding or wave winding.
Double layer lap or wave windings are generally used. A double layer
winding means that each armature slot will carry two different coils.

Armature core (rotor)

• Commutator and brushes: Physical connection to the armature
winding is made through a commutator-brush arrangement. The
function of a commutator, in a dc generator, is to collect the current
generated in armature conductors. Whereas, in case of a dc motor,
commutator 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 (or fixed) to the shaft.
• Brushes are usually 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.
Working Principle Of A DC Generator:

• According to Faraday’s laws of electromagnetic induction, whenever a

conductor is placed in a varying magnetic field (OR a conductor is moved in a
magnetic field), an emf (electromotive force) gets induced in the conductor.
The magnitude of induced emf can be calculated from the
emf equation of dc generator. If the conductor is provided with a closed path,
the induced current will circulate within the path.
• In a DC generator, field coils produce an electromagnetic field and the
armature conductors are rotated into the field. Thus, an electromagnetically
induced emf is generated in the armature conductors. The direction of induced
current is given by Fleming’s right hand rule.
• According to Fleming’s right hand rule, the direction of induced
current changes whenever the direction of motion of the conductor
changes. Let’s consider an armature rotating clockwise and a
conductor at the left is moving upward.
• When the armature completes a half rotation, the direction of motion
of that particular conductor will be reversed to downward. Hence, the
direction of current in every armature conductor will be alternating. If
you look at the above figure, you will know how the direction of the
induced current is alternating in an armature conductor.
• But with a split ring commutator, connections of the armature
conductors also gets reversed when the current reversal occurs. And
therefore, we get unidirectional current at the terminals.
Classifications Of DC Machines : (DC Motors And DC
Generators)
• Each DC machine can act as a generator or a motor. Hence,
this classification is valid for both: DC generators and DC motors.
• DC machines are usually classified on the basis of their field
excitation method. This makes two broad categories of dc machines;
(i) Separately excited and (ii) Self-excited.
• Separately excited DC machines: In separately excited dc machines,
the field winding is supplied from a separate power source. That
means the field winding is electrically separated from the armature
circuit.
• Self-excited DC machines: In this type, field winding and
armature winding are interconnected in various ways to achieve a wide
range of performance characteristics (for example, field winding in
series or parallel with the armature winding).
• In a self-excited type of DC generator, the field winding is energized by
the current produced by themselves. A small amount of flux is always
present in the poles due to the residual magnetism. So, initially, current
induces in the armature conductors of a dc generator only due to the
residual magnetism. The field flux gradually increases as the induced
current starts flowing through the field winding.
Self-excited machines can be further classified as –
• Series wound dc machines – In this type, field winding is connected in
series with the armature winding. Therefore, the field winding carries whole
of the load current (armature current). That is why series winding is
designed with few turns of thick wire and the resistance is kept very low
(about 0.5 Ohm).
• Shunt wound dc machines – Here, field winding is connected in parallel
with the armature winding. Hence, the full voltage is applied across the
field winding. Shunt winding is made with a large number of turns and the
resistance is kept very high (about 100 Ohm). It takes only small current
which is less than 5% of the rated armature current.
• Compound wound dc machines – In this type, there are two sets of
field winding. One is connected in series and the other is connected in
parallel with the armature winding. Compound wound machines are
further divided as -
• Short shunt – field winding is connected in parallel with only the armature
winding
• Long shunt – field winding is connected in parallel with the combination of
series field winding and armature winding

• According to this classification, a wide range of characteristics of DC

generators as well as of DC motors can be achieved.
EMF Equation Of A DC Generator
Consider a DC generator with the following parameters,

P = number of field poles

Ø = flux produced per pole in Wb (weber)
Z = total no. of armature conductors
A = no. of parallel paths in armature
N = rotational speed of armature in revolutions per min. (rpm)
• Now, Average emf generated per conductor is given by dΦ/dt (Volts) ...eq. 1
• Flux cut by one conductor in one revolution = dΦ = PΦ ….(Weber),
• Number of revolutions per second (speed in RPS) = N/60
• Therefore, time for one revolution = dt = 60/N (Seconds)
• From eq. 1, emf generated per conductor = dΦ/dt = PΦN/60 (Volts) …
(eq.2)
• Above equation-2 gives the emf generated in one conductor of the
generator. The conductors are connected in series per parallel path, and
the emf across the generator terminals is equal to the generated emf
across any parallel path.
• Therefore, Eg = PΦNZ / 60A
• For simplex lap winding, number of parallel paths is equal to the
number of poles (i.e. A=P),
Therefore, for simplex lap wound dc generator, Eg = PΦNZ / 60P
• For simplex wave winding, number of parallel paths is equal to 2 (i.e
P=2),
Therefore, for simplex wave wound dc generator, Eg = PΦNZ / 120
Characteristics Of DC
Generators
• The following three characteristics of DC generators are taken into
considerations:
(i) Open Circuit Characteristic (O.C.C.),
(ii) Internal or Total Characteristic and
(iii) External Characteristic.
Open Circuit Characteristic (O.C.C.) (E0/If)
• Open circuit characteristic 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.
• The O.C.C. curve is just the magnetization curve and it is practically
similar for all type of generators.
• The data for O.C.C. curve is obtained by operating the generator at no
load and keeping a constant speed.
• Field current is gradually increased and the corresponding terminal
voltage is recorded.
• For shunt or series excited generators, the field winding is
disconnected from the machine and connected across an external
supply.
• Now, from the emf equation of dc generator, we know that Eg = kɸ.
• Hence, the generated emf should be directly proportional to field flux (and
hence, also directly proportional to the field current).
• However, even when the field current is zero, some amount of emf is
generated (represented by OA in the figure below).
• This initially induced emf is due to the fact that there exists some residual
magnetism in the field poles.
• Due to the residual magnetism, a small initial emf is induced in the armature.
This initially induced emf aids the existing residual flux, and hence,
increasing the overall field flux.
• This consequently increases the induced emf. Thus, O.C.C. follows a straight
line. However, as the flux density increases, the poles get saturated and the ɸ
becomes practically constant.
• Thus, even we increase the If further, ɸ remains constant and hence, Eg also
remains constant. Hence, the O.C.C. curve looks like the B-H characteristic.
Internal Or Total Characteristic (E/Ia)

• An internal characteristic curve shows the relation between the on-

load generated emf (Eg) and the armature current (I a).
• The on-load generated emf Eg is always less than E 0 due to the
armature reaction.
• Eg can be determined by subtracting the drop due to demagnetizing
effect of armature reaction from no-load voltage E 0. Therefore,
internal characteristic curve lies below the O.C.C. curve.
External Characteristic (V/IL)
• An external characteristic curve shows the relation between terminal
voltage (Vt) and the load current (IL).
• Terminal voltage V is less than the generated emf Eg due to voltage
drop in the armature circuit.
• Therefore, external characteristic curve lies below the internal
characteristic curve.
• External characteristics are very important to determine the suitability
of a generator for a given purpose. Therefore, this type of
characteristic is sometimes also called as performance
characteristic or load characteristic.
Characteristics Of Separately Excited DC Generator
• If there is no armature reaction and armature voltage drop, the voltage
will remain constant for any load current.
• Thus, the straight line AB in above figure represents the no-load
voltage vs. load current IL.
• Due to the demagnetizing effect of armature reaction, the on-load
generated emf is less than the no-load voltage.
• The curve AC represents the on-load generated emf Eg vs. load current
IL i.e. internal characteristic (as Ia = IL for a separately excited dc
generator).
• Also, the terminal voltage is lesser due to ohmic drop occurring in the
armature and brushes. The curve AD represents the terminal voltage
vs. load current i.e. external characteristic.
Characteristics Of DC Shunt Generator
• To determine the internal and external load characteristics of a DC
shunt generator the machine is allowed to build up its voltage before
applying any external load.
• To build up voltage of a shunt generator, the generator is driven at the
rated speed by a prime mover. Initial voltage is induced due to residual
magnetism in the field poles.
• The generator builds up its voltage as explained by the O.C.C. curve.
When the generator has built up the voltage, it is gradually loaded with
resistive load and readings are taken at suitable intervals.
Connection arrangement is as shown in the figure below.
• Unlike, separately excited DC generator, here, I L≠Ia. For a shunt
generator, Ia=IL+If. Hence, the internal characteristic can be easily
transmitted to Eg vs. IL by subtracting the correct value of If from Ia.
• During a normal running condition, when load resistance is decreased,
the load current increases.
• But, as we go on decreasing the load resistance, terminal voltage also
falls. So, load resistance can be decreased up to a certain limit, after
which the terminal voltage drastically decreases due to excessive
armature reaction at very high armature current and increased I 2R
losses.
• Hence, beyond this limit any further decrease in load resistance results
in decreasing load current. Consequently, the external characteristic
curve turns back as shown by dotted line in the above figure.
Characteristics Of DC Series Generator
• The curve AB in above figure identical to open circuit characteristic
(O.C.C.) curve. This is because in DC series generators field winding
is connected in series with armature and load.
• Hence, here load current is similar to field current (i.e. I L=If). The
curve OC and OD represent internal and external characteristic
respectively.
• In a DC series generator, terminal voltage increases with the load
current. This is because, as the load current increases, field current
also increases.
• However, beyond a certain limit, terminal voltage starts decreasing
with increase in load. This is due to excessive demagnetizing effects of
the armature reaction.
Characteristics Of DC Compound Generator
• The figure shows the external characteristics of DC compound
generators.
• If series winding amp-turns are adjusted so that, increase in load
current causes increase in terminal voltage then the generator is called
to be over compounded. The external characteristic for over
compounded generator is shown by the curve AB in above figure.
• If series winding amp-turns are adjusted so that, the terminal voltage
remains constant even the load current is increased, then the generator
is called to be flat compounded. The external characteristic for a flat
compounded generator is shown by the curve AC.
• If the series winding has lesser number of turns than that would be
required to be flat compounded, then the generator is called to be
under compounded. The external characteristics for an under
compounded generator are shown by the curve AD.
Applications of DC Generators
• A few applications of DC generators are:
• The separately excited type DC generators are used for power and
lighting purposes.
• The series DC generator is used in arc lamps for lighting, stable
current generator and booster.
• DC generators are used to reimburse the voltage drop within Feeders.
• DC generators are used to provide a power supply for hostels, lodges,
offices, etc.