Veer Surendra Sai University of Technology

Burla, Odisha

Introduction to DC Machines

By
DR. RABINDRA KUMAR SAHU
Professor
Department of EEE, VSSUT, Burla

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 Overview of DC Machines
DC machine is one of the most commonly used machine
for electro-mechanical energy conversion.
DC machines are classified into two types such as (i) DC
generators, and (ii) DC motors.
(i) DC generator
The machine which produces DC electrical power is known
as DC generator.
An electric generator is a machine which converts
mechanical energy/power into electrical energy/power.
It works on the principle of Faraday’s laws of
electromagnetic induction, which states that “whenever a
conductor cuts the magnetic flux, an e.m.f. induced in it
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 Applications: DC generators are suitable for many
applications – including general lighting, battery charging,
giving excitation to the alternators, series arc lighting etc.
(ii) DC Motor
The machine which produces mechanical power is known
as DC motor.
An electrical motor is a machine which converts electrical
energy/power into mechanical energy/power.
It works on the principle of Lorentz Law, which states that
“the current carrying conductor placed in a magnetic and
electric field experience a force”
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 Applications: DC motors are suitable for many
applications – including conveyors, trolleys,
underground subway cars etc.
Any DC machine can act either as a generator or a
motor
A DC machine works as a DC generator when it is
driven by a prime mover.
The same machine works as a DC motor when
electrical energy is supplied to it.
Therefore, the constructional features of a DC
generator and DC motor are the same.
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 Construction of a DC Machine

Figure (1.1): Schematic diagram of a 4-pole DC Machine

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 DC machine consists the following main parts.
• Yoke
• Armature
• Armature winding
• Pole consists of pole core and pole shoe
• Field winding
• Commutator
• Brushes and Brush holder
• Inter poles
• Shaft and Bearings
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(i) Magnetic Frame or Yoke
• Outermost protecting cover for the DC machine.
• Circular steel ring, provides protection to all parts of the
machine from moisture, dust etc.
• Provides mechanical support to the field poles and necessary
magnetic path between the poles.
• Made up of with cast iron, cast steel, silicon steel, rolled steel
etc.

3/18/2024 Figure (1.2): Magnetic Frame or Yoke

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(ii) Armature core or Stack
• Rotating part (rotor) of the DC machine and is mounted on the
shaft.
• Cylindrical in shape with slots to carry armature winding.
• Made up of silicon steel laminations in order to reduce the
eddy current losses.
• Provided with air ducts for cooling purposes.

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Figure (1.3): Armature Core
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(iii) Armature winding
• Formed by placing copper coil/conductor in armature slots.
• 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.

3/18/2024 Figure (1.4): Armature Winding

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• It either generates or receives the voltage depending on
whether the unit is a generator or a motor.
• Made-up of with conducting materials like copper.
(iv) Poles and pole shoes
• Poles produce the magnetic flux when the field winding is
excited.

Figure (1.5): Field pole

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• Each pole is divided into two parts namely pole core and
pole shoe.
• Pole core is a part on which field winding is wound over.
• Pole shoe serves the following two functions:
(i) Distributes the magnetic flux uniformly in the air gap.
(ii) Supports the field winding.
• Poles are joined to the yoke with the help of bolts or
welding
• In modern design the pole is made-up of with thin
laminations of cast steel or cast iron.

(v) Field winding

• Field coils or field windings are located on the pole core
of the machine.
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• When the field winding excited, i.e. when the current is
passed through the field winding in a specific direction, it
sets up magnetic field (i.e. magnetize the poles) in the
machine.
• The field coils may be either shunt windings (in parallel
with the armature winding) or series windings (in series
with the armature winding) or a combination of both.
• Made up of with conducting materials like aluminum,
copper etc.

Figure (1.6): Field winding

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(vi) Commutator
• Commutator is a mechanical rectifier, which converts AC
voltage of the rotating conductors to DC voltage.
• Collects the current from the armature conductors and passes
it to the external load via brush.
• Cylindrical in structure and is made-up of copper or bronze.

Figure (1.7): Fully Assembled Commutator

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(vii) Brushes and brush holder
• Brushes conduct the current from the commutator to the
external circuit.
• Made-up of with material like carbon, graphite etc.
• A brush holder is usually a metal box that is rectangular in
shape.
• The brush holder has a spring that holds the brush in contact
with the commutator.

(viii) Inter poles

• Inter poles are similar to the main poles.
• These are connected between the yoke and main field poles.
• They have windings in series with the armature winding.
• Inter poles have the function of reducing the armature
reaction effect in the commutating zone.

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 Figure (1.8): Cut View of DC Machine
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(ix) Shaft and Bearings
• Shaft is made of mild steel with a maximum breaking
strength.
• Shaft is used to transfer mechanical power from or to the
machine.
• These are normally lubricated by grease or oil.
• The armature is mounted on a steel shaft, which is supported
between the two bearings.
• The bearings are either ball or roller type and 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.

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 Working principle of a DC generator

• An electric generator is a machine which converts mechanical

energy into electrical energy.
• All the generators work on a principle of dynamically induced
e.m.f. i.e. Faraday’s law of electromagnetism induction.
• It states that, whenever a conductor cuts the magnetic flux,
an e.m.f. induced; which will cause a current to flow if the
conductor circuit is closed.
• The direction of induced e.m.f and hence current is given by
Fleming’s right hand rule.
• Therefore, the essential components of a generator are: (i) a
magnetic field, (ii) conductor or a group of conductors, and (iii)
motion of the conductor w.r.t. magnetic field.
• Figure (1.9) shows a single loop rectangular copper coil
(ABCD) rotating about its own axis in a magnetic field
provided by either permanent magnets or electromagnets.

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 • The two ends of the coil are joined to two slip-rings which are
insulated from each other and from the central shaft.
• Two collecting brushes (carbon or copper) press against the
slip-rings; their function is to collect the current induced in
the coil to external load resistance.
• The rotating coil may be called armature and the magnets as
field system.

Figure (1.9)
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 • Figure (1.10) shows a
single loop coil forming
two sides AB and CD able
to rotate between north
pole ‘N’ and south pole ‘S’
of a permanent magnet.
Assume this as the
starting point and fixing
the direction of rotation
as clock-wise.
• In this position the
conductor coil sides ‘AB’
and ‘CD’ are parallel to
the magnetic flux and
Figure (1.10): Conductor moving
therefore does not cut the
parallel to the direction of flux (00 magnetic flux and the
position) induced voltage at this
instant is zero
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 • As the coil rotates from
the 00 position to 900 in
clockwise, the coil sides
begin to cut the magnetic
flux at a gradually
increasing rate and
conductor comes to the
position as shown in
figure (1.11).

• Hence the magnitude of

induced e.m.f. also
gradually increases and
becomes maximum when
Figure (1.11): Conductor moving the coil rotates by an
perpendicular to the direction of flux angle 900.
(900 position)
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 • As the coil rotates from
the 900 position to 1800,
again the coil sides AB
and CD are parallel to the
flux lines as shown in
figure (1.12).

• Under this condition the

flux linkage is gradually
decreases and hence the
induced e.m.f. also
gradually decrease and
becomes zero at 1800.

• At this point, the coil has

Figure (1.12): Conductor moving gone through a half-
parallel to the direction of flux (1800 revolution.
position)
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 • Again, as the coil rotates
from position 1800 to 2700
as shown in figure (1.13),
the induced e.m.f. starts
increasing from zero to
maximum and attains
maximum value at 2700,
but in opposite direction.

Figure (1.13): Conductor moving

perpendicular to the direction of flux
(2700 position)
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 • As the coil again moves
from 2700 to 3600 as shown
in figure (1.14), the induced
e.m.f. starts decreasing
from its maximum value
and reaches to zero again.

• So it completes the second

half revolution.

• Thus, during the second

half-revolution, coil sides
cut flux in directions
opposite to that which they
did in the first half
Figure (1.14): Conductor moving revolution, hence, the
parallel to the direction of flux (3600 polarity of the induced
position) voltage reverses.
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 • So from the above
discussion, it can be
concluded that the e.m.f.
induced in the armature
conductor of a DC generator
is alternating in nature as
shown in figure (1.15).

Figure (1.15): Sinusoidal wave

form of induced e.m.f. • It is seen that, the e.m.f.
induced in the conductors is
always sinusoidal and split
ring commutator converts
this sinusoidal e.m.f. into
unidirectional e.m.f. as
shown in fig.(1.16)
Figure (16): Wave form of unidirectional
current at the load
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Fleming's Right Hand Rule:
If the right hand is held with the thumb, fore finger and middle
finger mutually at right angles, as shown in the figure (1.17),
then
• The Thumb represents the direction of motion of the
conductor (F).
• The Fore finger represents the direction of Magnetic field (B).
• The Middle finger represents the direction of induced or
generated e.m.f/current (V or I).

Figure (1.17): Fleming’s right hand rule

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 EMF Equation of a DC Generator
Let Ф = flux per pole in Weber
Z = total number of armature conductors
= number of slots × number of conductors per slot
N = speed of the armature in rpm
A = number of parallel paths in the armature winding
P = number of poles of the generator
According to Faraday's law of electromagnetic induction,
dΦ
average e.m.f. induced per conductor, e =
dt
Total Flux= Flux produced by each pole × No. of poles
dФ = Ф×P=PФ Webers
Time taken by a conductor for completing one revolution
dt =60/N sec
dΦ PΦ PΦ N
∴e =
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dt 60 / N 60 26
 We know that here ‘Z’ conductors are distributed in ‘A’ parallel
paths. So effectively (Z/A) conductors need to be multiplied with
e.m.f induced in one conductor
Total e.m.f generated between the armature terminals is given by,
Eg = average e.m.f. generated per conductor × no. of conductors
in each parallel path = PΦN × Z = ΦZN ×  P 
60 A 60  A
Φ ZN  P 
∴ Eg = × 
60  A 

For lap winding, A=P

For wave winding, A=2
For a given DC generator, Z, P & A are constants and hence
generated e.m.f is
E ∝ ΦN = k ΦN ; where k is a const.
g
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Solved Problem-1: A 4-pole DC generator having wave wound
armature winding has 51 slots, each slot containing 20
conductors. Find the generated voltage in the machine when
speed is 1500 rpm and flux per pole is 60 mWb.

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Solution: Given that
No. of poles, P = 4, No. of slots = 51
No. of conductors/slot = 20
Total no. of conductors, Z = 51X 20 = 1020
Speed, N = 1500 rpm
Flux per pole, Ф=60 mWb
No. of parallel paths, A= 2 (for wave wound)

ΦZN  P  60 × 10−3 × 1020 × 1500 × 4

Eg = ×  = = 3060 V
60  A  60 × 2

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Solved Problem-2: An 8-pole DC generator has flux per pole of 40
mWb and winding is connected in lap with 960 conductors.
Calculate the generated e.m.f on open circuit when it runs at 400
rpm. If the armature is wave wound, at what speed must the
machine be driven to generate the same voltage.

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⇒ Solved Problem-2: An 8-pole DC generator has per pole flux of 40
mWb and winding is connected in lap with 960 conductors.
Calculate the generated e.m.f on open circuit when it runs at 400
rpm. If the armature is wave wound, at what speed must the
machine be driven to generate the same voltage.
Solution: Given that
P=8, Φ=40mWb, Z =960, A=P =8 for lap wound, N= 400rpm
Generated e.m.f.,
 P  40 × 10 × 960 × 400 8
−3
Φ ZN
Eg = ×  = × = 256 V
60  A 60 8
If the armature is wave wound, A=2, P=8
ΦZN  P 
Eg = × 
60  A 
60 A 60 2
⇒ N = Eg × = 256 × −3
× = 100rpm
Φ ZP 40 × 10 × 960 8
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⇒

Solved Problem-3: A DC generator has armature e.m.f. of 100V

when flux per pole is 20 mWb and speed is 900rpm. Calculate
e.m.f. generated when (i) speed is 1000 rpm with same flux, and
(ii) speed is 900 rpm but flux is 23 mWb

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⇒
Solved Problem-3: A DC generator has armature e.m.f. of 100V
when flux per pole is 20 mWb and speed is 900rpm. Calculate
e.m.f. generated when (i) speed is 1000 rpm with same flux, and
(ii) speed is 900 rpm but flux is 23 mWb
Solution: Given that

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⇒

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Types of DC Generator

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 Types of DC Generators

DC generators are generally classified into the following types

according to the methods of their field excitation.

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(1) Separately Excited Generators
Separately excited generators are those, whose field magnets are
energized from an external DC source as shown in figure (1.18).

Figure (1.18): Separately excited generator

Important relations:
Armature current, Ia = IL
Generated voltage, Eg= V+ IaRa+ BCD
Power developed in the armature, Pg= EgIa
Power delivered to load, PL= VIL

Note: BCD= Brush contact drop, generally 1V per brush 18-Mar-24 3

7
(2) Self-excited Generators
Self-excited generators are those, whose field magnets are
energized by the current produced by the generators
themselves.
Self-excited generators are classified according to the type of
field connection they use. There are three general types of
field connections: series-wound, shunt-wound (parallel), and
compound-wound.
Compound-wound generators are further classified as long-
shunt compound and short-shunt compound.

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(i) Series-wound Generator
In the series-wound generator, shown in figure (1.19), the field
windings are connected in series with the armature. .

Figure (1.19): Series wound generator

Important relations:
Armature current, Ia = Ise= IL
Generated voltage, Eg = V+IaRa+IseRse+BCD
Power developed in the armature, Pg= Eg Ia
Power delivered to load, PL= VIL 18-Mar-24 3
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(ii) Shunt-wound Generators
In a shunt-wound generator as shown in figure (1.20), the field
coils consist of many turns of small wire. They are connected
in parallel with the load. In other words, they are connected
across the output voltage of the armature.

Figure (1.20): Shunt wound generator

Important relations:
Armature current, Ia = Ish+IL
Shunt field current, Ish= V/Rsh
Generated voltage, Eg = V+IaRa+ BCD
Power developed in the armature, Pg= EgIa
Power delivered to load, PL= VIL 18-Mar-24 4
0
Solved Problem-4: A 4-pole DC shunt generator with wave
connect armature has armature and field resistance of 0.4Ω and
50Ω respectively and supplied to 50 lamps of 60W, 250V
each. Calculate the (i) armature current, (ii) current per path, and
(iii) generated e.m.f.

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 Solved Problem-4: A 4-pole DC shunt generator with wave
connect armature has armature and field resistance of 0.4Ω and
50Ω respectively and supplied to 50 lamps of 60W, 250V
each. Calculate the (i) armature current, (ii) current per path, and
(iii) generated e.m.f.
Solution: Given that
No. of poles, P =4
No. of parallel path , A =2 (wave wound)

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Solved Problem-5: A series DC generator delivers a current of 150
A at 230V. Its armature and series field resistances are 0.2Ω and
0.06Ω respectively. Find (i) armature current, and
(ii) generated e.m.f.

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Solution: Given that

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