READING MATERIAL FOR III SEM DIPLOMA EX
STUDENTS OF RGPV AFFILIATED COLLEGES
SUBJECT ELECTRICAL MACHINES I
Professor MD Dutt
Addl General Manager (Retd)
BHARAT HEAVY ELECTRICALS LIMITED
Professor(Retd) in EX Department
Bansal Institute of Science and Technology
KOKTA ANANAD NAGAR BHOPAL
Presently Head of The Department ( EX)
Shri Ram College Of Technology
Sub Code BE 305 Subject Electrical Machines I
UNIT II D.C Generators
RGPV DIPLOMA EX BRANCH
SEMESTER: THIRD SCHEME: Jul.09 COURSE CODE: 305 PAPER CODE: 6232
NAME OF COURSE: ELECTRICAL MACHINE – I
COMMON WITH PROGRAM (S): E01
Syllabus UNIT II
D. C. Generator -
Principle, construction, armature winding, types of winding, EMF equation, armature
reaction and commutation, interpoles and compensating winding.Types of generators,
characteristics and applications, losses and efficiency. Simple numerical.
S No Topic Page
1 Principle 3
2 Construction details of DC machines 3,4,5,6
3 Types of winding, EMF equation 7,8
4 Armature reaction and commutation 9,10
5 Interpoles and compensating winding 11
6 Types of generators 12,13
7 Characteristics and application 14,15,16,17
8 Losses and efficiency 18,19,20,21
9 Simple numerical problems Separately
10 References 22
Basic principle of operation of a D.C. Generator
When the armature rotated in a magnetic field, an EMF force is generated in the
armature conductors. Let us consider that. at speed N the armature is rotated in a
magnetic field having field strength . The EMF generated is proportional to
the field strength and rpm N . conductor are placed in a slot of armature and it is
acted upon the magnetic field from the north pole of the motor. By applying
R.H.S it is found that the conductor has tendency to generate EMF. Since the
conductor is placed on the slot at circumference of rotor , The EMF generated is
alternating in nature which converted to DC by the commutator action of
Construction Details of DC Machines
There are four main parts of a DC machine
4)Brush and Brush gear
Field System:- The purpose of field system is to create a uniform magnetic field
within which armature rotates. It consists of following four parts.
a) Yoke or frame
b) Pole cores
c) Pole Shoes
d) Magnetizing coils
Cylindrical yoke or frame is used which acts as frame and carries the
magnetic flux produced by the poles. Poles are used to carry coils of
insulated wires carrying the exciting current. The pole shoes acts as support
to the coils and spread out flux over the armature periphery more uniformly.
The magnetizing coils is to provide number of ampere turns of excitation required
to give the proper flux through the armature to induce the desired potential
ARMATURE:- It is the rotating part of the DC machine and is built up in a
cylindrical drum. The purpose of armature is to rotate the conductors in the
uniform magnetic field , It consists of coils insulated wires wound around a iron
core and is so arranged that the electric current are induced in these wires when
armature is rotated in the magnetic field. The armature core is made from high
permeability silicon sheet steel stampings.
A small airgap exists so that armature can rotate freely without rubbing or touching
poles. Armatures are LAP or WAVE wound.
COMMUTATOR:- The commutator is a form of switch (rotating) placed
between the armature and the external circuit so arranged that the input is fed
(incase of motor) and the output is taken out (in case of generator) through
commutator by brushes and brushgear. Two important functions it is doing in case
of DC machine.
1) It connects the rotating armature conductors to the external circuit through
2) It converts the alternating alternating current induced in the armature
conductors into unidirectional current to the external load circuit in
generating action, where as it converts the alternating torque into
unidirectional torque in motor action.
The commutator is of cylindrical shape and is made up of wedge shaped
hard drawn copper segments of mica. These segments are insulated from
each other by thin sheet of mica . The segments are held together by two
Vee rings that fits into the groove cut into the segments. Each armature coil
is connected to the commutator segments through riser.
BRUSHES :- The brushes usually made from carbon are pressed upon the
commutator and from the connecting link between armature winding and
external circuit. They are made from carbon because is conducting material
and at the same time in powdered form provides lubrication effect on the
commutator surface. The brushes are held in brush holders and brushgear on
END HOUSINGS:- End housings are attached to the ends of the main
frame and supports bearings. The commutator side supports brushgear
assembly. Where as NCE side supports only bearing.
BEARINGS :- The ball and roller bearings are fitted in the end housings.
The function of bearing is to reduce the friction between the rotating part
and stationary part.
SHAFT :- The shaft is made of steel having maximum breaking strength.
The shaft is used to transfer mechanical power from or to the machine. The
rotating parts like armature core, cooling fan etc are keyed to the shaft
Armature coils can be connected to the riser of commutator to
form either LAP or WAVE winding
The ends of each armature coils are connected to commutator
Segment some distance apart, so that only two parallel paths are
provided between the positive and negative brushes. Thus wave
wound machines have A=2, They are used for High voltage low
LAP WINDING The ends of each armature coil is connected to
adjacent segments on the commutator so that total number of
parallel path is equal to the total number pf poles, Thus for LAP
A=P The Lap winding is used for low voltage high current
Generated E.M.F. or E.M.F. Equation of a Generator
Eg = PNZΦΦΦΦ/60a
Φ = flux/pole in weber
Z = total number of armature conductors
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 Generated
e.m.f. Eg = e.m.f. generated in any one of the parallel paths i.e. E.
All current-carrying conductors produce magnetic fields. The magnetic field
produced by current in the armature of a DC generator affects the flux
pattern and distorts the main field. This distortion causes a shift in the
neutral plane, which affects commutation. This change in the neutral plane
and the reaction of the magnetic field is called ARMATURE REACTION
Armature reaction can be better understood with this figure:-
UNDESIRABLE EFFECTS OF ARMATURE REACTION
1. Armature reaction causes a net reduction in the field flux per
pole. Due to this net flux decrease, induced armature e.m.f.
decreases and also the torque decreases.
2. Distortion of the main field flux along the air gap i.e. MNA
axis shifted. Due to this there is a problem of commutation
which results in copper losses, iron losses, sparking etc.
In the armature conductors of a d c generator are alternating in nature .
The commutations process involves the change from a generated alternating
current to an externally applied direct current . These induced current flow in one
direction when the armature conductor are under north pole . They are in opposite
direction when they are under south pole . As conductor pass out of the influence
of north pole and enter the south pole , the current in them is reversed . The
reversible of current takes place along the MNA or brush axis . When ever a brush
spans two commutater segments , the winding element connect to those segments
is short circuited . By commutation we mean the change that takes in a winding
element during the period of short circuit by a brush. These changes are shown in
In position (a)The current I flowing towards the brush from L.H.S passes round the
coil in a clockwise direction
In position (b), this coil carries the same current in the same direction, but the coil
is to short circuited by brush
In the position (c) the brush makes contact with bars a and b, thereby short
circuiting coil 1.The current is still I from L.H.S and I from R.H.S.
It is seen that these two currents can reach the brush without passing through coil 1
In (d) shows that bar b has just left the brush and the short circuit of coil1 has
ended. It is now necessary for the current I reaching the brush from the R.H.S in
the anticlockwise direction.
It is seen from above that during the period of short circuit of an armature coil by a
brush the current in that coil must be reversed and also brought up to its full value
in the reversed direction. The time of short circuit is known as period of
METHODS OF IMPROVING COMMUTATION
There are three methods for getting sparkles commutation.
1. Resistance commutation
2. Voltage commutation
3. Compensating winding
Commutating or Inter poles
Interpoles are narrow poles attached to the stator yoke, and placed exactly
midway between the main poles. The compole windings are connected in
series with the armature, because the interpoles must produce fluxes that are
directly proportional to the armature current
The armature and interpole mmf’s are affected simultaneously by the same
armature current. Consequently the armature flux which tends to shift the
MNA is neutralized. The interpoles must induce a voltage in the conductors
undergoing commutation that is opposite to the voltage caused by the neutral
plane shift and reactance voltage.
i)For a generator, the polarity of the interpole must be the same as that of the
next main pole further ahead in the direction of rotation.
ii) For a motor, the polarity of the interpole must be the opposite as that of
the next main pole in the direction of rotation.
Compensating windings are the most effective way for eliminating the
problem of armature reaction and flash over by balancing armature mmf.
The compensating windings are placed in the pole faces parallel to the rotor
armature conductors. These windings are connected in series with the
armature windings. The compensating winding produces an mmf that is
equal and opposite to the armature mmf.
The major draw back with compensating winding is that they are very
costly. There use can be justified for the following special cases
a) In large machines subject to heavy overloads or plugging
b) In small motors subject to sudden reversal and high acceleration.
Separately excitated DC machine.
Ans:- There are two type of excitation , namely separately excited
and self excited machine.
he seethe field coils are energized by a separate DC source. The
connections showing the separately excited d.c. machine is shown
Separately-excited generators are those whose field magnets are
energised from an independent external source of d.c. current.
Series wound DC Generator?
Self-excited generators are those whose field magnets are energised
by the current produced
by the generators themselves. Due to residual magnetism, there is
always present some flux in the poles. When the armature is rotated,
some e.m.f. and hence some induced current is produced which is partly
or fully passed through the field coils thereby strengthening the residual
Three types of self-excited DC Motors or Generators are there
The field windings are connected across or in parallel with the
armature conductors and have the full voltage of the generator applied
the field windings are joined in series with the armature conductors
Compound wound DC Machines
Compound Wound It is a combination of a few series and a few
shunt windings and can be either short-shunt or long-shunt
In general, three characteristics specify the steady-state performance of a DC
1. Open-circuit characteristics: generated voltage versus field current at
constant speed. This is also called magnetizing characteristics
2. Internal characteristic: It is plot between generated voltage versus load
3. External characteristic or Load characteristic: terminal voltage load current
at constant speed.
The terminal voltage of a dc generator is given by
Open-circuit and load characteristics separately excited DC generator
It can be seen from the external characteristics that the terminal voltage falls
slightly as the load current increases. Voltage regulation is defined as the
percentage change in terminal voltage when full load is removed, so that
from the external characteristics,
( )[ ]
Self-Excited DC Shunt Generator
Maximum permissible value of the field resistance if the terminal voltage
has to build up. OPEN CIRCUIT
Basic performance of DC Generator what are main characteristics
Since the field current If in a shunt generator is very small the voltage drop IfRa
can be neglected and
V=Ea V=If Rf is a straight line and is called resistance line the figure gives
below the voltage build up in DC shunt generator for various field circuit
resistance. A decrease in resistance of the field circuit reduces the slope of the field
resistance line resulting in higher voltage. If the speed is constant An increase in
the resistance of the field circuit increases the slope of the field resistance line,
resulting lower voltage If the field resistance is increased to Rc which is termed as
the critical resistance of the field, the field resistance line becomes tangent to the
initial part of the magnetizing curve, when the field resistance is higher than this
value, the generator fails to excite .
The figure above shows the variation of no load voltage with fixed Rf and variable
speed of the armature. The magnetizing curve varies with the speed and its
ordinate for any field current is proportional to the speed of the generator. The
following conditions must be satisfied for voltage build up
1. There must be sufficient residual flux in the field poles.
2. The field terminals should be such connected that the field current increases
the flux in direction of residual flux
3. The field circuit resistance should be less than the critical field resistance.
Characteristics of COPOUND DC GENERATOR
Depending upon the number of series field turns, the cumulatively
compound generators may be over compounded, flat compounded and under
compounded. For over compounded generator the full load terminal voltage
is higher than No load voltage, for flat compounded generator the full load
terminal voltage is equal to the no load voltage. In an under compounded
generator the full load terminal voltage is less than the no load voltage.
LOSSES IN A DC MACHINES and efficiency
Following are the losses in the DC machines
1) Electrical or copper loss ( I²R Losses)
2) Core losses or Iron losses
3) Brush Losses
4) Mechanical losses
5) Stray load losses
ELECTRICAL LOSSES:- Windings having resistance consumes certain
losses, these are termed as copper losses because mostly windings are made
i) Armature copper loss Ia²Ra ( Ia is armature current)
ii) Shunt field copper loss Ish²Rsh
iii) Copper loss in the series field Ise²Rse
iv) Copper loss in the interpole winding which are in series with armature
v) Compound machines both series field and shunt field copper losses
are also there
vi) Copper losses are there in the compensating winding
The core losses are the hysteresis losses and Eddy current losses.
Since the machine usually operates at constant flux density and speed,
these losses are almost constant. These losses are about 20% of Full
BRUSH LOSES:- There is a power loss at the brush contact with
commutator and the carbon brushes. This loss can me measured by the
voltage drop at the brush contact and armature current.
Pbd = Vbd Ia
The voltage drop is more or less remains constant over a wide range
of Ia and it is assumed 2V ( approx)
MECHANICAL LOSSES:- The losses associated with mechanical
effect are called mechanical losses. These consists of friction losses at
bearing and windage losses ( fan losses) . the fans are used to take
away the heat produced due to I²R losses and iron losses inside the
STRAY LOAD LOSSES:- These are miscellaneous losses which are
due to the following reasons:-
1) Distortion of flux due to armature reaction
2) Short circuit currents in the coils due to commutation.
These losses are difficult to find out, However they are taken as 1
% of full load power output.
Let us assume that the
R = Total resistance
I = Output current
Ish = Current through the shunt field
Ia armature current I +Ish
V is the terminal voltage
Power loss in the shunt field
= V Ish
Mechanical Losses = Friction losses at bearings+ friction losses at commutator +
Stray losses Core losses mechanical losses and shunt field copper losses are
considered as combined fixed losses.
ή = Output / Input
= VI / ( VI + Ia²Rat +Pk =VbdIa
Ia = I + Ish
Since Ish compare to I is very small we can consider
Ia ≡ I
ή = VI/ ( VI+ I²Rat +Vbd I +Pk)
LOAD for Maximum efficiency
Ifl = Full load current at maximum efficiency
Im = current at maximum efficiency
Im ² Rat = Pk
Im ² = Pk/ Rat
Current at Maximum ή = F.L Current X( Pk/F.L Copper loss) ²
1. Basic electrical and Electronics Engineering By Pankaj Swarnakar and
Shiv Shankar Mishra Tech India publication
2. Electrical & Electronics Engineering By RK Chaturvedi and SK Sahdev
3. Electrical & Electronics Engineering By JB Gupta KATSON Books
4. Basic Electrical Engineering by Vincent Deltoro
5. Basic Electrical Engineering by De an Sen TMH Publication