READING MATERIAL FOR B.E STUDENTS
OF RGPV AFFILIATED
ENGINEERING COLLEGES PERSUING IN
ELECTRICAL AND ELECTRONICS BRANCH
Professor MD Dutt
Addl General Manager (Retd)
BHARAT HEAVY ELECTRICALS LIMITED
Professor(ex) 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 Ex503 Subject Electrical Machine II
UNIT I DC Machines I
EX 503 ELECTRICAL MACHINES II
DC MACHINES I
Basic construction of DC machines, Types of DC machines and method of excitation,
lap and wave winding, EMF equation; armature reaction and methods of limiting
armature reaction; Commutation process and methods for improving commutation;
Basic performance of DC generators and their performance characteristics; Metadyne
and amplidyne; permanent magnet DC motor; Brushless DC motor
S No Topic Page
1 Basic construction of DC machines 2,3,4
2 Types of DC machines and method of excitation 5,6,7
3 Lap and wave winding, EMF equation 7,8
4 Armature reaction and methods of limiting armature reaction 9,10,11,12
5 Commutation process and methods for improving commutation 12,13,14,15
6 Basic performance of DC generators and their performance
7 Metadyne and Amplidyne; 19,20,21
8 Permanent magnet DC motor 21,22
9 Brushless DC motor 23,24,25,26
Q1 Write down the magnetic field system for a D.C. machine?
Ans:- The magnetic field system is the stationary (FIXED ) part of the
machine. It produces the main magnetic flux. The outer frame or yoke is a
hollow cylinder of cast steel or rolled steel. An even number of pole cores
are bolted to the yoke, he yoke serves following purpose:-
a).it supports the pole cores and acts as a protecting cover to the machine.
b) It forms a part of the magnetic circuit.
Each pole cores have pole shoe having curved surface it serves:
i) It supports field coils
ii) It increases the cross sectional area of magnetic circuit and reduces
The pole cores are made of silicon sheet steel laminations that are
insulated from each other by thin layer of varnish and are riveted
Each pole core have one or more field coils placed over it to produce
Q2 Explain with neat diagram armature of a DC machine?
Ans The rotating part of the D.C machine is called the armature. The
armature consists of a shaft on which a laminated cylinder, called
armature core, is mounted with help of a key for small machines and for
large machines they are mounted on a spider. The armature laminations
are having slots on the outer surface. The laminations are insulated from
each other and tightly clamped together. The purpose of using laminated
core is to reduce the eddy current loss.
The insulated conductors are put in the slots of the armature core. The
conductors are wedged and bands of steel wire is used to prevent them
flying under centrifugal forces. The conductors are connected together.
The connection arrangement is called armature wining ( LAP AND
Q3 Construction of Commutator
Ans:- Alternating voltage is induced in the coil rotating magnetic field.
To obtain direct current in the external circuit a commutator is needed.
The commutator rotates with the armature. The commutator is made from
wedge shaped hard drawn silver bearing copper segments. The each
segments are insulated from each other by mica and the inner surface is
also insulated from shaft. The riser of the commutator is connected to the
ends of the armature conductors
The current is collected from armature winding by means of two or more
car4bon brushes mounted on the commutator. Each brush is supported in
the brush holders. Brush holders are mounted on the brush gear. It
connects the stationary circuit to rotating circuit of armature.
Q4 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 here
Separately-excited generators are those whose field magnets are
energised from an independent external source of d.c. current.
Q5 Series wound DC motor or Generator?
Ans 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 pole
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 across them
the field windings are joined in series with the armature conductors
Q6 Compound wound DC Machines
It is a combination of a few series and a few shunt windings and
can be either short-shunt or long-shunt
Q 7 What do you mean by WAVE and LAP Winding
Ans 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 machines.
Q8 Derive E.M.F equation of a Dc Machine
Ans Generated E.M.F. or E.M.F. Equation of a Generator
Eg = PNZ/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.
Q9 What do you mean by armature reaction:
Ans 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.
Q 10 Methods How to minimize armature reaction
Ans : Methods To Reduce Armature Reaction
By high Reluctance at POLE TIPS:
1. At the time of construction we use chamfered poles. These poles
have larger air gap on the tips and smaller air gap at the centre.
These poles provide non-uniform air gap. The effect of armature
reaction is more near to edge of poles and negligible near the
centre of pole.
If air gap is kept non uniform i.e., larger air gap at the edges(Pole
Tip) and smaller near the centre of the pole and then armature flux
near the pole tip decreases and armature reaction decreases.
2. By Laminated Pole Shoe:
We insert Laminated objects in the pole. By having Laminated
pole shoe the reluctance in the armature flux path increases.
Hence the armature flux gap gets reduced
3. By Reduction in Armature flux:
The effect of Armature Reaction is reduced by creating more
reluctance in the path of Armature flux. This is achieved by using
field Pole Laminations having several Rectangular holes punched
in them. It gives high Reluctance in the path of armature flux. Due
to this armature cross flux reduces whereas main field remains
4. By having Strong main magnetic field:
During the design of DC machine it should be ensured that the
main field m.m.f. is sufficiently strong in comparison with full
load armature flux.
Greater the main field, lesser will be the distortion.
5) By using Inter Poles and Compensating Windings
The effect of Armature reaction is reduced by interpoles placed in
between the main poles. The magnetic axis of interpoles is in the
Direction of q-axis. Interpoles windings are connected in series
with the armature winding. So that, interpoles flux is able to
neutralize the effect of armature flux. The interpoles are narrow
and tapered with large air gap.
Interpoles are added [NN & SS]
The Armature Reaction can also be limited by compensating
windings. It is the best and most expensive methods. The
compensating windings are connected in series with armature
Q11 What is commutation How it can be improve
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 figure.
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
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 commutation.
METHODS OF IMPROVING COMMUTATION
There are three methods for getting sparkles commutation.
1. Resistance commutation
2. Voltage commutation
3. Compensating winding
The method of improving commutation consists of using carbon brushes . This
makes the contact resistance between the commutator segment and brushes high.
This high contact resistance has tendency to force the current in the short
circuited coil to change according to the commutation requirements.
In this method arrangement is made to induce a voltage in the coil under going
commutation , which will neutralize the reactance voltage. The injected voltage
is in opposition to the reactance voltage and if the value of injected voltage is
made equal to reactance voltage ,quick reversal of current in the short circuited
coil will take place and there will be sparkles commutation.
The effect of armature reaction is to shift the MNA in the direction of rotation in
case of generator and against the MNA in case of Motor.
b) 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
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.
Q12 Draw the characteristics of a separately and shunt excited DC
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 current
3. External characteristic or Load characteristic: terminal voltage load current at
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 CHARACTERISTICS
Q13 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
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.
Q14 explain metadyne and amplydyne
An ordinary DC generator Can be modified to a matadyne by providing an additional
pair of brushes on D-axis as shown in figure. The quadrature axis(q axis) brushes are
short circuited and the output is
obtained from d axis brushes . The stator has control field winding , A current If
flows through the control field winding. When the generator is rotating at a constant
speed, the control field wing mmf, Ff induces an emf Eaq between the quadrature axis
brushes qq’, This emf is given by
Eaq = Kqf If
Where Kqf is constant If is the field current
Since the qq’ are short circuited, a quadrature axis armature current Iq flows and q axis
mmf Fq is established. Since the impedance of the short circuited path is low , only a
small control field current if will produce a much larger quadrature axis armature
current. Due to commutator action this magnetic field is stationary in space. An emf is
generated in the armature by its rotation in the quadrature axis flux, This emf appears
across the direct axis brushes dd’and is given by
Ead- KdqI q
Kdq is constant and I q is quadrature axis armature current
For a given Ff a load resistance steady values of Id and Iq are reached. Any increase in
Id reduces Eaq as seen by equation above this reduces Iq . Therefore Ead and Id are
reduced, hence for given value of control field excited current If the output current Id
remains substantially constant over a wide range load variation. That is a Metadyne
behave as constant current generator
The most common version of the matadyne is the amplidyne. The amplidyne consists
of the basic matadyne in which a compensating winding is connected in series with the
power output brush terminals. The d axis mmf which opposes the control field mmf in
matadyne is cancelled by the compensating winding . The compensating winding is
located in the d axis on the stator. The compensating winding carries the load current
Id and produces a flux which opposes the flux produced by the d axis armature current.
The negative feedback effect of the load current is therefore minimized .The d axis flux
now mainly depends on the field winding current. This gives a wider control over the
net d axis flux.
Q15 what do you mean by PMDC
A permanent magnet DC (PMDC) motor is the DC motor whose pole are made of
permanent magnets. Figure shows two pole PMDC motor. The permanent magnets of
the PMDC motor are radially magnetized and mounted on the inner periphery of
cylindrical steel stator. The stator also serves as the return path for the magnetic flux.
The rotor has conventional DC armature with commutator segments and brushes.
Most of the PMDC motors operates on 6V,12V or 24V dc supply obtained from
batteries or rectifiers.
The torque is produced by interaction between axial current carrying rotor conductors
and the magnetic flux produced by the permanent magnets. In a PMDC motor Flux
Φ is constant so the back EMF depends on
τ c= K1Ia
where K1 =K Φ is called speed voltage constant or torque constantt
V = K1N +IaRa
N =(V-IaRa)/ K1
1. There is no field circuit copper loss, this increases the efficiency
2. No space is required for field winding, these motors are smaller than
corresponding wound pole motors.
1. There is risk of demagnetizing of poles may be caused by large armature current,
2. The magnetic field is always present even when motor is not in use, The motor
is totally enclosed so that foreign magnetic matter may not enter or get attracted
3. The PM can produce high flux density as compare to externally supplied DC
shunt field . Therefore PMDC motor has lower induced torque compare to shunt
Q16 what is DC brushless motor
Ans Brushless DC Motor(BLDC)
• A BLDC is simply a normal dc motor turned inside out, that means the coil is
on the out side and the magnets are inside
• The stator consists of several coils which current is led through Creating a
magnetic field that makes the rotor turns
• Stator consists of stacked steel laminations with windings placed in
the slots that are axially cut along the inner periphery
• Rotor is made of permanent magnet and can vary from two to eight pole
pairs with alternate North (N) and South (S) poles. Ferrite magnets and
Rare earth alloy magnets are used in rotor
Unlike a brushed DC motor, the commutation of a BLDC motor is controlled
• It is important to know the rotor position in order to understand which winding
will be energized following the energizing sequence.
• Rotor position is sensed by different ways some of them are
• 1)Hall sensors 2)Optical encoders
• :-When a magnetic field applied to a system with electric current a hall voltage
Perpendicular to the field and to current is generated. This was discovered by
Edwin Hall in 1879.
• Halls Sensors sense the position of
the coils The Decoder Circuit turns appropriate switches on and off The voltage
• through the specific coils turns the motor
• Torque speed characteristics of BLDC
• Increased Reliability & Efficiency
• Longer Life
• Elimination of Sparks from Commutator
• Reduced Friction
• Faster Rate of Voltage & Current
• Requires Complex Drive Circuitry
• Requires additional Sensors
• Higher Cost
• Some designs require manual labor (Hand wound Stator Coils)
• Consumer: Hard Drives, CD/DVD Drives, PC Cooling Fans, toys, RC airplanes,
• Medical: Artificial heart, Microscopes, centrifuges, Arthroscopic surgical tools,
Dental surgical tools and Organ transport pump system.