Addis Ababa Science & Technology University
College of Electrical and Mechanical
Engineering
Department of Electromechanical Engineering
Electrical Machines
By: Birhanemeskel A. (MSc in Mechatronics)
1
Magnetics
Transformers
Induction Motors
Course
Outlines
DC Machines
2
Chapter-Four
DC Machines
➢ A direct-current (dc) machine is a versatile machine, that is, the same
machine can be used as a generator to convert mechanical energy to dc
electrical energy or as a motor to convert dc electrical energy into
mechanical energy.
➢ However, the use of dc machines as dc generators to produce bulk power
has rapidly disappeared due to the economic advantages involved in the
use of alternating-current generation, transmission, and distribution. This
is partly due to the high efficiency and relative simplicity with which
transformers convert voltages from one level to another.
3
➢ Today, the need for dc power is often met by the use of solid state–
controlled rectifiers.
➢ However, dc motors are used extensively in many industrial applications
because they provide constant mechanical power output or constant torque,
adjustable motor speed over wide ranges, precise speed or position control,
efficient operation over a wide speed range, rapid acceleration and
deceleration, and responsiveness to feedback signals.
4
DC-machines
➢ DC-Machines has both generation and motor actions;
➢ Generator action:
✓ An emf (voltage) is induced in a conductor if it moves through a
magnetic field.
➢ Motor action:
✓ A force is induced in a conductor that has a current going through it and
placed in a magnetic field.
➢ Any DC machine can act either as a generator or as a motor.
5
Generators
❖ “Whenever a conductor is moved within a magnetic field in such a way that
the conductor cuts across magnetic lines of flux, voltage is generated in the
conductor.”
➢ The AMOUNT of voltage generated depends on: 𝑒 = 𝐵𝑙𝑣𝑠𝑖𝑛𝜃
✓ The strength of the magnetic field,
✓ The angle at which the conductor cuts the magnetic field,
✓ The speed at which the conductor is moved, and
✓ The length of the conductor within the magnetic field
➢ The POLARITY of the voltage generated depends on the direction of the
magnetic lines of flux and the direction of movement of the conductor.
➢ To determine the direction of current in a given situation, the Right-Hand
rule for generator is used.
6
Right-Hand Rule for Voltage generation
✓ Thumb in the direction the conductor is being moved
✓ Forefinger in the direction of magnetic flux (from north to south)
✓ Middle finger will then point in the direction of current flow in an external
circuit to which the voltage is applied.
➢ Generator is an electrical machines that change mechanical energy in to
electrical energy by the principles of electromagnetic induction.
7
Generators
➢In general there are two types of generators
- DC Generator
- AC Generators
➢DC Generator converts mechanical energy to DC electrical energy,
➢While AC generator converts mechanical energy to AC electrical energy.
➢ The study of dc generator has to begin with a knowledge of the ac
generator. The reason is that the voltage generated in any dc generator is
inherently alternating and only becomes dc after it has been rectified by
the commutator.
Principles of operation of Generators
➢ Whenever a conductor cuts a magnetic flux an e.m.f is induced in it
by Electro-Magnetic Induction.
8
Generators
➢ Magnitude of the induced emf is equal to the rate of change of flux
linkage
𝑑∅
𝑒=−
𝑑𝑡
➢ Magnetic field variation can be caused by:
➢ a) Relative motion
➢ b) Varying flux
➢This e.m.f causes a current to flow if the conductor circuit is closed.
➢Hence the two basic fundamental element of an electrical generator are
i) A magnetic field.
ii)A conductor or conductors which can so move as to cut the flux.
9
Generators
10
Generators
➢ An elementary generator consists of a wire loop mounted on the shaft, so
that it can be rotated in a stationary magnetic field.
➢ This will produce an induced emf in the loop.
➢ Sliding contacts (brushes) connect the loop to an external circuit load in
order to pick up or use the induced emf.
➢ The pole pieces (marked N and S) provide the magnetic field. The pole
pieces are shaped and positioned as shown to concentrate the magnetic field
as close as possible to the wire loop.
➢ The loop of wire that rotates through the field is called the Armature.
➢ The ends of the armature loop are connected to rings called slip rings. They
rotate with the armature.
11
Generators
➢ An end view of the shaft and wire loop is shown below. At this particular
instant, the loop of wire (the black and white conductors of the loop) is
parallel to the magnetic lines of flux, and no cutting action is taking place.
Since the lines of flux are not being cut by the loop, no emf is induced in the
conductors, and the meter at this position indicates zero. This position is
called the NEUTRAL PLANE.
12
Generators
➢ The shaft has been turned 90° clockwise, the conductors cut through more and more
lines of flux, and voltage is induced in the conductor.
➢ At a continually increasing angle , the induced emf in the conductors builds up from
zero to a maximum value or peak value.
➢ Observe that from 0° to 90° , the black conductor cuts DOWN through the field. At the
same time the white conductor cuts UP through the field. The induced emfs in the
conductors are series-adding. This means the resultant voltage across the brushes (the
terminal voltage) is the sum of the two induced voltages.
➢ The meter at this position reads maximum value.
13
Generators
➢ After another 90° of rotation, the loop has completed 180° of rotation and is
again parallel to the lines of flux. As the loop was turned, the voltage
decreased until it again reached zero.
➢ Note that from 90° to 180° the conductors of the armature have been
moving in the same direction through the magnetic field. Therefore, the
polarity of the induced voltage has remained the same.
14
Generators
➢ As the loop continues to turn, the conductors again cut the lines of magnetic flux.
➢ This time, however, the conductor that previously cut through the flux lines
of the south magnetic field is cutting the lines of the north magnetic field,
and vice versa.
➢ Since the conductors are cutting the flux lines of opposite magnetic
polarity, the polarity of the induced voltage reverses. After 270° of rotation,
the loop has rotated to the position shown, and the maximum terminal
voltage will be the same as it was from A to C except that the polarity is
reversed.
15
Generators
➢ After another 90° of rotation, the loop has completed one rotation of 360°
and returned to its starting position.
➢ The voltage decreased from its negative peak back to zero.
➢ Notice that the voltage produced in the armature is an alternating polarity.
➢ The voltage produced in all rotating armatures is alternating voltage.
16
Generators
17
Generators
➢ Since DC generators must produce DC current instead of AC current, a
device must be used to change the AC voltage produced in the armature
windings into DC voltage. This job is performed by the commutator.
➢ The commutator is constructed from a copper ring split into segments with
insulating material between the segments. Brushes riding against the
commutator segments carry the power to the outside circuit.
➢ The commutator in a dc generator replaces the slip rings of the ac
generator. This is the main difference in their construction. The commutator
mechanically reverses the armature loop connections to the external circuit.
18
Construction of DC Machines
• The dc machines used for industrial
applications have essentially two major
parts:
a. Field system (Stator) and
b. Armature (Rotor)
a. The field system: - is located on
stationary part of the machine called
stator, and it is designed to produce
magnetic flux and therefore provides the
necessary
excitation
(cause)
for
operation.
19
Construction of DC Machines
20
Construction of DC Machines
a. Field system (Stator) consists of the following:
i) Yoke (Frame): is constructed from Cast steel and serves as a mechanical
support for entire assembly.
ii) Field winding: consists of a few turns of heavy wire for a series field or
many turns of fine wire for a shunt field. Essentially, the field coils are
electromagnets whose ampere turns provide a magneto motive force
(MMF).
iii) Main or Field pole: - constructed from laminated steel and bolted or
welded to the yoke after the assembly of field windings has been inserted
on them. It is designed to produce the main flux.
21
Construction of DC Machines
b.
i.
ii.
iii.
iv.
Armature or Rotor consists of the followings:
armature core with slots,
armature winding accommodated in slots,
brush, commutator and the shaft
Generally the armature is the rotating part of a DC machine where the
process of electromechanical energy conversion takes place.
v. The purpose of the armature is to rotate the conductors in the uniform
magnetic field and to induce an alternating emf in its winding.
The armature core: - constructed from laminated layers of steel to reduce
hysteresis and eddy current loss in the core.
22
Construction of DC Machines
Brushes
➢ Brushes are used to ensure the electrical connection between rotating
commutator and stationary external load circuit.
➢ Is needed to collect the current from rotating commutator or to lead the
current to it.
➢ Brushes are made from carbon or graphite compounds in the shape of
rectangular block.
➢ They are firmly held against the commutator by means of spring.
➢ Brush rings are located at the commutator end.
23
Construction of DC Machines
Commutator
➢ Commutator is mounted on the rotor of the DC machine and it performs
mechanical rectification of power with the help of brush.
➢ AC to DC in case of generator
➢ DC to AC in case of motors operation.
➢ It consists of copper segments, individually insulated from each other and
from the shaft.
➢ The ends of armature coils are connected to the commutator, which together
with the brushes rectifies the alternating emf induced in the armature coils.
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➢ Commutation
➢ The currents induced in armature conductors of any DC generator are AC.
➢ To make their flow unidirectional in the external circuit, we need a
commutator.
➢ These currents flow in one direction when armature conductors are under
N-pole and in the opposite direction when they are under S-pole.
➢ As conductors pass out of the influence of N-pole and enter that of S-pole,
the current in them is reversed.
➢ This reversal of current takes place along magnetic neutral axis or brush
axis, when the two commutator segments to which the coil is connected are
being short circuited by brush.
➢ This process by which current in the short-circuited coil is reversed while it
crosses the M.N.A. is called commutation.
25
Commutation
➢ The loop is parallel to the magnetic lines of flux, and no voltage is induced
in the loop
➢ Note that the brushes make contact with both of the commutator segments
at this time. The position is called neutral plane.
26
Commutation
➢ As the loop rotates, the conductors begin to
cut through the magnetic lines of flux.
➢ The conductor cutting through the south
magnetic field is connected to the positive
brush, and the conductor cutting through the
north magnetic field is connected to the
negative brush.
➢ Since the loop is cutting lines of flux, a
voltage is induced into the loop. After 90°
of rotation, the voltage reaches its most
positive point.
27
Commutation
➢ As the loop continues to rotate, the voltage
decreases to zero.
➢ After 180° of rotation, the conductors are
again parallel to the lines of flux, and no
voltage is induced in the loop.
➢ Note that the brushes again make contact
with both segments of the commutator at the
time when there is no induced voltage in the
conductor.
28
Commutation
➢ During the next 90° of rotation, the
conductors again cut through the magnetic
lines of flux.
➢ This time, however, the conductor that
previously cut through the south magnetic
field is now cutting the flux lines of the
north field, and vice-versa. .
➢ Since these conductors are cutting the lines
of flux of opposite magnetic polarities, the
polarity of induced voltage is different for
each of the conductors. The commutator,
however, maintains the correct polarity to
each brush.
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Commutation
➢ The conductor cutting through the north
magnetic field will always be connected to
the negative brush, and the conductor
cutting through the south field will always
be connected to the positive brush.
➢ Since the polarity at the brushes has
remained constant, the voltage will
increase to its peak value in the same
direction.
30
Commutation
➢ As the loop continues to rotate, the
induced voltage again decreases to zero
when the conductors become parallel to
the magnetic lines of flux.
➢ Notice that during this 360° rotation of
the loop the polarity of voltage remained
the same for both halves of the
waveform. This is called rectified DC
voltage.
➢ The voltage is pulsating. It does turn on
and off, but it never reverses polarity.
Since the polarity for each brush remains
constant, the output voltage is DC.
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Commutation
32
Commutation
33
There are two types of armature constructions
1.Ring type
2.Drum type
Ring type
a. Coils are wound around a hallow iron core.
b. Taps are taken at regular intervals to be connected to the commutator
segments.
c. Early design and seldom used today
Drum type
i. Current armature construction standard.
ii. The insulated coils are inserted into slots in the cylindrical armature drum
iii. The ends of the coil are connected in series at the commutator segments.
34
Armature types
Drum type
Ring type
35
There are two types of armature windings; Lap and Wave windings.
Lap winding
a) The end of one coil is connected to the start of next coil at the adjacent
commutator segment.
b) Number of parallel current path =Number of poles
i.e. A =p
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Construction of DC Machines
Wave winding
➢ The end of the first coil connected to the start of the second coil
under the next main pole.
➢ It has only two parallel current path regardless of the number of
poles.
i.e. A= 2
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Wave winding Vs Lap Winding
➢ In wave winding, for the given number of poles & armature conductors,
more EMF is induced than lap winding.
➢ For high voltage & low current wave winding is used.
➢ For low voltage & high current lap winding is used, because, it has more
parallel paths of conductors.
38
EMF Equation of DC Generator
Let ϕ = flux produced per pole in Weber,
Z = total number of armature conductors
P = Number of poles of the generator
A = number of parallel paths in armature
N = armature rotation in revolutions per minute (rpm)
➢ Now emf gets induced in the conductor according to Faraday’s law of
electromagnetic induction. Hence average value of emf induced in each
armature conductor is,
𝑑∅
𝑒 = 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑐𝑢𝑡𝑡𝑖𝑛𝑔 𝑡ℎ𝑒 𝑓𝑙𝑢𝑥 =
𝑑𝑡
39
EMF Equation of DC Generator
➢ Now consider one revolution of conductor. In one revolution, conductor
will cut total flux produced by all the poles i.e. ∅ ∗ 𝑝. While time requires
60
to complete one revolution is seconds, where N is speed in rpm.
𝑁
∅𝑝
𝑁
𝑒=
= ∅𝑝
60
60
( )
𝑁
➢ This is the emf induced in one conductor. Now the conductors in one
parallel paths are always in series. There are total Z conductors with a
parallel paths, hence Z/A number of conductors are always in series and
emf remains same across all the parallel paths.
➢ So, total emf induced is,
𝑁 𝑍 ∅𝑝𝑁𝑍
𝑒 = ∅𝑝
∗ =
60 𝐴
60 𝐴
40
𝑒 = ∅𝐾𝑎 𝜔𝑚
Where, 𝐾𝑎 =
𝑝𝑁𝑍
, 𝜔𝑚
60 𝐴
=
2𝜋𝑁
60
𝑟𝑎𝑑/𝑠𝑒𝑐
Example 4.1
A 4 pole, lap wound dc generator has a useful flux of 0.07 Wb per pole.
Calculate the generated emf when it is rotated a speed of 900 rpm with help of
prime mover. Armature consists of 440 number of conductors. Also, calculate
the generated emf if lap winding is replaced by wave winding.
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Classification of DC Generators
➢ According to the method of their field excitation dc machines are classified
as: Separately excited and Self excited
1. Separately excited
❖ The generator does not supply its own field excitation.
❖ The field winding is excited from independent separate external DC
source
❖ It needs external DC source in order to bring them in to operation.
❖ It is used in laboratories where large voltage variation is essential for lab.
experiment
42
Classification of DC Machines
1. Separately excited
43
Classification of DC Machines
1. Separately excited
Field Equation; 𝑉𝑓 = 𝑅𝑓 𝐼𝑓 Armature Equation; 𝐸𝑎 = 𝑉𝑡 + 𝑅𝑎 𝐼𝑎 = 𝐾𝑎 ∅𝑊𝑚
𝑉𝑡 = 𝐼𝑎 𝑅𝐿
Power developed in the armature = 𝐸𝑎 𝐼𝑎
Power developed in the external load = 𝑉𝑡 𝐼𝐿
44
Classification of DC Machines
2. Self-excited DC Machines
➢ The generator supplies its own field excitation.
➢ When the field winding is excited by its own armature, the machines is
said to be a self excited dc machine. i.e. their field and armature windings
are connected.
➢ In these machines, the field poles must have a residual magnetism, so that
when the armature rotates, a residual voltage appears across the brushes.
➢ Residual magnetism will be used to start generator operation, voltage
build up process.
➢ Residual magnetism- retained magnetism by an electromagnet after a
magnetizing force is disconnected.
45
Classification of DC Machines
2. Self-excited DC Machines
➢ Based on the connection of the field winding with the armature winding, a
self-excited dc machine can be sub-divided in to three groups.
Shunt wound
Series Wound
Compound Wound a) short shunt
b) long shunt
46
Classification of DC Machines
2. Self-excited DC Machines
Shunt-Wound generator
➢ The field winding consists of a large number of turns of fine wire and is
connected in parallel with the armature.
➢ As the resistance of the field winding is very large, the shunt (field) current
Ish is very small.
➢ The excitation current varies in accordance with the output voltage.
➢ A flow of armature current causes voltage drop across the armature, and in
turn causes a decrease in excitation current and output voltage.
➢ Hence, variation in load current causes output voltage fluctuation, requires
regulation
47
Classification of DC Machines
2. Self-excited DC Machines
Shunt-Wound generator
48
Classification of DC Machines
2. Self-excited DC Machines
Series Wound Generator
➢The field winding of series wound machine is wound with few turns of
thick wire. And connected in series with the armature.
➢So the resistance of the field winding is very low.
➢The current drawn by the generator passes through the field winding as
well as the armature. So
49
Classification of DC Machines
2. Self-excited DC Machines
Compound Wound Generator
➢Consist of series and shunt field coils.
➢The series field compensates the drop in voltage that takes place due to
armature resistance. Thus the output voltage is fairly constant
50
Example 4.2
1. A 12-pole DC shunt generator has 50 slots on its armature with 12
conductors per slot with wave winding. The armature and field winding
resistance is 0.5 ohm and 60 ohm respectively. The generator is supplying
a resistive load of 15 ohm at terminal voltage of 300 V when running at a
speed of 625 rpm. Find the armature current, the generated emf and the
flux per pole. See fig. a below.
2. A load of 7.5 kW at 230V is supplied by a short-shunt cumulatively compound DC
generator. If the armature, series and shunt field resistances are 0·4, 0·3 and 100 ohm
respectively. Calculate the induced emf and the load resistance. See fig. b below.
Fig. b
Fig. a
51
Characteristics of DC Generators
To determine the relation between different quantities of a DC generator, the
following are the important characteristics of DC generators:
i. No-load characteristics. It is also known as magnetic characteristics or
open-circuit characteristics (O.C.C.). It shows the relation between the noload generated emf in the armature (E0) and the field current (i.e., exciting
current) 𝐼𝑓 , at a specified speed.
ii. External characteristics. It is also called the performance characteristics. It
shows the relation between the terminal voltage (𝑉𝑡 ) and the load current
𝐼𝐿 .
iii. Internal Characteristics. It is also known as total characteristics. It gives
the relation between the emf actually induced in the armature (Emf ) and
the armature current 𝐼𝑎 .
52
Characteristics of DC Generators
i. No-load characteristics
It shows the relation between the no-load generated emf in the armature (E0) and the field
current 𝐼𝑓 , at a specified speed. To obtain this characteristics, proceed as follows:
Open the field winding of the generator and connect it to a separate DC source through a
rheostat as shown in Fig. below. Connect an ammeter in the field circuit and a voltmeter
across the armature. Reduce the field current to zero and run the armature at a specified
speed. Get the reading of voltmeter and mark the point ‘a’ on the graph. To plot the
characteristics take field current 𝐼𝑓 along X-axis and no-load generated emf (E0) along Yaxis. Increase the field current in steps and get the corresponding voltmeter readings. Plot
these values on the graph. The curve thus obtained shown below shows the no-load
characteristics or open circuit characteristics (O.C.C.) of the generator.
No-load characteristic curve
53
Characteristics of DC Generators
i. No-load characteristics
Analysis of the curve While analyzing the curve, the following points are worth noting:
i. The curve starts from point ‘a’ instead of ‘O’ when the field current is zero. It is
because of the residual magnetism of the poles.
ii. The initial part of the curve (ab) is almost a straight line because at this stage the
magnetic material is unsaturated and it has high permeability.
iii. After point ‘b’ the curve bends and the generated emf (E0) becomes almost constant. It
is because after point ‘b’, the poles (magnetic material) starts getting saturated.
No-load characteristic curve
54
Characteristics of DC Generators
ii. Internal and External Characteristics
➢ Internal characteristics gives the relation between the e.m.f. actually
induces in the armature and the armature current Ia.
➢ The e.m.f E will be less than E0 due to the effects of armature reaction.
Therefore, this curve will lie below the open circuit characteristic.
➢ External characteristics also referred to as performance characteristic or
sometimes voltage-regulating curve
➢ It gives the relation between terminal voltage V and the load current I.
➢ V will be less than E due to voltage drop in the armature circuit.
➢ Therefore, this curve will lie below the internal characteristic.
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DC Motor
➢ An electro-mechanical energy conversion device (electrical machine) that
converts DC electrical energy or power (𝐸𝐼) into mechanical energy or
power (𝜔𝑇) is called a DC motor.
➢ Electric motors are used for driving industrial machines, e.g., hammers,
presses, drilling machines, lathes, rollers in paper and steel industry,
blowers for furnaces, etc., and domestic appliances, e.g., refrigerators, fans,
water pumps, toys, mixers, etc.
➢ The block diagram of energy conversion, when the electro-mechanical
device works as a motor, is shown in figure below.
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DC Motor
➢ The construction of a d.c motor is the same as a d.c generator. The only
difference is that in a generator the generated e.m.f. is greater than the
terminal voltage, whereas in a motor the generated e.m.f. is less than the
terminal voltage.
Major types of dc motors
➢ Self excited dc motor
1. Series dc motor
2. Shunt dc motor
3. Compound dc motor
➢ Separately excited dc motor
➢ Permanent magnet dc motor
57
DC Motor
58
DC Motor
59
DC Motor
Compound Motors
60
DC Motor
Separately Excited Motors
61
Characteristics DC Motors
The performance of a DC motor can be easily judged from its characteristic curves, known
as motor characteristics. The characteristics of a motor are those curves which show
relation between the two quantities. On the basis of these quantities, the following
characteristics can be obtained:
I. Speed and Armature current i.e., N – Ia Characteristics: It is the curve drawn between
speed N and armature current Ia. It is also known as speed characteristics.
II. Torque and Armature current i.e., T–Ia Characteristics: It is the curve drawn between
torque developed in the armature T and armature current Ia. It is also known as
electrical characteristic.
III. Speed and Torque i.e., N–T characteristics: It is the curve drawn between speed N and
torque developed in the armature T. It is also known as mechanical characteristics.
62
Speed Control of DC Motors
The speed of a DC motor is given by the relation 𝑁 ∝
∴ 𝑁∝
𝐸𝑏
∅
where 𝐸𝑏 = 𝑉 − 𝐼𝑎 𝑅𝑎 .
𝑉−𝐼𝑎 𝑅𝑎
∅
From the above equation it is clear that the speed of DC motors can be controlled;
✓ By varying flux per pole ∅. This is known as flux or field control method.
✓ By varying the armature drop, 𝑅𝑎 ,i.e., by varying the resistance of armature circuit. This
is known as armature control method.
✓ By varying the applied voltage, 𝑉. This is known as voltage control method.
63
DC MOTOR STARTING
• If dc motor is directly connected to a dc power supply, the
starting current will be dangerously high.
• The current drawn by the motor armature is
• At stand still Eb=0, therefore,
Since is very small, the starting current drawn by the supply
voltage is very large.
• This excessive starting current will damage the motor
• Thus, the starting current can be reduced by
1. Reducing VT at starting:- requires a variable supply voltage
2. Increasing resistance in the armature circuit:- an additional
resistance Rex can be connected in the armature circuit
64
65