DC Machine Construction and Working

Electrical andElectronics Engineering
Unit 3: DCMachines
Prepared By,
Mr. Arun E. Sonkamble
B.E.(Electrical),M.E. (Power System),ISTE,IET
(Department of Electronic&TelecommunicationEngineering)

Generating and Motoring Action
For a motor the input energy is electrical energy and the useful output energy is mechanical
energy.
For a generator the input energy is mechanical energy and the useful output energy is electrical
energy.

Generating and Motoring Action
BASIS MOTOR GENERATOR
Function The Motor converts Electrical energy into
Mechanical Energy
The Generator converts Mechanical energy to
Electrical energy.
Electricity It uses electricity. It generates electricity
Driven
element
The Shaft of the motor is driven by the
magnetic force developed between
armature and field.
The Shaft is attached to the rotor and is driven
by mechanical force.
Current In a motor the current is to be supplied to
the armature windings.
In the generator current is produced in the
armature windings.
Rule
Followed
Motor follows Fleming’s Left hand rule. GeneratorfollowsFleming’s Right handrule.
Example An electric car or bike is an example of
electric motor.
Energy in the form of electricity is generatedat
the power stations.

DC GeneratorConstruction
A DC Generator is an electrical device which converts mechanical energy into electrical energy.
Itmainly consists of three main parts,i.e. magnetic field system,armature andcommutator andbrush gear.
The other parts of a DC Generator are magnetic frame and yoke, pole core and pole shoes, field or exciting
coils, armature core and windings, brushes, end housings, bearings and shafts.

Yoke:
 The outer frame of a dc machine is called as yoke.
 It is made up of cast iron (small generators) or cast or rolled
steel (large size generators).
 It acts as protecting shield for generator.
 It provides mechanical strength tothe whole assembly but also
carries the magnetic flux produced by the field winding.

Field winding/ Pole coils:
 They are usually made ofcopper.
 Field coils are former wound and placed on each
pole and are connected inseries.
 They are wound in such a way that, whenenergized,
they form alternate North and South poles.

Direction offluxis YOKE⇒ POLE ⇒ ARMATURE ⇒
NEXT POLE ⇒YOKE
Pole shoes function:
Poles are joined to the yoke with the help of bolts or welding.
• It carries and support the field winding.
• Tosupport the filed winding: Pole Core provides thisarea towound
the field winding.
• To spread out the flux in air gap: Pole core direct the magnetic flux
through the air gap, armature, and to the next pole.

Armature core/ rotor:
 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 to theshaft.

Armature winding:
 The armature core slots are mainly used for holding the armature windings. These are in a closed circuit
winding form, andit is connected in series to parallel for enhancing the sum of produced current.

Commutator:
 The working of the commutator is like a rectifier for changing AC voltage to the DC voltage within the
armature winding to across the brushes.
 It is designed with acopper segment, andeach copper segment is protected from each other with the help
of mica sheets. It is located on the shaft of the machine.

Brushes:
 The electrical connections can be ensured between the commutator as well as the exterior load circuit
with the help of brushes.
 They collect current from commutator. Made up of carbon or graphite.

DC Generator Construction_ (Draw in exam)

DCGeneratorConstruction_ InShort
Sr. No. Name ofpart MaterialUsed Function
1. Yoke cast iron (small
generators) or cast
or rolled steel (large
size generators).
• main cover of the DC Generator
• acts as protecting shield for generator
• provides a mechanical support for the poles.
• Italsocarries themagnetic fluxproduced bythepoles.
2. Field winding
/pole coils
Copper • Each pole core has one or more field coils (windings)
placed over it to produce a magnetic field.
• Field coils are former wound and placed on each pole
and are connected inseries.
• They are wound in such a way that, when energized,
they form alternate North and South poles.
3. Pole shoes Thin cast steel or
wrought iron
laminations which
are riveted together
under hydraulic
pressure
Poles are joined to the yoke with the help of bolts or
welding.
• Itcarries and support the field winding.
• Tosupport thefiledwinding: Pole Core provides this
area to wound the field winding.
• Tospread out the flux in air gap: Pole core direct the
magnetic flux through the air gap, armature, and to the
next pole.

Sr. No. Name ofpart Material Used Function
4. Armature
core/ rotor
built up of thin
laminated circular
silicon steel disks for
reducing eddy current
losses.
• Rotating part of DC machine.
• Itiscylindrical inshapewithslotstocarry armature
winding.
• Itmaybe provided with airducts for the axialair
flow for cooling purposes.
• Armature is keyed to theshaft.
5. Armature
winding
Copper • Heart of the DC Machine. It is laminated to reduce
eddy current losses.
• The armature core slots are mainly used for holding
thearmature windings. These are in a closedcircuit
winding form, and it is connected in series to parallel
for enhancing the sum of produced current.
6. Commutator Made from a number
of wedge-shaped hard
drawn copper bars or
segments insulated
from each other and
from the shaft.
• It is like a rectifier for changingACvoltagetothe
DC voltage within the armature winding toacross the
brushes.
• It is designed with a copper segment, and each copper
segment is protected from each other with the help of
mica sheets. It is located on the shaft of the machine.

EMF equation of DCmachine
Let,
P= Number of poles ofmachine
Φ= Flux per pole in weber
Z = Total number of armature conductor
N = speed of armature in RPM
A = Number of parallel paths in armature winding
In one revolution of the armature, flux cut by one conductor is
flux cut by one conductor = P Φweber ----------(1)
Time taken to complete one revolution is given as:
For N revolutions (i.e. speed) it takes time T.Hence in order tocomplete one revolution , it takes time say“t”
60
� =
�
��
�
----------(2) N : T
1 revolution : t
Hence t=1/Nmin
t = 60 sec/N

�
=
�P
Φ
��
∗
��
��
�
�
Therefore, the average induced e.m.f in one conductor will be:
Put (2) in (3) � =
60
�
PΦ
� =
�
�� ----------(2)
� PΦ
� =
60
�� --------(3)
�� --------(4)
The number of conductors connected in series in each parallel path = Z/A.
The average induced emf across each parallel path will be

�P Φ �
�� =
��
∗
�
��
�P Φ �
�� =
��
∗
�
��
If the DC Machine is working as a Generator, the induced emf is given by the equation shown below:
Where Eg is the Generated Emf
If the DC Machine is working as a Motor, the induced emf is given by the equation shown below:
In a motor, the induced emf is called Back Emf (Eb) because it acts opposite to the supply voltage.

Back emf in Motor
When the current-carrying conductor placed in a magnetic field, the torque induces on the conductor, the
torque rotates the conductor which cuts the flux of the magnetic field.
According to the Electromagnetic Induction Phenomenon “when the conductor cuts the magnetic field,
EMF induces in theconductor”.
The Fleming right-hand rule determines the direction of the induced EMF.
Itisseenthat thedirection oftheinduced emf isopposite tothe applied voltage. Thereby theemf is
known as the counter emf or back emf.
The back emf is developed in series with the applied voltage, but opposite in direction, i.e., the back emf
opposes the current which causes it.

What actually Back emf in Motor does?(Significance)
Back EMF represents that portion of the supply voltage which multiplied by the current equals the
mechanicalworkproduced(roughlyspeaking).(The rest of the supply voltage is usedupas I-Rdrop
and produces heat - it is wasted energy. So you really want your back EMF to be as close to the supply
voltage as possible).
The back EMF plays a self-regulating role by limiting current and energy flow through the motor. In
a parallel motor, it is only due to back EMF that if you apply a good voltage source to an armature with a
very low resistance, whether no-load or full-load, the motor would run nicely at a near-constant speed.

In order to understand the significance of back EMF, let us see what would happen if there were no
back EMF.
Let’s make up an example of afictitious parallel DC motor with fixed magnets, supply voltage E of12
V, Armature resistance of 0.1 Ohm.
If there is no back EMF, the armature will draw a current of 120 A.
E= I *R (Ohm’s Law)
It draws a power of 1440 watts (E x I) from the source
P= E*I � = � ∗ � = �� ∗ �� = ��
Produces a heating of 1440 Watts i.e. it is power loss I2R
All the power drawn from the source is gone into heating. There is no power left to do any mechanical work
(as per conservation of energyprinciple).
� = =
� ��
� �.�
= ��
�

However, all the voltage is not dropped by I.R in the armature.
As the armature starts moving, the movement of the armature in the field induces a voltage in the
armature winding that ends up opposing and cancelling out part of the supply voltage. This opposing
voltage is what we call the back EMF.
Now the supply voltage of 12V applied across the armature gets
split internally as a sumof twodrops-1.the I.R drop and2.the
back EMF drop.
E = Eb + (Ia * Ra )
E is supply voltage Eb is back EMF Ra is resistance of armature winding Ia is the armature current
In practice, the resistance of the armature is small enough that the I.R drop (at normal operating
currents) is really small compared to the supply voltage.
For the sake of simplicity, let’s try to neglect the I-R drop and see what happens !
E = Eb

What actually Back emf in Motordoes?
E=Eb
That is,inpractical terms, theback EMFalmost balances outthe supplyvoltage (leaving
only a small fraction of the supply voltage to be taken up as I.R drop in the armature).
When motor starts Armature isnot moving Eb=0
The armature current is very high, only limited by the small resistance of the armature.
The initial torque is also very high
As the armature accelerates due to the torque, and gains rotational speed, the back EMF grows.
E = Eb + (Ia * Ra ) Eb = E - (Ia * Ra )
As the back EMF grows, the current drops, and the torque also reduces. It all settles at the point
where the torque is equal to the load.
For a reasonable load and a small armature resistance, the back EMF would be nearly equal to
the supply voltage.

Torque Equation of DCMotor
Torque acting on a body is quantitativelydefined as theproduct of force acting onthe body and
perpendicular distance of the line of action of force from the axis of rotation.
The equation of torque is given by,
� = �� sin ------(1)
Qualitatively , torque is the tendency of a force to cause a
rotational motion, or to bring about a change in rotational
motion .
Visualize the below shown dc motor front-view . You will find that each
conductor experiences a force and the conductors lie near the surface of
the rotor at a common radius from its center. Hence torque is produced
at the circumference of the rotor and rotor starts rotating.
�

When a DC machine is loaded either as a motor or as a generator,the rotor conductors carry current. These
conductors lie in the magnetic field of the air gap.
Thus, each conductor experiences a force. The conductors lie near the surface of the rotor at a common radius
from its centre. Hence, a torque is produced around the circumference of the rotor, and the rotor starts
rotating.
When the machine operates as a generator at a constant speed, this torque is equal and opposite to that
provided by the prime mover.
When the machine is operating as a motor, the torque is transferred to the shaft of the rotor and drivesthe
mechanical load. The expression is the same for the generator and motor.
When the current-carrying current is placed in the magnetic field, a force is exerted on it which exerts turning
moment or torque F x r. This torque is produced due to the electromagnetic effect, hence is
called Electromagnetic torque.
The torque which is produced in the armature is not fully used at the shaft for doing the useful work. Some
part of it gets lost due to mechanical losses. The torque which is used for doing useful work in known as
the shaft torque.

To establish the torque equation, let us first consider the basic circuit diagram of a DC motor,and its voltage
equation.
E is the supply voltage Eb is the back emfproduced
Ia, Ra are the armature current and armature resistance
respectively
� = �� + �� ------(2)
But keeping in mind that our purpose is to derive the torque
equation of DC motor we multiply both sides of equation (2)
by Ia.
�� = �� + ��
2
∗ �� ------(3)
Where, �� is the electrical power input to the armature.
��
2
∗ ��is the copper loss in the armature.

We know that,
Total electrical power supplied to the armature = Mechanical power developed by the armature +
losses due to armature resistance
The mechanical power developed bythe armature is Pm, �� = �� ------------------ (4)
Also, the mechanical power that rotates the armature can be given regarding torque T and speed n.
��= �� = 2��
Where nis in revolution per seconds (rps) andTis in Newton-Meter.
Equate (4) and (5), we get �� = 2��
� =
��
2��
------(5)
ButEb is given by
�
� =
60

W
h
e
r
e
N
i
s
t
h
e
s
p
e
e
d
i
n
r
e
v
o
l
u
t
i
o
n
p
e
r
m
i
n
u
te (rpm) and
Where n is the speed in (rps).
�P Φ �
�� =
��
∗
�
��

Where N is the speed in revolution per minute (rpm) and
�
� =
60
Where n is the speed in (rps).
Hence we can write torque equationas,
� =
��
2��
ΦnZP
��
� =
2��
�
�
�
= �.
��
For a particular DC Motor, the number of poles (P) and the number of conductors per parallel path (Z/A)
are constant.
� =K*Φ
�
�
��
=
ΦnZP
�
��
��
�P Φ �
�� =
��
∗
�
��
�� ∗ �∗ P Φ �
�� =
��
∗
�
��
ΦZP��
� =
2��
0.159 ∗ ΦZP��
� =
�
ZP
� =
2��

Where,

� =K*Φ ��
Thus, from the above equation, it is clear that the torque produced in the armature is directly proportional
to the flux per pole and the armature current.
The direction of electromagnetic torque developed in the armature depends upon the current in armature
conductors. If either of the two (flux or current) is reversed the direction of torque produced is reversed
andhence the direction of rotation. But when both are reversed, anddirection of torque does not change.

Types of DCMotor
Separately Excited DC Motor
As the name signifies, the field coils or field
windings are energised by a separate DC source
as shown in the circuit diagram shown below:
Self-excited DC Motor
As the name implies self-excited, hence, inthis
type of motor, the current in the windings is
supplied by the machine or motor itself.
Shunt wound or shunt motor
Series wound or series motor
Compound wound orcompound motor.

Types of DCMotor
This is the most common types of DC Motor. Here the field winding
is connected in parallel with the armature as shown in the figure
below:

Types of DCMotor
The current, voltage and power equations for a shunt motor are
written as follows.
By applying KCL at junction in the above figure.
Where,
�� = �
� + �
� ……(1)
I or �� is the input line current
��is the armaturecurrent
�� or ��ℎ is the shunt field current
The voltage equations are written by using Kirchhoff’s voltage law (KVL) for the field winding circuit.
� = ��ℎ * ��ℎ
For armature winding circuit the equation will be givenas:
……(2)
� = �� + ��* �� ……(3)
�
�
��
ℎ
�
�

Types of DCMotor
The power equation is givenas:
Power input = mechanical power developed + losses in the
armature + loss in thefield.
�� = �� + ��
2
�� + ��ℎ
2
��ℎ ……(4)
From (2), � = ��ℎ * ��ℎ ��ℎ
=
�
��
ℎ
Put this in (4),
�� = �� + ��
2
�� + � ∗ �� = �� − ��
2
�� − � ∗ �� = �(�−��ℎ ) −��
2
��
From (1), �� = ��+
��
� = ��+ ��ℎ �� = �− �� = �� −��
2��
��= ��(� − �� ∗ ��)
�
�
��
ℎ
�
�

Types of DCMotor
From (3),
�� = ��(� − �� ∗ ��)
� = �� +��* �� = � − ��* ��
�� = �� ∗ ��
Multiplying equation (3) by
�
�
……(5)
we get the following equations.
� = �� + �
�* �
� ……(3) � ∗ �� = �� ∗ �� + ��
2
∗ �
�
……(6)
� ∗ �� = ��+ ��
2
∗ �� ……(7)
Where,
�
�
��
ℎ
�
�

Types of DCMotor
� ∗ �� is the electrical power supplied to the armature of the motor.

Concept of Load Torque
Torque has 2 main components: load torque and acceleration torque.
Load torque is the amount of torque constantly required for application and includes friction load
and gravitational load.
Load torque: The torque which is generated due to load connected to the motor.
DC motor is meant toproduce rotational kinetic energy andit does byexerting torque on theload.
That TORQUE, which it exerts upon the load through its shaft,is called the load torque.

Types of Loads
Definition: The device which takes electrical energy is known as the electric load.
In other words, the electrical load is a device that consumes electrical energy in the form of the current and
transforms it into other forms like heat, light, work, etc.

Types of Loads
Resistive Load
The resistive load obstructs the flow of electrical energy in the circuit and converts it into thermalenergy,
due to which the energy dropout occurs in the circuit. Loads consisting of any heating element are classified
as resistive loads.
The lamp and the heater, incandescent lights, toasters, ovens, space
heaters and coffee makers are the examples of the resistive load.
The resistive loads take power in such a way so that the current and the
voltage wave remain in the same phase. Thus the power factor of the
resistive load remains in unity.

Types of Loads
Inductive Load
The inductive loads use the magnetic field for doing the work.
These are found in a variety of household items and devices with moving parts, including fans, vacuum
cleaners, dishwashers, washing machines and the compressors in refrigerators and air conditioners. The
transformers, generators, motor are the examples of the load.
The inductive load has a coil which stores magnetic energy when the current pass through it. Thecurrent
wave of the inductive load is lagging behind the voltage wave, and the power factor of the inductive load is
also lagging.

Types of Loads
Capacitive Load
In the capacitive load, the current wave is leading the voltagewave.
The examples of capacitive loads are capacitor bank, three phase induction motor starting circuit, etc. The
power factor of such type of loads is leading. In engineering, capacitive loads do not exist in a stand-alone
format. No devices are classified as capacitive in the way light bulbs are categorized as resistive, and air
conditioners are labeled inductive.

Types of Electrical Loads in PowerSystem
Domestic load
The domestic load is defined as the total energy consumed by the electrical appliances in the household
work. It depends on the living standard, weather and type of residence.
The domestic loads mainly consist of lights, fan, refrigerator, air conditioners, mixer, grinder, heater, ovens,
small pumping, motor, etc. Most of the domestic loads are connected for only some hours during a day. For
example, lighting load is connected for few hours during night time.
The domestic load consume very little power and also independent from frequency. This load largely
consists of lighting, cooling or heating.

Commercial load
Commercial loadmainly consist of electrical loads that are meant to be usedcommercially, such as lightning
of shops, offices, advertisements, etc., Fans, Heating, Air conditioning and many other electrical appliances
used in establishments such as market restaurants, etc. are considered as a commercial load.
This type of load occurs for more hours during the day as compared to the domestic load.

Industrial Loads
Industrial load consists of small-scale industries, medium scale industries, large scale industries, heavy
industries and cottage industries.
The induction motor forms a high proportion of the composite load. Industrial loads may be connected
during the whole day.
The industrial loads are the composite load. The composite load is a function of frequency and voltage and
its form a major part of the system load.

Agriculture/Irrigation Loads
Motors andpumps usedin irrigation systems tosupply thewater for farming come under thiscategory.
Generally,irrigation loads are supplied during off-peak or night hours.
This type of load is mainly motor pumps-sets load for irrigation purposes. The load factor of this load is very
small e.g. 0.15 –0.20.

Some Other Classifications Of Electrical Loads
According To Load Nature
• Linear loads: Linear loads like transformers, motors follow ohm’s law (as long as their core isnot
saturated) i.e. their current change linearly with the change in applied voltage.
• Non-linear loads: non linear load like switching regulator current is not linearly changing with the
change in the applied voltage, it takes lesser current as applied voltage increases.
According To Phases
• Singlephaseloads
• Three phaseloads

Dynamics of Motor and Load Combination
When an electric motor rotates, it is usually connected to a load which has a rotational or translational
motion. The speed of the motor may be different from that of the load.
In the translational motion, the position of the body changes from point to point in space. The speed of the
load may be different from that of the motor.
If the load has different parts, their speed may be different. Some part of the rotor may rotate while others
may go through a translationalmotion.
J = Polar moment of inertia of motor load referred to
the motor shaft,kg-m2
ωm – instantaneous angular velocity of the motor
shaft, rad/sec.
T – the instantaneous value of developed motor
torque, N-m.
T1 – the instantaneous value of load torque, referred
to a motor shaft,N-m.

Torque Equation of Motor Load System of Fig. can be
described by the following fundamental torque
equation:
� − �1 =
�
��
� − � = �
�
�
� + � �
�
………….(1)
1
��
��
Equation (1) is applicable to variable inertia drives such as mine winders, reel drives, industrial robots.
For drives with constant inertia (a property of matter by which it continues in its existing state of rest or
uniform motion in a straight line, unless that state is changed by an external force.), (dJ/dt) = 0.
� −
�1
= �
�
�
�
�
�
� =
�1
+ �
�
�
�
�
�
………….(2)
In the above equation the motor torque is considered as an applied torque and the load torque as a resisting
torque.
Equation (2) shows that torque developed by motor is counter balanced by a load torque T1 and a dynamic
��
�

torque J(dωm/dt). Torque component J(dωm/dt) is called the dynamic torque because it is present only
during the transient operations. i.e., when the speed of the drive varies.

Case 2)When �<
��
��
i.e. thedrive will be decelerating and, particularly coming torest.
��
<
0
The acceleration or deceleration of the drive mainly
depends on whether the load torque is greater or less than
the motor torque.
� − �1 = �
��
��
Case 1) When � >
��
�
�
i.e. thedrive will be accelerating, in particular,picking, up speed toreach rated speed.
��
>
0

Characteristics of DC Shunt Motor
Generally, three characteristic curves are considered important for DC motors related to its performance
which are, (i) Torque vs. armature current, (ii) Speed vs. armature current and (iii) Speed vs. torque.
��
�
�
��ℎ
Here, supply voltage is constant. Same voltage is given to field winding
and armature.
The voltage equations are written byusing Kirchhoff’s voltage law
(KVL) for the field winding circuit.
� = ��ℎ *
��ℎ
�
��ℎ =
��
ℎ
…………….(1)
Where,
��ℎ = Filedcurrent
��ℎ = Field resistance which is
constant
V = supply voltage which is constant.

As V and ��ℎ are constant, the field
current ��ℎ is also constant.
As the field current ��ℎ is also
constant, flux Фis constant in DC
shunt motor.

For armature winding circuit the equation will be given as:
� = �� + ��* ��
But Back emf is:
�� = � − ��*
�
�
……(2)
Where K is constant and it is � =
P
∗
�
��
�� = �Φ � ��
As flux Фis constant in DC shuntmotor,hence �� = �� ……(3)
Where �� is constant and it is �� =�Ф
�P Φ �
�� =
��
∗
�
��
�
�
��
ℎ
�
�

�� = � − ��*
��
……(2) �� = �� ……(3)
The armature torque is directly proportional to theproduct of theflux andthearmature current i.e. Ta ∝ Φ.Ia
Ta ∝ Φ.Ia Ta =�� Φ.Ia
Where K2 is constant
As flux Ф is constant in DC shunt motor, hence Ta
=��.Ia
Where �� is constant and it is �� =��Ф
……(4)

Speed Vs. Armature Current (N-Ia) Refer following equations
�� = � − ��*
��
……(2) �� = ��
……(3)
Ta =��.Ia ……(4)
Put (3) in (2) �� = � − ��* �� = −
��∗
�
�
��
�
+
�
�
……(5)
Comparing equation (5) it with standard equation of a straightline, Y = mx+C
Y= �
��= −
��∗
��
��
� =
�
��

Speed Vs. Armature Current (N-Ia)
We find that the characteristic is a straight line with intercept
on speed axis equal to V/K1 , called no-load speed i.e when Ia is
negligibly small just to provide torque to overcome friction and
windage.
The characteristic show that dc shunt motor speed remains
almost constant from no-load ( Ia≃ 0) to full load ( Ia=rated
value) and that is why dc shunt motor is used in applications
where speed isrequired toremain almost constant throughout
the operation e.g Lathes, spinning and weaving machines,
drills etc.
If armature reaction is taken into account, speed drops
slowly, represented by the red curve.
V/K1

Torque Vs. Armature Current (Ta-Ia)
From (4) Ta =��.Ia
Therefore, in a dc shunt motor, the torque increases linearly
with armature current as shown by the black curve.
But, for larger Ia , the net flux per pole decreases due to the
demagnetizing effect of armature reaction and hence the curve
deviates from straight line as shown by red curve.

Speed Vs.Torque (N-T) Flux Фisconstantin DCshuntmotor. Refer following equations
�� = � − ��*
��
……(2) �� = ��
……(3)
Ta =��.Ia ……(4)
� = −
��∗ ��
��
�
+
�
�
……(5)
From (4) Ta =� .Ia � =
�� Put this in equation (5)
� �
�3
��
�3
∗ ��
�
� =− ��∗ ��
+
�
……(6)
� =−
�
�
+
�
�
��
��
��

From this, it is observed that speed N is proportional
to torque.

Speed Vs. Torque (N-T)
� = −
��∗
�
�
��
��
�
+
�
�
……(6)
From this equation speed-torque characteristics of dc shunt
motor can be drawn which turns out to be a straight line
shown by black curve.
When armature reaction is also considered i.e flux is no
longer constant , the speed-torque equation becomes
� = −
��∗ ��
�Ф��Ф
�
+
�
Ф
�� = �Ф
�� = ��Ф
Obviously, due to armature reaction, as flux decreases the

first term increases more than the second term , therefore the
speed drops more rapidly with increase in torque.

Speed control methods of DC shunt motor
��
� ∝
�
Back emf Eb of a DC motor is nothing but the induced emf in armature conductors due to rotation of the
armature in magnetic field. Thus, the magnitude of Eb can be given by EMF equation of a DC generator.
……(1)
(where, P = no. of poles, Ø= flux/pole, N = speed in rpm, Z = no. of armature conductors, A = parallel paths)
Eb can also be given as, �� = � − ��*
��
thus, from the above equations (1)
……(2)
……(3)
but, for a DC motor A, P andZ are constants ……(4)
This shows thespeed ofadcmotor isdirectly proportional tothebackemf andinversely proportional tothe
flux per pole.
� P Φ �
�� =
��
∗
�
��
�� ∗ �� ∗
�
� =
��

� ∝
��
�
……(4) �
� = � − �
�* �
� ……(2)
The above equation shows that the speed depends upon the supply voltage V,the armature circuit resistance
Ra, and the field flux Ф,which is produced by the field current.
In practice, the variation of these three factors is used for speed control. Thus, there are three general
methods of speed control ofD.C. Motors.
� − ��∗
��
� ∝
�
Resistance variation in the armature circuit: This method is called armature resistance control or
Rheostat control.
Variation of field flux Ф. This method is called field flux control.
Variation of the applied voltage. This method is also called armature voltage control.

Theclassification ofspeed controlmethods foraDCshuntmotor areasfollows. These twomethodsare:
1. Armature Control Methods
2. Field Control Methods
Armature Controlled DC Shunt Motor
a) Armature Resistance Control
b) Armature VoltageControl
Field Controlled DC Shunt Motor Variation in field fluxThis method is known as Field
Flux Control.

Re
Rheostat control.
In this method, a variable series resistor Re is put in the armature
circuit. The figure (a) above shows the process of connection for
a shunt motor. In this case, the field is directly connected across
the supply and therefore the flux Фis not affected by variation of
Re.
The voltage drop in Re reduces the voltage applied to the
armature, and therefore the speed is reduced.
Speed control of a d.c. Shunt motor by
armature resistance control.

Rheostat control.
Re
� = �� + ��* �� = � − ��*
�
�
……(2)
DC Shunt Motor
For armature winding circuit with presence of Re, the equation will be
given as:
� = �� +��* (�� + ��) �� = � − ��* (�� +��)
�
�
��
ℎ
�
�

Rheostat control.
A large amount of power (I*R) is wasted in the external resistance Re. i.e. ��* (�� + ��)

Rheostat control.
DC Shunt Motor
Here value of flux also changes due to change in armature current. But
that change will be less compare to change in Eb value.
� ∝ �� ∝ � − �� ∗ ��
As Eb value reduces due to addition of Re, speed decreases.
� − ��∗
��
� ∝
�
�
�
��
ℎ
�
�
Re

Rheostat control.
Drawbacks:
A large amount of power is wasted in the external resistance Re.
Armature resistance control is restricted to keep the speed below the normal speed of the motor and
increase in the speed above normal level is not possible by this method.
For a given value of Re, the speed reduction is not constant but varies with the motor load.

Speed control ofaD.C. shunt
motor by variation of field flux.
Since the field current produces the flux, and if we control the
field current then the speed can be controlled.
In the shunt motor, speed can be controlled by connecting a
variable resistor Rc in series with the shunt field winding.
In the diagram below resistor, Rc is called the shunt field
regulator.
To control the flux , the rheostat (variable resistor Rc) is added
in series with the field winding will increase the speed (N),
because of this flux will decrease. So, the field current is
relatively small and hence I2R loss is decreased.

The voltage equations are written byusing Kirchhoff’s voltage law (KVL)
for the field winding circuit.
� = ��ℎ *
��
ℎ
��ℎ
�
=
��
ℎ
…………….(1)
DC Shunt Motor
The voltage equations are written by using Kirchhoff’s voltage law (KVL)
for the field winding circuit when a variable resistor Rc in series with the
shunt field winding.
� = ��ℎ * (��ℎ +�� ) ��ℎ
=
�
(� +
� )
…………….(2)
�ℎ �
When Rc is added in series with field winding, current in equation (2) is less
compared with equation (1) for constant value of V. Hence flux decreases and
�
�
��
ℎ
�
�

in turn we can control speed.

Advantages:
This method is easy and convenient.
As the shunt field is very small, the power loss in the shunt field is also small.
The flux cannot usually be increased beyond its normal values because of the saturation of the iron.
Therefore, speed control by flux is limited to the weakening of the field, which gives an increase in speed(as N
is inversely proportional to flux Ф).
This method is applicable over only to a limited range because if the field is weakened too much, there is a
loss of stability.

In armature voltage control method the speed control is achieved by varying the applied voltage in the
armature winding of themotor.
This speed control method is also known
as Ward Leonard Method. This method
was introduced in 1891.

In the above system, M is the main DC motor whose
speed is to be controlled, and G is a separately excited
DC generator.
The generator G is driven by a 3 phase driving motor
which may be an induction motor or a synchronous
motor.
The combination of an AC driving motor and the DC
generator is called the Motor-Generator (M-G) set.
The voltage of the generator is changed by changing the
generator field current. This voltage when directly applied
to the armature of the main DC motor, the speed of the
motor M changes.
The motor field current Ifm is kept constant so that the motor
field flux ϕm also remains constant. While the speed of the
motor is controlled, the motor armature current Ia is kept
equal to its ratedvalue.

The generated field current Ifg is varied such thatthe
armature voltage Vt changes from zero to its rated
value. The speed will change from zero to the base
speed.
Since the speed control is carried out with the rated
current Ia and with the constant motor field flux, a
constant torque is directly proportional to the
armature current, and field flux up to rated speed is
obtained.
The product of torque and speed is known as power, and it
is proportional to speed. Thus, with the increase in power,
speed increases automatically.

Advantages of Ward Leonard Drives
Smooth speed control of DC motor over awide range in both thedirection is possible.
It has an inherent braking capacity.
Uniform acceleration is obtained
Disadvantages of Ward Leonard Drives
Larger size and weight.
The initial cost of the system is high as there is a motor-generator set installed, of the same rating as that of
the main DC motor.
Maintenance of thesystemisfrequent.
Higher losses and less efficiency.

Applications of Ward Leonard Drives
The Ward Leonard drives are used where smooth speed control of the DC motors over a wide range in both
the directions is required. Some of the examples are as follows:
 Rolling mills
 Elevators
 Cranes
 Paper mills
 Diesel-electric locomotives
 Mine hoists

Reversal of direction of rotation of DC motor
DC motors can turn in either direction (clockwise or counter-clockwise) and can be easily controlled by
inverting the polarity of theapplied voltage.
The direction of force, and therefore rotation, is explained using Fleming’sLeft-Hand Rule for Motors.
Your first finger represents the magnetic
field, pointing straightdown tothefloor.
Your middle finger represents the
current, pointing towards the
computer screen.
Your thumb represents the resulting
force, which points left.
This shows us that with the current flowing through the wire “into”the computer screen will cause a force
pushing left, in our model, this is equivalent to the motor turning counter-clockwise.

howwe changetheforce sothewire travelsin theopposite direction, causing our motor torotatein ‘reverse’.
We can use Fleming’sleft-hand rule again, with the same magnetic field, but this time use our thumbs to point
right instead of left. As a result, your middle finger should now point towards yourself, showing the current
flows out of thescreen.
This shows that in order to
make the motor rotate
clockwise, we must reverse
the flow of current (i.e.
changing the flow of current
changes the direction of the
force by 180 degrees).
Of course, the direction of current is controlled by the polarity of the voltage.
So in order to change the direction of rotation, we can simply reverse the voltage, causing the current to flow
in the opposite direction, changing the force by 180 degrees and the motor to be driven ‘backwards’.

It can be seen from the Fig. 2 that
if the direction of the main field in which current carrying conductor is placed, is reversed, force experienced
by the conductor reverses its direction (refer case a and c or b and d).
Similarly keeping main flux direction unchanged, the direction of current passing through the conductor is
reversed. The force experienced by the conductor reverses its direction. (refer case a and b or c and d).
However if both the directions are reversed, the direction of the force experienced remains the same.

So in a practical motor, to reverse its direction of rotation, either direction of main field produced bythe field
winding is reversed or direction of the current passing through the armature is reversed.
The direction of the main field can be reversed by changing the direction of current passing through the field
winding, which is possible by interchanging the polarities of supply which is given to the field winding.

Braking in DCmotor
A running motor may be brought to rest quickly by either mechanical braking or electrical braking.
Electrical Braking isusually employed inapplications tostop aunit driven bymotors in anexactposition or
to have the speed of the driven unit suitably controlled during its deceleration.
Electrical braking is usedin applications where frequent, quick, accurate or emergency stops arerequired.
Electrical Braking allows smooth stops without any inconvenience to passengers.
When aloaded hoist is lowered, electric braking keeps thespeed within safe limits. Otherwise, the machine or
drive speed will reach dangerous values.
When a train goes down a steep gradient, electric braking is employed to hold the train speed within the
prescribed safe limits.
Electrical Braking is more commonly used where active loads are applicable.
In spite of electric braking, the braking force can also be obtained by using mechanical brakes.

Braking in DCmotor
Disadvantages of Mechanical Braking
 It requires frequent maintenance and replacement of brake shoes.
 Braking power is wasted in the form of heat.
Types of Electrical Braking:
Electrical Braking:
Regenerative
Braking
Dynamic or
Rheostatic Braking
Plugging or
Reverse Current Braking.

Regenerative Braking in DC shunt motor
In Regenerative Braking, the power or energy of the driven machinery which is in kinetic form is returned
back to the power supply mains.
A machine operating asmotor maygo intoregenerative braking mode if its speed becomes sufficiently high so
as to make back emf greater than the supply voltage i.e., Eb > V.
� = �� + ��* �� = � − ��*
�
�
……(1)
Obviously under this condition the direction of Ia will reverse imposing torque which is opposite to the
direction of rotation as Ta ∝Φ.Ia.
�
�
��
ℎ
�
�

The situation is explained in figures (a) and (b).
The normal motor operation is shown in figure (a)
where armature motoring current Ia is drawn from
the supply and as usual Eb <V.
� = � ∗ � Φ
� �
Fig. (a) Machine operates asmotor
The question is how speed on its own become large enough tomake Eb < V causing regenerativebraking.
Such a situation may occur in practice when the mechanical load itself becomes active.
��P Φ �
�� =
��
∗
�
��

Fig. (a) Machine operates as motor
Imagine the d.c motor is coupled to the wheel of
locomotive which is moving along a plain track
without any gradient as shown in figure (a).
Machine is running as a motor at a speed of n1 rpm.
However, when the track has a downward gradient
(shown in figure b), component of gravitational force
along the track also appears which will try to
accelerate the motor and may increase its speed to n2
such that �� = ��2 >�.
In such a scenario, direction of Ia reverses, feeding
power back to supply.
Regenerative braking here will not stop the motor
but will help toarrest rise of dangerously high speed.
Fig.(b)Machineentersregenerative brakingmode.

The necessary condition for regeneration is thatthe back EMF Eb should be greater thanthesupply voltage so
that the armature current is reversed and the mode of operation changes from motoring to generating.
Applications of Regenerative Braking
 Regenerativebraking is used especially where frequent braking and slowing of drives is required.
 It is most useful in holding a descending load of high potential energy at a constant speed.
 Regenerative braking is used to control the speed of motors driving loads such as in electric
locomotives, elevators, cranes and hoists.
 Regenerativebraking cannot be used for stopping the motor.Itis used for controlling the speed above
the no-load speed of the motor driving.

Thank
You
22/01/2022 Department of Electronic Telecommunication Engg. 81

DC Machine Construction and Working

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to DC Machine Construction and Working

Similar to DC Machine Construction and Working (20)

Recently uploaded

Recently uploaded (20)

DC Machine Construction and Working