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DC MACHINES
ROTATING ELECTRICAL MACHINES
Applications such as light bulbs and heaters require energy in electrical form
...
One form of energy can be
obtained from the other form with the help of converters
...
The process
of translation is known as electromechanical energy conversion
...
In these machines the conversion is
reversible
...

If the conversion is from electrical to mechanical, the machine is said to act as a motor
...
Machines are called
ac machines (generators or motors) if the electrical system is ac and dc machines (generators or
motors) if the electrical system is dc
...
In the electrical system
the primary quantities involved are voltage and current, while the analogous quantities in the
mechanical system are torque and speed
...

ELECTROMAGNETIC CONVERSION
Three electrical machines (dc, induction, and synchronous) are used extensively for
electromechanical energy conversion
...
When a conductor moves in a magnetic field, voltage is induced in the conductor
...
When a current-carrying conductor is placed in a magnetic field, the conductor experiences a
mechanical force
...
In motoring action, the electrical system makes current flow through
conductors that are placed in the magnetic field
...
If the
conductors are placed on a structure free to rotate, an electromagnetic torque will be produced,
tending to make the rotating structure rotate at some speed
...
In generating action, the process is reversed
...
A voltage will be induced in the conductors that are rotating with the rotor
...
Moreover, the current flowing through the conductor will
interact with the magnetic field to produce a reaction torque, which will tend to oppose the torque
applied by the prime mover
...

The basic electric machines (dc, induction, and synchronous), which depend on electromagnetic
energy conversion, are extensively used in various power ratings
...

Stator: This part of the machine does not move and normally is the outer frame of the machine
...

Both stator and rotor are made of ferromagnetic materials
...
Conductors are placed in these slots
...
If the stator or rotor (or both) is subjected to a time-varying magnetic flux, the iron core is
laminated to reduce eddy current losses
...

The conductors placed in the slots of the stator or rotor is interconnected to form windings
...
The winding through which a
current is passed to produce the primary source of flux in the machine is called the field winding
...

Permanent magnets are used in some machines to provide the major source of flux in the machine
...
The three basic and
common ones are dc machines, induction machines and synchronous machines
...


DC MACHINES
The dc machines are versatile and extensively used in industry
...


2

Although a dc machine can operate as either a generator or a motor, at present its use as a generator is
limited because of the widespread use of ac power
...
Its speed can be controlled over a wide range with relative ease
...
DC
motors still dominate as traction motors used in transit cars and locomotives
...
The dc machine definitely plays an
important role in industry
...
The stator has salient poles that are excited by one or more held windings, called shunt field
windings and series field windings
...

The voltage induced in the turns of the armature winding is alternating
...
The
brushes are so placed that when the sides of an armature turn (or coil) pass through the middle of the
region between field poles, the current through it changes direction
...
As a consequence, the mmf due to the armature current
is along the axis midway between the two adjacent poles, called the quadrature (or q) axis
...

Brushes are made of carbon or graphite and are in rectangular block
...

ARMATURE WINDINGS
As stated earlier, in the dc machine the field winding is placed on the stator to excite the field poles,
and the armature winding is placed on the rotor so that the commutator and brush combination can
rectify the voltage
...
Before these are
discussed, some basic components of the armature winding and terms related to it are defined
...

3

A coil is formed by connecting several turns in series
...

There are a number of ways in which the coils of the armature windings of a dc machine can be
interconnected
...

LAP WINDING
In a lap winding the number of parallel paths (A) is always equal to the number of poles (P) and also
to the number of brushes
...


-

There are as many numbers of parallel paths in the armature as the number of poles
...


-

The emf between +ve and –ve brushes is equal to the emf generated in any one of the parallel
paths
...

NOTE: In wave windings;
-

Only two brushes are necessary, though their number may be equal to the number of poles
...
That is why this winding is known as two circuit or series winding
...


DC GENERATOR
An electrical generator is a machine which converts mechanical energy into electrical energy
...

Whenever a conductor cuts magnetic flux, dynamically induced emf is produced in it according to
faraday‘s law of electromagnetic induction
...

Parts of a dc generator
(i) Magnetic frame/Yoke
(ii) Pole core and pole shoes
(iii) Pole coil/field coil
(iv) Armature core
4

(v) Armature winding/conductors
(vi) Commutator
(vii) Brushes and bearings
Types of DC Generator
Based on the way in which generator field are excited, the generator may be classified as;
(a) Separately Excited Generator
...
You can
see the representation in the figure below
...
The higher the speed of rotation and current the higher
the output e
...
f
Note: Separately excited DC generators are rarely used in practice

Fig: Circuit Diagram for Separate Excited DC Generator
(b) Self-Excited Generator
...
Based on the way in which the field winding are connected to
the armature, these generators can be classified as;
(i)

Shunt wound: A shunt DC generator is shown in figure below in which the field
winding is wired parallel to armature winding so that the voltage across both winding
is same
...


5

(ii)

Series wound: A series DC generator is shown in figure below in which the armature
winding is connected in series with the field winding so that the field current flows
through the load as well as the field winding
...
Series generators are also rarely used except for special
purposes, eg: Boosters, etc
...
It has two field
findings namely Rsh and Rse
...

In compound generator, shunt field is stronger than series field
...
Compound generator is of two types (1) Short shunt and (2) Long shunt
...
It is shown in figure below:

6

Long shunt: - Here the shunt field winding is parallel to both armature and series field winding (Rse
is wired in series to the armature)
...
Its values depend on amount of current and value of
contact resistance
...
In practice, the brush
contact drop has the following constants, ie:- 0
...

GENERATED EMF OR EMF EQUETION OF A GENERATOR
As referred to the class session drived equation, the generator emf equation is:
Eg 

PNZ
60 A

Examples:
1
...
03ohms
...

2
...
05 ohms, 0
...

Calculate the generated emf and the armature current
...

3
...
05 ohms, 0
...
Calculate the induced emf and the armature current
...

4
...
What will be the voltage generated in the machine when driven at
1500 rpm? Assuming the flux per pole to be 7mWb
...
An 8 pole dc generator has 500 armature conductors and a useful flux per pole of 0
...

What will be the voltage generated if it is lap connected and running at 1200rpm? Also, what
will be the speed to be driven to produce the same emf if it is wave wound?
Characteristics of a DC Generator
Generally, following three characteristics of DC generators are taken into considerations: (i) Open
Circuit Characteristic (O
...
C
...

These characteristics of DC generators are explained below
...
Open Circuit Characteristic (O
...
C
...
This characteristic shows the relation between generated emf at no load (E0) and the
field current (If) at a given fixed speed
...
C
...
curve is just the magnetization curve and it is
practically similar for all type of generators
...
C
...
curve is obtained by operating the
generator at no load and keeping a constant speed
...
The connection arrangement to obtain O
...
C
...
For shunt or series excited generators, the field winding is disconnected
from the machine and connected across an external supply
...
C
...
curve of DC Generator
Now, from the emf equation of dc generator, we know that Eg = kɸ
...

8

However, even when the field current is zero, some amount of emf is generated (represented by OA
in the figure below)
...
Due to the residual magnetism, a small initial emf is induced in the
armature
...
This consequently increases the induced emf
...
C
...
follows a straight line
...
Thus, even we increase the If further, ɸ remains constant and hence, Eg also remains
constant
...
C
...
curve looks like the B-H characteristic
...


2
...
The on-load generated emf Eg is always less than E0 due to the armature
reaction
...
Therefore, internal characteristic curve lies below the O
...
C
...

3
...
Terminal voltage V is less than the generated emf Eg due to voltage drop in the armature circuit
...
External
characteristics are very important to determine the suitability of a generator for a given purpose
...

Internal and external characteristic curves are shown below for each type of generator
...
Thus, the straight lines AB in above figure represents the no-load voltage vs load
current IL
...
The curve AC represents the on-load generated emf Eg vs
...
e
...
Also, the terminal
voltage is lesser due to ohmic drop occurring in the armature and brushes
...


load

current

i
...


external

characteristic
...
To build up voltage of a shunt
generator, the generator is driven at the rated speed by a prime mover
...
The generator builds up its voltage as explained by the
O
...
C
...
When the generator has built up the voltage, it is gradually loaded with resistive load
and readings are taken at suitable intervals
...


10

Fig: Circuit for DC Shunt Generator Characteristic
Unlike, separately excited DC generator, here, IL≠Ia
...
Hence, the
internal characteristic can be easily transmitted to Eg vs
...


During a normal running condition, when load resistance is decreased, the load current increases
...
So, load resistance can be
decreased up to a certain limit, after which the terminal voltage drastically decreases due to excessive
armature reaction at very high armature current and increased I2R losses
...
Consequently, the external
characteristic

curve

turns

back

as

shown

by

dotted

line

in

the

above

figure
...
C
...
) curve
...
Hence, here load
current is similar to field current (i
...
IL=If)
...
In a DC series generator, terminal voltage increases with the load current
...
However, beyond a certain
limit, terminal voltage starts decreasing with increase in load
...

Characteristics of DC Compound Generator

The above figure shows the external characteristics of DC compound generators
...
The external characteristic for over
compounded

generator

is

shown

by

the

curve

AB

in

above

figure
...
The external characteristic
for

a

flat

compounded

generator

is

shown

by

the

curve

AC
...
The external characteristics for an under
compounded generator are shown by the curve AD
...

(ii)

Series Generator: Because of its raising characteristics they are used as a booster in a
certain distribution system especially in railways
...


Losses and Efficiency in DC Generator
(i)
Copper losses: This loss is due to the current flowing in the machine
...

(b) Field copper loss = If2Rf for shunt generator and Is2Rs for series generator
...
It is usually included in the armature copper
losses
...
It consists of
(a) Hysteresis loss which is due to the reversal of magnetization of the armature core
...
5Wb/m2), hysteresis loss is given by steinmetz
formula as;

13

1
...
When the armature core rotates it also cuts the magnetic flux,
hence the emf is induced in the body of the core according to law of electromagnetic
induction
...
This
current is known as Eddy current
...
This loss can be reduced by using thin courted
laminations which are insulated from one another by vanish coating
...
The lamination thickness varies from 0
...
01 to 0
...

(iii) Mechanical Losses: This consists of,
(a) Friction loss at bearings and commutator
...

Stray losses
Usually magnetic and mechanical losses are collectively known as stray losses
...

Constant or Standing losses (Wc)
As said above, field Cu loss is constant for shunt and compound generators
...
These losses together are known as standing or constant
losses
...

Condition for Maximum Efficiency
For the efficiency of the machine to be maximum, Variable Losses should be equal to the Constant
losses
...
A shunt generator delivers 195A at a p
...
The armature and shunt field resistance are
0
...
The iron and friction losses equal to 950W
...

(b) Cu losses
...

(d) Overall efficiency
...
A shunt generator has a full load current of 196A at 220V
...
If it has a full load efficiency of 88%, find the armature
resistance
...

15

3
...
If the armature resistance is
0
...
Allow a drop of 1V per brush
...
A long shunt compound wound generator gives 240V at full load output of 100A
...
1 ohm,
series 0
...
025 ohm, shunt field (including regulating resistance) 100
ohms
...
Calculate
full load efficiency of the machine
...
A four pole dc generator has a wave wound armature with 792 conductors
...
0121Wb
...

ARMATURE REACTION
In a DC machine, two kinds of magnetic fluxes are present; 'armature flux' and 'main field flux'
...

MNA and GNA
EMF is induced in the armature conductors when they cut the magnetic field lines
...

MNA (Magnetic Neutral Axis) may be defined as the axis along which no emf is generated in the
armature conductors as they move parallel to the flux lines
...

GNA (Geometrical Neutral Axis) may be defined as the axis which is perpendicular to the stator
field axis
...


16

Consider, no current is flowing in the armature conductors and only the field winding is energized (as
shown in the first figure of the above image)
...
The 'Magnetic Neutral Axis' (M
...
A
...
N
...

The second figure in the above image shows armature flux lines due to the armature current
...

Now, when a DC machine is running, both the fluxes (flux due to the armature conductors and flux
due to the field winding) will be present at a time
...

This effect is called as armature reaction in DC machines
...

Effects of Armature Reaction
(i) It demagnetizes or weakens the main flux
...

(ii) It cross magnetizes or distort it
...
N
...
gets shifted (M
...
A
...
Brushes
should be placed on the M
...
A
...

So, due to armature reaction, it is hard to determine the exact position of the MNA
(iii) The armature reaction shifts brush axis from GNA
...
Thus there will be an induced emf in the coil undergoing

17

commutation which will try to maintain the current in original direction
...

Methods of Reducing Armature Reaction
Usually, no special efforts are taken for small machines (up to few kilowatts) to reduce the armature
reaction
...

Compensating winding:
Now we know that the armature reaction is due to the presence of armature flux
...
Now, if we place another winding in
close proximity of the armature winding and if it carries the same current but in the opposite direction
as that of the armature current, then this will nullify the armature field
...
Compensating winding is
connected in series with the armature winding in such a way that it carries the current in opposite
direction
...
Winding on the interpoles is connected in series with the armature
...
Inter-poles nullify the quadrature axis
armature flux
...
The commutation in DC machine or more specifically commutation in DC
generator is the process in which generated alternating current in the armature winding of a dc
18

machine is converted into direct current after going through the commutator and the stationary
brushes
...

This transformation of current from the rotating armature of a DC machine to the stationary brushes
needs to maintain continuously moving contact between the commutator segments and the brushes
...


Then the coil is short circuited with the help of a brush for a very short fraction of time (1⁄500 sec)
...
After this short-circuit time the armature coils rotates under S pole and
rotates between a negative brush and its succeeding positive brush
...
This phenomenon of the reversal of current is
termed

as

commutation

process
...


The commutation is called ideal if the commutation process or the reversal of current is completed by
the end of the short circuit time or the commutation period
...

Methods of Improving Commutation
There are three methods of sparkles commutation:
1
...
Voltage Commutation
3
...
A DC motor converts DC electrical energy into
mechanical energy
...

In other words, when a magnetic field and an electric field interact, a mechanical force is produced
...
This is known as motoring action
...

20

Structurally and construction wise a direct current motor is exactly similar to a DC generator, but
electrically it is just the opposite
...
We can represent it by the block diagram
shown below
...
e
...

Parameter

K

relates

the

input

and

output

port

variables

of

the

dc

motor
...

Detailed Description of a DC Motor
To understand the DC motor in details let‘s consider the diagram below,

The circle in the center represents the direct current motor
...
On
the brushes, we connect the external terminals, through which we give the supply voltage
...
On the supply terminals, we represent the armature resistance
Ra in series
...
Electric current which flows through the
rotor armature via brushes, in presence of the magnetic field, produces a torque Tg
...

As the armature conductors are carrying currents and the armature rotates inside the stator magnetic
field, it also produces an emf Eb in a manner very similar to that of a generator
...


The back emf like in case of a generator is represented by

Where,
P = no of poles
φ = flux per pole
Z= No
...
of parallel paths
N = speed of the DC Motor
...
‘ that is whenever a direct
current motor rotates; it results in the generation of back Emf
...
So Eb is proportional to ω
...
Thus the voltage
difference between the supply voltage and back emf increases that means E − Eb increases
...
Thus a DC Motor is capable of maintaining the same speed under variable
load
...


22

Now since the armature winding electrical resistance Ra is small, this motor has a very high starting
current in the absence of back Emf
...

Now as the motor continues to rotate, the back emf starts being generated, and gradually the current
decreases as the motor picks up speed
...
Based on
that, there are three main types of DC Motors:
(i) Shunt wound DC motor
(ii) Series wound DC motor
(iii)Compound wound DC motor
DC Shunt Motor
Speed Control, Characteristics & Theory
A DC shunt motor (also known as a shunt wound DC motor) is a type of self-excited DC motor
where the field windings are shunted to or are connected in parallel to the armature winding of the
motor
...
Though there are separate branches for the flow of armature current and field current
as shown in the figure of below
...

This supply current in case of the shunt wound DC motor is split up into 2 parts
...
The
voltage across both windings remains the same
...


Now multiply by Ia both sides,
VIa = EbIa + Ia2Ra
Hence,
VIa = Electrical input to the armature
EbIa = Electrical equivalent to mechanical power developed in the armature
Ia2Ra = Cu loss in the armature
Now in general practice, when the motor is in its running condition and the supply voltage is constant
and the shunt field current given by,

But we know If ∝ Φ
i
...
field flux Φ is proportional to filed current If
Thus the field flux remains more or less constant, and for this reason, a shunt wound DC motor is
called a constant flux motor
...
Thus the field windings
are exposed to the entire armature current unlike in the case of a shunt motor
...
It
consists of all the fundamental components like the stator housing the field winding or the rotor
carrying the armature conductors, and the other vital parts like the commutator or the brush segments
all attached in the proper sequence as in the case of a generic DC motor
...


To understand that let us revert back into the above mentioned basic fact, that the motor has field coil
connected in series to the armature winding
...

1
...


2
...

3
...
To understand that better let‘s look
into the voltage and current equation of DC series motor
...


Let the supply voltage and current given to the electrical port of the motor be given by V and I
respectively
...

Now form the basic voltage equation of the DC motor
...

Rse is the series coil resistance and Ra is the armature resistance
...

Another interesting fact about the DC series motor worth noting is that, the field flux like in the case
of

any

other

DC

motor

is

proportional

to

field

current
...
e
...
And for this
reason, the flux produced in this motor is strong enough to produce sufficient torque, even with the
bare minimum number of turns it has in the field coil
...

Both the field coils provide for the required amount of magnetic flux that links with the armature coil
and brings about the torque necessary to facilitate rotation at the desired speed
...
Like a
shunt wound DC motor is bestowed with an extremely efficient speed regulation characteristic,
whereas the DC series motor has high starting torque
...

Though its starting torque is not as high as in case of DC motor, nor is its speed regulation as good as
a shunt DC motor
...

Types of Compound Wound DC Motor
The compound wound DC motor can further be subdivided into 2 major types on the basis of its field
winding connection with respect to the armature winding, and they are:

27

Long Shunt Compound Wound DC Motor
In case of long shunt compound wound DC motor, the shunt field winding is connected in parallel
across the series combination of both the armature and series field coil, as shown in the diagram
below
...

And Ia, Ise , Ish be the values of current flowing through armature resistance Ra, series winding
resistance Rse and shunt winding resistance Rsh respectively
...
And series field coil is exposed to the entire supply current, before
being split up into armature and shunt field current as shown in the diagram below
...
And Ia, Ise, Ish be the values of current flowing through armature resistance Ra, series
winding resistance Rse and shunt winding resistance Rsh respectively
...

And like in the case of a DC shunt motor,

Equation (2) and (3) gives the current equation of a short shunt compound wound DC motor
...

Cumulative Compounding of DC Motor
A compound wound DC motor is said to be cumulatively compounded when the shunt field flux
produced by the shunt winding assists or enhances the effect of main field flux, produced by the
series winding
...
This particular trait is not
really desirable, and hence does not find much of a practical application
...

Torque Speed and Power Relations
Understanding the torque equation and the relationship between speed and torque is an important part
of selecting and operating a DC motor
...
And when supply voltage is constant, speed is inversely proportional
to the load on the motor
...


The inverse relationship between speed and torque means that an increase in the load (torque) on the
motor will cause a decrease in speed
...
Ia and N ∝ Eb/ɸ

Where:
T = motor torque
V = supply voltage
ω = rotational speed
k = motor constant
R = resistance

30

Armature Torque (Ta)
Electrical power is converted into mechanical power in the armature, then if Ta be the torque
developed by the armature of a motor running at speed N in rps and Ta is in N-m, then; Power
developed at the armature is given by:Pdev  Ta  2N
but
Pdev  Eb I a
Eb I a
   Ninrps
2N
E I
 Ta  9
...
This is so because a part of it is lost to overcome the iron and mechanical (friction and
windage) losses
...

Thus, in the case of DC motors, the actual torque available at the shaft for doing useful mechanical
work is known as Shaft Torque
...
It is
represented by the symbol Tsh
...
p
...
p
...
The shaft torque is expressed as:
POUT  Tsh  2N
POUT
  N  m    Ninrps
2 N
POUT
Tsh 
  N  m    Ninrpm
2N / 60
P
 Tsh  9
...


31

Examples
1
...
8 ohms and field resistance of 200 ohms
...
46kW at 85% efficiency
...


A 4 pole, 400V shunt motor has 540 lap wound conductors
...
85kW
...
The armature resistance
is 0
...
Calculate the speed of the machine and torque
developed in Newton-Meter
...


A dc series motor takes 40A at 220V and runs at 800rpm
...
2 ohms and 0
...
5kW, find the torque
developed in the armature and the output of the motor
...

(ii) Speed vs Armature current (N -Ia) characteristics
...
These characteristics are determined by
keeping the following two relations in mind
...
Ia and N ∝ Eb/ɸ
These above equations can be studied at - emf and torque equation of dc machine
...
e
...

For a machine, P, Z and A are constant, therefore, N ∝ Eb/ɸ
Characteristics of DC Series Motors
Torque vs Armature Current (Ta-Ia)
This characteristic is also known as electrical characteristic
...
Ia
...
e
...
Therefore,
before magnetic saturation of the field, flux ɸ is directly proportional to Ia
...

Therefore, the Ta-Ia curve is parabola for smaller values of Ia
...
Therefore,
the torque varies proportionally to Ia only, T ∝ Ia
...

The shaft torque (Tsh) is less than armature torque (Ta) due to stray losses
...

In DC series motors, (prior to magnetic saturation) torque increases as the square of armature
current, these motors are used where high starting torque is required
...
Hence, for small currents speed is inversely proportional to ɸ
...
Therefore, when armature current Ia
is very small the speed becomes dangerously high
...

But, at heavy loads, armature current Ia is large
...


Due

to

decreased

Eb,

more

armature

current

is

allowed
...
From the above two characteristics of
DC series motor, it can be found that when speed is high, torque is low and vice versa
...
Though at heavy loads, ɸ
decreases in a small amount due to increased armature reaction
...
Hence, the Ta-Ia characteristic
for a dc shunt motor will be a straight line through the origin
...


Speed vs Armature Current (N-Ia)
As flux ɸ is assumed to be constant, we can say N ∝ Eb
...
But practically, ɸ as well as Eb decreases with increase in load
...
Generally, the speed
decreases only by 5 to 15% of full load speed
...
In speed vs
...

34

Fig: Speed Vs Armature Current Characteristic Curve (Shunt Motor)
Speed - Torque Curve for D
...
Shunt Motor
From the plot we can see as the load (Torque) on the shunt motor increase there is slight fall in the
motor speed
...
Therefore shunt motor will approximately be operated at same speed under no load and
full load conditions

Fig: Speed Vs Torque Characteristic Curve (Shunt Motor)
Characteristics of DC Compound Motor
DC compound motors have both series as well as shunt winding
...
And if the series flux is opposite to the direction of the
shunt flux, then the motor is said to be differentially compounded
...

Differential Compound DC Motors
Since in differential field motors, series flux opposes shunt flux, the total flux decreases with increase
in load
...

Differential compound motor are rarely used due to its poor Torque-Speed characteristics
...
But at very high load / torque the speed
is very high to damage the motor
...
Speed torque curve for differential compound motor
Cumulative compound motor
Cumulative compound motors are used where series characteristics are required but the load is likely
to be removed completely
...

These motors have generally employed a flywheel, where sudden and temporary loads are applied
like in rolling mills
...

(ii) When the load / torque are low the motor runs at the safe speed
...


Fig
...

36

Fig
...
C
...
This intentional change of drive speed
is known as speed control of a DC motor
...
This is different to speed regulation – where the speed is trying to be maintained (or
‗regulated‘) against the natural change in speed due to a change in the load on the shaft
...

37

Hence the speed can be varied by changing:
(i) The terminal voltage of the armature, V
...

(iii)The flux per pole, φ
...

Speed Control of DC Series Motor
Speed control methods for a DC series motor can be classified as:
(i) Flux control method
(ii) Variable resistance in series with the motor
Flux Control DC Series Motor
Variation of flux in series motor can be bought in any of these methods:
(i) Field Diverters
(ii) Armature diverters
(iii)Trapped field control
(iv) Paralleling field coil

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Variable resistance in series with the motor

Speed Control of DC Shunt Motor
(i) Variation of flux or flux control method
(ii) Armature or Rheostat control method
(iii)Voltage control Method
Variation of flux or flux control method
N α 1/ɸ

Fig: Flux control method
Armature or Rheostat control method
N α V – I a R a α Eb

Fig: Rheostat control method
Qn
...

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