Presentation on DC Machines

Contents :-
 DC motor and its working principle
 Fleming’s left hand rule for motors
 DC motor and its types
 Parts of dc motor
 DC motor speed control methods
 Back emf and its advantages
 DC generator and its working principle
 Fleming’s right hand rule for generators
 Parts of DC generator
 Losses in DC machines
 DC generator and its types

A direct current (DC) motor is a type of electric machine that converts electrical energy into mechanical
energy. DC motors take electrical power through direct current, and convert this energy into mechanical
rotation.
DC MOTOR

WORKING PRINCIPLE OF DC MOTOR
DC motors operate on Faraday’s principle of
electromagnetism which states that a current-
carrying conductor experiences a force when placed
in a magnetic field. According to Fleming’s “Left-
hand rule for electric motors,” the motion of this
conductor is always in a direction perpendicular to
the current and the magnetic field.
Mathematically, we can express this force as
F = BIL (where F is force, B is the magnetic field, I
stand for current, and L is the length of the
conductor).

The direction of rotation of a this motor is given by
Fleming’s left hand rule, which states that if the index
finger, middle finger, and thumb of your left hand are
extended mutually perpendicular to each other and if the
index finger represents the direction of magnetic field,
middle finger indicates the direction of current, then the
thumb represents the direction in which force is
experienced by the shaft of the DC motor.
Fleming’s left hand rule for motors

1. SEPARATELY EXCITED DC MOTOR
In this case the supply is given separately to the field
and armature winding. The main distinguishing fact in
these types of dc motor is that, the armature current
does not flow through the field winding, as the field
winding is excited from an external source of dc current
as shown in the figure alongside.

2. PERMANENT MAGNET DC MOTOR
It has the usual armature winding like the other
motors, but the field winding are not necessarily
present. In this radially magnetized permanent
magnets are mounted on the inner periphery of the
stator core to produce the required field flux. The
rotor however has a conventional dc armature with
commutator segments and brushes. The
diagrammatic representation of a permanent magnet
dc motor is given along.

3. SELF-EXCITED DC MOTOR
In this the shunt winding is connected partly or completely in series or parallel, to
the armature winding and so it can thus be subdivided as:
a. SHUNT WOUND SELF EXCITED DC MOTOR
In this case , the field winding are exposed to the entire
terminal voltage as they are connected in parallel to the
armature winding as shown in the figure. The shunt
wound dc motor is a constant speed motor, as the speed
does not vary here with the variation of mechanical load
on the output. It comes under the category of Self excited
DC Motor.

b. SERIES WOUND SELF EXCITED DC MOTOR
In this case, the entire armature current flows through the
field winding as its connected in series to the armature
winding. The series wound self excited dc motor is
diagrammatically represented for better understanding. In a
series wound dc motor, the speed varies with load. And
operation wise this is its main difference from a shunt wound
dc motor.

c. COMPOUND WOUND SELF EXCITED DC MOTOR
 The compound excitation characteristic in a dc motor is
obtained by combining the operational characteristic of
both the shunt and series excited dc motor. The compound
wound self excited dc motor or simply compound wound dc
motor essentially contains the field winding connected both
in series and in parallel to the armature winding. The
excitation of compound wound dc motor can be of two
types depending on the nature of compounding.
 Furthermore it is divided into:-
• Cumulative Compound motors
• Differential Compound motors

• Cumulative Compound DC Motor, in which the shunt field flux assists the main field flux, produced by the
main field connected in series to the armature winding. total = series + shunt.
• Differential compound dc motor, in which the arrangement of shunt and series winding is such that the field
flux produced by the shunt field winding diminishes the effect of flux by the main series field winding. The net
flux produced in this case is lesser than the original flux and hence does not find much of a practical
application. total = series – shunt
• Both the cumulative compound and differential compound dc motor can either be of short shunt or long
shunt type depending on the nature of arrangement.

Short Shunt DC Motor
If the shunt field winding is only parallel to the armature winding and
not the series field winding then its known as short shunt dc motor
or specifically short shunt type compound wound dc motor. Shunt is
basically an arm that is connected in parallel, so it is c a Short Shunt
DC Motor. DC Shunt motor is used in devices where Speed control is
essential. Its major application is in centrifugal pumps as they
produces constant flux.
Long Shunt DC Motor
If the shunt field winding is parallel to both the armature winding
and the series field winding then it’s known as long shunt type
compounded wound dc motor or simply long shunt dc motor.

A DC motor is constructed with:
 YOKE :-The magnetic frame or the yoke of DC motor made up of cast iron or steel and forms an integral
part of the stator or the static part of the motor.
Its main function is to form a protective covering over the
sophisticated inner parts of the motor and provide support to the armature. It also supports the field system by
housing the magnetic poles and field winding of the DC motor.
 Stator :-The stator is the stationary part of a motor, while the armature rotates. In a DC motor,
the stator provides a rotating magnetic field that drives the armature to rotate. ... One or more windings of
insulated wire are wrapped around the core of the motor to concentrate the magnetic field.
 Rotor :-The rotor contains coil windings that are powered by the DC current

 FIELD WINDING :-
The field winding of DC motor are made with field coils (copper wire) wound over the slots of the pole shoes in
such a manner that when field current flows through it, then adjacent poles have opposite polarity are produced.
The field winding basically form an electromagnet, that produces field flux within which the rotor armature of the
DC motor rotates, and results in the effective flux cutting.
 ARMATURE WINDING :-
The armature winding of DC motor is attached to the rotor, or the rotating part of the machine, and as a result is
subjected to altering magnetic field in the path of its rotation which directly results in magnetic losses. For this
reason the rotor is made of armature core, that’s made with several low-hysteresis silicon steel lamination, to
reduce the magnetic losses like hysteresis and eddy current loss respectively. These laminated steel sheets are
stacked together to form the cylindrical structure of the armature core.
 Poles of DC Motor :-
The magnetic poles of DC motor are structures fitted onto the inner wall of the yoke with screws. The construction
of magnetic poles basically comprises of two parts. Namely, the pole core and the pole shoe stacked together under
hydraulic pressure and then attached to the yoke. These two structures are assigned for different purposes, the
pole core is of small cross-sectional area and its function is to just hold the pole shoe over the yoke, whereas the
pole shoe having a relatively larger cross-sectional area spreads the flux produced over the air gap between the
stator and rotor to reduce the loss due to reluctance. The pole shoe also carries slots for the field windings that
produce the field flux.

 Brushes of DC Motor :-
The brushes of DC motor are made with carbon or graphite structures, making sliding contact over the rotating
commutator. The brushes are used to relay the current from external circuit to the rotating commutator form where
it flows into the armature winding. So, the commutator and brush unit of the DC motor is concerned with
transmitting the power from the static electrical circuit to the mechanically rotating region or the rotor.
 Commutator :-
The commutator of DC motor is a cylindrical structure made up of copper segments stacked together, but
insulated from each other by mica. Its main function as far as the DC motor is concerned is to commute or relay the
supply current from the mains to the armature winding housed over a rotating structure through the brushes of
DC motor

DC Motor Speed Control Methods:-
There are three main ways to achieve speed regulation in series DC motors–flux control, voltage control, and
armature resistance control.
1. Flux Control Method
In the flux control method, a rheostat (a type of variable resistor) is connected in series with the field windings.
The purpose of this component is to increase the series resistance in the windings which will reduce the flux,
consequently increasing the motor’s speed. N ∝ K E
b/Ø
As the magnetic flux depends on the current
flowing through the field winding, it can be varied
by varying the current through the field winding.
This can be achieved by using a variable resistor
in a series with the field winding resistor.
Initially, when the variable resistor is kept at its
minimum position, the rated current flows through
the field winding due to a rated supply voltage,
and as a result, the speed is kept normal. When
the resistance is increased gradually, the current
through the field winding decreases. This in turn
decreases the flux produced. Thus, the speed of
the motor increases beyond its normal value.

2. Armature Resistance Control Method:-
Speed of a dc motor is directly proportional to the back emf
Eb and Eb = V - IaRa. That means, when supply voltage V
and the armature resistance Ra are kept constant, then the
speed is directly proportional to armature current Ia. Thus, if
we add resistance in series with the armature, Ia decreases
and, hence, the speed also decreases. Greater the
resistance in series with the armature, greater the decrease
in speed.
N ∝ K E
b/Ø
Eb = V - IaRa

3. Voltage Control Method:-
Ward-Leonard System:
This system is used where very sensitive speed
control of motor is required (e.g electric
excavators, elevators etc.). The arrangement of
this system is as shown in the figure at right.
M2 is the motor to which speed control is
required. M1 may be any AC motor or DC motor
with constant speed.
G is a generator directly coupled to M1.
In this method, the output from generator G is fed
to the armature of the motor M2 whose speed is
to be controlled. The output voltage of generator
G can be varied from zero to its maximum value
by means of its field regulator and, hence, the
armature voltage of the motor M2 is varied very
smoothly. Hence, very smooth speed control of
the dc motor can be obtained by this method.

Back EMF in DC 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 the conductor”.
Advantages of Back Emf in DC Motor
1. The back emf opposes the supply voltage. The supply voltage induces the current in the
coil which rotates the armature. The electrical work required by the motor for causing the
current against the back emf is converted into mechanical energy. And that energy is
induced in the armature of the motor. Thus, we can say that energy conversion in the
DC motor is possible only because of the back emf.
The mechanical energy induced in the motor is the product of the back emf and the
armature current, i.e., EbIa.
Where Eb is the induced emf of the motor known as Back EMF, A is the number of parallel paths through the
armature between the brushes of opposite polarity. P is the number of poles, N is the speed, Z is the total number
of conductors in the armature and ϕ is the useful flux per pole.

In this case, the magnitude of the back emf is always
less than the applied voltage. The difference between
the two is nearly equal when the motor runs under
normal conditions.
The current induces on the motor because of the main
supply. The relation between the main supply, back
emf and armature current is given as
Eb = V – IaRa.

2. The back emf makes the DC motor self-regulating machine, i.e., the back emf develops the
armature current according to the need of the motor. The armature current of the motor is
calculated as:
Let’s understand how the back emf makes motor self-regulating.
 At starting: Motor speed is zero hence high torque is required at starting back emf is zero& hence
motor draws large armature current hence torque is high which meet the requirement at starting.
 At Normal condition: when armature rotates torque requirement reduces. As speed increases, back
emf develops, armature current decreases which decreases the torque.
 On the application of load: when load is applied speed decreases so torque requirement increases.
Now as speed decreases back emf decreases and hence armature current increases which increases
torque.
 On the removal of load: when the load is removed speed increases hence torque requirement is
low. As speed increases back emf increases which reduces the armature current & hence torque
decreases.

A DC generator is an electrical machine whose main function is to convert mechanical energy into electricity.
When conductor slashes magnetic flux, an emf will be generated based on the electromagnetic induction
principle of Faraday’s Laws. This electromotive force can cause a flow of current when the conductor circuit is
closed.
DC GENERATOR
Working Principle Of Dc Generator:-
The working principle of a DC generator is based on Faraday’s laws
of electromagnetic induction. When a conductor is placed in a
varying magnetic field, an electromotive force gets induced within
the conductor. This induced e.m.f magnitude is measured using the
equation of the electromotive force of a generator. If the conductor
is provided with a closed path, the induced current will circulate
within the closed path. In this generator, field coils will generate an
electromagnetic field as well as the armature conductors are turned
into the field. Therefore, an electromagnetically induced
electromotive force (e.m.f) will be generated within the armature
conductors. The path of the induced current is provided by Fleming’s
right-hand rule.

Fleming's right-hand rule (for generators) shows the direction of induced current when a conductor
attached to a circuit moves in a magnetic field. ... When a conductor such as a wire attached to a circuit
moves through a magnetic field, an electric current is induced in the wire due to Faraday's law of induction.
Fleming’s right hand rule for generators

The emf equation of dc generator according to Faraday’s Laws of Electromagnetic Induction is
Eg= PØZN/60 A
Where Φ is a flux or pole within Webber
Z is a total no. of armature conductor
P is a number of poles in a generator
A is a number of parallel lanes within the armature
N is the rotation of armature in r.p.m (revolutions per minute)
E is the induced e.m.f in any parallel lane within the armature
Eg is the generated e.m.f in any one of the parallel lanes
N/60 is the number of turns per second

Parts of a DC Generator:-
A DC generator can also be used as a DC motor without
changing its construction. Therefore, a DC motor,
otherwise a DC generator can be generally called a DC
machine. Below, we have mentioned the essential parts of
a DC Generator.
Stator:-
The main function of the stator is to provide magnetic
fields where the coil spins. A stator includes two magnets
with opposite polarity facing each other. These magnets
are located to fit in the region of the rotor.
Rotor:-
A rotor in a DC machine includes slotted iron laminations
with slots that are stacked to shape a cylindrical armature
core. The function of the lamination is to decrease the loss
caused due to eddy current.

 Armature Windings:-
Armature windings are in a closed circuit form and are connected in series to parallel for enhancing the sum
of produced current.
 Yoke:-
The external structure of the DC generator is known as Yoke. It is made of either cast iron or steel. It
provides necessary mechanical power for carrying the magnetic-flux given through the poles.
 Poles:-
The function of a pole is to hold the field windings. These windings are wound on poles and are either
connected in series or parallel by the armature windings.
 Pole Shoe:-
Pole shoe is mainly utilized for spreading the magnetic flux to avoid the field coil from falling.
 Commutator:-
A commutator works like a rectifier that changes AC voltage to DC voltage within the armature winding. 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.
 Brushes:-
The electrical connections can be ensured between the commutator as well as the exterior load circuit with the
help of brushes.

Losses in DC Machine:-
The losses that occur in a DC Machine is divided into five basic categories. The various losses
are Electrical or Copper losses (I2R losses), Core losses or Iron losses, Brush losses, Mechanical losses, Stray load
losses. These losses are explained below in detail.

1.Electrical or Copper Losses in dc machine:-
These losses are also known as winding losses as the copper loss occurs because of the resistance of
the windings. The ohmic loss is produced by the current flowing in the windings. The windings that
are present in addition to the armature windings are the field windings, interpoles and compensating
windings.
Armature copper losses = Ia
2Ra where Ia is armature current, and Ra is the armature resistance.
These losses are about 30 per cent of the total full load losses.
2.Magnetic Losses or Core Losses or Iron Losses in dc machine:-
The core losses are the hysteresis and eddy current losses. These losses are considered
almost constant as the machines are usually operated at constant flux density and constant
speed. These losses are about 20 per cent of the full load losses.
3.Brush Losses in dc machine:-
Brush losses are the losses taking place between the commutator and the carbon brushes. It
is the power loss at the brush contact point. The brush drop depends upon the brush contact
voltage drop and the armature current Ia. It is given by the equation shown below:

4.Mechanical Losses in dc machine:-
The losses that take place because of the mechanical effects of the machines are known as
mechanical losses. Mechanical losses are divided into bearing friction loss and windage loss. The
losses occurring in the moving parts of the machine and the air present in the machine is known as
Windage losses. These losses are very small.
5.Stray Losses in dc machine:-
These losses are the miscellaneous type of losses. The following factors are considered in stray load
losses.
•The distortion of flux because of the armature reaction.
•Short circuit currents in the coil, undergoing commutation.
These losses are very difficult to determine. Therefore, it is necessary to assign the reasonable value
of the stray loss. For most machines, stray losses are taken by convention to be one per cent of the
full load output power.

The DC generator can be classified into two main categories as separately excited and self-excited.
Separately Excited:-
In a separately excited type generator, the
field coils are energized from an
independent exterior DC source.
Self Excited:-
In a self-excited type, the field coils are
energized from the generated current
within the generator. These types of
generators can further be classified into
a series of wounds, shunt-wound,
and compound wound.
DC GENERATOR AND ITS TYPE:-

1.Separately excited DC generator:-
This dc generator has a field magnet winding which is excited using a separate voltage source (like
battery). You can see the representation in the below image. The output voltage depends on the speed
of rotation of armature and field current. The higher the speed of rotation and current – the higher the
output e.m.f
These are generators in which the field winding is excited by the output of the generator itself. As
described before – there are three types of self-excited dc generators – they are
1) Series
2) Shunt and
3) Compound.
2. Self Excited DC Generator:-

2.A shunt DC generator is shown in
figure (b), in which the field winding is
wired parallel to armature winding so
that the voltage across both are same.
The field winding has high resistance
and more number of turns so that only a
part of armature current passes through
field winding and the rest passes
through load.
1.A series DC generator is shown below in
fig (a) ,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. The field winding
is a low resistance, thick wire of few turns.
Series generators are also rarely used!

A compound generator is shown in figure below. It has two field findings namely Rsh and Rse. They are
basically shunt winding (Rsh) and series winding (Rse). Compound generator is of two types – 1) Short shunt
and 2) Long shunt
b) Long shunt:- Here the shunt field winding is parallel to both
armature and series field winding (Rse is wired in series to the
armature). It is shown in figure (2)
a) Short shunt:- Here the shunt field winding
is wired parallel to armature and series field
winding is connected in series to the load. It is
shown in fig (1).
c. Compound Generator:-

Presentation on DC Machines

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