Construction and Working Principle of DC Motors

A DC motor is an electromechanical energy conversion device, which converts

electrical energy input into the mechanical energy output.

The operation of the DC motor is based on the principle that when a current
carrying conductor is placed in a magnetic field, a mechanical force acts on the
conductor. The magnitude of the force is given by,

F=BIlNewtonsF=BIlNewtons

The direction of this is given by the Fleming's left hand rule.

Construction of a DC Motor

Here is the schematic diagram of a DC Motor

A DC motor consists of six main parts, which are as follows

Yoke

The outer frame of a DC motor is a hollow cylinder made up of cast steel or rolled
steel is known as yoke. The yoke serves following two purposes

It supports the field pole core and acts as a protecting cover to the
machine.
 It provides a path for the magnetic flux produced by the field winding.
Magnetic Field System

The magnetic field system of a DC motor is the stationary part of the machine. It
produces the main magnetic flux in the motor. It consists of an even number of
pole cores bolted to the yoke and field winding wound around the pole core. The
field system of DC motor has salient poles i.e. the poles project inwards and each
pole core has a pole shoe having a curved surface. The pole shoe serves two
purposes

 It provides support to the field coils.

 It reduces the reluctance of magnetic circuit by increasing the cross-
sectional area of it.

The pole cores are made of thin laminations of sheet steel which are insulated
from each other to reduce the eddy current loss. The field coils are connected in
series with one another such that when the current flows through the coils,
alternate north and south poles are produced.

Armature Core

The armature core of DC motor is mounted on the shaft and rotates between the
field poles. It has slots on its outer surface and the armature conductors are put in
these slots. The armature core is a made up of soft steel laminations which are
insulated from each other and tightly clamped together. In small machines, the
laminations are keyed directly to the shaft, whereas in large machines, they are
mounted on a spider. The laminated armature core is used to reduce the eddy
current loss.

Armature Winding

The insulated conductors are put into the slots of the armature core. The
conductors are suitably connected. This connected arrangement of conductors is
known as armature winding. There are two types of armature windings are used
– wave winding and lap winding.

Commutator

A commutator is a mechanical rectifier which converts the direct current input to

the motor from the DC source into alternating current in the armature winding.
The commutator is made of wedge-shaped copper segments insulated from each
other and from the shaft by mica sheets. Each segment of commutator is
connected to the ends of the armature coils.
Brushes

The brushes are mounted on the commutator and are used to inject the current
from the DC source into the armature windings. The brushes are made of carbon
and is supported by a metal box called brush holder. The pressure exerted by the
brushes on the commutator is adjusted and maintained at constant value by means
of springs. The current flows from the external DC source to the armature winding
through the carbon brushes and commutator.

Working of DC Motor

Consider a two pole DC motor as shown in the figure. When the DC motor is
connected to an external source of DC supply, the field coils are excited
developing alternate N and S poles and a current flows through the armature
windings

All the armature conductors under N pole carry current in one direction (say into
the plane of the paper), whereas all the conductors under S pole carry current in
the opposite direction (say out of the plane of the paper). As each conductor
carrying a current and is placed in a magnetic field, hence a mechanical force acts
on it.

By applying Fleming’s left hand rule, it can be seen that the force on each
conductor is tending to move the armature in anticlockwise direction. The force
on all the conductors add together to exert a torque which make the armature
rotating. When the conductor moves from one side of a brush to the other, the
current in the conductor is reversed and at the same time it comes under the
influence of next pole of opposite polarity. As a result of this, the direction of
force on the conductor remains the same. Therefore, the motor being rotating in
the same direction.
Applications of a DC Motor

The application of a DC motor depends on the requirement of the electrical

equipment and the characteristics of the DC motor. Here is a list of the fields
where the motors discussed above are used:

DC series motor

o Cranes
o Lifts and elevators
o Winching systems
o Hair driers
o Power tools

DC shunt motor

o Windscreen wiper drives

o Drills
o Conveyers
o Fans
o Centrifugal pumps
o Blowers

Compound DC motor

o Conveyers
o Stamping machines
o Compressors
o Heavy planners
o Rolling mills
o Presse

DC Generator

A DC generator is an electromechanical energy conversion device that converts

mechanical power into DC electrical power through the process of
electromagnetic induction.

A DC generator operates on the principle of electromagnetic induction i.e. when

the magnetic flux linking a conductor changes, an EMF is induced in the
conductor. A DC generator has a field winding and an armature winding.
The EMF induced in the armature winding of a DC generator is alternating one
and is converted into direct voltage using a commutator mounted on the shaft of
the generator. The armature winding of DC Generator is placed on the rotor
whereas the field winding is placed on the stator.

Construction of a DC Generator

Here is the schematic diagram of a DC Generator

A DC generator consists of six main parts, which are as follows

Yoke

The outer frame of a DC generator is a hollow cylinder made up of cast steel or

rolled steel is known as yoke. The yoke serves following two purposes

It supports the field pole core and acts as a protecting cover to the
machine.
 It provides a path for the magnetic flux produced by the field winding.
Magnetic Field System

The magnetic field system of a DC generator is the stationary part of the machine.
It produces the main magnetic flux in the generator. It consists of an even number
of pole cores bolted to the yoke and field winding wound around the pole core.
The field system of DC generator has salient poles i.e. the poles project inwards
and each pole core has a pole shoe having a curved surface. The pole shoe serves
two purposes
  It provides support to the field coils.
 It reduces the reluctance of magnetic circuit by increasing the cross-
sectional area of it.

The pole cores are made of thin laminations of sheet steel which are insulated
from each other to reduce the eddy current loss. The field coils are connected in
series with one another such that when the current flows through the coils,
alternate north and south poles are produced in the direction of rotation.

Armature Core

The armature core of DC generator is mounted on the shaft and rotates between
the field poles. It has slots on its outer surface and the armature conductors are
put in these slots. The armature core is a made up of soft iron laminations which
are insulated from each other and tightly clamped together. In small machines,
the laminations are keyed directly to the shaft, whereas in large machines, they
are mounted on a spider. The laminated armature core is used to reduce the eddy
current loss.

Armature Winding

The insulated conductors are put into the slots of the armature core. The
conductors are suitably connected. This connected arrangement of conductors is
known as armature winding. There are two types of armature windings are used
– wave winding and lap winding.

Commutator

A commutator is a mechanical rectifier which converts the alternating emf

generated in the armature winding into the direct voltage across the load
terminals. The commutator is made of wedge-shaped copper segments insulated
from each other and from the shaft by mica sheets. Each segment of commutator
is connected to the ends of the armature coils.

Brushes

The brushes are mounted on the commutator and are used to collect the current
from the armature winding. The brushes are made of carbon and is supported by
a metal box called brush holder. The pressure exerted by the brushes on the
commutator is adjusted and maintained at constant value by means of springs.
The current flows from the armature winding to the external circuit through the
commutator and carbon brushes.

Working of a DC Generator

Consider a single loop DC generator (as shown in the figure), in this a single turn
loop ‘ABCD’ is rotating clockwise in a uniform magnetic field with a constant
speed. When the loop rotates, the magnetic flux linking the coil sides ‘AB’ and
‘CD’ changes continuously. This change in flux linkage induces an EMF in coil
sides and the induced EMF in one coil side adds the induced EMF in the other.

The EMF induced in a DC generator can be explained as follows

 When the loop is in position-1, the generated EMF is zero because, the
movement of coil sides is parallel to the magnetic flux.
 When the loop is in position-2, the coil sides are moving at an angle to
the magnetic flux and hence, a small EMF is generated.
 When the loop is in position-3, the coil sides are moving at right angle to
the magnetic flux, therefore the generated EMF is maximum.
 When the loop is in position-4, the coil sides are cutting the magnetic flux
at an angle, thus a reduced EMF is generated in the coil sides.
 When the loop is in position-5, no flux linkage with the coil side and are
moving parallel to the magnetic flux. Therefore, no EMF is generated in
the coil.
 At the position-6, the coil sides move under a pole of opposite polarity
and hence the polarity of generated EMF is reversed. The maximum EMF
will generate in this direction at position-7 and zero when at position-1.
This cycle repeats with revolution of the coil.

It is clear that the generated EMF in the loop is alternating one. It is because any
coil side (say AB) has EMF in one direction when under the influence of N-pole
and in the other direction when under the influence of S-pole. Hence, when a load
is connected across the terminals of the generator, an alternating current will flow
through it. Now, by using a commutator, this alternating emf generated in the
loop can be converted into direct voltage. We then have a DC generator.

Applications of dc generators
1. They are used for general lighting.
2. They are used to charge battery because they can be made to give
constant output voltage.
3. They are used for giving the excitation to the alternators.
4. They are also used for small power supply (such as a portable generator).
5. They are used for supplying field excitation current in DC locomotives
for regenerative breaking.
6. This types of generators are used as boosters to compensate the voltage
drop in the feeder in various types of distribution systems such as railway
service.
7. In series arc lightening this type of generators are mainly used
8. Cumulative compound wound generators are generally used for lighting,
power supply purpose and for heavy power services because of their
constant voltage property. They are mainly made over compounded.
9. Cumulative compound wound generators are also used for driving a
motor.
10. For small distance operation, such as power supply for hotels, offices,
homes and lodges, the flat compounded generators are generally used.
11. The differential compound wound generators, because of their large
demagnetization armature reaction, are used for arc welding where huge
voltage drop and constant current is required.

Transformer

A transformer is a static device that transfers electric power between two

alternating current circuits with no change in frequency. The Voltage of
the circuit can be reduced or increased in accordance with the current
relationship. This is known as stepping up (increasing) the voltage and
stepping it down (decreasing).
 Principle – A transformer works on the principle of mutual induction.
Mutual induction is the phenomenon by which when the amount of
magnetic flux linked with a coil changes, an E.M.F. is induced in the
neighbouring coil.

Transformer Construction

There are three components of a Transformer:

 Iron Core
 Primary Winding
 Secondary Winding

Core

The core of the transformer is rectangular in shape and laminated. During the
transformer construction, it has to be designed in such a way that there are
fewer core losses during the operation of the Transformer. Core losses and iron
losses are a combination of all the losses that happen inside the core.

The core lets an alternating flux drive through it. This might cause energy loss
in the core due to hysteresis loss. So, you should choose a high-quality Silicon
Steel with low hysteresis loss to construct the core of a Transformer. This steel
is termed the Soft Steel Core of the Transformer.

The alternating flux produces certain currents known as Eddy currents. These
currents use electrical energy and cause certain losses, known by the name of
eddy current losses of the Transformer. The core must be manufactured as a
group of laminations. These successive laminations are electrically insulated to
reduce eddy currents. The insulation layer is made up of Varnish, which offers
high resistance to eddy currents.
Windings

There are two windings on the transformer i.e. Primary Winding and Secondary
Winding. The Primary Winding is connected to the input terminal and is
responsible for generating a self-induced EMF. The Secondary Winding is
connected to the output load. These windings are placed on the core and are
electrically insulated from each other and the core for proper functioning and
reduction in losses.

These coils have different numbers of turns compared to each other. The
Primary Winding of the Transformer has N1 turns. Similarly, the Secondary
Winding of the Transformer has N2 turns. Depending upon the operation of the
transformer, N1< N2, N1> N2, and N1= N2.

Applications of transformer

1. Transformers are being used in electrical power engineering to transform

power from either a produced voltage of roughly 11 kv to higher values
of 132kv,220kv,400kv,500kv,765kv As a result, vast volumes of power
may be sent across great distances to appropriate distribution sites while
saving a lot of money on transmission line costs as well as power failure.
2. Transformers have been employed at distribution centers to decrease this
high voltage toward a comfortable limit of 400/300 volts for usage
throughout households, offices, etc.
3. transformers are indeed employed in telecommunication as well as
instrumentation circuits, as well as control circuits.
4. Input transformers, interstage transformers, as well as outputting
transformers are commonly used in radio and television circuits.
Working Principle of 3 Phase Induction Motor

A 3 Phase Induction Motor works on the principle of electromagnetic induction.

When the three-phase winding of the stator is connected to the three-phase
supply, the three phase current in stator winding produces a rotating magnetic
field, which rotates round the stator at synchronous speed (N S). This rotating
field or flux passes through the air gap and cuts the rotor conductors, since rotor
conductors are stationary and flux is moving. As a result of relative speed
between the rotating field and stationary rotor conductors, e.m.f is induced in
rotor conductors. Since the rotor conductor circuit is closed, current flows in
that as a result of induced e.m.f. Thus, whenever a current carrying conductor is
placed in rotating magnetic field it produces its own flux. Due to this, a
magnetic flux called rotor flux is produced in rotor. Thus, a torque is produced
due to the interaction between the air gap flux and the rotor flux. The effect of
this torque is that the rotor starts rotating in the same direction as the rotating
magnetic field (according to Lenz law) and the speed of the rotor starts
increasing from 0 to N.

Since the rotor speed is increased it rotates in the same direction as the rotation
of magnetic field and the relative speed between the two decreases. As a result.
the induced e.m.f in rotor current and rotor frequency gets reduced and also the
torque gets reduced. But the rotor continues to accelerate and gains the speed.
Under no-load conditions equilibrium is reached when the developed torque
equals the torque due to losses. At equilibrium condition, the motor runs almost
at synchronous speed but not equal to synchronous speed. As soon as the motor
is loaded, the load torque plus the torque due to losses become greater than the
developed torque. Therefore. the motor slows down. However, a reduction in
rotor speed means an increase in relative speed as well as rotor current and
hence an increase in the developed torque. As a result of this, the developed
torque equals load torque plus torque due to losses. Hence equilibrium is
reached in loaded condition. The motor then runs at a new steady speed, which
is less compared to no-load speed. Thus, the speed ofmthree-phase induction
motor varies with the load.

Construction (or Parts) of 3 Phase Induction Motor

The main constructional features of a three phase induction motor are stator and
rotor. Usually stator receives three phase supply and working currents are
developed in rotor.

Stator

Figure 1.

Stator consists of cast iron stator frame and laminated steel stampings of
cylindrical stator core as shown in figure (1). These stator core steel laminations
have minimum hysteresis and eddy current losses compared to other materials
when alternating current passes through them. Stator core have the slots for
inserting the polyphase winding.Slots provided on the stator core are of three
types. They are open type, semi-closed type and closed type slots. Closed slots
facilitate uniform air gap thereby flux distribution is also uniform. They have a
limitation due to their high initial labour cost for winding and their increased
winding inductance. Open slots have the advantage of easy removal and
replacement of defective coils. But non-uniform air gaps are formed by open
slots. Among all the three, semi-closed slots are advantageous because of less
ripple content in their e.m.f waveform and less initial labour cost for designing.

Rotor
Induction motors are of two types depending upon the type of rotor used. The
two types of rotors are squirrel-cage type and slip ring type.

Squirrel cage type rotor has slots on its outer periphery which are circular in
shape. The rotor winding is placed in the rotor slots and its ends are shorted by
same winding material known as end rings as shown in figure (2). This
facilitates the rotor suitability for any number of stator poles and insertion of
external resistance to the rotor winding is not possible. Hence the starting torque
of squirrel cage type induction motor is double to that of full load torque only
for the application of rated voltage. Squirrel cage induction motors (SCIM) are
simple and economical in construction and require less maintenance.

Slip ring type rotor also have slots on its circular outer periphery. The rotor
winding which is to be placed in the rotor slots is similar to that of its three
phase stator winding so that number of rotor poles are equal to stator poles. Slip
ring type of rotors have a facility to insert external resistance at their output
terminals as shown in figure (3). Therefore its starting torque can be changed as
per the requirement, by placing suitable resistance in the rotor circuit.

Applications of 3 Phase Induction Motor

The following are the various fields of application of three phase induction
motors.

1. Fans
 2. Centrifugal pumps
3. Lift irrigation
4. Rice mills
5. Pumps
6. Cranes.

Alternator construction and working principle

A synchronous generator is a synchronous machine which converts mechanical

power into AC electric power through the process of electromagnetic induction.

Synchronous generators are also referred to as alternators or AC generators.

The term "alternator" is used since it produces AC power. It is called synchronous
generator because it must be driven at synchronous speed to produce AC power
of the desired frequency.

A synchronous generator can be either single-phase or poly-phase (generally

3phase).

Construction of Synchronous Generator or Alternator

As alternator consists of two main parts viz.

 Stator – The stator is the stationary part of the alternator. It carries the
armature winding in which the voltage is generated. The output of the
alternator is taken form the stator.
 Rotor – The rotor is the rotating part of the alternator. The rotor produces
the main field flux.
Stator Construction of Alternator

The stator of the alternator includes several parts, viz. the frame, stator core, stator
or armature windings, and cooling arrangement.

 The stator frame may be made up of cast iron for small-size machines and
of welded steel for large-size machines.
 The stator core is assembled with high-grade silicon content steel
laminations. These silicon steel laminations reduce the hysteresis and
eddy-current losses in the stator core.
 The slots are cut on the inner periphery of the stator core. A 3-phase
armature winding is put in these slots.
  The armature winding of the alternator is star connected. The winding of
each phase is distributed over several slots. When current flows through
the distributed armature winding, it produces an essential sinusoidal space
distribution of EMF.

Rotor Construction of Alternator

The rotor of the alternator carries the field winding which is supplied with direct
current through two slip rings by a separate DC source (also called exciter). The
exciter is generally a small DC shunt generator mounted on the shaft of the
alternator.

For the alternator, there are two types of rotor constructions are used viz. the
salient-pole type and the cylindrical rotor type.

Salient Pole Rotor

The term salient means projecting. Hence, a salient pole rotor consists of poles
projecting out from the surface of the rotor core. This whole arrangement is fixed
to the shaft of the alternator as shown in the figure. The individual field pole
windings are connected in series such that when the field winding is energised by
the DC exciter, the adjacent poles have opposite polarities.
The salient pole type rotor is used in the low and medium speed (from 120 to 400
RPM) alternators such as those driven by the diesel engines or water turbines
because of the following reasons −

 The construction of salient pole type rotor cannot be made strong enough
to withstand the mechanical stresses to which they may be subjected at
higher speed.
 If the salient field pole type rotor is driven at high speed, then it would
cause windage loss and would tend to produce noise.

Low speed rotors of the alternators possess a large diameter to provide the
necessary space for the poles. As a result, the salient pole type rotors have large
diameter and short axial length.

Cylindrical Rotor

The cylindrical rotors are made from solid forgings of high-grade nickel-chrome-
molybdenum steel.

 The construction of the cylindrical rotor is such that there are no-physical
poles to be seen as in the salient pole rotor.
 In about two-third of the outer periphery of the cylindrical rotor, slots are
cut at regular intervals and parallel to the rotor shaft.
 The field windings are placed in these slots and is excited by DC supply.
The field winding is of distributed type.
 The unslotted portion of the rotor forms the pole faces.
 It is clear from the figure of the cylindrical rotor that the poles formed are
non-salient, i.e., they do not project out from the rotor surface.
The cylindrical type rotor construction is used in the high-speed (1500 to
3000 RPM) alternators such as those driven by steam turbines because of the
following reasons −

 The cylindrical type rotor construction provides a greater mechanical

strength and permits more accurate dynamic balancing.
 It gives noiseless operation at high speeds because of the uniform air gap.
 The flux distribution around the periphery of the rotor is nearly a sine wave
and hence a better EMF waveform is obtained.

A cylindrical rotor alternator has a comparatively small diameter and long axial
length. The cylindrical rotor alternators are called turbo-alternators or turbo-
generators. The alternator with cylindrical rotor have always horizontal
configuration installation.

Working Principle and Operation of Alternator

An alternator or synchronous generator works on the principle of electromagnetic

induction, i.e., when the flux linking a conductor changes, an EMF is induced in
the conductor. When the armature winding of alternator subjected to the rotating
magnetic field, the voltage will be generated in the armature winding.

When the rotor field winding of the alternator is energised from the DC exciter,
the alternate N and S poles are developed on the rotor. When the rotor is rotated
in the anticlockwise direction by a prime mover, the armature conductors placed
on the stator are cut by the magnetic field of the rotor poles. As a result, the EMF
is induced in the armature conductors due to electromagnetic induction. This
induced EMF is alternating one because the N and S poles of the rotor pass the
armature conductors alternatively.
The direction of the generated EMF can be determined by the Fleming’s right
rule and the frequency of it is given by,

F =NsP/120

Where,

 Ns is the synchronous speed in RP

 P is the number of rotor poles.

The magnitude of the generated voltage depends upon the speed of rotation of the
rotor and the DC field excitation current. For the balanced condition, the
generated voltage in each phase of the winding is the same but differ in phase by
120° electrical.

Applications of Alternator
An alternator is mainly used for converting mechanical energy into electrical
energy in various applications such as:
 In automobiles
 In locomotives
 Power generation plants
 In Marine and navy boats
 Radiofrequency transmission