Transcript electrical machines - ce

ELECTRICAL MACHINES
An electrical machine is an electro-mechanical energy
conversion device.
The device which converts electrical energy to mechanical
energy is called a motor.
The device which converts mechanical energy to electrical
energy is called a generator.
MOTOR
Electrical
energy
input
Mechanical energy
Out put
GENERATOR
Mechanical energy
input
Electrical
energy
Out put
CLASSIFICATION OF ELECTRICAL MACHINES
ELECTRICAL MACHINES
Direct current machines
DC generator
Transformers
Alternating current machines
DC motor
Induction
machines
Synchronous
Machines
Fractional KW
Machines
DC GENERATOR
Principle of operation:
• DC generator works on the principle of Faradays laws of
electromagnetic induction.It states that
•“Whenever magnetic flux is cut by a moving conductor ,an emf
is induced in the conductor”.
•Direction of induced emf is given by Flemings Right Hand rule.
•Stretch out right hand thumb ,fore finger,middle finger mutually
perpendicular to each other.If the fore finger points in the
direction of flux and thumb in direction of motion of
conductor,then the middle finger will point in the direction of
induced emf.
Principle of working
Consider a single turn rectangular coil made of copper rotating about its own
axis in a magnetic field provided by either permanent magnet or electro
magnet. Consider a copper coil ABCD ,rotating in a magnetic field as shown in
slide.
The two ends of the coil are joined to two copper rings called slip rings
.Brushes of carbon ,press against slip rings for collecting current from the coil
to external load resistance RL.
Consider position of the coil(as shown in fig) as reference .In this position
plane of the coil is right angles to the magnetic lines of flux.In this position rate
of change of flux lis minimum and no emf induced in the coil. The position is
taken as the reference position.Measure the angle of rotation or time from this
position. As the coil rotates, the rate of change of flux and hence the induced
emf increases ,till at
maximum.
ᶱ
= 90 ,when the coil plane is horizontal ,the induce emf is
In the next quarter ,from ᶱ= 180 to 360 ,the variation of emf are similar
to first half.It is minimum when plane of coil is perpendicular to lines of
flux and maximum when plane of coil is parallel to the lines of flux.
however in both cases coil sides cuts the lines of flux in opposite
directions. Therefore emf in the coil is reversed.
Induced emf e= BlVsin ᶱ
where e= induced emf in volts
B= magnetic field intensity
l = length of the conductor
V= relative velocity b/w field and conductor in m/s
ᶱ = angle between plane of rotation and plane of flux.
SIMPLE LOOP GENERATOR
Elementary Generator
3
Simple d.c. generator
similar to a.c.
generator
different from
a.c. generator
commutator
Every time the coil passes through
the vertical, it reverses coil's
connections with outside circuit.
 I in the outside circuit always
flows in 1 direction.
Parts of a practical generator
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Magnetic frame or yoke
Pole-cores and Pole shoes
Pole coils or field coils
Armature core
Armature windings or conductors
Commutator
Brushes and bearings
A four pole DC generator
pole
yoke
Components of a dc generator.
Yoke or outer frame
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Outermost cylindrical part of
the machine.
FUNCTIONS:
Acts as the supporting frame
for the machine.
. Carry the magnetic flux
produced by the poles.
To provide mechanical
support to the poles.
Made of cast iron(small
machines) or cast steel(large
m/c)
Poles
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Pole consists of pole core
and pole shoe.
Field coil is wound on pole
core.
Poles are made of cast steel
and forged steel.
Pole is securely bolted to
the yoke.
FUNCTIONS:
Pole core carries a field
winding which is necessary
to produce magnetic flux.
It directs the flux through the
armature core.
Pole shoe
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Naturally pole shoe is a projection
over pole core and is always with
connection with pole body and fills
the gap between the yoke and the
pole body.
Functions of pole shoes
It supports the field coil.
It spreads out the magnetic flux in
the air gap.
It reduces the reluctance of the
magnetic path.
A combination of pole body and pole
shoe is termed as field magnet. This
behaves as a field magnet when
direct current is passed through the
field coil.
Field or Pole coils
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The fields coils are wound on pole cores.
The field or pole coils ,made from enameled copper
wire or strip.
FUNCTION:
Function of this coil is to produce the necessary flux
,when current is passed through it. Since it produces
the flux its also called exciting winding. The field coils
are so connected that they form alternate N and S
poles.
Armature
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Part of DC machine where an emf is induced
as it rotates relative to the main field.
Consists of toothed core ,a winding and a
commutator mounted on armature shaft.
Armature Core
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Armature core is cylindrical in
shape and is mounted on the
shaft. It consists of slots on its
periphery
to
house
the
conductors.
Made of cast iron or cast steel.
FUNCTIONS:
To
house
the
armature
conductors.
To provide a path of low
reluctance to the field from the
N-pole to S-pole ,through the
armature.
Armature windings
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A number of conductors ,housed on the armature core ,are
interconnected in a manner to add up the induced emf produced in the
conductors. The interconnected conductors constitute armature winding
FUNCTIONS:
generation of emf.
Carry the armature current supplied to the load in case of generators
and carry the current supplied in case of dc motors.
2 types of winding are there: lap and wave.
Lap winding
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In this connection ,the winding overlap.
The conductors are connected in series parallel
combination to increase the voltage and current rating.
Series connection increase vtg rating and parallel
increases the current rating.
Armature coils are connected in series through
commutator segments in such a way that armature
winding is divided into as many paths as the number of
poles of the machine .
Cont..
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In lap winding the total number of parallel paths =
no of poles’p’. Hence lap winding is preferred for
machines with high current rating.
Wave winding
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In this connection ,thw winding does not overlap.
Armature coils are connected in series through
commutator segments in such a way that the
armature winding is divided into two parallel paths
irrespective of the number poles of the machine.This
type of winding is preferred for low current high
voltage generators.
ARMATURE WINDINGS (Cont)
Lap winding
Wave winding
1) Number of parallel paths A=
No of poles ,P
1) A=2
2 ) No of brushes = p
2) no of brushes = 2
3) Preferred for high current –low
Voltage generators.
3) preferred for low current highvtg
Generators.
4) Used when current is grater than
500 A
4) used when current is less
than 500 A
Commutator
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Commutator is a most important and
vital part in a D.C generator without
which the generator fails to
work.Rectification of current is the
main function of the Commutator. It
converts the alternating current
induced in the armature winding in
to directional current.
It is of cylindrical structure and made
of wedge-shaped segments made of
high-conductivity copper. These
segments are insulated from one
another by thin layers of mica.Each
segment is connected to the
armature conductor by means of a
copper strip or plug.
Brushes and bearings
Brushes are stationary and rest on the surface of
the commutator.
 they are made of carbon.
 They are housed in a box type brush holders which
are open at both the ends.
FUNCTION:
 To collect current from commutator and make it
available to the external circuit.
BEARINGS:
 Ball bearings are used commonly because of their
reliability. Roller bearings are used for heavier
duties.
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Shaft
shaft is the central one over which the whole parts are
loaded.
.
DIFFERENT TYPES OF DC GENERATORS
Separately excited
Series wound
Self excited
shunt wound
Long shunt
compound wound
short shunt
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Dc generators are generally classified
according to the method used for field
excitation.
They are
1) separately excited generator
2)self excited generator
Separately excited generator
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In these the field magnets are energized from a separate
independent external source of dc current.
A separate dc source is used to drive the field current If.
The resistance Rf is connected to adjust the field current.
the emf generated in the armature drives the load.
Armature current and Ioad current is same.
Series wound dc generator
Long Shunt dc generator
Short shunt