DC GENERATOR - INTRODUCTION
An electrical generator is a device that converts mechanical energy to
electrical energy, generally using electromagnetic induction. The source of
mechanical energy may be a reciprocating or turbine steam engine, water falling
through a turbine or waterwheel, an internal combustion engine, a wind turbine,
a hand crank, or any other source of mechanical energy.
The Dynamo was the first electrical generator capable of delivering power
for industry. The dynamo uses electromagnetic principles to convert mechanical
rotation into an alternating electric current. A dynamo machine consists of a
stationary structure which generates a strong magnetic field, and a set of rotating
windings which turn within that field. On small machines the magnetic field may
be provided by a permanent magnet; larger machines have the magnetic field
created by electromagnets. The energy conversion in generator is based on the
principle of the production of dynamically induced e.m.f. whenever a conductor
cuts magneticic flux, dynamically induced e.m.f is produced in it according to
Faraday's Laws of Electromagnetic induction. This e.m.f causes a current to flow
if the conductor circuit is closed.
CONSTRUCTION OF D.C. MACHINES
A D.C. machine consists mainly of two part the stationary part called stator
and the rotating part called rotor.The stator consists of main poles used to produce
magnetic flux ,commutating poles or interpoles in between the main poles to
avoid sparking at the commutator but in the case of small machines sometimes
the interpoles are avoided and finally the frame or yoke which forms the
supporting structure of the machine. The rotor consist of an armature a cylindrical
metallic body or core with slots in it to place armature windings or bars,a
commutator and brush gears The magnetic flux path in a motor or generator is
show below and it is called the magnetic structure of generator or motor.
The major parts can be identified as,
1. Frame
2. Poles
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, 3. Armature
4. Field winding
5. Commutator
6. Brush
7. Other mechanical parts
Frame
Frame is the stationary part of a machine on which the main poles and
commutator poles are bolted and it forms the supporting structure by connecting
the frame to the bed plate. The ring shaped body portion of the frame which makes
the magnetic path for the magnetic fluxes from the main poles and interpoles is
called Yoke.
Why we use cast steel instead of cast iron for the construction of Yoke?
In early days Yoke was made up of cast iron but now it is replaced by cast
steel.This is because cast iron is saturated by a flux density of 0.8 Wb/sq.m where
as saturation with cast iron steel is about 1.5 Wb/sq.m.So for the same magnetic
flux density the cross section area needed for cast steel is less than cast iron hence
the weight of the machine too.If we use cast iron there may be chances of blow
holes in it while casting.so now rolled steels are developed and these have
consistent magnetic and mechanical properties.
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, poles:
Solid poles of fabricated steel with separate/integral pole shoes are fastened to
the frame by means of bolts. Pole shoes are generally laminated. Sometimes pole
body and pole shoe are formed from the same laminations. The pole shoes are
shaped so as to have a slightly increased air gap at the tips. Inter-poles are small
additional poles located in between the main poles.
These can be solid, or laminated just as the main poles. These are also
fastened to the yoke by bolts. Sometimes the yoke may be slotted to receive these
poles. The inter poles could be of tapered section or of uniform cross section.
These are also called as commutating poles or com poles. The width of the tip of
the com pole can be about a rotor slot pitch.
Armature
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, The armature is where the moving conductors are located. The armature is
constructed by stacking laminated sheets of silicon steel. Thickness of these
lamination is kept low to reduce eddy current losses. As the laminations carry
alternating flux the choice of suitable material, insulation coating on the
laminations, stacking it etc are to be done more carefully. The core is divided into
packets to facilitate ventilation. The winding cannot be placed on the surface of
the rotor due to the mechanical forces coming on the same. Open parallel sided
equally spaced slots are normally punched in the rotor laminations.
These slots house the armature winding. Large sized machines employ a
spider on which the laminations are stacked in segments. End plates are suitably
shaped so as to serve as ’Winding supporters’. Armature construction process
must ensure provision of sufficient axial and radial ducts to facilitate easy
removal of heat from the armature winding.
Field windings:
In the case of wound field machines (as against permanent magnet excited
machines) the field winding takes the form of a concentric coil wound around the
main poles. These carry the excitation current and produce the main field in the
machine. Thus the poles are created electromagnetically. Two types of windings
are generally employed. In shunt winding large number of turns of small section
copper conductor is used. The resistance of such winding would be an order of
magnitude larger than the armature winding resistance. In the case of series
winding a few turns of heavy cross section conductor is used. The resistance of
such windings is low and is comparable to armature resistance. Some machines
may have both the windings on the poles. The total ampere turns required to
establish the necessary flux under the poles is calculated from the magnetic circuit
calculations. The total mmf required is divided equally between north and south
poles as the poles are produced in pairs. The mmf required to be shared between
shunt and series windings are apportioned as per the design requirements. As
these work on the same magnetic system they are in the form of concentric coils.
Mmf ’per pole’ is normally used in these calculations. Armature winding As
mentioned earlier, if the armature coils are wound on the surface of the armature,
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An electrical generator is a device that converts mechanical energy to
electrical energy, generally using electromagnetic induction. The source of
mechanical energy may be a reciprocating or turbine steam engine, water falling
through a turbine or waterwheel, an internal combustion engine, a wind turbine,
a hand crank, or any other source of mechanical energy.
The Dynamo was the first electrical generator capable of delivering power
for industry. The dynamo uses electromagnetic principles to convert mechanical
rotation into an alternating electric current. A dynamo machine consists of a
stationary structure which generates a strong magnetic field, and a set of rotating
windings which turn within that field. On small machines the magnetic field may
be provided by a permanent magnet; larger machines have the magnetic field
created by electromagnets. The energy conversion in generator is based on the
principle of the production of dynamically induced e.m.f. whenever a conductor
cuts magneticic flux, dynamically induced e.m.f is produced in it according to
Faraday's Laws of Electromagnetic induction. This e.m.f causes a current to flow
if the conductor circuit is closed.
CONSTRUCTION OF D.C. MACHINES
A D.C. machine consists mainly of two part the stationary part called stator
and the rotating part called rotor.The stator consists of main poles used to produce
magnetic flux ,commutating poles or interpoles in between the main poles to
avoid sparking at the commutator but in the case of small machines sometimes
the interpoles are avoided and finally the frame or yoke which forms the
supporting structure of the machine. The rotor consist of an armature a cylindrical
metallic body or core with slots in it to place armature windings or bars,a
commutator and brush gears The magnetic flux path in a motor or generator is
show below and it is called the magnetic structure of generator or motor.
The major parts can be identified as,
1. Frame
2. Poles
www.BrainKart.com
, 3. Armature
4. Field winding
5. Commutator
6. Brush
7. Other mechanical parts
Frame
Frame is the stationary part of a machine on which the main poles and
commutator poles are bolted and it forms the supporting structure by connecting
the frame to the bed plate. The ring shaped body portion of the frame which makes
the magnetic path for the magnetic fluxes from the main poles and interpoles is
called Yoke.
Why we use cast steel instead of cast iron for the construction of Yoke?
In early days Yoke was made up of cast iron but now it is replaced by cast
steel.This is because cast iron is saturated by a flux density of 0.8 Wb/sq.m where
as saturation with cast iron steel is about 1.5 Wb/sq.m.So for the same magnetic
flux density the cross section area needed for cast steel is less than cast iron hence
the weight of the machine too.If we use cast iron there may be chances of blow
holes in it while casting.so now rolled steels are developed and these have
consistent magnetic and mechanical properties.
www.BrainKart.com
, poles:
Solid poles of fabricated steel with separate/integral pole shoes are fastened to
the frame by means of bolts. Pole shoes are generally laminated. Sometimes pole
body and pole shoe are formed from the same laminations. The pole shoes are
shaped so as to have a slightly increased air gap at the tips. Inter-poles are small
additional poles located in between the main poles.
These can be solid, or laminated just as the main poles. These are also
fastened to the yoke by bolts. Sometimes the yoke may be slotted to receive these
poles. The inter poles could be of tapered section or of uniform cross section.
These are also called as commutating poles or com poles. The width of the tip of
the com pole can be about a rotor slot pitch.
Armature
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, The armature is where the moving conductors are located. The armature is
constructed by stacking laminated sheets of silicon steel. Thickness of these
lamination is kept low to reduce eddy current losses. As the laminations carry
alternating flux the choice of suitable material, insulation coating on the
laminations, stacking it etc are to be done more carefully. The core is divided into
packets to facilitate ventilation. The winding cannot be placed on the surface of
the rotor due to the mechanical forces coming on the same. Open parallel sided
equally spaced slots are normally punched in the rotor laminations.
These slots house the armature winding. Large sized machines employ a
spider on which the laminations are stacked in segments. End plates are suitably
shaped so as to serve as ’Winding supporters’. Armature construction process
must ensure provision of sufficient axial and radial ducts to facilitate easy
removal of heat from the armature winding.
Field windings:
In the case of wound field machines (as against permanent magnet excited
machines) the field winding takes the form of a concentric coil wound around the
main poles. These carry the excitation current and produce the main field in the
machine. Thus the poles are created electromagnetically. Two types of windings
are generally employed. In shunt winding large number of turns of small section
copper conductor is used. The resistance of such winding would be an order of
magnitude larger than the armature winding resistance. In the case of series
winding a few turns of heavy cross section conductor is used. The resistance of
such windings is low and is comparable to armature resistance. Some machines
may have both the windings on the poles. The total ampere turns required to
establish the necessary flux under the poles is calculated from the magnetic circuit
calculations. The total mmf required is divided equally between north and south
poles as the poles are produced in pairs. The mmf required to be shared between
shunt and series windings are apportioned as per the design requirements. As
these work on the same magnetic system they are in the form of concentric coils.
Mmf ’per pole’ is normally used in these calculations. Armature winding As
mentioned earlier, if the armature coils are wound on the surface of the armature,
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