Construction Details Of A Dc Generator Or Motor :
Mainly The Dc Generator Or Motor Consists Of These Main Parts
1. Magnetic Frame or Yoke
2. Pole-Cores and Pole-Shoes
3. Pole Coils or Field Coils
4. Armature Core
5. Armature Windings or Conductors
6. Commutator
7. Brushes and Bearings
Of these, the yoke, the pole cores, the armature core and air gaps between the poles and the
armature core or the magnetic circuit whereas the rest form the electrical circuit.
1)Yoke :
The outer frame or yoke serves double purpose :
(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine
and
(ii) It carries the magnetic flux produced by the poles.
In small generators where cheapness rather than weight is the main consideration, yokes are made of
cast iron. But for large machines usually cast steel or rolled steel is employed. The modern process of forming the yoke consists of rolling a steel slab round a cylindrical mandrel and then welding it at the bottom. The feet and the terminal box etc. are welded to the frame afterwards. Such yokes possess sufficient mechanical strength and have high permeability
2)Pole-Cores And Pole-Shoes :
The field magnets consist of pole cores and pole shoes. The pole shoes serve two purposes
(i) they spread out the flux in the air gap and also, being of larger cross-section, reduce the reluctance
of the magnetic path
(ii) they support the exciting coils (or field coils) as shown in Fig
There are two main types of pole construction.
(a) The pole core itself may be a solid piece made out of either cast iron or cast steel but the pole
shoe is laminated and is fastened to the pole face by means of counter sunk screws as shown in
Fig.
(b) In modern design, the complete pole cores and pole shoes are built of thin laminations of
annealed steel which are rivetted together under hydraulic pressure The thickness of
laminations varies from 1 mm to 0.25 mm. The laminated poles may be secured to the yoke in any of
the following two ways :
(i) Either the pole is secured to the yoke by means of screws bolted through the yoke and
into the pole body or
(ii) The holding screws are bolted into a steel bar which passes through the pole across the
plane of laminations
3) Pole-Coils Or Field-Coils :
The field coils or pole coils, which consist of copper wire or strip, are former-wound for the
correct dimension (Fig). Then, the former is removed and wound coil into place over the
core as shown in above fig When current is passed through these coils, they electromagnetise the poles which produce the necessary flux that is cut by revolving armature conductors.
4) Armature Core :
It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic
flux of the field magnets. In addition to this, its most important function is to provide a path of very
low reluctance to the flux through the armature from a N-pole to a S-pole.
It is cylindrical or drum-shaped and is built up of usually circular sheet steel discs or laminations
approximately 0.5 mm thick. It is keyed to the shaft.
The slots are either die-cut or punched on the outer periphery of the disc and the keyway is
located on the inner diameter as shown. In small machines, the armature stampings are keyed directly
to the shaft. Usually, these laminations are perforated for air ducts which permits axial flow of air
through the armature for cooling purposes. Such ventilating channels are clearly visible in the laminations shown in Below Figures
Up to armature diameters of about one metre, the circular stampings are cut out in one piece as
shown in Fig.. But above this size, these circles, especially of such thin sections, are difficult
to handle because they tend to distort and become wavy when assembled together. Hence, the circular laminations, instead of being cut out in one piece, are cut in a number of suitable sections or
segments which form part of a complete ring
A complete circular lamination is made up of four or six or even eight segmental laminations. Usually, two
keyways are notched in each segment and are dove-tailed or wedge-shaped to make the laminations self-locking in position.
The purpose of using laminations is to reduce the loss due to eddy currents. Thinner the laminations, greater
is the resistance offered to the induced e.m.f., smaller the current and hence lesser the I^2 R loss in the core.
5) Armature Windings :
The armature windings are usually former-wound.
These are first wound in the form of flat rectangular coils and are then pulled into their proper shape
in a coil puller. Various conductors of the coils are insulated from each other. The conductors are
placed in the armature slots which are lined with tough insulating material. This slot insulation is
folded over above the armature conductors placed in the slot and is secured in place by special hard
wooden or fibre wedges.
6) Commutator :
The function of the commutator is to facilitate collection of current from the armature conductors. As shown in Art. , it rectified i.e. converts the alternating current induced in the armature
conductors into unidirectional current in the external load circuit. It is of cylindrical structure and is
built up of wedge-shaped segments of high-conductivity hard-drawn or drop forged copper.
These segments are insulated from each other by thin layers of mica. The number of segments is equal to the number of armature coils. Each commutator segment is connected to the armature conductor by
means of a copper lug or strip (or riser). To prevent them from flying out under the action of centrifugal forces, the segments have V-grooves, these grooves being insulated by conical micanite rings. A
sectional view of commutator is shown in Fig. whose general appearance when completed is
shown in Fig
7) Brushes And Bearings :
The brushes whose function is to collect current from commutator, are usually made of carbon or
graphite and are in the shape of a rectangular block. These brushes are housed in brush-holders
usually of the box-type variety. As shown in Fig. the brush-holder is mounted on a spindle and
the brushes can slide in the rectangular box open at both ends. The brushes are made to bear down on
the commutator by a spring whose tension can be adjusted by changing the position of lever in the
notches. A flexible copper pigtail mounted at the top of the brush conveys current from the brushes
to the holder. The number of brushes per spindle depends on the magnitude of the current to be
collected from the commutator.
Because of their reliability, ball-bearings are frequently employed, though for heavy duties, roller
bearings are preferable. The ball and rollers are generally packed in hard oil for quieter operation and
for reduced bearing wear, sleeve bearings are used which are lubricated by ring oilers fed from oil
reservoir in the bearing bracket.
Mainly The Dc Generator Or Motor Consists Of These Main Parts
1. Magnetic Frame or Yoke
2. Pole-Cores and Pole-Shoes
3. Pole Coils or Field Coils
4. Armature Core
5. Armature Windings or Conductors
6. Commutator
7. Brushes and Bearings
Of these, the yoke, the pole cores, the armature core and air gaps between the poles and the
armature core or the magnetic circuit whereas the rest form the electrical circuit.
1)Yoke :
The outer frame or yoke serves double purpose :
(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine
and
(ii) It carries the magnetic flux produced by the poles.
In small generators where cheapness rather than weight is the main consideration, yokes are made of
cast iron. But for large machines usually cast steel or rolled steel is employed. The modern process of forming the yoke consists of rolling a steel slab round a cylindrical mandrel and then welding it at the bottom. The feet and the terminal box etc. are welded to the frame afterwards. Such yokes possess sufficient mechanical strength and have high permeability
2)Pole-Cores And Pole-Shoes :
The field magnets consist of pole cores and pole shoes. The pole shoes serve two purposes
(i) they spread out the flux in the air gap and also, being of larger cross-section, reduce the reluctance
of the magnetic path
(ii) they support the exciting coils (or field coils) as shown in Fig
There are two main types of pole construction.
(a) The pole core itself may be a solid piece made out of either cast iron or cast steel but the pole
shoe is laminated and is fastened to the pole face by means of counter sunk screws as shown in
Fig.
(b) In modern design, the complete pole cores and pole shoes are built of thin laminations of
annealed steel which are rivetted together under hydraulic pressure The thickness of
laminations varies from 1 mm to 0.25 mm. The laminated poles may be secured to the yoke in any of
the following two ways :
(i) Either the pole is secured to the yoke by means of screws bolted through the yoke and
into the pole body or
(ii) The holding screws are bolted into a steel bar which passes through the pole across the
plane of laminations
3) Pole-Coils Or Field-Coils :
The field coils or pole coils, which consist of copper wire or strip, are former-wound for the
correct dimension (Fig). Then, the former is removed and wound coil into place over the
core as shown in above fig When current is passed through these coils, they electromagnetise the poles which produce the necessary flux that is cut by revolving armature conductors.
4) Armature Core :
It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic
flux of the field magnets. In addition to this, its most important function is to provide a path of very
low reluctance to the flux through the armature from a N-pole to a S-pole.
It is cylindrical or drum-shaped and is built up of usually circular sheet steel discs or laminations
approximately 0.5 mm thick. It is keyed to the shaft.
The slots are either die-cut or punched on the outer periphery of the disc and the keyway is
located on the inner diameter as shown. In small machines, the armature stampings are keyed directly
to the shaft. Usually, these laminations are perforated for air ducts which permits axial flow of air
through the armature for cooling purposes. Such ventilating channels are clearly visible in the laminations shown in Below Figures
Up to armature diameters of about one metre, the circular stampings are cut out in one piece as
shown in Fig.. But above this size, these circles, especially of such thin sections, are difficult
to handle because they tend to distort and become wavy when assembled together. Hence, the circular laminations, instead of being cut out in one piece, are cut in a number of suitable sections or
segments which form part of a complete ring
A complete circular lamination is made up of four or six or even eight segmental laminations. Usually, two
keyways are notched in each segment and are dove-tailed or wedge-shaped to make the laminations self-locking in position.
The purpose of using laminations is to reduce the loss due to eddy currents. Thinner the laminations, greater
is the resistance offered to the induced e.m.f., smaller the current and hence lesser the I^2 R loss in the core.
5) Armature Windings :
The armature windings are usually former-wound.
These are first wound in the form of flat rectangular coils and are then pulled into their proper shape
in a coil puller. Various conductors of the coils are insulated from each other. The conductors are
placed in the armature slots which are lined with tough insulating material. This slot insulation is
folded over above the armature conductors placed in the slot and is secured in place by special hard
wooden or fibre wedges.
6) Commutator :
The function of the commutator is to facilitate collection of current from the armature conductors. As shown in Art. , it rectified i.e. converts the alternating current induced in the armature
conductors into unidirectional current in the external load circuit. It is of cylindrical structure and is
built up of wedge-shaped segments of high-conductivity hard-drawn or drop forged copper.
These segments are insulated from each other by thin layers of mica. The number of segments is equal to the number of armature coils. Each commutator segment is connected to the armature conductor by
means of a copper lug or strip (or riser). To prevent them from flying out under the action of centrifugal forces, the segments have V-grooves, these grooves being insulated by conical micanite rings. A
sectional view of commutator is shown in Fig. whose general appearance when completed is
shown in Fig
7) Brushes And Bearings :
The brushes whose function is to collect current from commutator, are usually made of carbon or
graphite and are in the shape of a rectangular block. These brushes are housed in brush-holders
usually of the box-type variety. As shown in Fig. the brush-holder is mounted on a spindle and
the brushes can slide in the rectangular box open at both ends. The brushes are made to bear down on
the commutator by a spring whose tension can be adjusted by changing the position of lever in the
notches. A flexible copper pigtail mounted at the top of the brush conveys current from the brushes
to the holder. The number of brushes per spindle depends on the magnitude of the current to be
collected from the commutator.
Because of their reliability, ball-bearings are frequently employed, though for heavy duties, roller
bearings are preferable. The ball and rollers are generally packed in hard oil for quieter operation and
for reduced bearing wear, sleeve bearings are used which are lubricated by ring oilers fed from oil
reservoir in the bearing bracket.
The running with material of 75 structure centers performing superior to the koolmu centers when secure the lower center events than Xflux and it contains with iron, aluminum and silicon. It giving lower cost other decision to high flux and cerebrum boggling overhaul in center hardship.jovil winding machine
ReplyDeleteThe present close fixation winding machines are convulsively used with focuses with square kind of structure yet the new winding model has striking execution than existing winding machine.toroidal winding
ReplyDelete