DC Generator
Generator Principle
An electrical generator is a machine, which converts mechanical
energy (or power) into electrical energy (or power).
The energy conversion in a generator from mechanical energy to
electrical energy is based on the principle of the production of
dynamically induced electromotive force (e.m.f).
Whenever a conductor cuts magnetic 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.
Hence, two basic essential parts of an electrical generator are:
(a) A magnetic field, and
(b) A conductor or conductors, which can so move as to cut
the flux.
Simple Loop generator
Construction:
A single-turn rectangular copper coil
ABCD, as shown in Fig. 24.1a,
rotating about its own axis in a
magnetic field provided by either
permanent
magnets
or
electromagnets.
The two end of the coil are joined to
slip-rings ‘a’ and ‘b’ which are
insulated from each other and from
the central shaft.
Two collecting brushes (of carbon or copper) press against the sliprings.
Their function to collect current induced in the coil and to convey it to
the external load resistance R.
The rotating coil may be called “armature” and the magnets as “field
magnets”.
Working Principle
The coil sides AB and CD now represent by A and B
as shown in Fig.2.9. Imagine the coil to be rotating
in clock-wise direction.
When the coil sides A and B are at position 1, the
plane of the coil is at right angles to line of flux,
the flux linked with the coil is maximum but rate of
change of flux linkages is minimum.
It is so because in this position, the coil sides A
and B do not cut or share the flux, rather the slide
along them i.e. they move parallel to the lines of
flux. Hence there is no induced e.m.f in the coil.
As the coil continues rotating further, the rate of change of flux
linkages (and hence induced e.m.f in it) increases, till the angle
equals 90o.
At position 2, the voltage induced
in the coil is at a maximum
because the conductors are moving
at right angles to the lines of flux.
In this position, the coil plane is horizontal i.e.
parallel to the lines of flux. As seen, the flux
linked with the coil is minimum but rate of change
of flux linkages is maximum. Hence maximum
e.m.f is induced in the coil when in this position 2.
The direction of current in the coil can be obtained in accordance with
Feming’s right-hand rule.
In the next quarter revolution i.e. from 90o to 180o, the flux linked with
the coil gradually increases but the rate of change of flux linkages
decreases.
Hence, the induced e.m.f decreases gradually, the angles equals 180o (it
is reduced to zero value).
The voltage is again zero at position 3, just as it was
at position 1.
Coil side A is now at the bottom instead of at the top.
We find the first half revolution of the coil, no
e.m.f is induced in it when in position 1,
maximum when in position 2 and no e.m.f when
in position 3 in this half revolution the coil sides
A and B are moved in downward.
In the next half revolution i.e. from 180o to 360o,
the variations in the magnitude of e.m.f. are
similar to those in the first half revolution.
The e.m.f value is maximum when coil is in position 4 and
minimum when in position 5 (or 1).
The direction of current is opposite of the current which is found
for the first revolution.
Due to the direction of motion (upward direction) the current
direction is changed in the next half revolution.
Unidirectional Current Generation
We find that the current which is obtained from a simple generator
reverses its direction after every half revolution.
Such a current undergoing periodic reversals is known as alternating
current.
For making the flow of current
unidirectional in the external circuit, the
slip-rings are replaced by split-rings (or
commutator) as shown in (a).
The split-rings are made out of a
conducting cylinder which is cut into
two halves or segments insulated from
each other by a thin sheet of mica or
some other insulating material.
The ends of the coil are connected through the split-ring to the
brushes which lead to the external circuit.
Previous Fig. (a) can simply be drawn as Fig. 2.10.
While the coil moves from position 1 to position 2, the brushes
remain in contact with the split-ring segments and the current
direction remains as indicated although the magnitude decreases.
At position 2 the voltage
induced in the coil is zero,
and the current in the
external circuit is also zero.
At this instant segment A leaves brush 1 and makes connect with brush
2. Segment B leaves brush 2 and makes contact with brush 1.
As the coil moves from position 2 to position 3, segment A makes
contact with brush 2 only, while the current increases from zero and
leaves brush 2.
Of course, during this same period current returns to the coil through
brush 1 and segment B.
1
2
Again, when the current in the coil becomes zero, the segment in
contact with the brush changes, thereby maintaining a unidirectional
current in the external circuit.
This follows from the fact that wire 1 is always connected to the coil
side under the North Pole, while coil 2 is always connected to the coil
side under the South Pole.
Even now the current induced in the coil sides is alternating as before.
It is only due to the rectifying action of the split-rings (also called
commutator) that is becomes unidirectional in the external circuit.
Hence, it should be clearly understood that even the armature of DC
generator, the induced voltage is alternating.
Practical Generator
Fig. 5 shows a cross-section of a
typical
commercial
DC
generator,
simplified
for
emphasis of the major portions.
The basic principle underlying
construction and working of an
actual generator which consists
of the following essential parts:
1. Magnetic Frame or Yoke,
2. Pole Coils or Field Coils,
3. Pole-Cores and Pole-Shoes,
4. Interpole (Commutating Pole)
5. Compensating Winding
6. Armature or Rotor Shaft
7. Armature Core,
8. Armature Windings or Conductors,
9. Commutator,
10. Brushes and Brush Rigging, and
11. Bearing
Magnetic Frame or Yoke
Yokes are made of cast iron or cast steel or rolled steel.
The outer frame or yoke serves double purpose:
1. It provides mechanical support for the poles and acts as a
protecting cover for the whole machine, and
2. It carries the magnetic flux produced by the poles.
Pole Coils or Field Coils
To produce the flux line by means of an electromagnetic, a voltage
have to be supplied through a coil.
A coil consisting of many turns of fine wire is generally wound
around the core.
This coil is called shunt field.
Also around the core may be found a few turns of heavy wire.
This is called the series field.
These coils produce the magnotomotive force required to yield the
necessary flux cut by the rotating conductors.
Pole-Cores and Pole-Shoes
The field magnets consist of pole cores and pole shoes.
The core of the pole is built up of laminated steel and the shoe of pole
is curved to produce a more uniform magnetic field.
The pole shoes serve the following
purposes:
1. They spread out the flux in the air gap
and also, being of larger cross-section,
reduce the reluctance of the magnetic path,
and
2. They support the exciting coils (or field
coils)
Interpole
The interpole and its winding are mounted on the yoke of
the dynamo.
These are located in the interpolar region between the main
poles and are generally smaller in size.
The interpole winding is composed of a few turns of heavy
wire.
Since it is connected in series with the armature circuit so
that its magnotomotive force (mmf) is proportional to the
armature current.
Compensating Winding
Compensating windings are optional.
They are connected in the same manner at the interpole
windings but are located in axial slots of the field shoes.
Armature Shaft
The moving part of the DC generator is called the armature.
The armature consists of a shaft upon which all parts are mounted.
The armature shaft, which imparts rotation to the armature core,
winding, and commutator.
Armature Windings or Conductors
The armature winding 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.
The conductors are placed in the armature slots which are lined with
tough insulating material.
Armature Core
The material surrounding the shaft is laminated sheet steel and is
called the armature core.
This magnetic material is necessary to provide a path of low
reluctance to the line of flux from the poles.
The laminations are required to reduce the eddy current due to the
change of flux in the core.
Each junction point between coils is connected to a commutator.
The commutator segments are insulated from each other and the
shaft.
The segments form a ring around the shaft of the armature.
The armature serves the following functions:
1. It provides a low-reluctance path for the flux,
2. It holds the coils, and
3. It produces motion so that the coils can cut the flux.
Commutator
The function of commutator is to facilitate collection of
current from the armature conductors.
It rectifies i.e. converts the alternating current induced in
the armature conductors into unidirectional current in the
external load circuit.
The segments of commutator are insulated from each other
by thin layers of mica.
The number of segments is equal to the number of armature
coils.
Each commutator segments is connected to the armature
conductor by means of a copper lug or strip (or riser).
Brushes and Brush Rigging
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 and supported from the stator
structure by a rigging.
These brushes are housed in brushholders usually of the box-type
variety.
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 (the current is taken from the brush by
means of a flexible copper wire embedded in the brush, called the
pigtail) mounted at the top of the brush conveys current from the
brushes to the holder.
Bearings
The armature is supported at each end by a metal
framework called end bells.
The end bells contain the bearings in which the armature
rotate.
One end bell is left open or made with a cover that can be
removed to inspect the brushes.
The open end bell also aids in the natural cooling of the
generator.
Because of their reliability, ball-bearings are frequently
employed though for heavy duties, roller bearings are
preferable.
Rotor of a dc
machine
Cutaway view
of a dc
machine.
DC machine stator with poles visible.
Armature Windings
Coil: One or more turns of wire grouped together
and mounted on the drum wound armature in
order to cut lines of flux as shown in Fig. 4.1.
Coil side: Any side of coil that cuts lines of flux.
Winding Element: The side of coil is called a winding element.
Obviously, the number of winding elements is twice the number of
coils.
Conductor (or inductor): The length of a wire lying in the magnetic
field and in which an e.m.f. is induced, is called conductor (or
inductor).
Winding: The complete connection and location of all the coils on the
armature.
Pitch: A method of measurement. The pitch is measured as the unit of
coil sides, slots and commutator segments.
Front End Connection: A wire that
connects the end of a coil to a commutator
segment. This wire is located at that part
of the coil that is nearest the commutator.
Back End Connection: A wire or conductor that connects an inductor
on one side of the coil to an inductor on the other side of the coil. It is
on the end opposite to the commutator.
Pole Pitch: It may be variously defined as:
(a) The distance between identical points on adjacent poles i.e.
the periphery of the armature divided by the number of poles of the
generator.
(b) It is equal to the number of armature conductors (or
armature slots) per pole.
Coil Span or Coil Pitch (Ys): It is the distance, measured in terms of
armature slots (or armature conductors) between two sides of a coil.
It is, in fact, the periphery of the armature spanned by the two sides of
the coil.
Full-Pitched: If the coil pitch is equal to pole pitch, then winding is
called full-pitched.
It means that coil span is 180o electrical degrees.
In this case, the coil sides lie under opposite poles, hence the induced
e.m.f.s in them are additive.
Therefore, maximum e.m.f. is induced in the two coil sides.
Fractional Pitched: If the coil span is less than the pole pitch, then
the winding is called fractional-pitched.
In this case, there is a phase difference between the e.m.f.s in the two
sides of the coil.
Hence, the total e.m.f. round the coil, which is the vector sum of
e.m.f.s in the two sides, is less in this case as compared to that in the
full-pitched case.
Fractional-pitched windings are purposely used to effect substantial
saving in the copper of the end connections and for improving
commutation.
An armature wound with a fractional pitch is called a chorded
winding.
Back Pitch (YB): The number of
coil sides or slots spanned by the
back end connections.
Front Pitch (YF): The number
of coil sides or slots spanned by
the front end connections.
Both front and back pitches for
lab and wave winding are shown
in Fig. 24.25 and Fig. 24.26.
Pitch of Winding (Y) or Resultant Pitch (YR): It is the distance
between the beginning of one coil and the beginning of the next coil to
which it is connected.
Commutator Pitch (YC): The number of commutator segments spanned
from one end of a coil to the other end of the same coil.
From Fig.24.25 and 24.26 it is clear that for lap winding, YC is the
difference of YB and YF whereas for wave winding it is sum of YB and YF.
Single-Layer Winding: It is that
winding which one conductor or coil
side is placed in each armature slot.
Such a winding is not much used.
Two-Layer Winding: In this type of
winding, there are two conductors or
coil sides per slot arranges in two layers.
Usually, one side of every coil lies in the
upper half of one slot and other side lies in
the lower of half of some other slot at a
distance of approximately one pitch away.
The transfer of the coil from one slot to
another is usually made in a radial plane by
means of a particular bend or twist at the
back end.
The coil sides lying at the upper half of the
slots are numbered odd i.e. 1, 3, 5, 7, etc.
while those at lower half are numbered
even i.e. 2, 4, 6, 8, etc.
Degree of Reentrancy of an Armature Winding
A winding said to be singly re-entrant if on tracing through it once, all
armature conductors are included on returning to the staring point.
It is doubly reentrant if only half the conductors are included in tracing
through the winding once and so on.
Multiplex Winding
If there is only one set of closed winding, it is called simplex
winding.
If there are two such windings on the same armature, it is called
duplex winding and so on.
The multiplicity affects a number of parallel paths in the armature.
For a give armature slots and coils, as the multiplicity increases, the
number of parallel paths in the armature increases thereby increasing
the current rating but decreasing the voltage rating.
Lap and Wave Windings
Two types of end connections of windings
are employed, namely, the lap-wound
and wave-wound connections as shown
in Fig. 24.25 and 24.26.
Each
winding
can
be
arranged
progressively or retrogressively and
connected in simplex, duplex and
triplex.
The following rules, however, applying to
both types of the winding:
(a) The front pitch and back pitch are each approximately equal to the pole-pitch i.e.
windings should be full pitched.
This results in increased e.m.f. round the coils. For special purposes fractional
pitched windings are deliberately used.
(b) Both pitches should be odd; otherwise it would be difficult to place the coils
properly on the armature.
(c) The number of commutator segments is equal to the number of slots or coils (or
half the number of conductors) because the front ends of the conductors are
joined to the segments in pairs.
(d) The winding must close upon itself.
Uses of Lap and Wave Winding
The number of parallel path of wave winding does not depend on the number of
pole but the number of parallel path of lap winding depends on number of pole.
Each of the two parallel paths of wave winding contains conductor lying under
all the poles whereas in lap winding, each of the parallel paths contains
conductors which lies under one pair of poles.
So, for a given number of poles and armature conductors, the wave winding
gives more e.m.f. than the lap winding. Conversely, for the same e.m.f. lap
winding would require large number of conductors which will result in higher
winding cost and less efficient utilization of space in the armature slots.
Hence, means wave winding is suitable for comparatively low-current but high
voltage generators because it gives smaller parallel paths.
And, lap winding is suitable for comparatively low-voltage but high current
generators because it gives more parallel paths.
In wave winding, equalizing connections are not necessary whereas in a lap
winding they definitely are.
Any inequality of pole fluxes affects two paths equally; hence their induced
e.m.f.s are equal.
In lap-wound armatures, unequal voltages are produced which set up a
circulating current that produces sparking at brushes.