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442



ELEMENTS OF ELECTRICITY.



are shown graphically in Fig. 270. The broken and dotted curve
represents the rectified E. M. F. in the BE coil; the broken curve
represents the same in the AA coil. The maxima follow at inter-
vals of 90 and midway between these maxima, as indicated by
the intersection of the curves, the E. M. F.s are equal. The
unbroken portion of these curves represents the E. M. F. (and




Fig. 270.

current) in the external circuit. We therefore see that by inserting
the second coil we obtain a current which, while pulsating, does
not drop to zero as did the current from the original coil. If still
other coils be inserted between these two, we may obtain a cur-
rent which fluctuates less and less, and approaches constancy as
the number of coils is increased.

559. Open and Closed Coils. Consideration of Fig. 269 will
reveal the fact that except for the very brief instant when the
brushes slide across the gap between the commutator segments,
only one coil at a time supplies current to the external circuit.
Thus, while the coil BB is supplying current, the coil A A is open
at the commutator end and contributes nothing. The E. M. F.
induced in these coils while the corresponding commutator seg-
ments are not in contact with the brushes is represented by the
broken portions of the curves in Fig. 270. This E. M. F. is not
utilized. An arrangement in this manner of the coils of a generator
is called an open-coil winding.



R




Fig. 271.

We shall shortly see (Par. 569) that there is possible another
arrangement by which the various coils may be connected in
series and thus instead of being idle during a portion of the rotation
they all constantly contribute to a resultant E. M. F. This ar-



ELECTRO-MECHANICS. 443

rangement is called a closed-coil winding. Points on the curve
RR', Fig. 271, representing this resultant E. M. F. are obtained
by adding the corresponding ordinates of the component curves.
It is seen that as the number of coils is increased, not only
does the resultant E. M. F. increase but also the loops in the
curve RR' become greater in number and smaller in amplitude,
that is, the E. M. F. becomes less pulsating and more nearly
constant.

560. Essential Parts of D. C. Generator. The essential parts
of a D. C. generator are

(a) A magnetic field.

(b) Rotating coils.

(c) A commutator.

(d) Brushes.

The coils and commutator and the shaft to which they are
attached and with which they rotate are known collectively as
the armature. The coils are usually inserted in grooves or slots
in an enlarged portion of the shaft called the armature core. The
portions of the coils on the exterior of the armature core and
parallel to the axis of the shaft are called inductors.

561. The Field. The magnetic field in which the armature
revolves is produced by field magnets, which may be either per-
manent or electro-magnets. Permanent magnets can not be
controlled nor can they be made of the size and strength required
in large machines and they are therefore restricted to such small
generators as those used to operate the call bell of a telephone or
the sparking apparatus of a gasoline engine. In all important
generators, electro-magnets are employed. It is to this class of
generators that we refer in the following pages.

Whatever be the external appearance of the generator, analysis
will show that the field magnets are in principle horseshoe magnets,
each consisting of a yoke and two limbs, the ends of these latter
being shaped to embrace between them the revolving armature.
The field coils are wrapped about these limbs, or magnet cores.
In the simplest form of generator, as shown in Fig. 273, there are
but two magnet cores and the machine is designated as bipolar.
If there be more than one pair of cores, the machine is multipolar.
Whatever be the number of poles, they are alternately north and



444



ELEMENTS OF ELECTRICITY.



south. Fig. 272 represents the frame of a multipolar generator
of six poles. It will be seen that a similar arrangement would
result by grouping around a common center six horseshoe mag-
nets, the like poles of adjacent magnets being side by side.



YOKE




Fig. 272.

The magnet cores are made of soft annealed steel so as to be
free from hysteresis. They are frequently laminated so as to
avoid eddy currents. They terminate in soft iron pieces, shoes,
which perform several functions, (a) They hold in position the
field coils after these latter have been slipped over the cores, (b)
They diminish the air gap between the pole faces and the armature
core, (c) By the shape of their ends, or horns, they produce an
advantageous distribution of the flux.

562. Excitation of Field Magnets. For all D. C. generators the
field magnets are self-excited, that is, they are excited by current
from the machine itself.

Since the machine will not generate a current unless the field
be excited, and since the field is excited by the current drawn from



ELECTRO-MECHANICS.



445



the machine itself, it is not clear at first sight why a generator
ever produces a current. If the field magnets were of perfectly
pure soft iron, it is probable that no current would be produced
when the generator was set in motion, but the iron is not per-
fectly pure and there is always some slight residual magnetism
left in the cores (Par. 155), and when the machine is started,
this is sufficient to produce a small current through the field
coils. This strengthens the magnets which in turn increases the
current, and so on, a generator on starting "building up" grad-
ually, and frequently taking a minute or so to reach normal out-
put. This building up may sometimes be aided by the earth's
field.

563. Methods of Self-Excitation. There are three distinct
ways in which the coils of the field magnets may be wound and
the exciting current passed through them so as to obtain the
desired number of ampere turns. The corresponding generators
are said to be series wound, shunt wound, and compound wound
respectively.

In a series-wound generator, the entire current passes through
the field coils/ In Fig. 273, a represents diagrammatically a




Fig. 273.

series-wound, bipolar machine. The same current which passes
through the field coils flows through the external circuit, or the
field coils and the external circuit are in series. A still more
highly conventionalized diagram of the same machine is repre-
sented in 6.

In a shunt-wound generator, only a portion of the entire current,
from two to ten per cent, is passed through the field coils. These
coils are therefore in shunt with the external circuit. In Fig. 274,



446



ELEMENTS OF ELECTRICITY.



a represents a shunt-wound, bipolar machine, the shunt being
indicated by the dotted line, and b is a more conventionalized
diagram of the same machine. Since only a fraction of the entire
current passes through the field coils, in order to secure the neces-
sary ampere turns for the excitation of the magnet cores, there
must be many more turns in these coils than in the case of those of
a series-wound machine.




Fig. 274.

The field coils of a compound-wound machine combine series
and shunt windings. Thus in Fig. 275, a represents a compound-
wound, bipolar machine, the series winding being shown by the
heavy line and the shunt winding by the dotted line. For clearness
of the diagram, the windings are represented as on separate por-
tions of the cores.




Fig. 275.

There are two varieties of the compound windings, known as
compound short shunt and compound long shunt. If the shunt is
taken off across the brushes A and B, as shown in a and more
diagrammatically in b, it is a short shunt. If, as shown in c, one
end of the shunt be taken off beyond the series coil, it is a long shunt.
The diagrams b and c indicate the reason for these names. So far



ELECTRO-MECHANICS. 447

as the machine itself is concerned, there is but little difference
between long and short shunt, but, as will be shown in the next
chapter, there is a very great difference in the three classes of
machines and in the conditions under which each is to be used.

In the foregoing diagrams the yoke of the field magnets is
represented as above the armature, but this is simply for clearness.
While they may have any position, bipolar machines are usually
mounted with the yoke horizontal and below the armature, or,
less frequently, with the yoke vertical and to one side.

564. Control of Field. In connection with this subject, refer-
ence should be made here to control of field. Since the E. M. F.
developed in a generator varies with the rate of cutting of lines of
force, if the field be constant, the E. M. F. can be varied only by
varying the speed of rotation. Since, however, generators are

RHEOSTAT




FIELD
COIUS



Fig. 276.

usually run at a constant speed, the E. M. F. is varied by increasing
or decreasing the number of lines of force, that is, by varying the
field. In a shunt- wound machine, the current through the field
coils, and consequently the field, may be varied by means of a
rheostat in series in the shunt circuit, as shown in Fig. 276. In a




Fig. 277.

series-wound generator, the field may be varied by a rheostat in
parallel with the field coils as shown in Fig. 277. The greater the
current through the rheostat, the less through the field. These



448 ELEMENTS OF ELECTRICITY.

field rheostats are not attached to the generator direct but are
mounted upon a switchboard, an auxiliary piece of apparatus which
will be described later (Par. 579).

565. Armature Core. In Par. 560 we saw that the enlarged
portion of the shaft to which the rotating coils are attached is
called the armature core. This core has two separate functions to
perform, (a) It serves as a rigid base of attachment for these
rotating coils and is therefore cylindrical in shape, (b) As ex-
plained in Par. 145 and as shown in Fig. 182, it diminishes the air
gap between the poles, thereby reducing the reluctance in the
magnetic circuit and increasing the flux. It must therefore be of a
highly permeable material, such as soft iron. It not only increases
the flux but so directs it that the lines of force are most advan-
tageously situated for being cut by the rotating coils. For ex-
ample, in the multipolar machine shown in cross-section in Fig.
278, if the armature core were non-magnetic, the lines of force




would pass directly across the gaps abed and therefore would not
be cut by the coils, but this core being of iron, the lines pass into it
(Par. 145) as shown in the diagram and are cut by the coils as they
rotate.

The core being of a magnetic substance and lying between the
poles of the field magnets, it acquires polarity (Par. 119). As it
rotates, this polarity shifts and to avoid hysteretic losses (Par. 399)
its retentivity should be very small, that is, it should be made of
soft and pure iron.

Also, since it is a conductor rotating in a magnetic field, eddy
currents will be produced in it, and to reduce these it is laminated
or built up of thin sheets (Par. 429).

These sheets usually take the form of punchings. For small
machines they may be disc-shaped and perforated with a single



ELECTRO-MECHANICS.



449



hole for assembling upon the shaft, but for large machines they are
generally segments of a circle. On the outer periphery they are
provided with slots in which the coils are wrapped (Fig. 279) and
on the inner there are undercut grooves by which they are as-
sembled upon a spider which in turn is keyed to the shaft.

ARMATURE .. - LAMINATED CORE

CQMMUTATOR

SEGMENT
INSULATION



SHAFT




-SPIDER



K *- DUCTS FOR VENTILATION

Fig. 279.



Although this lamination diminishes the eddy currents it does
not entirely obviate them and to reduce their heating effect the
core is not built up solid but at intervals ventilating spaces are
left. The air currents enter between the spokes of the spider and
emerge through these ducts.

566. Classes of Armatures. Based upon the manner in which
the coils are wrapped upon the core, there are two distinct classes
of armatures, the ring wound and the drum wound, both shown
diagrammatically in Fig. 280. Should the coil after passing




RlNGj WINDINq DRUN WINDING

Fig. 280.

through a slot on the outer surface of the armature be threaded
back through the interior of the core (Fig. 279), then again out
through a slot and so on, in other words, should it be wrapped in
a continuous helix around the rim of the armature, just as a wire
might be wrapped around the rim of a wagon wheel to hold a tire



450



ELEMENTS OF ELECTRICITY.



TERMINALS




TERMINALS



-VENTILATING DUCT



in position, it is a ring winding. On the other hand, should the
coil, after passing through a slot, cross along a chord of the end
of the core and return by a slot on the other side, it is a drum

winding.

Electrically the two
windings do not differ
in principle but prac-
tically the drum wind-
ing is used almost to
the exclusion of the
ring winding.

One objection to the

Fig. 281. . -i. .-I

ring winding is that

the conductor of which the coil is composed must be put on by
threading it back and forth and bending it into place. This is
difficult with the large copper inductors now required; moreover,
any insulation about the coil would be injured in this process so
that insulation has to be
put on as the coil is
placed in position, and it
is difficult to fasten such
coils rigidly.

On the other hand, the
coils for a drum winding
being all alike may be.
made up on a form and
of as heavy material as
may be desired (Fig. 281).
They are then wrapped
with insulation, baked to
expel moisture and var-
nished. Finally they are
packed tightly into the
armature slots and held
securely in position by
wooden wedges inserted as shown in Fig. 282. As an additional
precaution, a certain amount of banding is usually wrapped about
the armature.

567. The Commutator. In Par. 556 we saw that the split ring,
or arrangement of copper segments by which the alternating cur-




Fig. 282.



ELECTRO-MECHANICS.



451



rent was rectified, is called the commutator. With the increase in
the number of coils, the number of segments also increases and
they finally reduce to relatively thin wedge-shaped copper plates
of the form shown in Figs. 279 and 283. In the upright portion or
neck of these segments there are cut mortises into which the coil
terminals are soldered.



,'COMMUTATOR SEGMENT
I




RINGj




Fig. 283.



The commutator is the weakest point about the armature. Not
only must the separate segments be assembled into a cylinder
which is firmly attached to the armature shaft but they must also
be perfectly insulated both from each other and from the shaft.
The segments, separated by sheets of mica, are arranged in a
cylinder, being held at one end by a hub or sleeve and at the other
end by a wedge ring, from both of which they are insulated by a
layer of a composition of mica and shellac. The sleeve and the
wedge ring are drawn tightly together by means of bolts, thus,
binding the segments rigidly together, and these are then turned
down to a perfect cylinder.

568. Brushes. The brushes are so named because in the earlier
machines they were of brass wire and resembled a stiff paint
brush. In the process of evolution these took the form (still used
in certain machines) of brass laminae like the leaves of a book,
then were made of copper gauze compressed into prisms. They
are now rectangular blocks of carbon, made somewhat in the same
manner as the carbons for arc lights (Par. 516) except that there
is sometimes incorporated a small amount of paraffine which acts
as a lubricant. They are held in brush holders which are provided
with springs by which the pressure of the brushes against the



452



ELEMENTS OF ELECTRICITY.



commutator may be regulated. The holders in turn are secured
to a rocker frame by which the brushes may be shifted bodily in the
direction of rotation of the commutator or in contrary direction.
The object of this adjustment is explained later (Par. 570). The
brushes must be proportioned to the current which they are to
carry and for heavy currents, instead of being of a single large
carbon block, each consists of a number of smaller carbons with
separate springs. These may be compared to the finger tips of a
hand pressing lightly upon the commutator. Should one be
momentarily jarred away from the commutator, the others pre-
serve a flexible contact and the circuit is not broken. It will be
shown later (Pars. 573 and 577) that, except for one class of drum
windings, there are required as many brushes as there are poles.

569. The Ring- Wound Generator. In the operation of a gen-
erator, the current flowing through the coils gives rise to conditions
which, since they necessitate certain minor corrections and ad-
justments, should be thoroughly understood. On account of the







Fig. 284.

greater simplicity of the diagram, these are most readily explained
by reference to the ring winding, but it must be remembered that
this is selected merely for ease of explanation and that the majority
of modern machines are drum wound.

Fig. 284 represents diagrammatically a bipolar, ring-wound
generator. In this diagram the extremities of each turn of the
winding are represented as connected to the adjacent commutator
segments, but in the actual machine there may be a number of



ELECTRO-MECHANICS. 453

turns between these tapping wires (see Fig. 286). The lines of
force of the field, as shown by the dotted lines, follow around the
rim of the armature core and therefore as the armature rotates,
only the outer portion of the coils cuts these lines, the remaining
portion being idle. These outer portions, the inductors, are per-
pendicular to the plane of the paper but, in order that they may
be seen, are shown as part of the helical winding.

Assuming the rotation of the armature to be clockwise, applica-
tion of the right hand rule (Par. 422) shows that the direction of
the induced E. M. F. in each inductor to the right of the sym-
metrical plane through the axis of the armature is from the ob-
server, while that hi each inductor in the left half is towards the
observer. Beginning at the bottom inductor on either side and
following around to the top, the instantaneous value of the E. M.F.
being generated in each, assuming the field to be uniform, is
proportional to the sine of the angle through which it has turned
from the symmetrical or neutral plane (Par. 552), and these
inductors being portions of a continuous helix, the total E. M. F.
in each half of the armature is the sum of these separate E. M. F.s.
If, therefore, brushes be applied at A and at B, the two segments
lying in the neutral plane, and be connected through an external
circuit, A being at a higher potential than B, a current will flow
out by A and returning by B will divide, one-half flowing up each
side of the armature and reuniting at A. In other words, the
halves are in parallel and afford two paths for the current through
the armature. An analogous arrangement would be the grouping,
shown at the right of Fig. 284, of sixteen cells, two in parallel and
eight in series, the variation in the E. M. F. of the individual cells
being indicated by the length of the lines representing the cells.

570. Armature Reaction. The tendency of the field magnets
of a generator is to magnetize the armature core by induction.
As shown in Fig. 285, a north pole would be induced at N' and a
south pole at S'. However, when the generator is in operation,
each half of the ring core is surrounded by many ampere turns and
is therefore powerfully magnetized. With clockwise rotation the
current in the armature alone would produce a north pole at N"
and a south pole at S" (Par. 404). The actual magnetization of
the ring is therefore the resultant of these two, and a north pole
will be found at some intermediate point as N" f and a south pole
at S'". As a result of this reaction between the original field and



454



ELEMENTS OF ELECTRICITY.



the armature field, the flux will be distorted as shown and the
neutral plane will no longer coincide with the symmetrical plane
but will be shifted forward in the direction of rotation to some
such position as CC. The brushes must now be shifted forward
until they coincide with this plane, or are even very slightly ahead
of it (Par. 572). This adjustment is made by means of the rocker
frame (Par. 568). The plane in which the brushes are finally
placed is called the commutation plane and the angle between this
and the symmetrical plane is called the angle of lead.




Fig. 285.



The advancing of the brushes and variations in the current
through the armature may cause further shifting of the plane of
commutation, but generators are now so constructed that when
the brushes have once been adjusted and the machine is run under
average conditions, no further movement is needed.

571. Commutation. Fig. 286 represents diagrammatically a
portion of the armature of a ring-wound generator in four suc-
cessive positions. For clearness of diagram, the brush is drawn
below the commutator segments. The broken and dotted vertical
line represents the neutral plane. With clockwise rotation and
field from left to right, currents will flow through the coils in the
direction indicated by the arrows.

In position a, the brush is in contact with segment G alone. Of
the total current delivered to the brush, one-half flows in from the
coil C, the other half from the coil B.



ELECTRO-MECHANICS.



455



In position 6, the armature has moved until the brush, still
retaining contact with G, has just established contact with F. As
before, one-half of the total current flows in from C, but the other
half, arriving from A, divides at F, a small but rapidly increasing
portion flowing direct to the brush, the diminishing remainder
flowing through B to G and thence to the brush. The reason why
at first only a small portion flows direct from F to the brush is
that F is then in contact with the brush along a narrow strip only
and the resistance of this contact is considerable. However, as
the armature continues to move, this resistance decreases and the
current through F increases, that through B decreasing accord-
ingly.




Fig. 286.



In position c, the brush makes equal contact with F and G, one-
half of the current flows through CG, the other half through AF,
and the current in B is zero.

In position d, the contact with F has increased and that with G
has dwindled to a narrow line. At this instant, the full current
from A flows through F, while the current from C, for reasons
explained above, divides at G, a diminishing portion flowing
through G direct to the brush, an increasing portion flowing
through B to F.

When finally the brush is in contact with F alone, the conditions
are as represented in a, that is, one-half of the total current flows
from A to F, the other half from B to F. Originally the current
in B flowed from left to right and it now flows from right to left,



456 ELEMENTS OF ELECTRICITY.

in other words, as the successive segments slip past the brush, the
current in the corresponding coils undergoes complete reversal.

When these changes of the current in the coil under the brush
take place as outlined above, the commutation is said to be perfect.

572. Sparking. There are certain conditions which interfere
with the realization of perfect commutation. The armature
revolves at high speed and the reversal of the current in a coil
often takes place in less than one-hundredth of a second. As
these coils frequently carry from fifty to one hundred amperes
and are wrapped about an iron core, the self -induced E. M. F. is
considerable. The effect of this E. M. F. is to oppose any change
in the original direction of the current flowing in the coil, therefore,
in position d in Fig. 286 the rise of the current from G into B is
retarded and the greater part of the current from C is forced to
flow from G direct into the brush. As the area of contact between
the brush and G decreases, the current density (number of amperes
per square centimeter of cross-section) may become so great as to
produce injurious heating of the brush and of the commutator
segments. Finally, as G separates from the brush, a momentary
arc is produced, its heat being sufficient to volatilize a small
portion of both the segment and the brush. Continuance of this
"sparking" will injure or destroy the commutator.



Online LibraryWirt RobinsonThe elements of electricity → online text (page 35 of 46)