THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE ANSWER
HAWKINS
ELECTRICAL GUIDE
NUMBER
TWO
QUESTIONS
ANSWERS
&
ILLUSTRATIONS
A PROGRESSIVE COURSE OF STUDY
FOR ENGINEERS, ELECTRICIANS, STUDENTS
AND THOSE DESIRING TO ACQUIRE A
WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE
by
HAWKINS AND STAFF
THEO. AUDEL & CO. 72 FIFTH AVE. NEW YORK.
COPYRIGHTED, 1914,
BY
THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
TABLE OF CONTENTS
GUIDE NO. 2.
=THE ARMATURE= 221 to 228
Definition--=how continuous current is obtained=--type
of armature--comparison ring and drum armatures--=why drum
armature is the prevailing type=--disc armatures--why disc
armatures were abandoned.
=ARMATURE WINDINGS= 229 to 256
Preliminary considerations--=winding diagrams and winding
tables=--lap and wave winding--angular pitch or spread of
drum coils--parallel or =lap winding=--series or =wave
winding=--double-windings--Siemens winding--objection
to Siemens winding--=chord winding=--multiplex
windings--number of brushes required--number of armature
circuits--=equalizer rings=--=drum winding
requirements=.
=THEORY OF THE ARMATURE= 257 to 282
Current distribution in ring and drum armatures--=connection
of brushes=--variation of voltage around the
commutator--=cross magnetization=; field
distortion--remedies for field distortion--angle of
lead--demagnetizing effect of armature reaction--=effect of
lead=--=eddy currents=; lamination--remedy for eddy
currents--magnetic drag on the armature--smooth and slotted
armatures--comparison of smooth and slotted armatures--magnetic
=hysteresis= in armature cores--=core loss= or iron
loss--dead turns--friction.
=COMMUTATION AND THE COMMUTATOR= 283 to 302
=Period of commutation=--commutating
plane--normal neutral plane--neutral plane--plane
of maximum induction--commutation--position
of the brushes--=sparking=--effect of
self-induction--construction of commutators--=points
relating= to commutators--types of commutator.
=BRUSHES AND THE BRUSH GEAR= 303 to 320
Classification--gauze brushes--wire brushes--strip
brushes--=carbon brushes=--adjustment--comparison of
copper and carbon brushes--size of brushes--number--=contact
angle of brush=---brush contact--drop in voltage at
brushes--=brush holders=--brush rigging--multipolar brush
gear.
=ARMATURE CONSTRUCTION= 321 to to 348
Parts--shaft--core--slotted core--=core
laminations=--core bolts--attachment to
shaft--insulation of core discs--teeth--advantages and
defects of slotted armatures--slotted cores; built
up construction--=ventilation=--=insulation
of core=--armature windings--construction of
inductors--objection to copper bars--=various windings=:
hand winding--evolute or butterfly winding--connectors--barrel
winding--bastard winding--former winding--former
coils--peculiarity of evolute coil--"straight out"
coil--=coil retaining devices=--driving horns.
=MOTORS= 349 to 388
Definition--=principles=--propelling drag--essential
requirements of construction--=the reverse electromotive
force=--hydraulic analogy--action of current supplied
to motor--=armature reaction= in motors--method
of starting a motor---classes of motor; =series,
shunt, and compound=--power of a motor--brake horse
power--mutual relations of motor torque and speed--=speed
regulation=--series parallel controller--=interpole
motors=.
=SELECTION AND INSTALLATION= 389 to 406
=General conditions governing
selection=--=construction=--efficiency--adaptation of
series and shunt motors--location--foundations--=erection
of dynamos and motors=--connecting up dynamos--marine
generating set--belt clamp--belt lacing--belt
speed--=points relating to belts=--gear drive--friction
drive--=electrical connections=.
=AUXILIARY APPARATUS= 407 to 430
=Switches=--switch classification and
construction--difficulty encountered in opening the
circuit--=various switches=: knife, snap, and
quick break types--=fuses=--circuit breakers:
maximum, minimum, reverse current, maximum and reverse
current, no voltage breaker--=discriminating cut
out=--time limit attachments--=rheostats=--starting
boxes--=switchboards=.
CHAPTER XVII
THE ARMATURE
The armature of a dynamo consists of coils of insulated wire wound
around an iron core, and so arranged that electric currents are induced
in the wire when the armature is rotated in a magnetic field or the
field magnets rotated and armature held stationary.
The commutator is in fact a part of the armature, but is of
sufficient importance to be considered in a separate chapter.
=Ques. What are the practical objections to the elementary armature,
described in fig. 165?=
Ans. It induces a very feeble current, which is not of constant
pressure, but pulsating; that is, it consists of two pronounced
impulses in each revolution as shown in fig. 168.
=Ques. Why does the elementary armature produce a pulsating
current?=
Ans. The pulsations are due to the coil moving alternately into, and
out of, the positions of best and least action in the magnetic field.
=Ques. How is a continuous current, or one of uniform pressure
obtained?=
Ans. If an additional coil be added to the elementary armature, at
right angles to the existing coil, and its ends suitably connected
to a four part commutator, as in fig. 185, so that one coil is in
the position of best action, while the other is in the position of
least action, the pulsations of the resulting current will be of
less magnitude. By increasing the coils and suitably altering the
construction of the commutator to accommodate the ends of these coils,
the resultant current may be represented by practically a straight
line, indicating the so called _continuous current_, instead of the
wavy resultant curve No. 6, as illustrated in fig. 187.
[Illustration: FIG 247.--Ring armature of four pole dynamo:
diagram of winding and connections, showing direction of the induced
currents. The currents in the windings under the upper N and S poles
are opposed to each other and flow to the external circuit by the
positive brush 1, and back to this half of the armature by the negative
brushes 3 and 4. At the same instant the opposed currents in the lower
windings flow to the external circuit by positive brush 2 and return
to the armature through negative brushes 3 and 4. The armature is
thus divided into four circuits and four brushes are required which
must be placed between the poles so as to short circuit the coils as
they pass through the neutral space. In this form of winding there is
no difference of potential between the + brushes, so that they are
connected in parallel, as are also the negative brushes, and then
to the external circuit. In multipolar machines there are as many
brushes as pole pieces. Since opposite commutator bars are of the
same potential on this four pole dynamo they may be joined by a cross
connecting wire and two brushes, as 2 and 4, dispensed with. This can
only be done when there is an even number of coils. The armature is
said to be "cross connected."]
An armature for practical use has a large number of coils,
suitably arranged upon an iron core, so that a large proportion
of them are always actively cutting the lines of force, or
moving into the positions of best action in the magnetic field.
=Types of Armature.=--Although there are many forms of armature,
all may be divided into three classes, according to the arrangement of
the coils or winding on the core, as:
1. Ring armatures;
2. Drum armatures;
3. Disc armatures.
Each of these forms of armature has its own special advantages
for particular purposes, the disc type being least in favor and
not having had any extensive application in this country.
[Illustration: FIG. 248.--Early form of Gramme ring armature,
the core being shown cut through, and some of the coils displaced to
make it clearer. The core, F, consists of a quantity of iron wire
wound continuously to form a ring of the shape shown by the section.
Over this is wound about thirty coils of insulated copper wire, B C D,
etc., the direction of the winding of each being the same, and their
adjacent ends connected together. The commutator segments consist of
a corresponding number of brass angle pieces, _m_, _n_, which are
fixed against the wooden boss, _o_, carried on the driving shaft.
The junction of every two adjacent coils is connected to one of the
commutator segments, as shown at _n_.]
=Ques. What is the comparison between ring and drum armatures?=
Ans. The drum armature is electrically and mechanically the more
efficient, possessing, as it does, possibilities in the way of better
mechanical construction of the core, and in the arrangement and fixing
of the inductors thereon not to be found in the ring form. Less wire
and magnetizing current are required for the field magnets for a given
output than with the ring armature. Drum winding is not so simple as
ring winding, and it is more difficult to ventilate a drum than a ring
armature, it being necessary to provide special ventilating ducts.
[Illustration: FIG. 249.--Modern form of Gramme ring armature.
The core consists of a number of thin flat rings of well annealed
charcoal iron, the outer diameter of each ring or disc being 11½
inches, and its inner diameter 9¼ inches. Sheets of thin paper
insulate each disc from its neighbors to prevent the flow of eddy
currents. The armature is mounted on a steel shaft to which is keyed
a four armed metal "spider," the extremities of whose arms fit into
notches cut in the inner edges of the soft iron core rings, so that a
good mechanical connection is obtained between the core and the shaft.
The spider is made of a non-magnetic metal, to reduce the tendency to
leakage of lines of force across the interior of the armature. The
armature inductors consist of cotton covered copper wire of No. 9
standard wire gauge, wound around the core in one layer, and offering
a resistance, from brush to brush, of 0.048 ohm. There are two
convolutions in each section, the adjacent ends of neighboring sections
being soldered to radial lugs projecting from the commutator bars.]
=Ques. Describe a ring armature.=
Ans. It consists essentially of an iron ring, around which is wound
a number of coils. These various coils are wound on separately, the
wire being carried over the outside of the ring, then through the
center opening and again around the outside, this operation being
repeated until the winding for that individual section is completed.
The adjacent coil is then wound in the same way, the ends of each being
brought out to the commutator side of the armature, the arrangement of
the coils on the ring and connections with the commutator being shown
in fig. 247, examples of actual construction being shown in figs. 248
and 249.
=Ques. For what conditions of operation is the ring armature
specially adapted, and why?=
Ans. It is well suited to the generation of small currents at high
voltage, as for series arc lighting, because the numerous coils can be
very well insulated.
[Illustration: FIG. 250.--Distribution of magnetic lines of
force through a Gramme ring. Since the metal of the ring furnishes a
path of least reluctance, most of the magnetic lines will follow the
metal of the ring and very few will penetrate into the aperture of
the interior. This condition causes a serious defect in the action of
ring armatures rendering the winding around the interior useless for
the production of electromotive force. Hence, in ring armatures only
about half of the winding is effective, the rest or "dead wire," adding
its resistance to the circuit, thus decreasing the efficiency of the
machine.]
=Ques. Why does a ring armature require more copper in the winding
than a drum armature?=
Ans. For the reason that those inductors which lie on the inner side of
the iron ring, being screened from practically all the lines of force,
as shown in fig. 250, do not generate any current.
Numerous attempts have been made to utilize this part of the
winding by making the pole pieces extend around the ring in
such a manner that lines of force will pass to the inside of
the ring, also by arranging an additional pole piece on the
inside of the armature, but mechanical considerations have
shown these methods to be impractical.
=Ques. Is any portion of the winding of a drum armature inactive?=
Ans. Yes; the end connectors do not generate any current.
[Illustration: FIG. 251.--Illustrating the principle of
Siemens' drum winding. In order to make the winding and connections
clear, one coil and the commutator is shown assembled, although the
latter is not put in place until after all the sections have been
wound, the ends of the wires being temporarily twisted together until
all can be soldered to the risers. The cores of these early machines
were of wood overspun circumferentially with iron wire before receiving
the longitudinal copper windings.]
=Ques. What is the chief advantage of the drum armature?=
Ans. It reduces considerably the large amount of dead wire necessary
with the ring type.
=Ques. How is this accomplished?=
Ans. By winding the wire entirely on the outer surface of a cylinder or
_drum_, as it is called, as shown in fig. 251, thus none of the wire is
screened by the metal of the core.
[Illustration: FIG. 252.--Elementary four coil drum winding,
showing the connections with the commutator segments, and directions of
currents in the several coils. The action of this type of armature is
fully explained in the text.]
Fig. 252 shows an elementary four coil drum armature. Starting
from the point _a_ and following the winding around without
reference at first to the commutator, it will be found that the
rectangular turns of the wire form a closed circuit, and are
electrically in series with one another in the order of the
numbers marked on them.
With respect to the connections to the four segments _w_, _x_,
_y_, _z_, of the commutator it will be found that at two of
these, _x_ and _y_, the pressures in the windings are both
directed _from_, or both directed _toward_ the junction with
the connecting wire. At the other two segments, _z_ and _w_,
one pressure is toward the junction and the other directed from
it. If, therefore, the brushes be placed on _x_ and _y_ they
will supply current to an external circuit, _z_ and _w_, for
the moment being idle segments.
=Disc Armatures.=--The inductors of a disc armature move in a
plane, perpendicular to the direction of the lines of force, about an
axis parallel to them as shown in fig. 253. The main difficulty with
this type has been in constructing it so that it will be strong and
capable of resisting wear and tear. It was introduced in an effort to
avoid the losses due to eddy currents and hysteresis present in the
other types of armature.
[Illustration: FIG. 253.--Disc armature of Niaudet. It is
equivalent to a ring armature, having the coils turned through an angle
of 90°, so that all the coils lie in a plane perpendicular to the axis
of rotation. The connections of the coils with each other and with
the commutator remain the same, the beginning and the end of adjacent
coils leading to a common commutator bar as shown. The magnetic field
is arranged by the use of two magnets, so arranged as to present the
north pole of one to the south pole of the other, and _vice versa._ In
the figure one of these magnets is considered as above the paper, and
the other below. If this armature be rotated through the magnetic field
as shown, a reversal of current takes place in each coil, when it is in
such a position that one of its diameters coincides with the pole line,
_NS_. If the brushes be set so as to short circuit the coils that are
in this position, the armature will be divided into two branchings, the
current flowing in an opposite direction in each, and a direct current
will flow in the exterior circuit.]
On account of the nature of the construction of a disc
armature, it is necessary that the coils subject to induction
occupy as small a space as possible in the direction of their
axes. This requirement, as well as the connection of the
inductors with each other and with the commutator, prevented
the general adoption of this form of armature, and subsequent
experience failed to justify the existence of the type.