Copyright
Wirt Robinson.

The elements of electricity online

. (page 36 of 46)
Online LibraryWirt RobinsonThe elements of electricity → online text (page 36 of 46)
Font size
QR-code for this ebook


If the brush be moved slightly forward in the direction of
rotation of the armature (as for example under coil C in position
d), the act of commutation will occur, not with a coil which is in
the neutral plane but with one in which there is being induced an
E. M. F. opposite in direction to the self-induced E. M. F. This
induced E. M. F. therefore opposes and assists in overcoming the
self -induced E. M. F. and removes this source of sparking. Gen-
erator brushes, therefore, are usually set slightly in advance of the
neutral plane.

573. Multipolar Generators. Fig. 287 represents diagram-
matically a four-pole ring-wound generator. Application of the
right hand rule shows that with clockwise rotation the direction
of the induced E. M. F. is as represented by the arrowheads. If
these be examined, it will be seen that the E. M. F. acts from the
coils to the commutator in two points, A and B, and from the
commutator to the coils in two other points, C and D. Therefore,
if brushes be applied at these four points and be connected through



ELECTRO-MECHANICS.



457



an external circuit, currents will flow out from A and B, and
return by C and D. With a six-pole machine six brushes are
needed and in general in ring-wound generators as many brushes
are required as there are poles. Brushes of like polarity are usually
connected to a common conductor, a ring, to which in turn the
corresponding lead is attached.

574. Advantages of Multipolar Machines. Multipolar ma-
chines possess some important advantages over bipolar machines
and most modern machines of appreciable power are of this type.

(a) When a coil of the generator represented in Fig. 287 has
rotated geometrically through 180, it has rotated electrically




Fig. 287.

through 360. With a six-pole machine, one-third of a revolution
carries it through 360 electrically. Therefore, with the same
number of lines of force from pole to pole, the same E. M. F. may
be developed by a four-pole machine with an angular velocity
only half as great as that of the bipolar machine. Or, if the
angular velocity of the two be the same, the multipolar machine
will develop the greater E. M. F.



458



ELEMENTS OF ELECTRICITY.



(b) Examination of the figure will show that the current coming
in to the machine divides equally between C and D and from each
of these points has two paths to the positive brushes A and B, in
other words, the current through the armature has as many paths
in parallel as the machine has poles. With the same sized in-
ductors, the resistance through the armature of a four-pole
machine is only one-half of that of a bipolar machine, or, with the
same total current, the inductors of the four-pole machine carry
only one-half the current as those of the bipolar. This is of great
importance in generators handling large currents.

Minor advantages of the multipolar machines are the more
advantageous distribution of the flux and the less weight of iron
required in the field magnets.

575. Drum Windings. The distinguishing feature of the drum
winding has already been given (Par. 566). Since the coils are
arranged with the inductors at opposite ends of a chord of the
armature core (Fig. 280), if the induced E. M. F. in one of these
inductors acts from front to rear, that in the other must act from
rear to front. Hence, the principle governing all drum windings
is that the coils must be so wrapped that the two inductors are
never simultaneously under like poles. There are a number of
different windings which fulfill this condition but they all belong
to one or the other of two general classes, wave winding and lap
These will be explained below.



576. Plane Development of Drum Winding. There are two
conventional ways of representing diagrammatically a drum
winding. The first is to develop the armature by placing it





LAP WINDING



Fig. 288.



WAVE



on its side and rolling it along on a plane. Fig. 288 represents
in simplest form such a development of a lap winding and of a
wave winding (both incomplete), and indicates the appropriate-
ness of these names.



ELECTRO-MECHANICS.



459



N


i
i


s


i

i


N


I

i
i


5



Fig. 289 represents a lap winding for a four-pole generator, the
armature carrying sixteen inductors and eight commutator seg-
ments. The coils are composed of inductors 1 and 6, 3 and 8, 5 and
10, etc. It will be noted that in each the two inductors are under
different poles. Furthermore, if we begin at inductor No. 1 and
follow the winding through, it will be seen that we pass in succes-
sion through all of the inductors and finally return to the starting
point; in other words, just as in the ring winding, the inductors are
in series and the winding is a closed coil (Par. 559).



IN! POLES



INDUCTORS



! COMMUTATOR BARS
BRUSHES



With rotation from left to right, the direction of the induced
E. M. F. is as indicated by the arrowheads, and by inspection the
position of the positive brushes is readily located at segments
3 and 7 and that of the negative brushes at segments 1 and 5.

577. Star Development of Drum Winding. An objection to the
foregoing diagram is that the windings are not represented as clos-
ing upon themselves. To remedy this, use is made of what may
be termed a star development. If we should stand a barrel on end,
cut all of the hoops except the one at the top, open out the staves
from the bottom until the head rested upon the ground with the
staves radiating like the petals of a daisy, we should have a star
development of the barrel. Applying this to an armature, the
commutator corresponds to the head of the barrel and the inductors
to the barrel staves. The inductors and their connections are
thus shown in their proper relation to the commutator segments
and the windings close, the only distortion occurring in the cross
connections at the back end of the armature. Such a projection




460



ELEMENTS OF ELECTRICITY.



of a lap winding for a four-pole machine is given in Fig. 290. The
heavy radial lines represent the inductors. To enable the eye to
trace the back connections with least difficulty, these latter are
drawn in a regular geometric pattern with salient angles.

With clockwise rotation, the direction of the induced E. M. F.
is as indicated by the arrowheads. If the circuits be traced, it will
be seen that there should be four brushes and that they should be
located as indicated in the diagram. There are, therefore, four




Fig. 290.

paths through the armature. For this reason, the lap winding is
frequently spoken of as a parallel winding. It is best suited for
the production of large currents at low voltage.

A star projection of a wave winding for a four-pole machine is
shown in Fig. 291. In addition to the manner in which it is put
on, this winding differs from the lap winding in several other
respects, particularly in requiring but one pair of brushes. The
positions of the positive and the negative brushes are shown in the



ELECTRO-MECHANICS.



461



diagram. Should an additional negative brush be introduced at ic
and connected to 6, it would be of na appreciable electrical effect,
for examination of the diagram will show that c and b are already
connected through the coil cdeb in which, at the instant shown,
no E. M. F. is being induced. The inductors of a wave winding
are therefore in series and there are but two paths through the
armature, for which reasons wave-wound armatures are best
suited for the production of small currents of high voltage.




Fig. 291.

578. Calculation of E. M. F. of Generator. The E. M. F. of a

generator may be calculated as follows:

Let <=flux from each pole
n = number of poles
n' = number of revolutions per second
n" = number of paths through the armature
N= number of inductors -



462 ELEMENTS OF ELECTRICITY.

' The number of flux lines cut by each inductor in one revolution
is n.<.

The number of flux lines cut by each inductor per second is
n'.n.Q.

The E. M. F. generated by each inductor is n f . n . </10 8 .

But since there are N/n" inductors in series, the total E. M. F.
. N . n f . n . <f> ,,

* ".io volts -

579. Switchboards. A generator may be called upon to furnish
current for various uses, as, for example, for lighting, for charg-
ing a storage battery, for running a motor, etc., etc., and it may
be required to do these things one at a time or in various com-
binations. Wires must therefore be run from the generator to
the lamps, battery, machines, etc., and there must be switches
in the various circuits. The generator must be supplied with
a field rheostat (Par. 564) by which its E. M. F. may be adjusted,
and this implies that it must also be equipped with a voltmeter
by which this E. M. F. may be measured. If a storage battery
is to be charged, its E. M. F. must be known before the current
from the generator can be turned on (Par. 245). It is also often
desirable to know the current flowing in any one of the circuits,
and for this there must be ammeters. Overload switches should
be inserted in the principal circuits and an underload switch
must be in the charging circuit for the storage battery (Par.
415). Should an attempt be made to connect these various
switches and instruments to the generator direct, the machine
would be hidden in a hopeless maze of wiring. These auxiliary
pieces of apparatus are therefore gathered together, taken to
one side and mounted upon a switchboard. Wires from the
machine, not exceeding three in number, are brought over in a
conduit and the distribution of electrical energy takes place at
the board. This distribution is usually made from two heavy,
parallel copper bars, called bus bars, which are connected to the
source of the electrical energy and which may be regarded as its
enlarged terminals.

Originally of minor consideration, the switchboard has now
risen to a position of importance second only to that of the
machine itself and frequently rivalling it in cost. It is composed
of panels of some non-conducting material, preferably marble,
upon the front of which are mounted the switches and instru-



ELECTRO-MECHANICS.



463



ments; the bus bars, wiring and connections being at the back. In
addition to a symmetrical distribution of the apparatus, it is cus-
tomary to arrange parallel wires of a circuit on direct-current
switchboards so that if they be horizontal, the upper one is the
positive wire; if they be vertical, the right hand one is positive.



LAMPS




STORAGE BATTERS GENERATOR

Fig. 292.

In drawings of switchboards, several conventions are observed.
Wires are always drawn as right lines which are perpendicular
or parallel to the lower edge of the board (Fig. 292). This is to
aid the eye in tracing the circuits. If two wires cross but are not
connected electrically, this fact may be indicated by a little arch
in one of the wires, or they may be assumed not to make connec-
tion unless a dot be made upon the point of intersection.



464 ELEMENTS OF ELECTRICITY.

580. Example of Switchboard. A switchboard by which the
current from a shunt-wound generator may be used to run a
number of lamps and charge a storage battery, either separately
or simultaneously, is shown in Fig. 292. The circuits are easily
followed by the eye and the use of the various switches will be
understood from the following:

To charge the battery:

(a) Close b to the left and read the battery voltage.
(&) Start the generator. Close b to the right and read the
generator voltage. Manipulate the field rheostat until
the generator voltage is about ten per cent greater than
the battery voltage.

(c) Close a, c, and last the underload switch.
To run the lights at the same time:

Close also d.
To run the lights separately:

With the above arrangement open c.

(It will be noted that the lights are now run through the under-
load switch. This is not correct. An additional switch should
be used by which the generator may be thrown direct on the bus
bars. It is omitted in the diagram to avoid overcrowding the
figure.)

The right hand ammeter reads the current from the generator.
To run the lights from the battery alone:

With all switches open, close c and d.
The left hand ammeter now reads the current from the battery.

581. Coupling of Generators; Three- Wire System. In Par.
502 it was shown that the successful transmission of electrical
power to a distance depended upon the employment of high
voltage, the loss of power in the leads varying inversely as the
square of this voltage. Alternating currents are easily stepped
up for transmission and as easily stepped down at the point
where they are to be utilized. In the case of direct currents the
transformation is much more troublesome and expensive. For
such currents, however, there has been devised a system by
which the voltage may be doubled and thus the advantage of
high voltage transmission be partly secured. This will be under-
stood from the following explanation. It is desired to operate
at a distance a number of 110 volt incandescent lamps. If two



ELECTRO-MECHANICS. 465

generators, A and B, each capable of delivering 110 volts to the
lamps, be connected in series as shown in Fig. 293, the voltage
between the leads will be 220. If the lamps between C and D
be arranged two in series, each will receive its required 110 volts,





rat



Fig. 293.

while the currents in the leads will be only one-half of that re-
quired by the same number of lamps arranged singly in parallel.
The leads therefore may be reduced three-quarters in size. If now
a third wire NN, the neutral, be inserted as shown in the figure,
it will be possible to have a different number of lamps on the
two sides. If there be more lamps above the neutral than below,
the excess current flows in on the neutral; if there be less above,
the excess current flows out on the neutral, in other words, the
neutral needs only be sufficiently large to carry the difference in
the currents required on the two sides. In practice, however,
it is made of the same size as the other two leads. Notwithstand-
ing the extra wire, the saving in copper in this three-wire system
is five-eighths, or 62.5 per cent, of the amount required in a two-
wire system for transmitting equal power. Against this saving
must be put the cost of the extra generator (though certain special
generators have been devised to supply a three-wire system
from a single machine), and the extra cost of installation and of
switches and switchboard appliances, so that frequently the
saving is more apparent than real. In addition to this, more
attention is required in regulating the two generators since with
unequal loads on the two sides of the neutral, the E. M. F. of
the generators must differ.

The principle involved has been applied abroad to a five-wire
system.



466 ELEMENTS OF ELECTRICITY



CHAPTER 41.

GENERATOR CHARACTERISTICS.

582. Adaptation of Generator to Work Required. Of the vari-
ous proposed classifications of direct current generators, the
most important is the one based upon the excitation of the field
magnets (Par. 563), that is, into series, shunt and compound
machines.

Each one of these classes possesses certain advantages and
disadvantages which render it more suitable for some purposes
and less so for others.

As an illustration, suppose we have at our disposal a series
generator and a shunt generator and are required to charge a
storage battery: which of the two should we use?

To prevent the storage battery from discharging back through
the generator, the voltage of the latter must be kept constantly
higher than that of the battery. Suppose we were to start with
the series generator. Its E. M. F. can not build up until a
current flows through the field coils, and no current can flow
through these until the external circuit is completed. There-
fore, should we simply start the generator and then switch it
on to the storage battery, the battery would discharge back
through the generator. We must then first build up its field
by sending the current through some external circuit other than
that which includes the battery and then, when the E. M. F.
has reached the proper point, switch the current in on the
battery.

Suppose this to have been done and that the connections are
as shown diagrammatically in Fig. 294. As the battery becomes
charged, its voltage rises, consequently the current sent through
it by the generator grows smaller. The current through AB
being smaller, the field gets weaker: the voltage of the generator
consequently falls; this again causes the current to decrease;
the field gets still weaker, and so on. In other words, the generator
unbuilds and "drops its load," and, unless there be an under-
load switch in the circuit, the battery will soon discharge back.



ELECTRO-MECHANICS. 467

A series-wound generator is therefore not fitted to charge a stor-
age battery.



P



FIELD BATTERY



Fig. 294.

On the other hand, suppose that we employ the shunt generator
and that it is connected as shown in Fig. 295. The generator is
started and, the current flowing through the shunt field A B,
the E. M. F. builds up rapidly. When the voltage has reached
the proper point, the switch S is closed and the current is thrown
in on the battery. As the battery becomes charged, its voltage
rises and this counter E. M. F. cuts down the current from the
generator but the effect is very different from that in the case
of the series generator. As the current from the shunt generator
decreases, its voltage increases. The explanation of this is as
follows. The E. M. F. of the generator at any instant is spent




Fig. 295.

in doing two things, driving the current through the resistance
of the armature coils and brush contacts (or through the internal
resistance of the machine), and driving it through the resistance
of the external circuit, including the overcoming of any counter
E. JVE. F. in that circuit. This will be recognized as but another
example of lost and useful volts as discussed in Par. 305. The
smaller the current through the armature, the smaller the lost
volts, or the internal drop Ir, and the more nearly the voltage
between A and B approaches the E. M. F. of the generator.
We see then that the voltage of the shunt generator always re-
mains greater than that of the battery and that the charging can
be done with safety.



468



ELEMENTS OF ELECTRICITY.



583. Characteristics. The advantages and disadvantages of
the various forms of generators may be discussed in a similar
manner to the foregoing. Where constancy of current is to be
maintained, a series generator is under certain conditions satis-
factory; where constancy of voltage is desired, a shunt or a
compound generator must be employed. However, we might
sometimes overlook some point in our discussion or might give
undue weight to some other, therefore, the most sure method
is actually to try the machine under varied conditions, keep a
record of the results, tabulate and compare these. If they can
be put graphically in the form of a curve, they give a clearer
conception of the working of the machine. Such curves are called
"characteristics" and much information can be derived from their
study.

584. Magnetization Characteristics. As an illustration of these
characteristics, suppose that we have a generator whose field is



AMPERES



Fig. 296.



excited from a separate source, such as a storage battery. We
rotate the generator at a constant speed, we excite the field by
various currents and we record the strength of the exciting cur-
rent and the corresponding voltage across the brushes of the
generator. Plotting this data with amperes as abscissae and the
corresponding volts as ordinates, we obtain a curve (Fig. 296)
which is called the "magnetization characteristic"

A study of this reveals (a) that with no current in the field
coils there is still a small voltage, OA, due to the residual magnet-
ism of the magnet cores (Par. 562), and (b) that as the amperes
in the field coils increase regularly, the voltage at first rises rapidly



ELECTRO-MECHANICS.



469'



and then more slowly. Reflection will show that this curve is
nothing more than the magnetization curve described and figured
in Par. 393.

585. Characteristic of Series Generator. Fig. 297 represents
diagrammatically a series generator run at constant speed and

FIELD




AMMETER
Fig. 297.

connected in circuit with a number of lamps in parallel and an
ammeter. A voltmeter is connected across the brushes. By
turning on lamps the resistance of the circuit is reduced and the
current thereby increased. This current is measured by the am-
meter and the corresponding terminal voltage is given by the
voltmeter. If the amperes be laid off as abscissae and the cor-
responding volts as ordinates, the resulting curve, ABMN, Fig.
298, is the external characteristic, so called because, as was pointed



D AMPERES
Fig. 298.

out above (Pars. 461 and 582), the voltage read by the voltmeter
is not the total E. M. F. of the machine but only the IR drop
over the external circuit, in other words, the useful volts. Should
we wish to represent the total E. M. F., the internal drop, or
lost volts 7r, must be added to the external drop.



470 ELEMENTS OF ELECTRICITY.

Since r, the internal resistance of the machine, is constant,
the internal drop varies directly as the current and is represented
in Fig. 298 by the straight line OF. If the ordinates of OF be
added to the corresponding ordinates of the curve ABMN, the
resulting curve OH is the total E. M. F. curve or the internal
characteristic. Were it not for the effects of armature reaction,
this curve would agree with the magnetization curve described
in the preceding paragraph.

Examination of the external characteristic shows that the
machine should be operated with currents corresponding to the
flatter portion of the curve, for if the current falls below KO,
slight changes in the current produce great fluctuations in the
voltage and the operation of the machine is unstable.

' .586. Critical Resistance. From the figure, MD/DO is the
tangent of the angle MOD, and since MD represents E. M. F.
and OD represents current, E/I = tan 0. But from Ohm's law
E/I = R, hence, at any point upon the external characteristic
the corresponding external resistance is proportional to the tan-
gent of the angle which the ordinate at that point subtends.

As the external resistance is increased, the angle 6 increases
and the point M moves towards B. Finally, a very slight in-
crease in 6 will cause M to drop to the origin. There is therefore
for a series generator an external resistance, the critical resistance,
beyond which the generator will not operate. Reflection will
show the correctness of this conclusion since the resistance must
always be small enough to permit a sufficient current to flow
through the field coils and produce the necessary strength of field.

587. Characteristic of Shunt Generator. If a shunt gener-
ator be connected up as shown in Fig. 299 and data be obtained




AMMETER N



Fig. 299.

and characteristic plotted as described in Par. 585, the resulting
curve (Fig. 300) will be seen to differ widely from the one obtained
from the series machine. To begin with, the voltage is a maximum



ELECTRO-MECHANICS.



471



when there is no current in the external circuit. As the current
is increased, the voltage falls quite regularly until a final point is
reached when a further decrease in the external resistance causes
both the current and the voltage to drop and if all resistance be
removed, the machine unbuilds entirely and the curve returns
upon the origin.



Fig. 300.

The foregoing results are brought about by two causes. First,
the current through the shunt grows smaller and the field con-
sequently weaker. Whatever decreases the difference of potential
between A and B (Fig. 299) decreases the current through the
field coils. With no current in the external circuit, the full E. M.
F. of the machine is available for driving current through the
shunt. When, however, a current flows through the armature,
the available E. M. F. is the total E. M. F. diminished by the
internal drop, 7r, which last varies directly with the current.
At first, as the field current weakens, the voltage is not greatly
affected since the field magnets are being worked on the upper
part of the magnetization curve. When, however, the magnet-
ization falls below the bend of the curve, it drops rapidly as the
exciting current decreases.



Online LibraryWirt RobinsonThe elements of electricity → online text (page 36 of 46)