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ELECTRO-MECHANICS. 515



CHAPTER 44.

ALTERNATING CURRENT GENERATORS.

636. Alternators. The fundamental principles of alternating
current generators have already been brought out in the chapter
treating of direct current generators. It was there shown that
the currents generated in the revolving armatures described were
all alternating and to rectify them an especial contrivance, the
commutator, was required. It would therefore seem that should
the commutator be discarded and collector rings (Par. 553) be
substituted in its place, we would obtain an alternating current
generator, or, as it is more briefly named, an alternator. However,
we also saw that in the D. C. generators the fields were self-
excited, the direct current for this purpose being drawn from the
commutator. The discarding of the commutator therefore in-
volves a change in the methods of exciting the field coils, and for
this and for other reasons it is necessary to consider these machines
a little more in detail.

637. Field Excitation of Alternators. In most alternators the
field coils are excited by a current drawn from a separate source,
such as from a battery or from a small D. C. generator. This
auxiliary generator, the exciter, may be operated in a number of
ways, (a) It may be entirely independent, (b) It may be driven
by a belt from a pulley on the shaft of the alternator, (c) It may
be mounted upon an extension of this shaft, (d) It may form an
integral part of the armature of the alternator itself. In this case
the armature must be provided with both collector rings and
commutator, the field current being drawn from the latter.

638. Compound Alternators. As the current through an alter-
nator increases, the internal drop also increases and consequently
the ^voltage across the brushes diminishes. We have seen (Par.
511) how important it is in the case of electric lighting (for which
alternating currents are largely used) that the voltage delivered
to the lamps should be constant. To secure this constancy of
potential, the voltage across the brushes must not only not fall



516 ELEMENTS OF ELECTRICITY.

with increase of current but must actually rise. In direct current
generators this is secured by compounding (Par. 588). This
remedy is not directly applicable to alternators but there are
several ways in which an approximation to it may be obtained.
One of these is shown diagrammatically in Fig. 336 which repre-
sents the armature of an eight-pole alternator. The current
leaving the armature windings at the coil A, before reaching the
corresponding collector ring passes through the primary of a step
down transformer B. The core of this transformer is attached
to the armature spider or forms a part of it and therefore rotates




SERIES FIELD COIL5
Fig. 336.

with the armature. The current from the secondary is taken to
a commutator CD which is mounted upon the armature shaft
close to the collector rings but which, for the sake of clearness, is
represented in the diagram as moved off to the right and turned
sidewise to the observer. This commutator has only as many
segments as the alternator has poles, and the alternate segments
are connected together as shown. Brushes pressing against it
deliver a rectified current to the series field coils. An increase
in the current in the external circuit, and hence in the primary of
the transformer, causes an increase in the current through the
secondary, and hence through the field coils, which in turn causes
the desired rise in voltage.

It is sometimes possible to dispense with the transformer and
to take the current direct from the coil A to the commutator.
As a rule, however, alternators generate a high voltage current
which, besides being dangerous, is apt to cause excessive sparking



ELECTRO-MECHANICS. 517

at the commutator. For these reasons the transformer is to be
preferred.

639. Alternators Usually Multipolar. It is in general necessary
that an alternator should be multipolar. This will be seen from
the following. A small alternator may be driven at 1800 revolu-
tions per minute. This speed may be exceeded by some of the
turbine driven machines but is near the limit for the average
small generator and much above the limit for large machines. At
this rate, a point on the circumference of a twelve inch armature
is travelling faster than a mile per minute. But at 1800 revolu-
tions per minute the frequency of the current from a bipolar
machine is only 30. This, we have seen (Par. 621), is too low for
the operation of an incandescent lamp. Moreover, frequencies
as high as 120 are often required. Since the speed of the alter-
nator can not be increased, such frequencies can be obtained
only by increasing the number of poles. In some of the larger
modern alternators, the number of poles has approached one
hundred.

640. Classes of Alternators. As shown above, the classifi-
cation of D. C. generators according to the method of field excita-
tion into series, shunt and compound machines is not applicable
to alternators. They may, however, be divided into two general
classes; (a) those with stationary field 'and revolving armature,
and (b) those with stationary armature and revolving field. Of
this second class there is a subdivision, the inductor alternator,
in which, although the field revolves, the exciting current passes
through a single coil which is stationary (Par. 643). From an
electrical standpoint, there is no call for these divisions, the
principle being the same in all, but each possesses certain minor
advantages and it is therefore desirable to consider them separately
though briefly.

It will shortly be shown that alternators may be designed to
deliver a single current or two or more separate and distinct
currents which differ in phase and accordingly they are also
classed as single phase or as polyphase.

641. Alternators with Revolving Armatures. The simplest
form of alternator with revolving armature is figured and ex-
plained in Par. 553. The majority are multipolar. A diagram-



518



ELEMENTS OF ELECTRICITY.



matic end view of such a machine is given in Fig. 337, and in Fig.
338 a similar four-pole machine is shown as rectified, that is, the
field, the armature and the collector rings are represented as
having been straightened out. With clockwise rotation, the coils




O



FIELD

CURRENT



Fig. 337.

A, B, C, D move from left to right as indicated by the large
arrow. Application of the rule given in Par. 421 shows that at
the instant represented a clockwise E. M. F. is induced in A and
in C and a counter-clockwise E. M. F. in B and in D, but since




000



Fig. 338.

these coils alternate in the direction of their winding, they add
their respective E. M. F.s. The current passes into the external
circuit from the collector ring EE through the brush G and returns
through the brush H which is in contact with the ring FF. The
direction of this current is reversed as A passes beneath the center
of S.



ELECTRO-MECHANICS.



519



642. Alternators with Revolving Field. In alternators with
revolving field, the field may retain its relative position exterior
to the armature, but far more frequently they interchange places
and the revolving field is internal. Roughly speaking, the field
core resembles the hub of a wheel whose spokes have all been
sawed off to a length of two or three inches. The field coils are
wrapped about these spokes, alternating in direction so as to
obtain the desired polarity. The exciting current is brought in
and taken out by means of a pair of slip rings (identical in opera-
tion with collector rings). Fig. 337 would represent such a ma-
chine if the field circuit and the external circuit were interchanged,




Fig. 339.

that is, if the exciting direct current were brought in through the
collector rings and if the present field circuit were used as the
external circuit. The armature, Fig. 339, is built up of laminated
punchings, spaces being left for ventilation. The coils are placed
in slots and held in position by wedges.

The great advantage of this form of alternator is that the cur-
rent, which we have seen is usually of high voltage, is taken off
through fixed connections, which may be insulated to any desired
degree, and only the relatively small exciting current passes
through the sliding contacts on the slip rings.



520



ELEMENTS OF ELECTRICITY.



643. The Inductor Alternator. The inductor alternator, shown
diagrammatically in section in Fig. 340, possesses the advantage
of having no sliding contacts and therefore requires
no collecting rings or brushes and is free from the
sparking which occurs in other machines. It con-
sists of an inductor, a rotating toothed soft-iron disc
around whose edge there is a deep groove. In this
groove lies the annular field coil C which is fastened
to the frame work and therefore does not rotate
with the inductor. When a current flows through
C, the inductor becomes magnetized, its faces
being of opposite polarity and hence the teeth on
one side being all of like polarity. The frame
work which surrounds this revolving inductor
has inward projections corresponding to the mov-
ing poles, and upon these projections the armature





Fig. 340.



coils are wrapped. Since the poles on each side do not alternate
in polarity, there is no reversal of flux through the armature coils
but this flux rises and falls and thus produces an alternating cur-
rent in the coils.

644. Polyphase Alternators. Suppose that the ends of the two
coils in Fig. 269, instead of terminating in the commutator seg-
ments as shown, should each be connected to a separate collector
ring as shown in Fig. 341. There being no electrical connection
between these coils, a pair of brushes C could be applied to the
rings of the coil B and lead current from this coil into an external
circuit. A second pair of brushes D could be applied to the rings
of A and lead current from A into an entirely separate external
circuit. As the armature rotates, an equal E. M. F. is generated
in each coil but the currents in the respective circuits vary with
the resistances of these circuits and are entirely independent of
each other, in fact, the machine is electrically equivalent to two
separate and distinct machines, the only connection between the
two being that they generate equal E. M. F.s of equal periodicity.
If the E. M. F. curves of the two coils be plotted on a common
axis of time, it will be seen that their maxima occur at a constant
phase difference of 90. Since the machine thus generates two
distinct currents of different phases, it is called a two-phase or a
di-phase alternator.

Theoretically, other coils could be inserted midway between



ELECTRO-MECHANICS.



521



those of Fig. 341 and still others between these, each with its own
collector rings and each supplying a separate external circuit with
current differing in phase from the currents from the other coils.
Practically, the distinct windings of such alternators rarely exceed




Fig. 341.

three. Those which generate more than one current are designated
as polyphase; those which generate but one are, in contra-distinc-
tion, called single phase.

It can be shown that to generate these polyphase currents it is
not necessary that the windings for each phase should be entirely
separate. For example, as shown in Fig. 342, by tapping a ring-




Fig. 342.

wound armature at four points 90 apart and by connecting each
of the tapping wires to a collector ring, we obtain a two-phase
alternator. At the instant shown in the diagram the leads A are
carrying the entire current, the current in the leads B being zero
since the points to which their tapping wires are connected are



522



ELEMENTS OF ELECTRICITY.



momentarily at the same potential. When, however, the armature
has turned through an angle of 90, these conditions are reversed
and the leads B will carry the entire current, while the current
in A will be zero.

While polyphase currents are used to a limited extent in a three-
wire lighting system, their principal use, as will be explained in
the following chapter, is for the operation of alternating current
motors.

645. Tri- Phase Alternators. In its most general form, the
armature of a tri-phase alternator carries three distinct windings
spaced 120 apart and supplied with six collector rings by which
currents can be distributed to three separate circuits. The E. M.
F. generated by such an alternator is shown in Fig. 343, the sine
waves being of equal amplitude but differing in phase by 120.

If the resistances of the three circuits are equal, then the cur-
rents are also equal and the circuits are said to be balanced. In
such a case the curves in Fig. 343 may be taken as representing




Fig. 343.

the currents also. Examination of this figure will show that at
any point along the horizontal axis, the sum of the ordinates is
zero. For example, at A and C where the current in one of the
circuits is zero, the currents in the other two circuits are both
equal and opposite, and at B and D where the current in one of
the circuits is a maximum, the sum of the currents in the other
two circuits is equal and opposite. It is therefore possible when
the circuits are balanced to discard three collector rings and three
lead wires, for whether the current goes out on one or on two wires,
an equal current comes in on the remaining wires or wire. The
arrangement of such a three-wire three-phase system is shown in
Fig. 344.



ELECTRO-MECHANICS.



523



Should the circuits not be balanced, it is still possible to reduce
the number of leads and collector rings from six to four, the fourth
wire serving as a common return for the excess current of the
other three.

646. Tri-Phase Delta Connection. In Fig. 344, a represents
diagrammatically a ring wound armature tapped at three points
120 apart, each tapping wire terminating in a collector ring.
These rings, A, B, C, for the sake of clearness are represented as




Fig. 344.

separated from their common axis. The same armature is repre-
sented in 6 in a still more highly conventionalized form, the curved
portions between the tapping wires being straightened out and
the rings being drawn at the vertices of the resulting triangle.
This diagram also shows the three leads running from these rings
and the arrangement of lamps so as to produce a balanced system.
On account of the shape of the diagram this is called a
^-connection, sometimes also a mesh-grouping. At one instant
the entire current flows out on A and returns through the lamps
D and E; at another instant it flows out on C and returns
through D and F; at still another it flows out on B and returns
through E and F; at all others, a varying current flows through
each lamp.

At the instant represented in a, Fig. 344, the armature coils
between B and C are sending current out by C, and the coils
between A and C (except the few to the left of the neutral plane)
are contributing to this current. The currents in these two por-
tions of the armature windings do not reach their maxima simul-
taneously but the total resultant current is a maximum when
these component currents are equal which is the case at A, the
60 phase in Fig. 343 The maximum current in the leads is
therefore 2 sin 60 = V3 times the maximum current in one portion



524



ELEMENTS OF ELECTRICITY.



of the armature windings. The maximum E. M. F. between any
two of the leads is, however, no greater than that in one portion
of the armature windings.

647. Tri-Phase Y-Connection. Suppose that in addition to
tapping the ring-wound armature in three points, as described
in the preceding paragraph, we cut the winding at these points
and connect the corresponding ends together as shown in Fig.
345 a. The current entering at B (at the instant represented in




Fig. 345.

the diagram) flows to the common junction at the center where it
divides, a portion going to A, the remainder to C. This arrange-
ment, shown still more diagrammatically in b, is called a Y-con-
nection, sometimes also a star grouping.

The E. M. F. of the coils between B and a is now in series with
that of those between a and A and of those between a and C,
excepting in both cases the few turns to the left of the neutral
plane. The maximum E. M. F. between the leads of b is therefore
the sum of the E. M. F.s in two of the three portions of the arma-
ture windings at the moment when the E. M. F. in the third
portion is zero. This is represented by the double ordinate at A
in Fig. 343. But A being at the 60 phase, this double ordinate
is 2 sin 60 = \/3, or the maximum E. M. F. between any two of
the leads is V3 times the maximum E. M. F. developed in a
single portion of the armature windings.

On the other hand, since at any one instant never more than
two of the portions of the armature windings can combine in
delivering current, and since these two portions are always in
series, the maximum current in the leads is the same as the maxi-
mum current in any one of these portions.

It will be noted that in the A-connection the current is V3
times the maximum of that in the armature coils, while in the



ELECTRO-MECHANICS. 525

Y-connection the voltage is V3 times the maximum of that in
these coils. The power, IE, developed by the two arrangements
is therefore the same.

648. Transformation of Direct and of Alternating Currents.

We have seen that the secret of the electrical transmission of
power is the employment of currents of high potential (Par. 502) .
On account of freedom from trouble caused by sparking at the
commutator, it is true as a general statement that an alternating
current can be turned out at a higher voltage than can a direct
current. Whether the current produced by a generator be direct
or alternating, it is often desirable to raise its voltage still higher
before sending it out on the line, and whether this be done or not,
it is almost always necessary at the distant end of the line to
reduce the voltage to fit the standard machines or lamps with
which it is to be used. In this transmission, therefore, a current
must be stepped up at the sending station and stepped down at
the receiving station.

In the case of direct currents, this transformation is effected by
motor generators (Par. 605). These machines are costly, their
operation involves a considerable loss of power and they require
as much attention as the generator itself. If power is to be dis-
tributed among scattered buildings, a motor generator and an
engineer would be required in each, also space for installation of
the machine. On the other hand, these changes in alternating
currents are made by transformers which are relatively inexpen-
sive, require little or no attention and have an efficiency in some
cases exceeding 98 per cent. They may be placed wherever
needed and occupy but little room since they are usually mounted
against a wall or upon a pole like a letter box. For these reasons,
for the transmission of power to a distance, the alternating current
has a great advantage over the direct.

649. Transformers. The principle of transformers was out-
lined in Par. 431 but they are considered here again in order that
some additional facts about their use may be brought out. That
they rightfully fall under the heading of the present chapter, the
following will show. An alternator is a machine which induces
an alternating E. M. F. by rapidly varying the magnetic flux
through a coil. From this point of view, a transformer is also an
alternator, the E. M. F. in the secondary being induced by the



526



ELEMENTS OF ELECTRICITY.



changing flux produced in it by the primary. Moreover, since the
transformer has no moving parts (and is hence sometimes called
static), there is no loss of energy in overcoming friction, etc., and
by proper design the combined losses due to magnetic leakage,
eddy currents, hysteresis and resistance may be reduced to less
than two per cent, so that we may say that the transformer is the
most efficient of machines.

Transformers are of two types, the core or ring transformer
(Fig. 204) and the shell transformer (Fig. 205). The shell trans-
former is the more frequently used but, for the sake of clearness,
the following diagrams represent ring transformers.

In the actual construction of the shell transformer, the coils are
usually wound in separate portions which are thoroughly insulated
and then sandwiched together, after which they are placed in a
form and the laminated iron punchings of which the shell is com-
posed are built up around them. The completed coils are then
put in an iron case which is usually filled with oil. This serves a
double purpose; it aids the insulation of the coils, prevents the
penetration of moisture into the wrappings and prevents excessive
heating of the coils. In some of the larger transformers, the oil
itself is cooled by water circulating in pipes which pass through
the oil. In others, the oil is omitted and cooling is brought about
by currents of air driven over the coils.

If a current be sent through the primary of a transformer, it
will produce in the core a certain number of lines of force. These
lines, as shown in Fig. 346, penetrate every turn of the coils in















1




\




,


r-


c






p <




<


-4-


I S




A


c


r+H

li_








1 <


i






"*-


j <


t






*,_


-< -<


1
_'





Fig. 346.

both primary and secondary. An equal E. M. F. is therefore
induced in every turn. If this E. M. F. be e, and if there be Af'
turns in the primary and N" in the secondary, the E. M. F. in the
primary is E'= N'e, that in the secondary is E" = N"e, whence

E' : E"=N' : N"



ELECTRO-MECHANICS.



527



or, as already shown (Par. 431), the E. M. F.s in the two coils are
to each other as the number of turns in the respective coils.

650. Operation of Transformer. In Par. 431 it was shown
that the work done in the primary of a properly designed trans-
former is equal to that done in the secondary. It follows from
this principle that the current in the primary varies with the




Fig. 347.

current in the secondary and that when the secondary circuit is
open there should be no current in the primary. This can be
shown experimentally by the arrangement shown in Fig. 347.
With the switches in the secondary circuit open, the ammeter in
the primary circuit indicates the merest trace of a current. Reflec-
tion will show that the primary, a coil of small resistance wrapped
about a soft iron core, is nothing more nor less than a choke coil
as described in Par. 621, and that the current is cut down by the
choking effect. The small current which does get through, the
"no load current," is just sufficient to maintain the magnetic flux.

If now one of the switches in the secondary be closed, the
ammeter will indicate a current through the primary. If a second
switch be closed, the current through the primary is doubled ; if a
third switch be closed, it is trebled, in other words, the current
through the primary adjusts itself to conform to the current in
the secondary, or, the primary acts as an automatic valve and
permits only so much current to flow through it as is needed to
supply the demands of the secondary.

This very remarkable property may be explained as follows.
When an E. M. F. is impressed upon the primary, the secondary
circuit being open (Fig. 346), a current flows and produces within
the primary a magnetic flux. The lines of force, as shown by the
arrowheads, travel around the magnetic circuit and enter the
primary from below. This sets up an induced E. M. F. in the
primary opposite to the actual E. M. F. (Par. 421) and conse-



528 ELEMENTS OF ELECTRICITY.

quently cuts down the current in the primary. An E. M. F. is also
set up in the secondary but produces no current since this circuit is
open. When, however, the secondary circuit is closed, a current
flows as indicated by the arrowhead. This current produces lines
of force opposite in direction to those from the primary, that is, it
diminishes the number of lines from the primary (Par. 418, 6).
This in turn diminishes the choking effect and allows a larger
current to flow through the primary.

From the facts brought out above it will be seen that in any
given transformer the voltage in the secondary varies directly
with the voltage in the primary; on the other hand, the current
in the primary varies directly with the current in the secondary;
in other words, the primary determines the voltage; the secondary



Online LibraryWirt RobinsonThe elements of electricity → online text (page 40 of 46)