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determines the current.

651. Connection of Transformers. On account of the choking
effect described in the preceding paragraph, transformers are not
connected in series but in parallel. Fig. 348 represents an alter-





Fig. 348.

nator delivering high potential current to two mains and through
transformers connected in parallel distributing energy from these
mains to the stations A and B.

652. Auto-Transformers. The transformers described in the
preceding paragraphs are used when the voltage in the primary
is to be very materially changed, as for example when it is to be
increased or diminished tenfold, or, as a minimum, when it is to
be doubled or halved. Smaller changes in voltage may be made
by means of resistance, but this we have shown to be wasteful.
A better method is to use the so-called auto-transformer, shown
diagrammatically in Fig. 349. This is a transformer in which the
primary and the secondary coils are combined in one. In prin-
ciple it does not differ from the ordinary transformer. As explained
in Par. 649, when a current is sent through the primary coil, an
equal E. M. F. is developed in every turn. The E. M. F. in the



ELECTRO-MECHANICS.



529



secondary therefore varies directly with the number of turns
tapped by it. In the diagram, the secondary is used to step down
the voltage in the primary. If the current were delivered to the




Fig. 349.

secondary and drawn from the primary, the voltage would be
stepped up.

653. Rectification of Alternating Current. An alternating
current may be rectified in several ways. It has already been
shown (Par. 556) how it may be rectified at the point of origin by
means of a commutator. It is, however, frequently desirable to
transmit the current to a distance as alternating and to rectify
it at the receiving station. In this case it may be rectified by (a)
mechanical means, or by (b) electro-chemical means.

An alternating current is rectified mechanically by means of a
synchronous converter, also called a rotary converter. Briefly ex-
plained, this is a generator with both commutator and collector
rings. The alternating current is delivered to the collector rings
and the machine operates as a motor. While so operating, direct
current is drawn from the commutator.

Alternating current may also be rectified mechanically by a
motor-generator (Par. 605), the motor being driven by the alter-
nating current and direct current being drawn from the com-
mutator of the generator at the opposite end of the shaft.

The electro-chemical rectifiers are of several kinds. In one, the
current is passed through a cell containing electrodes of aluminum
and of lead or steel, the aluminum having the property of per-
mitting the current to pass when it is the cathode but suppressing
it when it is the anode. Allied to this is the mercury arc rectifier
which will now be described.

654. The Mercury Arc Rectifier. In the description of the
mercury vapor lamp (Par. 527), it was shown that the resistance



530 ELEMENTS OF ELECTRICITY.

to the passage of the current was confined mainly to the surface
of the negative electrode and was so great that several thousand
volts were required to break it down, but that once that it had
been broken down, a current could be maintained by a small
voltage provided that this current did not fall below a certain
minimum. If it fell below this, the negative electrode resistance
was re-established and the current was interrupted.

This principle is utilized in the mercury arc rectifier, an appara-
tus for the conversion of alternating currents into the relatively
small direct currents such as are employed in charging the smaller
storage batteries. It may be used with either single phase or
polyphase currents. Its operation will be understood from the
following. Fig. 350 represents diagrammatically one of these




Fig. 350.

converters. It consists of a pear-shaped exhausted glass globe
of about nine inches in diameter. Through its top extend the
terminals A and B which connect on the interior with the iron
electrodes C and D. A third terminal enters below and connects
with the mercury electrode E. Suppose that desiring to charge
the storage battery F by means of current from an alternator M,
we should make connections as shown on the right of the diagram.
No current can flow in either direction until the negative electrode
resistance at either D or E be broken down. Suppose that as
explained below this resistance be broken down at E. Current
will now flow through the circuit in the direction BDEF but will
continue to flow for only a small fraction of a second. As soon
as the voltage between D and E drops to about ten volts, the



ELECTRO-MECHANICS.



531



resistance at E is re-established and the current is interrupted.
When the E. M. F. reverses, no current can flow, for the resistance
at D has not yet been broken down. We see then that this ar-
rangement could not be used. Now suppose a direct current
generator G to be connected as shown on the left of the diagram.
When the resistance at E has once been broken down, direct
current from G will flow steadily in the direction ACEF. If now
the alternator be turned on, the alternating E. M. F. in the
direction BDE can send a current through the circuit because the
direct current from G, by preventing the resistance at E from
reasserting itself, keeps open the road through E, but the alternate
impulses in the reverse direction can send no current since the
resistance at D prevents. It is thus seen that by such an arrange-
ment the alternating current from one-half of each cycle could be
used to charge the battery.

The illustration above is purely hypothetical but is intended
to bring out the fact that if in any manner the resistance at the
negative electrode can be kept broken down, than the apparatus
becomes selective in its operation and permits current to pass in
one direction but not in the other, in other words, it becomes a
rectifier.

655. Rectification of Single Phase Current. The arrangement
of the converter to rectify a single phase current is shown in Fig.
351. The leads from the alternator M terminate in the electrodes




Fig. 351.

C and D, but at A and B branches are thrown off which include
the inductance coils G and H and unite at J. To one side of the
electrode E there is an auxiliary mercury electrode F which is



532 ELEMENTS OF ELECTRICITY.

connected through a resistance R with the wire from A to G.
The globe is mounted so that it may readily be tilted.

To charge a storage battery, the battery is connected between
E and J as shown. The globe is then tilted until the mercury in
E connects with that in F. At this instant the current passes
through the path MARFEJHBM. The globe is now released
and as the thread of mercury between E and F is broken, an arc
is produced, some of the mercury is ionized and the vapor in the
globe is thereby rendered a conductor. The path of the current
is now MACEJHBM, but the E. M. F. acting in this direction
soon dies down and then reverses, that is, acts in the direction
MBD. The inductance of the coil H now comes into play and
prolongs the current through H, a momentary current flowing
around the circuit JHBDEJ. Before this delayed current has
died down to the point where the resistance of E is re-established,
it is picked up by the growing E. M. F. in the direction MBD, the
circuit now being MBDEJGAM. At the next reversal, the in-
ductance of the coil G comes into play, and so on, these induced
delayed currents fulfilling the part of the direct current described
in the preceding paragraph and keeping the path through E open.

656. Comparison of Alternating and Direct Currents. Alter-
nating current generators, since they require no commutator, are
somewhat cheaper to construct than those for direct current, but
this may be counterbalanced by the cost of the separate field
exciter. The great advantage of alternating currents is the ease
with which they may be transformed and the simplicity and the
efficiency of the static transformers used for this purpose. On
the other hand, they can not be used in electrolytic work nor in
charging storage batteries and alternating current motors fall
behind direct current motors both in efficiency and in speed
regulation. While most incandescent lamps operate equally well
with either kind of current, the arc lamp mechanism for alternating
currents is not so satisfactory as that for direct currents. As a
general statement therefore, alternating current is most suitable
where power is to be transmitted to a distance; in all other cases
direct current is to be preferred.



ELECTRO-MECHANICS. 533



CHAPTER 45.

ALTERNATING CURRENT MOTORS.

657. Alternating Current Motors. The electrical conditions
encountered in motors designed for use with alternating currents
are particularly complex. The interaction of the flux of the field
coils and that of the armature coils, one or both of which may be
shifting, the inductance, hysteresis and eddy currents necessarily
developed in a machine in which alternating currents flow through
coils embracing soft iron cores, render the mathematical treatment
of the problem more intricate than is desirable in an elementary
text book. In the following pages therefore, we can do no more
than glance at the fundamental principles of a few of the simpler
forms.

658. Classes of Alternating Current Motors. Alternating cur-
rent motors are usually classed under the following heads:

(a) Series motors.

(b) Synchronous motors.

(c) Repulsion motors.

(d) Induction motors.

The distinction between these will be brought out as we proceed.

659. Series Motors. In Par. 604 it was shown that changing
the direction of the current supplied to a shunt motor did not
alter the direction of rotation. The same could have been shown
for the series motor. At first sight, therefore, it would seem that
whether supplied with direct or with alternating current, these
motors would operate equally well. In the case of the shunt
motor however, the inductance of the field coils is much greater
than that of the armature coils. The current through the field
coils therefore lags much more than that through the armature
(Par. 617). The torque is a maximum when the armature current
and the field flux reach their maxima simultaneously, but since
the field and the armature currents differ in phase, but little power
is developed. The shunt motor, therefore, is not used with alter-
nating currents.



534



ELEMENTS OF ELECTRICITY.



In the series motor, the field and armature coils being in series,
there can be no phase difference and the above objections do not
apply. When used with single phase alternating currents, series
motors develop great starting torque and possess the advantages
and disadvantages described in Pars. 602 and 603. They are
therefore largely used as railway motors. The A. C. motors
differ from the D. C. motors in certain minor arrangements by
which the tendency of the A. C. machine to excessive sparking is
reduced. Also, as in all other A. C. machines, the field cores must
be laminated.

660. Synchronous Motors. Suppose Fig. 352 to represent a
rectified portion of the alternator shown in Fig. 337. AB repre-
sents the field which is excited by direct current and whose polarity





Fig. 352.

therefore does not vary. CD represents a portion of the revolving
armature, the coils supplied with alternating current from a
distant source. At the instant -shown in the diagram, it will be
seen that each pole of the armature experiences a force which
tends to move CD from left to right. If CD does not move, it
will at the next reversal of the current be urged in the opposite
direction, or from right to left. Suppose it begins to move from
left to right. If before the moving coil E arrives beneath F, the
current through E reverses, the polarity of E also reverses and
E will be driven back from F, in other words, the movement of
CD will be checked. If E passes under F without reversing, it
will be pulled back as soon as it begins to emerge on the other
side. If it reverses as it passes under F, it will be pushed ahead.
The frequency of the alternating current supplied to the arma-
ture being constant, the relative positions of the fixed field poles
and the rotating armature coils at the instant when the current



ELECTRO-MECHANICS.



535



in these latter reverses depends upon the angular speed of the
armature. The effect of variation in this speed can be shown
graphically as follows. In Fig. 353 AB represents the fixed field,
and a, b and c represent the successive positions of an armature
coil moving at three different speeds. If the armature be rotated






Fig. 353.

slowly, the angular distance between reversals is small; if it rotates
rapidly, this angular distance is large. In a, the armature is
turning slowly and the polarity of the coil reverses when the coil
has travelled through less angular distance than that separating
the field poles. In 6, it is turning rapidly and the reversals occur
at angular distances apart greater than that between the field
poles. In c, the reversals occur at the same angular distance
apart as that separating the poles. In this last case, the armature
coil passes over the distance between two successive north poles
of the field in the same time that a coil of the distant alternator
supplying current to the armature passes over the distance be-
tween two successive north poles of its field, in other words, the
armatures of the motor and of the alternator rotate in electrical
synchronism.

For the sake of clearness only forces of attraction are represented
in these diagrams. It is seen at a glance that only in the case of



536 ELEMENTS OF ELECTRICITY.

the synchronous rotation is the torque the same in direction for
the successive positions of the rotating coil. If, therefore, an
alternator be brought up to synchronous speed and then supplied
with alternating current, it will continue to rotate. Such machines
are called synchronous motors. They differ in a few minor details
from alternators. Either the field or the armature may revolve
and they may be driven by either single phase or polyphase
currents.

661. Operation of Synchronous Motors. A serious objection
to the single phase synchronous motor is that it can not of itself
start from rest. An auxiliary motor is required to bring it up to
synchronous speed before the current is turned on. The polyphase
machines will start up of themselves, but even with these it is
usual to employ an auxiliary starter.

Since these motors must maintain synchronous speed, it follows
that their speed does not vary with variations in the load. The
question then arises how is the supply of power varied to meet
the different demands made upon it. The force on an inductor
of the armature being I. H. I (Par. 591) varies directly as the
current. .The current varies as the difference between the im-
pressed E. M. F. and the back E. M. F. (Par. 593). The impressed
E. M. F. is delivered by the alternator and is constant. The back
E. M. F. varies with the speed of rotation of the armature, hence
also is constant. If these two E. M. F.s reached their maxima
and minima simultaneously, in other words, if they were in phase,
the difference between them, and hence the current, would be a
minimum. If, however, the armature coils should fall back a few
degrees in angular position, still preserving synchronous rotation,
the two E. M. F.s would no longer be in phase, their difference
would increase and a greater current would flow. When, therefore,
a load is thrown on a synchronous motor, the armature drops
back a few degrees and thus exerts a greater torque. If the load
be excessive, the machine is thrown out of synchronism and stops.

662. The Repulsion Motor. The repulsion motor, shown
diagrammatically in Fig. 354, consists of an ordinary D. C.
armature placed in a field produced by a single phase alternating
current. As the field alternates, an E. M. F. is induced in every
coil in the armature except in the two at the opposite ends of the
horizontal diameter. The direction of these E. M. F.s for an in-



ELECTRO-MECHANICS.



537



creasing flux from N is indicated by the arrowheads in the dia-
gram. No current is produced since the E. M. F.s in the two
halves of the armature are equal and opposed. If a brush be ap-
plied to the commutator so as to touch two adjacent segments, a
current will be produced in the coil thus short-circuited. If the




brushes be applied to the terminals of the coils A and B, the
resulting flux in these coils will be opposite and parallel to the
field and hence no torque will be developed. If, however, the coils
D and E be short-circuited, the flux in these coils, as shown in
the diagram, will be oblique to the field, D will be repelled from
N and E will be repelled from S and clockwise rotation will ensue.
When the field is reversed, the flux in the coils is also reversed
and the rotation will continue in the same direction. As thus
described, only the coils in the positions D and E contribute to
the torque. If the brushes be enlarged so as to short-circuit a
number of adjacent coils, all of these coils will contribute to the
turning moment. Finally, if the brushes be connected as shown,
currents will flow through the remaining coils and the torque will
be correspondingly increased.

It will be noted that there is no direct electrical connection with
the armature of this machine and that the currents are produced
by induction. It is therefore a true induction motor.

663. Principle of Induction Motor. The principle of the
induction motor will be understood from the following.

SNS, Fig. 355, represents a series of magnetic poles, alternating
in polarity and moving steadily from right to left as indicated by
the arrow. Beneath these there is what may be compared to a



538



ELEMENTS OF ELECTRICITY.



copper ladder with heavy copper rungs. Consider the opening
A BCD in this ladder. At the instant shown it is penetrated by
the lines of force from N, but as AT is moving off to the left, the
number of lines embraced is decreasing and there is therefore
induced a clockwise current in the direction ABCD (Par. 421).
In the adjacent opening ABEF, the number of lines embraced
is increasing and there is therefore induced a counter-clockwise
current in the direction ABEF, that is, the E. M. F. in the copper
surrounding both of these openings produces a current from A to




O



O O



Fig. 355.

B. Since AB is a conductor carrying a current and placed in a
magnetic field, it experiences a force urging it to follow along after
the moving pole (Par. 352). In a similar manner it can be shown
that the rungs under the south poles are also urged to the left.
Reflection will show that this movement is also a consequence of
Lenz's law (Par. 430).

Although the induced E. M. F. be small, the currents in the
copper rungs are, on account of the low resistance, very large and
the force I. H. I on the rungs (Par. 356) is also large.

Suppose now the copper ladder to be bent into a cylindrical
shape and fixed upon an axis like a squirrel cage (Fig. 356), and
suppose the moving poles to be formed into a ring surrounding this
cylinder, and their movement of translation to be converted into a
movement of rotation. Corresponding rotation will be produced
in the squirrel cage, which by a suitable pulley or by gearing could
be made to do mechanical work. We have thus produced rotation



ELECTRO-MECHANICS.



539



in the cage by rotating the magnetic field about it, but the thought
arises at once that the energy expended in rotating the field
might better have been applied to the cage direct. However, it
will now be shown that it is possible to produce a rotating field
without resorting to mechanical rotation.




Fig. 356.

664. Production of Rotating Field. Suppose Fig. 357 to repre-
sent a ring wound stationary iron frame and suppose there are
connected to the winding at points 90 apart the leads CC' and
DD' from a two phase alternator similar to the one shown in
Fig. 341. Suppose we start with the armature in the position



c'




shown in that figure. At this instant the current in the leads CC'
is a maximum and that in DD' is zero. Application of the right
hand rule shows that the current entering at C and leaving at C'
produces a south pole at C and a north pole at C'. The current



540 ELEMENTS OF ELECTRICITY.

entering at C now begins to decrease, while an increasing current
starts in at D. Currents leave by C' and D f and a north pole is
produced between C' and D f .

When the current at C has dwindled to zero, the entire current
enters at D and leaves by D f . A north pole is therefore produced
at D'.

Without carrying this explanation farther, it is seen that during
one complete cycle a north pole starts at C' and travels in a
clockwise direction entirely around the iron frame, a corresponding
south pole keeping pace at the opposite end of the diameter of the
ring. The effect is therefore the same as if the frame work had
held a pair of permanent magnetic poles and had been rotated
through [360. In the actual case, however, there has been no
mechanical motion and no waste of energy in overcoming friction,
such as would have occurred had the heavy iron frame been
rotated. A squirrel cage placed inside of this ring would have
been rotated by the rotating poles.

The rotating field described above was produced by a two-phase
current. It may also be produced by a tri-phase current.

665. The Induction Motor. The induction motor is based
upon the foregoing principles. The rotating cage, although it
resembles the armature of other motors, is not strictly an armature
since it has no electrical connection with the power circuit. It is
therefore called the rotor, the surrounding magnetic field being
called the stator. This is the usual arrangement but it is quite
possible to have the field the rotating member.

The inductors are of copper as described above, and in order to
insure penetration by the lines of force of the field, the interior
of the rotor is built up of laminated iron (Par. 565), in fact, the
copper inductors are generally embedded in slots in this laminated
core. The stator is likewise laminated and, instead of being ring
wound as described in the preceding paragraph, it is wrapped
somewhat as shown in Figs. 337 and 339.

If the rotor revolved synchronously with the rotating field,
there would be no cutting of lines of force by the inductors, hence
no induced current and no torque developed. In order then to
develop torque the rotor must run below synchronism. It would
therefore seem that the slower the rotor turned the greater would
be the torque, but this is not correct. Examination of Fig. 355
will show that as the pole N moves over the interval ABEF, an



ELECTRO-MECHANICS. 541

upward flux is produced, that is, a flux tending to demagnetize N.
The force on an inductor is I. H. I (Par. 356), but when the speed
falls below a certain point, the field H is demagnetized more
rapidly than / increases and the total force therefore falls off. If,
therefore, an increasing load be applied to one of these motors, it
will slow down until the maximum torque is developed, after
which, if the load be further increased, it will come to a stop.

If one of these motors is to start from rest under load, as for
example in operating an elevator, it is desirable that the maximum
torque should be exerted at starting. This may be attained by
constructing the rotor so that the resistance of the inductors may
be varied. The resistance is introduced at starting and the cur-
rents through the inductors are thus kept down so that the de-
magnetizing effect described above will not be too great. As the
motor gathers headway, this resistance may be cut out.



HIGH POTENTIAL. 543



PART VI.
HIGH POTENTIAL.



CHAPTER 46.

DISCHARGE OF ELECTRICITY THROUGH GASES.

666. High Potential. The two following chapters, with which
we conclude this book, treat of the discharge of electricity through
gases and of electrical oscillations. While these subjects stand
somewhat apart from the divisions which we have hitherto con-
sidered, they can not be said to be very intimately interrelated,
and they are here classed under one heading mainly because
their most characteristic phenomena are usually produced by
the use of high voltages. The title "high potential" must not
therefore be regarded as descriptive but rather as used to avoid



Online LibraryWirt RobinsonThe elements of electricity → online text (page 41 of 46)