5. _The collecting-brushes_ that rest on the commutator cylinder and
take off the current of electricity generated by the machine.
Fig. 218 shows a dynamo of the early Edison type—the names of the
principal parts are given in the note below, as well as those of the
other parts of the machine.
[Illustration: FIG. 218.]
_This is a two-pole machine_, direct current; the figure is introduced
to show the “parts” only—as this dynamo has been largely superseded by
others of the four pole type.
NOTE.—A, Magnet yoke; B, Magnet and field piece; C, Pole piece;
D, Zinc field piece; E, Armature; F, Commutator; I, Quadrant; JJ,
Brushes; K, Adjusting handle for the brushes; L, Switch pivot; M,
Pilot lamp receptacle; N, Negative lug; O, Switch lever; P, Positive
lug; Q, Positive terminal; R, Negative terminal; S, Negative rod; T,
Pole piece; UU, Bearings; X, Slides for belt tightener; VVV, Driving
pulley; Y, Connecting blocks, one on each side of machine.
_An electric motor_ is a machine for converting electrical energy into
mechanical energy; in other words it produces mechanical power when
supplied with an electric current; a certain amount of energy must be
expended in driving it; the _intake_ of the machine is the term used
in defining the energy expended in driving it; the amount of power it
delivers to the machinery is denominated _its out-put_.
The difference between the out-put to the intake is the real
_efficiency_ of the machine; it is well known that the total efficiency
of _an electric distribution system_, which may include several
machines, usually ranges from 75 to 80 per cent., at full load, and
should not under ordinary circumstances fall off more than say 5 per
cent. at one-third to half load; the efficiency of motors varies
with their size, while a one horse-power motor will, perhaps, have
an efficiency of 60 per cent., a 100 horse-power may easily have an
efficiency of 90 per cent. and the larger sizes even more.
The general and growing application of electric power to the driving
of all kinds of machinery including pumps makes _the question of
motor driving one of the most important in the power field_. For
many purposes, a single speed is sufficient, but for others, it is
imperative that the speed should be variable; and for still others,
though not absolutely necessary, a speed adjustment is very desirable.
While the _direct-current motor_ has been in this field so long that
its properties are well known and its possibilities fully developed, in
the operation of motors located in the immediate neighborhood of the
generator the _alternating-current motor_ has marked advantages where a
large area of territory has to be covered and the conditions are nearly
uniform, that is to say—
Where the current has to be transmitted a long distance and the load is
approximately constant, the alternating system is preferred, as it can
be operated with small main lines or conductors. This effects a saving
in copper, over the direct system which requires larger conductors.
[Illustration: FIG. 219.]
[Illustration: FIG. 220.]
Fig. 217, on page 247, shows _a four-pole generator_ designed to run by
a belt or directly connected to an engine. The five parts named, as the
principal parts of a dynamo, are all shown in the figure. The machine
is arranged ready to be bolted to the floor.
Fig. 219 on the opposite page is the _armature_ which is made up of
coils of insulated wire, the free ends of which, see Fig. 220, are
united to the arms of the _commutator bars_. When the armature is
finished, as shown, the wire forms an unbroken circuit.
[Illustration: FIG. 221.]
Fig. 221 is intended to represent another form of armature, but the
principle upon which it operates, is the same, as the other shown. A
A, represents the wire coils of the armature, B, is the shaft with its
journals, C, is the commutator. All commutators both for generators and
for motor armatures are insulated by mica between the bars.
[Illustration: FIG. 222.]
Fig. 222 shows _a woven wire brush_. The brushes on the dynamo, page
247, are made of carbon.
[Illustration: FIG. 223.]
Fig. 223 shows an alternating _induction_ motor. Induction is a
property by virtue of which an electric current is transferred from one
conducting line to another _without any metallic connection_; it is
that influence by which a _strong current_ flowing through a conductor
controls or affects _a weaker current_ flowing through another
conductor in its immediate neighborhood,—the strong current remaining
unaffected.
[Illustration: FIG. 224.]
Fig. 224 exhibits the armature for the above alternator; it is
familiarly called a “squirrel cage armature” on account of its
resemblance to the wheel in a squirrel’s cage.
Its peculiar construction enables it to run without producing any
sparks; this feature renders it safe to run where there are explosive
gases which might be ignited by an electric spark. In the machine the
bearings are cast solid with the end shields, thus assuring perfect
alignment when properly turned. Another feature is the automatic self
adjusting bearings which are lubricated mechanically by rings resting
upon the shaft. These rings were formerly a failure, but by the use of
mineral oils are now a success.
This machine is one of the simplest designs of alternating motors, the
example, Fig. 223, is one developing one hundred horse-power.
[Illustration: FIG. 225.]
Fig. 225 shows a revolving _field_ with “spider.” In this construction
of generators or motors the field revolves in place of the armatures,
the first object of this design is to reduce the high rotative speed;
it is also claimed to have a better electrical efficiency.
The field spider consists of an extra heavy cast iron pulley which
is keyed to the shaft; the low speed at which it runs permits the
employment of bolts to secure the field coils and laminated pole
pieces to the rim of the spider, as shown in the engraving. With this
construction each individual pole piece can be removed and replaced
independent of the others.
_The laminated pole piece_, one of which is shown in detail in Figs.
226-229, takes its name from the fact that it is built up of a large
number of layers of soft sheet iron, which it has been demonstrated
give a better electrical efficiency than a solid iron. Soft iron is the
most magnetic of all metals and is better suited for pole pieces than
steel.
It should be understood that each individual pole piece is insulated
from the others as well as from the spider. The pieces of sheet iron
are stamped out—like washers and are cut apart and the ends united so
as to form a continuous coil, like a coil of wire and each coil is
isolated; mica is used between the layers.
[Illustration: FIGS. 226-229.]
Fig. 230 is designed to illustrate the front of a continuous current
two wire switchboard with circuit breakers; these are made up
usually of marble or slate so that they will not burn; the Insurance
Underwriters require a non-combustible material at this place, as well
as hangers, and insulators used for conductors.
_The Switches_ shown in the middle of the board, are enlarged in Fig.
232, and are used for closing the connections with the generators and
lines running to various parts of the field to be lighted or furnished
with power.
The switch handles are made usually of wood or hard rubber; the blades
are of copper. The connections are soldered into the sockets shown
upon the ends of the screws which project beyond the back of the
switch-board.
The upper row of figures as shown in Fig. 230 and enlarged in the
engraving, 231, are _circuit-breakers_. The use of these is analogous
to that of the safety-valve upon a steam boiler, so that when the
pressure in the circuit exceeds that at which it is set the “breaker”
opens the circuit and thus prevents damage.
[Illustration: FIG. 230.]
In this case, the main contact is formed by means of a laminated
brush while the final stroke is made on carbon, the motion of this
breaker is by means of a toggle-joint which so multiplies the power
applied that it does not require much of an effort to close it; this
device maintains the same speed in operating the breakers when the
circuit-breaker is tripped.
_A Rheostat_ is a device for controlling the amount of electricity in
a conductor—by the insertion of coils of wire in a box—which may be
successively switched in or out of the main circuit by means of a lever
and button-switch. The best place to install a rheostat is on a wall
or post, as the resistance transforms a portion of the electric energy
into heat, which heat must be dispersed into the atmosphere.
_A transformer_ is an induction coil employed usually for lowering
electric pressure, but it may also be used for raising the same, in
which case it is sometimes called a _booster_. _A compensator_ is a
transformer which works automatically.
[Illustration: FIG. 231.]
[Illustration: FIG. 232.]
_Ammeters_ record _the quantity of current flowing through the circuit,
in amperes_. _Voltmeters_ record _the pressure_ or strength of the
current in _volts_.
An _Ampere_ is an electric current which would pass through a circuit
whose resistance is one ohm under an electro-motive force of one volt.
A _Volt_ is an electro-motive force of sufficient strength to cause _a
current of one ampere to flow against a resistance of one ohm_.
The _ampere_ is the unit for calculations relating to _the quantity_
or volume of a current; the _volt_ is the unit for calculating the
_pressure_ or strength of the current.
_The action of the electric current_ in producing rotation in an
electric motor is really quite simple. While many electrical problems
are comparatively complicated, the principal elements in the operation
of electric motors may be readily understood. The fundamental fact
in this connection is the relation between an electric current and a
magnet.
If a piece of round bar iron be surrounded by a coil through which
an electric current passes, it becomes a magnet. In Fig. 233 the
passage of a current through the coil of wire around the iron bar in
either direction, _renders the iron a magnet_, with all its well-known
properties. It will attract iron, and the space surrounding it becomes
magnetic. Iron filings will arrange themselves in the direction shown
by the dotted lines in the figure. One end of the magnet is the North
or positive + pole and the other the South or negative - pole.
If a wire, such as _CD_, be moved past either pole of the magnet, there
will be a tendency for current to flow in the wire either from _C_ to
_D_ or _D_ to _C_, according to the character of the pole past which it
is moved, and to the direction of the movement. If the ends of the wire
_CD_ are joined by a conductor, so that there is a complete circuit, a
current of electricity will flow through this circuit.
This circuit may be a simple wire, as shown by the line _CEFD_, or
it may be the wire coils on machines enabling the current to produce
mechanical work, or it may be electric lamps producing light. The
indispensable feature is that there shall be a complete unbroken
circuit from _C_ to _D_ for the current to flow, no matter how
complicated or how long this circuit may be.
This description of a dynamo and motor carries with it all of the
elementary theory of electric generators and motors that is necessary
for an attendant to know in order to take reasonably intelligent care
of electric machines. Further useful knowledge must be acquired by
studying the different types of electric motors and dynamos. All these
other types of _direct current_ machines have the same elementary
theory, although their construction may be quite different.
By suitable illustrations the operation of the electric motor as
applied to pumps will be easily understood; its application to other
machines is the same in theory and practice.
“_Why an electric motor revolves_” is a question well worth careful,
and, if necessary, long study.
[Illustration: FIG. 233.]
The reason why there is a tendency for an electric current to flow
in the wire _CD_ when it is moved in the vicinity of a magnet is not
fully known. There are several theories, all more or less complicated,
and depending upon pure assumptions as to the nature of an electric
current. For practical purposes it matters little what the reason is,
_the fact that current flows when there ts an electric pressure in
a closed circuit_, is the important thing, and it serves all useful
purposes to know that current does flow, and that its direction and
amount are always the same under similar circumstances. There are many
facts in mechanics that are accepted and used practically, about which
little is known as to their fundamental and primary causes, and this
fact about motors and dynamos is, therefore, only one of many which all
must accept without a full and complete explanation.
_The intensity of the electric pressure_, or electro-motive force,
depends upon the velocity of revolution of the wire sections in the
armature and upon the strength of the magnets, and the quantity of
current depends upon the electro-motive force and upon the amount of
the resistance in the circuit. Other things being equal, the current,
flowing through a long small wire, or greater resistance, will be less
than through a short, thick wire, or a less resistance.
Having seen that when a wire is moved in the vicinity of a magnet an
electric pressure is produced which will cause a current to flow in
a closed circuit, one can easily conceive of many ways in which, by
combining magnets and wires so that there will be a relative motion
between them, a current of electricity may be generated. In order to
cause a continuous flow the relative motion must be continuous; and if
the current is to be uniform the motion must be uniform.
[Illustration: FIG. 234.]
Two electro-magnets are shown in Fig. 234, in which the North pole
of one magnet is near the South pole of the other, and the magnetic
field between the two lies in the approximately straight lines between
the two magnets, as indicated by the dotted lines. If the wire _CD_
be moved across this field and its ends be joined, as by the metallic
circuit _CEFD_, a current will flow in this circuit. The wire _CD_ may
be made to revolve around the wire _EF_, passing in front of one pole
and then in front of the other pole, as in Fig. 235. The current in
the circuit will pass in one direction when the wire is passing one
pole, and in the other direction when it is passing the other pole.
The connection between this elementary arrangement and the dynamo
is easily recognized. In the dynamo a magnetic field is produced by
electric magnets, called “pole pieces,” and a considerable number of
wires similar to the wire _CD_ are placed upon an armature so that they
revolve in front of these poles. Each individual wire produces current
first in one direction and then in another direction, as explained
above; but if there be many wires there will always be the same number
in front of the North, or positive pole, and the same number in front
of the South, or negative pole, so that the total or resultant action
is practically uniform, and may be made to produce a continuous
current. Such a machine is the common direct current dynamo, or motor.
[Illustration: FIG. 235.]
A dynamo transforms mechanical into electrical energy, and a motor
transforms electrical into mechanical energy. The two operations are
reversible, and may be effected in the same machine; a dynamo may be
used as a motor, or a motor may become a dynamo.
_A dynamo is a motor when it is driven by a current of electricity, and
it is a dynamo when it is driven by mechanical power and produces an
electric current._ If a motor be driven by an engine, it can deliver
a current of electricity which is able to operate other motors or
electrical apparatus or lights. A simple form of electric machine is
shown in Fig. 236, which is a general form of the electric motor. In
this there are two projections of steel, _H_ and _G_, which are made
electro-magnets by the current flowing through the wires wound around
them from any source of electricity, such as a battery at _I_ and _J_.
These magnets have poles facing toward an armature, _K_, on a shaft.
The poles _G_ and _H_ are called the “salient” poles; the poles _M_ and
_P_ are called the “consequent” poles. The magnetic flow or field is
shown by the dotted lines. On the periphery of the armature are wires
in the slots shown. As this armature revolves, there will be a tendency
for electricity to flow through the wires.
[Illustration: FIG. 236.]
In order to distribute a current of electricity through these wires it
is necessary to make a complete circuit. As each of the wires in the
slots passes in front of a pole, a pressure or electro-motive force
will be generated, and its direction will depend upon whether the pole
is a North or a South pole, _i. e._, + or -.
NOTE.—In the above illustrations I and J represent the ordinary
electric battery; in electrical literature such marks always indicate
a battery.
The pressure or electro-motive force generated in the wires moving in
front of the North, or positive field poles, will be in one direction,
while that of those in front of the South, or negative field poles,
will be in the opposite direction. Therefore, if two such wires be
connected together at one end of the armature, the free terminals
of the wires at the other end of the armature will have the sum of
the electro-motive forces generated in the two wires. The wires so
connected can be considered as a turn of a single wire instead of two
separate wires, and this turn may be connected in series with other
turns, so that the resulting electro-motive force is the sum of that
in all the turns and all the wires so connected. It is customary to
connect the coils of an armature so that the electro motive force
given is that obtained from half the coils in series. The other half
of the coils is connected in parallel with the first half, so that the
currents flowing in the two halves will unite to give a current in
the external circuit equal to twice the current in the two armature
circuits or paths.
It is evident that, as the armature revolves, wires which were in front
of the positive pole will pass in front of the negative, and that
in order to maintain the electro-motive force it will be necessary
to change the connections from the armature winding to the external
circuit in such a way that all the wires between the two points
of connection will have their electro-motive forces in the proper
direction. The connection to the armature must therefore be made not
at a definite point in the armature itself, but at a definite point
with reference to the field magnets, so that all the wires between two
points or contacts shall always sustain the same relation to the field
magnets.
For this purpose a device known as a “commutator” is provided. The
commutator is made up of a number of segments, as shown at _A_, in Fig.
237, which are connected to the armature winding. On the commutator,
rest sliding contacts, or brushes, which bear on the segments and are
joined to an external circuit, making a continuous path through which
current may flow. As the commutator revolves, the different segments
come under the brushes, so that the relative position of the armature
wires between the brushes is dependent on the position of the brushes.
The armature wires which connect the brushes are those sustaining the
desired definite position to the field magnets, so that the currents
from the armature at all times flow properly into the external circuit,
although individual armature wires carry currents first in one
direction and then in the other direction, depending on the character
of the pole in front of which they may be moving.
[Illustration: FIG. 237.]
On two-pole machines there are two brush-holders, each containing one
or more brushes. On the four-pole machine there may be either two or
four brush-holders, and on a six-pole machine, either two, four, or six
brush-holders.
A single path of the current through the commutator and armature
winding is shown by the arrows on Fig. 237. The brushes _B_ and
_C_ are placed on the top side of the commutator to make them more
accessible, and this shows a peculiar but simple armature winding.
For the sake of simplicity, the batteries _I_ and _J_, of Fig. 236, are
not used on common forms of generators or motors, but the current that
flows from the armature through the commutator is made to flow through
the electro-magnets either in whole or in part. If all of the armature
current flows around the electro-magnets or fields of the machine, it
is a “series” machine; if only a part of the current is used in this
way, it is a “shunt” machine; that is, some of the current is “shunted”
through the fields. Sometimes both the shunt and series windings
are used, and in that case the machine is called a “compound wound”
machine. Such a machine has a large wire through which the main current
passes, and a fine wire through which the shunted current flows. Fig.
237 shows how the commutator and the fields are connected, and how the
current flows from the wires in the armature through the commutator in
a series machine.
If the current delivered by a dynamo does not flow in the desired
direction, it can be reversed by shifting the wires in the binding
posts or by throwing a switch. If the motor does not revolve in the
desired direction, it can be made to do so by reversing the connections
to the armature or field-coils; so that, without knowing which way a
current of electricity is to be generated, any practical man can make a
motor revolve in a proper direction by simply changing its connections.
It is natural that a machine which gives out electric energy when
driven by an external power, should, when electric energy is delivered
to it, reverse its action and give out mechanical power and do work.
Perhaps the simplest way to explain the cause of the movement of
an electric motor, when supplied with a current, is to compare its
action to the well-known attraction of unlike poles or magnets and
the repulsion of like poles. Unlike poles are North and South; like
poles are two North or two South. In all motors a current through the
field causes a North or South pole to be maintained, and a current
through the armature and brushes causes an opposite polarity. These
constantly-maintained unlike poles attract each other and pull the
armature around on its axis.
It has been explained that if a motor be driven by a belt an
electro-motive force is produced and the machine acts as a dynamo.
It is also a fact that an electro-motive force is produced whether
the power for driving the machine is received from a belt or from
the electric current,—that is, whether the machine be driven as a
dynamo or as a motor. In a dynamo, however, the current follows the
direction in which the electro-motive force is acting. In a motor, the
electro-motive force produced has a direction opposed to that of the
flow of current. This may be illustrated by the following experiment.
Two similar machines are driven independently at 600 revolutions and
give an electro-motive force of 100 volts. Similar terminals of the two
machines are connected together; no current flows between the machines,
because the two pressures are the same and are in opposite directions.
If now the belt be thrown off from one machine, its speed will begin to
fall; this will lower its electro-motive force below that of the other
machine or dynamo, but will not change the direction of the force.
There will now be a difference of pressure in favor of the machine
which is driven, and it will deliver a current through the other
machine and run it as a motor. The speed of the motor will continue to
fall until the difference in pressure or electro-motive force between
the two machines is only sufficient to cause the flow of enough current
to keep the motor running against whatever frictional resistance, and
other resistance there may be. The electro-motive force generated in
the motor, which is against, or counter to that of the current in the
circuit, is called the “counter electro-motive force.”
In order to determine how fast a motor will run without doing work
under any given pressure, it is not necessary to know anything
about the dynamo that furnishes the pressure. The pressure alone
is sufficient to determine the speed of the motor. For instance, if
a motor will give a pressure of 500 volts when running free at 100
revolutions, it will always run at about 100 revolutions when not doing
work on an electric circuit where the pressure is 500 volts.