OPERATION OF DYNAMOS
Before Starting a Dynamo or Motor.--When the machine has been securely
fixed, it should be carefully examined to see that all parts are in good
order. The examination should be made as follows:
1. The field magnet circuit should first be inspected to see that none
of the wires or connections have broken or have become loose, and that
the coils are correctly connected;
2. The caps of the bearings should be taken off, and these and the
journals carefully cleaned of all grit and dirt. They should then be
oiled, and the caps replaced and screwed up by hand only;
3. The gaps between the outer surface of the armature and the polar
faces should be examined in order to ascertain whether any foreign
body, such as a small screw or nail has lodged therein. If such be
the case, it should be carefully removed with a bit of wire;
4. The guard plates protecting the armature windings should be removed,
and the windings carefully inspected by slowly rotating the armature,
to see that they are not damaged, and that the insulation is perfect.
The armature should then be finally rotated by hand to see that it
revolves freely, and that the bearings are securely fixed;
5. The commutator should be examined to see that it is not damaged in
any way through one or more of the segments being knocked in, or the
lugs being forced into contact with one another;
6. The brush holders and brushes should be inspected to see that the
former work freely on the spindle, and that the hold off catches work
properly, are clean and make good contact with the brush holders or
flexible leads;
7. Having ascertained that the machine is not injured in any way, and
that the armature revolves freely, the brushes should be adjusted.
In the subsequent working of the dynamo it will of course be
unnecessary to follow the whole of these proceedings every time the
machine is started, as it is extremely unlikely that the machine will
be damaged from external causes while working without the attendant
being aware of the fact.
Adjusting the Brushes.--The _adjustment of the brushes_ upon the
commutator requires careful attention if sparking is to be avoided. There
are two adjustments to be made:
1. For pressure; The brushes must bear against the commutator segments
with sufficient pressure for proper contact.
2. For lead. The brushes must have the proper angular advance (positive
or negative, according as the machine is a dynamo or motor) to prevent
sparking.
Ques. At what point on the commutator should the brushes bear?
Ans. The points upon the commutator at which the tips of the brushes
(carried by opposite arms of the rocker) bear, should be, in bipolar
dynamos, at opposite extremities of a diameter. In multipolar dynamos the
positions vary with the number of poles and the nature of the armature
winding.
Ques. What provision is made to facilitate the correct setting of the
brushes?
Ans. Setting marks are usually cut in the collar of the commutator next
to the bearing.
[Illustration: Figs. 664 and 665.--Diagrams illustrating how to set
brushes. Some brush holders require brushes set _with_ the direction of
rotation of the commutator, and others, set _against_ the direction of
rotation. In fig. 664 is shown a brush holder of the first class, which
must always be set as indicated by the arrow. If set in the opposite
direction, trouble will ensue, as an inspection of the figure will show,
because the surface of the commutator and the brush would form a toggle
joint, and the brush would tend to dig into the commutator and either
break itself or bend the brush rigging. In fig. 665 is shown a brush
holder of the second type. This brush is set against the direction of
rotation, but an inspection of the cut will show that there is, in this
case, no tendency for the brush to dig into the commutator surface. Each
type of brush holder, of which there are several, should be adjusted as
recommended by the manufacturer to secure proper working.]
Ques. How are the brushes set by these marks?
Ans. The tips of all the brushes carried by one arm of the rocker are set
in correct line with the commutator segments marked out by one setting
mark, and the tips of the brushes carried by the other arm or arms are set
in correct line with the segments marked out by the other mark or marks.
If one or more of the brushes in a set be out of line with their
setting mark, it will be necessary to adjust the brushes up to this
mark by pushing them out or drawing them back, as may be required,
afterwards clamping them in position. When adjusting the brushes,
the armature should always be rotated, so that the setting marks are
horizontal. The rocker can then be rotated into position, and the
tips of both sets of brushes conveniently adjusted to their marks. In
those brush holders provided with an index or pointer for adjusting
the brushes, the setting marks upon the commutator are absent, length
of the pointer being so proportioned that when the tips of the brushes
are in line with the extreme tips of the pointers, the brushes bear
upon the correct positions on the commutator.
[Illustration: Fig. 666.--Method of soldering cable to carbon brush. Drill
a hole in the end, also in the side of the brush, as shown in the sketch,
and after thoroughly tinning the "pig-tail," place it in the end hole and
fill the holes up with solder through the side hole. Another method is to
drill a hole through the carbon so that the cable will just slip through,
countersink the edge of the hole a little, clean the cable thoroughly and
pass it through the hole. Then with any good flux and solder, fill the
countersunk part on both sides.]
Ques. What should be done after adjusting the brushes to their correct
positions upon the commutator?
Ans. Their tips or rubbing ends should be examined while in position to
see that they bed accurately on the surface of the commutator.
In many instances it will be found that this is not the case, the
brushes sometimes bearing upon the point or toe, and sometimes
upon the heel, so that they do not make contact with the commutator
throughout their entire thickness and width. The angle of the rubbing
ends will therefore need to be altered by filing to make them lie
flat.
Ques. How is the proper brush contact secured?
Ans. When the brushes do not bed properly they should be refitted to
secure proper contact.
Ques. How is the pressure adjustment made?
Ans. This is effected by regulating the tension of the springs provided
for the purpose upon the brush holders.
Ques. With what pressure should the brushes bear against the commutator?
Ans. The tension of the springs should be just sufficient to cause the
brushes to make a light yet reliable contact with the commutator.
The contact must not be too light, otherwise the brushes will vibrate,
and thus cause sparking; nor must it be too heavy, or they will press
too hard upon the commutator, grinding, scoring and wearing away the
latter and themselves to an undesirable extent, and moreover, giving
rise to heating and sparking.
The correct pressure is attained when the brushes collect the full
current without sparking, while their pressure upon the commutator is
just sufficient to overcome ordinary vibration due to the rotation of
the commutator.
[Illustration: Figs. 667 to 669.--Method of winding cables with marlin.
When connecting the feeders and dynamo and service leads to a switchboard,
the wires are often _served_ with marlin. By serving is meant to tightly
wrap the wires of each set together with marlin. A tool for serving may be
made as in fig. 667, using a piece of oak 2 ins. wide, 7/8 in. thick and
14 ins. long, having four holes drilled through it, as shown. The marlin
is passed through the holes, commencing at the hole nearest the handle,
the object being to cause a strain on the marlin at the point where it
passes around the wire, so that the marlin may be wrapped tightly. It
is necessary to serve the first four or five inches by hand, pushing the
winding into the conduit as far as possible. This acts as an additional
protection to the wires where they leave the conduit. The serving is
continued, as in fig. 668, to within four or five inches of the first
lug by means of the serving tool, passing the ball of marlin around the
wires with the serving tool. The wires are then bent in shape, as in fig.
669. To serve the wires properly it is necessary to tie the ends of the
wires taut. The wires should be straightened and run together so as to
be parallel, being bound with tape at different points to keep them so.
When the serving is complete the marlin should be thoroughly painted with
a moisture resisting compound. The marlin serving will stiffen the wires
and they can be bent very neatly to avoid touching the bus bars of the
board. When painted the marlin hardens so that it is difficult to bend
the wires after the paint has dried. It then requires a strong pressure
to bend them. The marlin acts as an additional insulation and mechanical
protection to the wires, and while no harm would result from the wires
coming in contact with the bars while thus protected, it looks better to
bend them so as to avoid touching the bars.]
Direction of Rotation.--This is sometimes a matter of doubt and often
results in considerable trouble. As a general rule, a dynamo is intended
to run in a certain direction; either right handed or left handed
according to whether the armature, when looked at from the pulley end,
revolves with or against the direction of the hands of a clock. Dynamos
are usually designed to run right handed, but the manufacturers will make
them left handed if so desired.
It may be necessary to reverse the direction of rotation of a dynamo,
if the driving pulley to which it has to be connected happen to revolve
left handed, or if it be necessary to bring the loose side of the belt
on top of the pulley, or to place the machine in a certain position on
account of limited space. The direction of rotation of ordinary series,
shunt, or compound bipolar dynamos may be reversed by simply reversing the
brushes without changing any of the connections, then changing the point
of contact of the brush tips 180°.
In multipolar dynamos, a similar change, amounting to 90° for a four pole
machine, and 45° for an eight pole machine, will reverse their direction
of rotation. It will be understood that under these conditions, the
original direction of the current and the polarity of the field magnets
will remain unchanged.
This rule does not apply to arc dynamos and other machines, which have
to be run in a certain direction only, in order to suit their regulating
devices.
If the direction of current generated by a dynamo be opposite to that
desired, the two leads should be reversed in the terminals, or the
residual magnetism should be reversed by a current from an outside source.
[Illustration: Fig. 670.--Method of assembling core discs. For this
operation two wooden "horses" should be provided to support the core at
a convenient height, as shown in the illustration.]
Starting a Dynamo.--Having followed the foregoing instructions, all keys,
spanners, bolts, etc., should be removed from the immediate neighborhood
of the machine, and the dynamo started.
[Illustration: Figs. 671 and 672.--Tinning block for electric soldering
tool. It is made with two soft bricks. One brick is used to support the
soldering tool, and the other to contain the tinning material and to
furnish a material which will keep the copper bit bright enough to receive
its coating of "tin." Fig. 671 represents a section of the tinning brick,
which is scooped out on top as shown by the lower line. Into one end
of the hollow in the brick, some sal-ammoniac is placed to help tin the
copper bit. Sal-ammoniac is a natural flux for copper and aids greatly
in keeping the tool well tinned. Next, some melted solder is run into the
hollow of the brick, and lastly enough resin to fill the cavity nearly to
the top. When the tool is not in use, the electricity is switched off and
the tool permitted to lie in the resin. If it be desired to repair the
tin coating a little when the tool is in use, the latter is rubbed on the
brick below the layer of solder, and the layer of resin. If the tool be in
very bad condition, it may be pushed into the sal-ammoniac once or twice
and then rubbed in the solder again. It requires but little heat to keep
the brick and its contents ready for use. In fact, the brick is a fair
non-conductor of heat and prevents the escape of heat from one side of the
tool. When momentarily not in use, the tool remains in the solder which
becomes melted underneath the layer of resin. When the copper bit becomes
too hot, it will begin to volatilize the resin, thus calling attention to
this fact, whereupon, the electricity should be turned off from the tool.]
Ques. How should a dynamo be started?
Ans. A dynamo is usually brought up to speed either by starting the
driving engine, or by connecting the dynamo to a source of power already
in motion. In the first case, it should be done by a competent engineer,
and in the second case by a person experienced in putting on friction
clutches to revolving shafts, or in slipping on belting to moving pulleys.
[Illustration: Fig. 673.--Connections for two shunt wound dynamos to
run in parallel. The positive lead of each machine is connected to the
same bus bar. In starting, if but one machine is to be used, the dynamo
is first brought up to speed and the voltage regulated by means of the
rheostat R and the voltmeter V. The main switch is then thrown in. The
connections for the field are taken off the dynamo leads so that the
opening of the main switch will not open the field circuit and for this
reason the field will begin to build up as soon as the machine is started.
When but one of the machines is running, the idle machine is brought up to
speed with the main switch open, and the voltage regulated by means of the
rheostat and voltmeter until the voltages of the machines are the same.
Then the main switch is thrown in and the load on the machines (which
is ascertained by the ammeters) is equalized by means of the rheostats.
Should there be any great difference in voltages, the higher one will
run the other as a motor without changing the direction of rotation. The
field current will remain unchanged, and the armature current of the low
dynamo will be reversed, which will cause it to run as a motor in the same
direction as it ran as a dynamo. When dynamos feeding current to motors
are to be shut down, the switches on the motors should first be opened.
Otherwise some of the motor fuses will blow. As the voltage goes down the
motors will draw more current to do the work. If a plant be shut down with
the motor switches "in" it will generally be found impossible to start a
shunt dynamo, the low resistance in the mains not allowing enough current
to flow around the shunt fields to energize them.]
Ques. Should the brushes be raised out of contact in starting?
Ans. The brushes should not be in contact in starting if there be any
danger of reverse rotation, as might happen when the dynamo is driven
by a gas engine. Aside from this, it is desirable that the brushes be
in contact, because they are more easily and better adjusted, and the
voltage will come up slowly, so that any fault or difficulty will develop
gradually and can be corrected, or the machine stopped before any injury
is done.
[Illustration: Fig. 674.--Connections for two shunt dynamos to run on
the three wire system. The two machines are connected in series, three
wires being carried from them, one from the outside pole of each machine
and one from the junction of the two machines. The voltage between the
outside wires is equal to the combined voltage of the two machines and the
voltage between the outside and the central or neutral wire is equal to
the voltage of the corresponding machine. If the load on each side of the
system be equal, there will be no current in the neutral wire, while if
the loads be unequal, the neutral wire will have to carry the difference
in current between the two outside wires.]
Ques. How should a series machine be started?
Ans. The external circuit should be closed, otherwise a closed circuit
will not be formed through the field magnet winding and the machine will
not build up.
Ques. What is understood by the term "build up"?
Ans. In starting, the gradual voltage increase to maximum.
[Illustration: Fig. 675.--Connections for two compound wound dynamos to
run in parallel. The series fields of the machines are connected together
in parallel by means of wire leads or bus bars, which connect together
the brushes from which the series fields are taken. This is known as
the equalizer and is shown by the line running to the middle pole of the
dynamo switch. By tracing out the series circuits it will be seen that
current from the upper brush of either dynamo has two connections to its
bus bar. One of these leads through its own field, and the other, by means
of the equalizer bar, through the fields of the other dynamo. As long as
both machines are generating equally there is no difference of pressure
between the brushes of either, but should the voltage of one be lowered,
current from the other would flow through its fields and thereby raise
the voltage, and at the same time reduce its own until both are equal.
The equalizer may then be called upon to carry much current, but to have
the machines regulate closely it should be of low resistance. It may also
be run as shown by the dotted lines, but this will leave all the machines
alive when any one is generating. The ammeters should be connected as
shown. If they were on the other side they would come under the influence
of the equalizing current and would indicate wrong, either too high or
too low. The equalizer switch should be closed a little before the main
switches are closed.]
Ques. How should a shunt or compound machine be started?
Ans. All switches controlling the external circuits should be opened, as
the machine excites best when this is the case. If the machine be provided
with a rheostat or hand regulator and resistance coils, these latter
should all be cut out of circuit, or short circuited, until the machine
excites, when they can be gradually cut in as the voltage rises.
When the machine is giving the correct voltage, as indicated by the
voltmeter or pilot lamp, the machine may be switched into connection
with the external or working circuits.
Ques. In starting a shunt dynamo, should the main line switch be closed
before the machine is up to voltage or after?
Ans. If the machine be working on the same circuit with other machines, or
with a storage battery, it is, or course, necessary to make the voltage of
the machine equal to that on the line before connecting it in the circuit.
If the machine work alone, the switch may be closed either before or after
the voltage comes up. The load will be thrown on suddenly if the switch be
closed after the machine has built up its voltage, thus causing a strain
on the belt, and possibly drawing water over the engine cylinder. On the
other hand, if the switch be closed before the voltage of the machine has
come up, the load is picked up gradually, but the machine may be slow or
may even refuse to pick up at all.
Ques. Why does a shunt machine pick up more slowly if the main switch be
closed first?
Ans. Because the resistance of the main line is so much less than that
of the field that the small initial voltage due to the residual magnetism
causes a much larger current in the armature than in the shunt field. If
this be too large, the cross and back magnetizing force of the armature
weakens the field more than the initial field current strengthens it, and
so the machine cannot build up.
Ques. If a shunt dynamo will not pick up, what is likely to be the trouble?
Ans. The speed may be too slow; the resistance of the external circuit
may be too small; the brushes may not be in proper position; some of the
electrical connections in the dynamo may be loose, broken or improperly
made; the field may have lost its residual magnetism.
[Illustration: Figs. 676 and 677.--Diagrams of ground detectors. Fig. 676,
a ground detector switch suitable for mounting on a switch board. The two
arms pivoted at their upper ends are connected with an insulating bar A
and make contact at their lower ends with two brass strips and a contact
button, which are connected to the bus bars and ground, respectively.
When the arms are moved to the left, the positive bus bar is connected to
the ground through the voltmeter V. In fig. 677 is another form of ground
detector. This is known as a lamp ground detector. On a 110 volt system
two ordinary lamps are connected in series, while the line connecting the
lamps is connected to the ground through a snap switch S. When current
is on, the two lamps will burn with equal brilliancy, but at a lower
candle power. When the switch S is closed, if the two lines be clear, the
brilliancy of the lamps will not be affected, but if there be a ground on
the positive side, one lamp will burn brighter, the brightness depending
on the resistance of the ground. If there be a dead ground, the lamp will
burn to its full candle power.]
Ques. What is the indication that the connections between the field coils
and armature are reversed?
Ans. If the machine build up when brought to full speed, the connections
are correct, but if it fail to build up, the field coils may be improperly
connected.
[Illustration: Fig. 678.--Method of correcting reversed polarity in large
shunt dynamo by transposing the shunt field leads, and then starting up
the machine. As soon as the voltmeter registers any voltage, the dynamo
may be stopped and the field leads restored to their original position,
when it will be found that the residual magnetism in the pole pieces
will usually bring the dynamo up to its polarity and proper voltage.
This method has the disadvantages, of the uncertainty as to the machine
building up, and that a temporary wire must probably be run from the
switchboard to one terminal of the field circuit, which is usually
connected to a terminal back of the dynamo frame, so that the flow of
current through the field coils may be reversed. With dynamos having
laminated field magnet cores of comparatively low residual magnetism,
this method may suffice, but in the case of solid field magnetic cores it
is not practical. A better method is to disconnect the shunt field leads
and temporarily extend them to some other source of direct current. If
the current be of higher voltage than the coils are designed for, as for
instance 110 volt dynamo and available current 500 volt, caution must be
exercised and a suitable resistance be provided to protect the coils. A
500 volt coil, however, may be supplied from 110 volt circuit, providing
the field winding to be energized is equipped with a cut off switch
having a discharge resistance, so that it may be used to close and break
the circuit when the temporary leads have been connected. If the field
windings be not so provided, a bank of lamps or some other non-inductive
resistance must be connected across the leads between the field magnet
coils and the point at which the circuit is to be opened and closed. This
is to provide a path for the discharge of the induced electromotive force.
The circuit should not remain closed more than a few seconds if the full
voltage can be applied. It is well, however, to leave the current on long
enough to run the machine up to about half speed and make sure, by means
of a voltmeter, that the polarity has been corrected. When this has been
ascertained the dynamo should be stopped and the field winding leads
returned to their proper terminals. Then the voltage will be brought up
in the right direction, provided the work has been done correctly.]
This can be tested by connecting a voltmeter across the terminals
of the armature, or by means of a magnetic needle placed at a short
distance from one of the pole pieces in such a position that it
does not point to the north pole. If the field coils be improperly
connected, the current due to the initial voltage will weaken the
field magnetism and thus prevent the machine building up, and when the
field circuit is closed the voltmeter reading will be reduced, or the
magnetic needle will be less strongly attracted.
Ques. What will be the result if the connections of some of the field
coils of a dynamo be reversed?
Ans. If one-half the number of coils oppose the other half, the field
magnetism will be neutralized and the machine will not build up at all;
but if one of the coils be opposed to the others, the machine might build
up, but the generated voltage will be low, and there will be considerable
sparking at some of the brushes.
Ques. How may it be ascertained which coil is reversed?
Ans. In all dynamos there should be an equal number of positive and
negative poles, and in almost all of them the poles should be alternately
positive and negative. Therefore, if a pocket compass be brought near the
pole pieces, and it show that there are more poles of one kind than the
other, the indication is that one or more of the coils are reversed, and
the improper sequence of alternation will determine which one is wrongly
connected.
Ques. When a dynamo loses its residual magnetism, how can it be made to
build up?
Ans. By temporarily magnetizing the field. To do this a current is passed
through it from another dynamo, or from the cells of a small primary
battery. Usually, this will set up sufficient initial magnetism to allow
the machine to build up. The battery circuit should be broken before the
machine has built up to full voltage.
Ques. What should be done if a dynamo become reversed by a reversal of
its field magnetism due to lightning, short circuit, or otherwise?
Ans. The residual magnetism should be reversed by a current from another
dynamo, or from a battery; but if this be not convenient, the connections
between the machine and the line should be crossed so that the original
positive terminal of the dynamo will be connected to the negative terminal
of the line, and vice versa.
[Illustration: Fig. 679.--Method of correcting reversed polarity in
compound wound dynamo. The polarity may be reversed without disconnecting
or changing the wire. The figure shows two compound dynamos, and essential
connections. The current from any machine connected to the equalizer bar
by its equalizer switch will divide, a portion going through the series
field winding of the other machines connected to the bus bar, the division
being determined by the resistance of the different sets of coils. For
instance, assume that No. 1 dynamo has had its polarity reversed and
that No. 2 is running connected to the bus bar. The method of reversing
the polarity of No. 1 machine is as follows: No. 1 machine should be at
rest and then make sure that the circuit breaker and negative switch
are open and that any other special connections to other machine or
station lighting circuits are open. Then close the positive and equalizer
switches, thus allowing a part of the current from the other dynamo to
pass through the equalizer connection and through the series field winding
of No. 1 machine in the usual direction, which will magnetize the magnetic
core. If No. 1 machine be a large unit and No. 2 a small unit, it will be
necessary to cut out the resistance of the shunt field circuits by means
of the rheostat, if it be desired to maintain its bus bar voltage at its
normal point. This will rob the series winding of any other machines which
may be connected to the bus bars and will lower the voltage slightly. No.
1 machine is then brought up to full speed when it will be found to have
recovered its correct polarity. The positive switch may be readily opened,
watching the bus bar voltage closely as it will rise when the current is
restricted again to the series field winding of the other machines. The
dynamo will then be ready to cut in with the other machines as soon as the
voltage has been brought up to the proper point, or it may be shut down
until required.]
Ques. Can a dynamo be reversed by reversing the connections between the
field coils and the armature?
Ans. No, for if these connections be reversed, the machine will not build
up.
Ques. Will a dynamo build up if it become reversed?
Ans. Yes.
Ques. Then what is the objection to a reversed dynamo?
Ans. Since the direction of current of a reversed dynamo is also reversed,
serious trouble may occur if it be attempted to connect it in parallel,
with other machines not reversed.
Attention while Running.--When a dynamo is started and at work, it will
need a certain amount of attention to keep it running in a satisfactory
and efficient manner. The first point to be considered is the adjustment
of the brushes. If this be neglected, the machine will probably spark
badly, and the commutator and brushes will frequently require refitting
to secure good contact.
Ques. What may be said with respect to the lead of the brushes?
Ans. The lead in all good dynamos is very small, and varies with the load
and class of machine. The best lead to give to the brushes can in all
cases be found by rotating the rocker and brushes in either direction
to the right or left of the neutral plane until sparking commences,
increasing with the movement. The position midway between these two points
is the correct position for the brushes, for at this position the least
sparking occurs, and it is at this position that the brushes should be
fixed by clamping the rocker.
[Illustration: Fig. 680.--Method of taking temperature. In taking the
temperature of a hot part, it is convenient to use a thermometer in which
the scale of degrees has been etched on the stem. Bind this to the heated
part, having first taken the precaution to cover the bulb with waste
to prevent the radiation of heat and take the reading when the column
of mercury has ceased to rise. The question which most often presents
itself to the attendant is how hot can the various parts of a dynamo or
motor become and yet be within the safe limit. The degree of heat can
be determined by applying the hand to the various parts. If the heat be
bearable it is entirely harmless, but if the heat become unbearable to the
hand for more than a few seconds, the safety limit has been reached and
the machine should be stopped and the fault located. Of course when the
solder begins to melt at the commutator connections and shellac begins to
"fry out" of the armature and an odor of burnt cotton begins to pervade
the air, the safe limit has been far exceeded, and in most cases, as a
matter of fact serious damage is the result. To be more definite, _no part
of the dynamo or motor should be allowed to rise in temperature more than
80 degrees F. above the temperature of the surrounding air_, excepting
in the case of commutators where no solder has been used to connect the
leads. These can be allowed to rise to a still higher temperature.]
Ques. How does the lead vary in the different types of dynamo?
Ans. In series dynamos giving a constant current, the brushes require
practically no lead. In shunt and compound dynamos the lead varies with
the load, and therefore the brushes must be rotated in the direction of
rotation of the armature with an increase of load, and in the opposite
direction with a decrease of load.
In cases where the dynamos are subjected to a rapidly varying or
fluctuating load, it is of course not possible to constantly shift
the brushes as the load varies, therefore the brushes should be
fixed in the positions where the least sparking occurs at the moment
of adjustment. If at any time violent sparking occur, which cannot
be reduced or suppressed by varying the position of the brushes
by rotating the rocker, the machine should be shut down at once,
otherwise the commutator and brushes are liable to be destroyed,
or the armature burnt up. This especially refers to high tension
machines.
Ques. What should be done if the brushes begin to spark excessively?
Ans. First, look at the ammeter to see if an excessive amount of current
is being delivered; second, see if the brushes make good contact with the
commutator, and if the latter have a bar too high, or too low, and an open
circuit.
[Illustration: Figs. 681 and 682.--Remedies for leakage of oil from
self-oiling bearings. If there be sufficient space, a metal ring may be
attached to the shaft as in fig. 681. With this arrangement the high speed
of the shaft will carry the oil outside of the ring and throw it off in
the oil reservoir. Another way is to insert a tin apron, as shown in fig.
682 at T, which will serve to drain the oil which may creep along the
shaft, and also cut off the draft from the pulley which may suck the oil
out of the bearing. Sometimes a tin fan is attached to the pulley, which
tends to drive the oil back into the bearing, and which also assists in
keeping the box cool.]
Ques. What should be done if the current be excessive?
Ans. If the current exceed the rated capacity by more than 50 per cent.,
and continue for more than a few minutes, the main switch should be
opened, otherwise the machine may be seriously injured.
Ques. How does an excessive current injure a dynamo?
Ans. By causing it to overheat which destroys the insulation of the
armature, commutator, etc.
Lubrication.--The shaft bearings of dynamos may be lubricated by sight
feed oilers or oil rings. The latter method is almost universally used.
An oil well is provided in the hollow casting of the pedestals as shown
in fig. 728. Oil rings revolve with the shaft and feed the latter with
oil, which is continuously brought up from the reservoir below. The dirt
settles to the bottom and the upper portion of the oil remains clear for
a long period, after which it is drawn off through the spigot and a fresh
supply poured in through openings provided in the top. The latter are
often located directly over the slots in which the rings are placed, so
that the bearings can be lubricated directly by means of an oil cup, if
the rings fail to act or the reservoir become exhausted.
[Illustration: Fig. 683.--Imaginative view of a shaft showing its rough
granular structure. In operation these minute irregularities interlock and
act as a retarding force, or frictional resistance. Hence, the necessity
for lubrication--a lubricant presents a thin intervening film against
which the surfaces rub.]
Ques. What kind of oil can should be used in filling the reservoir, or
oil cups?
Ans. One made of some non-magnetic material such as copper, brass, or zinc.
If iron cans be used, they are liable to be attracted by the field
magnets, and thus possibly catch in the armature.
Ques. What is the indication of insufficient lubrication?
Ans. The bearings become unduly heated.
Ques. What precaution should be taken with new dynamos?
Ans. They are liable to heat abnormally and for the first few days they
should be carefully watched and liberally supplied with oil.
After a dynamo has been running for a short time under full load, its
armature imparts a certain amount of heat to the bearings, a little
more also to the bearing on the commutator end of shaft; beyond
this there is no excuse for excessive heating. The latter may result
from various causes, some of which are given with their remedies, as
follows:
1. A poor quality of oil, dirty or gritty matter in the oil;
2. Journal boxes too tight;
3. Rough journals, badly scraped boxes;
4. Belt too tight;
5. Bearings out of line;
6. Overloaded dynamo;
7. Bent armature shaft.
Ques. What is the allowable degree of heating?
Ans. It may be taken as a safe rule that no part of a working dynamo
should have a temperature of more than 80° Fahr. above that of the
surrounding air.
Accordingly, if the temperature of the engine room be noted before
applying the thermometer to the machine, it can at once be seen if the
latter be working at a safe temperature. In taking the temperature,
the bulb of the thermometer should be wrapped in a woolen rag. The
screws and nuts securing the different connections and cables should
be examined occasionally, as they frequently work loose through
vibration.
[Illustration: Fig. 684.--Diagram illustrating forces acting on a dynamo
armature. In the figure the normal field magneto-motive force is in the
direction of the line 1, 2, produced by the field circuit G, if there were
no current in the armature. But as soon as the armature current flows, it
produces the opposing force 3, 4, which must be combined with 1, 2 to give
the resulting force to produce magnetism and hence voltage. The resultant
1, 5, if 3, 4 be large enough, does not differ much from the original
force 1, 2. Or, expressed in a more physical way, the brushes E, F, rest
on the commutator and all the turns embraced by twice the angle 6, 3, F,
oppose the flow of flux through the armature core as well as all the turns
embraced by twice the angle, 7, 3, E. The remaining turns distort the
flux, making the pole corners at A and B denser, and at C and D rarer. So
that all the effect is to kill an increase of flux, or voltage. This cross
magnetism tends also to decrease the flow of flux, for the extra ampere
turns required to force the flux through the dense pole tips are greater
than the decreased ampere turns relieved by the reduction of flux at the
other pole tips; this follows, since iron as it increases in magnetic
density requires ampere turns greater in proportion than the increase of
flux.]
Instructions for Stopping Dynamos.--When shutting down a machine, the load
should first be gradually reduced, if possible, by easing down the engine;
then when the machine is supplying little or no current, the main switch
should be opened. This reduces the sparking at the switch contacts, and
prevents the engine racing.
When the voltmeter almost indicates zero, the brushes should be raised
from contact with the commutator. This prevents the brushes being damaged
in the event of the engine making a backward motion, which it often does,
particularly in the case of a gas engine. On no account, however, should
the brushes be raised from the commutator while the machine is generating
any considerable voltage; for not only is the insulation of the machine
liable to be damaged, but in the case of large shunt dynamos, the person
lifting the brushes is liable to receive a violent shock.
Ques. What attention should the machine receive after it has been shut
down?
Ans. It should be thoroughly cleaned. Any adhering copper dust, dirt,
etc., should be removed from the armature by dusting with a stiff brush,
and the other portions of the machine should be thoroughly cleaned with
linen rags. Waste should not be used, as it is liable to leave threads or
fluff on the projecting parts of the machine, and on the windings of the
armature, which is difficult to remove.
Ques. What attention should be given to the brushes and brush gear?
Ans. They should be examined and thoroughly cleaned. If necessary the
brushes should be refitted and readjusted. All terminals, screws, bolts,
etc., should be carefully cleaned and screwed up ready for the next run.
The brush holders should receive special attention, as when dirty, they
are liable to stick and cause sparking. All dirt and oil should be removed
from the springs, contacts, pivots, and other working parts.
It is advisable at stated intervals to entirely remove the brush
holders from the rocker arms, and give them a thorough cleaning by
taking them to pieces, and cleaning each part separately with emery
cloth and benzoline or soda solution.
Another point to which particular attention should be given is the
cleaning of the brush rocker. This being composed wholly of metal, and
the two sets of positive and negative brushes being only separated
from it by a few thin insulating washers, it follows that if any
copper dust given off by the brushes be deposited in the neighborhood
of these washers, there is considerable liability for a short circuit
of the machine to occur by the dust bridging across the insulation.
Ques. What further attention should be given?
Ans. It is a good plan, when the machine has been thoroughly cleaned and
all connections made secure, to occasionally test the insulation of the
different parts. If a record be kept of these tests, any deterioration of
the insulation can at once be detected, localized and remedied before it
has become sufficiently bad to cause a breakdown.
As a means of protecting the machine from any moisture, dirt, etc.,
while standing idle, it is advisable to cover it with a suitable
waterproof cover.