CARE OF THE COMMUTATOR AND BRUSHES
For satisfactory operation, the brushes and commutator must be kept
in good condition. To this end the main thing to be guarded against is
the production of sparks at the brushes. If care be taken in the first
instance to adjust the brushes to their setting marks, and to regulate
their pressure upon the commutator, and afterwards to attend to the lead
as the load varies, so that little or no sparking occurs, and also to keep
the brushes and commutator free from dirt, grit, excessive oil, etc., the
surface of the commutator will assume a dark burnished appearance and wear
will practically cease. Under these circumstances the commutator will run
cool, and will give very little trouble.
In order to maintain these conditions it will only be necessary to see
that the brushes are kept in proper condition and fed forward to their
setting marks, as they wear away, and that the commutator is occasionally
polished.
If the pressure of the brushes upon the commutator be too great, or
their adjustment faulty, or the commutator be allowed to get into a
dirty condition, sparking will result, and, if not at once attended to
and remedied, the brushes will quickly wear away, and the surface of the
commutator will be destroyed. As this action takes place, in the earlier
stages, the surface of the commutator will become roughened or scored,
resulting in jumping of the brushes, and increased sparking; in the later
stages, the commutator will become untrue and worn into ruts, moreover,
owing to the violent sparking which takes place through this circumstance,
the machine will quickly be rendered useless[D].
[D] NOTE.--In operating dynamos having metal brushes, it is of importance
to keep the commutator smooth and glossy. To accomplish this, it is
necessary to keep the commutator and brushes clean and free from grit,
and to occasionally lubricate the commutator with some light oil, such
as ordinary machine oil. This should be done daily if the machine be
in constant use. Keep the brushes resting upon the commutator with just
enough pressure to insure a good firm contact. This will be found to be
much less than the springs are capable of exerting. A good method to
follow in cleaning the machine is as follows: Loosen the brush holder
thumb screws and tilt the brushes off the commutator (or, if box brush
holders be used, take them out of their holders). Then run the machine and
hold a clean cloth against the commutator. After the commutator is clean,
hold against it a cloth or piece of waste moistened with machine oil and
reset the brushes. If for any reason the brushes begin to cut or score
the commutator, it may be readily detected by holding the finger against
the commutator; the ridge may be easily felt by the finger. This should
be attended to at once in the following manner: Tilt back the brushes (or
if box brushes are used take them out of their holders), and hold lightly
against the commutator a piece of No. 00 sandpaper well moistened with
oil, passing it back and forth until the surface is perfectly smooth. Then
wipe off the commutator with a clean piece of cloth or waste and lubricate
with another clean piece moistened with oil and reset the brushes.
Ques. How is the commutator easily tested as to the condition of its
surface?
Ans. It is readily tested by resting the back of the finger nail upon
it while in motion; the nail being very sensitive to any irregularities,
indicates at once any defect.
Ques. What causes grooves or ridges to be cut in the commutator?
Ans. They result from using brushes with hard burnt ends which are not
pliable; also by too great a pressure of the brush upon the commutator
surface.
Sparking at the brushes is expensive and detrimental, chiefly
because it results in burning the brushes and also the commutator,
necessitating their frequent renewal. Every spark consumes a particle
of copper, torn from the commutator or brush. The longer the sparking
continues, the greater the evil becomes, and the remedy must be
applied without delay.
Ques. What kind of oil should be used on the commutator?
Ans. Mineral oil.
Ques. What attention should be given to the brushes?
Ans. At certain intervals, according to the care taken to reduce sparking
and the length of time the machine runs, the brushes will fray out or wear
unevenly, and will therefore need trimming. They should then be removed
from the brush holders and their contact ends or faces examined. If not
truly square, they should be filed or clipped with a pair of shears, the
course of treatment differing with the type of brush.
If the machine be fitted with metal strip brushes, frayed ends should
be clipped square with a pair of shears, the ends thoroughly cleaned
from any dirt or carbonized oil, and replaced in their holders. Gauze
and wire brushes require a little more attention. When their position
on the commutator has been well adjusted and looked after, so that
little or no sparking has taken place, it is generally only necessary
to wipe them, clean the brushes and clip off the fringed edges and
corners with the shears, or a pair of strong scissors. If, however,
the machine has been sparking, the faces will be worn or burnt away,
and probably fused. If such be the case, they will need to be put in
the filing clamp, and filed true.
A convenient method of trimming carbon brushes, or of bedding a
complete new set of metal brushes, is to bind a piece of sandpaper,
face outwards, around the commutator after the current has been
shut off, and then mount the carbon or metal brushes in the holders,
adjusting the tension of the springs so that the brushes bear with a
moderately strong pressure upon the sandpaper. Then let the machine
run slowly until the ends of the brushes are ground to the proper
form. Care should be taken, however, that the metal dust given off
does not get into the commutator connections or armature windings, or
short circuiting will result.
If the contact faces of the brushes are very dirty and covered with
a coating of carbonized oil, etc., it will be necessary to clean them
with benzoline or soda solution before replacing.
[Illustration: Fig. 710.--Bissell brush gear. The brushes are held in
the brushholders radially and work equally well with armature running in
either direction. Brushes can be renewed and adjustment made while machine
is in operation.]
Ques. Describe a filing clamp.
Ans. As usually constructed, it consists of two pieces of metal, both
shaped at one end to the correct angle, to which the brushes must
be filed. One of the pieces of metal (the back part) has a groove
sufficiently large to accommodate the brush, which is clamped in position
by the other piece of metal and a pinching screw.
If the clamp be not supplied with the machine a convenient substitute
can be made out of two pieces of wood about the same width as the
brush. One end of each piece is sawn to the correct angle, and the
brush placed between the two.
In filing, the brush is fixed in the clamp, with the toe or tip
projecting slightly over the edge of the clamp, and the latter being
fixed in a vise, the brush is filed by single strokes of a smooth file
made outwards, the file being raised from contact with the brush when
making the back stroke.
[Illustration: Fig. 711.--Jig for filing brushes to the correct bevel;
used with copper brushes to fit them to the commutator.]
Sparking.--In all well designed machines there are certain positions upon
the commutator for the brushes at which there will be no sparking so long
as the commutator is kept clean and in good condition. In other dynamos,
badly designed or constructed, sparking occurs at all positions, no
matter where the brushes are placed, and in such dynamos it is therefore
impossible to prevent this no matter how well they are adjusted.
[Illustration: Fig. 712.--Commutator clamp; a useful device for holding
the segments firmly in position in taking out the end rings of the
commutator to repair for internal grounds. It is made of 2 × 1/8 inch
sheet steel, with a 1/2 inch screw. The illustration clearly shows the
adjustable fastening. The notches fit around rivets on one side of each
fastening, which can be moved by removing the two cotters. The clamp is
made loose or taut by screwing the bolt in the nut.]
Ques. What two kinds of sparking may be generally distinguished?
Ans. One kind of sparking is that due to bad adjustment of the brushes,
and a second kind, that due to bad condition of the commutator.
Sparks due to bad adjustment of the brushes are generally of a bluish
color, small when near the neutral plane, and increasing in violence
and brilliancy as the brushes recede from the correct positions upon
the commutator.
When sparks are produced by dirty or neglected state of the
commutator, they are distinguished by a reddish color and a
spluttering or hissing. When due to this last mentioned cause, it is
impossible to suppress the sparking until the commutator and brushes
have been cleaned. In the former case, the sparks will disappear as
soon as the brushes have been rotated into the neutral points.
Another class of sparks appear when there is some more or less
developed fault, such as a short circuit, or break in the armature or
commutator. These are similar in character to those produced by bad
adjustment of the brushes, but are distinguished from the latter by
their not decreasing in violence when the brushes are rotated towards
the neutral plane.
Having distinguished the classes of sparks which appear at the commutator
of a dynamo, it remains to enumerate the causes which produce them. These
are:
1. Bad adjustment of brushes;
2. Bad condition of brushes;
3. Bad condition of commutator;
4. Overload of dynamo;
5. Loose connections, terminals, etc.;
6. Breaks in armature circuit;
7. Short circuits in armature circuit;
8. Short circuits or breaks in field magnet circuit.
Bad Adjustment of Brushes.--When sparking is produced by bad adjustment
of the brushes, it may be detected by rotating or shifting the rocker, by
the indication that the sparking will vary with each movement.
To obtain good adjustment of the brushes, it will be necessary to rock
them gently backwards and forwards, until a position is found in which
the sparking disappears.
Ques. If, in rocking the brushes, a position cannot be found at which the
sparking disappears, what is the probable cause of the trouble?
Ans. The brushes may not be set with the proper pitch, that is they may
not be separated a correct distance, or the neutral plane may not be
situated in the true theoretical position upon the commutator through some
defect in the winding, etc.
In this last named case, the brushes may be strictly adjusted to their
theoretically correct positions before starting the machine; then,
when the machine is started and the load put on, violent sparking
occurs, which cannot be suppressed by shifting the rocker. If,
however, one set of brushes only be observed, it will generally be
found that, at a certain position, the sparking at the set of brushes
under observation ceases or is greatly reduced, while sparking still
occurs at the other set. When this position is found, the rocker
should be fixed by the clamping screw, and the brushes of the other
set at which sparking is still occurring adjusted by drawing them back
or pushing them forward in their holders until a position is found
at which the sparking ceases. Correct position of the brushes and
the suppression of sparking is a matter of importance, and any time
spent in carefully adjusting the brushes will be amply repaid by the
decreased attention and wear of the brushes and commutator.
[Illustration: Figs. 713 to 715.--Brushes making bad contact. A brush
making a bad contact, as only at the shaded portion of figs. 713 and 714,
will not allow the short circuited coil enough time to reverse, causing
sparking and heating. The latter will also result from bad contact on
account of the surface being too small for the current to be carried off.
This form of bad contact is worse than that shown in fig. 715, where the
area of contact surface only is lessened. If the brushes do not make good
contact, they should be ground down.]
Bad Condition of Brushes.--If the contact faces of the brushes be fused or
covered with carbonized oil, dirt, etc., there will be bad contact which
is accompanied by heating and sparking. Simple examination will generally
reveal whether this be the case. The remedy is to remove the brushes, one
at a time if the machine be running, clean, file if necessary, trim, and
readjust.
If the brushes be exceedingly dirty, or saturated with oil, it will be
necessary to clean them with turpentine, benzoline, or soda solution,
before replacing.
Bad Condition of Commutator.--If the surface of the commutator be rough,
worn into grooves, or eccentric, or if there be one or more segments
loose or set irregularly, the brushes will be thrown into vibration,
and sparking will result. A simple examination of the commutator will
readily detect these defects. A rough and uneven commutator is due to bad
adjustment of brushes, bad construction of commutator, and to neglect
generally. If allowed to continue, it results in heavy sparking at the
brushes, and the eventful destruction of the commutator. The fault may be
remedied by filing or re-turning the commutator.
[Illustration: Fig. 716.--Rough and grooved commutator due to improper
brush adjustment and failure to keep brushes in proper condition.]
Ques. How is an untrue commutator detected?
Ans. If the commutator be untrue, the fact will be indicated when the
machine is slowed down by a visible eccentricity, or by holding the hand,
or a stick in the case of a high tension machine, against the surface
while revolving, when any irregularity or eccentricity will be apparent
by the vibration or movement of the stick. The only remedy for an untrue
commutator is to re-turn it in the lathe.
Ques. What should be done in case of high segments?
Ans. They should be gently tapped down with a mallet, and if possible the
clamping cones at the commutator end should be tightened.
If it be impossible to hammer the segments down, they should be
filed down to the same diameter as the rest of the commutator, or the
commutator re-turned. For low segments, the only remedy is to pull
out the segments, or turn commutator down to their level.
Ques. Explain the term "flats on the commutator."
Ans. This is the name given to a peculiar fault which develops on one or
more segments of the commutator. It is not confined to dynamos of bad
design or construction, but frequently appears on those of the highest
class, and may be recognized as a "pitting" or "flattening" of one or more
segments.
Ques. What is the effect of flats on the commutator?
Ans. Sparking at the brushes.
Ques. What are the causes which produce flats?
Ans. Periodical jumping of the brushes due to a bad state of the
commutator, bad joint in the driving belt, a flaw, or a difference in the
composition of the metal of the particular bar upon which it appears. But
more frequently flats may be traced to a more or less developed fault,
such as a break, either partial or complete, in the armature coil.
The break may occur either in the coil itself, or at the point where
its ends make connection with the lug of the commutator, or at the
point where the lug is soldered to the segment.
Ques. What should be done in case of flats?
Ans. The brushes should be examined to see if any periodical vibration
take place. If such be the case, the cause should be removed, the flat
carefully filed or turned out, and the brushes readjusted.
If it be due to a difference in the composition of the metal of which
the segment is made, the flat will exist as long as the particular
segment is in use, and will need periodic attention.
With hard drawn copper or phosphor bronze segments, this fault is
rarely due to this last mentioned cause. It is more frequently due to
bad soldering, of the conductors to the lugs, or of the lugs to the
segments. In all cases of flats, if the disconnection in the armature
circuit be not complete, and cannot be readily located, the effect of
re-soldering or sweating the ends of the coils into the lugs should
be tried. Flats may also frequently be cured by drilling and tapping
a small hole in the junction between the lug and the segment, and
inserting a small screw, or bit of screwed copper or brass wire,
afterwards filing down level with the surface of the commutator.
[Illustration: Figs. 717 and 718.--Method of repairing broken joint
between commutator segment and lug. To repair such a break push asbestos
in between adjacent bars, so that heat from the torch will not affect
them. Asbestos should also be worked in at the back if possible, for
the purpose of keeping solder from places where it would cause trouble.
Then unsolder the armature leads from the lug and remove the latter.
Next, with specially made cape chisels, cut in a slot in the commutator
bar for a new lug. Care and skill are required not to destroy the mica
insulation between the segments. The slot should be cut one-quarter to
three-eighths inch deep. The connector is then soldered in place. With
care a satisfactory connection can be made in this way, which will last
well. If it do not last, the trouble in almost every case is due to poor
soldering. Short circuits sometimes occur after this operation, because
of solder falling in at the back and lodging on lower connections. In
large machines, the excessive current flowing is quite likely to melt this
solder, and the machine may buck, throwing out the melted solder, after
which it may be all right again. While the bar connector is out, however,
asbestos should be packed in back of it to prevent this occurrence, which
may be a serious affair. All surplus solder and the asbestos packing
should be removed after the connection is finished, and the connections
cleaned with compressed air. The armature should be turned over slowly,
air being applied all the while.]
Segments Loose or Knocked In.--When the segments are loose, it is an
indication that the clamping ring or cone has worked loose. This should
therefore be tightened up, and the commutator re-turned if necessary.
Ques. How should low commutator segments be treated?
Ans. The commutator surface may be turned down to the level of the low
segment, or the latter may be pulled out again to its former level, this
latter being the preferable method, if it can possibly be effected.
Ques. How is a commutator segment pulled out to its correct position?
Ans. A hand vise is firmly clamped to the lug, or a loop of copper wire is
passed round the conductor where it joins the commutator. A bar of iron,
to act as a lever, is supported on a fulcrum over the commutator, and one
end of the bar is passed through the loop or vise. Pressure is applied
to the other end which will generally bury the segment up to its proper
position.
How to Re-turn a Commutator.--In re-turning the commutator, the armature
should first be carefully taken out of the armature chamber, avoiding
knocks or blows of any kind. The whole of the winding should then be
wrapped in calico or canvas before the armature is put into the lathe,
to prevent any particles of metal becoming attached to the surface of the
armature at the time the commutator is being turned. The armature should
on no account be rolled upon the floor, or subjected to blows or knocks
while being put into the lathe.
In re-turning the commutator, a sharp pointed tool should be used
with a very fine feed. A broad nosed tool ought not to be used, as
it is liable to burr over the segments. After turning, the commutator
should be lightly filed with a dead smooth file, and finally polished
with coarse and fine sandpaper. After the commutator has been turned
and polished, the insulation between the segments should be lightly
scraped with the tang of a small file to remove any particles of metal
or burrs which might short circuit the commutator.
The points where the armature wires are soldered to the lugs should
also be carefully cleaned with a brush, and should then receive a coat
or two of shellac varnish.
While the commutator is being turned, care should be taken that the
setting marks for the adjustment of the brushes are not turned out if
these be present. The same care should be used in putting the armature
back into the armature chamber as was used in taking it out, otherwise
the insulation may be damaged.
[Illustration: Figs. 719 and 720.--Bissell commutators. The segments are
of hard drawn copper and are insulated from each other and from the shell
by mica.]
Ques. Should the commutator be run without any lubricant?
Ans. In most cases it will be found that a little lubricant is needed
in order to prevent cutting the brushes, cutting the commutator; this is
especially the case when hard strip brushes are used. The quantity of oil
applied should be very small; a few drops smeared upon a piece of clean
rag, and applied to the commutator while running, being quite sufficient.
Ques. What kind of oil should be used on the commutator?
Ans. Mineral oil, such as vaseline, or any other hydrocarbon. Animal or
vegetable oils should be avoided, as they have a tendency to carbonize,
and thus cause short circuiting of the commutator, with attendant
sparking.
[Illustration: Figs. 721 to 723.--Method of repairing a large hole burned
in two adjacent bars of a commutator. Fig. 721 shows the hole. The first
operation is to clean carefully and tin the surface of the hole. The two
bars are then wedged apart and mica strips, A B, fig 722, of the requisite
size and thickness forced in. The commutator must now be warmed up as much
as possible by means of soldering irons, and strips of mica, C D, E F,
fig. 723, placed at the front and back of the hole, being kept in position
by pieces of wood W, solder is poured into the hole from a ladle, using
a rough mica funnel to guide it.]
Overload of Dynamo.--It may happen, through some cause or other that a
greater output is taken from the machine than it can safely carry. When
this is the case, the fact is indicated by excessive sparking at the
brushes, great heating of the armature and other parts of the dynamo, and
possibly by the slipping of the belt (if it be a belt driven machine),
resulting in a noise. The causes most likely to produce overload are:
1. Excessive voltage;
2. Excessive current;
3. Reversal of polarity of dynamo;
4. Short circuits or grounds in dynamo, or external circuits.
Ques. What is the indication of excessive voltage?
Ans. It is indicated by the voltmeter, or by the brilliancy of the pilot
lamp.
[Illustration: Fig. 724.--Method of smoothing commutator with a stone. The
proper stone to use is made out of white sandstone similar to that used
for grindstones, but a trifle softer. It is dove-tailed into a holder,
as shown in the illustration, and held in place by a set screw. When
being used, one knob is grasped in one hand and the other knob in the
other hand, the stone being moved back and forth along the length of the
commutator. As the stone will become coated with copper at first, it must
be cleaned frequently by means of coarse sandpaper. The fine dust from
the stone will get under the brushes and wear them to a very close fit.
After using the stone, finish with fine sandpaper.]
Ques. What are the causes of excessive voltage?
Ans. Over excitation of the field magnet or too high speed.
In the former case, resistance should be introduced into the field
circuit to diminish the current flowing therein if a shunt machine; or
if a series machine, a portion of the current should be shunted across
the field coils by means of a resistance arranged in parallel with
the series coils; or the same effect may be produced in both cases by
reducing the speed of the armature if this be possible.
If due to excessive speed, which will be indicated by a speed
indicator, the natural remedy is to reduce the speed of the engine
driving the dynamo, or, if this be not easily done, insert resistance
into the dynamo circuit, as described above.
Ques. What are the causes of excessive current?
Ans. If the dynamo be supplying arc lamps, the excessive current may
possibly be caused by the bad feeding of the lamps. If this be the case,
the fact will be indicated by the oscillations of the ammeter needle, and
the unsteadiness of the light.
If incandescent lamps be in the circuit, the fault may be caused
by there being more lamps in circuit than the dynamo is designed to
carry. Under such circumstances, another dynamo should be switched
into circuit in parallel, or, if this be not possible, lamps should
be switched off until the defect is remedied.
When motors are in the circuit, sparking frequently results at the
dynamo commutator, owing to the fluctuating load. In such cases the
brushes should be adjusted to a position at which the least sparking
occurs with the average load.
Ques. What may be said with respect to reversal of polarity of dynamos?
Ans. When compound or series wound dynamos are running in parallel, their
polarity is occasionally reversed while stopping by the current from the
machines at work.
Loose Connections, Terminals, etc.--When any of the connecting cables,
terminal screws, etc., securing the different circuits are loose, sparking
at the brushes, as a rule, results, for the reason that the vibration
of the machine tends to continually alter the resistance of the various
circuits to which they are connected.
When the connections are excessively loose, sparking also results at their
points of contact, and by this indication the faulty connections may be
readily detected. When this sparking at the contacts is absent, the whole
of the connections should be carefully examined and tested.
Breaks in Armature Circuit.--If there be a broken circuit in the armature,
as sometimes happens through a fracture of the armature connections, etc.,
there will be serious flashing or sparking at the brushes, which cannot be
suppressed by adjusting the rocker. As a rule it results in the production
of "flats" upon one or more bars of the commutator.
[Illustration: Fig. 725.--Sandpaper holder for commutator. The sandpaper
is made fast on top by a clamp and screw. The two face blocks are
pivoted and adjust themselves to the commutator, and will fit any size of
commutator. If it have four brushes, the lower block will go in between
the brush-holders.]
Ques. How may such sparking be reduced without stopping the machine?
Ans. By placing one of the brushes of each set a little in advance of the
others, so as to bridge the gap.
Short Circuits in Armature Circuit.--This fault is indicated by sparking
at the commutator, and in bad cases by an excessive heating of the
armature, dimming of the light and slipping of the belt, and in the case
of a drum armature, by a sudden cessation of the current.
[Illustration: Fig. 726.--Sandpaper block. It is made to fit the surface
of the commutator. At S is a saw cut into which the ends are pushed after
being wrapped around the block. The latter should be cut down on the
dotted lines to form a handle.]
Short Circuits or Breaks in Field Magnet Circuit.--Either of these faults
is liable to give rise to sparking at the commutator. If one of the
coils be short circuited, the fact will be indicated by the faulty coil
remaining cool while the perfect coil is overheated. The fault may arise
through some of the connections to the coils making contact with the
frame of the machine or with each other. To ascertain this, examine all
the connections, and test with a battery and galvanometer. A total break
in one or more of the field coils may readily be detected by means of the
battery and galvanometer.
A partial break is not, however, so readily discovered, for the
reason that the coil wires may be in sufficiently close contact to
give a deflection of the galvanometer needle. The only methods of
detecting this fault is by measuring the resistance of the coils with
an ohmmeter or Wheatstone bridge, or by placing an ammeter in circuit
with each coil in turn, and comparing the amount of current flowing in
each. If the partial break be not accessible, the only way to remedy
the fault is to rewind the coil, and the same applies to a break in
the interior of the coil.
Short Circuits in Commutator.--These are of frequent occurrence, and
result in heating the armature and sparking at the brushes. They are
caused either by metallic dust or particles lodging in the insulation
between the segments, or by the deterioration of the commutator
insulation.
To remedy, the insulation between the segments should be carefully
examined, and any metallic dust, filings, or burrs cleaned or scraped
out. When the commutator is insulated with asbestos or pasteboard (as
is oftentimes the case in dynamos of European make), short circuits
very frequently occur through the insulation absorbing moisture or
oil, which is subsequently carbonized by the sparking at the brushes.
In faults of this description the only remedy is to expel all moisture
from the commutator insulation by means of heat, and scrape out all
metallic dust which may be embedded in the surface of the insulation.
If this do not effect a cure, it will be necessary to dig out the
insulation, as far as possible, with a sharp tool, and drive in new
insulation. Oil should not be used on commutators insulated with these
materials, but only asbestos dust or French chalk.