ARMATURE TROUBLES
A large proportion of the mishaps and breakdowns which occur with dynamos
and motors arise from causes more strictly within the province of the man
in charge than in that of the designer. The armature, being a complex and
delicately built structure, is subject in operation to various detrimental
influences giving rise to faults.
Many of the faults which occur are avoided by operators better informed as
to the electric and magnetic conditions which obtain in the running of the
machine, especially the mechanical stresses on the copper inductors due
to the magnetic field and the necessity of preserving proper insulation.
The chief mishaps to which armatures are subject are as follows:
1. Short circuits;
a. In individual coils;
b. Between adjacent coils;
c. Through frame or core;
d. Between sections of armature;
e. Partial short circuits.
2. Grounds;
3. Breaks in armature circuit.
Short Circuit in Individual Coils.--This is a common fault, which makes
its presence known by a violent heating of the armature, flashing at the
commutator, flickering of the light on lighting circuits, and by a smell
of burning varnish or overheated insulation. When these indications are
present, the machine should be stopped at once, otherwise the armature is
liable to be burnt out. The fault is due either to metallic dust lodging
in the insulation between adjacent bars of the commutator, or to one or
more convolutions of the coils coming into contact with each other, either
through a metallic filing becoming embedded in the insulation or damage
to the insulation.
[Illustration: Fig. 700.--Method of locating short circuited armature
coil. Disconnect the external and field circuits from the armature, and
pass a large current--say from 20 to 100 amperes--from a battery (B) or
another dynamo through the whole armature by means of the brushes. Then,
having previously well cleaned the commutator, measure the difference of
potential between adjacent segments all round the commutator (C), by means
of a voltmeter or galvanometer (G), the terminals of which are connected
to adjacent segments, as shown. The short circuited coil or coils will
be located by the difference of potential between the corresponding
segments being little or nothing. It may be remarked, however, that this
is not always a decisive test. In some cases the short circuit may be
intermittent, or may disappear as soon as the armature ceases to rotate.
In such cases, the short circuit is caused by the wire coming into contact
through the action of the centrifugal forces developed by the rotation of
the armature.]
Ques. How is the faulty coil located?
Ans. When the machine is stopped, the faulty coil, if not burnt out, can
generally be located by the baked appearance of the varnish or insulation,
and by its excessive temperature over the rest of the coils, being
detected also by the baked appearance of the varnish or insulation.
Ques. What should be done if the machine do not build, and it be suspected
that the fault is due to short circuited armature coils?
Ans. The field magnets should be excited by the current from a storage
battery or another dynamo, and, having raised the brushes from contact
with the commutator, the armature should be run for a short time. In
stopping, the faulty coil or coils may be located by the heat generated
by the short circuit.
When the dynamo is started for the purpose of localizing a short
circuit, precautions should be taken, and the machine only run for a
few minutes at a time until the faulty coil is detected.
When the faulty coil has been located, the insulation between the
segments of the commutator to which its ends are connected should be
carefully examined for anything that may bridge across from segment
to segment, and scraped clean. If the commutator be apparently all
right, the fault probably lies in the winding. The insulation of the
winding should be carefully examined, and any metallic filings or
other particles discovered therein carefully removed, and a little
shellac varnish applied to the faulty part.
[Illustration: Fig. 701.--Test for break in armature lead. Clean the
brushes and commutator, and apply current from a few cells of battery
having a telephone receiver in circuit as shown in the figure. If the
machine have more than two brushes, connect the leads to two adjoining
brushes and raise the others. Now rotate the armature slowly by hand and
there will be a distinct click in the receiver as each segment passes
under the brushes until one brush bears on the segment at fault, when the
clicking will cease. In making this test, the brushes must not cover more
than a single segment.]
Ques. If the insulation on adjacent conductors has been abraded, how
should it be repaired?
Ans. A small boxwood or other hardwood wedge, coated with shellac varnish
should be driven in tightly between the wire; this will generally be
sufficient.
[Illustration: Fig. 702.--Bar to bar test for open circuit in coil or
short circuit in one coil or between segments. If, in testing as in fig.
701, on rotating the armature completely around, the receiver indicate no
break in the leads, connect the battery leads directly to the brushes, as
shown in the above figure, and touch the connections from the receiver
to two adjacent bars, working from bar to bar. The clicking should be
substantially the same between any two commutator bars; if the clicking
suddenly rise in tone between two bars, it indicates a high resistance in
the coil or a break (open circuit).]
Ques. If a faulty coil cannot be quickly repaired and the dynamo be
needed, what should be done?
Ans. The coil may be cut out of circuit, and the corresponding commutator
segments connected together with a piece of wire (of a size proportionate
to the amount of current to be carried), soldered to each. It will not be
necessary to cut out and remove the entire coil.
If the active portions only be separated so that they do not form a
closed circuit, it will answer the purpose. If the wires be cut with
a chisel at the point where they pass over the ends of the core, and
the ends separated, it will be quite as effective as removing the
entire coil. It is wise, of course, to rewind the coil at the first
opportunity.
[Illustration: Fig. 703.--Alternate bar test for short circuit between
sections. Where two adjacent commutator bars are in contact, or a
coil between two segments becomes short circuited, the bar to bar test
described in fig. 702 will detect the fault by the telephone receiver
remaining silent. If a short circuit be found, the leads from the receiver
should then include or straddle three commutator bars, as here shown. The
normal click will then be twice that between two segments until the faulty
coils are reached, when the clicking will be less. When this happens,
test each coil for trouble and, if individually they be all right, the
trouble is between the two. To test for a ground place one terminal of
the receiver on the shaft or frame of the machine, and the other on the
commutator. If there be a click it indicates a ground. Move the terminal
about the commutator until the least clicking is heard and at or near that
point will be found the contact. Grounds in field coils can be located in
the same manner.]
Short Circuits between Adjacent Coils.--In ring armatures the presence of
this fault does not necessarily imply that the machine will not build;
in drum armatures, wound into a single layer of conductors, it entirely
prevents this occurring.
Reference to a winding diagram will show that adjacent coils are during
a certain period of the revolution at the full difference of pressure
generated by the machine. Hence, if any two adjacent coils be connected
together or short circuited, the whole of the armature will be practically
closed on itself, any current generated flowing within the armature only.
Large drum armatures wound with compressed and stranded bars and
connectors are particularly susceptible to this fault, a slight blow
generally forcing one or more of the strands into contact with the
adjacent bars, thus short circuiting the armature, and rendering it
practically useless so far as the generation of current is concerned.
In this class of short circuit in drum armatures, the method of locating
the faulty coils by exciting the field, and running the armatures on open
circuit, does not apply, for the reason that the whole armature will be
heated equally.
A method of locating such fault is illustrated in fig. 704. This
applies to drum wound armatures. Faults of this description can
frequently be discovered by a careful inspection of the windings of
the armature without recourse to testing. When located, the fault
can usually be repaired with a hardwood wedge, as explained above,
or a piece of mica or vulcanized fibre cemented in place with shellac
varnish.
[Illustration: Fig. 704.--Method of locating short circuits between
adjacent armature coils. Fasten a monkey wrench to the rim of the pulley,
or a crank to the shaft. Now, excite the fields, and, to make the effects
more marked, connect the coils in parallel. When this has been done it
will require considerable force to rotate the armature, and then it will
move quite slowly, except at one position. When this position has been
found, mark the armature at points in the center of the pole pieces at
points A and B and at both ends of the armature. The explanation is that
both halves of the armature oppose one another at this position; but when
not at these points a continuous circuit is formed, and the resultant
magnetic effect is considerable. The "cross" or "short" circuit is nearly
always found on the commutator end in the last half of the winding, where
the wires pass down through the first half terminals. This applies to
an unequal winding. In armatures where the windings are equal, it is as
liable to occur at one point as at another. With this method a defect can
be found and remedied in a few moments, for it has always been a simple
matter to repair it when discovered. These results can be observed in a
perfect armature by connecting the opposite sections of the commutator.]
Short Circuits between Sections through Frame or Core of
Armature.--Detection of this fault can be effected by the methods
described above, and by disconnecting the whole of the armature coils
from the commutator and from each other, and testing each separately
with a battery and galvanometer coupled up as in fig. 705, one wire being
connected to the shaft and the other to the end of the coil under test.
As a rule, there is no way of remedying this fault other than unwinding
the defective coils, reinsulating the core, and rewinding new coils.
[Illustration: Fig. 705.--Method of locating short circuits between coils
through armature core. The galvanometer, battery and coil to be tested
are connected in series as shown, and then the unconnected terminal of the
galvanometer is brought into contact with the shaft. If then some portion
of the insulation of the wire has been abraded or destroyed, thus bringing
the bare wire into contact with the metal core, as at A in the figure,
the needle of the galvanometer will be deflected since a closed circuit is
formed through the core and wire. If the insulation be perfect, the needle
will not be deflected. It will thus be seen that in the conductivity test
(fig. 700) it is necessary that the needle should be deflected, or turned,
to prove that all is right, while in the insulation test the converse
holds good; if the needle be deflected, it proves that the insulation is
broken down.]
Short Circuits between Sections through Binding Wires.--This fault is
the result of a loose winding, and is caused by the insulation upon which
the binding wires are wound giving way, thus bringing coils at different
pressures together. As a consequence of the heavy current which flows,
the binding wires are as a rule unsoldered or burned. The location of the
fault can therefore be effected by simple inspection. To remedy, it will
be necessary to unwind and rewind on new binding wires, on bands of mica
or vulcanized fibre, soldering at intervals to obviate flying asunder.
Partial Short Circuits in Armatures.--This is usually due to the presence
of moisture in the windings. To remedy the fault, the armature should be
taken out and exposed to a moderate heat, or subjected to a current equal
to that ordinarily given by the dynamo. Under the action of heat or of
this current the moisture will be gradually dispersed. When thoroughly
dry, and while still warm, a coat of shellac should be applied to the
whole of the windings.
[Illustration: Fig. 706.--Method of testing for breaks. The instruments
are connected as shown. B is the battery, G the galvanometer, and S the
coil of wire being tested. One terminal of the battery is connected to a
terminal of the galvanometer, and the other to one of the ends of the coil
under test. The other terminal of the galvanometer is connected to the
other end of the coil. If the connecting wires be making good electrical
contact with the respective terminals, and the wire of coil being tested
be unbroken, the needle of the galvanometer will be deflected as soon
as a closed circuit is made by the end of the coil coming into contact
with the galvanometer terminal. If the wire of the coil be broken in some
part or the ends of the connecting wires do not make good electrical
contact with the terminals, the needle will not be deflected. In order
to prevent mistakes, it is advisable to test the battery and galvanometer
connections and contacts by short circuiting or bringing the ends of the
wire connecting the terminal of the galvanometer and negative pole or
the battery together before starting to test the circuit or coil. If the
needle be deflected, the connections are all right; if not deflected,
there is a bad contact somewhere, which must be made good before the test
can proceed.]
Burning of Armature Coils.--The reason for the burning of an armature
coil may be explained as follows: The coil, segments, and the short
circuit between the segments form a closed circuit of low resistance so
that it is only necessary to have a low pressure set up in the active
portion of the coil to force a very large current through the coil and
the short circuited commutator bars. The heating effect of this current
is sufficient to burn out the coil.
[Illustration: Fig. 707.--Watson field coils. Automatic machinery is
employed to wind these coils; after winding, they are bound with tape,
then baked to expel all moisture, and while hot, are saturated with
an insulating compound and again baked for twelve hours to make them
practically oil and water proof. Heavy flexible leads are brought out to
avoid danger of breaking or other damage.]
Cutting Out Damaged Armature Coils.--To cut out a damaged coil from
an armature, first, disconnect the coil from the commutator, and after
cutting off the leads, insulate the exposed parts with tape. Then connect
the commutator bars (which were connected with the leads) with a wire of
the same size as the wire winding.
To remove the coil entirely, cut the band wire or remove the wedges, and
lift up a sufficient number of leads and coils to permit of the removal
of the damaged coil.
Grounds in Armatures.--These faults occur when the armature coils become
connected to the frame or core of the armature. When this grounding is
confined to a single coil, it is not in itself liable to do damage. A
simple method of locating a grounded coil is illustrated in fig. 708.
[Illustration: Fig. 708.--Method of locating grounded armature coil. B
is a battery or dynamo circuit giving a current of a few amperes through
the armature by its own brushes (1 and 2). At G is placed a roughly
made galvanometer, to carry some 25 amperes or so, one terminal being
in connection with the shaft of the armature, and the other attached to
a movable brush 3. Since the function of the particular galvanometer is
simply to show a deflection when a current is passing, and to mark zero
when there is none, a coil of thick wire with a pocket compass in the
center will do all that is required, but care must be taken to remove
it sufficiently far away from the disturbing effects of the armature
magnetism. The manner of testing is as follows: Assume a steady current to
be flowing from battery B through the armature; touch the commutator with
brush 3, and a current will flow through G. Slowly rotate the armature
or the brush 3 until the galvanometer G shows no deflection. The coil
in contact with 3 will be found to be _grounded_. A hand regulator or
rheostat R may be inserted in series with the battery or dynamo circuit
to regulate the strength of the current passing.]
Ques. What is the advantage of this test?
Ans. The damaged coil can be located without unsoldering the coils from
the commutator, which is sometimes a difficult operation without proper
tools; further, the fault can frequently be repaired without disconnecting
any of the wires if its exact position be determined.
Magneto Test for Grounded Armatures.--A magneto test for grounded
armatures is not to be recommended, as armatures often possess sufficient
static capacity to cause a magneto to ring even though there be no leak.
This is due to the alternating current given by the magneto for when the
circuit has capacity it acts as a condenser and at each revolution of the
armature of the magneto a rush of current goes out and returns, charging
the surfaces of the conductor alternately in opposite directions, and
ringing the bell during the process.
[Illustration: Fig. 709.--Method of binding armature winding. Complete
appliances for handling armatures in making repairs are usually not
available with most street railway companies, since they are so seldom
required. When needed, therefore, some temporary contrivance must be
resorted to for help in the dilemma. Should an armature burn out, some
local concern that makes coils and rewinds armatures may be available to
do the work; again, it will be necessary to send to the manufacturers for
a man, as soon as coils can be made ready for the work. In no case should
any but an experienced man be given charge of this work. But if there be
any doubt as to whether the armature is really burnt out, let a competent
man be the judge. When a large armature needs repairing, a pair of chain
tongs can be used on some part of the shaft when putting in the coils,
and a block and tackle, as shown, can be used, when putting on the band
wires. Do not finish one band and then cut off the wire, but run it over
for the next, etc. Then solder and trim off the wires.]
Breaks in Armature Circuit.--A partial or complete break in the armature
circuit is always accompanied by heavy sparking at the commutator, but
not, as a rule, by an excessive heating of the armature or slipping of
the belt, and this enables the fault to be distinguished from a short
circuit. The faulty part can always be readily located by the "flat" which
it produces upon the surface of the commutator. The armature circuit being
open at the faulty part, heavy sparking results at every half revolution
as the brushes pass over it, and as a consequence the corresponding
segments become "pitted" or "flattened" with respect to the others; they
may easily be discovered on examination.
Breaks in the armature circuit may occur in either the commutator
or in the coils of the armature. To ascertain whether it be in the
latter, carefully examine the winding of the faulty coil.
The defect may be sought for more particularly at the commutator end
of the armature, as breaks in the wire are most frequent where the
connections are made with the commutator segments. If no break can
be discovered, try passing a heavy current through the faulty coil by
means of the brushes.
If a partial break exist with sufficient contact to pass a current,
the coil will be heated at that point and may be discovered by running
the fingers over the coil.
When located, the fault may be repaired by rewinding the coil, or
carefully cleaning the broken ends and jointing.
The fault may also be temporarily repaired by soldering the adjacent
commutator segments together without disconnecting the coil.