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:
- Short circuits;
- In individual coils;
- Between adjacent coils;
- Through frame or core;
- Between sections of armature;
- Partial short circuits.
- Grounds;
- 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.
Fig 700Method of locating short circuited armature coil Disconnect the external and field
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.
Fig 701Test for break in armature lead Clean the brushes and commutator and apply
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.
Fig 702Bar to bar test for open circuit in coil or short circuit in one coil or between segments
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.
Fig 703Alternate bar test for short circuit between sections Where two adjacent commutator
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.
Fig 704Method of locating short circuits between adjacent armature coils Fasten a monkey
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.
Fig 705Method of locating short circuits between coils through armature core The
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.
Fig 706Method of testing for breaks The instruments are connected as shown B is the
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.
Fig 707Watson field coils Automatic machinery is employed to wind these coils after
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.
Fig 708Method of locating grounded armature coil B is a battery or dynamo circuit
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.
Fig 709Method of binding armature winding Complete appliances for handling armatures
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.
