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. 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. 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. 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.
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 uselessD.
[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.
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.
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.
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:
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.
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.
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.
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.
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.
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:
Ques. What is the indication of excessive voltage?
Ans. It is indicated by the voltmeter, or by the brilliancy of the pilot lamp.
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.
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.
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.
The excessive heating of the parts of dynamos and motors is probably the most frequent and annoying fault which arises in operation. When the machine heats, it is a common mistake to suppose that any part found to be hot is the seat of the trouble. Hot bearings may cause the armature or commutator to heat, or vice versa.
All parts of the machine should be tested to ascertain which is the hottest, since heat generated in one part is rapidly diffused. This is best done by starting with the machine cold; any serious trouble from heating is usually perceptible after a run of a few minutes at full speed with the field magnets excited.
Heating may be due to various electrical or mechanical causes, and it may occur in the different parts of the machine, as in:
Ques. How is heating detected?
Ans. By applying the hand to the different parts of the machine if low tension, or a thermometer if high tension, and also by a smell of overheated insulation, paint, or varnish.
Ques. What should be done if the odor of overheated insulation, paint or varnish be noticeable?
Ans. It is advisable to stop the machine at once, otherwise the insulation is liable to be destroyed.
Ques. What is the allowable rise of temperature in a well designed machine?
Ans. It should not exceed 80° Fahr., above the surrounding air, and in the case of the bearings, this temperature ought not to be reached under normal conditions of working.
If this limit be exceeded after a run of six hours or less, it indicates a machine either badly designed and probably with the material cut down to the lowest possible limit with a view to cheapness, or some fault or other which should be searched for and remedied as early as possible, otherwise the machine will probably be destroyed.
Ques. How should the rise of temperature be measured?
Ans. It is not sufficient to feel the machine with the hand, but special thermometers must be placed on the armature winding, immediately on stopping the machine, covering them with cotton or wool to prevent cooling. Readings must be taken at short intervals, and continued till no further rise of temperature is indicated.
Heating of Connections.--A rise of temperature of the connections may be due to either excessive current, or bad contacts, or both. The terminals and connections will be excessively heated if a larger current pass through them than they are designed to carry. This nearly always proceeds from an overload of the dynamo, and if this be rectified, the heating will disappear.
If the contacts of the different connections of the dynamo be not kept thoroughly clean and free from all grit, oil, etc., and the connections themselves be not tightly screwed up, heating will result, and the connections may even become unsoldered.
Heating of Brushes, Commutator and Armature.--When heating occurs in these parts, it may be due to any of the following causes: 1, excessive current; 2, hot bearings; 3, short circuits in armature or commutator; 4, moisture in armature coils; 5, breaks in armature coils; 6, eddy currents in armature core or conductor.