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7
MAINTENANCE OF ELECTRICAL EQUIPMENT and
MEASUREMENT OF INSULATION RESISTANCE
 
A. MAIN MOTORS AND GENERATORS
 
7A1. Inspection and cleaning. Frequent inspection and cleaning are necessary to insure trouble-free operation and long life of motors and generators. Machines should be examined for cleanliness, proper lubrication, tightness of connections, and freedom from moisture before every start.

Inspect the commutator frequently for uniform, hard surface gloss. Check for serious roughness and dirtiness of the slots between segments. Examine the brushes for wear and freedom in the brush holders. Check the brush holder spring tensions and see that pigtail connections are tight. Inspect the windings for presence of dirt and oil and clean them if necessary. Accumulations of brush dust must be removed. Check the bearings for adequate lubrication, signs of wear, and condition of the journal surface; scoring is likely to be an evidence of the presence of foreign particles in the lubricating oil. Joints in connections and at terminals must be inspected to make certain they are tight. Careful inspection must be made to insure that there is no oil leakage into the machines. Check to see that the joints on all covers and shields are tightly sealed. Inspect the equipment in operation for sparking, vibration, and temperature. Cleanliness is one of the most important factors in proper maintenance of motors and generators. Keep both the interior and exterior of the machines free from water, salt, lint, dust, dirt, and particularly, oil.

Most of the casualties to main motors and generators of submarines may be attributed to lubricating oil or other foreign matter reaching the commutator, armature, or field coils. This gradually breaks down the insulation and finally results in burned-out coils or armatures. The penetrating and damaging effect of oil in electrical apparatus is universally known and must always be carefully guarded against. This is especially true in the submarine service where

  electrical machinery is operated under adverse conditions, due to the continual moisture, and the many sources from which lubricating oil may find its way inside the machine casing.

Loose dust or foreign particles located in accessible parts of the machine may be removed by wiping with a clean dry cloth. Cheesecloth is recommended for this purpose. Do not use a cloth that deposits lint.

Compressed air is effective in removing loose foreign matter from inaccessible locations. Its use, however, is not recommended unless the machine can be opened sufficiently to permit air and dirt to escape. There is always danger of blowing abrasive particles into insulation or beneath insulating tapes.

The use of suction is preferable since there is less possibility of damaging insulation. A flexible tube attached to the suction side of a portable blower makes a suitable vacuum cleaner for this purpose. Grit, iron dust, and copper particles should be removed by this method only, whenever possible.

If the accumulation of dirt on insulation surfaces contains grease or oil, a solvent is usually necessary to remove it. An approved nontoxic and nonexplosive solvent must be used and then only sparingly. The solvent should be applied by moistening a lintless cloth with the fluid and lightly rubbing the surfaces to be cleaned. Excessive use of solvent may soften the insulation. After cleaning, the surfaces should be dried thoroughly to remove all traces of solvent.

CAUTION. Carbon tetrachloride is one of the best solvents for this purpose; but it must not be used in confined spaces and must never be taken to sea in a submarine due to its toxic properties. Crews of submarines have been poisoned by its fumes.

 
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7A2. Definition of insulation resistance. When a constant potential is impressed across insulation, the current which flows is inversely proportional to the resistivity of the insulation. Depending upon the physical arrangement of the conductors and insulation, the paths followed by the current may become somewhat complicated. In general, however, the current flow is through the body of the insulation or over its surface, or through a combination of both. The resistance opposing this flow of current is defined as insulation resistance.   1. A direct reading ohmmeter of the hand-driven generator type (megger).

2. The ground detector system (Sections 3A15, 3C4, and 3C5) for main propulsion motors and generators by converting the voltmeter readings into resistance values.

3. A voltmeter (high-resistance type) or milliammeter and a d.c. voltage supply.

4. A resistance bridge.

5. A direct indicating ohmmeter of the generator, battery, or electronic type.

Figure 7-1. Leakage paths in cable construction.
Figure 7-1. Leakage paths in cable construction.
Insulation resistance may be measured without damage to the insulation. A correct interpretation of such a measurement is usually the most convenient measure of the condition of the insulation. It can be used as a guide in determining when cleaning, drying, or overhaul is necessary, thereby preventing further development of conditions which might eventually lead to insulation failure in service. A properly interpreted reading may also eliminate needless shutdowns, overhauls, or renewals to improve insulation resistance that is entirely adequate.

7A3. Methods of measuring insulation resistance. Insulation resistance may be measured by various instruments such as:

  In the second method, the circuits to be tested must be energized to cause a deflection. This method, therefore, provides a convenient means of testing for grounds and measuring insulation resistance with no interruption in service. Methods 1 and 2 are the most commonly used.

If a megger is used, the circuit must be deenergized while the instrument is used to take readings, and the hand-driven generator should be cranked as long as practicable to obtain a steady reading. Subsequent tests should be made in the same manner so that readings will be comparable.

If the portable voltmeter method is used,

 
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care should always be exercised to restrict the applied voltage to a value commensurate with the condition of the insulation. It should also be noted that the resistance of the voltmeter has a direct bearing on the accuracy of the results. A voltmeter having a sensitivity of 100 ohms per volt does not permit measurements in excess of 2 megohms with any degree of accuracy for an applied voltage of 500 volts. The maximum resistance that can be measured with voltmeters having a sensitivity higher than 100 ohms per volt increases in direct ratio to the sensitivity in ohms per volt.

The instruments used in insulation resistance testing should be well maintained and periodically checked to insure that the rated voltage is delivered and that the instrument is in calibration.

7A4. Records of insulation resistance measurements. Suitable forms, such as the Megger Test Record (Figure 7-2), are provided for keeping accurate records of measured values. When properly filled out, the forms give the

  apparatus or circuit, the date, and the condition under which the reading was taken. Any change that may have taken place can thus be noted by comparison with previously recorded values on file.

7A5. Factors affecting resistance values. The principal factors that may influence values of insulation resistance measured in service are:

1. Connected cable and electrical apparatus. Other apparatus connected in the circuit may have an important bearing on observed values. For example, when measurements are taken on a generator connected to a switchboard, the value obtained includes not only the resistance of the generator circuits but also that of the bus work of the switchboard, all apparatus connected to the bus, and the generator cables. Since the insulation resistance of all this equipment is in parallel, the measured value may be quite low, but no conclusion as to the condition of the generator may be drawn from the value obtained. The reading indicates merely that the

Figure 7-2. Megger test record card.
Figure 7-2. Megger test record card.
 
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insulation resistance of the circuit, as a whole, is low.

For preliminary significant measurements, the machine should be isolated only to the extent of opening line switches, circuit breakers, and conductors. The insulation resistance measurement taken in this manner will still include the effect of connected cables and equipment that cannot be conveniently disconnected. For this reason, further isolation must be undertaken if precise readings of the apparatus in question are to be obtained. Armature windings, for example, may be further isolated by lifting all brushes off the commutator; shunt field circuits may be broken up by disconnecting the leads connecting successive poles; and cables may be isolated by completely disconnecting the cable at both ends. The degree of isolation must be progressive if it is to determine accurately the weak spots. As more isolation is undertaken, higher resistance values should be expected within the component part of the circuit involved, because of the reduction of possible parallel current paths to ground.

NOTE. Before proceeding with complete isolation, corrective measures, such as elimination of excessive moisture in the insulation, condensation on its surfaces, and removal of accumulated foreign matter, should be undertaken. Tests may then show sufficient improvement in the insulation resistance to eliminate the necessity of breaking the internal connection within the machine.

2. Moisture. Moisture content has a significant effect on insulation resistance and must be taken into account. All insulating materials absorb moisture from the atmosphere, some more readily than others. For example, cotton, paper, and asbestos insulation materials absorb moisture more readily than does mica. Vacuum pressure impregnated insulation keeps out moisture more effectively than built-up or immersion impregnated insulation. Insulation that has cracked or is otherwise damaged usually is more susceptible to moisture absorption, other conditions being equal.

Normally the moisture may be driven off or evaporated by the application of heat. Heat may be applied internally by the passage of

  current through the conductors or externally by heaters used to raise the temperature in the affected area. If, however, in addition to moisture, the insulation has deteriorated from exposure to oil, acid, or other harmful matter, the insulation resistance probably cannot be restored to its original value.

3. Temperature. The resistance of any insulating material varies with temperature. The resistance of copper and other common con ducting materials increases with temperature rise, the resistance of insulation decreases as the temperature rises. The presence of moisture in the insulation also greatly affects the values of insulation resistance at different temperatures. Temperature must always be taken into consideration when observed values of insulation resistance are being interpreted. When readings are taken at intervals, the values may be properly compared only when taken at approximately the same temperature or when due allowance is made for differences in temperature. Similarly, readings taken at room temperature must be compared only with previous, readings under the same conditions of humidity.

4. Cleanliness. The condition of the insulation influences the value of insulation resistance. Foreign matter such as dust, salt, carbon, or copper dust form conducting paths. The presence of oil or moisture acts as a binding agent and encourages the accumulation of such foreign matter, increasing the conductivity of the paths. The windings of rotating electrical machinery particularly collect such deposits in service. Other factors remaining constant, the relative variations in insulation resistance over a period of time are an indication of the degree of cleanliness of the insulation. A winding that may be in good condition in all other respects may have a low insulation resistance caused solely by deposits of foreign matter. After a thorough cleaning, the value may increase to an acceptable amount.

5. Condition of insulation. Any insulating material deteriorates with age, due to the individual or combined effects of heat, moisture, vibration, mechanical injuries, oxidation, and chemical action from acid or alkali fumes, salt, air, oil, and so forth. The rate of deterioration

 
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depends upon the conditions to which the insulation is exposed, such as location, type of service, atmosphere, and the amount of care. Although deterioration is inevitable, the life of the insulation may be lengthened appreciably by constant intelligent maintenance suited to the service conditions imposed.

6. Residual charges. Residual charges of static electricity, if present in a winding, affect insulation resistance measurements and should therefore be removed by grounding the conductors for a few minutes before measurements are made.

7. Construction. In the case of rotating electrical machinery, the dimensions, shape, number of turns, type of insulation, and process of manufacture influence the insulation resistance of the windings of machines. Windings in large or low-voltage machines will have inherently lower insulation resistances than those in small or high-voltage machines. Field windings will have inherently higher values than direct current armature windings due to the numerous creepage paths at the commutator connections.

  The types of bonding and coating varnishes and the drying processes used also have considerable influence. Duplicate machines constructed in the same shop may differ in their insulation resistance because of the variations that occur in their manufacture.

Before tests are made, detail drawings should be consulted to ascertain what type of insulation is under test.

8. Summary. Because of the various factors enumerated in the foregoing section, no rigid rule or formula has been established regarding acceptable values of insulation applicable to all types of machines. For main propulsion d.c. motors and d.c. generators, as well as for any motor rated at or above 50 hp, and generators rated at 35 kw or more, a table is supplied outlining the minimum acceptable insulation resistance of the various circuits. For smaller machines, the operating personnel must be guided by comparing measured values of insulation resistance with similar data previously recorded, noting also the particular conditions under which they were obtained.

Figure 7-3. Minimum insulation resistance of dry direct current propulsion motors and generators based on readings of 25 degrees C or 77 degrees F.
Figure 7-3. Minimum insulation resistance of dry direct current propulsion motors and generators based on readings of 25 degrees C or 77 degrees F.
 
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Figure 7-4. Effect of temperature on insulation resistance of insulated windings.
Figure 7-4. Effect of temperature on insulation resistance of insulated windings.

7A6. Explanation for use of table (Figure 7-3) for d.c. propulsion motors and generators. a. General. The values Ra, Rc, Rf, and Rj, indicate the minimum desirable insulation resistances under operating conditions for the circuits shown. When values less than these are obtained, action to further investigate the cause and remedy it, as indicated below, is necessary. It is recommended that whenever insulation resistance values less than Ra, Rf, Rj are obtained, the equipment concerned should be cleaned at the first available opportunity.

b. Armature circuit complete. Before any cleaning is attempted, measure insulation resistance of armature circuit complete, including armature, compensating fields, commutating fields, series fields, brush rigging, and connections to machine terminals. This resistance is measured by connecting the testing instrument between one armature terminal and ground. If the measured value is equal to or greater than Ra, but cleaning appears desirable, an attempt should be made to clean the machine in place without disassembly except for the removal of the access plates. If the measured value of

  insulation resistance, after cleaning, is equal to or greater than Rb, the machine should be placed back in service; if the measured value of insulation resistance is less than Rb, the several parts of the armature circuit should be disconnected and each part measured separately to determine if any one part of the circuit is causing the trouble. After the several parts are isolated from each other, if one particular part is found to be causing the trouble, that part should be treated individually. When the several parts of the armature circuit have been disconnected, and the low insulation resistance still cannot be attributed to any particular part of the circuit, the machine should be recleaned to insure that it has been properly done. If after a thorough check of the cleaning, the insulation resistance of the armature circuit complete is still less than Rb and the trouble cannot be isolated, the machine should be removed and reconditioned in a yard or base shop at the first opportunity.

If the measured value of insulation resistance for the armature circuit complete is less than Ra before cleaning, the several parts of the armature circuit should be disconnected from each other and each part should be treated as outlined below.

c. Armature alone. If the insulation resistance of the armature alone is equal to or less than Rc before cleaning, it should be cleaned in the vessel. If the insulation resistance of the armature alone when cleaned is equal to or greater than Rd, the armature is suitable for service.

If the insulation resistance of the armature alone when it has been cleaned, is less than Rd, the armature should be removed at the first available opportunity to a yard, base, or tender for reconditioning. After reconditioning, the insulation resistance should not be less than Re. After such reconditioning has been completed the armature alone should be given a shop high-potential test of 2/3 (2E + 1,000) volts, E being the operating voltage of the machine.

d. Armature circuit less armature. If, previous to cleaning, the insulation resistance of the armature circuit less armature is equal to or less than Rf, it should be cleaned in the vessel. If the insulation resistance of the armature

 
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circuit less armature when cleaned is equal to or greater than Rg, that part of the equipment is suitable for service. If, after cleaning, the insulation resistance of the armature circuit less armature is less than Rg, the various parts of the circuit should be isolated to determine if one part is causing the trouble. In some cases, the low insulation resistance may be caused by dirt, oil, or defective insulation at one spot such as in one pole, or at one brush rigging stud, and so forth. If the low insulation resistance cannot be traced to some particular part or spot, all parts of the armature circuit less armature should be removed at the first opportunity for reconditioning. After reconditioning, the insulation resistance of the armature circuit less armature should not be less than Rh. The reconditioning should be followed by a shop high-potential test of 2/3 (2E + 1,000) volts.

e. Shunt fields. If the insulation resistance of the shunt field circuit complete prior to cleaning is equal to or less than Ri, the shunt fields and connections should be cleaned in place. If the insulation resistance of the shunt field circuit complete after cleaning is equal to or greater than Rk, that part of the equipment is suitable for service.

If the insulation resistance of the cleaned shunt field circuit complete is less than Rk, each shunt field coil should be disconnected and measured separately to determine if one coil is causing the trouble. If the cause of the low insulation resistance can be traced to one pole, that pole should be removed for reconditioning or the coil should be replaced with a spare. If the cause of the low insulation resistance cannot be traced to one coil, all coils should be removed for a yard, base, or tender reconditioning. After reconditioning, the insulation resistance of the shunt field circuit complete should not be less than Rl.

f. Example of above discussion. Assume that a submarine requires the cleaning or overhaul of a propulsion generator. The rating of the generator is 415 volts, 1100 kw. The temperature of the generator is 25 degrees C, or 77 degrees F, and the machine is dry.

R = 415 / ((1100/100) + 1000) = 0.410 megohms

  Therefore the applicable minimum values at 25 degree C are:

Ra.41 X.3= 0.123 megohms
Rb.41 X1.5= 0.615 megohms
Rc.41 X.45= 0.185 megohms
Rd.41 X2.25= 0.923 megohms
Re.41 X5.0= 2.05 megohms
Rf.41 X.45= 0.185 megohms
Rg.41 X2.25= 0.923 megohms
Rh.41 X5.0= 2.05 megohms
Ri.41 X2.0= 0.82 megohms
Rk.41 X5.0= 2.05 megohms
Rl.41 X10.0= 4.1 megohms

1. Armature circuit complete. Assume that the following conditions prevail:

a) Measured value of the insulation resistance of the armature circuit complete is 0.160 megohms. This value is greater than Ra (0.123 megohms); and the armature circuit complete should be cleaned in place.

b) After cleaning, the measured value of the insulation resistance of the armature circuit complete was 0.450 megohms which is less than the minimum Rb (0.615 megohms).

c) The armature alone was disconnected from the armature circuit complete and was measured alone. A value of 1.2 megohms, which is greater than Rd (0.923 megohms), was obtained, indicating that the armature was satisfactory for service.

d) The measured value of insulation resistance of the armature circuit less armature was found to be 0.75 megohms which is less than Rg (0.923 megohms), indicating that the armature circuit less armature needed additional cleaning or that there was some isolated low-resistance path. The compensating windings, the commutating windings, and the brush rigging were disconnected from each other and measured separately. The compensating winding measured 4.0 megohms, the commutating winding measured 1.0 megohms, and the brush rigging measured 4.0 megohms, indicating that a low-resistance path to ground was somewhere in the commutating pole winding. Each commutating pole winding was disconnected and measured separately and it was found that one commutating field pole had lower insulation

 
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resistance than any of the other commutating field poles. Upon further investigation, it was found that one of the less accessible spots of the pole had not been adequately cleaned. After cleaning, the insulation resistance of the pole in question was measured and found to be equal to all of the other poles. All parts of the armature circuit less armature were then reconnected and the insulation resistance measured. A value of 1.2 megohms which is greater than Rg (0.923 megohms), was obtained, indicating that these parts were satisfactory for service. The armature was then connected in the circuit and the armature circuit complete gave a measured insulation resistance value of 0.750 megohms which is greater than Rb (0.615 megohms), and the armature circuit complete was ready for service.

2. Shunt field circuit. The measured value of insulation resistance of the shunt field circuit complete before cleaning was 0.10 megohms which is less than the minimum value of Ri (0.82 megohms). Each shunt field coil was disconnected and tested separately; one coil was found to have much lower insulation resistance than any of the other coils. The defective coil was removed and it was found that the insulation between the coil and the metal pole piece had been damaged, allowing a low-resistance path to ground. The damaged insulation was renewed and all the shunt field coils were cleaned and reconnected. The insulation resistance then measured 3.50 megohms, which indicated that the shunt field circuit complete was ready for service.

7A7. Repairing defective insulation. Windings should be cleaned and dried before any repairs are attempted. When a defect is located, either a permanent or a temporary repair should be made as circumstances will permit. All connections should be maintained tightly and suitably taped where necessary. Wedges should be maintained tightly in their slots and any loose space should be filled with slot fillers. Binding bands and bolted and soldered connections should be checked because the effect of magnetic stresses, vibration, and cycles of temperature variation constantly tend to loosen bands and connections. Field coils should be checked for tightness on field poles and for evidences of

  bruises due to retainers. Insulation will occasionally require a coating of insulation varnish. Only high-grade air-drying insulating varnish must be used. Apply two thin coats only, suitably thinned in accordance with the directions of the varnish manufacturer. Care should be taken to avoid clogging air vents, and any excess varnish should be removed before it sets. All insulating surfaces such as mica, cone extensions, brush insulation, and so forth, should also be coated with varnish. It is essential that varnish be applied only on clean, dry surfaces after all necessary repairs and cleaning have been effected. Varnish may be applied either by spraying or with a brush. It should be noted that the application of varnish will not permanently increase the insulation resistance or dielectric strength of the insulating material and accordingly cannot be used as a substitute for repairing or replacing defective insulation.

7A8. Condensation. To prevent condensation during extended shutdown periods, the temperature within the machine must be kept higher than the outside temperature. Condensation can be prevented, or eliminated, if found, by circulating heat through the machine. A convenient means of heating the machines is to leave the shunt fields energized at a low current. Do not exceed the maximum field current allowed in the manufacturer's instruction book for nonrotating machines.

CAUTION. Always secure the circulating water to the main motor or generator coolers when the machines are secured. If this precaution is not observed, condensation may take place on the cooler core.

Moisture absorbed by insulation or condensed on its surfaces may result in short circuits or grounds. The dielectric strength of the insulation is lowered temporarily while moisture is present and may be permanently lowered if deterioration occurs. For these reasons, moisture should not be allowed to accumulate, and a machine should not be placed in service without first making certain that the insulation is dry.

Insulation may be dried out with a hot air heater, allowing the hot air to enter through a port at the bottom of the machine and the

 
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moisture-laden air to escape through a port at the top. If insulation resistance is not too low, reduced current may be passed through the shunt field coils. The voltage and current should be gradually raised as the machine dries. Constant circulation of air is important. If an outside source of air is used, make certain that the air is clean and free of moisture. It is a good practice to energize with a low current daily all fields on machines that are not in use in order to dry them and keep them above room temperature.

The insulation resistance should be measured before, and at intervals during, the drying process. The interval between readings may vary with the rate of drying and the convenience in making the measurements. The insulating resistance decreases rapidly at the early stages of drying; but as the temperature becomes constant and evaporation progresses, the insulation resistance begins to increase, rapidly at first, then at a slower rate. When the readings reach a constant value and are sufficiently high, the drying-out process is complete and may be discontinued. Complete drying may take 24 hours or longer, depending on the heat and air circulation.

7A9. Brushes and rigging. a. General. The brush rigging is doweled in its proper position by the manufacturer. A machine must never be operated unless these dowels are tight and the rigging properly positioned. In the event of a change of position, old marks must be obliterated and new reference positions definitely determined and marked.

Brush brackets should be kept in their original positions so that they are square with the commutator segments and so that the distances between the brushes around the commutator are equal.

Brush holders are removable and fit in grooves to assure proper alignment. Brushes should not be loose in the holders, nor should they be so tight that they do not move freely. There should be a clearance of 0.005 in. to 0.025 in., measured in line with the shaft, between the brush and the holder. The clearance in the

  other direction, perpendicular to the commutator bar, between the brush and the holder should be 0.005 in. to 0.014 in. Check the brushes frequently to see that they are not sticking in the holders, that leads are firmly attached to the brushes and the holders, and that the pigtails are not rubbing on any part of the machine. Worn-out brushes must be replaced before they reach the end of their travel and break contact with the commutator.

Figure 7-5. Brush removal.
Figure 7-5. Brush removal.

If sparking of the brushes is encountered, check for the following possible causes:

1. overload

2. incorrect positioning of the brush rigging

3. worn-out, burned, or incorrectly fitted brushes

4. brush holder brackets out of alignment

5. rough, dirty, or insufficiently undercut commutator

6. open circuit or loose connection in the armature

7. loose connection between pigtail and brush or pigtail and holder

 
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NOTE. Brushes having loose pigtail connections, while they may not spark themselves, will often cause other brushes to spark because the defective brushes do not take their share of the load.

b. Procedure for reassembling brush holders. Whenever it becomes necessary to disturb the original adjustment of the brush holders, the following procedure should be followed in reassembly:

1. Set up the brackets with the brush holders in place and wrap a long strip of paper around the whole circumference of the commutator. Mark the lapping points of this paper; lay it on a flat surface, and divide the space between the marks into as many equal spaces as there are brush arms. Mark each division point, wrap the paper around the commutator, and adjust the brush brackets until the toes of the brushes of the different brackets just touch the marks. All brush holders should be the same distance from the commutator - not less than 0.080 in. or over 0.100 in. The toes of all brushes on one bracket must be in line with the edge of one commutator segment. If a bracket is out of line, loosen the bolts and adjust to the proper alignment by shimming or filing under the bracket head. Occasionally slight filing to increase the clearance in the bolt hole may be necessary. Correct staggering of the brushes has been provided for by suitable drilling of the brush holder brackets.

2. After the brushes have been properly spaced, they must be sanded to fit the curvature of the commutator. Fine sandpaper may be used. Do not use emery or carborundum. Remove the carbon dust with a cloth as it will cause serious trouble if allowed to collect on the winding. To fit the brushes with sandpaper, lift two or three of the brushes sufficiently to permit a sheet of sandpaper to be inserted between the brushes and the commutator face with the abrasive side of the paper toward the brushes. Move the sandpaper along the commutator face in the direction from the heel of the brush to the toe; release the brush pressure as the paper is drawn back. It is important to keep the paper down on the commutator face to avoid rounding the edges of the brushes. The brushes in one side of

  the holders should be sanded separately from those on the other side, moving the sandpaper always toward the center of the holder with the drag on the brush also toward the center. Continue the sanding operation until the brushes make firm, even, and complete contact with the commutator face.

c. Spring tension. Frequent adjustment of the spring tension is not necessary, but it is advisable to check the springs and possibly the tension when the brushes are worn down approximately halfway. The brush pressure should be about 2.5 psi of contact area between the brush and commutator. A small spring balance may be used for checking the brush spring pressure as shown in Figure 7-6.

Figure 7-6. Method of measuring brush spring pressure.
Figure 7-6. Method of measuring brush spring pressure.

d. Brush yoke setting. Shifting of the brushes around the commutator effects both the compounding and commutation. In a generator, the armature current reduces or increases the main field magnetization, depending upon whether the brushes are ahead of, that is, shifted in the direction of rotation of, or behind the true neutral point, thus having considerable influence on the compounding. To prevent sparking, the brushes must be held in such a position that the armature coils short circuited by the brushes are under the influence of the

 
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commutating poles. Occasional shifting from an exact center to produce slight changes in compounding is permissible. Even the most careful setting with a tram is subject to slight errors, and, for a final adjustment, slight changes in brush position may be necessary.

If a brush rigging has been completely disassembled, it will, of course, be necessary after assembly to locate the proper setting of the yoke. On all machines, the mechanical neutral is determined by the factory marks on the armature slots and commutator bars. Rotate the armature until two slots, which are marked, are equidistant from the center lines of two cominutating poles. Set the brushes of the stud between the two commutating poles at the center of the group of commutator bars which are marked on the ends. This will be over the center of the bar which is stamped with an identifying mark. The setting obtained in this way is approximate only, and must be checked by observation of the machine under load.

Figure 7-7. Factory mark on armature slots and commutator bars.
Figure 7-7. Factory mark on armature slots and commutator bars.

e. Rotating the brush rigging. Geared brush yokes used on General Electric machines are rotated by means of a pinion gear and wrench which are supplied as special tools. First, the upper cover on the side of the machine on which the yoke clamping arm is located must be removed. Next, the clamping arm is removed, the clamping bolt at the top of the yoke loosened, and the two flexible connections disconnected. The pinion is engaged with the gear teeth on the yoke and the pinion bracket

  secured to the frame. The rigging can now be rotated with a wrench.

On the other type machines, the rigging is rotated by removing the dowel which secures the yoke and inserting a steel bar in the holes provided on the rim of the yoke.

Figure 7-8. Wrench and pinion gear installed for rotating G.E. main motor brush rigging.
Figure 7-8. Wrench and pinion gear installed for rotating G.E. main motor brush rigging.

Figure 7-9. Wrench and pinion gear installed for rotating G.E. main generator brush rigging.
Figure 7-9. Wrench and pinion gear installed for rotating G.E. main generator brush rigging.

 
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7A10. Care of commutators. a. General. Successful operation and long life of a machine depend largely on keeping the commutator surface clean and free from oil and dirt. This does not mean that a commutator should be kept bright and shiny. The proper color of the commutator after the machine has been run for some time should be uniformly medium or dark chocolate.

The commutator should be wiped occasionally with a piece of dry canvas. Waste or soft linty material must never be used. Oil, vaseline, or any of the so-called commutator compounds must not be used.

Sandpaper should be used lightly on the commutator, if at all, and emery cloth must never be used. Emery is a metallic conductor and, if lodged between segments, causes short circuits. If it does become necessary to use sandpaper to smooth a commutator, the paper should be fitted in a wooden block, shaped to the curvature of the commutator.

If the mica between the segments becomes higher than the copper, a hacksaw blade with the set ground off may be used for undercutting the mica. Good judgment should govern the frequency of this treatment; undercutting the mica too frequently makes the slots too deep and permits a dangerous amount of carbon dust to collect in the undercut. After cutting down the mica, it is desirable to bevel the corners on the bars very lightly and to sand the commutator lightly to remove any rough spots from the edges of the segments.

A freshly turned commutator, or one on which the surface has been renewed, should be run under light load for approximately 24 hours. The commutator surface should then have a uniform polish. During the initial period of running, the commutator surface should be wiped with dry canvas at frequent intervals in order to remove any carbon deposit. Do not use waste or other linty material. No lubricants of any kind should ever be applied to a commutator. The brushes are self-lubricating and may leave a soft black deposit on the commutator when first placed in service. This deposit should be wiped off. The dry canvas or other nonlinty

  material used for wiping may be wound around a block and held against the commutator.

When in service, the commutator should maintain a dull polished surface. Blackening of all the bars indicates poor adjustment of the commutating field or incorrect brush pressure. Blackening of groups of bars at regular intervals may be due to the same cause or to poor brush contact. Blackening at irregular intervals indicates a rough or eccentric commutator that can be corrected satisfactorily only by stoning or cutting. This is a major repair and is usually performed by a tender or at a naval shipyard.

b. Brush vibration and sparking. Noisy brushes are generally the result of a rough commutator or too much clearance between the commutator and brush holders. Under some conditions, brush vibration accompanied by noise may appear at light loads. This is characteristic of some brushes and will disappear as soon as the brushes carry appreciable current. Brush vibration frequently causes sparking. Sparking of any kind should be watched closely to determine whether or not the bars are being damaged.

Due to slight mechanical unbalance, commutators may possibly run with an eccentricity of several thousandths of an inch at some speeds. This is not necessarily cause for concern, unless other damaging effects are noted. No attempt should ever be made to tighten or loosen the commutator clamping bolts for any reason.

c. Machine vibration. The source of any appreciable vibration of a machine should be located and corrected. A small amount of vibration may be expected from the diesel engine, but since all rotating parts of the generator are carefully balanced before installation, any existing vibration is usually the result of shaft misalignment. Newly fitted oil seals which rub on the shaft may also cause vibration.

7A11. Air gaps. Shims are provided between the poles and the frame for adjustment of the air gaps. The normal air gap for the main and commutating poles varies in generators of different manufacture. Refer to the manufacturer's instruction book for specific dimensions. When assembling any pole, the air gaps of the other

 
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poles on the same machine should be measured at the same time and the loose pole set to agree.

In measuring the air gap, it is important that the poles be concentric about the armature. In case all poles are removed at once and reassembled, the air gap should be set to the factory specifications if a tapered gage is used, and to shipyard readings if feeler gages were used originally, and are being used again. The air gap of a pole should be recorded, preferably before removal, with gages that are to be used for reassembly and the gap then reset to the original setting. Air gaps must be measured over a tooth on the armature which has been scraped clean of varnish. The location of this tooth is indicated by a mark on the armature next to the core on each end. Measurements are made by rotating this tooth under each pole, measuring from the same tooth to the pole in each case. The frame head supporting the bearings and the bearing housing are doweled after the air gaps have been adjusted at the factory. In adjusting the air gap, it is not necessary to allow for movement of the shaft in the bearings due to rotation. The air gaps are sufficiently large so that normal bearing wear will not have any influence on the operation. It is more important that all the air gaps be uniform than that their average be equal to the designated nominal value.

The commutating poles are provided with both magnetic and nonmagnetic shims. Whenever they are removed, the same thickness of nonmagnetic shims should be replaced as were removed.

7A12. Bearings and lubrications. a. Main motor bearing lubrication. The bearings of geared motors are fed from the reduction gear lubrication system. The pressure supplied by the main pump is adequate for lubrication down to the dead slow speed (38 propeller rpm). When operating at dead slow speed the oil pressure is extremely low. However, if a continuous flow of oil can be observed in the oil sight flow indicators, the bearings are adequately lubricated. The standby lubricating oil pump is used to replace the main pump when the oil pressure drops below 5 pounds, at which time an alarm warns the electrician on watch that the pressure

  is low. The standby pump is also used to pre-lubricate the bearings after a shutdown. The bearings of direct drive motors are lubricated from separate motor-driven pumps. The pump controllers have a selector switch by which the pumps may be run at slow speed in order to obtain the quietest operation. This condition should never be used at shaft speeds in excess of 80 rpm. The oil flow at full speed should be approximately 1 1/4 gallons per minute for the journal bearings and 2 1/2 gallons per minute for the thrust bearing.

b. Main generator bearing lubrication. The main generator bearings are the same type as those used on the main motors but they are lubricated from their respective main engine lubricating systems. The bearings are designed to operate with a 10 to 15 psi oil pressure at the bearing. Flow through the bearing should not be less than a quart per minute at normal speed. Any pressure that results in the required flow is satisfactory. Possible plugging is avoided by the size of the oil feed lines and the openings in the bearings which are not less than 3/16 in. in diameter. The flow of oil in passages of this size is not limited sufficiently at practical feed pressures. A bypass is, therefore, installed in the piping to divert a part of the flow around the bearings in order to prevent overlubrication and the possibility of excess oil entering the generator.

c. Temperatures of oil and bearings. The temperature of oil supplied to the bearings should not exceed 130 degrees F. The maximum safe operating temperature of the bearings is 180 degrees F.

d. Causes of overheated bearings. Overheated bearings may result from a number of different causes, among which the following are most frequently found:

1.insufficient oil
2.inferior grade of oil
3.dirt and grit in oil
4.clogged oil lines
5.poorly fitted bearings
6.bearings too tightly set up
7.scratched or corroded journals
8.conduction from overheated electrical parts
9.misalignment of shafting
 
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Dirt may cause the oil sight glass to indicate oil when none is present. A clogged top vent will have the same effect. Lack of end play will cause binding or heating, the trouble becoming aggravated as the shaft expands. A bent shaft will cause vibration and grinding at the journals. All of these troubles should be guarded against by frequent, intelligent inspections. Until a machine is available for overhaul, overheating may often be checked by the use of a liberal supply of fresh, cool oil, or in an emergency, by the use of water. The electrical parts should be kept clear of either oil or water.

c. Removal of bearings. The commutator end bearing housing on generators and motors is enclosed by a cover plate which must be removed to gain clearance for lifting the bearing housing over the bearing. Lifting jacks are provided for lifting the rotor slightly. This permits rotating the lower half of the bearing to the top of the shaft for removal. During removal, care must be taken to prevent damage to the external surface of the bearing which fits the housing. The lifting jack must be slacked off after the bearing is replaced. Serious damage will result from neglect of this precaution.

7A13. Cooler maintenance. a. Cleaning. Periodic cleaning of water tubes is necessary to remove any foreign matter carried in by the cooling water. Access to the tubes on all types of coolers is obtained by removing the water boxes or headers. The interior of the tubes may be cleaned with nonabrasive brushes, rubber plugs, compressed air, or by any standard approved method used for cleaning condenser tubes.

Strainers in the water inlet line and the inside of the core tubes should be cleaned as

  frequently as necessary to provide an unrestricted flow of water.

b. Prevention of moisture condensation. Condensation of moisture in the air cooling system must be prevented in order to avoid the possibility of water being carried into the generator or motor and deposited on the windings. Since it is difficult to determine accurately the temperature at which condensation will occur, the best practice is to adjust the cooling water flow until it is just sufficient to maintain the temperature of the air out of the machine at about 10 degree F below the maximum allowed in the manufacturer's instruction book.

CAUTION. Whenever the load is changed, the temperature should be checked immediately and the cooling water adjusted accordingly. Failure to do this will cause great changes in the injection temperature of the cooling water.

c. Control of cooling water. The flow of cooling water is controlled by valves and, in the case of the motors, also by speed control of the pump. The piping is arranged so that any cooler section may be cutout and the machine operated on the remaining section or sections. When operating on reduced coolers, the machine temperatures must be watched closely and the load reduced if necessary.

d. Zinc plates. Each cooler section contains protective zinc plates which protect the cooler tubes from the electrolytic action caused by salt water. These plates must be inspected at regular intervals and replaced when approximately 75 percent of the plate has been dissolved. Neglect of this inspection and renewal leads to serious cooler deterioration and possible damage to the motor or generator through the leakage of the cooling water into the machine.

 
B. CABLES
 
7B1. Insulation resistance measurements of cables. a. General. The primary purpose in making insulation resistance measurements of cable installations is to determine the condition of the cable in order that deterioration, which would result in eventual failure, may be discovered and remedied. Insulation resistance and methods of measuring its values are explained in Sections 7A2 and 7A3.   b. Factors affecting resistance values. The following factors must be considered in measuring insulation resistance of cables:

1. Other apparatus connected. Any equipment connected in the circuit when a measurement is made will result in a reading that will include the connected equipment. For example, when measuring the insulation resistance of the positive cable connecting a generator to a

 
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switchboard, the cable should be disconnected at each end. If this is not done, the measurement will include the insulation resistance of the bus work, all apparatus connected to the bus, the generator, and the negative cable. Since the insulation resistance of this other apparatus is in parallel with that of the cable, the measured value of the combination may be considerably below the value that would be obtained if the cable were disconnected and measured separately.

For convenience, initial measurements may be made with the cable only partially isolated by opening switches, circuit breakers, or other disconnecting devices in the circuit. If the value then obtained is satisfactory as compared to previously recorded values that were obtained under the same conditions, or to limiting values, no further isolation of the cable will be necessary. Otherwise, it will be necessary to completely disconnect the cable and measure it alone before a conclusion can be drawn as to its condition.

2. Total quantity (number and length) of cable. When insulation resistance of cables is to be measured, its length must be taken into account. The total insulation resistance of a particular length of cable is the resultant of a number of small parallel individual leakage paths distributed along the cable sheath. In order to have a common unit of comparison, the cable insulation should be expressed in ohms or megohms per foot of length. This is determined by multiplying the measured insulation resistance of the cable by its total length. It should be noted that in so far as insulation resistance measurements are concerned, it makes no difference whether the cables are in series or in parallel, and consequently the total length should include the sum of all the lengths of cable connected at the time of measurement. For example, if 2 cables, each 100 ft long, are connected together, even at only one end, at the time of measurement, the total length is 200 ft and the insulation resistance per foot is 200 times the measured value. The foregoing should not, however, be confused with the total length of individual conductors when considering multiple conductor cable. For convenient comparison purposes, the data applicable to multiple

  conductor cable are based on the insulation resistance between all the conductors connected together and the sheath or ground. Thus, when reference is made to total length of multiple conductor cable, it means the length represented by the sheath and not by the sum of the length of individual conductors within that sheath. For example, the total length of 300 ft of MHFA-7 (7-conductor cable) is 300 ft, not 7 times 300 ft. Consequently, the insulation resistance per foot with all conductors connected together, is 300 times, not 2100 times, the measured value.

3. Type of cable. Insulation resistance varies considerably with the nature of the insulating material employed and the construction of the cable. It is possible, therefore, to judge the condition of a cable as determined by its measured insulation resistance only when it is considered in relation to the typical characteristics of the particular type of cable in question. The heat and flame resistant cables (type HF series) are now in general use. The curves shown in Figure 7-10 are applicable only to the type specified.

4. Temperature. Fairly accurate temperature measurements on the sheath of the cable must be made in order to permit a reliable interpretation of the insulation resistance measurements. The temperature should be measured by means of thermometers, attached to the cable sheath, or armor, at several points along the length of the cable. An average is then made of these values. The thermometer bulb should be placed in direct contact with the sheath, or armor. Scrape away the paint at the point of contact. Hold the thermometer in place with pads of felt or other, heat insulating material placed over the bulb and secured with tape. The number of thermometers used and their location should be such that they indicate a representative average of the sheath temperature of the entire cable being measured.

The effect of temperature on insulation resistance of SHFA and SHFL type cables is graphically illustrated by the curves shown in Figure 7-10 which show the resistance changes which may occur in the normal operating temperature range as measured at the cable sheath.

 
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Figure 7-10. Insulation resistance vs. sheath temperature, SHFA, SHFL, sizes 650 and 800.
Figure 7-10. Insulation resistance vs. sheath temperature, SHFA, SHFL, sizes 650 and 800.
 
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Curve A of Figure 7-10 is the characteristic curve of insulation resistance and temperature for normal types SHFA and SHFL, size 650 and 800 cables. In referring to the curve, it should be noted that the insulation resistance falls rapidly with increase in temperature. Curve B of Figure 7-10 indicates a safe minimum insulation resistance for the cables, when used at submarine propulsion voltages.

c. Procedure. The procedure in measuring insulation resistance should be as follows:

1. Disconnect the cable from other equipment, in so far as practicable, and make a record of the connections remaining.

2. Measure the average sheath temperature.

3. Ground the cable for a few seconds to remove any static charge.

4. Measure the insulation resistance by means of a suitable instrument.

For single conductor cable (SHFL, SHFA, SDGA) there is but one insulation resistance to measure; that between the conductor and armor of lead sheath. For multiple conductor cables (MHFA, THFA, etc.) the insulation resistance

  should be measured from all conductors connected together to the armor, or to the metallic structure, or ground, to which the cable is attached if the cable is without armor. Measurements should also be made from each conductor to every other conductor. For example, in a 3 conductor cable this results in 4 measurements from armor or ground to conductors 1, 2, and 3 connected together; from conductor 2 to conductor 3. The lowest of these values should be used as the measured value.

5. Determine the total length of the cable in the circuit.

6. Multiply the total length by the measured resistance, thus obtaining the resistance in megohms per foot.

7. Compare the measured megohms per foot with the minimum safe megohms per foot indicated by the applicable curve at the measured average sheath temperature.

8. If previous measurements were made of exactly the same installation with the same equipment in the circuit and at the same temperature, compare the present resistance values with the previous values and note what change has occurred.

 
C. AUXILIARY MOTORS AND MOTOR GENERATOR SETS
 
7C1. General maintenance of auxiliary motors and motor generator sets. a. Cleaning. The interior and exterior of the machines must be kept clean at all times. Inspect the machines daily for presence of dirt, oil, and moisture, and wipe the machines thoroughly if such foreign matter is found.

b. Insulation resistance. Moisture on the commutator, armature, or field coils causes leakage paths that lower the insulation resistance and result in a ground. Periodic checks of the insulation resistance should be made and recorded, following the same general procedure as outlined for main motors in Section 7A3. Since no specific acceptable values can be established such periodic tests are useful in detecting weaknesses of insulation or accumulations of moisture or dirt. Then by comparing readings with those previously recorded under approximately similar conditions of temperature and humidity,

  it can be determined when cleaning, drying, or other servicing of the machine is necessary.

If a test indicates that the insulation resistance is below an acceptable value, all parts should be wiped with clean cloths. Do not use a cloth that will deposit lint in the windings. If the insulation resistance remains low, the windings should be cleaned with an approved solvent solution. The commutator heads and cross connectors should also be thoroughly cleaned.

Dry the windings as outlined in the section that follows (7C1c) until the insulation resistance becomes constant; then coat windings and adjacent parts with a high-grade air-drying varnish. Never apply varnish over damp or dirty parts, and do not depend on insulating varnish alone to increase the insulation resistance. All parts must be cleaned and defects repaired before varnish is applied.

 
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c. Drying windings. Windings may be dried by circulating hot air through the machine by means of a fan. A spare heater or a bank of lights may be used as the source of heat. Care should be taken to see that the heat is distributed evenly so that all parts will have the same temperature and dry evenly.

Drying can also be accomplished by passing reduced current through the shunt field coils. It should be noted, however, that short circuits may develop in the coils if this method is used while the coils are wet or actually grounded.

When drying a coil with power applied, check the temperature of the coils every 15 minutes for a few hours. If the temperature increases to a point where the coil is too hot to touch, shut off the current.

NOTE. Do not continue drying after the insulation resistance becomes constant. If insulation resistance is still low, determine which parts of the machine are defective and make any necessary repairs.

d. Armature. It is important that air spaces between coils be open for free circulation of air. This is also true of the openings between the shaft and core plates. Do not allow dirt or other foreign material to accumulate on the armature, particularly in locations where it will restrict the free circulation of air. It is of major importance that no oil or dirt accumulate above the commutators. Large creepage distances are provided between the commutator bars and heads. Keep this portion dry and occasionally coat with special insulating compound supplied for this purpose. Armature coils should be cleaned regularly and should occasionally be thoroughly dried and varnished in accordance with general instructions. A vacuum cleaner with a small inlet is effective in cleaning between the coils, and is much more desirable than compressed air. Steel banding wire should be checked regularly and replaced if any bands show signs of defect.

e. Commutators. Successful operation and the longevity of machines depend largely on the degree to which the commutators are kept clean and free from oil or dirt. Wipe the heads and ends of V-rings frequently to keep them in good

  condition. Use special insulating compounds for these parts. Keep the undercut between copper segments clean. This will prevent burning between bars and possible short circuiting of armature coils.

Do not sandpaper or stone commutators unless their condition makes it necessary. Never use emery cloth or paper on a commutator. If sanding or stoning should be necessary, make sure that the paper or stone fits the surface of the commutator.

After the machines have run for a short period, commutators usually acquire a dull brown finish. This is the proper surface. Do not try to keep the commutators bright and shiny. If the mica between commutator segments becomes higher than the copper, the slots should be undercut. This is easily done with a hacksaw blade which has the set ground off. Excessive undercutting with a sharp instrument must be avoided because it will wear down the mica to such a point that an excess amount of carbon dust may collect in the undercut. After the mica has been cut down with a saw blade, file, or other instrument, it is desirable to sand the commutator lightly to remove any burrs on the edges of the copper segments.

If commutators have high bars or are rough, they should be ground smooth, otherwise the condition may cause excessive sparking and heating of the commutators or severe burning of the bars and possible loosening of the armature leads. Grinding should be done at as high a speed as practical, and with extreme care. Make several light passes over the commutator. When finished, undercut the mica and sand the commutator lightly. Canvas should be fitted around the armature so that copper dust will not enter the spaces between the windings. If practical, a vacuum cleaner should be placed in a position to collect the flying dust. When finished, the machine should be thoroughly cleaned. Compressed air should not be used for this purpose as it will blow the copper dust through the entire machine. Commutator clamping nuts should not be disturbed.

f. Main poles and coils. Main poles are

 
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held to the motor frames by bolts that may be removed with standard wrenches. The coils and adjacent parts should always be kept clean. Dirt must not be allowed to collect between the coils and magnet frames. If dirt cannot be readily removed, release the holding bolts and wipe the coils and frames with a cloth. Dry the coils, coat with air drying insulating varnish, and tighten the bolts. Make certain that the bolts are thoroughly tightened, and that there is no misalignment of the poles.

If a coil should become damaged, remove it, together with the pole piece, from the machine. This can be done without removing the armature. When removing the coil from the pole, or when reassembling, do not damage the sheets of insulation wrapped around the pole.

Should it be necessary to replace any of the taping, dry the coil thoroughly and coat it with air drying insulating varnish before retaping. Never apply varnish to a damp coil, for it will tend to seal in the moisture and may cause trouble. When removing a coil, be sure to check the air gap before releasing the holding bolts. Then disconnect the cross-connectors and check the markings to make certain that the connectors will be replaced in their proper positions. When the bolts have been released, observe the number and thickness of the shims. Withdrawing or replacing a coil should be done very carefully so that the pole will not damage the armature. The use of a sheet of pressboard rubbed with paraffin and inserted in the air gap often makes this operation easier.

g. Commutating poles and coils. Commutating poles are held in the same manner as the main poles. The method of removing and replacing a commutating coil is similar to that for a main field coil.

The voltage drop across any one commutating coil will be very low, so there is little or no chance of short circuited turns. However, the voltage to ground may be practically as high as the line voltage. Also, the commutating coils are of taped copper bar. Therefore, it is important that no dirt collect on the insulating plates or lodge between the plates and magnet frames.

  If it is impossible to remove readily the dirt between the plates and the frame, the poles may be released in the same manner as the main poles. Coils are dipped in insulating compound and baked and are therefore, a solid piece. Disassembly of a coil from a pole may be easier if the coil is heated after it has been removed from the frame. Coils are fitted on the poles from the frame side so that it is unnecessary to remove the pole shoes. The method of removing a commutating coil is similar to that of removing a main field coil. When tightening cross-connectors to terminals of a commutating coil, use a feeler gage to insure full contact. Commutating poles are provided with both magnetic and nonmagnetic shims. The same thicknesses of both types should be replaced as were removed.

h. Brushes and rigging. The brush rigging is of rigid construction in order to eliminate vibration. All studs must be kept tight. Insulator plates must be cleaned regularly. The machines should never be run until the brush rigging is locked in its proper position.

Brush holders should be kept in their original position so that they are square with the commutator segments and so that the distances between the brushes around the commutator are equal. Brush holders are removable and fit in grooves to assure proper alignment.

The brushes furnished for each application are of a type and grade selected to give best operation. The type and grade should not be changed without consulting the manufacturer of the equipment. Brushes should not be loose in the holders, neither should they be so tight that they do not move freely. New brushes should be sanded to fit the surface of the commutator. Care must be taken during this operation to make sure that carbon dust is not blown through the machine. Spring tension on the brushes should be approximately 2 psi of brush surface. If brushes spark, due to a rough commutator, little or nothing is accomplished by setting the springs for higher tension. Continuous operation under these conditions will cause excessive heating of the commutator which may result in loosening of the armature leads. Correct the trouble at its source.

 
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D. AUXILIARY CONTROL EQUIPMENT
 
7D1. Magnetic contactor starting panels. a. Cleaning and inspection. Control equipment should be cleaned and inspected regularly to prevent breakdowns or serious shutdowns. Dust that collects on the working parts of a controller must be removed. Excessive wear of moving parts can be avoided if parts are kept free of foreign matter.

b. Lubrication. The armature lever shaft of contactors should be lubricated occasionally. A light engine or machine oil should be used. The quantity of oil used should be kept to an absolute minimum so that the oiled parts do not become dust collectors. Oil must not be allowed to collect on the sealing surfaces of operating magnets since improper operation of the device will result.

c. Contacts. Contacts should be renewed before their wear allowance is completely gone. When copper contacts become badly roughened or burned, they should be smoothed off with a fine file, taking care to remove only as little copper as is necessary to reestablish good contact. Silver contacts should not be filed except in extreme cases of roughness. A silver contact,

  although badly oxidized, can still make good contact.

The main and auxiliary contactor contact surfaces must be kept clean and uniformly bearing. The contact springs should be kept at an even tension or replaced if found defective. Protective overload and no-voltage devices should be inspected and tested periodically. On these tests, the proper operation and sequence of the contactors should be noted. The need for minor repair or adjustment to one contactor will often disable a panel. The flexible connector terminals should be kept tight and other possible sources of open circuits watched.

d. Arc chutes. On contactors equipped with blowouts, the arc shields should be replaced before the material is burned away enough to allow the arc to touch the blowout pole piece.

e. Insulation. The insulation on wires and coils may sometimes be damaged due to vibration and friction against other parts. Parts with damaged insulation should be reinsulated as soon as the damage is discovered, and wherever possible, the cause of the damage should be removed,

 
E. PANELS AND SWITCHBOARDS
 
7E1. General maintenance of panels and switchboards. Panels and switchboards should be wiped frequently with a soft brush having no metallic binding. If it is necessary to clean off anything other than dust, a soft flannel cloth or a piece of chamois should be used. Cotton waste or cloths that leave lint must not be used.

Frequent examination must be made to insure that all connections are tight.

The condition of the wires behind the board should be checked periodically.

The tendency of the ship's structure to weave sometimes causes enough movement of the wires behind the board to result in their abrasion and consequent breakdown.

  Surface moisture must be kept at a minimum on all panels to hold up the circuit insulation resistance readings. Its presence on a panel will often account for low circuit insulation resistance readings. If it becomes necessary to remove moisture, use a flannel cloth.

Alcohol must never be used for cleaning panels. This substance is not only inflammable, but its use will break down the finish surfaces of panels and of the instruments mounted on them.

The use of an approved lacquer is recommended since it not only improves appearance, but also produces a polished surface which does not absorb and hold moisture.

 
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F. HEATING UNITS
 
7F1. General maintenance. Before any attempt is made to work on a heater or switch, all power lines to the unit must be disconnected.

Immersion type heating units used in lubricating oil heaters should be removed and inspected periodically for the presence of carbon on the heater blades. This deposit is caused by continuous contact with oil. If an accumulation

  of carbon is found, the blades must be scraped clean, then reinstalled.

Contacts in the terminal box and switch should be checked and tightened if necessary.

Care must be taken to see that immersion type units are not turned on unless they are immersed. The elements burn out quickly when current is applied to a dry unit.

 
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