If there is a single rule which applies to all maintenance proce dure in all plants and under all conditions, it is be careful. Care lessness and failure to observe safety precautions are two things that the maintenance man cannot afford.
8 · 1 GENERAL PROCEDURE
The first procedure in any organized maintenance of equipment should be periodic inspection to prevent serious trouble from arising. This inspection should include not only electrical equip ment but the machine as well, should point up the wear and tear on the electrical equipment, and should provide a basis on which replacement of parts and correction of danger spots can be taken care of before they cause serious trouble.
One of the greatest causes of failure of control systems is the presence of dust, grease, oil, and dirt, which must be removed periodically in order that the equipment may function properly. The removal of dust and dirt may be accomplished by dusting or wiping with rags, but this is not always effective with oil and grease. These substances generally should be re moved by the use of a solvent such as carbon tetrachloride. Care should be exercised whenever these solvents are used, be cause the inhaling of any appreciable quantity of their fumes is quite likely to be very harmful. Therefore, adequate ventila tion should always be provided.
Periodic inspection should always include a check for over heating of electrical equipment and mechanical parts, because excess heat is always an indication of trouble to come. The value of checking for excess heat depends upon your knowledge of the proper operating temperature of bearings, coils, contacts, transformers , and the many other pieces of equipment associated with machinery, motors, and control.
Bearings of motors and mechanical equipment should be checked for proper lubrication. It is very seldom, however, that bearings of electrical equipment such as starters and switches are oiled. They are designed to operate dry , and, generally speaking , oiling the bearings will eventually cause a gum to form , causing the equipment to malfunction.
Another frequent cause of failure of control equipment is loose bolts and electrical connections. Each connection should be periodically checked for tightness, and the inspection should include the checking of possible loose bolts and nuts on the equipment.
Short circuits and grounds in the electrical wiring may be prevented by proper inspection of insulation and by using a Megger insulation tester on motors and cables in associated equipment.
If you are to maintain the same equipment over a period of time, the first law to follow is to be familiar with your equip ment. Know your equipment mechanically and electrically so that you will sense trouble before it develops.
The second law is to be observant. Whenever you pass a ptece of equipment for which you are responsible, listen and look. Quite often this is all that is necessary to tell you that trouble is on its way. Good maintenance procedure can be summed up in a very few words: Keep it tight, keep it clean, keep it lubricated, and inspect it frequently.
8 · 2 MAINTENANCE OF MOTOR STARTERS
The most frequent trouble encountered with motor starters is contact trouble. Contacts should be inspected for excessive burn ing or pitting and for proper alignment. If they are pitted, copper contacts may be filed, but care must be exercised not to remove too much contact surface or to change their shape appreciably. Copper contacts are subject to heat and oxygen on closing and opening of the circuit, and copper oxide may be formed on the sur face of the contact. This oxide is an insulator which must be removed if it covers a large part of the contact surface. Most contacts made of copper are arranged to be of the wiping type, which allows the mechanical closing of the contacts to remove this oxide as it forms. If the contacts are silverplated, the silver oxide is a good conductor and need not be removed; in fact, silver contacts should never be filed.
The contacts should be inspected not only for pitting but for proper alignment and for proper contact pressure. Improper alignment or lack of contact pressure will cause excessive arcing and pitting of the contacts.
8 ·3 CAUSES OF TROUBLE
One of the most frequent causes of failure of automatic equip ment is improper adjustment of contacts and time-delay circuits. Generally, the manufacturer of contollers for automatic equip ment will supply the proper contact-clearance distances and other information necessary for the proper timing of the circuit. This information should be readily available to the maintenance man so that he can periodically correct these adjustments. A check of these adjustments should be part of the regular inspec tion of this type of equipment.
The second most prevalent cause of trouble in motor starters and contactors is coil burnout. Coils on modern starters are well built and well insulated, which has eliminated considerable trouble due to vibration and moisture. Coils are still subject to burnout, however, chiefly because of one of two things. The most frequent cause of coil burnout is failure of the contactor magnetic circuit to close, causing a gap in this circuit which in creases the normal current through the coil to dangerous levels. The normal current to start the movement of a magnetic pole piece may be as high as 40 or 45 amp, but as the magnetic circuit is closed, this cu rrent usually drops to a very low value of 1 to 11/z amp, which is all that is required to maintain the magnetic circuit. If this circuit does not close, the coil will maintain a cur rent somewhere in between these two values , which can very easily cause it to overheat and burn out its windings.
The second most frequent cause of coil burnout is improper voltage . If the voltage applied to the coil is exceedingly high, the current through the coil can reach dangerous levels and cause it to bu rn out. If the voltage applied to the coil drops so low that the magnetic circuit cannot be completed, we have a gap which will cause exceedingly high currents and cause coil burnout. In view of the above-mentioned causes, the proper pro cedure when it is found that a coil has burned out on a starter is to check the mechanical linkage to see that the contactor can close completely and to check the voltage applied to the coil under load to see that it is sufficient but not excessive. Check for spring tension to see that the springs themselves are not causing the magnetic circuit to remain partially open .
Should the contactor be equipped with flexible leads, they should be checked for fraying and broken strands and should be replaced if these conditions exist. Should the starter be equipped with arc shields, they should be inspected for proper alignment around the contacts. They should be checked for ac cumulations of dust and dirt, and if carbon deposits have built up on the inside of these shields, these deposits should be care fully removed, since carbon reduces the arc path and can be the cause of serious arc-overs, particularly under high-voltage conditions.
Spring tension for proper contact pressure is very important in a starter and should be checked against manufacturer’s stan dards if they are available. They should at least be checked to see that each contact has approximately the same spring ten sion so that the contact pressure will be equal on each contact. Improper or unequal spring tension is one of the most common causes of contact chatter and starter hum, so be sure that when these conditions exist, you check the spring tension on every contact to determine if it is sufficient and that they are all equal.
8·4 MAINTENANCE OF RELAYS
Generally speaking, the maintenance of voltage relays is the same as that for motor starters and contactors with only the additional precaution that, in general, relays operate on lower currents and thus are provided with less power. This lower power demands a smoother, easier operating mechanical linkage and mechanism and thus requires more careful attention to the matters concerning it.
Current relays must be checked to see that they are receiving the proper amount of current for closing their contacts and that the spring tension and contact spacing are correct to give the proper pull-in and drop-out currents. Wear of contact surface and change in spring tension can cause a geat deal of variance in these values of pull-in, drop-out, and differential currents, which may make the circuit operate in a manner detrimental to the equipment.
Overload relays are devices which normally do not operate for long periods of time; therefore, they are subject to accumula tions of corrosion, dust, and dirt, which must be removed after periodic checking to see that they can operate when needed. If proper equipment is available, overload relays should be tripped by current periodically to see that they do function. Ex cessive tripping of overload relays is generally not an indication of relay failure so much as it is of overload on the circuit. The maintenance man should first determine the current value at which the overload unit actually trips and compare this with the allowable current to determine whether the fault lies with the overload unit or with the circuit itself.
Time-delay relays, whether of the pneumatic type or the dash pot type, require periodic adjustment to compensate for normal changes in their operating characteristics. The dashpot relay should be checked for dust and other foreign matter in the oil reservoir, since any impurities in the oil will affect the accuracy of the timing of the dashpot.
Quite frequently, relay contacts may be of the make-before break or break-before-make type, and here again spring tension and contact spacing become very important and require a check to determine that they are functioning as they were intended to.
8 ·5 MAINTENANCE OF PILOT DEVICES
Generally speaking, pilot devices require very little maintenance other than a check of their mechanical operation and their con tact condition . Where the pilot device is a form of pressure switch or vacuum switch, the range of its operation should be checked occasionally to see that the contacts open and close at the pressure they were set up to operate on. The contact surfaces should be examined to see that they have not accumu lated a coating of copper oxide, dust, or oil. They should be operated through their pressure range several times to check for consistency of operation.
Float switches are subject to troubles because of float rods being bent or because of water in the float due to a leak. A check of the proper operation of the float, the float rod, and the mechanical linkage to the float switch itself will determine the amount of wear and can generally indicate a replacement of parts before any serious trouble can develop. Of course, con tact condition is a must on checking this as well as other pilot devices.
When limit switches are an integral part of a control system, they are a very likely source of trouble because they perform many thousands of operations per day on an active piece of equipment. They are prone to mechanical failure because of worn bearings and cam surfaces as well as contact surfaces and spring tension. The only solution to limit-switch failure is fre quent and accurate inspection to determine their mechanical and electrical condition. When the mechanical condition of a limit switch becomes questionable, it should be replaced or re paired before it causes serious trouble with the other equipment.
8 ·6 MAINTENANCE OF BRAKES AND CLUTCHES
The chief cause of failure of brakes is, of course, worn brake lining or brake disks, as the case may be. This is an inexcusable cause of failure, however, if periodic inspection is employed. Never allow brake lining to reach the dangerous condition of wear.
The second most prevalent cause of brake failure is excessive wear and improper adjustment of the linkage from the electric solenoid or other operating device to the brake shoe or brake disk. These must be maintained in their proper mechanical align ment and condition. Improper linkage adjustment is a frequent cause of coil burnout on brake solenoids, since it may not permit the proper closing of the magnetic circuit, which in turn causes excessive current to flow in the solenoid coil.
Solenoid-operated clutches are subject to the same types of trouble as solenoid-operated brakes. Therefore, the inspection and maintenance procedure for these units should be the same as that for brakes.
Summary While this chapter has attempted to point up some of the basic principles of good maintenance, the actual maintenance of a specific piece of equipment must be determined by its op erating cycle, the complexity of its equipment, and the amount of maintenance time available. The chief difficulty in most main tenance situations is a lack of the understanding of the word “maintenance,” which means to maintain the equipment in operation as compared with repairing the equipment after it has broken down. Again, inspect it, keep it clean, and keep it tight, and you will be doing maintenance, not repair.
Review Questions
1 . What is the chief cause of coil burnout on starters, contactors, and relays?
2. Can copper oxide be the cause of trouble on contacts?
3. Should silver contacts be filed frequently?
4. What are some of the results of improper spring tension on starters?
5. What is a probable cause of contact chatter and starter hum on motor starters?
6. What would you expect to be the result of low voltage applied to the coil of a magnetic starter?
7. What is the proper procedure to determine the cause of too frequent tripping of overload relays?
8. What is a likely result of having a float half full of water when if operates a float switch?
9. What is likely to happen if an accumulation of carbon is allowed to form in the arc shields of a starter or contactor?
10. What is the best method of removing oil and grease from contacts and other surfaces where it might be harmful?
11. When using cleaning materials, what precautions should betaken?
12. What will be a likely result of poor adjustment of the link age on a brake?
13. What causes a change in the timing on dashpot-type time delay relays?
14. What two adjustments are likely to change the setting of the pull-in, drop-out, and differential currents of a current relay?
15. What is the difference between maintenance and repair as applied to control circuits and components?