Motors and Generators:General Inspection

General Inspection

The fundamental justification for the inspection and maintenance of motors and generators is to prevent service interruptions resulting from equipment failure. A definite program of inspection and maintenance should be organized so that all apparatus is assured of attention at stated periods; these periods should be adjusted to meet the actual need that experience over a number of years as indicated is necessary. To assure adequate inspection, it is essential that an inspection record be kept for each piece of apparatus.

Maintenance should be supplemented by visual inspection of all areas that experience has shown to be vulnerable to damage or degradation. Obviously, this necessitates scheduling disassembly of the apparatus at the time the electrical tests are made. Following is a general maintenance guide that is applicable to all motors and generators. Visual Inspection

The most significant parts on which inspection should be made are the (1) armature (or stator) windings, (2) field winding (or rotor), (3) brush rigging and collector rings or commutator surfaces.

Armature windings

Check for the following signs of deterioration:

• Deterioration or degradation of insulation resulting from thermal aging. Examination of coils might reveal general puffiness, swelling into ventilation ducts, or a lack of firmness of the insulation, suggesting a loss of bond with consequent separation of the insulation layers from themselves or from the winding conductors or turns.

• Girth cracking or separation of the ground wall from wound coils.

This is most likely to occur on long stator coil having asphaltic-type bonds. Particular attention should be paid to the areas immediately adjacent to the ends of the slots. Where considerable cracking is observed, it is recommended that the wedges at the ends of the slots be removed, as dangerous cracks may also have occurred just within the slots.

• Contamination of coil and connection surfaces by substances that adversely affect insulation strength, the most common being carbon dust, oil, and moisture contamination.

• Abrasion or contamination of coil and connection surfaces from other sources, such as chemicals and abrasive or conducting sub- stances. Such effects are aggravated in the case of motors used in adverse atmospheric industrial applications, such as chemical plants, rubber mills, and paper manufacturing facilities, and wastewater treatment installations.

• Cracking or abrasion of insulation resulting from prolonged or abnormal mechanical stresses. In stator windings, looseness of the bracing structure is a certain guide to such phenomena and can itself cause further mechanical damage if allowed to go unchecked.

• Eroding effects of foreign substances embedded or lodged against coil insulation surfaces. Particularly damaging are magnetic particles that vibrate with the effects of the magnetic field in the machine.

• Insulation deterioration due to corona discharges in the body of the medium voltage machine or end windings. These are evidenced by white, gray, or red deposits and are particularly noticeable in areas where the insulation is subject to high electrical stresses. Some experience is required to distinguish these effects from powdering, which can occur as a result of relative vibratory movement between hard surfaces and which can be caused by loose end-winding structures.

• Loose slot wedges or slot fillers that, if allowed to go uncorrected, may themselves cause mechanical damage or reduce the effectiveness of stator coil retention against short-circuit and other abnormal mechanical forces.

• Effects of overspeeding may be observed on DC armatures by distortion of the windings or commutator rises, looseness or cracking of the banding, or movement of slot wedges.

• Commutators should be checked for uneven discoloration, which can result from short-circuiting of bars, or for pinholes and burrs resulting from flashover.

• Risers (connections between commutator bars and coils in slots) may collect carbon deposits and cause electrical leakage and subsequent failure.

Field windings

In addition to insulation degradation from causes similar to those listed under armature windings, close attention should be paid to the following in field windings:

• Distortion of coils due to the effects of abnormal mechanical, electrical, or thermal forces. Such distortions might cause failure between turns or to ground.

• Shrinkage or looseness of field-coil washers. This permits coil movement during periods of acceleration and deceleration, with the probability of abrading turn insulation, and breaking or loosening of connections between coils.

• Breakage or distortion of damper bars due to overspeed or abnormal thermal gradients between bars and the connecting end ring. Such breaks are often difficult to observe in machines that have operated in contaminated conditions and usually occur near the end ring or at the end of the pole piece. Low-resistance measurements between bar and end ring by means of a micro-ohmmeter, or digital low resistance ohmmeter, or similar instrument provides a means of detection.

• Loose damper bars with related burning of the tips of the pole-piece laminations. Among other cases, this could occur as a result of incorrect swaging or other means of retention of the bar during manufacture.

• In cylindrical-pole (or round motor) windings, evidence of heating of wedges at their contact with the retaining-ring body and half-mooning or cracks on the retaining rings can be caused by high circulating currents due to unbalanced operation or sustained single-phase faults close to the generator, such as in the leads or generator bus.

• Condition and tightness of end-winding blocking, signs of movement of the retaining-ring insulating liner, and any other looseness should be noted.

• Powered insulation in air ducts is evidence of coil movement. Red oxide at metallic joints is evidence of metal parts.

• Check tightness of field lead connections and condition of collector lead insulation.

Brush rigging

• Brush rigging should be checked for evidence of flashover.

• Before disassembly, the brush boxes should be checked to ensure that the clearance from the collector or commutator surface is in line with the manufacturer’s recommendations. They should be checked to see whether the brushes are free riding and that excessive carbon buildup is not present.

• Brushes themselves should be checked to see whether any excessive edge chipping, grooving, or double facing is evident.

• Brush connections should also be checked.

Voltage checks

• Unbalanced voltage or single-phase operation of polyphase machines may cause excessive heating and ultimate failure. It requires only a slight unbalance of voltage applied to a polyphase machine to cause large unbalanced currents and resultant overheating. In such cases, the power supply should be checked and rectified if even the slightest unbalance is found.

• Single-phase power applied to a three-phase motor will also cause excessive heating from failure to start or from unbalanced currents.

• Unbalanced currents may also be caused by attempts to operate machines having one or more coils disconnected or cut out of one or more phases. If the unbalance is appreciable, the machine should be rewound.

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