Motor and Generator Maintenance
This section deals with the inspection and maintenance of motors and generators of all sizes except steam and gas turbines. To obtain maximum efficiency and reliability of motors and generators, they have to be operated and maintained correctly. When motors and generators are maintained, many pre- cautions must be followed to avoid damage. Usually this damage results from maintenance personnel lacking thorough knowledge of motor design, construction, application, and correct maintenance. The purpose of this section is to provide general maintenance and failure mechanism information common to most types of motors and generators. The information is divided into several subsections; the first two sections provide information on the failure mechanism and overall general inspection for all types of machines. Also, the reader should refer to Section 1.8 in Chapter 1 for additional information on causes of insulation degradation and failure modes of motors. The remaining sections deal with particular types of machines and components.
Failure Mechanisms
The failure mechanisms of the machine are divided into stator winding, rotor winding, and exciter. These are discussed in Sections 10.9.1.1, 10.9.1.2, and 10.9.1.3.
Stator-Winding Insulation
The failure mechanisms of stator winding are (1) age deterioration, (2) electrical cause, (3) mechanical causes, (4) thermal causes, and (5) environmental contamination.
The age-related deterioration causes brittleness, shrinkage, and cracks in insulation. The electrical causes are corona, slot discharge, lightning, switching surges, single-phasing, unbalance voltages, overheating effects, and test failures. The single phasing and voltage unbalance can be caused by either problems in the utility distribution system or the in-plant distribution system. Voltage unbalance causes negative sequence currents, which cause overheating of the remaining phase windings and the stator. The negative sequence currents also cause rotor overheating, which in turn causes stator induced currents that can lead to stator-winding failure. Rotor heating may result in rotor vibration and shaft/bearing overheating, which can result in machine-bearing failure. Similarly, the overloading problems can be caused by low voltage on the incoming utility line supplying the plant or facility, or problems in the in-plant distribution system. The effects of overloading are stator-winding overheating, mechanical stresses on winding end turns and individual coils. This in turn results in deterioration of the turn-to-turn, coil-to-coil, phase-to-phase, and coil-to-ground insulation. The mechanical causes are vibration, loose ties and wedges, broken amortisseur bars, fan blades, loose iron, loose connections, close-in, or out-of-step synchronizing, and foreign objects. The thermal causes are overloading, overheating from short-circuited laminations, thermal cycling, loss of cooling, overheating from failure of strand insulation, and tape separation. The environmental and contamination causes are conducting dust or particles, moisture, oil, and magnetic particles.
Rotor-Winding Insulation
The failure mechanisms of rotor insulation are (1) age deterioration, (2) electrical causes, (3) mechanical causes, (4) thermal causes, and (5) environmental contamination.
The age deterioration causes are the same as discussed for the stator. The electrical causes are starting transients, switching surges, and high voltage induced from the stator faults. The mechanical causes are vibrations, high resistance connections, cracked or broken lead support insulators, collar deterioration, broken amortisseur bars, close-in unbalanced faults, broken banding wire, and loose mechanical parts. The thermal causes are excessive field current, loss of cooling and unbalanced faults. The environmental factors are moisture, bridging of magnetic pole gaps, or groups of energized parts by foreign objects or conductive dust.
Exciter Insulation
The failure mechanisms of exciter insulation are age deterioration, electrical causes, mechanical causes, thermal causes, and environmental contamination as discussed under stator and rotor insulation.