Chapter 10. Installation and maintenance of electric motors
Installation of bearings and pulleys
Important checks at the time of commissioning
Maintenance of electric motors and their checks
Maintenance of bearings
General problems in electric motors and their remedy
Winding temperature measurement at site
Analysis of insulation failure of an MV motor at a thermal
Prevention from shaft currents
Bearing currents due to internal causes:
In an electric motor the stator windings are in a circular form. Each phase is wound identically and spaced equally to each other. Under ideal conditions the electric field produced by these current-carrying conductors must be balanced and neutralized, as in a three-core cable, producing no residual field. (For more details see Section 28.8.) But this is not always so. While some field in space may always be present in all ratings, it assumes cognisable strength in large LV and MV machines (ž 2 MW), using circular laminations and all machines using segmented stator punchings.* Such space fields induce an e.m.f. between the two shaft ends. This is not a direct result of proximity but may be due to asymmetry in the magnetic circuit because of harmonics present in the system, a non-ideal flux distribution, due to slight disorientation or variation in the punching for the slot positions, or an eccentric air gap between the stator and the rotor. The phenomenon of disorientation in the punchings for the slot positioning is independent of motor current and influences large LV and MV machines equally.
1.Cognisable shaft currents may exist in large LV and MV motors of 2000 kW and above, using circular laminations and all motors with segmented laminations due to the magnetic field caused by asymmetries.
2. The problem of shaft currents may also be due to dielectric leakages that may take cognisance in MV motors of 3.3 kV and above. This can be prevented by grounding one of the bearings to prevent the leakages.
3. The bearing insulation is therefore determined by the manufacturer while checking the shaft voltage at the works. This forms a routine in-house test for all MV and large LV motors.
4. The grease lubricating film inside the bearings is too thin, of the order of 1 mm – 20 mm and may break down at very very low induced voltages, causing bearing circulating currents and arcing between the races and the rolling elements. The situation getting worse with non-uniform film thicknesses and presence of foreign conducting particles inside the bearing. To detect shaft currents, the normal procedure of leading manufacturers is to measure the shaft voltage end to end, with a full voltage applied to the motor terminals. If this is 300–350 mV or more, it will indicate that the bearings require insulation, as illustrated in Figure 10.8. On very large motors, using segmented punchings, shaft voltages even of the order of 1 to 2 V have been noticed. These voltages are highly detrimental to the life of bearings and are undesirable. As standard practice, all such motors are provided with a bearing insulation by the motor manufacturers. The insulation is generally provided between the bearing and the end shield at the non-driving end (NDE) of the motor (for ease of maintenance and replacement) as illustrated at location 3 in Figure 10.8.
Prevention from shaft currents caused by PWM inverter drives