Page 259 - Industrial Power Engineering and Applications Handbook

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Table 10.3 Quantity of grease and regreasing intervals

Bearing bore To be injected after running hours Quantity of grease Quantity of initial
dia. d (mm) (g) grease
(8)
Speed zp.m + 750 1000 1500 3000
45
8000 ~ 8000
50
I
60
65 6o
I
70 40
75 8o
80 100
90
140
95
188
100
80 220
110 260

120
320
130
Courtesy M/s Siemens

motors of 2.4 kV and above. This can be prevented by
grounding one of the bearings to prevent the leakages.
The bearing insulation is thus determined by the
manufacturer while checking the shaft voltage at the
works. This forms a routine in-house test for all HT
and large LT motors.
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
Figure 10.7 Even if both the bearings of the motor are
insulated, there will be a current path as shown detrimental to the life of bearings and are undesirable.
As standard practice, all such motors are provided with
cally. Creepage of oil, water, moisture, dirt or metallic a bearing insulation by the motor manufacturers. The
particles may also short-circuit the insulation and defeat insulation is generally provided between the bearing
its purpose. Therefore, the insulating lining, wherever and the end shield at the non-driving end (NDE) of
provided, must be protected and kept clean. the motor, as illustrated at location 3 in Figure 10.8.

Summary 10.4.6 Reasons for high bearing temperature
1 Cognisable shaft currents may exist in large LT and This may be due to
HT motors of 2000 kW and above, using circular
laminations and all motors with segmented laminations Excessive quantity of grease causing churning
due to the magnetic field caused by asymmetries. Inadequate grease due to deterioration or leakage
2 The problem of shaft currents may also be due to Misalignment, causing friction and excessive axial
dielectric leakages that may take cognisance in HT forces