Page 256 - Industrial Power Engineering and Applications Handbook

Installation and maintenance of electric motors 10/235
through cable lugs to ensure a proper grip as shown in - Sprin pressure to be maintained around 150-200
P
Figure 10.4. A poor cable termination will mean arcing g/cm for proper contact of the brushes with the
and localized heat and may lead to joint failure. slip-rings.
For oil-filled control equipment such as autotransformer - Cleanliness of slip-rings and the brushes.
starters or oil circuit breakers (BOCBs or MOCBs, - Slip-rings, if roughened by arcing, must be cleaned
Chapter 19), insulating oil should be checked with fine glass paper, having a similar curvature to
periodically for its insulating properties. Leading the rings using a wooden block on which the glass
manufacturers of this equipment indicate the number paper can be wrapped. Emery cloth should not be
of switching operations under different conditions of used.
load and fault, after which the oil must be replaced - When replacing with the new brushes, the new
and these must be followed. brushes must be first ground to acquire a curvature
Each electric motor and connected control gear is similar to that of the rings.
grounded separately at least two points. The ground - Brush lifting and short-circuiting devices can be
resistance should be checked to ensure continuity of employed for motors required to run continuously
ground conductors. Refer to Chapter 21 for more details for a long period to minimize wear and tear of slip-
on grounding requirements. rings and brushes. However, when speed control is
Insulation resistance of the motor windings between required or switching operations are frequent,
phases and phase to ground should be checked and continuous contact brush gear assembly must be
made up in the event of deficiency, according to Section employed.
9.5.
Slip-ring motors need a regular and meticulous check
of brushes, brush holder unit and slip-rings for 10.4 Maintenance of bearings
cleanliness, accurate contacts, brush curvature, wear
and tear of slip-rings and arcing (Figure 10.5). The Grease may leave skin effect on the races of the bearings
following procedure may be followed: if the motor is stored idle for a long period. This may
cause noise during operation and overheating of the
bearings. After a long period of storage grease may also
dry and crack, and produce these effects. To detect this,
bearing covers may be opened and the condition of the
grease and any skin effects checked. If such marks are
visible, the bearings must be taken out and washed
thoroughly in petrol or benzene to which is added a few
drops of oil, and then re-greased with a recommended
grade and quantity of grease. Quantities of grease above
recommended levels may cause heat the same way as
quantities below recommended levels.
Recommended brands and grades of lithium-based rust-
inhibiting bearing grease conforming to IS 7623 are given
below:

Figure 10.4 Connection of cables in motor terminal box Figure 10.5 Slip-ring assembly with brush lifting and short-
through cable lugs circuiting gear