Specifications

165
turn round and come to rest with the heaviest portion in the lowest position, and the hollower
portion will occupy the top position. Small counter-weights shall be fixed on the top part(lesser
weight portion) of the armature. The greater the distance from the centre, the smaller should be
the weight. An alternative method is to remove some weight from the heavier portion of the rotor
by drilling a hole in the end supports or by chipping, as found convenient. The operation should
be repeated until the rotor can come to rest equally well in all positions.
Dynamic balancing : This means carrying out the balancing operation when the rotor is actually
rotating. Although a body may appear well-balanced by the static test, any little unbalance becomes
prominent at high speeds. Special dynamic balancing machines are available by means of
which the exact amount of weight to be added (or removed) as well as its location can be
accurately determined. The rotor is mounted on a pair of pedestals carrying spring suspension
systems, and driven at any speed required. The machine consists of a visual indicator which
amplifies the vibration felt on the bearings due to the unbalance of the rotor. A suitable device is
also incorporated by means of which a counter-weight of the right value may be temporarily
inserted at the right place on the same shaft to which the armature is coupled, until all oscillations
are neutralized. After the weight of the counter-weight, the correct radial length and angle of
location are determined, the armature is removed and a permanent counter-weight fixed at the
correct point, and a test conducted once again to confirm the correction. When balanced in this
way, the armature will run very smoothly at all speeds without any vibration. Care must be taken
in fixing the counter-weights properly so that they do not fly off at high speeds.
For balancing large armatures which may not be possible to be moved easily, special
electronic equipment are available, so that the balancing operation may be performed ‘on site’
on the machine itself.
15.8 Preventive Maintenance of Electrical Equipments
Maintenance usually consists of regularly scheduled inspection, greasing, oiling and possibly
minor repairs. Most causes of failure of alternator and electrical equipment are poor maintenance
procedure, which involves flushing out oil wells, greases cups, and checking of rotor and slip
rings for concentricity.Shop overhaul is essential for all electrical equipment at least once in five
years.
To avoid major repairs:
 Check all connections and wiring.
 Make sure that moisture does not penetrate the winding insulation. Presence of moisture
lowers insulation resistance. Test insulation using a megger.
 Remove the moisture by heating the windings using hot bulbs or applying low voltage
to winding to develop heat and dry. Do not allow the temperature to rise above 90
0
C,
which may damage the insulation.
 Dust in the machine should be removed by using a blower with low pressure of air.
 Remove grease and oil using carbon tetrachloride(CTC). While using CTC the area
should be well ventilated to avoid fumes and toxics.