Preventive Maintenance of Electric Motor | Points to be attend during periodical maintenance: general
cleaning and blowing off of dust with compressed air at a pressure of 80 to 100
lb. per sq.in.
i.
Checking of air gap
ii.
Greasing or oiling of bearings
iii.
Measurement of insulation resistance of windings
iv.
Attention to slip ring or commutators
v.
Checking up of carbon brushes and brush gear
vi.
Checking current taken by motor
vii.
Checking tightness of terminal connections
viii.
Checking if motor is operating smoothly and without vibration.
N.B.
whenever any motor is inspected, it is equally important to check the starter
and control gear.
Air Gap Measurement -
Air
gap depends upon the size of the motor. AC motors have much small air gaps than
DC motors. The gap between the rotor and the stator varies from a few mils
(thousands of an inch) to 50 miles or more depending upon the size of the
motor. The air gap is measured by inserting long steel feeler gauge leaves in
the air gap between the rotor and the stator and ascertaining the maximum
thickness of the feelers that can passed. At least four readings should take at
different points around the periphery of the motor, i.e. top, bottom, front and
back. Unless acre is taken in measuring the gap, the results will not be
consistent.
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| Preventive Maintenance of Electric Motor 1.1 |
Ball & Roller Bearings Usage
Precautions in Fitting Bearings
Ball
and roller bearing are manufactured to very close tolerances and are therefore
easily damaged by careless handling and fitting. Therefore, utmost care is
required in fitting up and maintaining them.
The following points require special attention:
a.
The bearing housing and the shaft end, over which the bearing fits, should be
thoroughly cleaned so that the bearing fits neatly and just push tight.
Bearings should never be driven tight, because it will distort the race and
damage the bearing. Therefore, the fitting surfaces should be carefully checked
and any burs or surface injuries should be cleaned up with a smooth file and
finished with fine emery where necessary.
b.
Scrupulous cleanliness is essential in handling ball and roller bearings. Grit
and dirt are enemies to all bearings. Stored bearing should not be removed from
their boxes until they are required for fitting and only clean white cotton
fluff less cloth (never cotton waste) should be used in their vicinity.
c.
When fitting new bearing, the protecting oil put in by the manufacture need not
be washed off as it makes a good lubrication for the first few hours of
running. Grease, of the best quality, should be lightly packed into the bearing
itself and the bearing then fitted into position in its housing. The inner race
may be pressed on to the shaft but if this not practicable; it can be fitted
into position by lightly tapping with a wooden mallet over a tube passing over
the shaft end. If a tube is not available, a copper drift may be used. Care must
be taken to ensure that the bearing is square on the shaft. Wooden blocks
should not be used unless they are of very hard wood and show no tendency to
splinter done. In handling ball or roller bearing, avoid dust and grit as
plague.
Bearing Problems
Alignment of Directly Coupled Motors
The
alignment is easily checked by laying the edge of a steel foot rule against the
sides of the two flanges and checking whether the steel edge bears fully
against the sides of the two flanges or if there is any gap. The gap can be
readily seen if a light is placed on the opposite side. Any variation in levels
is corrected by suitable steel shims. The alignment should not only be correct
in the vertical and horizontal planes but the axis of both the shafts should be
in the same line and not make an angle with each other. This can be checked by
measuring the gap between the flange faces at four points, i.e. top, bottom,
front, and back.
Static and dynamic balancing of motors
Balancing
However
carefully constructed, a motor armature shaft has necessarily some unequal
distribution of weights in its body, which results in its axis of gravity being
slightly off centre and out of line with the axis of rotation. Therefore, when
the armature rotates, centrifugal force is created which act upon bearings.
This causes the whole machine to vibrate.
Its
intensity varies at different speeds and becomes maximum at some critical
speeds due to the effects of resonance. The amount of unbalance determines the
degree of vibration. For smooth running and long useful life, the rotating
parts should be properly balanced. Then the machine will run smoothly without
producing any oscillation, vibration or noise.
Balancing
consists of re-adjusting the distribution of masses in the body in such a way
as to bring the axis of gravity to coincide with the axis of rotation. This is
done by placing a counter-weight on or removing some weight from some part of
the armature in such a way that the unbalanced centrifugal force is cancelled
out. To do this is necessary to determine precisely where the counter-weight is
to be placed or removed, and what the weight should be.
Read Also : Types of Motor Enclosures
Static Balancing
In
static balancing, the rotor is supported on a pair of perfectly horizontal
knife edges. If the armature is in perfect balance, the rotor should rest in
any position. If, on the other hand, the rotor is not well-balanced and has
uneven distribution of weight, the rotor will turn round and come to rest with
the heaviest portion in the lowest position, and the hollow over portion will
occupy the top position.
After
noting the lowest point of the armature, small counter-weight should be fixed
on the top part of the armature and a test conducted again. The operation
should be repeated until the rotor can come to rest equally well in all
positions. The amount of weight added and its location has to be found by the trial
and error method, remembering that 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.
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| Preventive Maintenance of Electric Motor 1.2 |
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 rest, any little
residual balance becomes prominent at high speeds. Special dynamic balancing
machines are available by means of which the exact amount of weight to be added
as well as its location can be accurately determine. The rotor is mounted on a
pair of pedestals carrying spring suspension systems, and driven at any speed
required. The machine incorporates 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 trace of vibration. Care must be taken in securing the
counter-weights properly so that they do not fly off at high speeds.
For
balancing large armatures which may not permit of being moved and placed on the
balancing machine, special electronic equipment has been developed making use
of vibration detectors and cathode ray oscilloscopes, so that the balancing
operation may be performed in ‘situ’ on the machine itself.
Final Word
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