Page 263 - Industrial Power Engineering and Applications Handbook

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between the insulation, the copper conductor and the 3 Flash-over in terminal box
iron core during normal running. This may cause This may be due to fly ash, overvoltages or voltage
loosening of bondage between them, leading to surges. For prevention see Section 7.18.
vibrations and shrinkage of the insulation on cooling.
This can result in cracking of the insulation, exposing Mechanical failures
it to the environmental pollution discussed later, and
eventual failure. Rotor rubbing the stator
Prevention of insulation failures Sometimes, as a result of an unbalanced magnetic
With the use of surge arresters and surge capacitors field, causing an air gap eccentricity or excessive
(Section 17.10) shaft deflection, the motor is not able to maintain
By monitoring the insulation condition of the the small air gap between the rotor and the stator
windings during maintenance, at least once a year, and this may lead to failure.
which can be carried out by measuring (a) the It is also possible that after long running hours the
polarization index (Section 9.5.3) and (b) the balls or the rollers of the bearing have given way.
dielectric loss factor, tan 6 (Section 9.6) and making As a consequence of misalignment in the coupling.
up the insulation as in Section 9.5.2, when the Prevention
condition of the insulation is acceptable and only Ensure that the supply voltage is balanced.
its level is less than permissible. Check bearings and air gap during maintenance, at
A d.c. insulation resistance test or polarization least once a year.
index reveals only the surface condition of the Ensure an accurate alignment of load.
insulation and does not allow a realistic assessment Rotor stampings are loose or rotor bars are damaged
of internal condition. Loss tangent values are true Misalignment causes vibrations, which may even-
reflections of the insulation condition to detect tually lead to failure. The vibrations may also cause
moisture content, voids, cracks or general deteri- cracks between the rotor bars and the end rings.
oration. The tan Gversus test voltage curve may be Frequent starts and stops may also cause this because
drawn and compared with the original curve provided of excessive heat.
by the manufacturer, and inferences drawn regarding Prevention
the condition of the insulation. The different starting Check for accurate alignment.
tan Gvalues will reveal the condition of the insulation Check the rotor’s condition during the annual
in terms of amount of contamination, as noted in maintenance.
Table 10.4 (See IEE, Vol. 127, May 1980). Environmental pollution
Failure may be caused by coal dust, fly ash and
moisture. Pollution may weaken the insulation,
particularly of a protected type motor and result in a
failure at some stage.
Prevention
Table 10.4 Conditions of insulation in terms of starting Blow the surface clean with air at brief intervals.
(tan 4
Ageing
Starting value of tan 6% Amount of contamination Overvoltages, voltage surges and overheating of
windings over many years of operation may dry and
0-4 Low void content shrink the insulation and develop cracks. Through
4-6 Clean these cracks, moisture and dust can penetrate and
6-10 Some dirt destroy the insulating properties of the insulation
10-14 Dirt and moisture resulting in an eventual failure of the insulation.
14-16 Gross contamination Field experience has revealed that one of the major
16-20 Heavy deposit of oily dirt causes of failure of an HT motor is weak insulation,
Above 20 Severe oil and carbon contamination
caused by environmental pollution and ageing.

Relevant Standards
~~
IEC Title IS BS IS0
60072- 111 99 1 Dimensions and output series for rotating electrical machines. 1231/1991 BS 5000-10/1989 -
Frame numbers 56 to 400 and Flange number 55 to 1080 BS 4999-141/1987
60072-2/1990 Dimensions and output series for rotating electrical 1231/1991 BS 5000-1011989 -
machines. Frame number 355 to 1000 and Flange BS 4999-103/1987
number 1 180 to 2360
60072-3/1994 Dimensions and output series for rotating electrical 996/1991 BS 5000-1111989
machines. Small built in motors. Flange number
BF 10 to BF 50
60 136/1986 Dimensions of brushes and brush holders for electrical 991911991, BS 4999-147/1988 -
machinery 13466/1992