Page 262 - Industrial Power Engineering and Applications Handbook

Installation and maintenance of electric motors 10/241
insulation impregnation and baking, can be washed noted above it obviously does not measure the temperature
and dried, to work again. Dismantle the motor, take of the hottest spot inside the stator. The readings may, at
out the rotor and bearings. Wash the stator and rotor best, reflect the temperature of the stator housing instead
with clean water to remove all the mud and silt. Pad of the stator winding. As a rough estimate, we can take
it dry with cloth. Blow warm air over the stator and a temperature gradient of 30% between the surface and
the rotor and heat them gradually, adopting the the windings to obtain a near-realistic temperature of the
procedure in Section 9.5. Unless they show permanent winding. If 8is the thermometer reading, then the winding
watermarks or rust or scratches, the bearings can also temperature may be around (W0.7) "C and it should be
be washed dry and regreased as shown in Section 10.4. less than the safe working limits. For example, for class
11 Cast iron body, feet or ribs etc. found broken or E insulation W0.7 I 105°C or 8 I 74"C, and for class B
cracked during transit or otherwise. Replacement of insulation 8/0.7 I 110 "C, i.e. 8 I 77°C.
the motor in such cases may not be practical. This illustration is for general guidance when the motor
However, using the motor may not be advisable in is checked at site for its operating temperature. The
view of a weaker foundation and insufficient cooling. thermometer reading should not exceed the above figures.
In such cases the broken parts can be welded using However, a few degrees above these figures may be
cast iron electrodes. Cracks, however, cannot be permissible, and this will depend upon the wall thickness
remedied. Unless the cracks are wide and may cause of the stator housing, the air duct between the housing
extensive damage during operation, the body may and the stator core, the design of the cooling ribs and the
still not require replacement. Minor cracks, however, effectiveness of the cooling fan etc., which only the
which do not impair the motor's perfor-mance or manufacturer can confirm.
cause development of further cracks, may be
compromised. Corollary
A surface temperature as high as 70°C or more is obviously
a very high temperature and such a surface cannot be
10.6 Winding temperature easily touched. It is therefore natural for a human hand
measurement at site to feel very hot when touching the surface of a running
motor. But to derive a conclusion from this may be
Sometimes the motor may appear to be running misleading.
overheated. In fact it may not be so. The easiest way to
measure the temperature at site is by a thermometer which
can be conveniently inserted into the hole of the lifting 10.7 Analysis of insulation failures
.-
hook. In very small motors where a lifting hook may not of an HT motor at a thermal
have been provided, a small oil cavity can be drilled in power station
between the top fins allowing the thermometer to be
embedded there (Figure 10.10).
A powerhouse (thermal) application is the most stringent
application, as discussed in Section 7.19. Based on field
10.6.1 Temperature correction data collected from various installations by different
When the temperature is measured by a thermometer as agencies the general insulation failures observed may be
attributed to the following.

I I I
Thermometer Electrical failures
1 Failure at overhangs
In protected motors failure may be due to accumula-
Cavity with oil
tion of fly ash at the overhangs (modern installations
use only enclosed motors). Fly ash becomes an
extremely good conductor when damp. The failure
will generally occur when a motor is switched on
after a prolonged shutdown.
The motor may also fail due to system overvoltages,
such as during a fast bus transfer (Section 7.19) or
due to voltage surges (Section 17.3).

Coil puncture is also a cause of insulation failure. It
may be a result of poor impregnation at the manu-
Thermometer 2 Failure of a coil
facturing stage, or due to overvoltages, voltage surges
reading "C and ageing. In resin-poor insulations, where the whole
stator and rotor, after impregnation, becomes a solid
mass, the chances of an insulation failure are remote.
Figure 10.10 Measurement of winding temperature at site by In a formed coil (resin-rich) design, however, there
thermometer method may be a differential expansion (thermal effect)