Page 193 - Industrial Power Engineering and Applications Handbook

Special-purpose motors 71 173

Terminal block

Upper ball bearing

Lower ball bearing

Inner mechanical seal

Figure 7.5(b) Sewage submersible pump and its cross-sectional view (Courtesy: SU Pumps)

temperature etc. on the performance of such motors is as liquid into the bearing housing and vice versa to prevent
for standard motors, discussed in Chapter 1. The general any possible contamination.
routine tests and methods of conducting them are also
similar to those for standard motors discussed in Cooling
Chapter 11.
The motor stator windings and bearings are cooled through
the pumped liquid passing externally around the motor
Protection body through heat conduction. The interior of the motor
The protection for such motors is also as for standard body (housing) is normally filled with a fluid to facilitate
squirrel cage motors. See Chapter 12. rapid dissipation of heat. The general practices are:
Rewinding of stator 1 Water-filled motors: These are initially filled with
clean water. Distilled water was originally used as an
The maintenance of such motors at site is easy, since the initial filler, but with the passage of time, clean water
stator can be wound with readymade PVC insulated was found to be a better substitute and now only
winding coils, and does not need a varnish impregnation clean water is used as an initial filler.
and subsequent baking etc., unlike a standard motor. It is 2 Oil-filled motors: Instead of water oil is now used
thus easy to rewind them at site. with oil and mechanical seals at the bearings to prevent
leakage of oil through them.
Bearings 3 Air-filled: The latest practice is to keep the housing
empty. This arrangement is found to be economical
The bearings are water lubricated. The typical materials and the motor operates at higher efficiency.
of construction are carbon, copper alloys, bakelite and
ceramics. The mechanical seals, like a double oil seal Motor shaft
protected with a cap called a Sand Guard, are robust and
perfect in sealing the motor to prevent the entry of pumped The motor with a short shaft length has a shaft of stainless