Page 190 - Industrial Power Engineering and Applications Handbook

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The centrifugal basket is a dead weight to be accelerated greater depths than this, one has no option but to lower
to the maximum spinning speed. The motor operates for the pump into the well, in other words, to lower the
short durations at different speeds and varying loads. It entire pump house below ground level, which is not
is required to accelerate heavy inertia loads at each speed, economical, practical nor advisable. Moreover, as the
and is normally designed for multi-speeds such as, 4/81 groundwater table may be receding rapidly one cannot
24/48 poles, 6/12/28/56 poles or 6/12/24/48 poles etc., be certain of an ideal depth for such pump houses. A
depending upon the type of centrifuge. The rotor is given depth considered ideal today may not be so with in a few
special consideration at the design stage to take account years as the water level may recede further. Better
of the excessive heating due to rapid speed changes, alternatives are found in a deep well turbine and a
braking and acceleration of heavy masses of massecuite submersible pump, described briefly below.
and basket etc. (e.g. by better bracing, high-resistance
rotor bar material, better heat dissipation etc.). During 7.5.1 Deep-well turbine or a vertical wet
one complete cycle of massecuite there is a wide pit pump
fluctuation in load and the motor speed and the motor
operates at different h.p. A normal overcurrent release Use of vertical hollow shaft motors
(OCR) therefore cannot protect the motor. Use of RTDs
and thermistors as discussed in Section 12.7, can provide With the use of such pump sets, the pump alone is lowered
total protection against such variable load drives. into the pit and the prime mover is mounted above ground
The method discussed above is a conventional one to level. These pumps can lift water or any other liquid
achieve required speed variations. With the application from a depth of more than 10 m. They are used extensively
of newer technologies, speed variations may be achieved for irrigation, domestic use, sewage disposal, etc., and
more accurately and promptly with a single-speed motor, are easy to install and maintain. They have an extra-long
by the use of the following: drive shaft and an extra number of bearing assemblies to
hold the long drive shaft in position and to eliminate the
1 Variable drive fluid couplings (see Section 8.4.1(2)) risk of excessive shaft vibration and hunting around its own
These may not prove to be as effective from the point axis. They are built to maintain permanent shaft alignment,
of view of energy conservation, as the motor will have high thrust capacity and are compact in size.
always be running at its rated speed and engagement The shaft of the pump goes through the motor shaft to
of the coupling alone will vary the output speed. the top of the motor and is bolted there. The pump shaft
2 Static drives using solid-state technology (see Section can be adjusted at the top to set the impeller by tightening
6.2) This is the best method for achieving the required or loosening the nut holding it. This also eliminates the
speed variations, not from the point of view of quicker use of a flexible coupling between the motor and the
and smoother speed variations, but of total energy pump. These motors are always constructed in a vertical
conservation even at low outputs. flange design and are provided with heavy thrust bearings
to take the additional load of the pump impeller, its shaft
Note and the fluid in the shaft. These motors are produced in
Application of solid state technology squirrel cage design for simplicity and also because they
For all the applications discussed above, which may require special have to drive only a light-duty load and operate at a
starting and/or pull-out torques or speed variations, the use of static fixed speed. They are also provided with an anti-reverse
drives is more appropriate today. With the use of this tcchnology, ratchet arrangement to prevent the rotor from rotating in
a standard motor can be made to perform any required duty, except the reverse direction caused by backflow of liquid in the
the constructional features and the applicable deratings as discussed
in Chapter 1. See also Example 7.1. event of an abrupt shutdown and during an accidental
The use of special motors was more relevant until the 1980s, phase reversal. A reverse rotation may cause the pump
when solid-state technology was still in its infancy and was not so shaft to unscrew. Since the motor is vertical flange
widely applied. With the advent of static drives, as discussed in mounted and the pump shaft passes through the motor’s
Sections 6.2-6.4, the use of standard motors is gradually becoming hollow shaft, it is called a vertical hollow shaft motor
more common for all these applications. The drive itself can alter (see Figures 7.3 and 7.4).
the supply parameters to the required level to make a standard
motor operate and perform within desired parameters, besides
conserving energy. The purpose of describing a few of these 7.5.2 Submersible pumps using submersible motors
applications is only to indicate their non-standard features, where
a standard motor with normal controls may not be able to perform A more economical alternative is found in a submersible
the required duties. pump where the pump, directly coupled with the prime
mover, is slid into the tubewell through narrow pipes.
7.5 Motors for deep-well pumps Narrow pipes are easy to sink into rocky terrain or very
deep water levels. They are less expensive and are easy
to install due to the elimination of the need for a pump
These pumps are used to lift deep groundwater or any house. Once the unit is slid into the well it requires little
other liquid from hard or rocky soil. Moreover, the liquid maintenance. (See Figures 7.5-7.7.) Such pumps have a
level may be so deep that it may prevent the use of a standard centrifugal multistage arrangement, and the
centrifugal pump. Theoretically, the maximum depth from motors are required to work under water or any other
which water can be lifted against atmospheric pressure liquid. These motors have an exclusive application for
is 9.8 m (32 feet). To lift water or any other liquid from submersible pumps.