VARIABLE SPEED DRIVE CONSTANT
PRESSURE SYSTEM
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 The latest and most efficient method of water supply is by using electrically driven pumps with variable speed drives so as to maintain a fixed predetermined constant pressure inspite
of variable consumption. The equipment basically comprises of centrifugal pumps driven by
standard squirrel cage induction motor. The variable speed is achieved by a frequency
convertor which changes the electrical frequency from standard 50 cycles to a figure both
higher or lower so as to achieve speed for say, 4- pole motors from a standard 1500 RPM
synchronous at 50 cycles upto 2000 RPM on the higher side and more or upto 1000 RPM or
less
on the lower side. This is automatically achieved through a PID (Potential Integral
Differential) controller and apressure transducer. The pressure transducer is a device which
senses variable pressure and converts this variable pressure to electrical signals either
variable voltage or variable amperes. The signal from the pressure transducers is fed to the
PID controller from which it is transferred to the variable speed drive, thereby altering the
speed of the motor to increase or decrease depending on increase or decrease of
consumption.

         Normally in a large system combination of fixed speed and variable speed pumps can
be used with a PLC (Programmable Logic Controller) which is like a computer. With this
equipment 1,2 or 3 additional fixed speed pumps can be automated in such a manner that
initially the variable speed pump starts. If this pump
cannot cope with the consumption and
demand a Fixed Speed Pump would cut in and the speed of the variable speed drive would
automatically drop so as to maintain a constant pressure. Similarly, if the demand increases
for more than the capacity of two pumps, a third pump can cut in etc. On the other hand if
the capacity drops to a very low value such as at night smaller idler pumps can cut in to
maintain the pressure and prevent larger capacity pumps running at very low speed.
         The advantages of the variable speed drive constant pressure system which is now
fast replacing the large Hydropnuematic System are as follows:

Very appreciable reduction in electric energy as compared to Hydropnuematic System
where the pumps have to start and stop.

Elimination of maintenance of air compressors and prevention of air absorption in water
which is a cause for corrosion.

As bulky pressure tanks are eliminated, substantial space saving is achieved.

It should however, be noted that the cost of Variable Speed Drive System is economical
only in respect of larger systems as the frequency converter, PLC etc. are higher priced.
Therefore for the smaller water supply system the Hydropnuematic System of latest design
with precharged autoclaves are still extensively used.

 
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