Pump Motors
17
Level 2
A.O.Smith
The primary function of the pressure tank is to
protect the pump motor. Without a tank, pres-
sure in the system would drop to zero almost
immediately when a faucet was opened. Then,
almost as quickly, the cut out pressure would be
reached and the pump would shut off. The cycle
would repeat as long as the faucet remained
open. This is the same thing that happens when
a pressure tank becomes waterlogged.
Short or rapid cycling is hard on the pump
motor whose windings are subjected to starting
or in rush current levels on an almost constant
basis. The start capacitor will often fail because
its duty cycle is exceeded. They are rated at
twenty starts of three second duration per hour.
Rapid cycling causes heat to build up which may
cause failure. Premature wear also occurs on the
start switch.
When a pump’s flow decreases, the pressure
increases to a point known as shut off. This pres-
sure may be many times the cut out pressure set-
ting. Water hammer is caused by flowing water
in a system suddenly being stopped. Water ham-
mer can cause sudden pressures as high as 760
PSI with system pressure of only 60 PSI. Since
there is a delay between the time the pressure
switch senses cut out and the time the pump
actually stops, water hammer will occur.
The shut off point may be determined by a
gauge with the pump running against a closed
discharge. This point could also be found by
lengthening a vertical discharge pipe to the point
where the pump is unable to cause water to flow
from the top.
