A.O. Smith ST1302 Manual page 7

Pump Motors
An open impeller puts a great deal more force
on the motor bearings than does the closed
impeller. A 203 bearing is often used with closed
impeller pumps while open impeller may require
a larger 304 bearing on shaft end. Closed
impeller pumps are used extensively today.
The performance of the pump and motor is
affected by another major factor, the speed at
which the impeller is driven. The capacity of an
impeller varies in proportion to the change in its
speed.
EXAMPLE:
The capacity of an impeller is 50 gallons per
minute at a motor speed of 3450 RPM. Its
capacity will drop to 25 GPM if the motor
speed is reduced to 1725 RPM.
The above example is what takes place when
using a 2-speed motor (3450/1725) for energy sav-
ings during low traffic periods on pool or spa
motors. The load on the motor is reduced drasti-
cally but you also do not get the turn-over rate of
the high speed.
In the original applications, each motor is test-
ed with a specific pump, under a range of operat-
ing conditions. Each impeller requires a specific
amount of horsepower to turn at a given speed.
If a weaker replacement motor is selected the
pump's output will not just be reduced, the motor
will be overloaded. The motor is trying to run at
its normal speed around 3450 RPM, and the
impeller needs more power to be driven at that
speed. If a stronger motor is selected, there will
not be a significant increase in the pump's out-
put. The stronger motor may run a little faster,
but the impeller does not need the added horse-
power to run around 3450 RPM. In summary,
the motor and impeller should be considered a
matched set.
Level 2
5
7
8
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