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When changing a sheave size, or increasing the setting on a variable
pitch sheave, be careful not to overload the motor. With reasonable accur
acy, you can substitute "motor amps" in the above equation to replace horse
power.
Example: A motor with a 9.0amp nameplate rating is using a 5inch
sheave to drive a fan. The motor is slightly oversized, and the actual
amperage draw is 6.2 amps. If the motor sheave size is increased to
6 inches, will the motor be able to handle the increase.
Solution: The increase in fan rpm is the same proportion as the
increase in the motor sheave diameter, so
(rpm2/rpml) = 6.0/5.0 = 1.2
The new amperage draw on the motor will be:
= 6.2 x (l.2)3
amps2
= 10.7 amps
The increase in motor sheave diameter will require that the motor be
changed to a larger horsepower size, which provides at least 10.7 amps.
Table 24 is provided to simplify the process of figuring the rpm ratio
squared and cubed.
TABLE 24
Tabulated Data Representing the Result
of Application of the Fan Laws
217


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