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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 9.0-amp nameplate rating is using a 5-inch
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 2-4 is provided to simplify the process of figuring the rpm ratio
squared and cubed.
TABLE 2-4
Tabulated Data Representing the Result
of Application of the Fan Laws
2-17
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