Service Factor (SF) indicates how much above the nameplate rating a motor
can be loaded without causing serious degradation. On high service factor
motors (1.3 - 1.5) I have seen the nameplate amps shown at SF rather than at
rated HP. If the amps are at SF the efficiency is not as bad as you may
My Delta contractor saw has a 1.5HP, 3450RPM, 1.15SF, 115/230V motor with
12.8FLA. That works out to be a 76% efficiency.
On the other hand, my pool pump is a 3/4HP, 1.5SP 115/230V motor that is
rated at 9.3 Amp (at SF). If I run the calculation assuming that it is
9.8FLA the efficiency is 52%. However, if I adjust for the fact that the
current is at SF (1.125 HP) the efficiency works out to be about 78%.
"It ain't what you don't know that gets you into trouble. It's what you know
for sure that just ain't so." --- Mark Twain
NEMA defines service factor as:
"The service factor of a general-purpose motor is a multiplier which,
when applied to the rated horsepower, indicates a permissible horsepower
rating loading which may be carried under the conditions specified for
the service factor."
For a 2-pole, 1/2 through 1 HP motor the SF is 1.25;
1/6 - 1/3 HP it's 1.35;
1/20 - 1/8 HP it's 1.4.
Thanks. You are right, the efficiency seems awful and it appears that it
really is. 18 amps is pretty high for a 1.5 hp motor. The motor on my jet
saw says 1 3/4 hp, 12 amps at 115 volt. Jet puts pretty good motors on
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