# Useful link for calculating HP

After several posts asking about Motor Horsepower, I found the simplest explanation on this website.
http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm
Dave
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Thanks Dave...good reference.
Bob S.

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Bob S. wrote:

Yep, great link. I just found out my TS motor has 54% efficiency... Time to go shopping?
--
gabriel

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How did you calculate the efficiency?
Bob

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Bob Davis wrote:

Using the HP equation:
hp = v * i * eff / 746
Plug in my values:
1.5hp = 115V * 18A * eff / 746
Do a little algebra:
eff = (1.5hp * 746) / (115V * 18A)
Therefore:
eff = 0.54 = 54%
--
gabriel

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gabriel,
What is the service factor on the nameplate? 1.0, 1.15, etc.
-- Al Reid

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Al Reid wrote:

Hmm... have to check, I'm not in front of the saw right now... What's a service factor?
--
gabriel

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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 think.
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%.
-- Al Reid
"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

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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.
RB
gabriel wrote:

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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 their equipment.
Bob

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Dave

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