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Old 12-14-2009, 04:26 PM
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Originally Posted by Old Rotor Flap
Several decades ago manufactures rated electric motor in HP. It was just too basic for the average consumer and therefore it was necessary to find a way to express features in less useable terms.

Enter amp. ratings.

Look at a real motor data plate, like a Dayton motor. Lots of information including HP and amp. ratings and rpm. The consumer power tool market avoids as much comparative information as possible. Consumers who make critical comparisons in search of quality are confused and shouldn't be exposed to tedious details.

Look at the air compressors at Home Depot and Lowes. They're rated at some virtually useless HP number, one that rates it's HP when it's not actually compressing air. I've seen'em rated at 7.5 peak HP. Yet, when you look at the motor (usually Emerson = Ember-soon?) the HP line on the tag says SPL. SPL sounds like another word for Trust Me, which is the only Yiddish I know and it translates to fy. I've seen the same compressor that had the same size Emerson motor where the tag said 5 HP. They must have let one slip through to the compresser company.

Amperage rating is one measure of bearing inefficeincy. A failing bearing will increase amp. draw. Want a higher amp rating for your product? Put cheap bearings or bushings in it.
A HP line on the tag that says SPL means "special". You guys all have good points, but you need to look at the torque the motor is producing to know how good the motor is. A one horsepower motor produces 3 ft. lbs of torque at 1800 RPMs. At 3600 RPMs it produces 1 1/2 ft lbs of torque. That means that a 2 HP motor would produce 3 ft/lbs of torque at 3600 RPMs, and a 4 HP motor would produce 6 FT/lbs of torque at the same speed. The motor manufacturers do fudge the horsepower ratings based on peak amperage draw which is not how to rate a motor because their ratings are not based on electrical fact.

In Vince's example, a 1 HP motor draws 16 amps at 120 volts. At 240 volts, that same motor would only draw 8 amps. It is still doing the same amount of work only at half the amperage draw. That means at 120 volts it would be close to tripping or tripping a 15 amp breaker in the breaker panel just at full running speed regardless of load. My air compressor is rated at 5 "peak" horsepower and runs with no problem on a 20 amp single pole circuit. If you think about it for a minute, that doesn't make any sense. To get the ratings they claim, they lock up the motor and then measure how much amperage is being drawn with the motor completely stalled. That will give you greater peak amperage ratings, but doesn't really tell you how well the motor performs when actually doing work with the saw.

No one lives forever, the trick is creating something that will.
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