Posted by David Sherman on Thursday, April 12, 2007 at 01:28:57 :
In Reply to: off topic horsepower question posted by ccsammy on Wednesday, April 11, 2007 at 22:31:17 :
In theory, horsepower is the same regardless of whether it's electric, gas or diesel. In practice, rated horsepower is only at one particular speed and each type of motor has it's own torque/hp/rpm curve.
So first of all, you ned to make sure your gas/diesel motor puts out 15hp at 1750 rpms. A lot of small engines are rated at considerably higher speeds. When you're well below the peak of the hp/rpm curve, hp is roughly proportional to rpm. So, if the gas motor is rated 15 hp at 3600 rpm, for example, it'll only do about half that at 1750.
The second thing that makes a difference is how hard the load is to start. A blower, pump, or generator is an easy load to start because it takes basically no torque to move it slowly. An air compressor is a hard load to start because it has to work against the air pressure in the receiver as soon as the motor kicks in.
The third factor is that a lot of manufacturers fudge their hp ratings. Sears tools and appliances are notorious for that. I've seen vacuum cleaners with "4 hp" motors. To get 4 real hp at 120 volts it would be drawing 30 amps, which you just can't get out of a 16 gage cord plugged into a 20 amp outlet. Lawn mower motor ratings seem almost as optimistic at times. Industrial electric motor ratings are honest, and good motors also specify their duty factor. "Compressor duty" motors need have a periodic cool-down time, but most others can run continually. Your 15 hp 3 phase motor is undoubtedly a decent industrial motor that will put out 15 honest hp all day long even at the low end of the line voltage. Being as how it runs at 1750 rpm, it's undoubtedly a squirrel-cage induction motor, which means it doesn't havee spectacularly high starting torque. If you find a gas or diesel motor whose manufacturer's hp curve shows it putting out 15 net flywheel hp at 1750 rpms, it will surely work to replace your electric motor. The catch is that you have to know the net flywheel hp at the speed you intend to run it, which is likely to be considerably less than the big bright number printed on top of the air cleaner. A dealer should be able to get that information fairly easily, though.
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