Originally Posted by
Tim Bridge
Mike, since I am a master electrician I tend to get my info from the National Electric Code.
They have tables that are used for the calculations of full loads for motors.
I was using the 115 volt single phase columns for my amperage.
I am not sure where they get their numbers from, or the efficiency they use.
From my experience with the motors that I have wired and trouble shot, their numbers are close to the nameplate ratings on the motors that are used in commercial and industrial applications.
Those numbers don't make sense, Tim. Check the mathematics I gave you in my previous post. It's clear that a 1HP, 120V motor should not draw 16 amps at full load.
A 1 HP, 120V motor operating at 80% efficiency would draw about 7.8 amps. At 70% efficiency it would be about 8.9 amps. Electric motors, even small ones, are generally at least 70% efficient. We know that electric motors are more efficient than 40%. The problem with very low efficiency would be heat. All those excess amps would turn into heat. So if the motor was drawing 16 amps, while only needing 6.5 amps to produce 1 HP, those other almost 10 amps would be heating the motor. 9.5 amps times 115V is a bit over 1,000 watts. That's a lot of heat for a motor the physical size of a 1 HP motor to dissipate.
The motor in a shop vac gets cooled by the air flow but you'd feel the exhaust air to be pretty warm. 1,000 watts are essentially a blow dryer. And that amount of heat generated inside the motor would really affect the life of the motor. Blow dryers use resistance elements to generate that much heat.
Mike
[Energy is conserved. A 1 HP motor puts out 746 watts of mechanical power, by definition. Any additional power input to the motor turns into heat. A motor is a device to turn electrical energy into mechanical energy. A generator is a device to turn mechanical energy into electrical energy.]
Last edited by Mike Henderson; 05-12-2016 at 10:48 PM.
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