Sunday, April 8, 2018

On The Subject Of Horse Power

Horse power is a subject often discussed and rarely agreed upon because of manufacturers different methods of rating their machinery. Here is an article from Amateur Work on the origins of the term and the basics of how the numbers were determined.


So continuing into the physics of power and horse power, here is an excellent write up from an excellent book "The How And Why Of Mechanical Movements" (see above post). Don't be scared away by the technical insinuation here, this book is another one of those great books I wish I would have discovered at a younger age. The explanations are clear without a lot of tech speak and lots of illustrations to help clear up the tech speak that there is, lol.


 
So why bother with a pony brake, why not just determine HP based on the stated current requirements. Because motors are not, and probably never will be 100% efficient. Friction, power factor, and the big one, wire resistance all contribute to lower available power at the shaft output. A 100% efficient 1.5 hp motor would draw 1119 watts, at 115 volts that equals 9.73 amps. However in real life a sealed 1.5 hp motor will draw + or - 15 amps depending on the manufacturer. At 115 volts this equals 1725 watts and an efficiency of 65%. Some open motors will draw as low as 13 amps indicating efficiencies of around 80% but for shop use, if you can, stick with sealed motors. Dusty conditions such as in the base of a bandsaw or other such application can produce enough fine dust to produce a dust explosion.

If superconductivity at room temperature can ever be developed in materials that can be used to build motors with, then 100% efficiency will be close. It certainly would be world changing, in everything from the electrical grid to the cellphone in your hand.