Showing posts with label motors. Show all posts
Showing posts with label motors. Show all posts

Wednesday, November 7, 2018

Motor Horsepower, Again?

So in issue # 4 of Woodcuts, is another article on motor horsepower, to compliment the two I have previously posted. This one deals with universal motors such as the one found in your router. Your 3 1/4 hp labeled plunge router, doesn't actually put out 3 1/4 hp as claimed. If it did you would trip your breaker as soon as you did any work with it, and if plugged into a heavier circuit would burn out pretty quick. The consensus is consistent with other types of motors, they are actually 1.5 hp.

This is a short but very clearly articulated opinion on motor hp. The author Peter Scott, if he is not a teacher, he missed his calling. The very short, but straight to the point explanation of "power factor" (a concept understood by few) is clearly communicated. My college instructor didn't come anywhere near this clear an explanation.

100% efficient electrical usage runs at a power factor of 1 or "unity". Resistance circuits such as heaters run at a power factor of 1, all of the incoming energy is converted to heat. Motors are different, Peter explains it well.

Case In Point: I have talked about operating the power house at Port Arthur Shipyard before. Much of the equipment in this power house was still original from the pre-WW1 era. To save on our energy bills the power house monitored "power factor". Induction motors have poor power factor, synchronous motors run very close to unity. When we were running all three 300 hp AC to DC converters, to supply the dry dock and foundry with DC power, the big induction motors would pull the power factor below .85. At that point we would shut down one of our induction motor compressors, and start a 300 hp synchronous motor, running a Joy compressor. The synchronous motor would bring the plant power factor back up to .9 to .92, from .80 to .85 saving the shipyard a good amount of money on their hydro bill.

So without further ado here is that short but well communicated article on the subject matter.


Saturday, September 22, 2018

Belts / Pulleys / Motor Mounts

So powers back on , out almost 24 hrs., must have been a lot of trees down out there, I seem to have gotten by unscathed. Could have been worse, the Ottawa area got hit with two tornadoes that leveled a small neighborhood and knocked the power out to half a million people. It was part of the same storm system.

So here is chapter 37 from "Complete Book Of Home Workshops". This chapter covers belts, pulleys, and motor mounts. For the novice setting up a home hobby shop much of this information is must know, for the long experienced, much of it is considered basic, but basic, like "common sense", sometimes isn't so common. If you get your information from interactive sites, there is always that one character who gets joy from convincing you, the wrong answer is the right answer.

So here's 8 pages of motor and tool speed set-up, common sense information.










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.