Need help understanding electric motors

[Scott Brandstetter]

I started a thread where I am going to build an edge sander from plans bought online. The plans call for a 3/4 HP electric motor at 3450 rpm but in the instructions they state that 3450 is too fast for sanding so they use a 4.5 inch pulley and 3 inch pulley to slow the speed to half. I am looking at motors online (I admit, this is not in my knowledge base) and found that there are 3/4 hp motors with 3450 rpm and then there are 1.5 hp at 1750 rpm.

I don't understand the relationship of HP to RPM. I also question, if the ultimate goal is to get the speed of the sander (belt moving across the wood presented) is considered best at 1750 RPM, why not use a motor rated at this speed and do away with the pulleys and belt.

I hope this makes sense. Any help is appreciated.

[Gerry Grzadzinski]

Electric motors typically come in both 1725rpm, and 3450 rpm. You can get either model in any horsepower. Not sure why, but 1725rpm motors tend to cost a little more than 3450rpm motors.

It looks like almost all edge sanders that are sold use 1750rpm motors. I suspect that when Woodsmith designed their sander, they started with a 3450rpm motor, and then found out that they made the wrong choice, and adjusted the pulley to accommodate the motor they had.

[Malcolm McLeod]

Not to split hairs, but your speed ratio will be ('driver dia'/'driven dia') = (3/4.5) = (2/3). So final RPM is 3450 * 2/3 = 2300.

[Mike Jungers]

AC motor speed is a function of the frequency (60hz) of the applied power and the number and arrangement of the poles (coils) inside the motor. A 1750 RPM motor has more poles than a 3450 RPM motor and typically will cost more for the same HP. There's waaay more to motors than this but this is the gist of it.

[Curt Harms]

Like Mike says, 3450 RPM motors are 2 pole, 1725 RPM motors have 4 poles thus cost somewhat more. There are online calculators to determine driver/driven pulley combinations to yield a desired RPM. Here's the first one that comes up on google:

http://www.blocklayer.com/pulley-belteng.aspx