1725 or 3450 RPM motor?

[RSnow]

I am building a wide belt sander and I am planning to use a 3 HP motor. My question is should I go with a 3450 or a 1725 rpm motor?

For the 3450 motor I will use a 3” pulley on the motor and a 5” pulley on the drum.
For the 1725 motor I would use a 6” pulley on the motor and a 5” pulley on the drum.
With both of the scenarios I’ll end up with the same surface feet per minute, but
Looking at these scenarios I see that with a 3450 will have a small pulley driving a larger pulley which takes less effort to drive the drum than the 1725 which has a large pulley driving a small pulley.

What am I missing here???

 

[edison123]

A question ?

Muthu

http://www.edison.co.in

 

[RSnow]

Two Questions…
1) Does a 1725 RPM motor produce twice the torque of a 3450 RPM motor for any given HP?
2) What are the limitations of one VS the other?
Thanks

 

[jraef]

HP = tq x RPM/5250, so tq = HP x 5250/RPM

So if the HP stays the same, a higher RPM will have less shaft torque. But you will regain that torque in the greater reduction ratio of your pulley if you want the final speed to be the same. So from that standpoint, 6 of one, half dozen of the other.

However the 3450RPM motor will draw slightly less current, so your overhead (parasitic) losses in the motor will be slightly lower. On the flip side, your higher reduction ratio will mean higher mechanical losses (belt heat). So if you plan on running a lot, the lower RPM motor will likely have a slight overall throughput efficiency advantage. If you only run occasionally, you may never notice the difference.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[edison123]

As Jeff says, you won't lose torque in either of the method. A 4 pole motor has less wear and tear and noise and a less stringent alignment requirement than a 2 pole motor.

Muthu

http://www.edison.co.in

 

[MikeHalloran]

Have you done the calculations for the drive belts both ways? ISTR some special steps for speedup drives.

In a sander, motor noise is not going to be a deciding factor. ;-)



Mike Halloran
Pembroke Pines, FL, USA

 

[rawelk]

The 1750 RPM motor would be a better choice also due to belt wrap considerations.

 

[RSnow]

Thanks for the information Guys.

Sorry rawelk…I am not following what you mean by “Belt Wrap considerations”

 

[itsmoked]

All he means is that you can just use two of nearly the same size pulleys which is always better for belt life.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Belt wrap has to do partly with the arc that the belt contacts the pulley. With a drive with a large difference in pulley sizes and close centers, the smaller pulley may have traction issues because the wrap or arc of contact is too little. On the other hand, the torque or drive tension will be less at double speed. And what itsmoked said.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rawelk]

RSnow, it is as itsmoked and waross said. In your case the difference in pulley size isn't much in either case (problems develop when trying to transfer considerable power with a small pulley on one side, and especially when motor and driven shafts are physically close to one another), but is slightly better for the 1750 RPM example.

Another facet is it'll probably take a bit longer, and be twice as large a speed change to go from 0 to 3450 RPM than from 0 to 1750 RPM, and thus encourage a bit more belt slip throughout each start-up.

I haven't used any other manufacturer's, but Gates had good belt drive design software. I don't see it on their site anymore, and appears they've turned it into an online web-based designer. Haven't used this one yet so don't have an opinion.

http://www.gates.com/dfproweb/index.cfm