kevin24018
spending too much on rocks
Member since February 2012
Posts: 284
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Post by kevin24018 on Mar 15, 2012 10:02:19 GMT -5
hope this is the correct section....
when choosing a motor for a polisher or grinder what horse power should one look for?
seems the idea rpm is like 1725.
what I have learned is 3 phase motors require special wiring etc and are way more than an average person like me can deal with.
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Post by jakesrocks on Mar 15, 2012 10:42:45 GMT -5
1/3 HP should be enough for your needs. Any commonly available single phase motor should work, as long as you can shield it from water and grit spray. 1725 RPM is just about right for your needs. You can always slow it down for polishing by using a smaller pulley on the motor, or larger on the arbor.
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Post by johnjsgems on Mar 15, 2012 10:45:38 GMT -5
Unless you are in a commercial building you wouldn't have 3 phase power available. Typically any 6" wheel unit and saws through 10" will work with a 1/3 hp. 10" with abusive hand cutting would be better with 1/2 hp. 8" units with more than two wheels would be better with 1/2 or even 3/4 hp. Bigger units and bigger saws are better with capacitor start motors. Make sure the motor is either "Open Drip Proof" or "Totally Enclosed Fan Cooled" and listed as continuous duty. Resilient mount motors will vibrate less than rigid mount. 1725 rpm is ideal for almost all lapidary applications and pulley choices can vary speed to exactly what you need.
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getyourbone
starting to shine!
Member since February 2010
Posts: 44
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Post by getyourbone on Mar 15, 2012 23:40:29 GMT -5
I've had a question swimming around in my head that I think may make sense to ask here...
Is there a reason to NOT use a 3450 rpm motor and reduce it to a needed rpm?
I have passed up good deals on 34,35,3600 rpm motors because of how high the rpm. all else equal is a 3450 rpm motor reduced to 1725 with pulley size the same as a "native" 1725 rpm motor?
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Post by johnjsgems on Mar 16, 2012 8:43:32 GMT -5
I just bought a nice HP 10" saw last weekend. With it was a 3/4 hp capacitor start motor 3450 rpm. All the speed charts I have are for 1725 motors but it seems to me if the speed can be reduced to 1725-2200 it should work OK. I'll likely use a smaller motor and save the 3/4 hp for something else but a 3450 rpm pulley chart would be very handy. A 1725 with 1.5" on motor and 3" on pulley is listed as 875. I'm guessing same pulley set up with 3450 would reduce speed in half. Then again, I'm pretty not smart.
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kevin24018
spending too much on rocks
Member since February 2012
Posts: 284
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Post by kevin24018 on Mar 16, 2012 9:48:01 GMT -5
I too have thought about a higher rpm reduced down, but I have to wonder if those 3500 rpm motors have as much torque as the 1750, I'm thinking they don't and that's why the lower rpm motors are used and perfered.
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Post by catmandewe on Mar 16, 2012 9:57:13 GMT -5
I think you are right on about the torque Kevin. I have tried the 3450 motors and used pulleys to slow them down, they do work for a while but they burn out fairly easily. It works good until you put a really hard or large rock in it then the motor starts smelling and pretty soon its a goner.
Tony
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Post by catmandewe on Mar 16, 2012 9:58:08 GMT -5
Of course that was on a saw, maybe a grinder won't load it like a saw does.
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Post by parfive on Mar 16, 2012 10:11:30 GMT -5
3450 motor reduced to 1725 doubles the torque at the driven shaft. Hp remains the same.
Just like amps and volts – one goes up, the other goes down. Power (watts) is unchanged.
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getyourbone
starting to shine!
Member since February 2010
Posts: 44
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Post by getyourbone on Mar 16, 2012 10:33:40 GMT -5
So how does hp figure into this? A factory 1/2 hp 1725 rpm would have twice the torque as 1/2 hp 3450 rpm of the of the same design? Part of the reason I ask is that I am amazed that Diamond pacific gets away with a 1/4 hp motor on the genie.
I do realize that there is a real world difference between a harbor freight rating and a industrial - Baldor rating. By the way what rpm does the genie have?
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kevin24018
spending too much on rocks
Member since February 2012
Posts: 284
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Post by kevin24018 on Mar 16, 2012 11:29:22 GMT -5
Generally speaking i think when they make motors they tend to give the lower rpms more torque because of what they are ment to do. If a motor has low rpm chances are its because it needs to work hard, not fast. Look at fan motors very high rpm but very little torque. This isn't always true but as a general rule I think it is. just my opinion.
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Post by johnjsgems on Mar 16, 2012 12:19:19 GMT -5
Genie is 1725.
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Post by parfive on Mar 16, 2012 12:32:08 GMT -5
I run six wheels with a 1/8 hp motor no problem. 14” saw has a 1/3 hp motor, no problem.
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kevin24018
spending too much on rocks
Member since February 2012
Posts: 284
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Post by kevin24018 on Mar 16, 2012 14:14:42 GMT -5
I run six wheels with a 1/8 hp motor no problem. 14” saw has a 1/3 hp motor, no problem. are these "stock" motors or what did they come off of? what's the label say on the 1/8? rpms etc
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morsefire
noticing nice landscape pebbles
Member since November 2011
Posts: 83
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Post by morsefire on Apr 8, 2012 19:17:57 GMT -5
I think 735 watts = 1 horse. Or something close to that.
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dshalldms
starting to spend too much on rocks
Member since January 2008
Posts: 113
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Post by dshalldms on Nov 2, 2012 5:26:30 GMT -5
Hi,
Here in the UK a standard single phase motor has nominal speeds as follows:-
230v 50cycles 2 pole 2900rpm standard torque 230v 50cycles 4 pole 1450rpm twice the torque 230v 50cycles 8 pole 725rpm four times the torque
The HP rating of the motor would remain the same regardless of the number of poles. Only the speed and torque change.
This goes a long way towards explaining why some manufacturers get away with using smaller HP motors and others over engineer their products by using larger ones.
The speeds would be slightly difference in the USA because of the electrical standards.
Regards,
Derek
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jamesp
Cave Dweller
Member since October 2012
Posts: 36,159
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Post by jamesp on Nov 3, 2012 10:02:48 GMT -5
The Heating and Air industry uses a lot of 1000 and 725 rpm motors that are 1/8 to 1/2 horse single phase for the fans often operating of 110 ac .Scrap metal places are uaually dismantling roof top units for copper AL and steel.I really like the slow speed for lapidary stuff.And talk about cheap.
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slabbercabber
starting to shine!
Member since March 2010
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Post by slabbercabber on Nov 3, 2012 17:49:05 GMT -5
I think 735 watts = 1 horse. Or something close to that. Yes 745.6 to be precise. but there is an efficiency loss. Generally around 9 amp draw for a one horse motor. Efficiency goes down with fractional motors. Motors are wound with more coils to slow the speed. This increases the cost and as a result slower motors are often built to higher standards of quality. The best indicator would be service factor. Never install a motor with sf less than one for a continuous duty use. 1.15 or more is better.
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