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Motor size on homemade disc sander?

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Forum topic by Ben Winchester posted 12-03-2015 11:22 AM 1139 views 0 times favorited 24 replies Add to Favorites Watch
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Ben Winchester

22 posts in 620 days


12-03-2015 11:22 AM

Hey guys, I’d love to make a 12” disc sander, and I’m wondering if the motor I have laying around will be sufficient. It’s a 1/3 hp, but it spins at 15k rpm. It’s an old condenser fan motor. If I play with some pulleys to get the rpm down to a suitable level, will it have the torque I need?

-- Measure once, cut once, hope for the best.


24 replies so far

View dhazelton's profile

dhazelton

2570 posts in 1990 days


#1 posted 12-03-2015 12:50 PM

I have an old Sears combination belt/disc sander and the motor I put on was a 1/3 hp Dunlap and it works fine. You just need a light touch and don’t bear down too hard on it. 15K is crazy fast though. You may want to look for another motor.

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MrUnix

5427 posts in 1892 days


#2 posted 12-03-2015 01:20 PM

A 15,000 rpm fan motor? Does it run on standard 120v AC or is it a DC motor?
(post a picture!!!)

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View Gixxerjoe04's profile

Gixxerjoe04

849 posts in 1270 days


#3 posted 12-03-2015 01:28 PM

Do you have a lathe? I thought about making an attachment to make a 12” disc sander on it, haven’t got around to it yet though. 15k does sound really fast.

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Ben Winchester

22 posts in 620 days


#4 posted 12-03-2015 01:58 PM

I dont have a lathe, but yeah 15000 is crazy fast, thats why i was thinking I could reduce it down with pulleys. I just wonder if it will be easy to stall it. I did some figuring and based on the formula T = Php 63025 / n i come up with a very small number.

And yes, its a standard 115v motor. I was surprised by the rpms too,but its for a condenser so maybe it has to turn fast to run a cooling unit

-- Measure once, cut once, hope for the best.

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Alexl

59 posts in 634 days


#5 posted 12-03-2015 02:44 PM

No way that is actually 15k loaded. a typical condenser fan is 18” in diameter (this is even on the smaller side). This diameter at 15k rpm produces a tip speed of 362 meters/second. The speed of sound is 340 m/s. I am pretty certain that nobody wants a condenser unit that produces sonic boom! (not to mention it would destroy itself)

beyond this, for 60hz AC, the maximum speed with 2 poles is 3,000 RPM.. and this is without any slip.

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dhazelton

2570 posts in 1990 days


#6 posted 12-03-2015 11:13 PM

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MrUnix

5427 posts in 1892 days


#7 posted 12-03-2015 11:21 PM

Yeah, it’s a brush type motor which is not the same as your typical induction motor and doesn’t follow the same rules :) You might be able to use one of those router speed controllers to slow it down as well, but I still don’t think it would make a good disc sander… those things don’t really have a lot of torque.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

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bigblockyeti

4359 posts in 1414 days


#8 posted 12-03-2015 11:28 PM

Given the resources and effort required, I’d be very tempted to find a 1hp induction motor that spins at the right rpm (1725) thus requiring no reduction, and has a shaft and bearings of appropriate size to mount a disc right to the motor. I see them all the time on CL for $50 or less.

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bigblockyeti

4359 posts in 1414 days


#9 posted 12-03-2015 11:29 PM

The maximum speed for an induction motor with two poles at 60hz is 3600 rpm, the maximum speed for 50hz is 3000 rpm.

View JoeinGa's profile

JoeinGa

7683 posts in 1700 days


#10 posted 12-04-2015 12:19 AM

I started out trying a small motor, wasn’t worth a crap. Here’s what I ended up with …

http://lumberjocks.com/JoeinGa/blog/34349

-- Perform A Random Act Of Kindness Today ... Pay It Forward

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TheFridge

7258 posts in 1179 days


#11 posted 12-04-2015 12:34 AM

I think a 5hp might be sufficient.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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Ben Winchester

22 posts in 620 days


#12 posted 12-04-2015 12:59 AM



I think a 5hp might be sufficient.

- TheFridge

Sweet I’ll grab one from work! SAND. EVERYTHING!!

I still don t think it would make a good disc sander… those things don t really have a lot of torque.

Cheers,
Brad

- MrUnix

Hurrah! Thats all I needed to hear! The magic “T” word! I’ll accept that this motor isnt appropriate for a disc sander. Do you think there is any shop application for thislittle guy?

-- Measure once, cut once, hope for the best.

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exelectrician

2327 posts in 2120 days


#13 posted 12-04-2015 01:30 AM

I had a 12” sander once. the problem is that the speed of the disc at the outer edges almost always led to burning of the wood. Now I have a 8” sander everything works so much better. This is one more case where bigger is Not better.

-- Love thy neighbour as thyself

View Alexl's profile

Alexl

59 posts in 634 days


#14 posted 12-04-2015 12:45 PM



Yeah, it s a brush type motor which is not the same as your typical induction motor and doesn t follow the same rules :) You might be able to use one of those router speed controllers to slow it down as well, but I still don t think it would make a good disc sander… those things don t really have a lot of torque.

Cheers,
Brad

- MrUnix

Ah, my mind was on induction motors, thanks for setting me straight. What the hell kind of fan is it supposed to be spinning though? When someone tells me condenser fan, i think HVAC or refrigeration. I have never seen a fan for one of these that would be small enough to spin at 15k


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Alexl

59 posts in 634 days


#15 posted 12-04-2015 12:46 PM



The maximum speed for an induction motor with two poles at 60hz is 3600 rpm, the maximum speed for 50hz is 3000 rpm.

- bigblockyeti

Doh! They say the better you get at higher level math, the worse you are at simple calculations. I am going to go with this excuse :)

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