How do you slow down an electric motor?

I recently bought out a couple of pieces of equipment from a metal shop where the owner was retiring. One is a vertical metal cutting bandsaw that weighs like a ton, and cuts great. The other piece is an equally heavy 6 x 48 inch belt sander/ 12 inch disc sander combo. It also is heavy cast iron, and the 6 x 48 sanding belt drives directly off of a steel drum on one side of the motor and the disc drives directly off the other side. There are no belts, pulleys etc.

The thing is downright dangerous..........it spins at way too high of a speed, so much so that it scares me to get near it when it is running. I put a good Norton disc on it and it shapes metal like there is no tomorrow, but the other night Dan was shaping a piece of steel and the sanding disc exploded. It knocked his safety glasses off and put a cut on his face. Luckily, it isn't a deep cut and I think he won't have any scarring. He said he will never use that thing again, and I can't blame him. I considered just tossing it in the dumpster so that no one would ever get hurt again, but it really is a nice quality piece, and if I can get the speed under control I think it will be ok.

I have researched on the internet either a variable or fixed controller to drop the rpms maybe in half, but am coming up empty. The motor is of unknown HP as the self adhesive info tag is long gone, but it is a 120 volt motor, maybe 3-5 HP if I had to guess. I've looked in Graingers catalog and still can't find what I know must exist somewhere. Not sure if the control would work by cutting down the voltage or the pulses, or whatever. Cutting down the voltage would be a problem because when we were first trying the sander out we used a 25 foot extension cord and it overheated the cord and the motor stopped running.............it draws a bunch of juice.

So what am I looking for and where do I find one? Thanks for any help you can give.

Don

Is it direct drive off the motor, or belt driven??? If it's belt driven then maybe a pulley change would work... Otherwise, find a hand held tachometer, check the motor rpm and buy an equal sized motor with lower rpm.

What you can use is a variable frequency drive. You might call these guys http://www.variablefrequencydrives.n...encydrives.htm to see if they can help, but I expect they are going to need nameplate information to tell you anything. I have no clue on their cost, but expect it might be cheaper to just change the motor to a half or quarter speed unit if you can find one that fits.

That's the problem........it is a direct drive deal with the rest of the sander built around the motor in the middle. All the brackets and framework are an integral part of the motor, just one big cast iron affair. The base is even part of the motor.

What is confusing is that this thing was manufactured as what it is, so how did it run for so many years spinning away at that speed? The old guy who I bought it from said something about it "having some issues with the sanding belt" when we looked at it, but I found two bearings in the top drum that are worn, and replacing those should make the belt run truer. He failed to tell me this thing kills people! :eek::D

Don

sounds like the same belt sanded like i have .i do not run the disc sander any more after having the same thing happen to me .there was a sound coming off the sanding disc look over and BANG 80grit sanding disc to the face hit me so hard it look the the safety glasses off my face hit me in the lip and teeth so hard i was thinking i open my lip up. it was stringing could not tell at first . but i did not do any thing to my good looks:3dSMILE:but i did get very sick looking at the safety glasses .if i did not have them on i would not have the use of that eye . you can not slow it down to much with out hurting the tq of the sander the ac motor needs to run at max rpm for tq if you cut it down with a speed controll it may not work as good . i have a ac motor machine with a frequency drive on it did not work that hot step it down with pulleys .it still needs more tq .with a dc motors work better with a controller and you can turn up the tq at low rpm i have used the baldor and the danfoss drives

find any electric motor service company--they will know what wires to couple to change speeds or voltage

I also have a 6x48 belt sander with 12 in disc (belt driven) but I don't use the disc anymore---too many times a slight change in where you are sanding will change the force direction and you'll lose your hold on whatever your trying to sand/grind. And the belt sands in a straight line vs circular for the disc--makes it easier to control finish of what ever your working on

That might be the thing, just forget that the disc is there and use the belt only. Even that part is a little intimidating because it whips around pretty fast too. But at least if the belt breaks (which has happened twice already......but that is because the top roller bearings are loose and letting it wobble a little at max speed) it won't do the damage the flying disc will. I really would like to get this thing working right as it removes metal like right now.

I thought of calling an electric motor shop as you suggested and asking them what can be done to the motor to slow it down.........I think I'll try that. Funny part is, the 76 year old man I bought it from earned his living using it for the past who knows how many years. Maybe he just ducks faster than we do.:LOL::LOL:

Don

All you need is an inverter. As long as you keep the motor within 20% or so of its rated speed, you have have adequate torque and other operational characteristics. You can go even slower, but I personally like to keep it within 20%.

If you have three phase power, then I have a nice supply of inverters, could hook you up. If not, then you're not out of luck, they do make single phase inverters. ( Variable Frequency Drives. ) It would be probably more expensive to get a DC motor and drive than just an inverter. I promise you would love the machine after you put a speed drive on it. You might find yourself adjusting it constantly for different jobs, you you might find a happy medium and leave it, but either way the machine won't be in the dumpster.

I am unclear on how a sanding disc can explode? Are you talking about the steel plate upon which you attatch a sticky sandpaper sheet? yikes.

a sanding disc will explode/disentigrate when you run something across the center of it where it is sanding in multidirections instead of at the outter edge where as you would only be applying force in a smaller arc---at the center the forces are omni directional

the sanding disc they call them PSA like DA sand paper but with a stonger backing the metal backer did not blow up the paper lost the hook up on the face plate .so i got a close up look at it as it hit me. i have 4 machine that run a frequency drive one 3 phase AC and one DC .the dc just seams to work better for me and small feed motors for cutting speed on the mills .i wish my AC motor work as good as my DC but will not.i know some things about this stuff. just enough to get in trouble

5 hp motors that run on 120 vac are pretty rare since they draw so much current.

First thing to check: has the motor been changed? 120 VAC single phase motors normally come in two flavors; 1725 rpm and 3450 rpm. Obviously, if some nimrod changed motors and put a 3450 in place of a 1725, things get awfully fast! You should be able to find at tag....if not, borrow a mechanical tachometer that you hold against the end of the motor shaft. If you cant find one, take the motor to an electric motor repair guy....they are normally really cheap to deal with and can probably either fix your motor or suggest what to do. Clamp the leads and determine the draw:

AMP DRAW
Motor Size 120 volts 240 volts

1/4 hp. 6 amp. 3 amp.
1/3 hp. 7 amp. 3½ amp.
1/2 hp. 10 amp. 5 amp.
3/4 hp. 14 amp. 7 amp.
1 hp. 16 amp. 8 amp.
1½ hp. 20 amp. 10 amp.
2 hp. 24 amp. 12 amp.
3 hp. 34 amp. 17 amp.
5 hp. 56 amp. 28 amp.


mike in tucson

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Originally Posted by pat mccarthy

i have 4 machine that run a frequency drive one 3 phase AC and one DC .the dc just seams to work better for me and small feed motors for cutting speed on the mills .i wish my AC motor work as good as my DC but will not.

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Might be the inverter. What HP and what brand is it? Oh, and what means for the speed control? Do you spin a potentiometer, or dial it in on the keypad?

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Originally Posted by firebird77clone

Might be the inverter. What HP and what brand is it? Oh, and what means for the speed control? Do you spin a potentiometer, or dial it in on the keypad?

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AC 3 phase motor i think 4 hp 1750 rpm step down about 450 rpm .the frequency drive is a baldor key pad and has a rpm read out with a speed dail on the machine . can get the numbers if needed .thanks

Don, there should be a shop around where you live that could fix this for you. It sounds like a nice sander, just might have been hotrodded by a past owner, maybe Tim the Tool Man Taylor!:LOL:
I've had luck with fixit shops staffed by older folks who were around when some of my vintage woodworking tools were being manufactured. The young guys just look for a manual and replacement part, the older folks came from a generation where you fixed it, not replaced it! Glad no one was seriously hurt!

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Originally Posted by pat mccarthy

AC 3 phase motor i think 4 hp 1750 rpm step down about 450 rpm .the frequency drive is a baldor key pad and has a rpm read out with a speed dail on the machine . can get the numbers if needed .thanks

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If you are running a 1750 rpm motor at 450 rpm, that is 26% of its rated speed, and you are probably having torque issues.

Baldor is a good name in inverters, I have had a lot of luck with them in the past. Dig out the manual, and see if there are some parameters you can adjust to improve the torque at low speed.

If you can give me the model #, I might be able to tell you which parameter(s) to adjust.

here is mine i try to get a shot at the rpm of the motor looks like 3000 rpm

Thanks guys for all the suggestions and info. Not sure which direction I am going to go, but you all have given some good sources to start checking. I do have one of those handheld tachometers that you hold up against a spinning shaft and will check it when I get to the shop today.

Pat, mine looks VERY much like the one you pictured, in fact I think they are the same. 3000 rpms seems about right. Here are a couple of pictures of mine. I have the top portion of my belt assembly removed as I am replacing the bearings.......it was wandering and it looks like the bearings being worn might be the culprit.

Don

Man that band saw would come in handy. It was a pain cutting the spacers for my alternator bracket with a hack saw!

Steve, that horizontal bandsaw in the one picture has been a lifesaver. We've worked that thing hard for about 4 or 5 years and it has improved the quality and ease of our work so much. It flips up to become a vertical bandsaw too, but it was always marginal at that aspect.........runs too fast and blade too wide. So I have been looking for a used metal cutting vertical one for a while. Finally found the grey one in the other picture, bought it from the same older guy who was closing up shop. We've only begun to use it, but love it already. The speed is super slow and it cuts curves like one should.

Tools really do make the job so much easier. What I would give for one of those waterjet babys like you see on American Chopper ! :D:D

Don

yep it the same one and it is 3000 on rpm .it will burn wood if you try to sand it:LOL: them two screws at the top tune in the belt to track see my belt is not very wide they will move when the belt starts to wear in and start to track dif .i know 2 guy s that have the same sander s and you have to play with it some .here a tip leave the side tin cover off till you get the belt to track a fresh belt will cut thru it if track is off. i can tell you i have had the belt sander for some time about 5 years it runs fine .but it a china motor and if you try to slow it down i would bet you will burn it up .just get use to the speed thats where it needs to be .i run 40 grit on the belt .you have to watch it when the belt is fresh. i have had it take 1/8 plate steel eat it and spit it back at me .do not sand any thing that can get thru the air gap from the table and belt:eek:

I agree Pat, these things are wicked. I put a hand held tach on it tonight and the tach goes to 3500 and it passed it up by 500 rpms, so I guess it runs at 4000! I did play with the belt adjustment trying to get it to track straight, but the problem is, when you first start the motor the belt goes to the left, then as the motor speeds up it goes to the right. I watched it spin, and the bottom drum tracks straight, but the top one wobbles a little. I pulled the bearings out and they seem to have some slop, so I am going to hit the bearing house and replace them.

If that doesn't do it, I am going to have the top drum trued up on a lathe, but I can't see how a steel drum would have gotten deformed. We've pretty much decided to concentrate on getting the belt portion working and forget the disc part. We already have one 12 inch disc sander that runs at a sane speed, so we will continue to use that one.

Don

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Originally Posted by Itoldyouso

I agree Pat, these things are wicked. I put a hand held tach on it tonight and the tach goes to 3500 and it passed it up by 500 rpms, so I guess it runs at 4000! I did play with the belt adjustment trying to get it to track straight, but the problem is, when you first start the motor the belt goes to the left, then as the motor speeds up it goes to the right. I watched it spin, and the bottom drum tracks straight, but the top one wobbles a little. I pulled the bearings out and they seem to have some slop, so I am going to hit the bearing house and replace them.

If that doesn't do it, I am going to have the top drum trued up on a lathe, but I can't see how a steel drum would have gotten deformed. We've pretty much decided to concentrate on getting the belt portion working and forget the disc part. We already have one 12 inch disc sander that runs at a sane speed, so we will continue to use that one.

Don

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the track is very touchy look at the side there a flat plate .the guy i know added another spring plate to it he did seam to think that help. some belts just do not track as well as other sanding belts. that is why mine is so thin .the best thing is to do is just bump the start button and see wich way she going and move the thum wheel the other way. them two up top just a very very small bit till you can hold the button on and have it track good .on the top drum you do not want to turn it flat. it should have some crown to it it will hold track .you cut that out it will be very hard to keep true or put some more crown in

you may want to play with the flat plate the belt backer so it not holding up the belt .we all pick up are belt sanders at the same time. the other two guys did not work as good as mine .my budy was not happy .but i played with it a bit the othe two guys pick up a wear backer made from teflon from grizzy tool i think

That was one thing we thought of, it seems too tight against the belt. BTW Pat, what HP are these things?

Don

if i go to the shop today i get that for you

Thanks.

Don

here is what is on the motor tag.. belt speed 5000 sf.p.m
motor r.p.m 3000
1 hp motor.
cycles 60/65
voltage 110/220
use only wheel suiltable for 3600 rpm:eek:we know thats right**):eek:

Dan mentioned last night that he sort of noticed a nick in the sanding disc before he started using it, so maybe that was why it fractured. He didn't think anything of it at the time, but in hind sight it may be been the problem.

Thanks for that info Pat. Now I kind of have an idea of what I'm dealing with.

Don

Don,
You can buy one of these for $300....
http://grizzly.com/products/Combinat...-Series/G1014Z
Mine is about 12 years old, has been through about a half dozen belts, and is doing fine.

Yeah, I would have been better off, I paid $ 250 for the one I bought because I was in a tool buying mood. :D We have some Grizzly tools, like our metal brake, and their stuff is nice.

I may give this one to someone I really don't like very much.:rolleyes: Reminds me of a 12 gauge shotgun that I sawed off short one time years ago. . Made a pistol grip on it ,barrel was like 12 inches long, kept it next to the bed for protection. One day a buddy talked me into shooting it so we went to a dump and made a target out of a 55 gallon drum. When I fired it the d*** thing blew right out of my hands and the hammer acted like a can opener, slicing my hand between the thumb and finger.:eek: Didn't even hit the barrel!:o Went home with a rag wrapped around my bleeding hand.

I never fired that gun again and said that if someone broke in I would hand them the gun and tell them to shoot me.......they would be hurt more than I would. :D That gun now resides in some long forgotten river, in a million pieces. This sander reminds me of that gun.

Don

I think we've all owned a tool or two that seems to be Satan spawn! I had a belt sander once that had a habit of getting too much wood off too fast. The damb thing would practically rip your arms off on contact. Bought a more expensive one that has always worked great. I think the original had too much rpms like yours! Mine was a hand belt sander. In the mid west I hear they actually have sander races!:HMMM::D

I was going to say the best way to slow it down is to unplug it..................

But somebody might punch me in the mouth if I say that so I'm just not going to say it.

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Originally Posted by pro70z28

I was going to say the best way to slow it down is to unplug it..................

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Kinda technical stuff for the Friday night crowd!!!! Hope there's instructions you can download somewhere?????

I looked for plug pullers, but all the youtube stuff was just switch flippers.

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Originally Posted by pro70z28

I looked for plug pullers, but all the youtube stuff was just switch flippers.

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am i missing something ? hell this was a post i started to slow down my tobin arp ? i just pulley it down over 100+ set of heads latter its fine :) ok ok i have old timers but i did start a post about slowing down a AC motor ?? sorry :(

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Originally Posted by pro70z28

I looked for plug pullers, but all the youtube stuff was just switch flippers.

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So, which end of the plug did you end up pulling??? Like I said, purdy technical for a Saturday night!!!!:whacked::confused::confused::whacked:

A controller that will handle the load is going to cost more than the tool is worth. Chuck it and find a replacement, or strip the guts out of the motor and use it for a jackshaft and drive it with another motor and a belt drive so you can change the RPM with pulley ratios.

i am NOT an electrician.
couldnt a guy just put a rheostat in the power cord?

you guys may want to look back at this post started back in 6-14-2009 this was on a belt sander that don posted on thats is the same RPM as mine that i have had for 6years now i use every week at the shop .he never posted back on his belt sander so he must be use to the RPM by now that it runs at .the belt sanders use a china motor on them its best not to play with them much as it could burn up it .it needs the rpm so it can make power and take the load.slowing it down may put more heat in to the motor and burn it out ?????