Thread: How do you slow down an electric motor?

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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!

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 looksbut 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.

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

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.

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?

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?

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!
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!

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

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.