Thread: How do you slow down an electric motor?

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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.

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. 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. 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. Didn't even hit the barrel! 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. 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!