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Hi everyone. A friend has given me an inexpensive grinder motor and wheels, that he was planning on using as a boat anchor. I have several sanding mops that I use mostly for scroll saw work and to do this, I have to change them out on my drill press. The grinder motor spins way to fast to use for some of the delicate parts that come off a scroll saw, so I was thinking about buying one of the speed controllers for a router that are available. Finally, the question...do you think it would be safe to run the grinder with this speed controller, or would it burn out the motor windings? :help:

As always, THANKS for the help. I wish everyone a good day.
 

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Speed controllers only work on universal type motors. I believe that if the motor has a starting circuit that kicks out once the motor is up to speed it won't work. It may depend on how old it is so more info and maybe a picture or two might be necessary.
 

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All the bench grinders I have seen are induction motors, which like Chuck said do not work with speed controllers.
The only way you might use it is to use step pulleys and a belt and a separate grinder stand, which is a lot of fussing around. Another idea is to get an old drill motor and build a stand to hold it. Then use the speed controller and chuck up the sanding attachments.
Herb
 

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Rick; another approach might be to use the motor with a small pulley to drive a shaft (large pulley) on pillow blocks, with your buffing wheels on that. Remember the shoemakers used to have that arrangement in their work area? Several different 'tools' on a long shaft? Somebody here might have a better description. As an afterthought, I think lapidary hobbyists do the same thing.
 

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Yeh, but I couldn't resist ...that is one pretty piece of equipment, Herb!
You could accomplish the same thing with the line shaft and stepped pulleys off the salvaged motor...like a drill press setup
 

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do you have a lathe that you could use?
 

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First off, I would NOT recommend trying this.....

That said, a guy I knew had a combination mill/metal lathe that had mechanical, but limited, speed control, with an induction motor. He put one of the router speed controls that you can get cheap at Harbor Freight. He makes sure it is set a full speed, turns the machine on, and proceeds to slow the motor with it. I was amazed he did that with out smoking the motor!

So, while it TECHNICALLY worked, he is very likely adding a LOT of extra heat to the motor's windings. I can't imagine that will continue to work as a long term solution for him, and if I were to guess, he will end up with a bad motor of continued use with it. Not to mention the drastic decrease in working power of the machine.

So again, I would NOT recommending putting a speed control on a universal motor, as you risk permanently destroying it.
 

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First off, I would NOT recommend trying this.....

That said, a guy I knew had a combination mill/metal lathe that had mechanical, but limited, speed control, with an induction motor. He put one of the router speed controls that you can get cheap at Harbor Freight. He makes sure it is set a full speed, turns the machine on, and proceeds to slow the motor with it. I was amazed he did that with out smoking the motor!

So, while it TECHNICALLY worked, he is very likely adding a LOT of extra heat to the motor's windings. I can't imagine that will continue to work as a long term solution for him, and if I were to guess, he will end up with a bad motor of continued use with it. Not to mention the drastic decrease in working power of the machine.

So again, I would NOT recommending putting a speed control on a universal motor, as you risk permanently destroying it.
The start circuit in an induction motor is normally engaged any time that the motor speed is below about 80% of it's plated speed. The speed of the motor is primarily a function of the supply frequency, with the motor load having a slight second order effect. Increasing or decreasing the voltage applied only affects the current drawn, lower voltage means higher current needed to produce the same power, means more heat produced in motor windings, leading to baking the insulation. That's why a lot of moderate to high power tools come with recommendations about not using them with long (or any) extension cords. The voltage drop in the cord an be underestimated by the lay person, which results in a low supply voltage to the motor, excess current draw through the cable, increased voltage drop in the cable, in an every repeating cycle until a breaker trips, cables fail, or the motor bakes.
 

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A hand drill is a universal motor so a speed control will work with them. How about building a frame to hold one of them so that you can run it hands free?
 
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