I use my Dremel a lot, A LOT, and I use it so much on the slowest speed that the first "stop" on the speed slide often doesn't work or it cuts out while using it so I move to the next speed and it is faster but sometimes it drops to the slowest speed while being on the second "stop". This is an "older" (maybe from 2000-2004) 120v model.

I've taken this apart a few times and looked at the speed selector switch and it looks like a linear (straight) slide resistor with a copper surface and maybe graphite slide. The slowest stop has a black spot where it looks like it is either warn or something. I've used a wire brush on it and cleaned it very well a few times. I looked at a new switch and it was almost the price of a new unit and the new units aren't as nice IMHO.

I think the speed switch is a potentiometer though I haven't measured the resistance on it. If it is a resistor that controls the speed, I've read that this often leads to greatly reduced torque at lower speeds. I'm wondering if I used a PWM with DC power (maybe just rectified 120?) if I would get more torque at lower speeds (same rotational speed) than with the resistor switch. Also, would adding a capacitor somewhere in the existing circuit help anything, or if I tried DC adding the cap definitely changed the voltage in the power supply I just made. IDK if that would make the DC voltage too high if I went with a PWM, but if that is the case, I could limit not use 100% of the rectified/capped DC w/PWM to the proper pulse/% that would = the power of the AC voltage * amps.

So if it draws 120vac * 2.2 = 264 watts.

With rectified DC with a cap I found the voltage to be about 168-170 so if it was drawing 2.2 amps the wattage would be 372 watts so I could limit the PWM to stop at 70% of a full pulse width which would equal the same wattage as the AC with the same amperage. Though it seems like something is wrong and somehow the rectification of DC is producing more power/wattage.

So are any of these ideas worth trying or looking into or is there something else that can be done, like just adding a capacitor to the unit as it is?

 

High torque + Low speed with Dremel can't be done. At least not easily. Some sort of DC PWM control might help, but I do not consider that simple.

Of course, the ultimate question is why do you need slow speed?

I have used Dremels for >65 years.

 

I too have used my 10yr+ model without really any problem, never opened it up, but I would assume that a Triac is involved for speed control.
Generally, especially with a series motor, there has to be some kind of feedback device in order to accurately control RPM.
Max.

 

You could build an external speed controller with PWM or keep it lenear like this.

 

Dremmel isn't designed for low speed high torque. It's design is intended to let the high speed do the work. Y'know, force over area X time. If you want higher torque, the motor isn't designed for that sort of application. You'd have to go with some sort of gear reduction. 2:1 (5000 RPM to 2500 RPM) means half as fast but twice the torque. 10:1 will give you ten times the torque but also take ten times longer to do the work.

Bottom line: The machine is capable of doing so much work over a given period of time. Slow it down and it takes longer. But you can apply more force and reduce the amount of time. In the end the work done is the same.

So why do you want to slow it down?

 

I've read that this often leads to greatly reduced torque at lower speeds.

The reason for that is "Inverse Square Law". Lower speed means you've reduced the amount of energy, which means lower magnetic force. Slow it down by half and you get 1/4 the work.

Here's how magnets work: Two magnets opposing each other held at 1 inch apart exert a force against each other. Let's call it M (or 1M). Force the magnets to half the distance and now you're getting 4M of resistance. It's a squaring function. A one inch square has one square inch. A two inch square has four square inches. Don't believe me? Draw a square one inch wide by one inch high, then draw it again, only use two inches. Now calculate how many square inches there are in a 2 inch square.

So running a motor at a reduced voltage doesn't mean it just runs slower it means it runs weaker too.

 

Thanks for all the replies, many of them touch on the exact problem and explain why it is happening (Thanks TonyR).
The reason I need slower speeds with reasonable tourque is because I often work with plastics whether it is cutting, grinding, sanding, shaping, etc and the problem with the plastics I work with (and most I've worked with) is that these tools melt the plastic very quickly at high speeds and I've found that using a sanding drum works best at the slowest setting but the problem is that I spend 1/2 the time waiting for the tool to speed back up and very little pressure applied to the drum slows the rotation considerably.

Now I don't think I'll have a problem with the melting if the speed didn't decrease the way it does, though I'm not sure. It really depends on what you are working on, the size of the surface, so I can usually move around and not linger in one spot to melt it. Slower speeds also help reduce the dust with various materials as well.

IDK, maybe I'm not using it the best way and using a higher speed may be better, but there are many things where high speeds are problematic due to human error (cutting too much of the part) and a slow speed seems to work best if it didn't slow and take so much time to speed back up.

 

The article I posted (post #4) shows ways to control the voltage supplied to the motor ro maintain a low speed so as the load increases the voltage increases so maintaining the torque at low speed.

 

Maybe get a Dremel flex shaft like this one
Wadoy Rotary Tool Flex Shaft Attachment for Dremel Wen Roybi Keyless Chuck Collet Grinder
and attach it to a lower RPM motor or a multi-speed drill press.

 

You need a geared adapter. I bought one at the 99cent store like 5 years ago. But for 1/4" shafts. You need 1/8" shafts if for Dremel
Unable to find an exact picture or the product to link, but mine looks like this :

Mine is for drills, suspect that for a high rpm Dremel, its gears would not survive.

These exi$t:
----> http://www.directindustry.com/prod/laudenbach-formtechnik/product-110475-1571413.html
and:

I have available 3 like these, new, in box, geared 128:1 :
----> https://www.google.com/search?q=esc...CigB&biw=1305&bih=619#spd=5045975742830504364

They are like low rpm Dremels, the size of about a 'C' cell. And could work with :
----> https://www.aliexpress.com/item/5Pc...3309-42b4-930b-e38071099b54&priceBeautifyAB=0

 

There at the russian internet circulates the wonderful diagram about 494 sinchronous detector generating a PWM pulse and in pause measuring the motor response. Thus it is able to provide about full power even at 1% of nominal rpm while keeping those rpm enormously stable. Only bad thing is that there is VERY sophisticated to boost that circuit over some 20...40 Volts. However, Your case, most of dremels are working on 19 Volts so its accurate for You.
Idea itself is let the PWM output steers the p-channel IRF9530 what steers motor. But motor anti-splash diode provide a voltage toward identical (this is very important note) transistor, what after it integrates the pulses, via potentiometer of negative loop sensitivity gives the signal back to IC input for PWM adjust, or determins an adaptation to inertia of motor= minimizes the overregulation. Yet at frequency adjustment chain another potentiometer gives choice for rpm what must be kept. I shall try to find this circuit, as it had been never published in Latin based websites, sadly.

 

Here is newer version of circuit http://artradiolab.ru/3.6.htm

The right one circuit is those hardly end part in article, with 2 pieces IRF9540 while the most first illustrates the wrong concept yet widely re-published at the www. All article is about why the first one is not good enough. May use a Google translator for to read the text. Its worth to.