1. Home
  2. ⏣ General Metalworking
  3. Speed control on the 398 and 400 Dremel models

Speed control on the 398 and 400 Dremel models

Hi all, my Dremel 398, like all Dremel 398's I guess, has what I'd define a lousy speed control: if e.g. I set the speed to 5000 rpm, and then put some strain on it, it's not capable of keeping the 5000 rpm set, but it will slow down to e.g. 3000 rpm. But then engine would be perfectly capable of keeping 5000 rpm under that strain, proof is that instead of 5000 I set it to e.g. 8000, then under the same identical strain, it will run at 5000 rpm. What it would need is a sense of its REAL rotational speed, so with a feedback system it could keep it up. But it doesn't. Now there's the new Dremel 400, and from what I've read they've improved the speed mechanism. Over what? Over the first Dremels in history, or over the 398 I own?

So my question is: has anyone the new 400 and can tell me if it has TRUE speed regulation?

If so, I'm gonna purchase one immediately, otherwise there's no point to "upgrade" my 398 with a 400!

Thank you very much, Andrea

Reply to
andrea
Loading thread data ...

Dude! Your expecting way too much out of a cheap DC motor and an even cheaper chopper drive.

Reply to
starbolins

Starbolins, betraying lack of knowledge but seeming to come on as "informed", blurted "Dude! Your expecting way too much out of a cheap DC motor and an even cheaper chopper drive."

It would appear to me conditions you stated re. regulation, are normal for the Dremel tool. Without knowing the amount of "strain" (load ??) placed on the tool the speed reduction you see is probably normal.

Bob (knows how to spell "you're") Swinney

Reply to
Robert Swinney

blurted "Dude! Your

Dremel tool. Without

reduction you see is probably

I don't think there are enough dremel users who are unhappy with it to justify dremel changing it.

It is Easy to do with a comparator circuit feeding back to the speed control and i dont think much more than a dual op amp but there is not a lot of extra space in a dremel to shoehorn a circuit in and i don't think its worth it for the effort involved.

the cost of the "constant speed" dremel would be negligible too at the manufacturing side

Frankly it works ok for me when i need it just the way it is

Reply to
Brent

Dremel tool. Without

reduction you see is probably

I wonder how many people would pay the price for a dremel with active speed control? It probably wouldn't cost much today to implement. Optical encoder or tach feeding back to a PWM speed control.

Of course, I just adjust how I lean into the work and do it by ear....

Wes

Reply to
Wes

Well, you're probably right on a technical level.

But as a practical matter, I just don't rely on Dremels to do more than occasional small stuff.

The one time I did try to do some real work with a Dremel it overheated, then fried. I replaced it and had the same problem.

So I saved up my pennies and over the years bought:

- a Foredom for most of the occasional light stuff.

- something called Scintilla. Swiss, and I think they were taken over by Bosch. I use this for almost everything else.

- an Air Turbine tool which I never seem to need.

- a Porter-Cable laminate trimmer. I use this when I need a 1/4" shaft cutter.

My suggestion to the OP is to save your pennies and...

DOC

blurted "Dude! Your

Dremel tool. Without

reduction you see is probably

Reply to
doc

I have a 398 too. It seems like it does have feedback speed control of a fairly basic sort: the motor drive current is proportional to the error between the speed you set and the current speed. With no load, you get the set speed. But when you load it, it slows down until the speed error gets large enough to increase the current enough to provide enough torque under load to maintain the new lower speed.

In other words, slowing down is a necessary part of providing more torque with a simple proportional-based speed control. The speed error can be reduced by increasing the gain of the speed-control loop (so current changes faster as a function of speed error) but too much gain can create problems.

It is possible to build more sophisticated speed controls that add a term based on the integral of the error. With a constant load, this will eventually bring the speed error to zero. I donèt know if the 400 does this; I have not used one.

Dave

Reply to
Dave Martindale

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.