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

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

On Sep 2, 2:25 pm, "Robert Swinney" wrote:
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

wrote in message

oups.com...

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

"Robert Swinney" wrote:

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.


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

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




On Sep 2, 2:25 pm, "Robert Swinney" wrote:
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

wrote in message

oups.com...

writes:

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.

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