DIYbanter - View Single Post - Speed control on the 398 and 400 Dremel models

Dave Martindale

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