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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Speed control for small d.c. motors
Date: Thu, 26 Sep 2002 01:25:40 GMT
NNTP-Posting-Date: Wed, 25 Sep 2002 18:25:40 PDT
John Woodgate wrote:
> I read in sci.electronics.design that Tony Williams
> wrote (in <firstname.lastname@example.org.
> uk>) about 'Speed control for small d.c. motors', on Wed, 25 Sep 2002:
> >One of those 8-ohm motors had an Ra that was much
> > higher (>50 ohms) at low current. Which is why I
> > noted the measurements as being "at stall". It
> > was as though the commutator needed a significant
> > wetting current.
> That's typical, yes.
This seems to have to do with the rotation angle of the commutator. The
only brush motor I have at home (I am drowning in DC brushless motors)
is a 24 volt servo motor with a 1024 line encoder with index. It is
about 20 ohms but at various rotational positions, as high as 150 ohms
can be measured. This shows why the some of the highest quality servo
motors you can get DC brushless types.
The variability of resistance with angle in brush motors is less of a
problem with spring loaded carbon brushes than it is with wire brushes.
The problem I have seen with BEMF control with small cheap motors is
that with little inertial load and at low speeds, the motor torque
ripple causes large speed variation anyway. A turntable or the like has
high enough inertia to reduce the speed ripple due to torque ripple to
very small values. Of course, a lot of small DC brushless motors have
far less torque ripple than brush motors of comparable size.
Unfortunately, brushless motors require more support circuitry. The
drive circuit I currently use uses a Unitrode (TI) UC3625 for
commutation and safety circuits and 6 MOSFETs for up and down drivers. I
developed much of my spindle drive code with a simpler DC brushless
motor driver using junk box NPN and PNP transistors for up and down
drivers and a few gates to decode hall switch states into coil states.
As I recall, one or two 7486s will decode all required states and some
inverters drive the transistors.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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