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Ned Simmons

On Fri, 03 Oct 2008 15:45:37 GMT, wrote:

I've got a couple DC servo motors that are in great shape but old
enough that Electro-Craft no longer has any info on them. So I would
like to know if there is a way to figure out what max voltage and
current might be. Can using the motor as a generator be used as a way
to determine the approximate proper voltage and current?

I'd first compare the motor to motors of similar size and construction
to get an estimate of torque rating and max RPM.

Then experiment using the relationships
Ke = RPM/volt
Kt = lb*in/amp
to get values for Ke and Kt.

Either rotating the motor at a known speed and measuring the output
voltage, or powering the motor with a known voltage and measuring RPM
will give you Ke.

To determine Kt you'll need to control torque and measure current, or
vice versa. The easiest way would probably be to drive the motor with
an adjustable current source with a torque arm and weight attached to
the shaft. Adjust the current to balance the weight with the arm
horizontal.

Keep in mind that DC motors will have both a continuous and a peak
current rating. The continuous current is mainly a function of winding
resistance and the ability of the motor to dissipate heat. The ratio
between continuous and peak current is generally an indication of
magnet material and the resistance of the magnets to demagnetization
as a result of high current in the windings. You might see a ratio of
3:1 in low end motor, while a high performance, low inertia motor
might be as high as 7:1 or more.

--
Ned Simmons