DIYbanter - DC servo motor question

On Sat, 04 Oct 2008 05:27:58 GMT, wrote:

On Fri, 03 Oct 2008 12:36:27 -0400, Ned Simmons
wrote:

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
Thanks for the reply Ned. These motors came from a lathe that
originally had a Bandit control. Later these same lathes were equipped
with a Fanuc 5T control and Fanuc motors. If I short the wires on the
Electro-Craft motors and spin the shaft by hand there is a lot of
drag, but the Fanuc motors have very little drag when the shaft is
spun by hand. I'll try powering them both with the same voltage and
compare speed and then I'll measure the current drawn with the torque
arm setup you described.
Thanks Again,
Eric

PM servo motors are designed for equal performance in

either direction of rotation. This means that they are

necessarily low speed (5000 Rpm) machines because the brush

angle cannot be offset to reduce high speed sparking. Typical

rated power is at 3,000 RPM. so the voltage needed to produce

3,000 RPM should be pretty close to the motor rated voltage.

Check the stalled resistance. Ohmeter readings can be very

unreliable - measure the effective resistance by monitoring the

terminal voltage when passing 1 amp or so through a stalled

motor

..
http://xs.to/xs.php?h=xs432&d=08400&...ng_-002155.jpg

shows the average relation between motor rated power,rated

voltage and typical winding resistance. It will not of course

give a precise answer because individual motor designs can be far

from average but it will be a LOT better than eyeball estimates.

For a closer figure you need to guesstimate the motor

maximum current. Copper has a temperature co-efficient of 0.4%

/deg C. If you now increase the stalled current (in stages to

allow time for the temperature to stabilise) until the measured

R increase by 30% the winding temperature will be about 100 deg

C which is pretty safe - High temperature servo motors can

withstand up to 150 deg C. This process is not as long winded

as it sounds - you can usually get a pretty close estimate with

no more than two trial currents. For best accuracy use the

brushes to feed the current through the armature and measure the

voltage drop directly on the relevant commutator segments.

Jim