Re: [CAD_CAM_EDM_DRO] Re: servo voltage question

cnc_4_me wrote:

>Mike, are you talking about this motor...And what pulley ratio are
>you running and what rapids do you have...
>
>900 RPM AMETEK, #965922-101. Permanent magnet DC motor rated 38 VDC
>nominal. Reversible, ball bearing. Max. rated rpm is 1300 max.
>Armature current before permanent demagnetization is 12 amps. Torque
>constant is 50 oz-in per amp. Terminal resistance 20 ohms +-20%.
>Voltage constant is 37 VDC per 1000 rpm. When tested on 38 VDC, we
>recorded the following data: No-load spedd 1100 rpm @ 0.260 amp. With
>a 100 oz-in load the shaft speed was 900 rpm @ 2.3 amps. When tested
>as a generator at 1000 rpm, the output was 5.8 amps @ 12 VDC. Shaft:
>5/8" dia. X 1-5/16" long. The opposite end of the shaft has a turned
>down diameter and a flat mounting area with two sets of tapped holes.
>This was probably originally intended for an encoder or tachometer
>mounting. This shaft has an outside dia. of 3/16" and is approx. 3/8"
>long. We do not have the encoders whtat were mounted on this motor,
>but suitable encoders are probably available from encoder suppliers
>if required. The motor has five #10x32 tpi tapped mounting holes on
>the shaft end of the motor body. Electrical connections are via 3-
>1/2" long flexible wires. Dimensions: 4" dia. X 4-1/2" long
>(excluding shafts). Stock #DCM9951 $44.95
>
>

Probably, yes, he is. But, there is an error in the above data. It is very
difficult to get an accurate reading of the terminal resistance of a brush
motor. The 20 Ohm rating above has to be wrong. Trying to shove 12 A
through a 20 ohm motor would dissipate 2880 W and cook the motor in
a fraction of a second. But, the actual resistance is about 4-5 Ohms, which
is still quite high. If you drive 12 A through this motor is will take
a minimum
of 48 V (12 * 4)! So, the manufacturer's ratings are actually
contradictory,
though you can't erase the magnets with 36 V. In general, this motor
will perform
better with at least a 2:1 belt reduction. With a 2:1 reduction, (and
supplying
enough voltage to spin the motor, UNDER LOAD, up to its RPM rating of
900 RPM, you'd get 450 RPM at the leadscrew. That will give 90 IPM with
a 5 TPI screw.

Jon