Re: [CAD_CAM_EDM_DRO] Re: volts/rpm

mariss92705 wrote:

> Hi,
>
> A good (and accurate) way to measure armature resistance is to chuck
> up the motor in a drill press (use low RPMs) and measure the DC open-
> circuit voltage on the motor leads.
>
> Then comes the important part. Make sure the motor is secured in a
> vise. Switch your meter to DC Amps and place it across the motor
> leads. Measure the short-circuit current.
>
> Take the voltage reading from part (1) and divide it by the current
> reading from part (2). Ohm's Law now gives you an accurate
> calculation of the motor's armature resistance.
>
> What can screw up the readings is part (2). If you notice the drill
> press motor labor and drop RPMs, or worse yet, the test motor gets
> loose from the vise and spins, be prepared to extract your meter from
> the nearest wall where it will be imbedded. Your readings will not be
> accurate then (or ever again with that meter).
>
> I speak from hard-won and expensive experience.

Shorting out motors with mechanical power applied only works for
small motors. With the larger ones, you need to apply an appropriate
dummy load (or have a big DC machines power lab at your disposal).
With a dummy load, you need to measure armature voltage and
current without changing the setup to get the right reading.

I'm not talking about locomotive traction motors here, modest servo
motors for a bridgeport size mill will also deliver LOTS of amps if
dead-shorted at 1000 RPM.

Jon