Re: [CAD_CAM_EDM_DRO] volts/rpm

ccs@... wrote:

> Is the voltage I measure on a dc servomotor being rotated by a lathe
> spindle at a known rpm essentially the same voltage I would need to
> apply to make it turn at that rate, provided I can source and (it can
> handle) sufficient current for the torque load?

Almost. There is IR drop due to the resistance of the armature winding,
and there is armature reaction (depolarization of the field magnets).
The armature reaction within the normal limits of the motor should be
miniscule, but the IR drop is not always so low. You can measure
armature resistance, but it is tricky because the brushes don't give
very good contacts when not moving.

> Is there any easy way to estimate the current handling
> of a motor with no specs whatsoever?

Looking at the windings to see the wire size will give some idea.
The general size of the motor will also give an idea of its ability
to shed heat.

> I know someone - I think Mariss - posted an excellent description of
> how to determine servomotors specs this way, but I have not been able to
> find it in my search of the list archives.

The first measurement you mention is measuring Kv, the volts/RPM, which
is one of the prime measurements of a motor. The second is Kt, which
indicates
torque/Amp, in some convenient units. You can do this with a balanced arm
and a weight.

Determining the safe peak current of the motor is essentially a destructive
measurement, and I don't know of a safe (for the motor) way to do it. You
can get a motor that looks "big enough", and then load it up to some value
at which it powers the machine effectively. If the motor doesn't overheat
or lose performance, then that is likely to be an acceptable limit for the
motor.
If the Kv drops, ie. now you get less volts/RPM, then the magnets have
been damaged, and you exceeded the ratings. Oops!

Jon