Yes ! But you need to know at least the voltage or the current, and you
need a adjustable mechanical brake (Prony or other).

If you know the voltage, then apply it and measure rpm w/o load. There is
no rule of thumb but it is mostly so that a series wound (or permanent
magnet) motor will give max output input power/(torque x rpm) at about 1/2
to 2/3 of max unloaded rpm. So use the brake to get that rpm and compute
current and torque at the same time, then derate a little bit and check
for long term effects (heat).

Of course the right way to do this is to plot the motor characteristic
(torque vs I and rpm vs torque and voltage). You will quickly find the
relevant peaks. The absolute max. current can be determined from the wire
gauge used.

Note that servos for temporary (low duty cycle) operation will totally
mislead you when plotted like this. You will know that something strange
is on when the max output does not tally with the wire gauge and size of
motor and the thing gets very hot very fast.

Characteristic plotting works for any electrical machine and for thermal
(combustion) engines. You can improvise a Prony brake using a smooth drum
wheel, a leather belt and a weight table. However this may be dangerous
depending on the specs of the motor.

This is all based on the idea that the original designers went for best
efficiency. I don't know what happens if you plot machines designed in
certain countries where efficiency is not paramount for certain reasons
... ;)

Peter

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