finding the right voltage for a dc motor

Hi,
I was given two small dc motors. However, I have no idea if these are 12v
or 24v or anything else. I was thinking about trying it with 6, then 12
then 18...but how do I know when to stop? Is there a way to find out
without risking burning the motors?

Many thanks

 

I'd measure the stall current. The max effciency operation will be the
voltage that gives half the stall current when operating with a load.
Max voltage would be twice the operating voltage, to give some headroom.

 

I suppose I should add measure the stall current at some low voltage
that doesnt cause the motor to get too hot. Then the working and max
voltage mentioned above can be found. Sorry about the confusion. Need
to be able to go back and edit this stuff somehow.