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How is the technique and method to do measurement test in order to determine the optimum operating voltage of any type of a dc motor ?
as always shown to us in motor specs e.g.

 DC motor RS 385, brushed
 Voltage 12 - 24 V
 No load 4 - 15 W 
 Load    10 - 24 W

Any truth on the technique and/or method is appreciated

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Actually the current is the most important as it generates heat loss. But the final parameter is the temperature rise, which is the most important. You can drive the motor back and forth with very high current as long the temperature remains in the spec. limits which is determined by insulation class.

Usually, motor has data for rated nominal current. This is the continuous current that would cause motor heat within spec. limits. at declared environment temperature.

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What is optimum in your context?

For optimum efficiency, run the motor as fast as possible, which means low torque loading and high voltage, and adapt the torque requirement at the load to the motor's ideal torque using gearing. Run the motor fast and gear down.

Look at the motor's datasheet and note peak efficiency is somewhere close to the unloaded speed. (This will also show that if you get TOO close to the unloaded speed, like 90%, efficiency reduces again in a brushed motor, because the friction of the brushes steals torque from the output)

This ties in with Marko's answer in that waste heat comes from current, and gearing to reduce torque reduces current.

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