Determine motor RPM...

[WilliamD]

I know that DC motors will increase in RPM, as you increase the voltage. I'm trying to figure out, based on all the DC motors I have and have taken apart, is what determines if one develops full RPM at 12 volts, or 120 volts, etc.

I have a DC servo motor that runs 12V, and high amps. I have a treadmill motor that runs 6000 RPM at 120V. Then I have a "normal" DC motor that runs 3200 RPM at 120V.

What determines this? I really don't notice anything that different between them. Any help solving this or pointing me to other documentation would be extremely helpful!

[Al_The_Man]

The basic construction of pole strength and armature winding characteristics determine the motor characteristics, when you apply a voltage to a DC motor the motor will increase in speed until the generated voltage approaches the applied voltage, i.e. the voltage will be almost equal, the difference between the two is the voltage the motor sees.
To increase the speed, the applied voltage has to be increased until again the generated voltage is again (almost) equal.
If you know the maximum rpm of the motor, you can find out the max. applied voltage to reach this.
Turning the motor over at a known rpm and reading the generated voltage at this RPM, you can extrapolate what the final max voltage can be applied.
Al.

[WilliamD]

Thanks! That makes a bit more sense. I still have a ways to go understanding all about motors though.