How can I calculate the RPM of a motor running at less than Maximum voltage?

I'm working on a project that needs a certain RPM of motor. I am looking at motors from http://servocity.com/html/3-12v_gear_motors.html. I will be running these motors at ~5v. Is there an equation I can use to calculate this sort of thing?

iceng5 years ago
It is a small DC brush motor with a gear reduction train mounted.
They say these motors are reversible meaning the brushes are set in a
neutral position setup ( not the most efficient ) hardly matters in these
small motors.

The speed of an armature motor is primarily a function of several variables
they are friction and voltage and the permanent magnetic flux in the motor.
You can not easily change the flux or the friction leaving voltage. 

The speed is a result of the magnetic force the armature exerts against 
the permanent magnets ( probably NIB type ) surrounding the rotor
( armature ) and that magnetic force is generated by the current running
through the armature wire coils.
A first approximation to speed is to treat the armature as a resistor.
Which means if you half the voltage, so will you half the current and the
associated magnetic motive force will half the speed.  

Motor speed is proportional to armature Voltage.

At no load the inductive armature component also affects this speed
meaning  the change in speed is not a one to one ratio and the speed
wont drop as much.

However as you put load on the motor shaft the armature becomes
much more resistive ( less inductive ) and the first approximation of speed
proportional to voltage is a good one after all.

Hope this helps



Under load, all bets are off.

seandogue5 years ago
Not really. The best way is to measure it using a retro-reflective sensor or similar feedback mechanism.