What variable resistor to control motor speed?

if i know the voltage (3v), current (100mA) and terminal resistance (27.6 Ohm) of a small dc motor how do i work out what variable resistor rating to use to control it?What variable resistor to control motor speed?
It depends on the stall voltage of the motor, which depends on the motor and the mechanical load. I would guess at a 20 or 50 ohm variable resistor in series with the motor.


Using a variable resistor is a very inefficient way to control a motor because the resistor dissipates power. Also, low resistance values like this are hard to find. A better way would be to use a 555 timer as an oscillator with variable pulse-width, driving the motor with a common-emitter NPN transistor.


What variable resistor to control motor speed?
Is it a shunt wound motor or a series wound motor.





For a shunt wound motor i.e where the field coils are in parallel with the rotor, which is essentially a constant speed device the speed N is proportional to Voltage(V) - armature current(Ia) x armature resistance(Ra). You need to measure Ra to find the range of resistance required which must be put in the armature circuit. The torque is proportional to Ia so this will drop as you increase Ra.





The characteristic of a series motor where the field coils are in series with the rotor is different and speed is proportional to 1/Ia. This is why you should never run a series DC motor on no load as Ia will be very small and the speed will be very high. A diverter resistance across the armature would give speeds lower than normal as this will increase the current through the motor and again you need Ra to determine the range of resistance required.