StephenDoty said:
so even if I get a 40amp 0-30v power supply and set it to 12v, the motor will pull only 3 amp from the power supply even if it is on 40amps?
You stated that you have a 12 volt 3 amp motor, but you didn’t mention at what RPM the motor draws 3 amps. The current that a motor's armature draws varies with armature RPM.
The commonplace PMDC motors for instance, draws maximum current at start-up (armature motionless). As the armature begins to increase in RPM, a back emf voltage is generated that is proportional to the armature's angular velocity. This generated back emf voltage opposes the applied voltage thereby reducing the amount of current that will be drawn through the armature windings from the applied voltage source.
The electric fuel pump on my vehicle draws a steady 3 amps at its high RPM operating RPM, but when 12 volts is initially applied (armature motionless), the instantaneous current draw briefly exceeds 12 amps. The fuse is only rated at 10 amps, but it's a slo-blo fuse. The fuel pump armature accelerates so quickly that the current very rapidly decreases to the steady 3 amps, which prevents the fuse from producing enough heat to blow the fuse. Even so, my vehicle’s battery and its electrical wiring are capable of handling this load.
If your particular motor happens to also draw an instantaneous current that just briefly draws 10 amps or more, it may cause your power supply to crowbar and shut down. In this case, you may have to install a series resistor and a 3-position switch. The positions would be for: Off, Start, & Run. Upon start-up, the switch position places a current limiting resistor in series with the motor’s armature windings so as to limit the initial high start-up current. Once the motor has come up in speed (its current demand reduced), simply flip the switch to Run and the series resistor is eliminated and the motor runs directly from 12 volts with its normal torque and velocity for the given physical load.