Using a motor with DC

Question

I am using a Lego DC motor with a 9V battery which can provide up to 1A of current. I called Lego support and they said the motor draws 300mA of current. What does that mean? If I plug the motor to the battery will it use 300mA of current or will it use as much as my battery can provide and damage itself?

I know a motor has an internal resistance that's why I make this question: if is has na internal resistance it can control the amount of current that will consume without any external resistor, right? Am I right?

Answer 1

In general, it should be OK to run your motor from a 9V 1A power supply. The 300mA rating ought to be the current it draws while running. Stall current (how much current it draws if you block it from turning) may be higher, in which case it might get hotter than it should - possibly burning out if you leave it blocked too long.

If the motor has a thermistor, then the stall current will be limited and you won't have to worry about burning out the motor.

The site @codo referenced (Lego motor tests) gives more detailed information about the various motors, including which ones have thermistors.

Don't block the motor from rotating and you should be fine with the power supply you have. If you expect blockages to happen, use a motor with a thermistor or use a power supply that limits the current.

Answer 2

It is up to you to provide a supply that provides correct voltage, and capable of providing up to the required current. (Note the asymmetry!) If the voltage is correct the motor will draw (only) the current it needs.

Answer 3

The motor internal resistance will be relatively low, probably allowing it to draw much more than an amp when it starts. But that doesn't matter from a battery (though it might cause some sorts of power supply or adapter to panic and shut down).

When the motor is running it actually takes much less current - 300ma in your case - provided it's not too heavily loaded. So, run the motor on its own, and notice how fast it runs when it's not driving anything.

If you slow it down with your fingers (don't do this with a big motor like a drill!!!) it will draw more current as it slows down, and if it stalls altogether it can easily take an amp or more. So when you use it, make sure it is lightly enough loaded to run fast - if you need to drive a heavier load, like running a car around the carpet, that's what gears are for.

(If you can measure voltages and currents, say so and we can add more detailed explanations)