# Driving Micro Metal Gearmotors with special circumstances

Will there be any problems if one of the Micro Metal Gearmotors was powered from a 12V supply but the current and speed was limited by the motor driver? To be more specific, the current would be limited will below the nominal shall current (at 6V), and the speed would never exceed the free-run speed nominally at 6V. I am not concerned with efficiency. I know the current limiting will limit the acceleration of the motor. I want to make sure that the brushes and/or bushings in the motor are not damaged or worn prematurely.

Hello.

Your plan to effectively limit voltage to the motor sounds reasonable. One way you could accomplish this is by limiting the duty cycle of the PWM signal.

- Grant

I plan on limited the speed by having a fixed duty cycle. The current limiting is accomplished by the motor driver (A4953) “disabling” the h-bridge for a fixed off time of 25uS when the current crosses the set point. The motor will “see” 12V briefly when at or near the maximum speed controlled by the fixed duty cycle. I am just wondering if that will be a problem.

`current limit set point = 0.2A motor resistance = 6V - 0.7A = 8.5ohm`

12V will be across the motor when the following is true.
`(12V - BEMF) / 8.5ohm < 0.2A (I believe the inductance is low enough that it can be ignored.)`

I am not sure what you mean when you say “at or near the maximum speed controlled by the fixed duty cycle” I took a quick look at the datasheet for your motor driver and it seems like you are only referring to how the current limiting on it works. Just in case, please note that to control the speed of the motor you will have to supply a variable duty cycle to the IN pins of your driver.

When you apply a PWM signal to a motor the inductance of the motor should smooth out the output voltage and current of the driver, so for a duty cycle of 50% the motor should only see an effective voltage of half the input voltage. I expect the motor to be fine being driven like that.

- Grant