DC Motor Driver v1.1 with 6V RM2/RM3 in GM3

I bought the GM3 gear-motor with the standard 6V RM3 motor before I realized there was a 12V version. I also bought the RM2 [www.solarbotics.com] at the same time to replace the RM3 to get more torque and speed.

I just put a DC Motor Driver v1.1 [reprap.org] together, and I was about to connect it to a gearmotor and the Arduino to test it, but I realized it was using a 12V power supply.

I've searched around and there is some discussion about it, but what exactly would happen if I used the 6V motor? Would it burn out after a short while (how long)? Or can I reduce the voltage by keeping the analog/PWM signal for speed (PWM_A) below a certain level and still operate at a good speed?


Also, I've noted that the stall current of the RM2 is much higher than that of the standard RM3 [www.solarbotics.com]. Even at 1V, the stall current is above the max current for the motor driver. Should I be worried about this and not use the RM2? Or is a stall unlikely?

Joshua Merchant Wrote:
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>
> I've searched around and there is some discussion
> about it, but what exactly would happen if I used
> the 6V motor?
>
It would run roughly twice as fast and give you a lot more torque. It would also tend to overheat under load and would burn out a LOT faster.
>
> Would it burn out after a short
> while (how long)?
>
LOL! Do it and tell us.
>
> Or can I reduce the voltage by
> keeping the analog/PWM signal for speed (PWM_A)
> below a certain level and still operate at a good
> speed?
>
Yes indeed! Use the PWM signal to keep 12 v. on half the time and you will get the equivalent of 6 v.
>
> Also, I've noted that the stall current of the RM2
> is much higher than that of the standard RM3
> [www.solarbotics.com]
> if. Even at 1V, the stall current is above the max
> current for the motor driver. Should I be worried
> about this and not use the RM2? Or is a stall
> unlikely?
>
I've used the RM2 and the RM3 both to power the extruder. They are much of a muchness in terms of performance. Mind, I had both of mine equipped with 12 v motors. They have better brushes, I'm told, and have a good service life.

I used the 6V GM3 as an extruder motor and found that the brushes wear out after a few tens of hours of use. I haven't worn out a 12V version after more than 100 hours of use.

When you start a motor it takes its stall current momentarily so the driver should have a peak current rating bigger than the stall current of the motor at the supply voltage.

Using PWM to reduce the speed is not the same as reducing the voltage. If you slow down a motor by reducing the voltage, the current goes down. If you do it with PWM the current goes up but is flowing for a shorter time. As the motor slows down its back emf reduces so the current taken rises.

Because power is I^2R you get can higher dissipation in the motor and driver as you reduce the PWM ratio.

Edited 1 time(s). Last edit at 08/30/2008 04:11AM by nophead.

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