Poor brush lifetime of Micro Metal Gearmotor HP

I am working with a few Micro Metal Gearmotor HP (30:1 and 10:1) and so far 3 of them have failed within an hour of operation.
The reason is the destroyed brushes that no longer make contact. The motors operated at 8-9V up to a minute of continuous operation and with significant time to cool down.

The description mentions: “in general, these kinds of motors can run at voltages above and below this nominal voltage, so they should comfortably operate in the 3 – 9 V”.

I believe that either the brushes are inappropriate to withstand the power draw or this batch is defective.

Has anyone else experience brush failure?

I am sorry you are having problems with your motors. We have had no indications of a systematic defect in our motors. What is your load, and what are the motors doing during that minute of continuous operation? Can you quantify “significant time to cool down”?

- Grant

The motors are on a line following robot of ~0.15kg. The robot operates no more than 1min. During that time the motors operate at 50% PWM duty cycle with bursts of 100%. I have not measured the load on the motors but it seems that the motors have enough power to spin the wheels without visible stalling.
After each run the robot is shut down for approximately more than 2 minutes. I believe it is enough to cool down the motors.

Like I previously mentioned, we have had no indication of a systematic defect in our motors, and we know of many examples of people successfully using them in line following robots (including our own). Running them at 9V will shorten their life span, but I would not expect that effect to be as dramatic as you are describing. Could you post a video showing your line follower in action? At the moment, my suspicion is that it is either badly tuned or somehow otherwise rapidly alternating the direction of the motors. Abrupt increases in speed or changes of direction produce large current spikes that can quickly degrade the brushes.

- Grant

Indeed, the robot changes the direction and the velocity of the motors quite fast. This is necessary in order to achieve quick response times, and unavoidable because of the relative low power. It is possible that the robot would benefit from more powerful motors. Here is a video,

I would like to stick to these motors because of their high power to weight ratio. By limiting the maximum current to the motors, would there be any significant improvement to their lifetime? Of course there would be a decrease in the torque.

There are a couple things you could do to extend the life of the motors. One is to lower the supply voltage or limit the maximum duty cycle of the PWM (which has the same effect as lowering the supply voltage). The second would be to implement acceleration limiting by ramping up the speed (e.g. rather than letting it go straight from 0 to 100%), which would reduce the magnitudes of the current spikes experienced by the motors.

By the way, we are currently working on making versions of those gearmotors with carbon brushes, which should substantially increase their lifetimes in high-performance applications.

- Grant

I have already set a limit to maximum duty cycle and the acceleration.

Let us know when the new motor are available.

Sorry for not posting as soon as we released them, but the micro metal gearmotors with carbon brushes are now available. You can read more about them in this blog post.

- Grant

Great job! I will try them out.