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I'm dealing with an application in which I use small DC brush geared motors to move a robot. The motors are driven by a standard FET driver. I do have the option to coast or brake the motor after every operation. Brake would be preferable since the robot would stop directly after a "forward" or "backward" operation. What I'm wondering is: would the brake operation affect negatively the lifespan of the motor or gearhead? Or is this effect negligible?

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The forces of breaking a motor by shorting its electrical connections are the same as starting the motor with full voltage applied. Actually, they'd only be the same in a ideal motor. In any real motor the breaking forces would be a little less due to the less than 100% efficiency of the motor.

Motor torque is proportional to the current. During electronic breaking, that current is the generator voltage applied to the coils resistance. During startup, that current is the driving voltage applied to the coils resistance. Unless the mechanical load is providing significant power on its own, the breaking current, and hence the breaking torque, won't exceed the initial stall current when the full voltage is applied.

This is all a long way of saying that the maximum mechanical stress is more likely at startup with full voltage, than when electrically breaking by shorting the motor connections.

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