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We were using a regular POLOLU d.c motor whose stall current is 0.36A. We want to use this motor in a robotic hand where the motor will be stalled while holding the object. We are limiting the current at 0.11A because according to datasheet at this current we get the maximum efficiency. Is it safe to do this? I mean stalling it at low currents? If not what can we do to tackle this situation?

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Electric motors designed for stall torque can handle stall torque just fine. One giveaway sign of high stall torque motors is forced cooling (cooling with a powered fan and not just a fan mounted passively on the axis of the motor). One of the issued with stalling motors is heat, as the stopped rotor does not facilitate and airflow and the winding are more prone to overheating.

If your motors stall but the maximum specified stall current is not exceeded you will not damage the motor. Please take care as some motors specify more then one maximum stall current, larger ones (if there are more) can only be applied for a limited amount of time.

Efficiency is not directly related to the stall torque, it is safe to apply the maximum continuous stall torque current to the motor for a prolonged time without any damage.

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    $\begingroup$ That means I can safely stall a motor at lower current, probably 50% of the maximum stall current. I am planning to do it by using a current limiting motor driver. I will set it to 50% of the total maximum stall current allowed for this d.c motor. $\endgroup$ Oct 20 '20 at 6:57
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    $\begingroup$ You can set it to 100% of the stall torque, if the stall torque specified in the motor datasheet does not mention any time limitations. $\endgroup$
    – 50k4
    Oct 20 '20 at 7:14
  • $\begingroup$ Hey I was wondering, what happens if the motor is backdriven? $\endgroup$ Oct 21 '20 at 5:53
  • $\begingroup$ Goes into generator mode, most probably... $\endgroup$
    – 50k4
    Oct 21 '20 at 5:55
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Efficiency is power output divided by power input. Electrical power, your input, is

$$ P = IV \\ $$

Mechanical power, your output, is

$$ P = Fv \\ $$ or $$ P = \tau \omega \\ $$

that is, force times speed or torque times angular speed. If you are stalled, then your speed is zero. If your speed is zero then your power is zero and your efficiency is zero.

As 50k4 said, you can do whatever the datasheet says you can do. The keyword to look for is continuous. That's what you should be able to safely do, forever, without risk of damage.

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