I don't currently fully understand how a DC brushed motor reaches maximum mechanical power at around 50% of their stall torque. I know that a decrease in the speed of the rotor will decrease back-EMF and in turn, increase current, thus the increase in power.

However, I don't exactly understand why the maximum power stops at around 50% of the stall torque. What determines the stop at 50%?

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    \$\begingroup\$ See lancet.mit.edu/motors/motors3.html for useful graphs and details \$\endgroup\$ Mar 9, 2021 at 7:33
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    \$\begingroup\$ It's just another instance of the Maximum Power Transfer theorem at work. en.wikipedia.org/wiki/…. \$\endgroup\$
    – user16324
    Mar 9, 2021 at 12:36
  • \$\begingroup\$ Note that this is only true at constant voltage, and the rated voltage is a somewhat artificial limit. If you really want to get the maximum amount of power out of a motor safely, and you pay close attention to the real limits, then you can get closer to the stall torque at a higher speed. Assuming a constant voltage is what creates the linear torque-speed curve and that puts the max power at 50% \$\endgroup\$ Mar 9, 2021 at 13:58

1 Answer 1


It's because, for a DC motor, the torque curve and speed curve are both linear and of opposite polarity slopes and both curves also start and end at the same place. 100% torque = 0% speed, and 100% speed = 0% torque.

Those two facts mean that the adding them up will always equal 100%. Specifically, 50% of one corresponds to 50% of the other, and since power = torque x RPM, this just happens to be when the multiplication of the two is the maximum.

At one extreme end, you have 100% torque but 0% speed so the multplication gives you zero output power.

At the other extreme end you have 100% speed but 0% torque so the multplication again gives you zero output power.

enter image description here Taken from: https://www.vexforum.com/t/motor-torque-speed-curves/21602

Geometrically, it is identical to why a square is the rectangle with the most area if the sum of all sides is a fixed number. If one side is speed and one side is torque with power being the area, and the sum of length and width must be 100, a square of 50 x 50 gives the most area.


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