I need to limit the power of my brushless motor at 700 watts.

I don't have a fixed motor or propeller. For any configuration the max consumed power need to be less or equal 700 W.

The max current of the motors is ~50 A, but the voltage can be changed.

I looked for commercial power limiters but I didn't find any that would meet the high current with the adjustable power value.

The project is a aircraft model and I can use and/or develop hardware and software periferics.

The circuit:


A problem I have to avoid is the controller stick from having deadzone, as this can cause the pilot to lose track of the controller action.

  • \$\begingroup\$ Can the ESC limit current? If so, that may work well enough. \$\endgroup\$
    – rdtsc
    Nov 10, 2021 at 18:20
  • 1
    \$\begingroup\$ If you need a true motor power limit, the right place for it is in the ESC. \$\endgroup\$
    – John D
    Nov 10, 2021 at 18:23
  • \$\begingroup\$ I was thinking of something between ESC and Battery/PWM from the radio. It is dificult to development a ESC? And how i select the limit power on the ESC? \$\endgroup\$
    – Arcaniaco
    Nov 10, 2021 at 18:33
  • \$\begingroup\$ Read the Datasheet for the ESC, it should have all of the information you are looking for, if not, select a different ESC. You could add a link to it in your question, too. \$\endgroup\$
    – Tyler
    Nov 10, 2021 at 19:00
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    \$\begingroup\$ Developing an ESC is a fairly complex project. We don't provide designs here or provide links to completed designs. A purchased ESC will probably have a means of limiting torque or current, but not power. Battery current is probably the best indication of total power delivered by the battery. However a given current level will result in less power as the battery voltage declines. Total battery power is power used by the propellor plus motor losses plus ESC losses. You could test a motor-ESC-propellor combination at various speeds then limit the speed to whatever results in 700 W use. \$\endgroup\$
    – user80875
    Nov 10, 2021 at 19:31

1 Answer 1


Total power delivered from the battery is the power used by the propellor plus motor losses plus ESC losses. A purchased ESC is likely able to limit the current output to the motor by limiting the motor speed. That will not be a very effective means for limiting power.

Battery current is an indication indication of total power delivered by the battery. However a given current level will result in less power as the battery voltage declines as it discharges. There will also be a voltage drop as current increases with load. You could more accurately calculate battery power by multiplying battery current by battery voltage.

The result of the battery power estimate or calculation could be used to reduce the speed command when the power exceeds a set limit. The system would essentially prevent the propellor speed from exceeding the speed that results in the set power limit whatever that speed happens to be under the prevailing conditions.

  • \$\begingroup\$ It would basically modularize the pwm signal according to power, right? If so, I understand and believe it works, thank you. For implementation I only think about use via software, would you have an idea of ​​how to do this with hardware resources only? \$\endgroup\$
    – Arcaniaco
    Nov 11, 2021 at 23:51
  • \$\begingroup\$ I think a speed error signal would modulate the pwm. A power error signal would override the speed command when power feedback exceeds a set power level. If a purchased control system already controls speed, it may be possible to add a hardware power override control loop to that. \$\endgroup\$
    – user80875
    Nov 12, 2021 at 1:23
  • \$\begingroup\$ It was not clear what the error signal would look like. When you say speed are you talking about RPM or PWM modulation? Because for a fixed PWM signal, the RPM will vary depending on the wind speed, the error is not linear and I have no way to obtain the wind speed. The solution of measuring voltage and current, and using a controller to calculate the power by modularizing the pwm seems to be the best option, but more expensive too. \$\endgroup\$
    – Arcaniaco
    Nov 12, 2021 at 11:26
  • \$\begingroup\$ Speed is motor shaft speed RPM. Speed feedback can be obtained from a shaft speed sensor or from motor back EMF. A purchased ESC should have that built in. \$\endgroup\$
    – user80875
    Nov 12, 2021 at 13:10
  • \$\begingroup\$ The EMF i can get with a sensor or need to calculate? How? \$\endgroup\$
    – Arcaniaco
    Nov 12, 2021 at 14:17

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