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The input of a ESC is a PWM signal.

Does this signal correspond to a motor speed?

If that is the case, if I send a constant 10% duty cycle to the ESC, then will it try to keep the same motor speed regardless of motor load? I assume that it is different for different ESCs, but in general?

On my quad-copter, if I set the throttle at 20%, then the motor will spin the same speed under no load and light load, but if I squeeze the motor shaft hard with my fingers it slows down. Is that because it is not powerful enough so that the friction is too great? But if I increase the throttle, I cannot hold it back any longer.

It therefore seems like an ESC is partly a speed controller and partly a power output controller. Is that correct? What is the relation between speed control and power control?

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You feed an ESC with a 'radio control' PWM signal that's a pulse 1 to 2 ms long, repeated fairly frequently, indicating how fast to run the motor.

ESCs are designed assuming a human or some other control is in the loop. You set the throttle, see how your craft behaves, and then adjust the throttle accordingly. Rinse and repeat.

This means that an ESC doesn't have to control power, or speed, or torque, in any accurate, formulaic, fed-back way. As long as the input to output characteristic is monotonic, reasonably linear, reasonably stable, it will serve its purpose.

To this end, most ESCs control the effective voltage to the motor. They do this by superimposing a PWM onto the motor drive signal (that's completely unrelated to the control input PWM) in addition to commutating the motor drive. For a constant loading, this means it controls the speed. As the load increases, the speed will drop a little due to motor resistance, until a current limit is reached at which the speed will drop significantly as the motor stalls.

That current limit could be a limit programmed into the ESC, and so represents a constant maximum torque the motor can produce. At low settings, it will be the effective voltage divided by the motor resistance, which means your motor will not be able to reach the same torque it could at higher settings (your stopping the shaft with your fingers experiment).

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  • \$\begingroup\$ Thanks for your explanation. So, does the motor continuously 'slip'? I (apparently wrongly) think of a three-phase motor as being synchronous with the driving phases, like a stepper motor. I know stepper motors will slip if too much torque is required of them. \$\endgroup\$
    – bob_monsen
    Jan 3, 2022 at 18:55
  • \$\begingroup\$ @bob_monsen With a stepper, the drive waveform is driven independently, and the motor can follow it or not, so slip or skip. With ESC+BLDC, the drive waveform is derived from the motor change of position (sensorless) or motor position (sensored), so as the motor slows due to load torque exceeding electrical torque, the drive waveform slows as well to keep in sync. Below a minimum speed, a sensorless ESC gives up, so stalls rather than slips. A sensored or independently driven ESC might be said to slip if the electrical torque was insufficient for the load. \$\endgroup\$
    – Neil_UK
    Jan 4, 2022 at 5:11

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