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I'm building an octocopter, and can't quite understand what an ESC (electronic speed controller) is used for. A brushless outrunner motor has three cables. Is there one common ground, one power supply (DC) and one cable to communicate with the ESC?

Does the ESC simply use the information gathered from the motor to decide when to turn the power on to get the maximum effect out of the motor? So the ESC is simply a PWM which synchronizes the pulses with the motor?

The ESC also has some extra features like turning down the power when the voltage is below a threshold, and using low resistance to brake the motor. If you don't need these features, is using a PWM just as good as an ESC?

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    \$\begingroup\$ A brushless motor is essentially a 3-phase AC synchronous motor. An ESC is a 3-phase generator for it and cannot be replaced by the PWM controller you would use for a (brushed) DC motor. \$\endgroup\$ – Brian Drummond Mar 13 '14 at 16:25
  • \$\begingroup\$ @BrianDrummond Thanks for clarifying that it is an AC motor. I've tried searching for this on Google, but some said it is a 3-phase AC motor, while others said it is a DC motor where the third cable is for communication. \$\endgroup\$ – Friend of Kim Mar 13 '14 at 17:01
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A BLDC motor is a 3-phase AC motor, which is why there are three wires.

An ESC is a set of 3 half-bridges that drive the motor phases to create a rotating magnetic field. Below is a schematic from here that should give you some idea.

enter image description here

If you were to feed PWM to two wires of a BLDC motor it would do very little except get hot and stay in one place.

Ideally, the output would like this (the frequency is varied to change the motor RPM, 60Hz is shown).. image borrowed from here.

enter image description here

A video showing what the non-ideal waveforms look like is here:

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  • \$\begingroup\$ Thank you for a good answer. I'm pretty new to electronics, and do not fully understand schematics yet. I've tried to learn what an ESC does, but some say it's DC, some say it's AC. Thank you for a clarification here. \$\endgroup\$ – Friend of Kim Mar 13 '14 at 16:47
  • \$\begingroup\$ Some say the ESC is intelligent in that it controls the input based on some information gathered from the motor. Is the ESC simply generating three AC phase shifted supplies? And it also serves as a PWM to control the speed of the motor? \$\endgroup\$ – Friend of Kim Mar 13 '14 at 16:50
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    \$\begingroup\$ They could measure the back-EMF of the motor phases to implement space-vector modulation (SVM). I don't know if the hobby ESCs do that. \$\endgroup\$ – Spehro Pefhany Mar 13 '14 at 17:05
  • \$\begingroup\$ How does the ESC know which frequency to use? \$\endgroup\$ – Friend of Kim Mar 13 '14 at 17:35
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    \$\begingroup\$ From the rpm you command to the ESC. \$\endgroup\$ – Spehro Pefhany Mar 13 '14 at 17:57
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The PWM signal from your radio or in the case of a drone, your flight controller is fed to a microchip mostly an ATMEL but any MCU that can generate a Sign wave can be used, like a PIC microchip. The MCU generate the three phases at a frequency that is derived from the PWM input. The three sign waves that is then used to drive your FET's and that is driving the motor, the higher the frequency the faster the motor spin.

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    \$\begingroup\$ Actually just as in a mechanical commutator the frequency is determined by the measured rotation of the motor - unlike with an induction motor, a permanent magnet motor cannot "slip", but only generate useful torque when running synchronously. \$\endgroup\$ – Chris Stratton May 24 '17 at 5:52

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