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I have just built an electric skateboard based on a Hobbyking 50A ESC, controlling a 400Kv outrunner-type brushless motor. The skateboard requires a little nudge at the beginning to run regardless of the speed setpoint, otherwise it struggles and vibrates. This is not an issue of torque capability, as it can carry me up a slope without issue once it has non-zero speed.

I'm starting to think, in spite of discussions I find on the web saying these sorts of controllers use sensorless position feedback algorithms, that the stator rotating field is set to rotate at a given speed (i.e. fixed frequency) regardless of the rotor position - resulting in very little torque when driving very high load inertia at zero speed. I was expecting some kind of closed loop control of the stator field with respect to the rotor field (90°), wouldn't you?

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    \$\begingroup\$ If you need high torque at zero speed (which you do) then you need a sensored motor and controller. Probably what you have is for extreme geared down, or flight use which requires much less zero speed torque. What you have is a constant frequency motor controller. \$\endgroup\$
    – Jack Creasey
    Mar 30, 2017 at 23:06

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I was expecting some kind of closed loop control of the stator field with respect to the rotor field

Indeed, there is. The ESC senses the back-electromotive force (back-EMF) produced by the motor, and keeps track of the rotor position using that information.

The back-EMF is a term for the voltage induced into the stator windings due to them being exposed to a time-varying magnetic field, in turn created by the spinning permanent magnet rotor. In essence, the back-EMF is the voltage that would be present at the terminals of an extenally spun motor without anything being connected to it.

If the motor is at rest, the rotor isn't spinning, so there is no back-EMF to sense. The only thing that the ESC can do is to blindly spin the rotor by driving the motor phases open loop, until back-EMF builds up enough for the ESC to figure out the true position of the rotor. This works acceptably with just a propeller or the inertia of a lightweight transmission and chassis of a RC car as a load, not so much with a high geared motor driving a human.

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  • \$\begingroup\$ Your last paragraph answers the question - that's basic: since it's based on back-EMF sensing, without speed no back-EMF, and hence no closed loop control. I would have thought that without back-EMF, the controller could have assumed the rotor was still at the zero position, and set the stator field 90° from there - but now that you mention it there is no such thing like zero position since the rotor can start anywhere so there is really no other option than driving with a fixed frequency. In a nutshell, good call. \$\endgroup\$
    – Mister Mystère
    Mar 31, 2017 at 14:34
  • \$\begingroup\$ Ground RC vehicles traditionally use motors with hall sensors, as that is required at low speed. If they keep using the hall sensors or switch to back EMF at higher speed is an interesting question. \$\endgroup\$
    – Chris Stratton
    Mar 31, 2017 at 16:19

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