A project that I'm working on requires a wheel/cylindrical drum that can be spun by a motor, but must also be freely spun by a person when it isn't powered. Once It gets up to speed, it must coast freely for a while before stopping, much like a buddhist prayer wheel.
I thought that a brushless dc motor was the best way to achieve this, but I am having difficulty getting any usefull amount of torque out of them. I wasn't sure how slowly an off the shelf ESC could operate, and since I need this to spin at relatively low speeds (100-300 rpm), I have just been driving them with a microcontroller and power mosfets. I can get them to spin, but as soon as I apply light pressure with my hand they stop, and attaching an empty pop can prevents them from moving altogether. I think that my problem is lack of feedback (no hall sensors or back EMF), but from what I've read it appears that back EMF for sensorless control only works at higher speeds, and all the motors with hall sensors that I've found look like overkill for what I'm doing (it shouldn't take a $100+ motor to spin an empty soup can around...).
My questions are:
- Are ungeared brushless dc motors the best choice for spinning a cylinder but also allowing it to rotate freely when not powered?
- What factors determine the torque of a BLDC motor?
- What is the simplest way to drive a BLDC motor at reasonably low speeds? Are ESC's only good for the higher speeds like you find in RC airplanes?
- Where can I get motors with hall sensors that aren't industrial grade? They must also not be too stiff so that they can spin freely.
Here are some specific parts that I'm using:
Motor:
http://www.hobbyking.com/hobbyking/store/__8622__2213N_800Kv_Brushless_Motor.htmlMosfets: IRLB8721PbF
http://www.infineon.com/dgdl/irlb8721pbf.pdf?fileId=5546d462533600a40153566056732591Microcontroller: Teensy 3.2
https://www.pjrc.com/teensy/teensy31.html