Driving BLDC motors while allowing them to spin freely when unpowered - Page 1
Forum section: electronics / beginners

author: ottopasuuna
author_rank: Newbie
timestamp: June 20, 2016, 05:15:25 pm
content: 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: IRLB8721PbFhttp://www.infineon.com/dgdl/irlb8721pbf.pdf?fileId=5546d462533600a40153566056732591Microcontroller: Teensy 3.2https://www.pjrc.com/teensy/teensy31.html

author: Gyro
author_rank: Super Contributor
timestamp: June 20, 2016, 05:44:48 pm
content: Any particular reason why it needs to be a brushless motor? An ironless rotor brushed motor would probably spin more freely when unpowered and would have better torque at low rpm (and certainly starting a 'flywheel' from a standstill).

author: ottopasuuna
author_rank: Newbie
timestamp: June 20, 2016, 09:37:23 pm
content: No it doesn't have to be brushless. The only reason I initially ruled out brushed motors was that all the ones I have dealt with previously were geared and didn't spin freely, and I had not heard of ironless/corelss brushed motors... Guess I failed at my research stage. They seem like a much better option for what I'm doing; I suspected I was overcomplicating things considerably.Do you know where I can get ironless brushed motors for a reasonable price? I found Maxon motors which look nice, but they're a little pricey.

author: Gyro
author_rank: Super Contributor
timestamp: June 20, 2016, 09:57:05 pm
content: I'm only familiar with them because they were used on a project I was involved with.QuoteAh, good question. The other manufacturer is Portescap, in fact I think they invented them. There are quite a lot of hits for surplus "escap motor" on ebay in the US, they look cheaper than Maxons but I don't know what size you need, or what postage would be like for you (pricey to the UK).Here's a brief overview of ironless motors...http://www.designworldonline.com/Ironless-DC-Motors-Deserve-a-Second-Look/

author: The Soulman
author_rank: Super Contributor
timestamp: June 21, 2016, 08:19:37 am
content: Quote from: ottopasuuna on June 20, 2016, 05:15:25 pmThere are two types of brushless motors, synchronous and asynchronous, the synchronous motor (you have) NEEDS to be controlledin a synchronized way either with feedback from hall sensors or back-emf, either way fairly low rpms are achievable.Self start is more difficult without hall sensors but doable.If you are really interested on every aspect to control a synchronous motor:https://endless-sphere.com/forums/viewtopic.php?f=30&t=36602

author: Jeroen3
author_rank: Super Contributor
timestamp: June 21, 2016, 09:54:37 am
content: Use an AC motor instead. They don't have magnets and can coast smoothly to a stop.You'll need a more complicated full bridge driver with pwm of several khz. Depening on the motor voltage.Or you buy an VFD.