An unusual departure from woodworking (how motors work) - Page 1
Forum section: electronics / general

author: IanB
author_rank: Super Contributor
timestamp: June 22, 2014, 01:48:36 am
content: Not so much new to learn for technical people, but very entertaining to watch:Did you know that the rotor from a 120 V motor will spin when powered from a 1.5 volt battery?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 22, 2014, 02:52:57 am
content: The brushes/commutator are acting as a mechanical inverter. Nowadays, much work is done to replace those with a solid state inverter in order to make it usable in high use applications.

author: GeoffS
author_rank: Unknown
timestamp: June 22, 2014, 03:50:13 am
content: Lovewoodgears.ca, my second most visited website.

author: amyk
author_rank: Super Contributor
timestamp: June 22, 2014, 12:53:59 pm
content: Quote from: IanB on June 22, 2014, 01:48:36 amIt's a high speed high-power motor, so the bearings are very low friction and the only significant friction is from the brushes - which he substituted with wires having much lower contact pressure. A 1kW motor at 120V will produce approximately 0.1W at 1.2V - still quite enough to spin.

author: zapta
author_rank: Super Contributor
timestamp: June 23, 2014, 05:26:46 pm
content: Quote from: IanB on June 22, 2014, 01:48:36 amExcellent video. Low budget but yet good video quality and easy to follow explanation. Now I understand why AC brush motors cannot have permanent magnet stators.

author: AlanR
author_rank: Regular Contributor
timestamp: June 24, 2014, 12:40:46 pm
content: The "AC brush motors" are actually series wound DC motors. In fact any wound field type DC motor will work on AC. Why? Because the magnetic field polarity on the armature and the stator field are changing together. The result is the vector forces are in the same direction. If you hook a battery up to a universal motor with leads reversed or not, it will still rotate the same direction. Now if you make the armature or stator magnetic field stationary by not changing the polarity on one or the other, then it will change direction of rotation of the motor if you change polarity on which ever part you did not make the polarity stationary for.Honestly though, we should not be using brushed motors in power tools anymore for 2014. We should have already switched to 400 Hertz, 3-phase AC induction motors because with today's chip integration, it should be possible to make a single chip solution that only requires a DC filter capacitor (full wave diode bridge built into the the chip) and a heatsink to cool this chip. The other components would be a tachometer to get RPM speed of the motor for closed looped feedback for precise torque control and a linear travel potentiometer for the trigger button when variable speed is needed.In fact I even drew up a chip outline for what such a chip would have for pins that I submitted to all the major power tool makers. I doubt they'll switch. Could be a conspiracy to keep the tools with brushed motors so that they will have a short life expectancy. After all, we live in a disposable society.....

author: amyk
author_rank: Super Contributor
timestamp: June 25, 2014, 12:27:06 pm
content: Induction motors don't have the torque characteristics of brushed motors, and the control electronics are another (expensive) point of failure.

author: Zero999
author_rank: Super Contributor
timestamp: June 26, 2014, 07:38:42 pm
content: In my experience, the armature overheats or the switch breaks more often than the brushes go.A brushless motor drive will also have to have power factor correction, which is mandatory, if it's over 75W, which will push up the price more.

author: GeoffS
author_rank: Unknown
timestamp: June 27, 2014, 10:25:46 am
content: Matthias has just posted a second part to how motors work, this one on AC induction motors.

author: AlanR
author_rank: Regular Contributor
timestamp: June 27, 2014, 11:43:20 am
content: Quote from: amyk on June 25, 2014, 12:27:06 pmWith closed loop feedback vector control, an AC induction motor can perform and do everything a DC motor can do. You are thinking of the motor from the perspective of direct connection to the power supply. DC motors are in fact starting to be replaced in many variable torque, constant torque, variable speed, and servo applications that they once ruled. Microprocessors have come a long way with the ability to do very fast calculations. So AC systems are viable today for 99% of all applications that used brushed motors. The control electronics are one chip solution I was talking about to increase reliability. Corporations I can think of that can produce such a one chip solution are Power Integrations, Texas Instruments, or Linear Technology. I've seen these three do some amazing things with silicon that was not possible 10 years ago.

author: ConKbot
author_rank: Super Contributor
timestamp: June 27, 2014, 11:52:52 am
content: Quote from: Hero999 on June 26, 2014, 07:38:42 pm+1 Ive never seen a cordless or corded drill with worn out brushes. Ususally just smoky smelling to flat our burnt windings. (usually related to mixing paddles and drywall compound)Though I have seen some angle grinders with worn out brushes, though once the replacements came in, it was just a flathead screwdriver to replace them.

author: AlanR
author_rank: Regular Contributor
timestamp: June 27, 2014, 12:19:06 pm
content: All the more reason to switch. They do no incorporate current limiting or even current fold back as the motor gets hotter in brushed motor tools. We need to move forward and STOP with the brush motor garbage. If we still use universal motors in power tools in the year 2020, then I have lost faith in humanity in knowing what progress means. There is no reason that a 400 Hertz AC induction motor power tool cannot be price competitive. A 400 Hertz motor has been around since WW2 when a lightweight motor and generator had to be made for portable RADAR systems in aircraft. With this technology being very mature, it should be cheap. Single chip solutions that incorporate microprocessors and power devices are also not expensive and are getting cheaper as the years go forward.