Working with larger motors  - Page 1
Forum section: electronics / beginners

author: zrlopez
author_rank: Contributor
timestamp: October 02, 2020, 11:49:47 pm
content: Hey everyone, I started poking around and attempting to make measurements on an ac fan motor with my multimeter, because I wanted to know such things as how to calculate the torque I could get from it. Then I realized, man, I don't know much about how to work with, tear down, or make measurements on these larger motors with my oscilloscope and multimeter. More importantly, I need to know how to handle them safely.My first struggle was measuring voltages off the motor because the wires were not exposed. So I had to crack open the case in order to get access to the black and red wires. Does anyone have any good resources that talk about tear down, repair and measurement techniques for larger ac motors? I want to know how to incorporate more motors into my circuit builds

author: james_s
author_rank: Super Contributor
timestamp: October 03, 2020, 03:22:58 am
content: I'm not sure what you're trying to measure. There's voltage going into the motor and current the motor is drawing, there isn't really anything electrical to measure inside the motor.

author: zrlopez
author_rank: Contributor
timestamp: October 05, 2020, 12:08:00 am
content: I was having trouble getting access in the motor for my multimeter probes because the wires were encased. So I started thinking about safe and proper techniques to make measurements of the winding resistance, torque, slip, armature voltage etc., etc.. Smaller dc/ac stepper and servo motors that you get in kits are pretty easy to measure because the black, red and brown wires are exposed. But larger motors you wanna sometimes poke around the windings with your probe to look for shorts and open circuits and stuff like that

author: Nusa
author_rank: Super Contributor
timestamp: October 05, 2020, 02:48:34 am
content: Larger AC motors are quite likely to be 3-phase. Do you know one when you see one?

author: james_s
author_rank: Super Contributor
timestamp: October 05, 2020, 03:26:28 am
content: That depends on your definition of "larger". 3 phase motors are very common in industrial settings and BLDC motors are getting more and more common in smaller applications. Most AC motors used in domestic settings are either series wound universal, or single phase shaded pole or PSC induction motors.

author: IanB
author_rank: Super Contributor
timestamp: October 05, 2020, 03:29:54 am
content: Matthias Wandell on YouTube has several videos where he explains how different kinds of motors work, how to figure out the wiring, start them, reverse them, etc.He has also done experiments with speed and power. For example, to measure torque you can attach some kind of load to the shaft like a fan and simultaneously measure rpm and input power. Torque can be directly calculated from that, if you make an allowance for the motor efficiency.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 05, 2020, 10:49:17 am
content: Quote from: zrlopez on October 02, 2020, 11:49:47 pmi sometime come across with the same problem on some other things. the problem lies when we thing too much in "voltage" term. we can safely assume a motor is pure inductive load, much like a simple inductor. we wont get anything if we measure the "voltage" on its 2 pins, except the voltage of its source, ie the mains 120-240Vac in your case. and we cant either dismantle the inductor without destroying it in hope to get anything meaningfull in the middle of it if we keep thinking in term of "voltage". to solve it, we need to think in term of "current", if you can see how much current the device draw then you will be able to correlate with torque. DMM can be used in AC current/ampere measurement setup by breaking the mains circuit and putting it in series, but thats a bit fiddly and some safety concern involved. if you have budget or too carefull about your life, you can get current probe such as Hantek CC-65 (can be connected to or view with a DSO, as well as a DMM with a BNC to DMM jack adapter) or Uni-T UT210E to do measurement, they are not so expensive you just put them around the wire without have to break any circuit or safety insulation. ymmv.

author: zrlopez
author_rank: Contributor
timestamp: October 10, 2020, 09:17:37 pm
content: Thank you for that tip. I'll make sure to check Matthew Wandell.