Locked Rotor Detector circuit for small single phase induction motor - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: December 08, 2021, 09:41:21 pm
content: Has anyone here designed a locked rotor detector circuit for small single phase induction motor?I'm thinking a Hall effect current sensor and the cheapest uC with an A/D that you can actually purchase with the present supply line issues.Thoughts?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2021, 10:24:24 pm
content: I suppose that would do. Or even dumber: current transformer and latching relay?Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 08, 2021, 10:45:19 pm
content: Depending on the small 1 phase motor type, there may not be much difference ( only a few %) between magnitude of run current and start/locked current. The phase angle changes a bit.I took some readings , made vector diagrams some years ago of a capacitor run motor, but can't find the dir I put it in.I'll post up if I find it.

author: Circlotron
author_rank: Super Contributor
timestamp: December 08, 2021, 11:11:33 pm
content: Power factor at locked rotor would likely be pretty awful. Feed the motor with several tens of volts from a transformer and put a scope on it and see how many degrees the current lags the voltage. If it is significantly different from normal running then you might have a doable method. Although... at normal running speed and low load the power factor will be low too, so maybe not so good.Is it the kind of single phase motor that has a separate starting winding and a centrifugal switch? If so, could you get inside the motor and bring out an extra wire from the starting winding to give an indication of when the switch is on because of very low rpm?

author: Circlotron
author_rank: Super Contributor
timestamp: December 08, 2021, 11:15:48 pm
content: Is your application to detect if the motor stops revolving, or to detect if it never starts in the first place? Knowing if it is only one of those and not both might simplify the solution.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2021, 11:51:17 pm
content: Yeah, assuming a "thermally protected" induction motor -- i.e. one that actually draws a lot of LRA. "Small" in this case would be probably 100-1000W. I mean, in the grand scheme of things, single phase motors are already quite "small", so I should probably assume even less -- but it would be nice to have the type given.For the other case ("impedance protected"), it simply doesn't matter, it's a non-issue -- it'll get toasty, sure, but it won't catch fire. You may still be able to sense it electronically (see above), but a far easier method is to just stick a motion sensor on the shaft and not care further.Typically "impedance protected" types are shaded-pole motors. They're very inefficient to begin with, so are ineffective over some tens of watts.Also, is this a design or maintenance problem? It matters, as a lot of us here are designers, and may assume anything about the design is up for grabs. Whereas in a maintenance context, you might have to put in a lot of work to add that rotation sensor.Tim

author: strawberry
author_rank: Super Contributor
timestamp: December 09, 2021, 12:18:19 am
content: if start current is present longer than 8s or motor stuck while running and start current is present= fault

author: Jester
author_rank: Frequent Contributor
timestamp: December 09, 2021, 09:47:33 am
content: This will be for a horizontal bandsaw with a capacitor start induction motor about 1/2 HP. These saws are gravity fed and turn the motor off when the cut is complete, so are often setup, started and then left unattended. Every once in a blue moon a chip will come off and jam the blade. Apparently the end result is often a burned up motor, I guess the thermal protection is inadequate.My first thought was some sort of motion sensor on the passive wheel, then thought it might be simpler to implement if I don’t need to run a cable for the sensor and simply measure the voltage and current, ignore the current during start-up and monitor for locked rotor current assuming I can differentiate between FLA and LRA.

author: Circlotron
author_rank: Super Contributor
timestamp: December 09, 2021, 10:05:28 am
content: Has the motor got a plastic fan inside a cowling on one end? If so, you could glue a small magnet to one of the fan blades and poke a threaded hall effect sensor through a hole drilled in the cowling and feed the pulses to a cct that detects if the pulses stop or go below a certain frequency.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 09, 2021, 09:26:11 pm
content: I wonder if there's shenanigans you could do with a magnet (DC or AC?) against the blade. Point being this would also protect against stalls in the drive train, saving belts/pulleys/tires as well as motors. Something like, when the blade is moving, does that block the field from a permanent magnet, as sensed by a Hall effect sensor on the other side? The blade being steel, this might be a bit tricky.In any case, eddy currents are dragged by the blade's relative motion, and this can be measured. If nothing else, something like a guitar pickup, but with AC excitation in the middle, and diffing the signal from two flanking coils (upstream and downstream), that should do it.Could potentially wind the whole thing on a single half 'E' core..?Tim