ADXL335 accelerometer for detecting a stalled motor? - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: February 13, 2022, 03:03:25 pm
content: I'm looking for a non-invasive way of telling when a 1kw mains induction motor has stalled at startup, or the bearings are failing. A stalled motor gives a distinctive low frequency rumble. A running motor with failing bearings emits a high pitched whine. Would something like the ADXL335 do the job? Datasheethttps://www.analog.com/media/en/technical-documentation/data-sheets/adxl335.pdfIn the case of a stall at startup the goal is to recognize the condition within a second or so so I can stop it before the thermal current breaker trips.I have a working motor (used in a centrifugal water pump), but I don't have the ability to artificially force a stall or retrofit it with failing bearings so I'll need to 'train' the system to recognize a healthy motor and assume that anything out of the ordinary means trouble.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 13, 2022, 03:24:16 pm
content: But stall is trivial to detect from current, and almost all larger ACIM already fit the motor protection switch which is basically a fancy, adjustable automatic fuse. This doesn't even cost much, normal off-the-shelf product.For the high whine of bearings, you'd need something with more BW than an accelerometer, like a microphone or piezo transducer.

author: e100
author_rank: Frequent Contributor
timestamp: February 13, 2022, 04:01:09 pm
content: Quote from: Siwastaja on February 13, 2022, 03:24:16 pmI'm in Australia, the worst place in the world for DIY electrical. Legally only a licensed electrician is allowed to touch the wiring and all the hardware has to be approved for use in Australia, hence the requirement for a non-invasive solution.

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2022, 04:11:03 pm
content: So you cant even add a clip-on current transducer that fits over the insulation?e.g.https://www.sparkfun.com/products/11005N.B. it must be fitted on a single wire carrying AC current, If Line and Neutral in a cable both pass through it, the currents cancel out and it doesn't work.

author: e100
author_rank: Frequent Contributor
timestamp: February 13, 2022, 04:29:16 pm
content: Quote from: Ian.M on February 13, 2022, 04:11:03 pmCorrect, modifications of any kind, such as stripping the outer insulation jacket to reveal the 3 insulated conductors, can only be done by a licensed electrician.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 13, 2022, 04:45:12 pm
content: Regulations are everywhere. Just Do The Right Thing and don't get caught.

author: e100
author_rank: Frequent Contributor
timestamp: February 13, 2022, 04:55:33 pm
content: Quote from: Siwastaja on February 13, 2022, 04:45:12 pmIn Australia unapproved electrical work automatically invalidates your house insurance. Would you take the risk?The are also large fines and the possibility of a prison term. Australia really is the worst place for DIY electrical.

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2022, 05:14:38 pm
content: Quote from: e100 on February 13, 2022, 04:29:16 pmI assume you also cant *legally* open the panel or junction box to clip a current transformer like the above, on where the wires are already separated by the licenced electrician?That makes it a bit harder.You could pick up the magnetic field of the small loop area between Line and Neutral in the cable where it is accessible to *anyone*, by using two hall effect sensors carefully positioned either side of the cable jacket, held by a non-conductive clip or even cable-tying the cable to your PCB so it lies between the hall sensors. Rotate the whole assembly round the cable for maximum output. With correct sensor orientation external magnetic fields can be made to cancel out. Unfortunately it still wont work reliably if the cable is steel armored or in steel conduit.

author: tooki
author_rank: Super Contributor
timestamp: February 13, 2022, 06:52:00 pm
content: Couldn’t you just detect the mechanical motor movement (or lack thereof) using some kind of noninvasive sensor like inductive, magnetic, or optical?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 13, 2022, 07:20:25 pm
content: Quote from: e100 on February 13, 2022, 04:55:33 pmSame here. And the answer is yes.Does not matter because insurance companies are total scam anyway, if anything happens then you can only start a legal fight to get what belongs to you, and only lose. Besides, in an old house there always are some undocumented installations, so no one can say if an installation is legal or not. Just make sure your piece of wire won'tactuallycause the fire - if that happens, it's only right that you'll face all the consequences. Similarly, I loathe the system where a piece of paper (sole purpose of which is to keep trade union mafia happy) is a license tokilldo any illegal deathtrap with no risk of any legal consequence. It should be the other way around, professionals should be monitored and punished by at very least revoking their licenses if their installations are not up to code. Yet it's the normal people who are scared about even farting near a fuse box.Anyway, the only way anyone can show something is illegal is if the installation is not to the code. So make your illegal installations properly, and to code. But not too well because they stand out compared to the hacks made by professionals.I have saved in excess of $10000 by doing a few simple electric installations instead of paying to professionals for the same job, with the added benefit that unlike when done by professionals, my own installations are properly done, to the code, and safe, so the house won't burn down at least due to those installations. As soon as I pay the last installments of my mortgage, I'm extremely happy to terminate the only insurance I have, which is just to satisfy the bank. The saved money allows me to cover for unexpected loss of property or whatever. The insurance system here is only meant to cover very small (a few hundred $) loss of property because this is all stupid people care about, no one thinks about large damages.In this kind of banana country, it's best to mind one's own business and just Do The Right Thing and not care about stupid made up requirements others. But of course, the requirement is then that you really know what you are doing. For the Average Joe, maybe all the regulations are net positive, after all. The last thing I want is anyone doing death traps freely. That's why the punishment should be due to dangerous installation, not due to lack of paper. This would also make professionals responsible for their work, but the trade unions are strictly against this as their job is to protect the professionals.For some, the optimum case is paying professionals to do the job "for the paperwork", then redo it all from scratch, properly. Most of the mere mortals just have to live with poor workmanship and ridiculously expensive everything. I feel so sad for them.In any case - motors that may stall require those protection devices. If you can't install one because you can't afford a professional and don't have guts to do the right thing yourself, do you realize that the end result also compromises safety*? Instead of a proper protection device, you are planning to design some complex DIY hack. This is the typical outcome of heavy regulation: loss of safety.*) unless you are really sure the motor is internally thermally protected. Still, protection device would increase safety.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 14, 2022, 01:52:36 am
content: Could you get the motor wired to be plug in, then add a smart plug with power monitoring?

author: Someone
author_rank: Super Contributor
timestamp: February 14, 2022, 03:26:32 am
content: Quote from: NiHaoMike on February 14, 2022, 01:52:36 amThis has been the workaround in industrial/commercial settings for a long time, make everything possible a plug in "appliance" and the regulations mysteriously vanish.

author: e100
author_rank: Frequent Contributor
timestamp: February 14, 2022, 04:50:09 am
content: Quote from: NiHaoMike on February 14, 2022, 01:52:36 amI'm not aware of any domestic smart plugs that give you the 100 millisecond sample time interval you need to detect a startup stall. They are basically designed for measuring energy consumption over longer time scales so as not to confuse the user with readings that jump around.

author: Berni
author_rank: Super Contributor
timestamp: February 14, 2022, 06:52:12 am
content: Well what are you going to do when you detect a stall? You need to be invasive to cut power to the motor and save it from death.You don't need 100ms resolution to detect a stall. The motor will constantly be pulling massive amounts of current from mains while it is in a stall. This is why stalls are so deadly for motors, since its consuming lots more than its rated power, at the same time all that power is going into purely heat and at the same time there is no cooling air being moved by the fan. Because of this the motor overheats in a matter of minutes, the enamel wire catches fire and finally shorts out to blow the fuse, but at this point the motor is charcoal.So id say if a smart power meter notices the motor drawing a lot more current than normal for 10 seconds straight its safe to say the motor is not having a good time and should be shut down. Lots of smart plugs also have a relay inside that can be used to cut the power to the motor.As a bonus this can also detect if the pump part is having a bad time. For a centrifugal pump the output pipe bursting can overload the pump causing the motor to draw too much power. If the input of the pump gets blocked or draws air then the motor current will drop while the pump starts to destroy its seals. If the pump is normally moving a liquid with consistent viscosity and flow rate then the current consumption will also be very consistant, so its easy to calibrate that as the normal value and just call anything that is +/- 25% from that as being a fault.

author: e100
author_rank: Frequent Contributor
timestamp: February 14, 2022, 09:44:31 am
content: Quote from: Berni on February 14, 2022, 06:52:12 amThe circuit is switched by an Australian approved smart plug. No electrician involved.Quote from: Berni on February 14, 2022, 06:52:12 amAs I said in my original post the goal is to detect the stall and cut power before the thermal breaker trips after a second or so. Resetting the breaker is a manual process and the cheap breakers built into Australian approved power sockets only survive 6 or so cycles before going out of spec.

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2022, 10:19:18 am
content: Assuming the smartplug's got load current sensing, hack the smartplug (or get a hackable one). Adding local motor overload protection in software shouldn't be too tough to do once you've figured out the toolchain, and doesn't leave any physical evidence lying around for any anal-retentive electrical inspector to see. Worst case, you *might* have to swap out a cap or two defining the current sensor's frequency response, but that's not an obvious physical change.

author: Berni
author_rank: Super Contributor
timestamp: February 14, 2022, 12:18:15 pm
content: Ah i thought the thermal protection was in the motor.Well in that case sounds like the circuit breaker in the socket is already taking care of it. It is a good thing that it requires a manual reset so that a human being comes over to check the state of the motor, then the human is also present when the motor is restarted to shut it back down if something is seriously wrong. The limited number of cycles also doesn't sound like an issue because a pump shouldn't be seizing up this often in the first place. If a centrifugal pump fails to start so often, then it might be a sign of other issues like water getting into bearings, stiff seals, bad start capacitors, junk inside the pump..etcIf the protection trips for seemingly no reason, you reset it and it runs fine for a long time again this might be the sign that the protection is going bad, or it might hint at the pump being overloaded and drawing too much current. Centrifugal pumps actually require resistance in the output pipe, without it the flowrate becomes too high and the pump becomes too hard to spin for the motor. This can be solved with a smaller pipe or slightly closing a valve.