Sensing stalled ac induction motor - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: February 23, 2023, 06:03:57 pm
content: I have a 4x6 horizontal bandsaw for metal cutting powered with a 1/2 HP single phase 120Vac induction motor. Anyone that has used one these knows sooner or later a small wedge of metal from what is being cutoff ends up jamming the blade and because these saws can be left unattended while cutting can result in the motor burning up or the worm gear getting trashed.So I want to make a simple circuit probably using a hall effect current sensor to figure out when the motor has stalled and then cut the power ASAP. The worm gear is more of an issue than the motor is in some ways because the worm gear can get trashed very quickly.So my thought de jour on implementing this is hall effect device driving an analog input on a cheap uC. Need to ignore start-up current for perhaps 1s and then trip on anything over some threshold might be as simple as say anything over 10A for 100ms. Power supply might just be capacitor fed from line voltage. Enough to energize a relay.Seems simple enough, can anyone spot any obvious flaws in the concept?

author: Terry Bites
author_rank: Super Contributor
timestamp: February 23, 2023, 07:02:08 pm
content: The motor current will rise sharply as it aproaches the stall condition. Start up from 0RPM also draws a lot of current- its also a temporary stall condition.You need a delay to prevent an over current breaker tripping during start up.You can get breakers with delay. They trip your contactor via the Emergency stop loop.Seach for "motor protection relays"Loads on aliexpress.Personally, I wouldn't piss off my insurer. Say OMRON K8AK-AW, K8DT-AS.

author: james_s
author_rank: Super Contributor
timestamp: February 23, 2023, 08:37:56 pm
content: That was my thought, a circuit breaker, or even better, one specifically designed for motor protection. The locked rotor current of an induction motor will typically be several times the nominal rating.

author: ConKbot
author_rank: Super Contributor
timestamp: February 23, 2023, 09:25:39 pm
content: https://www.grainger.com/product/SQUARE-D-NEMA-Magnetic-Motor-Starter-5E748Yeah, I'd be looking for an off the shelf solution for something like this. You're barking up the right tree, it's just a solved problem. That may not be the exact model that suits you, but a contactor/overload relay combo like that will do what you wana, once the full load amps are dialed in.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 23, 2023, 10:18:21 pm
content: Hi Jester,As other members already stated above, using a dedicated thermal+magnetic breaker will protect your motor for sure. Such kind of device is really reliable and easy to wire/setup.It will trigger well before a stall condition, thus preventing the windings from frying and the worm gear to break.Simply set the current on the circuit breaker according to the nominal current indicated on the motor plate.Quote from: ConKbot on February 23, 2023, 09:25:39 pmGraingers' stuff is as affordable as usual...Many brands manufacture this type of devices and an acceptable price range is 80-130 USDhttps://www.rockwellautomation.com/en-us/products/hardware/allen-bradley/circuit-and-load-protection/circuit-breakers--circuit-protection/circuit-breakers--motor-protection.htmlhttps://www.allied-automation.com/wp-content/uploads/2015/02/MITSUBISHI_Motor-Circuit-Breakers-MMP-T-Series.pdfHave also a look on Craigslistor equivalentto find a second-hand device.

author: james_s
author_rank: Super Contributor
timestamp: February 23, 2023, 11:22:14 pm
content: I don't know how Grainger survives. Their prices on just about everything are obscene. Maybe if you're a really big industrial customer they give you a break, I don't know. My friend's shop used to buy things there sometimes before they realized they were getting fleeced.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 23, 2023, 11:55:58 pm
content: Quote from: james_s on February 23, 2023, 11:22:14 pmWell, they sell a very wide range of parts, devices, and other stuff.For industrial buyers, if they have a serious problem and are stuck, Grainger has the part available off the shelf for immediate delivery.They sell mainly top-notch parts provided by first tier manufacturers.If time matters, cost is not really an issue.Dealing with multiple distributors, sellers, orders and invoices can also be really PITA.For amateurs and private users, leafing through their catalog is an incredible experience, but the story usually stops there.

author: Jester
author_rank: Frequent Contributor
timestamp: February 24, 2023, 12:52:30 am
content: I don't know guys, I'm seeing prices $300-$697 plus the cost of a CT, that's more than what I paid for the bandsaw.The moment I hear thermal, I think slow, perhaps that's just terminology. Once "primed" ie. the motor is spinning, I would prefer to be able to trip in 100ms or soI figure I could make this for about $30 for parts and a little bit of code;

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 01:18:46 am
content: You could do it with a $15 circuit breaker, some of them are pretty fast, especially the magnetic type. I'm thinking the sort that are used in RVs and boats. You don't even need something fancy, even a slow thermal breaker ought to be fast enough to prevent damage to the motor. You just need to size it correctly.

author: themadhippy
author_rank: Super Contributor
timestamp: February 24, 2023, 01:41:12 am
content: Quoteand tend to trip out every other time you hit the start button.If the bandsaw already has a contactor fitted you may find there's an overload available that wires straight into it.Another option,instead of monitoring the current is to detect when the motor isnt rotating

author: jonpaul
author_rank: Super Contributor
timestamp: February 24, 2023, 03:56:59 am
content: Bonjour, à DIY solution may be a safety or fire hazard. Most motors are designed to be stalled without damage. Check nameplate of the motor.Your electric panel mfg supplies ,many types of breakers, with combinations of fast, slow, magnetic, thermal trip.See Website of electric breakers for motor stall protection for the motor types used. Eg, Eaton, Square D, Cutler Hammer breakers specifically designed for motor stall trip service.Especially good forum, see Mike Holt site and electricians forums.https://www.mikeholt.com/Bon courageJon

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 24, 2023, 05:11:23 am
content: For protecting the motor you do not need a fast acting gadget. The motor is not directly damaged by too much current, but too much current will lead to an eventual overheating, and it is the heat that damages the motor. And because the motor has a big thermal mass, it heats up slowly. That is why all those thermal cutouts have plenty of time to react.For the worm gear it is differently. This may get damaged by a shockload, and even 100ms may be far to slow. If some piece of metal breaks and jams the saw, something on the electrical side will not be of any protection. Even if the power was cut of immediately, the inertia of the motor itself may be enough to cause the damage. so you need to go in another direction for the solution.A breaking pin is a simple and cheap option.A friction clutch may work, maybe it's possible to lower blade tension, so the blade slips in case of a fault.And these cheap saws should never ever run unattended anyway. You should at least stay within hearing distance while it is running. For a "lights out" solution, you need a whole different set of safety precautions.

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 05:55:00 am
content: Quote from: themadhippy on February 24, 2023, 01:41:12 amThat's why I said sized appropriately. The breaker in the panel doesn't trip every time the saw is started, but there's a reasonable chance it would if the motor was stalled.There is also the ebay option, something like this for examplehttps://www.ebay.com/itm/144673701088I'm not endorsing that specific item, it's just an example.

author: Whales
author_rank: Super Contributor
timestamp: February 24, 2023, 06:02:51 am
content: I also recommend standard DIN-railMCBs(like you use for house wiring). You can get them in various amperages, various trip speeds, they have safety ratings and they're very cheap. So cheap that I'd recommend buying a fewdifferent instant trip speedsto find out what works best (I suspect type D but might as well try a type C as they're more common).EDIT: Maybe you don't care about instant trip speed, only slow thermal trip, so type D might be best. That way you can closer match the current rating.For current rating: choose a breaker that matches the appliance's specc'd rating. A fuse or breaker's current rating is what it can run indefinitely at without tripping (mostly).For a box to mount the breaker in you probably want a "surface mount box". You can get very small ones pretty cheap, they come with blanking inserts for the unused slots.

author: Jester
author_rank: Frequent Contributor
timestamp: February 24, 2023, 10:18:14 am
content: I'm much more concerned about the worm gear. The motor would likely be fine with a thermal (breaker) solution.When the blade gets jammed, the motor usually continues to turn, either the blade slips against the pulleys or the steel drive gear keeps turning and quickly eats the driven brass gear. It's far from a good design and the brass gear costs about $100. The saw provider had 200 brass gears in stock and they are available on Amazon that tells you something.I usually stay and observe while cutting, or at least stay within earshot, but even a couple of seconds can be too long to prevent damage to the driven gear.I don't plan to sell these just make one so I don't have to get a new gear again or experience the down time. IMHO the solution I'm thinking about will be safer than the present "thermal" protection offered by the breaker. I'm not concerned about the safety of whatever I implement, it will be safe. Once running I will try trip if greater than 2x nominal current for 2 cycles and see what happens.I guess I will build it and see how it goes. Thoughts on beefy relay vs. beefy triac?

author: Whales
author_rank: Super Contributor
timestamp: February 24, 2023, 11:14:40 am
content: QuoteEep. The motor itself has inertia. Perhaps even a single skipped tooth (milliseconds) is enough to write off the brass part? In that case the inertia of the motor might make this an impossible problem unless you also implement reverse electric drive to stop/brake the motor's intertia stupidly quickly (within a few tens of degrees?).I don't think this is a problem best addressed with electronics. Mechanics might be better.Perhaps a limited torque clutch between the motor and the worm gear? If it stalls then the motor just spins freely, not harming anything. There are infinite designs of these from the simple (piece of plastic tube pushed over two axles) to complex.My bandsaw doesn't use a worm gear, the small induction motor direct drives the main wheel, but its a small saw anyway (1.4m circumference blade). Sorry your design has these problems, that replacement worm price is evil. I wonder if a 3d printed part would be strong enough (then you could just treat them as disposable).

author: Siwastaja
author_rank: Super Contributor
timestamp: February 24, 2023, 12:21:41 pm
content: Quote from: Whales on February 24, 2023, 06:02:51 amNo. Standard circuit breaker is required anyway for every circuit for protecting wiring against fire, but is not very useful for motor stall protection. Instead, use one designed for motor protection. It's usual off-the-shelf product. It's like standard MCB, but curve is better shaped for induction motor protection AND it is adjustable, there is a screw for sensitivity adjustment you can experimentally tune to prevent false triggers but react to stall in time.OP should look into these products and if they still wish to build their own, use the existing product as motivation / reference.Random results from Google:https://kentstore.com/3rv2021-1ja10/https://www.electromaterial.com/PROTECTION-AND-CONTROL/HAGER/MOTOR-PROTECTION-HAGER

author: Jester
author_rank: Frequent Contributor
timestamp: February 24, 2023, 01:07:10 pm
content: Quote from: Siwastaja on February 24, 2023, 12:21:41 pmNowhere did I suggest not having a circuit breaker. In fact I stated a circuit breaker would offer protection for the motor.Many of the suggested solutions are three phase, this is a single phase motor.The driven gear will not be destroyed in 32ms but could be damaged if it runs for several seconds.

author: Jester
author_rank: Frequent Contributor
timestamp: February 24, 2023, 01:12:22 pm
content: Quote from: jonpaul on February 24, 2023, 03:56:59 amI think your first sentence is nonsense.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 24, 2023, 03:30:56 pm
content: Quote from: Jester on February 24, 2023, 01:12:22 pmI agree. The idea that there are zero people in this world capable of designing a safe motor protector circuit is ridiculous. Some people have designed the off-the-shelf units, after all.Motor protector does not replace a fuse (or MCB) which is of course needed for fire safety, but acts in series with it. This makes achieving safe design somewhat easier than designing an MCB.

author: Jester
author_rank: Frequent Contributor
timestamp: February 24, 2023, 05:29:55 pm
content: Quote from: Whales on February 24, 2023, 11:14:40 amThe inertia is a good point to consider, I expect that when the blade is jammed the friction at the worm gear will act as a significant brake. For testing I will put a sensor and see how fast or slow it stops.The saw in question is for metal and requires a much slower blade speed, hence the 20:1 worm gear. I'm guessing your direct drive saw is for wood or thin aluminum?I'm not sure but I suspect that if the blade jams just as the leading edge of the drive gear (that is pretty close to a knife edge) is just approaching the first tooth of the driven gear then there is a tendency for that edge to act like a knife and start shearing off that first brass tooth, once started it continues a bit like peeling an orange. On the other hand if this occurs while the leading edge is already engaged it does not have the same opportunity to start shearing and may simply throw the blade or stall the motor.

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 05:43:17 pm
content: Well I suppose you could measure blade rotation by putting a sensor on one of the other wheels the blade rides on and trip when you stop seeing pulses from that. This seems like a bad design in general though if stalling it destroys a gear, especially if the gear costs more than a few dollars.

author: themadhippy
author_rank: Super Contributor
timestamp: February 24, 2023, 06:00:32 pm
content: QuoteMore common here to have the overload sensor open the control circuit

author: Whales
author_rank: Super Contributor
timestamp: February 24, 2023, 11:31:40 pm
content: QuoteYes, mine is only good for light duty. The motor starts stalling if you try to cut anything too thick or too fast. I think I've gone as far as 1cm thick aluminium plate before (not sure of the alloy) but it requires patience and constant lube.QuoteI like the idea of throwing the blade being a featureKind of like how phillips head screws cam-out.Hope you find a good fix.

author: amyk
author_rank: Super Contributor
timestamp: February 25, 2023, 11:13:24 pm
content: The brass gear may be a sacrificial part. If you don't want it to be, then make an equivalent out of steel and mount it with a shear pin.

author: james_s
author_rank: Super Contributor
timestamp: February 26, 2023, 12:34:57 am
content: I think he said it costs $200 though, that's a heck of an expensive sacrificial part when a $1 key would do. Might not be trivial to make a replacement worm gear though.

author: Infraviolet
author_rank: Super Contributor
timestamp: February 26, 2023, 04:37:24 am
content: I'll just add a quick suggestion, shaft encoder on one of the rotating parts. If you could attach some magnets to a fast turning shaft you could use that hall sensor you already mentioned to detect them passing, too long a period without a signal edge and you cut the power via a relay, probably with a software rule somewher not to worry about insufficiently frequent signal edges durng the first X seconds of spinning up time after the saw is turned on.

author: elekorsi
author_rank: Regular Contributor
timestamp: February 26, 2023, 10:44:21 am
content: Quote from: Siwastaja on February 24, 2023, 03:30:56 pmThis is not true, typical motor protection switch (for example Schneider GV2, Siemens 3RV2) has build in thermal AND magnetic trip. It has a function of circuit breaker, so the fuse is not required. Of course you must stay inside of allowed short circuit rating, same as for all protection devices.Another thing is thermal overload relay, which doesn't switch power contacts, but only AUX contacts. In that case you need a seperate protection element (fise, MCB) and contactor which coil is wired through those AUX contacts of the thermal relay. (example: Schneider LRD, Siemens 3RU2)

author: jonpaul
author_rank: Super Contributor
timestamp: February 26, 2023, 04:32:01 pm
content: for a 1/2 hp motor on a saw, DIY stall protection is risky from a safety, liability and insurance aspect.Wiser to use a properly rated UL/ETL circuit breaker or motor protection relay.just my opinionj

author: james_s
author_rank: Super Contributor
timestamp: February 26, 2023, 05:29:30 pm
content: Quote from: elekorsi on February 26, 2023, 10:44:21 amOnly if it's installed in the panel feeding the circuit the motor is on. Typically a motor protection device is located near the motor. You still need a breaker to protect the wiring.

author: james_s
author_rank: Super Contributor
timestamp: February 26, 2023, 05:30:04 pm
content: Quote from: jonpaul on February 26, 2023, 04:32:01 pmHow is it more risky than the existing arrangement that has no protection?

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2023, 05:58:56 pm
content: Quote from: Jester on February 24, 2023, 10:18:14 amHave you asked the manufacture for their recommendation? They might be able to suggest a specific part.I agree with the other posts regarding a sheer pin/clutch/speed sensor being a good alternative.Going back to the electrical route. How about a fast acting circuit breaker, bypassed with a contactor driven by the start switch? As long as the start switch is held for long enough for the motor to spin up, it won't trip on start, or add a timer relay, to the contactor's coil. A fast magnetic breaker should cut the power quickly enough to stop the motor, before there's any damage. There should obviously be another breaker in the circuit, which is not bypassed, for protection against fire.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 26, 2023, 07:20:56 pm
content: Quote from: james_s on February 26, 2023, 05:30:04 pmThis, plus suggesting a circuit breaker for motor stall protection. A bad idea, especially for the 1/2hp motor which needs quite low (and accurately adjustable) threshold current. A circuit breaker would simply do nothing in case of stall.

author: Jester
author_rank: Frequent Contributor
timestamp: February 26, 2023, 11:30:26 pm
content: I'm not a gear expert but from what I have read the worm pair is a friction drive so the material of the worm needs to be significantly harder than that of the driven gear. Apparently using a worm gear in this application is simply a cost saving measure, worm gears are more typically used for things like lifting a drill press table ie, intermittent use not constant use. The replacement worm gears are expensive and a PITA to change.I'm going to make one as described and see what happens. If it works it will be worth the effort.Thoughts on a beefy TRIAC vs. A beefy relay?

author: james_s
author_rank: Super Contributor
timestamp: February 27, 2023, 03:03:01 am
content: I've seen worm gears in lots of long duty cycle applications, they're used when there is a need for a large ratio and they also cannot be back driven by applying force to the output shaft, a benefit in some applications. Kitchen mixers, automotive wipers, power windows, low speed industrial gear motors, motorized toys, etc.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: February 28, 2023, 05:46:00 pm
content: How about a shaft mounted zero speed switch. I’ve used them in the past to sense when a motor has stopped and safe to reverse.

author: floobydust
author_rank: Super Contributor
timestamp: February 28, 2023, 08:46:54 pm
content: OP can you give us the motor nameplate data. 1/2HP 115VAC I would expect FLA around 3.7-4.4A? 1,800RPM?I would do a simple experiment, measure motor current with nothing being cut and then jammed.The torque limiting-factor is either mechanical - belts/pulleys slipping, gears stripping etc. or electrical where the motor stalls or overload trips.Frequently, a whimpy motor is used as the protective element for the entire machine.If you want torque limiting, this is (perfectly) done using sensors for motor power and speed.Motor current can work (use a CT) but the power factor can give poor results at low torque and worse if the gearbox is lossy i.e. worm gear has lots of frictional losses so no load (current) torque can look high.Speed is important if the motor has relatively high slip and RPM sags, or careful there is no place for slip in the drive.You don't want a pulley/belt/blade slipping yet motor speed/load all looks good and you just roast the mechanical. I've seen many belts burned up this way.A triac is fine, dissipates around 1W/A for heatsink requirements. Or use a relay if that heat is a problem. Motor current alone can do quite well for a ballpark of machine load. If you measure it, that will tell us how complicated or simple this could be.

author: Infraviolet
author_rank: Super Contributor
timestamp: March 01, 2023, 06:12:27 pm
content: Worm gears, for long life, need to be run fairly slowly don't they? I've seen plenty of examples where over-torquing a worm gear setup somewhat won't damage it at all, (though may overheat whatever motor is driving the worm), but running really fast wears it away (or even melts it with plastic worm/wheel pairs).If this whole situation is a problem specific to the worm gear of this setup, what about buying some spur gears and stacking them in a compound setup to give a similar reduction ratio to the worm? Though I'd tend to think an encoder to monitor things would be simpler, mounted on the fastest spinning part you can access.

author: Jester
author_rank: Frequent Contributor
timestamp: March 18, 2023, 06:08:06 pm
content: Works as planned. At least if the blade jams I know it's going to shut down in about 8ms.

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2023, 12:35:28 am
content: What did you end up using?