Sensing stalled ac induction motor

[Jester]

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?

[Terry Bites]

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.

[james_s]

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.

[ConKbot]

https://www.grainger.com/product/SQUARE-D-NEMA-Magnetic-Motor-Starter-5E748
Yeah, 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.

[timeandfrequency]

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.

https://www.grainger.com/product/SQUARE-D-NEMA-Magnetic-Motor-Starter-5E748

Graingers' stuff is as affordable as usual...

Many brands manufacture this type of devices and an acceptable price range is 80-130 USD
https://www.rockwellautomation.com/en-us/products/hardware/allen-bradley/circuit-and-load-protection/circuit-breakers--circuit-protection/circuit-breakers--motor-protection.html
https://www.allied-automation.com/wp-content/uploads/2015/02/MITSUBISHI_Motor-Circuit-Breakers-MMP-T-Series.pdf

Have also a look on Craigslist or equivalent to find a second-hand device.

[james_s]

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.

[timeandfrequency]

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.

Well, 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.

[Jester]

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 so

I figure I could make this for about $30 for parts and a little bit of code;

[james_s]

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.

[themadhippy]

You could do it with a $15 circuit breaker, some of them are pretty fast, e

and 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

[jonpaul]

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 courage

Jon

[Doctorandus_P]

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.

[james_s]

and 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

That'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 example https://www.ebay.com/itm/144673701088

I'm not endorsing that specific item, it's just an example.

[Whales]

I also recommend standard DIN-rail MCBs (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 few different instant trip speeds to 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. 

[Jester]

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?

[Whales]

but even a couple of seconds can be too long to prevent damage to the driven gear.

Eep.  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).

[Siwastaja]

I also recommend standard DIN-rail MCBs (like you use for house wiring).  You can get them in various amperages, various trip speeds,

No. 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

[Jester]

I also recommend standard DIN-rail MCBs (like you use for house wiring).  You can get them in various amperages, various trip speeds,

No. 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

Nowhere 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.

[Jester]

any DIY solution is a safety and fire hazard

Your electric panel mfg has,many types of breakers, in combination of fast, slow, magnetic, thermal trip.

The catalog of electric breakers will guide you to the motor stall protection for the types and HP you use.

Check

Square D, Cutler Hammer

Especially good forum,and advise for electric is the Mike Holt site.

Bon courage

Jon

I think your first sentence is nonsense.

[Siwastaja]

any DIY solution is a safety and fire hazard

I think your first sentence is nonsense.

I 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.

[Jester]

but even a couple of seconds can be too long to prevent damage to the driven gear.

Eep.  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).

The 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.

[james_s]

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.

[themadhippy]

Motor protector does not replace a fuse (or MCB) which is of course needed for fire safety, but acts in series with it.

More common here to have the overload sensor open the  control circuit

[Whales]

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?

Yes, 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.

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.

I like the idea of throwing the blade being a feature    Kind of like how phillips head screws cam-out.

Hope you find a good fix.

[amyk]

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.