Overload Protection for Table Saw Motor - Page 1
Forum section: electronics / general

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: September 08, 2018, 09:51:38 pm
content: Hi all:I recently picked up a '70s vintage Sears table saw that got thoroughly flooded in a hurricane. I liked it because of its cast-iron top and its ZERO price tag, but in trying to get it working I have run into a problem. The motor behaves erratically, and so I have decided to replace the power switch, start capacitor and overload protection relay, then see if the motor itself is OK. I may also open the motor housing and check the bearings for wear and the windings for corrosion; if need be I'll try to find a replacement.Sears used to sell a module incorporating the switch, cap and relay, but no more. So I need to select an appropriate overload protection relay. I searched here for threads dealing with this topic and got a little bewildered by "information overload"... Then it occurred to me that, owing to themarch of progress,a better/cheaper solution may be available. Any ideas would be most welcome.Motor specs: 120 VAC, 10A, 3450 RPM, single-phase.

author: tautech
author_rank: Super Contributor
timestamp: September 08, 2018, 10:17:37 pm
content: A bad Start/Run centrifugal switch can cause unexplained behavior.Either the Start/Run contacts or the centrifugal mechanism itself.Check for broken/missing springs and free unrestricted operation of the mechanical parts.Many motors have thermal overload circuitry inbuilt in the form of a bimetal thermal switch and that could be damaged from immersion and subsequent rust or the iron/steel parts.Once all these things have been checked as operational I'd still be taking this motor to a rewinder and have them do insulation and current draw checks to ensure it's safe to use.

author: Ian.M
author_rank: Super Contributor
timestamp: September 08, 2018, 10:22:58 pm
content: Tautech just posted much the same, but I've got a few extra comments.Odd are there's a centrifugal switch inside the motor housing that's responsible for disconnecting the start capacitor. Its contacts will certainly need to be cleaned, and you'll probably need to clean and lubricate its mechanism.If the flood was salt or brackish water it would probably be best to replace the motor, otherwise it will need stripping down and the bearings will need to be thoroughly flushed, degreased and re-lubricated if sleeve bearings or if ball bearings, replaced. Theoretically, sealed ball bearings might have survived, but seals over 40 years old will almost certainly have failed.There's no reason not to try to strip down the switch and overload relay - you are planning to replace them anyway so if you FUBAR them its no great loss, but with a little effort you can probably get them usable for testing. A 40+ year old start capacitor is probably bad even without the flood. If you cant get them usable, their internal condition may indicate that *ALL* other electrical parts on the saw need replacement.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: September 09, 2018, 01:37:42 am
content: Thanks for the timely and thoughtful replies, tautech and Ian.M.What I'm hearing from the both of you is that the motorandthe control circuitry are probably toast. Good thing I didn't spend any money! I kinda suspected that myself, but before I go sourcing another motor I will follow your advice and do the recommended inspections.Had I known that the table saw was direct drive (with the motor inside the housing) instead of the expected belt drive (externally mounted), I might have let it die a natural death in the landfill. Finding anexternalreplacement motor would be less of a hassle, since its form factor wouldn't matter; but finding a motor just the right size for the housing and the arbor is going to be tricky. Maybe paying somebody to refurbish the one I have is the best course of action.The good news is that this beast is equipped with a nice flat cast iron table with only surface rust. Definitely a keeper.

author: ignator
author_rank: Regular Contributor
timestamp: September 09, 2018, 03:05:56 am
content: Did you pull the motor apart and clean it as suggested? Ball bearings can be cleaned if you carefully remove the shields, de-grease, look for corrosion, and if OK, re-grease them, and install the shields.I would not give up on it, nor replace any parts until you have verified the start switch is working, that the start capacitor is not shorted or open (and I'm guessing that may be the only component to replace, as it is not a sealed component).You indicate minor surface rust, so it must not have been flooded too long. If the stator and rotor are not seriously corroded, then the varnished motor windings are probably OK.The overload is most likely a Klickson bimetal switch, just flush any dirt out to ensure it can operate properly.I have a direct drive saw that has an aluminum top (junk, but works), as long as you use a sharp blade you will be happy.

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2018, 03:09:49 am
content: I would bet the motor is not going to be too hard to repair, I've repaired a few really nasty rusted motors including one of the pump motors in my hot tub that was so rusted I had to turn down the output shaft on a lathe and shrink fit a stainless sleeve. After washing out the motor and cleaning off the rust it has been fine for more than a decade now.Open it up, wash it out with clean water, if it's the normal type that has a centrifugal starting switch inside it make sure that mechanism is not binding and the contacts are clean.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: September 09, 2018, 03:03:54 pm
content: ignator and james_s:Thanks for the encouragement! My good friends at eBay found me a replacement capacitor for about six dollars (free shipping!); this component, as ignator suggested, was the one most likely to fail after forty-plus years, even had the saw never been used. As also pointed out by ignator, the device that Sears calls an "overload protection relay" is in all likelihood a Klickson bimetal switch.I should note that the motor itself was NOT submerged in the flood, and I have not given up on it yet but will test it once I have the new start cap.In my haste to shorten my original post, I excised the question of "overload protection" which is in the title. I did read some other posts here and elsewhere, but am still a little iffy on what my parameters should be. In order to true up this thread, here is some background information for new readers. Basically, electric motors like the one in question can draw a lot of current, and power tools like drill presses and table saws by their nature are prone to locking up if a drill bit or saw blade lodges in the material being worked.This, in turn, can lead to very bad stuff happening, including heat buildup which could potentially destroy the motor and/or showers of flying shrapnel which might causegrievous bodily harmto the operator. That, as I understand it, explains the need for overload protection. So, let's say I'm feeling lucky about the motor itself; it may be just fine as is. But since I paid nothing for the table saw, I am of a mind to shell out a few dollars to purchase a brand-new switch and a suitable overload protection device, and here is where my confusion lies. There are various approaches to power-tool safety switches and circuit protection; some cost WAY more than I would ever pay for a brand-new pro-grade table saw. So, can anybody steer me in the direction of an appropriate relay or other device that will keep my saw happy?I know this is not a woodworkers' forum, but for the sake of others who may read the title and come looking for advice, I'd like to get some feedback from the forum members that might potentially apply to devices other than my old consumer-grade table saw. (Incidentally, somebody who lives less than twenty miles from me is selling the same model saw on eBay for $40! Wish I'd seenthatlisting first...)

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2018, 04:04:24 pm
content: Honestly I wouldn't worry too much about overload protection, if you want you could add a circuit breaker that will trip if the motor is overloaded but keep in mind the use case here. This is a table saw, it's not going to be running unattended and in most cases it's probably not going to run for more than a minute or two at a time in a home shop. It's highly unlikely that something is going to go seriously wrong without you noticing and they draw enough power in the first place that a fault like the start winding getting stuck on will probably trip the branch circuit anyway.

author: Ian.M
author_rank: Super Contributor
timestamp: September 09, 2018, 05:28:14 pm
content: You need to pull the motor end bell anyway to check the centrifugal switch mechanism, as it may have some light corrosion from excessive humidity after the flood, and its a fair bet that after 40 years its contacts will need cleaning and dressing. While you are in there, you could look at the possibility of adding a thermal fuse in series with the common and in thermal contact with the main winding on the stator if you are paranoid about motor burnout. However if you keep your blades sharpened with a proper set on their teeth so they don't bind in the cut, and avoid cutting green hardwood or anything else that's likely to cause the motor to bog down, there is very little risk of a motor burnout.Single phase induction motors typically don't 'grenade' as their speed is limited by the supply frequency, and their rotor is a lot more robust than other types of motor that use wound rotors. Just about the only way for one to fail dangerously would be if it had bearings in really poor condition (or loose metal debris inside the housing) that caused the rotor to seize suddenly enough at full speed for the inertia of the blade to sheer the arbour.I'd be far more concerned about the adequacy of a 40 year old blade guard (if its even got one) and the condition and security of the fence and riving knife, as they are far more likely to cause hazards in operation than any motor fault.

author: ignator
author_rank: Regular Contributor
timestamp: September 09, 2018, 07:40:04 pm
content: The only time I've tripped the overload protection is when ripping long boards. I was pushing too fast with too dull of a blade, in too hard of wood (Douglas Fir). I was making 2x2s from 2x4s.The only issue I can think you will have with your existing overload trip, is the contacts may be corroded, so if they trip, and you reset, it may leave a resistive connection that will trip easily in the future. That's a big what if. I would not worry about it until it's a problem. And the worse thing that would happen without the overload trip is the motor windings will smoke and burn up. You will smell it before critical failure.

author: rbm
author_rank: Regular Contributor
timestamp: September 09, 2018, 09:42:48 pm
content: Since you're into this so far, I'd recommend increasing the safety of the tablesaw by buying or building a magnetic motor starter. These are available as complete units, sized for the rating on the motor to which they are connected. The ready units have bulit-in overload protection. Alternatively, it's possible to wire up a contactor which will accomplish the same task. Some contactors have builtin overload switches as well, that will cut power to the holding circuit if the motor stalls. The advantage of a magnetic motor starter is that power to the motor is cut if the electrical supply fails, the overload trips or the operator hits a cutoff switch. The motor will not restart when the trip condition resets (i.e. electrical supply returns, overload condition clears or the cutoff switch is released. I have a simple motor start circuit on my tablesaw wired up to a remote cutoff switch that I can activate with my knee or foot. It allows me to keep my eyes constantly on the work and ensure no accident happens as might happen with a regular switch should I have to divert my attention to finding the off switch.I've attached a PDF document that shows the various circuits. Westinghouse or Telemetrique make motor start contactors.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: September 10, 2018, 12:40:26 pm
content: RBM: Thanks for posting that .pdf. The magnetic motor starter is the device I had in mind. Seems to me, based on reading the article, that this unit serves two very different functions: On the one hand, it is a "soft start" which protects the motor from inrush current spikes, while on the other it provides a "kill switch" which when activated removes the motor completely from the power grid. Have I got that right?Anyway, a cursory eBay search turned up an 00 model, which I believe would be appropriate for the motor I have, with a price in the $50 range. A little spendy, in my opinion, but who can put a price on personal safety? I DO like the foot- or knee-activated kill switch idea, even if it may be a bit of (heh-heh) "overkill", because in my experience, accidents happen when you're least prepared to deal with them (tired, bored, emotionally preoccupied), and a big red panic button can be a life-saver in that one instance when you really need it. With regard to personal safety, people run the gamut from "Macho Cavalier" to "Nervous Nellie"; on that scale, my needle points slightly toward the NN side.Getting at the innards of the motor had proven to be a bit of a hassle, owing to the fact that it's bolted to a pair of lead screws for controlling blade tilt and height, and the one controlling blade height appears to have been slightly bent...WTF? Plus, the whole carriage assembly is a rusty, sawdust-encrusted mess. I am approaching this project as a trial run; I'm learning tips and tricks that will almost certainly come in handy as other vintage power tools come into my possession. For the time being, I can't really justify the $50 expenditure for a device to protect a $50 table saw that presently isn't even operable, and may never be.

author: james_s
author_rank: Super Contributor
timestamp: September 11, 2018, 04:18:50 am
content: Quote from: Dumpsterholic on September 10, 2018, 12:40:26 pmNot really, the main purpose of a magnetic starter is that it requires line voltage to latch it on. If the saw loses power for any reason, the switch drops out and the saw will not start up unexpectedly when power is restored.They also normally incorporate overload protection and will cut out if the motor draws too much current.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: September 11, 2018, 02:51:25 pm
content: james_s:Thank you for clearing that up. Without casting aspersions on other, more safety-minded folk, I honestly don't see the need for either function. IF I had children, and IF I let them run around my workshop unattended, and IF I were prone to leaving my table saw plugged in with the blade protruding from the table, then the latch-on aspect might seem worthwhile; otherwise it seems superfluous to me.I suppose that, were I cutting a board and somehow lost power for a brief moment before it came back on, with the board still engaged with the saw blade, that might cause a problem when the saw restarted, sending the wood flying toward me at high speed. Then again, if the subterranean lizard people suddenly drilled up through the floor of my workshop, it wouldn't matter whether or not I had my aluminum-foil PPE in place--I would still fall into their clutches and be dragged down their tunnel, subject to torture, mutilation, genetic experiments, watching endless reruns of "The View", or worse.As for overload protection: The old-timers I've spoken to have all said that having a sharp blade and a firmly locked-down rip fence, both properly aligned perpendicular to the cut, will prevent most accidents. Also, not cramming the work piece into the blade too quickly, not cutting warped, green, or over-sized stock, and listening carefully to the motor, will give me as broad a margin of safety as can be hoped for. Unless those pesky lizard people come after me!As an update: Without undo cursing, I did manage to open up the motor case. There was quite a bit of fine sawdust inside, but the bearings seemed to spin freely and I saw no signs of heat or corrosion damage. The erratic behavior of the motor (stopping & starting, spinning backwards & forwards) can all be explained by a failed capacitor/start relay combination, or so I have been led to believe. Once I have the new cap, I'll do a bench test with motor clamped down somewhere and nothing attached to the arbor.

author: james_s
author_rank: Super Contributor
timestamp: September 11, 2018, 03:17:05 pm
content: Well people did manage to survive for decades without them, I suppose the most likely use case would be a power outage that occurs while you're sawing, you might fumble around in the dark trying to exit the room and not remember to turn off the saw. Most consumer type power tools I've seen don't have one though so I'm not going to push you to add one.I also agree that the overload protection is not really mandatory. Unless you are a clueless noob which it doesn't sound like you are, you'll recognize the sound and feel of the motor being overloaded and stop pushing. Most motors have some kind of thermal cutout built right into them, and a really serious overload will trip the branch circuit.