Table saw trips GFCI and noise problems

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Forum topic by gafortiby posted 11-05-2015 02:13 AM 2536 views 0 times favorited 20 replies Add to Favorites Watch
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5 posts in 1133 days

11-05-2015 02:13 AM

My Dewalt DW745 table saw keeps tripping my GFCI. Sometimes it trips the GFCI at load, sometimes at startup. I changed circuits, same result. I replaced the GFCI outlet, same result.

The DW745 is a universal motor type, 15Amp. It is on a 15Amp circuit by itself. The blade is brand new. I am just cutting dry pine boards with a normal feed rate. Nothing fancy.

The saw never tripped the GFCI in the first 100+ hours of use connected to my shop-vac and cleaned every night. Then I lent the saw to my friend who kept it for a month and he used it to build some plywood cabinets. I have no idea how it was used but when I got the saw back it was caked with sawdust in every crevice, slot and gear. I am not sure how he managed to cut anything since the rip fence was seized up due to sawdust accumulation. I cleaned everything but since then, it trips the GFCI frequently. Perhaps the motor had overheated? Table saw skills notwithstanding, this is a good friend and no I am not going to ask him to fix it.

I do not want to use the saw without GFCI, especially since this “job-site” table saw does NOT HAVE a ground plug. It has a 2-prong plug, not a 3 prong plug.

I think there is something wrong with the motor. The brushes are in good condition but just in case I cleaned the commutator which was slightly dirty, but it made no difference. Sometimes I can cut for an hour before the GFCI trips. Sometimes it trips on my first cut. Sometimes it trips as soon as I start the motor before the wood touches the blade.

My gut says the insulation is breaking down due to heat, there is a suspicious looking spot on insulation for the winding where it looks like the insulation has a gap. I have not checked it with a multimeter. I only have a cheap multimeter so no Megger test for me. I am planning to attempt a fix by putting some motor insulating varnish (class F) on top of the spot. If the insulation breakdown happens to be on the surface of the winding i could get lucky.

What other places do I need to check? Recall that I have checked the house circuits and they are fine. None of my other tools trip the GFCI. I am using an extension cord 12 gauge 25 feet, tried with other extension cords and same result. I think the problem must be in the table saw.

I have “infinite resistance” at the prongs with the switch off and zero resistance with switch on. Infinite resistance is just 2M ohms for my meter so that’s all I can test for now.

20 replies so far

View Pezking7p's profile


3217 posts in 1850 days

#1 posted 11-05-2015 02:22 AM

I think you’re spot on with overheated motor and insulation breakdown. But also check that the starter switch that disengages the starter capacitor is moving freely and is set correctly.

I have no suggestions, though, other than looking for a replacement motor.

Could also be a good time to shop for an upgrade if you’re in to that type of thing.

-- -Dan

View Robertd's profile


59 posts in 2384 days

#2 posted 11-05-2015 02:23 AM

check the start capacitor, if it is defective the motor will draw more current to start and trip the gfci.
Bob D

View MrUnix's profile


7041 posts in 2398 days

#3 posted 11-05-2015 02:50 AM

Not specific you your particular saw, but pretty much any saw that has one of those universal motors like yours – the most common point of failure is associated with the rear bearing, and it’s heat that will cause the grief.

The problem is that the rear bearing is supported by a rubber cup inserted into a plastic housing. Here is what the rear bearing looks like with the rubber cover pulled off:

For infrequent, light and intermittent use, it’s fine. But as soon as you try to use it for any length of time, or on really heavy or thick stock, that bearing heats up (as all bearings do), and the rubber/plastic can’t handle the heat. I wouldn’t be surprised if your buddy went gorilla on that saw, particularly with the condition you got it back in. Unfortunately, the only way to check is to pull the motor apart. Not real hard, just 5 screws, but still a PITA.


PS: If the motor housing was something like cast aluminium instead of plastic, those motors would last much, much longer… but would probably cost twice as much to produce… molded plastic is cheap in bulk :)

-- Brad in FL - In Dog I trust... everything else is questionable

View SuperCubber's profile


1042 posts in 2483 days

#4 posted 11-05-2015 03:28 AM

I’m not electrician, but could it have something to do with operating a 15 amp saw on a 15 amp circuit? Shouldn’t it be on a 20 amp circuit.

I’m certain I’ve read, from several reliable sources, that doing so will shorten the life of the motor. Maybe the undue strain on the motor caused some extra heat buildup.

-- Joe | Spartanburg, SC | "To give anything less than your best is to sacrafice the gift." - Steve Prefontaine

View Redoak49's profile


3662 posts in 2187 days

#5 posted 11-05-2015 12:59 PM

The problem is clear…do not lend tools to friends. It is difficult but can prevent problems like this.

I was cutting a tree down with my chainsaw and a neighbor asked to borrow it. Both for safety and liability issues I told him that I would come over and help him. I will help someone with a job but not usually loan a tool. It is real awkward if the tool is damaged and you ask them to replace it.

Based on the post, the saw was abused and not even returned clean.

View Tennessee's profile


2893 posts in 2713 days

#6 posted 11-05-2015 01:14 PM

I agree with Brad, the motor is failing. Either a bad bearing or insulation breakdown.

And the 15 amp circuit has nothing to do with it. The breakers pop on amperage, not voltage. But it should have a 20 amp breaker if it is rated at 15 amps.

I understand your friendship, but I agree with Redoak. I have friends like that. I like them as people a lot. I’ll help them out in a heartbeat. I recently gave one who needed a portable CD player an extra one I had. Just gave it to him. Later he told me it overheated and died. A battery powered unit? So I asked him if the batteries overheated. He said no, he found an AC adopter around his house and it had the “right plug”, so he used it. I asked him if he had checked the output DC voltage on the adopter and he looked at me with blank eyes. Nuff said…

I have only one trusted neighbor who is also very respectful of tools that I will lend to. All others, they get me in the bargain and I am using the tool. Friend or not, I would hesitate to ever lend that friend a tool again, but that is your call. And it seems like you need a new motor…

-- Tsunami Guitars and Custom Woodworking, Cleveland, TN

View TinWhiskers's profile


179 posts in 1151 days

#7 posted 11-05-2015 02:15 PM

If you are worried about the tripping I thnk your first step is to put a grounded plug on. Would a Ground Fault breaker work without a ground? Just learned garages are required by code to have GFIs. Millions of us don’t. Try a portable GFI on a regular 20A circuit. Best test I use is to smell the motor. Hot motor smell is distinct. There are devices that you can plug the cord into to read amps.

View rwe2156's profile


3168 posts in 1680 days

#8 posted 11-05-2015 02:15 PM

From the sound of it I’m pretty sure the motor was overheated the motor by working it too hard.
These types of saws are not designed for ripping 8 foot sheets of plywood one after the other, and if they were feeding it too fast, then there you go, the motor got burnt up.

Big clue is when motors fail they often start drawing more amps.

Unless Brad is correct about what happened (I suspect its more than an overheated bearing) replacing the motor will be cost prohibitive and you will end up buying a new saw.

Time to have a talk with your pal…..

-- Everything is a prototype thats why its one of a kind!!

View TinWhiskers's profile


179 posts in 1151 days

#9 posted 11-05-2015 04:00 PM

View gafortiby's profile


5 posts in 1133 days

#10 posted 11-05-2015 04:06 PM

Thanks for the ideas.

This is a universal motor so there is no starter capacitor to check.
I don’t smell anything from the motor and I do know what a burning or overheating motor smells like.
The motor housing is plastic but the rear bearing rotates smoothly with no noise or play when I rotate by hand.
I woul love to run a 20 amp circuit but don’t have one, eventually I should add one to my garage shop.

I do have a kill-a-watt meter so I tired measuring current draw through that. the motor draws about 9-10 amps when spinning freely and only slightly more when cutting. So I don’t think the motor is totally burnt out and it seems safe to run on the 15 amp circuit as long as I don’t have other stuff on the same branch.

I learned a while ago that never lend anything, be it a book or $5. The real options are give or don’t give. So I took a risk lending the tool out, in my mind I was prepared to write it off forever because even though it’s a reliable friend you never know with electronics. Having gotten the tool back I’m just happy it runs at all.

I’m going to try the motor varnish and if that doesn’t work maybe it’s time to get an induction motor driven saw.

View dhazelton's profile


2793 posts in 2496 days

#11 posted 11-05-2015 04:19 PM

Have you even TRIED plugging the saw directly into another circuit? If you have a non-GFCI outlet upstream from that outlet I would try that to see if it trips the circuit breaker. GFCIs go bad. And yes, it should be on a 20 amp circuit. 15 is for general lighting. You should put a 20 amp circuit in your space for a table saw. Unless you plan on standing outside in the rain running your saw I wouldn’t worry about the plug.

View pintodeluxe's profile


5798 posts in 3012 days

#12 posted 11-05-2015 04:56 PM

An electrician can drop in a 20 amp dedicated circuit in about an hour. There is nothing more annoying than a tool tripping the breaker at an inopportune moment.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View gafortiby's profile


5 posts in 1133 days

#13 posted 11-05-2015 10:14 PM

FIXED! maybe.

I “ripped” a few pieces of 2×4 this morning at full blade depth (3”) successfully without tripping the GFCI. It is really resawing but since I don’t have a bandsaw I use my table saw for this often.

This morning I put the saw, by itself, on a previously-not-tried 15A non-GFCI circuit that normally powers my sprinkler system. The receptacle is in the garage but not GFCI because it is an exempt dedicated appliance circuit. I confirmed that this circuit had no other upstream or downstream outlets or switches and it is only about 10 feet away from our house main.

I put a plug-in GFCI unit (not a surge protector nor a AFCI but a real GFCI plug unit) and plugged the table saw into that. Plus I adjusted my feed rate to be slow as possible without burning. Did not trip. It turns out that outlets I was trying before is on the same branch as my basement mini fridge (pulling just 80 watts). Maybe it’s the fridge! That fridge never trips a GFCI by itself but maybe it was the fridge motor turning on-off in conjunction with the high table saw load that kept tripping things earlier.

Another thing I did: The wood was burning more than when I first bought the saw. So I checked everything and the riving knife was knocked out of alignment and causing some binding. I had already replaced the blade but didn’t think to check the adjustable riving knife.

I’ll have to see if I get any trips with continued usage. Knock on wood!

You guys are right though, I should get a dedicated 20 amp circuit. I am planning on this soon.
Actually TWO 20amp circuits, one for table saw and another one for simultaneous dust collection.

View MrUnix's profile


7041 posts in 2398 days

#14 posted 11-05-2015 10:46 PM

Unless Brad is correct about what happened (I suspect its more than an overheated bearing) replacing the motor will be cost prohibitive and you will end up buying a new saw.


Just to clarify… it’s not the bearing specifically, but the way it is housed. When the bearing heats up, the rubber cup around the bearing (and I’ve seen them with hard plastic or even metal on one particular saw) deforms… get it hot enough, and the plastic motor housing holding it all in place starts to warp. It doesn’t take much to throw things out of whack, and if it gets bad enough, it can let the armature start making contact with stuff it is not supposed to contact. That might be the source of the burned or otherwise damaged windings that were seen on that motor. I’ve seen them where the motor housing looked like it had been hit with a torch it was so melted from the heat generated, and the motor would shoot sparks out the vent holes when running.

Typically, as bearings start to age, they start to run a bit hotter, which accelerates the problem even more as time goes on. Usually when things start getting really bad, you will get the smell of burnt/melted plastic, but not always. If the plastic housing looks good, then all may not be lost.

The motor housing is plastic but the rear bearing rotates smoothly with no noise or play when I rotate by hand.

That doesn’t really determine much other than it hasn’t become totally trashed. A bearing with just a little bit of grease left in it will turn freely and not make any noise. If you yank the armature and spin the outer race by hand, it should not free spin more than a revolution or two… anything more and it’s time to replace as the grease is gone (and will run a lot hotter than it should). Fresh bearings have a bit of resistance to spinning due to the grease. That motor has some pretty inexpensive bearings in it… front is (or should be) a standard 6201-2RS (rubber seal) and the rear (in the plastic housing) should be a 6200-ZZ (although I’ve seen sealed ones in there as well). You can get both from a reputable supplier for a few bucks each, if that. Just verify what is there if you do decide to replace them.


-- Brad in FL - In Dog I trust... everything else is questionable

View gafortiby's profile


5 posts in 1133 days

#15 posted 11-06-2015 12:31 AM

Brad, thanks great information. Yes on this motor the bearing is basically housed in a thin plastic cup and if the cup were to crack or melt even slightly then the motor would definitely spin out of round.

It is good to know that the bearings can be replaced. Although if it froze and heated up the damage would be done before I would get around to replacing the bearings. It almost sounds like I would need to replace them preventatively.

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